Cyril Nicard

Matthieu Siebentritt

Coralie Jung

Audrey Le Fichant

Vincent Feynerol

Benjamin Bauer

Aude Nadler

Céline Arrighi

Pauline Ronfart

Paula Vieira Salomao

Nicolas Tran

Jean-Yves Joëlle

Ma Soeur Aline

Neveu Ronan

Ainsi Que

Nicolas Montesinos

Industrial PhD -Impact of the microstructure of ZnAlMg coatings for steel on the formation mechanisms and performance of Cr-free conversion coatings, ArcelorMittal MaizièresResearch

Keywords: Advanced level C1, Beginner level, Beginner level

Comprehension of the conversion coatings mechanisms on multiphase ZnAlMg coated steel, ArcelorMittal Maizières Research, IRCP (I2E), PSL o Understand Ti-based and phosphate-based conversion coatings formation mechanisms on multiphase ZnAlMg materials o Investigate an alternative conversion coating application process, improving corrosion resistance of painted ZnAlMg and suitable for industrial process o Determination of the corrosion mechanisms of Ti-based conversion coatings on painted ZnAlMg coated steels 2015 (6 months) Master thesis training -Applicability of SAF 2304 duplex stainless steel regarding its corrosion resistance in the GDI, Sandvik Materials Technology, Sandviken, Sweden o Evaluation of the pitting corrosion resistance of SAF 2304 in contact with ethanol-based biofuel o Determination of the pitting corrosion resistance of SAF 2304-containing an intermetallic phase formed during brazing 2014 (5 months) R&D training -Study of the twins created after tensile testing in a betametastable Ti-12Mo (%) alloy,

and mapping however have shown that PCC grains form a homogeneous layer on all phases while the distribution of TiCC on ZnAlMg phases is heterogeneous, binary eutectic phases being better covered than monophase Zn dendrites. The heterogeneous CC distribution was consistent with the worsened blistering resistance of TiCC-treated ZnAlMg surfaces compared to PCC, verified by electrochemical measurements, LEIM and outdoor corrosion tests.

The origin of the homogeneous or heterogeneous distribution of CC on ZnAlMg was proposed to be related to the ratio between the ZnAlMg microstructure fineness and the conversion coating elements size. For PCC, the galvanic coupling inside the eutectic phases and between the micrometric-sized ZnAlMg phases has little influence on the distribution of micrometric phosphate grains. For nanometric TiCC, the galvanic coupling inside binary eutectic phases can be considered as the origin of different precipitation kinetics and CC distribution. This way, the galvanic coupling inside phases can be critical for CC-containing nanometric-sized elements.

The hypothesis was confirmed by electrochemical measurements on pure phases simulating ZnAlMg microstructure and by comparing conversion coating precipitation on ZnAlMg materials and pure phases.

A solution was proposed to overcome the microstructure/CC elements size ratio issue, using a Cu or Fe cementation pretreatment prior to TiCC procedure. SEM, AES and XPS confirmed the deposition of metallic Cu or Fe during pretreatment resulting in a finer distribution of local cathodes/anodes on ZnAlMg surface as well as a resulting homogeneous distribution of the TiCC on ZnAlMg. LEIM experiments demonstrated the improved blistering resistance of ZnAlMg coated steel with the Cu pretreatment while scab test outdoor exposure confirmed the blistering corrosion resistance gain after 1 year of testing. 37: Predominant equilibrium species for 113 mM Zn 2+ + 57 mM Al 3+ + 57 mM Mg 2+ + 112 mM PO4 3-+ 66 mM Ti 4+ + 88 mM Mn 2+ + 420 mM F -using the Hydra-Medusa software and associated database of equilibrium constants at 25 °C. Complexes used in the simulation include (a) for Zn species alone: ZnO (cr), Zn(OH)2, Zn(OH)3 -, Zn(OH)4 2-, Zn4(OH)4 4+ , ZnOH + , Zn2(OH)6 2-, ZnOH 3+ ; (b) for Al species alone: Al(OH)3, AlOOH, Al(OH)4 -, AlOH2 + , Al13O4(OH)24 7+ , Al2(OH)2 4+ , Al3(OH)4 5+ , AlOH2 + ; (c) for Mg species alone: MgO, Mg(OH)2, MgOH + , Mg4(OH)4 4+ ; (d) for Ti species alone: TiO2, TiO(OH)2, TiO(OH)3 -, TiOOH + ; (e) for Mn species alone: Mn(OH)2, MnOH + , Mn(OH)3 -, Mn(OH)4 2-, Mn2(OH)3 + , Mn2OH3 + ; (f) for P species alones: H3PO4, H2PO4 -, HPO4 2-; (g) for F species alone: HF, H2F2, HF2 -; (h) mixed species: Zn3(PO4)2.4H2O, Mn3(PO4)2, MnHPO4, ZnF2, ZnF + , AlF2 + , AlF3, AlF4 -, AlF5 2-, AlF6 3- , MgAlO4, MgF2, MgF + , MnF + , MnF2, AlPO4, TiF6 17: Binding energies of compounds with energies fitting pure phases and ZnAlMg peaks after standard degreasing and immersion in the TiCC bath ................................................... 159 Table 18: Binding energies of compounds with positions fitting P 2s, Mn 2p and F 1s peaks on ZnAlMg after Cu2+-modified alkaline degreasing and immersion in the TiCC bath ........... 162

List of figures

List of tables

Abbreviation list

ZnAlMg: ZM: Zinc-Aluminium-Magnesium coating for steel typically containing 1.5-11 wt. Al wt. % in the automotive and pre-painted market. Thanks to the Al concentration increase and the addition of Mg, an improved corrosion resistance was allowed, reducing the sacrificial coating thickness as well as production costs of galvanic layers and increase of the lifetime of ZnAlMg coated steels.

To ensure the paint adherence on the ZnAlMg substrate, conversion coatings (CC) are applied prior to the different paint coatings. Due to the current European regulations, restrictions are in place about the use of phosphate and Cr VI -based CC [1], [2]. Therefore, new alternative CC are being developed for the last decade to replace the toxic Cr VI -based CC formulations. In the pre-painted market, Ti-based conversion coatings (TiCC) are regarded as a promising alternative.

However, in preliminary industrial tests made by ArcelorMittal Maizières Research on painted ZnAlMg coated steel dedicated to pre-painted market with TiCC, sometimes delamination and blisters were observed near the artificial scratches made down to the steel to simulate accidental defects that can occur in service life.

Unlike TiCC-treated ZnAlMg coated steel, ZnAlMg ones treated with phosphate-based CC (PCC) exhibit an enhanced blistering corrosion resistance. In addition, HDG coated steels treated with TiCC are not subject to blistering corrosion either. This way, the blistering issue results from a lack of adherence between the ZnAlMg coatings dedicated to the pre-painted market and the TiCC. One of the possible reasons of the latter can be an inhomogeneous TiCC distribution on the multiphase ZnAlMg substrate because of micro galvanic coupling between phases during the CC precipitation.

The identification of the origins of the different blistering resistance of painted TiCC and PCC-treated ZnAlMg, in particular, the impact of the microstructure on the CC distribution and performance and looking for a solution for improvement of TiCC performance were in the main scope of this PhD. The work was conducted in collaboration between ArcelorMittal Maizières Research and l'Institut de Recherche de Chimie Paris de l'Ecole Nationale Supérieure de Chimie de Paris. To achieve the objectives, several steps were necessary:

-The presence of preferential corrosion paths leading to the blistering on painted ZnAlMg coated steel treated with TiCC was confirmed.

-The reasons of formation of homogeneous PCC on multiphase ZnAlMg substrate were understood.

-The inhomogeneity of TiCC on multiphase ZnAlMg coating obtained by a standard treatment procedure was confirmed and the role of substrate phase composition and spatial distribution of these phases in the formation mechanism of TiCC was understood. The inhomogeneous distribution of cathodic zones was shown to be the main reason for the inhomogeneity of TiCC distribution.

-Based on the elucidated mechanisms, a pretreatment was proposed improving the TiCC The last chapter ("Conclusions and perspectives") summarizes the general conclusions brought by the thesis, their importance regarding the corrosion resistance of painted ZnAlMg coated steels and the perspectives of the study.

Several appendices give some additional details like design of specific experiments or technics, parameters used for XPS peaks deconvolution etc.

Chapter 1: State of the art

This chapter is a literature review about the structure and fabrication process of painted ZnAlMg coated steel. Additional research was carried out to compile the well-established degradation mechanisms leading to delamination and blistering on painted metallic substrate. This work was used as a basis for the methodology selected in the PhD. As the thesis will be presented in publication formatted chapters, it is worth noting that each chapter will include a short focused bibliographic introduction.

Basic knowledge about ZnAlMg sacrificial coatings for steel 1.Development of ZnAlMg sacrificial coatings

Historically, aluminium was the first alloying element to be incorporated to the galvanizing process at concentrations higher than the usual 0.2 wt. % Al used in continuous Hot Dip Galvanized steel (HDG). Notorious ZnAl sacrificial coatings in use nowadays are Galvalume ® (1972) and Galfan ® (1980) containing respectively 55 wt. % and 5 wt. % of Al [1]- [3].

It was later on noticed that adding up to 0.1 wt. % of Mg could significantly improve the corrosion resistance of Galfan ® in atmospheric and aqueous environments [4], leading to the commercialization of Super Zinc ZnAl4.5Mg0. 1 (1985). This promising discovery opened the way to the development of ZAM ® (1998) by Nisshin Steel, exhibiting 6 wt. % Al and 3 wt. % Mg [5]. Not only the corrosion resistance of the sacrificial coating was improved but the cathodic protection induced by the magnesium was also significantly increased. It led European steel companies but also Nippon Steel Corporation to manufacture new compositions of ZnAlMg from the early 2000s with various Al and Mg proportions [1]- [7]:

ZnAlMg microstructure

The microstructure of materials is strongly influenced by their production process. ZnAlMg coatings are produced by continuous hot dip galvanizing.

ZnAlMg production by Hot Dip Galvanizing

Whether the steel is galvanized with the classic zinc-based coating (Zn-0.2 wt. % Al) or with a ZnAlMg layer, a similar Hot Dip Galvanizing process is used (Appendix 1). In the case of ZnAlMg coatings, the immersion bath is constituted of liquid zinc, aluminium and magnesium elements in a concentration defined for each commercial composition. In the framework of this PhD, ZnAlMg compositions with Al ranging between 1 and 11 wt. % and Mg up to 3 wt. % were studied, whether dedicated to the automobile or pre-painted market.

In the Figure 1 is displayed a scheme of the coating process. Both faces are coated with the same ZnAlMg coating thickness. The line speed is typically in the order of 100 m/min. First, the sheet surface must be free of contaminants and oxides to form a solid bonding between the zinc layer and the steel surface. To do so, an alkaline cleaning is performed to remove impurities and aluminium oxide. The steel sheet is heated in an annealing furnace to impart it the desired strength and formability. A protective atmosphere based on nitrogen and hydrogen

(5 % H2 and 95 % N2) is used in the furnace to prevent the formation of an oxide layer on the steel surface.

In order to make sure that the correct ZnAlMg coating thickness is applied on the surface, "gas knives" are used as they inject air on both sides of the steel sheet to remove ZnAlMg excess [11].

Parameters such as the bath composition, bath temperature and dipping time have to be controlled to get the targeted microstructure.

ZnAlMg microstructure as a function of Al and Mg contents

In ZnAlMg coatings microstructures, four solid phases can be found, namely a Zn-rich phase, Al-rich phase, Zn2Mg and Zn11Mg2. While Zn11Mg2 is rarely formed, other phases can form Zn dendrites, binary eutectics Zn-Zn2Mg, binary eutectoid Zn-Al and/or the ternary eutectic Zn-Al-Zn2Mg depending on the ZnAlMg coating composition (Figure 2). In the case of low magnesium-alloyed compositions, the ternary eutectic phase is considered as a binary Zn-Al phase due to the very low Mg concentration measured.

The variation of the chemical composition of the phases compared to the equilibrium values was attributed to the rapid solidification of the ZnAlMg coating [12]. It is also responsible for the fine and complex microstructure of the coating. Ratio of Al and Mg are usually adjusted depending on the desired mechanical, physicochemical and other properties. In the pre-painted market, Al and Mg contents were selected to allow a high flexibility associated with a corrosion resistance twice better than HDG one [5], [9]. For low Mg-containing ZnAlMg compositions, the binary Zn-Zn2Mg phase is mainly located at the surface of the coating, forming a continuous and surface located phase (first micron in depth), while Zn dendrites and the binary ZnAl phase form islands in the microstructure.

Corrosion resistance of bare ZnAlMg compared to HDG

It has been largely reported in the literature, through natural exposition and accelerated corrosion tests (VDA test, salt spray testing, scab test, Cl --free test etc), that ZnAlMg coated steels exhibit an enhanced corrosion resistance compared to Hot Dip Galvanized ones (HDG)

in their un-painted state ( [1], [3], [7], [8], [14]- [17] etc). However, different interpretations are proposed to explain this corrosion resistance gain.

The study of the corrosion resistance of ZnMg alloys compared to HDG coatings confirmed the beneficial effect of Mg alloying. It was explained that the formation of a dense layer of simonkolleite Zn5(OH)8Cl2•H2O, replacing poorly resistant ZnO on ZnMg surface, suppresses the O2 reduction [18] which is the main reaction controlling the corrosion rate of the substrate in neutral and/or slightly alkaline environment [19]. A long-lasting cathodic protection was attributed to the newly formed simonkolleite. In addition, the formation of Mg(OH)2 was reported by Hosking et al. to limit the O2 reduction [20]. It was also proposed that the role of Mg 2+ ions was to buffer the pH around 10 by the formation of Mg(OH)2 as well as to react with carbonate ions, preventing CO3 2-from reacting with simonkolleite or other Zn-based products, hence stabilizing them [14].

Considering the contribution of each alloying element in the improved corrosion resistance of ZnAlMg coatings compared to HDG ones, often the type of corrosion products formed on ZnAlMg was claimed to be at the origin of the corrosion resistance gain. Indeed, the formation of compact Layered Double Hydroxide (ZnAl-LDH) and simonkolleite corrosion products at the expense of poorly protective ZnO has been reported in many publications focusing on the characterization of the corrosion behavior of ZnAlMg compared to HDG coatings [1], [7], [8], [15], [17], [20]- [22].

Some other effects were also reported, for instance the released Mg 2+ was proposed to induce the formation of Mg-containing corrosion products but also to react with CO3 2-or SO4 2-to stabilize corrosion products such as simonkolleite or zinc hydroxysulfate by preventing them from reacting with Zn to form soluble or low protective complexes [22]. The presence of Al was proposed to promote dense LDH formation at advanced stages of corrosion, as already reported in the literature [1], [7], [15], [21], while at initial stages of corrosion Al stays in insoluble form and selective dissolution of Zn and Mg occurs [22]. The comparison of ZnAlMg coated and HDG coated steels has also demonstrated that the corrosion resistance gain is dependent on the exposure conditions and chloride content [8]. It was demonstrated that in environments involving a high time of wetness and high chloride load, the corrosion resistance gain of ZnAlMg over HDG was very relative.

In general terms, it has been commonly admitted that the formation of compact corrosion products acting as a barrier to the oxygen reduction on ZnAlMg surface is the source of the improved corrosion resistance of ZnAlMg coatings compared to HDG coatings.

Hence, it enabled reducing by a factor of two the ZnAlMg coating thickness/weight while keeping an equivalent corrosion resistance as HDG coatings [5], [9]. As mentioned earlier (section 1.2.1.), the ZnAlMg, applied by hot dip galvanizing process, enables the formation of a sacrificial coating with a thickness of 10 µm [5], [9]. To ensure the paint adherence on the ZnAlMg coating and provide corrosion protection to the painted ZnAlMg coated steel, conversion coatings (CC) are applied prior to the paint. They are inorganic thin insoluble oxide layers with thicknesses ranging from 10 nm up to 1 µm depending on the type of conversion coating applied on the metal surface [24], [25]. Different layers of paint can be applied on the CC depending on the targeted physical, mechanical properties, corrosion resistance etc of the painted ZnAlMg coated steel among which a primer and a top coat. First, the primer, directly in contact with the CC, is applied to provide corrosion protection and ensure a good adhesion of the paint by wetting the surface. A top coat is commonly applied as the finish coating, promoting the cosmetic appearance and surface resistance required for the system. Depending on the exposure conditions, they might also exhibit UV, bacteria, condensation resistance properties etc. The paint thickness is comprised between 5 and 30 µm for the primer and 15 and 200 µm for the finishing coat.

Painted ZnAlMg coated steels

Depending on the targeted application, both CC and paint layers can be deposited using different processes. Indeed, in the automotive sector, the immersion procedure is in use [25] while in the pre-painted market, coatings are applied by coil coating [10].

For painted Zn-based coated steel, the overall paint adherence and corrosion resistance is ensured by CC. They have been extensively studied in the literature and their formation mechanisms on metal substrates is of electrochemical nature. They act as a physical and insoluble barrier limiting the O2 reduction on the metal surface, thus the corrosion rate [23], [24], [26]. Although various types of CC exist depending on the underlying metal substrate and application process, a common deposition mechanism pattern was observed for all of them.

Basics of conversion coatings procedure and formation mechanisms

Before the CC procedure, the galvanized steel surface is usually prepared by an alkaline degreasing to remove oil, grease and metallic oxides on the top of the sample. It is particularly efficient for aluminium and zinc oxides dissolution as they are very soluble in alkaline media [27]. The alkaline degreasing solution (10 < pH < 13) is composed of a base associated to surfactants and cleaning agents [28].

After the pretreatment, the conversion coating procedure is usually considered as a two-step mechanism. The first step is the establishment of cathodic and anodic sites on the surface, Chapter 1 : State of the Art responsible for pH increase and Zn-ions release (Fig. 4). The metal dissolution corresponds to the anodic reaction while the reduction of hydrogen ions or the reduction of oxygen constitutes the cathodic reaction. The interaction between the conversion coating electrolyte and the substrate defines cathodic and anodic areas on the surface. The pH increase acts as the trigger for the CC precipitation. Once a threshold pH is reached, the precipitation of the CC takes place, forming insoluble oxide layers of various thickness and isolating the underlying metal from the atmosphere. In function of the chemical composition of Chapter 1 : State of the Art the bath and the formed layer, different types of conversion coatings are known for galvanized steels. They will be discussed in the following sections.

Chromiun (VI) CC

Historically, chromium (VI) CC has been the most widespread conversion coating as it provides excellent corrosion protection and adhesion for polymeric-based paint [23]. The CC versatility enables using it over a wide range of metal substrates such as steel [23], Zn-based [28], [29],

Al-based [31]- [33], Mg-based [34] alloys etc. Their composition is considered as the simplest since the oxidant is strong enough to lead to the metal dissolution and the conversion coating formation. Cr VI CC are aqueous solutions of chromic acid H2CrO4, chromium salts (sodium chromate Na2CrO4, potassium chromate K2CrO4 or dichromate), hydrofluoric acid HF and phosphoric acid H3PO4 [23]. The 2-step deposition mechanism of Cr VI on HDG substrate can be described as follows:

-Anodic dissolution at anodes:

Zn → Zn + 2e (1-1)
Cr VI ions are very strong oxidants. Considering their concentration in the solution, the oxygen reduction and the hydrogen evolution are negligible. The reduction of Cr VI , present either as CrO4 2-or Cr2O7 2- [27], [33], occurs from pH = 2 [23]:

-Oxidant reduction at cathodes from pH = 2:

CrO + 5H + 3e → Cr(OH) + H O (1-2)
Or

Cr O + 8H + 6e ↔ Cr O . H O + 3H O (1-3)
The resulting CC thickness is about 100 nm [23]. In addition, Cr VI CC are known for exhibiting a "self-healing" property in case the coating presents a defect down to the steel [23], [31], [32].

The self-healing ability of the conversion coating is built during the redox reaction occurring on the surface [32]. Indeed, as the CC is forming, Cr VI ions get trapped inside the conversion layer. When a defect is created, the release of Cr VI that diffuse to the defect and react with the metal substrate can form a new CC in the defect, hence provide corrosion protection.

Although Cr VI CC proved their efficiency over the years, they represent a non-negligible carcinogenic, mutagenic and toxic threat [35], [36]. Legislations designed by the REACH organization (Registration, Evaluation, Authorization and Restriction of Chemicals) have required the development, and subsequent experimentation of various alternative Cr VI -free CC formulations.

Phosphate CC

Phosphate CC (PCC) are among the most used CC for a wide range of substrates such as iron, steel, Zn, Al and Mg-based alloys [37]- [42]. The trication phosphating is the most common chemical conversion process in the automotive industry. PCC coatings exhibit thicknesses in the micrometric range. The PCC composition consists of a phosphate source, Zn cations and accelerators (NO3 -, NO2 -etc.) [37], [43]. Accelerators such as chlorates, nitrites or nitrates…also serve to prevent the reduction of H + into hydrogen gas which disturbs the phosphating process. Additional cations (Ni 2+ , Mn 2+ etc.) can be added to the basic formulation.

While both, Mn 2+ and Ni 2+ , are considered to decrease the grain size of the precipitated phosphates, Ni 2+ is expected to act as a cathode and accelerate the surface reaction during the phosphating process but seal the pores of the PCC, limiting the anodic undermining [44]- [47].

For HDG, the PCC formation mechanism is represented by the following reactions were proposed [37], [38], [48], [49]:

-Anodic dissolution:

Zn → Zn + 2e (1-4)
-Cathodic reduction leading to pH increase:

NO + 2H + 2e → NO + H O (1-5) Or NO + 8H + 6e → NH + H O (1-6)
-PCC precipitation in pH range from 4.7 to 8.5 [49]):

2PO + 3Zn + nH O → Zn (PO ) . nH O (Hopeite) (1-7)
In the case of a tri-cation phosphating, the reaction becomes:

(3 -x -y)Zn (1-8)

A phosphate surface exhibit the presence of PCC grains as shown in the Figure 5 Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] To ensure a homogeneous distribution of the conversion coating on the surface, activators such as sodium titanium phosphates Na4TiO(PO4)2 are commonly used as an activation step immediately before phosphating. The deposition mechanism of the activator on the surface is described as follows 1) particles of titanium phosphates adsorb on the surface of the metal substrate due to their low solubility and help the germination of the phosphate crystals. Then,

2) a cationic exchange between sodium and metal cations occurs that leads to the adsorption of the salt MexTiO(PO4)2 on the surface [START_REF] Manchet | Mécanismes de corrosion et de délamination d'un composite acier/polymère pour le marché automobile[END_REF]. The information gathered throughout the literature reported a similar behavior of the activator.

Van Roy et al. demonstrated that zinc phosphate crystals preferentially nucleate on adsorbed activator particles during the PCC procedure [START_REF] Van Roy | Study of the phosphating treatment of aluminium surfaces: role of the activating process[END_REF]. According to Ogle et al., the activation step helps increasing the number of phosphate crystal per unit area while decreasing the crystal size, overall coating weight and minimize reaction time [37]. This interpretation was confirmed by Erbe et al. who compared the zinc phosphate distribution with and without activation step on an aluminium-silicon coated steel [START_REF] Erbe | Electrochemically triggered nucleation and growth of zinc phosphate on aluminium-silicon-coated steel[END_REF]. Indeed, they observed that the crystal size was reduced when an activation step was included in the process and that the size distribution of the crystals was more homogeneous.

Phosphating is a reliable conversion coating which does not rely on the use of Cr VI . However, the trication phosphating involving the use of nickel also includes danger [START_REF] Inrs | Nickel et ses oxydes -Fiche toxicologique N°68[END_REF]. Indeed, nickel of TiCC on HDG coated steel and demonstrated the formation of a mixed manganese/titanium phosphate Ti/Mn(PO4)x, zinc phosphates Zn3(PO4)2 and zinc hydroxides Zn(OH)2 using FT-IRRAS and X-ray Photoelectron Spectroscopy (XPS) [START_REF] Wilson | Formation of ultra-thin amorphous conversion films on zinc alloy coatings: Part 2: Nucleation, growth and properties of inorganicorganic ultra-thin hybrid films[END_REF].

Similarly, as for Cr VI -CC and PCC, the precipitation of TiCC on HDG is considered as a multistep process consisting of 1) the metal substrate dissolution accompanied by local alkalization from cathodic reactions leading to 2) the CC precipitation as titanium oxide (1-9 and 1-10) at pH = 2.5 -8.5 [24], [START_REF] Li | Characterization and performance of a Zr/Ti pretreatment conversion coating on AA2024-T3[END_REF].

-Anodic dissolution:

Zn → Zn + 2e (1-9)
-Cathodic reduction leading to pH increase:

O + 4H + 4e → 2H O (1-10) 2H + 2e → H (1-11)
-TiCC precipitation:

TiF + 4OH → TiO . H O + 6F (1-12)
The conversion coating is reported to be composed of two different phases. The first one corresponds to the different phosphates, oxides and hydroxides while the second one refers to the organic coating flocculating with titanium oxide. The conversion of the second phase starts at pH=7 when the organic phase flocculates with titanium oxide before leading to a precipitation at pH=9 [START_REF] Manchet | Mécanismes de corrosion et de délamination d'un composite acier/polymère pour le marché automobile[END_REF].

Globally, despite the various CC formulations, a common precipitation mechanism pattern exists between them. It is based on a 2-step mechanism: 1) the dissolution of the metal substrate at anodes accompanied by the oxidant reduction leading to a pH increase at cathodes and 2) the CC precipitation from a threshold pH. It makes the replacement of harmful CC such as Cr VI and PCC easier. However, an evaluation of the corrosion resistance of CC is necessary to ensure their ability to replace Cr VI -CC or PCC for the desired application.

3. Degradation mechanisms of painted ZnAlMg coated steels

Blistering corrosion of painted ZnAlMg coated steel with TiCC

Through natural exposure testing in an environment with a corrosivity index of C3 (for pure Zn) or C5 (for steel) according to the ISO 9223 standard, the corrosion resistance of painted ZnAlMg coated steel treated with TiCC was evaluated. After 22 months of natural exposure in a marine environment, blisters can be observed in case of a down to steel defect (Figure 7). In case of exposure in a semi-tropical environment, blisters can appear after 8 months of testing.

In the framework of this PhD, blisters are associated with circled local paint decohesion which are distant from the down to steel defect and not directly with it, unlike delamination. The blistering corrosion has already been discussed in the literature [START_REF] Petrie | Osmotic blisters in coatings and adhesives[END_REF]- [START_REF] Doherty | A quantitative study of blister growth on lacquered food cans by scanning acoustic microscopy[END_REF] but no study reported the case of blistering occurring only if the defect goes down to the steel.

General blistering degradation mechanisms

Different mechanisms have been described in the literature to explain the coating disbondment on metallic substrate ([60]- [START_REF] Leidheiser | Towards a Better Understanding of Corrosion beneath Organic Coatings[END_REF]) Mechanisms taking place near the defect will participate in the delamination development while degradation mechanisms occurring away from the defect will enable blisters growth.

Anodic and cathodic delamination

Anodic delamination

Anodic delamination is observed when a ZnAlMg coated steel, painted or not, exhibits a down to steel scratch. Indeed, when an electrolyte gets in contact with ZnAlMg and the steel, a galvanic coupling is defined between the two metals. It will lead to the preferential dissolution of the zinc-based coating as its own galvanic potential is lower than the steel one: The contact between the two metals will induce an increase of the dissolution rate of zinc and a decrease of the iron one compared to the dissolution rate they have when they are not in contact with each other. This way, the zinc gets sacrificially consumed to protect the steel. The dissolution of the sacrificial coating creates paint delamination as shown in Figure 8. In a neutral environment, the corrosion rate is defined by the cathodic reaction which is the oxygen reduction. The corrosion mechanism is the following:

-Anodic dissolution of the metal:

Me → Me + ne (1-13)
-Oxygen reduction at the cathode:

1 2 O + H O + 2e → 2OH (1-14)
The anodic delamination is also characteristic of a strong interface between the ZnAlMg and the paint. Indeed, the conversion coating is not affected by the dissolution and is adherent on the paint when delamination starts [START_REF] Amirudin | Corrosion mechanisms of phosphated zinc layers on steel as substrates for automotive coatings[END_REF]- [START_REF] Marcus | Corrosion Mechanisms in Theory and Practice[END_REF]. After paint removal, the coating is supposed to be located on the paint side.

For painted ZnAlMg steel with a down to steel defect, the anodic delamination takes place close to the defect, where the electrolyte is in contact with the steel and creates the galvanic coupling.

Simultaneously, at the delamination front, the cathodic delamination is predominant.

Cathodic delamination:

The paint delamination results from a local alkalization of the media leading to the destruction of the interface between the polymer and the metal hence an electrolyte inflow. At the front delamination, the metal potential is more cathodic than closer to the defect. A galvanic coupling is created between the metal substrate located at the delamination front and the metal closer to the defect. In the confined zone, the oxygen concentration brought in by the electrolytic inflow is lower than near the scratch. In addition, corrosion products form a natural barrier that slows chlorides and hydroxyl ions from reaching the delamination front. At the delamination front, the pH evolution is governed by buffering reactions, diffusion rates of reactants to the front and hydroxyl ions, oxygen reduction rate and volume of liquid trapped under the paint [START_REF] Amirudin | Corrosion mechanisms of phosphated zinc layers on steel as substrates for automotive coatings[END_REF]- [START_REF] Marcus | Corrosion Mechanisms in Theory and Practice[END_REF].

The initiated oxydo-reduction reactions are described here after (Figure 9):

-Anodic dissolution close to the defect ("open area"):

Me → Me + ne (1-15)
-Oxygen reduction at the delamination front ("confined zone"): Although a system exhibiting a scratch down to the steel will facilitate the incorporation of water through the scratch, water can also flow in through the paint which acts as a semipermeable membrane [START_REF] Amirudin | Corrosion mechanisms of phosphated zinc layers on steel as substrates for automotive coatings[END_REF]- [START_REF] Marcus | Corrosion Mechanisms in Theory and Practice[END_REF] hence also leading to blistering.

1 2 O + H O + 2e → 2OH (1-16)

Osmotic blistering

The driving force of the blistering formation is the concentration gradient of soluble products under the paint and the atmosphere [START_REF] Petrie | Osmotic blisters in coatings and adhesives[END_REF], [START_REF] Greenfield | The protective action of organic coatings on steel: A review[END_REF]. Indeed, polymers can be described as a semipermeable membrane which allows water to flow in through the paint toward the coating and the substrate. If soluble products are present between the paint and the substrate, the penetration of water will lead to an under-paint solution with a high soluble products concentration. The dilution of the highly concentrated solution is the equilibrium the system will tend to reach. A water uptake through the paint will lead to the growth of blisters. Water soluble products can be of very different nature (inorganic salts, corrosion products, water soluble solvent traces etc) and be incorporated inside the structure in each step of the manufacturing of painted steels, but also during their period of use as corrosion products can form. It has been reported that the concentration of corrosion products is a more prevailing parameter in blister formation than the presence of these products themselves at the paint/substrate interface. It means that soluble products are very likely to be found one way or another under paint but the concentration they reach is a more important parameter to be considered [START_REF] Doherty | A quantitative study of blister growth on lacquered food cans by scanning acoustic microscopy[END_REF].

One must also keep in mind that a lot of other parameters will influence the blistering ability of a painted material. Indeed, the CC and paint application modes and the internal stress induced from the paint application can be linked with the severity of blistering [START_REF] Prosek | The role of stress and topcoat properties in blistering of coil-coated materials[END_REF], [START_REF] Perera | Physical ageing of organic coating[END_REF], [START_REF] Piens | Effect of coating stress on adherence and corrosion prevention[END_REF]. However, these other aspects will not be further detailed in this literature survey.

Cathodic blistering

The corrosion mechanism leading to cathodic blistering is similar to the cathodic delamination.

The production of hydroxyl ions is responsible for the metal/polymer interface destruction. The blistering development can be described according to Figure 10.

Mechanical blistering

The mechanical blistering is related to the production of corrosion products which will put pressure at the metal/polymer interface until the coating disbondment is initiated [START_REF] Amirudin | Corrosion mechanisms of phosphated zinc layers on steel as substrates for automotive coatings[END_REF].

Publication focusing on the comprehension of the degradation mechanisms of painted metallic substrates showed that a weakness at the interface between the metal and the polymer is required for the corrosion to propagate [START_REF] Petrie | Osmotic blisters in coatings and adhesives[END_REF], [START_REF] Liu | Blistering failure analysis of organic coatings on AZ91D Mg-alloy components[END_REF], [START_REF] Greenfield | The protective action of organic coatings on steel: A review[END_REF].

Interface weakness of multiphase substrates treated with CC

The formation of blisters was not reported for painted HDG coated steels treated with TiCC. In addition, the blistering issue is not observed for painted ZnAlMg treated with PCC. The blistering evidences the lack of adherence and corrosion resistance of painted ZnAlMg coated steels treated with TiCC.

In addition, a ZnAlMg multiphase substrate can induce the galvanic coupling between phases hence impact the CC deposition mechanism.

Lostak et al. described the deposition mechanism of a ZrCC on a ZnAlMg substrate composed of a zinc rich-phase, aluminium rich-phase and a eutectic Zn-MgZn2 with EDS, AFM and SKPFM [START_REF] Lostak | Zr-based conversion layer on Zn-Al-Mg alloy coated steel sheets: insights into the formation mechanism[END_REF]. They reported that a heterogeneous ZrCC precipitation was taking place on all phases. The zinc-rich phase was found to be the most noble and exhibited the highest surface coverage rate while alloyed Zn-Zn2Mg and Al-rich phases were less covered in comparison.

They proposed that the heterogeneous precipitation was linked to the ratio of Zn exposed during the CC procedure. The coverage of alloyed phases was initiated after the anodic dissolution of Mg-rich and Al-rich phases respectively leading to a sufficient enrichment of exposed Zn for the CC to start.

Similar observations were made on different substrates exhibiting a galvanic coupling inside the microstructure.

Andreatta et al. [START_REF] Andreatta | SKPFM and SEM study of the deposition mechanism of Zr/Ti based pre-treatment on AA6016 aluminium alloy[END_REF] studied the effect of the microstructure of AA6016 alloy on the formation of Zr-TiCC using SEM-EDS, SKPFM and electrochemical measurements. They observed that the CC was preferentially formed on cathodic Al(Fe, Mn)Si intermetallics contained in the alloy. Nordlien et al. and Lunder et al. [START_REF] Nordlien | Formation of a zirconium-titanium based conversion layer on AA 6060 aluminium[END_REF], [START_REF] Lunder | Formation and characterisation of Ti-Zr based conversion layers on AA6060 aluminium[END_REF] draw a comparable observation when evaluating the Zr-TiCC distribution on AA6060 with SEM-EDS, GD-OES and TEM. They concluded that the precipitation of the Zr-TiCC was taking place on and around cathodic α-Al(Fe,Mn)Si intermetallics first.

Interesting solutions have been investigated to improve coating thickness and homogeneity.

Lostak et al. tested the addition of Cu 2+/ Fe 3+ ions to the CC bath to enable the precipitation of finely distributed Cu/Fe cathodes on HDG surface [START_REF] Lostak | Formation and characterization of Fe 3+ -/Cu 2+ -modified zirconium oxide conversion layers on zinc alloy coated steel sheets[END_REF]. It led to a CC film thickness increase due to the enhanced precipitation kinetics.

Regarding PCC, we did not see any communication about the heterogeneous distribution of PCC on multiphase Zn or Al based substrates. It evidences the satisfying PCC distribution and corrosion resistance performance on these substrates. One work related to the impact of the microstructure of a die-cast AZ91D alloy on the Mn PCC distribution was carried out by Zhou et al [START_REF] Zhou | Structure and formation mechanism of phosphate conversion coating on die-cast AZ91D magnesium alloy[END_REF]. They proposed that cathodic β phase of the alloy was first covered by flower-like phosphate nuclei at the expense of the anodic primary α phase. The resulting Mn PCC morphology was amorphous.

Conclusions of the State of the Art

As mentioned here above, different contributions and phenomena are simultaneously occurring on painted ZnAlMg coated steels during the corrosion process and several parameters are required for the blister formation.

In addition, the literature has evidenced the link between the blistering phenomenon and the interface metal/polymer weakness hence setting the focus on the CC. The impact of a multiphase substrate on the formation of TiCC or ZrCC has already been reported for various types of substrates. However, little information is available about the effect of multiphase substrates on the formation of PCC. It raises the following questions which will be addressed through this PhD thesis:

-What are the origin and degradation mechanisms leading to blistering corrosion of painted ZnAlMg coated steels treated with TiCC?

-Is there an effect of the microstructure on PCC coating formation on ZnAlMg coated steel? Can it impact the coating performance?

-Is the TiCC distribution related to ZnAlMg microstructure on painted ZnAlMg coated steels?

-If so, how to overcome the impact of ZnAlMg microstructure to obtain a homogenized TiCC distribution with improved corrosion resistance performance? The objective of this chapter is to give an overview of materials and techniques used in this work. The reason of their use will be explained. Details and specific parameters of measurements are given in the experimental section of the respective result chapters.

Materials

HDG and ZnAlMg materials were provided by an ArcelorMittal industrial line whose production is dedicated to the pre-painted market. Various ZnAlMg modalities containing 0-5 wt. % of Al and 0-3 wt. % of Mg. Industrial hot dip galvanizing process (Appendix 1) was used to prepare ZnAlMg materials based on the process described in the WO 2013/160567 AI patent [1].

Phases identified in model coatings microstructure are listed in the Table 3. Further details about ZnAlMg coatings microstructure is available in Appendix 2.

Pure metals were supplied by Goodfellow and the other phases were synthesized in collaboration with the University of Chemistry and Technology of Prague in Czech Republic (UCT Prague). Details on fabrication process are given in Chapter IV, V and VI. In addition, complementary information regarding pure phases microstructure is gathered in Appendix 3.

Characterization of phases after corrosion

Different characterization techniques were used to obtain complementary information.

Scanning Electronic Microscopy (SEM) coupled with EDS, Energy Dispersive Spectroscopy [2], [3], was used to verify the presence or absence of CC on ZnAlMg coatings or corrosion products on painted ZnAlMg coated steels. It wa also used to check the surface state as well as to obtain a first idea of the elemental composition of the observed phase or CC. For semi quantitative elemental analysis, 15 keV acceleration tension was used. In order to reduce electronic interaction volume EDS measurement were also made at 5keV. Depth penetration of electrons (R) can be calculated with Kanaya-Okayama Formula [4] (obtained from Bethe Formula [5]):

𝑅 = 0.0276 𝐴 𝐸 (𝑍 . 𝜌) (2-1)
where 𝑅 is the depth penetration, 𝐴 is the Atomic Weight (g/mole), n is a constant (=1.68), E0

is the electron beam energy (keV), 𝑍 is the atomic number and 𝜌 is the density (g/cm 3 ). This equation applied to pure Zn-based substrate gives R ~ 1.2 µm at 15 keV and R ~ 180 nm at 5keV.

Conversion coating distribution determination

Auger Emission Spectroscopy (AES) was also used to evaluate elemental concentrations of TiCC on ZnAlMg surfaces and can be used to obtain chemical state information from ZnAlMg surfaces after TiCC treatment. In association with Ar + sputtering, a depth distribution evaluation Chapter 2 : Materials and Methods of the TiCC on each phase of ZnAlMg coatings was performed thanks to the high spatial resolution achievable with AES (≈ 10 nm). Using this technique, quantitative elemental information can also be calculated.

The AES principle is based on the Auger effect and schematized in Figure 11 [6]. A high energy electron beam (2-10 keV) is used to ionize the sample surface. A core level electron is then ejected creating an inner-shell vacancy on the L3 level as visible on Figure 11. To fill in the energy gap on that level, a higher energy electron is transferred from the M1 level to the L3 one.

The released energy causes the emission of an Auger electron from the M2,3 level.

𝐸 = 𝐸 -𝐸 -𝐸 , (2-2)
Where Eka is the kinetic energy of an Auger electron, EL3 the energy of the L3 level, EM1 the energy of the M1 one and EM2,3, the energy level of the M2,3 level.

Conversion coating chemical composition evaluation

X-ray photoelectron spectroscopy (XPS), is a characterization tool used for extreme surface analysis (5 -10 nm) [7]. It gives access to the composition of the TiCC as well as the oxidation state of the element present on the surface. The principle relies on the irradiation of the sample with an X-ray beam generated by an electron gun bombarding a metallic target.

From the photon irradiation, the atoms of the sample surface are ionized by a photoelectric effect. The excited atoms will emit photoelectrons during their relaxation and their kinetic energy Ek is measured. According to Einstein's relation, the characteristic binding energy Eb can be calculated:

𝐸 = hν -(𝐸 + 𝜙) (2-3)
where ℎ𝜈 is the energy of the photon from the X-ray source and 𝜙 the energy required to extract the electron from vacuum which depends on the spectrometer (considered as negligible for the calculations). XPS can be used to determine the surface composition, evaluate elements and contamination concentrations quantitatively but also the chemical state of elements and their local bonding environment.

Conversion coating thickness measurement

GD-OES (Glow-Discharge Optical Emission Spectroscopy) in association with DiP (Differential interferometry Profiling) operation was used to perform elemental depth profiles of the TiCC on a ZnAlMg surface and evaluate its thickness. Samples are exposed to a plasma in a primary vacuum chamber, slowly sputtered by argon ions and neutral species accelerated into the plasma. The extracted elements are then excited by the plasma and will, in return, emit a characteristic photon out of which the wavelength can be calculated using Max Planck's equation [8]:

𝐸 = ℎ𝑐 𝜆 (2-4)
where E is the photon energy, h the Planck's constant (h = 3.336 x 10 -11 s.cm -1 ), c the speed light (X) and λ the wavelength.

From this technique it is possible to identify the nature of the material, perform quantification and determine the elemental depth distribution (Figure 13). The precise description of the used electrochemical sequence is given in the experimental section of each part chapter of this manuscript.

Local electrochemical analyses

𝒊 𝒍𝒐𝒄 (𝒓) = (𝝋 𝟏 (𝒓) -𝝋 𝟐 (𝒓))𝝈 𝒔 𝒅 (2-5)
where 𝜑 (𝑟) -𝜑 (𝑟) is the potential difference between the two microelectrodes and d is the distance between two Ag wires center. The local impedance was defined as:

𝒛(𝒓) = 𝑽 -𝜱 𝒓𝒆𝒇 𝒊 𝒍𝒐𝒄 (𝒓) (2-6)
where 𝑉 -𝛷 represents the potential difference between the WE and the RE in the solution.

Scanning Kelvin Probe (SKP) is a non-destructive technique used to measure the Contact Potential Difference (CPD) between the Kelvin probe and the surface sample tested [11]. This potential corresponds to the opposite of the Volta Potential.

Introduction

Coil-coated steel sheets are largely used in the building and construction industry as cladding, sandwich panels, tiled roof etc. The coil coating process ensures a high cost efficiency as the production is run without interruption, a low environmental impact and a surface quality in the end line. In addition, the coating weight can be continuously checked. The coil-coating process of multiphase ZnAlMg sacrificial coatings replacing monophase Zn-based HDG coatings has led to the appearance of blisters distant from the down to steel scratch after several months of outdoor exposure. One of the most common triggers for the blistering is surface contamination with soluble products, responsible for osmotic pressure [2], [8], [10] or corrosion process [6], [7].

Water, oxygen and local loss of adhesion between the polymer and the metal are required for blister development.

Under the blister, simultaneous electrochemical reactions can take place, creating corrosion products [6], [7]:

-Anodic dissolution:

Me → Me + ne (3-1)
-Cathodic reduction leading to pH increase:

O + 2H O + 4e → 4OH (3-2)
Blistering however had never been reported in the case of coil coated monophase Zn-based HDG with the same conversion coating and polymeric paints system. This indicates a possible link between the multiphase metal/polymer interface robustness and the emergence of blisters.

The objective of the present chapter is to identify the degradation mechanisms of painted

ZnAlMg coated steels which can occur in the presence of defects in the paint and sacrificial coating going down to the steel substrate. In order to localize the degradation and understand its formation, the morphology of the degraded samples was characterized using SKP and scanning electron microscopy while anodic galvanostatic polarization was used for an accelerated simulation of blistering for painted ZnAlMg coated steel.

Chapter 3: Comprehension of the degradation mechanisms of TiCC-treated and painted ZnAlMg coated steels 44

Experimental 2.1. Materials

ZnAlMg multiphase coatings were used for blistering studies and individual pure phases were used to confirm galvanic coupling.

Painted ZnAlMg materials

The ZnAlMg coated steel with a thickness of 500 µm was supplied by ArcelorMittal in the skinpassed state with a roughness of 0.95 ± 0.03 µm. ZnAlMg coatings were produced by hot dip process with a total thickness around 10 µm. The selected composition contained Al < 5 wt. % and Mg < 3 wt. %. Details about microstructure characteristics are summed up in Table 4. The samples of HDG were twice thicker, however the corrosion resistance of both materials without paint is supposed to be equivalent [11]. Further details about ZnAlMg coatings microstructure are available in Appendix 2.

Samples are coated with a 10 nm-thick conversion coating before the application of a polymer coating by roll-coater (thickness: 45 µm). Samples dimensions for the outdoor natural exposure were 100x200 mm. Characteristics regarding the pure phases are displayed in Table 4 and complementary information regarding pure phases microstructure is gathered in Appendix 3. The preparation of the phases is detailed in chapter 5 in which they are used as main materials. In addition, Al (99.999 % purity) supplied by GoodFellow was used.

Model phases

Outdoor natural exposure

Samples were exposed in a marine environment with a corrosivity index of C3 (for pure Zn) or C5 (for steel) according to the ISO 9223 standard for 24 months under a 5°-slope to evaluate their corrosion resistance when simulating the water retention occurring on tiled roofing. Prior to the exposure, a 6-cm long down to steel scratch was made manually with 1 mm Sikken blade and scratches depth were controlled using a Surtronic S 128 portable device. Two replicates were exposed for reproducibility purposes.

Electrochemical measurements

A three-electrode cell was used with a saturated calomel electrode (SCE) as the reference electrode, a platinum wire as the counter electrode and the sample as the working electrode.

Open Circuit Potential (OCP) measurements were performed using a Biologic VSP potentiostat.

The sacrificial coating dissolution was simulated by galvanostatic polarization of painted HDG and ZnAlMg pellets with a diameter of 32 mm with preliminary made artificial scratches.

Scratches up to the steel (length: 15 mm) were made manually with a 1 mm Sikken blade. The scratches depth was in average 50 µm controlled using a Surtronic S 128 portable device.

Samples were immersed in 1 L of NaCl 1 wt. % pH 7 electrolyte and a +1500 µA.cm -2 current was applied for 72 h on the coating. This current density is typical of what can be measured during a galvanic coupling between ZnAlMg coatings and steel when considering an oxygen reduction current density of 30 µA.cm -2 on the steel. Three replicates were tested in this experiment.

Surface characterization

Samples were cut using a hand shear to cut blisters in the center and observe their cross sections.

Samples with a thickness of 1 mm were finally polished by ionic beam using a JEOL SM-09010 cross polisher starting on the inferior face, opposite to the observation one. Polishing was carried out for 15 h at a voltage of 6 kV and a current of 200 µA.

The distribution of the TiCC deposited on model phases was investigated using a Field Emission Gun Scanning Electron Microscopy (FEG-SEM) on a JEOL 7800F device at an accelerating voltage of 5 kV, coupled with a Brucker Xflash 6160 Energy Dispersive Spectroscopy (EDS) device using a Si(Li) detector and equipped with QUANTAX 1.9 software (Bruker AXS).

Scanning Kelvin Probe measurement

The surface potential evaluation was performed with a VersaSCAN Electrochemical Scanning system manufactured by AMETEK Scientific Instruments. A Kelvin probe with a diameter of 500 µm was placed at 150 ± 20 µm of the sample surface and a mapping was carried out with a step size of 500 µm along both X-and Y-directions and a probe moving at a 500 µm/sec between 2 measurements. The sensitivity threshold was chosen at 50 mV and the overall scanned surfaces were 20x11 mm. Data treatment was made using the VersaSCAN software.

Results

Blisters morphology after field exposure

First of all, one should note that no blister was visible on the samples for which the scratch did not reach the steel substrate after any exposure. For the samples with scratch down to steel and exposed for 24 months in marine environment, a cross section was carried out along blisters and up to the down to steel scratch as shown in Figure 16. The local paint decohesion is also visible near well-developed blisters although no paint blistering is detectable from the top (bottom left picture). These areas with a weak adherence could correspond to early stage blisters which could further grow due to hygroscopic behavior of corrosion products and the osmotic effect.

Connectivity between blisters after field exposure

To verify if the blisters are indeed connected with the artificial scratch and between each other, a SKP analysis was performed on a painted ZnAlMg material with large blisters distant from the scratch. The obtained map is shown in Figure 18. In Figure 18, the grazing light photo of the blistered surface is displayed at the left corner while the topography mapping is visible in the center with the associated scale being in the top of the It is visible in the right image that the large blisters distant from the scratch match the low surface potential areas colored in green and blue while the yellow areas are consistent with an uncorroded coating and the red zone corresponds to the steel substrate. The corroded areas are substantially more extended than the blisters visible in grazing light. Some close but distinct blisters appear to be connected which confirms the presence of the corrosion path linking blisters. Finally, in the red-circled areas, a link between the down to steel scratch and blisters distant of 5-10 mm from the scratch is visible.

Considering that blisters do not form when no scratch is made, it is possible to suppose that galvanic coupling between the coating and steel enhances corrosion processes and that the blistering development is due to an enhanced anodic reactivity of the coating.

Artificial blisters formed under anodic polarization

To evaluate if indeed the anodic reactivity enhances blistering on multiphase substrate, galvanostatic anodic polarization tests were made on ZnAlMg coated and HDG steels with the same Ti-based conversion coating and industrial paint. The applied anodic current value was calculated in order to compensate the oxygen reduction current density (30 µA.cm -2 ) in the exposed steel area in the scratch. Samples after 72 h of polarization are shown in Figure 19. The mapping was done in the area of the delamination front where the corrosion is partial and no paint decohesion was detected. The oxygen signal in Figure 20 is associated with the presence of corrosion products and it is clear from Figure 20, that the oxygen distribution (light blue) closely follow the distribution of magnesium (dark blue) near the surface, confirming the selective corrosion of Zn2Mg phase [12].

The anodic behavior of Zn2Mg phase and its preferential corrosion is well known in the literature for unpainted ZnAlMg alloys and from electrochemical characterization of pure phases [12]. Measured in this work, corrosion potentials and OCP values presented in Figure Finally, it seems that under paint corrosion also promotes selective dissolution of Zn2Mg.

Discussion

The presented results argue in favor of the enhancement of corrosion propagation on multiphase ZnAlMg coating by anodic polarization. Indeed, the corrosion developed under anodic polarization by an external current and in an initial defect in the coating were linked with the galvanic coupling between the steel substrate and the ZnAlMg coating. This anodic undermining leads to initial delamination around the scratch. Then both, anodic undermining and cathodic delamination can take place. The multiphase ZnAlMg microstructure induces a galvanic coupling between the anodic Zn-Zn2Mg phase and the rest of the microstructure, fueling the anodic corrosion of Zn-Zn2Mg. At the same time, in the confined zone, a gradient of oxygen concentration between the center of the delaminated zone and the edge of the delamination front induces a slowly renewed oxygen concentration zone with a higher pH resulting from the oxygen reduction. The production of hydroxyl ions can initiate a cathodic delamination process from damaging the polymer/metal interface bonding, dissolving oxides etc [6], [7], [9], [13], [14]. The transport of water, oxygen and cations is also facilitated by a defect in the coating [23].

Chapter 3: Comprehension of the degradation mechanisms of TiCC-treated and painted ZnAlMg coated steels 52

The development of blisters will be also influenced by the presence of surface contamination with soluble products or a corrosion reaction on-going under the paint [2]. Indeed, because polymeric coatings act like semi-permeable membranes that will let water and oxygen flow [7],

[8], [10], [12], water and oxygen can get in contact with the polymer/metal interface on every location of the surface and the presence of soluble species will enhance osmotic pressure [7],

[8], [10] but also electrolyte conductivity and hence corrosion rate.

The inhomogeneous distribution of conversion coating or its low chemical stability can also contribute to the process leaving uncoated areas locally and enhancing the electrolyte uptake from the scratch [15]- [22]. Indeed, in the case of a multiphase coating, the conversion coating distribution can be heterogeneous on the surface, creating locally uncoated areas that could constitute weak points [15]- [22]. This hypothesis will be studied and confirmed in a following chapter of this PhD.

Globally, the combination degradation mechanisms leading to the formation of blisters is summed up in Figure 22 here after: The SKP mapping demonstrated the existence between the visually isolated blisters of the interconnected path characterized by lower than the surrounding matrix Volta potential values.

SEM observations of cross sections revealed that these areas contain corroded ZnAlMg coating under intact paint and that initial lack of adherence can originate from these corroded areas.

Similar to unpainted ZnAlMg corrosion, it seems that under paint corrosion of these coatings starts with selective corrosion of Zn2Mg phase.

Galvanostatic anodic polarization confirmed that delamination from a multiphase ZnAlMg coated steel is more severe than from a monophase HDG steel. This fact is in favor of the hypothesis that a galvanic coupling between phases of the coating can enhance reactivity.

Another hypothesis explaining the role of the multiphase microstructure can be related to nonhomogeneous conversion coating on multiphase substrate resulting in local lack of adherence.

The latter can contribute to easy access of water and oxygen to the ZnAlMg substrate but also to a localization of the corrosion process.

The exact role of the multiphase microstructure in the performance of painted ZnAlMg coated steel still needs to be understood. Indeed, it cannot be ruled out that the starting point of the corrosion propagation under the paint could be a combination of the initial inhomogeneity of the coating, its low chemical stability and the ZnAlMg substrate corrosion. Considering that the Ti-conversion coating thickness is nanometric, it is difficult to make a distinction between the degradation of the metallic substrate and of the conversion layer under the paint. The issue of conversion coating distribution on multiphase substrate will be addressed in the next chapters of the thesis.

Introduction

Phosphate conversion coatings (PCC) are among the most used conversion coating for galvanized steel and have been broadly studied on various substrates [1][2][3][4][5][6]. PCC are inorganic layers of micrometric thickness applied for pre-painted materials in order to improve corrosion resistance and ensure paint adherence on metallic substrates.

The PCC composition consists of a phosphate source, Zn cations and accelerators (NO3 -, NO2 - etc.) [1], [7]. Additional cations (Ni 2+ , Mn 2+ etc.) can be added to the basic formulation. While both, Mn 2+ and Ni 2+ , decrease the grain size of the precipitated phosphates, Ni 2+ is expected to accelerate the surface reaction during the phosphating process and seal the pores of the PCC, limiting the anodic undermining [8]- [11].

The PCC formation mechanism is a complex series of processes. For HDG, the following reactions were proposed [1][2][3][4], [10], [12]- [14]:

1. Anodic dissolution:

𝑍𝑛 → 𝑍𝑛 + 2𝑒 (4-1)
2. Cathodic reduction leading to pH increase:

𝑁𝑂 + 2𝐻 + 2𝑒 → 𝑁𝑂 + 𝐻 𝑂 (4-2)
Or 𝑁𝑂 + 8𝐻 + 6𝑒 → 𝑁𝐻 + 𝐻 𝑂 (4-3)

3. PCC precipitation in pH range from 4.7 to 8.5 [15]):

2𝑃𝑂 + 3𝑍𝑛 + 𝐻 𝑂 → 𝑍𝑛 (𝑃𝑂 ) . 𝑛𝐻 𝑂 (4-4)
Additionally, the formation of ZnNi alloys at the Zn/PCC interface was communicated [11]:

𝑥𝑍𝑛 + 𝑦𝑁𝑖 + 2(𝑥 + 𝑦)𝑒 → 𝑍𝑛 𝑁𝑖 (4-5)
The PCC process consists of several steps, including alkaline degreasing, rinsing, activation, phosphating and final rinsing. The Ti-based activation plays an important role for nucleation of PCC crystals on HDG [12], [16], [17].

Chapter 4: On the effect of multiphase microstructure of ZnAlMg substrate on the Ti-based activation and phosphate conversion coating distribution 59

Nowadays, HDG steel is more and more replaced by multiphase ZnAl and ZnAlMg coatings which offer longer corrosion resistance and reduction of the coating weight [18]- [21]. New ZnAlMg coatings have a complex multiphase microstructure composed of Zn dendrites, and binary and ternary eutectics formed by Zn and Al-rich phase and/or intermetallic Zn2Mg [19], [22], [23]. The reactivity of these phases being different [24]- [30], it can result in a micro galvanic coupling between phases and a heterogeneous spatial distribution of cathodes and anodes. The inhomogeneous surface reactivity can impact the PCC formation at different steps, including alkaline degreasing, activation or phosphating.

In the present work, we aim to verify if the interactions of PCC with multiphase ZnAlMg substrates are homogeneous and if the activator has an impact on the homogeneity of the layer via surface characterizations at different surface treatment steps of model ZnAlMg multiphase coatings and pure phases simulating ZnAlMg microstructures.

Experimental

Materials

Two types of materials were tested: model ZnAlMg multiphase coatings and individual pure phases.

Model ZnAlMg materials

Three types of ZnAlMg coated steel with a thickness of 500 µm were supplied by ArcelorMittal.

ZnAlMg coatings were produced by hot dip process with thickness ranging between 5 and 10 µm. 3 different compositions with different microstructures were selected containing Al < 5 wt.

% and Mg < 3 wt. %. The difference between the three compositions was in their phase composition which is summed up in Table 5. In coatings B and C, the typical size of phase was up to10 µm. In coating A, the microstructure was finer with phases below 1 µm. Only the composition A possessed the binary eutectoid ZnAl. All coatings were skin-passed resulting in a roughness comprised between 0.6 and 1.3 µm. Further details about ZnAlMg coatings microstructure is available in Appendix 2. Samples with a size of 100 mm×200 mm were cut from the supplied substrate and cleaned with a petroleum ether solvent to ensure complete removal of any oils or surface contamination.

Pure phases

For mechanistic purposes, Zn, ZnAl and Zn2Mg model phases with compositions matching the different phases contained in ZnAlMg coatings were prepared. Alloys were weighed in masses according to their planned composition and heated at 500 °C in furnace. Liquid mixtures were then filtered through ceramic filters (61 wt. % Al2O3 + 39 wt. % SiO2) with 10 pores per square inch porosity. The composition of the obtained phases is summarized in Table 6. The composition, crystallinity, grain size and orientation and hardness of all model phases were characterized by atomic absorption spectroscopy, SEM, EBSD, XRD and nanoindentation techniques. Complementary information regarding pure phases microstructure is gathered in Appendix 3. Samples were polished with SiC polishing discs (up to 4000 grit). After polishing, all samples were rinsed using ethanol and dried with compressed air.

Bication Zn-Ni phosphating solutions

Solutions used to prepare PCC on pure phases and model ZnAlMg materials are listed in Table 7. To determine the total concentration of phosphoric acid in the bath and the one available for the phosphating, total and free acidity were controlled. Titrations were performed using a 0.1 N NaOH aqueous solution for 10 mL of PCC bath reaching values of 30-32 mL for total acidity and 2-3 mL for free acidity respectively. Phenolphtalein and bromophenol blue indicator solutions were used to observe the color change during the titration of the total and free acidity respectively.

All baths were used maximum 12 h after mixing and renewed every 24 h. Fresh activator baths were used maximum 4 h after preparation while in order to verify the impact of the activator bath ageing on PCC distribution, the aged bathes were used 19 days after mixing. -Osmosis water rinsing at room temperature -Drying in hot air stream For ZnAlMg coatings, the alkaline degreasing was made by spraying on both faces of the sample for 12 sec. Three replicates were performed to control the reproducibility.

Electrochemical measurements

Open Circuit Potential (OCP) measurements were performed using a Biologic VSP potentiostat. A three-electrode cell was used with a saturated calomel electrode (SCE) as the reference electrode, a platinum wire as the counter electrode and the sample as the working electrode. Three replicates were made for this experiment.

Surface characterization

The morphology and distribution of the Zn-Ni phosphate conversion coating deposited on model phases were investigated using a Field Emission Gun Scanning Electron Microscopy The binding energy scale was calibrated on the neutral carbon set at 284.8 eV. Regarding reproducibility, the majority of quantitative and qualitative measurements were replicated at least 5 times.

Results

Characterization of the activated surface before immersion in PCC bath

The SEM and EDS are inefficient to evaluate the presence and the density of the activator nuclei on different phases of ZnAlMg coatings because the size of the nuclei is less than several nm and the total quantity of detected Ti is significantly less than the detection limit of EDS. Hence, the Ti concentration on different model phases and chemical form were evaluated by X-Ray Photoelectron Spectroscopy after alkaline degreasing and immersion in the activator bath. To verify if the activator nuclei are deposited only by physical process or if a chemical modification is also possible, a reference non-reacted activator was also characterized after evaporation at 35 °C for 50 h to determine its composition, compare it with as-received powders and evaluate the water impact on the chemical composition. Considering the low Ti concentration, the quantification error could be high but the order of magnitude still can be considered. As visible in Table 8, a higher concentration of Ti is measured on Zn compared to Zn2Mg and ZnAl. On the ZnAl phase, only one measurement enabled the detection of Ti on the phase surface which is why more detailed work was performed only on Zn and Zn2Mg phases. The chemical form of Ti is also different between Zn and Zn2Mg as there is more TiO3 2-than TiO2 on Zn. Taking into account that the total quantity of Ti measured on Zn2Mg is smaller than on Zn and that it also contains unreacted activator Na4TiO(PO4)2, the proportion of the activator chemically bound with the surface is higher on Zn than on Zn2Mg. In the case of dissolved and solvent evaporated activator, the proportion of TiO2 is of 13.76 at. % which shows that the contact of the activator with the metallic substrate might not be necessary to ensure the formation of TiO2.

Chapter 4: On the effect of multiphase microstructure of ZnAlMg substrate on the Ti-based activation and phosphate conversion coating distribution 67

OCP evolution in PCC bath

The evolution of the OCP of ZnAl, Zn and Zn2Mg model phases with the immersion time in the PCC bath after alkaline degreasing and rinsing at 55 °C and room temperature respectively is shown in Figure 25. The evolution of the OCP in a form of a sharp peak for Zn and ZnAl phases within the first 20 sec of immersion can be interpreted as a result of the fast precipitation kinetic of the phosphate conversion layer on phases alloyed with Al compared to Mg-alloyed phases. The appearance of such a peak was associated in the literature to a Ni 2+ diffusion limitation as the phosphating is carried out without stirring [39]. A similar experiment carried out on ZnAlMg model materials exhibited an OCP evolution close to the evolution of pure Zn2Mg phase. As for Zn2Mg, the OCP evolution is slower than for the other 2 phases owing to the preferential Mg dissolution in the phase, [13], delaying the formation of hopeite because of the lower dissolution of Zn and hence lower supersaturation rate by Zn 2+ . Mg phosphates solubility being more than 10 times higher than the solubility of hopeite [40], [41], the formation of the PCC layer can be delayed. Concerning the importance of the activator, it is visible in Figure 26, that without activator, the precipitation of scattered and large PCC grains is observed on all ZnAlMg compositions. For composition A, it is not possible to determine if there is a preferential precipitation on a specific Like on ZnAlMg materials, phosphates on model pure phases exhibited large and scattered grains of variable diameter (Figure 27). In contrast, a significant PCC grain size variation on Zn, ZnAl and Zn2Mg pure phases is visible, highlighting the different interaction between the PCC bath and the model phase depending on whether the phase is highly alloyed (ZnAl and Zn2Mg) or almost mono-component (Zn).

Surface observations by SEM

By comparison of the fully covered surfaces on model phases (Figure 27) and coatings (Figure 26) treated with a fresh activator, the grains look thinner and noticeably smaller on pure phases.

Still, a similar PCC grain density and morphology is observed on all pure phases and grains diameter is ranging between 1 and 2 µm. As for HDG coatings, for ZnAlMg coatings the activator seems to be a key factor to favor nucleation vs grain growth to ensure homogeneous distribution of the coating on different ZnAlMg materials and pure phases. The evolution of the covering rate as a function of immersion time for different coatings and immersion conditions is in a form of the precipitation kinetics in Figure 28.

Discussion

Impact of the activator on the PCC growth mechanism and grain shape

The role of the activation step in the phosphating process has already been studied for various types of PCC on HDG, steel [12], [16], [17] and Al [42]. The literature attributes the mechanism of activation in several steps -Adsorption of nano-particles of Na4TiO(PO4)2 activator to the surface [16].

-Transformation of Na4TiO(PO4)2 into Zn2Ti(PO4)2 when the activated surface gets in contact with Zn 2+ ions contained in the PCC bath [12], [16].

The growth of Zn3(PO4)2.4H2O on the formed nuclei is reported to be epitaxial on the Zn2Ti(PO4)2 nuclei [12], [16], [42].

This mechanism and the idea of the epitaxial growth are commonly accepted but we were not able to find a published result confirming the hypothesis of the epitaxial growth with the activator nuclei, except the reference [42] demonstrating that on Al substrates and with disclike Ti-based activator, the PCC grains grow directionally keeping a disc-like form. The results presented in this paper for alloyed substrates in conditions when PCC is not yet covering demonstrate that the disc-like and platelet-like shape of crystals can be obtained on the Znbased materials without activator (Figure 26 and Figure 27). Additionally, in case of aged activator (ageing decreases the concentration of potential nuclei), the PCC formed discs with diameter of about 1 µm on the model Zn2Mg phase while significantly better formed hexagonal platelets with diameter about 2 µm were formed on Zn and ZnAl substrates.

By consequence, the effect of the nucleating agent on the grain shape is not necessarily related to the epitaxy between the activator nuclei and the hopeite forming on it. The role of the activator for the shape of the PCC crystals can be hence limited to an increased number of PCC crystals which nucleate in the first seconds of the phosphating process regardless of the exact chemical composition of the nucleus. By consequence, ZnTiO3 and TiO2 present on the surface after activation step can serve as nuclei even if they do not exhibit a similar crystallography or stoichiometry as ZnxNiy(PO4)2 and in this case, the first nuclei of hopeite can be spherical.

Titanium phosphates can be also formed by the reaction of TiO2 or Ti salts with phosphoric acid in phosphating bath [43][44][45][46] favoring the epitaxial growth and formation of disc-like crystals from the first moments of grain growth.

In case of ZnAlMg substrates, at initial stages of PCC formation even with previous activation step, the visible PCC crystals on these materials were spherical. Such a morphology has never been reported for PCC on Zn. The apparent incoherence however can be a consequence of the fact that the surface observations reported in the literature were usually made in conditions of high surface coverage (typical observation times were from 10 seconds to several minutes). In such conditions the nucleation processes are already less important and the grain growth is well advanced. From our results, it seems that the induction period before the nucleation and the exponential crystal growth [47] on Zn2Mg phase and ZnAlMg materials is slower than on pure Zn and that the minimum size for a stable spherical nucleus on Mg-rich substrates is higher than on Zn. Indeed, spherical PCC crystals were observed on the alpha Mg phase in AZ31 Mg-Al alloy treated without preliminary activation [48]. The reason of longer induction period can be in a selective Mg dissolution from Mg-containing Zn materials [13] [24] which delays the increase of Zn concentration near the surface. The solubility of Mg phosphates being 10 times higher than the solubility of hopeite, more time is necessary to reduce Mg concentration in solution and to achieve Zn access to the surface.

To sum up, the most important characteristic of the activator for multiphase substrate should be hence its homogeneous distribution on different phases. From this point of view, the differences in the Ti chemistry and its quantity on the activated surfaces of different pure phases is important: the total quantity of Ti deposited on different phases decreases in order Zn>Zn2Mg>>ZnAl and the fraction of the chemically bound Ti decreases in the same order. As a result, the phase sensitivity to the activator ageing (by agglomeration of colloids [16] or by pH increase [49]) increases in order: Zn<Zn2Mg<ZnAl. The weakest activation of ZnAl phase can also explain why after 32 seconds of "slowed" PCC treatment at 40 °C, the multiphase coating with the composition A, which contained high surface fraction of ZnAl phase present as a binary eutectoid, was not completely covered while the compositions B and C, exempt of the ZnAl binary eutectoid, had been already completely covered by PCC.

Impact of the activator on the PCC distribution

As presented in section 4.1, the interaction of the activator with the phases composing ZnAlMg alloyed coatings looks different however it does not disturb the formation of a homogeneous layer of PCC on coatings with different microstructures. In this section, we will try to formulate some hypotheses explaining the apparent incoherence between the incongruent interaction of The good coverage of Zn dendrites could be explained by a high concentration of Ti-activator chemically bound to the surface. However, on ZnAl phase and on the eutectic phases in coatings, including the binary eutectoid-containing ZnAl phase, despite a lower activator concentration expected on ZnAl, a full coverage is achieved. This effect can be attributed to the size of the PCC grains after completion of the PCC procedure compared to the ZnAl phase dimensions in the material. As schematically represented in Fig. 7, at an advanced stage of the conversion reaction, PCC crystals are significantly bigger than the size of the phase which is not activated. This phase can be protected by the crystals deposited on the neighboring phases.

In case of eutectic phases with micrometric lamellas width and in case of ZnAl phase with the size less than the size of the PCC grains (about 1.5 µm diameter after 12 sec of immersion at 60 °C), these phases can be hence covered by PCC even if their interaction with the activator is not efficient. Such an observation can be through for both, ZnAl and Zn-Zn2Mg binary phases.

The proposed scheme can explain how, despite the ZnAlMg substrate and the activator deposition heterogeneities, a complete coverage of the multiphase ZnAlMg substrate can be ensured by the combination of the coarse size of PCC grains and relatively fine microstructure of the coating. It also reveals that, potentially, the formation of nanometric conversion layers on multiphase substrates might be more complex than the PCC formation.

Conclusion

In this work, the distribution of PCC at different surface treatment steps on model ZnAlMg materials as well as model phases simulating the different phases of ZnAlMg microstructures was characterized and the effect of the substrate microstructure on the phosphating process was understood.

• No preferential deposition of PCC on a specific phase was detected when working in standard conditions (with activator -immersion for 12 sec in PO4 ZnNi bath at 60 °C).

• Highly alloyed phases (ZnAl and Zn2Mg) were found to be more sensitive to the activator bath ageing than Zn. The impact was measured on both grain density and shape (round-like grains instead of plates) leading to different PCC nucleation kinetics between all 3 model phases PCC nucleation kinetics: Zn > ZnAl > Zn2Mg

The variation of reactivity of the model phases with the PCC bath was attributed to the concentration and type of Ti-based compounds formed on the surface during the activation step.

•

Introduction

Ti-based conversion coatings (TiCC) are nanometric coatings providing corrosion resistance and paint adherence to HDG [1], [2], aluminium [2][3][4][5][6] or ZnAlMg substrates [7]. It is generally assumed that thin Zr-based CC (ZrCC) and TiCC, deposited by immersion procedure [8] or by coil coating [9], follow a similar precipitation mechanism as they both exhibit a partial orbitald filling (Ti: [Ar] 3d² 4s²; Zr: [Kr] 4d² 5s²). The precipitation of TiCC on HDG is considered as a multi-step process consisting of 1) the metal substrate dissolution (5-1) accompanied by local alkalization from cathodic reactions (5-2 and 5-3) leading to 2) the CC precipitation as titanium oxide (5-4) at pH = 2.5 -8.5 [10].

-Anodic dissolution:

Zn → Zn + 2e

(5-1)

-Cathodic reduction leading to pH increase:

O + 4H + 4e → 2H O (5-2)
2H + 2e → H (5-3)

-TiCC precipitation:

TiF + 4OH → TiO . H O + 6F (5-4)
The TiCC procedure consists of several steps, including alkaline degreasing, rinsing, immersion in the TiCC bath and final rinsing.

Considering the low thickness of TiCC and the large number of elements composing the formulation, it is challenging to define the precise chemical composition of the conversion coating and its distribution on multiphase substrates. However, several works were published for Al-based [4]- [6] and Zn-based substrates [11]- [12].

ZnAlMg coatings have a complex multiphase microstructure composed by Zn dendrites, and binary and ternary eutectics formed by Zn and Al-rich phase and/or intermetallic Zn2Mg [13][14][15]. The reactivity variation between phases can result in a micro-galvanic coupling between phases and a heterogeneous spatial distribution of cathodes and anodes on the multiphase substrate and by consequence an inhomogeneous coating deposition [4]- [7], [16].

Chapter 5: Impact of phases in ZnAlMg coatings on the distribution of Ti-based conversion coatings 85 On Al alloys [4], [5], the ZrCC preferential precipitation was observed on and in the vicinity of cathodic intermetallic particles.

Recent work on ZnAlMg substrates has also shown the preferential precipitation of ZrCC on

Zn-rich phases at the expense of Al-rich and Mg-rich ones [7].

The impact of the microstructure on the CC precipitation and spatial distribution over multiphase substrates had already been reported for chromate-based CC [16].

However, the thickness of chromate CC, around 100 nm [16], is about 10 times higher than for TiCC or ZrCC. The impact of a heterogeneous distribution of CC on ZnAlMg on the paint adherence and local under paint corrosion hence can be more critical for TiCC and ZrCC.

The objective of the present work is to evaluate the impact of the phase composition and distribution in the ZnAlMg substrate on the distribution and chemical composition of TiCC. To do so, the distribution and chemical composition of the precipitated TiCC was studied on synthetic pure phases composing ZnAlMg and on a multiphase coating combining different surface analytical techniques.

The reactivity of the ZnAlMg material phases during the TiCC precipitation and possible effect of galvanic coupling between and inside phases were evaluated through electrochemical measurements.

Experimental

Materials

Two types of materials were tested including model ZnAlMg multiphase coatings and individual pure phases.

Model ZnAlMg materials

ZnAlMg coated steel with a thickness of 500 µm were supplied by ArcelorMittal in the skinpassed state with a roughness of 0.95 ± 0.03 µm. ZnAlMg coatings were produced by hot dip process with thickness around 10 µm. The selected composition contained Al ≤ 5 wt. % and Mg < 3 wt. %. Phase composition and details about microstructure of the materials are summed up in Table 9. Further details about ZnAlMg coatings microstructure is available in Appendix 2.

Chapter 5: Impact of phases in ZnAlMg coatings on the distribution of Ti-based conversion coatings 86 Samples with a size of 100 mm×200 mm were cut from the supplied substrate and subjected to a petroleum ether solvent cleaning process to ensure complete removal of any oils or surface contamination. During OCP measurements, the exposed area (A= 3 cm²) of the ZnAlMg samples was defined using insulator tapes containing precut portholes (PortHoles ™ Electrochemical Sample Masks).

Model phases

For mechanistic purposes, Zn, ZnAl and Zn2Mg model phases with compositions relevant to the ZnAlMg coating were synthetized. Depending on their composition, alloys were weighed in masses and heated at 500 °C. Liquid mixtures were then filtered through ceramic filters consisting of 61 wt. % Al2O3 and 39 wt. % SiO2 (10 pores per square inch porosity). The composition, crystallinity, grain size and orientation and hardness of all synthetized phases were characterized using atomic absorption spectroscopy, SEM, EBSD, XRD and nanoindentation techniques. The composition of the obtained phases is summarized in Table 9. Complementary information regarding pure phases microstructure is gathered in Appendix 3. In addition, Al (99.999 % purity) supplied by GoodFellow was used. It should be noted that the studied ZnAlMg material and ZnAl phase are multiphase while Zn and Zn2Mg are monophase. Chapter 5: Impact of phases in ZnAlMg coatings on the distribution of Ti-based conversion coatings 87 Samples were polished with SiC polishing discs (up to 4000 grit). After polishing, all samples were rinsed using ethanol and dried with compressed air. Prior to immersion, the reacting area (A= 1 cm²) of the pure phases was defined using insulator tapes containing precut portholes (PortHoles ™ Electrochemical Sample Masks).

TiCC solutions

Solutions used to deposit TiCC on pure phases and model ZnAlMg material are listed in Table 10.

Table 10: Main components and their concentration in solutions used for the TiCC procedure

Bath composition determination for Hydra-Medusa calculations

The predominant equilibrium species which could be expected from thermodynamics were calculated using the Hydra-Medusa software [17]. For the calculation, the concentrations of Ti 4+ , PO4 3-and Mn 2+ in the 20 wt. % TiCC-bath were evaluated using ICP-AES. It was assumed that the totality of the fluoride ions available in the bath originate from the hexafluoro titanic acid (H2TiF6). Hence, the fluoride concentration was defined as 6 times the Ti 4+ one. Details about elements concentration are available in Appendix 6.

Concentrations of the substrate elements (Zn 2+ , Al 3+ and Mg 2+ ) getting dissolved during the TiCC precipitation and incorporated in the conversion coating thickness were determined from the titration of the TiCC bath by a NaOH 1M solution. When reaching pH=7, the total acidity contained in the TiCC bath is neutralized. Considering that the neutralized acidity is twice the Zn concentration dissolved at the anodes (see reactions 5-1 and 5-2), it is possible to evaluate the total Zn concentration that is susceptible to be dissolved in the bath. The evaluation of the Zn/Al and Zn/Mg ratios was made by bath analysis using ICP-AES after the immersion of a ZnAlMg material sheet in the TiCC bath.

The resulting elemental concentrations used for the Hydra-Medusa software simulation are shown in Table 11.

Table 11: Elemental concentrations for the Hydra-Medusa software simulation of the precipitated species in TiCC

Electrochemical measurements

Open Circuit Potential (OCP) measurements were performed using a Biologic VSP potentiostat. A three-electrode cell was used with a saturated calomel electrode (SCE) as the reference electrode, a platinum wire as the counter electrode and the sample as the working electrode. Anodic and cathodic polarizations were carried out at a 2 mV/sec sweeping rate without stirring. Three replicates were made for this experiment.

Surface characterization

The distribution of the Ti-based conversion coating deposited on model phases was investigated using a Field Emission Gun Scanning Electron Microscope (FEG-SEM) JEOL 7800F at an accelerating voltage of 15 kV, coupled with a Brucker Xflash 6160 Energy Dispersive Spectroscopy (EDS) device using a Si(Li) detector and equipped with QUANTAX 1.9 software (Bruker AXS). Samples were observed not later than 4 h after the conversion coating procedure.

A minimum of 4 replicates were made to control the reproducibility.

GD-OES chemical analyses were performed using a Horiba Jobin Yvon GD-Profiler 2 GD-OES instrument to evaluate the distribution of P, Ti, Mn and F within the thickness of TiCC.

Samples were analyzed under a pressure of 700 Pa and a power of 30 W. The sampling time interval was 0,01 sec for the first 3 min then 0,1 sec until the end of the profile acquisition.

Parameters that could possibly influence the eroded mass of material per sec are the relative sputtering rate, the material density, the interaction with Ar + ions etc. Analyses were reproduced twice for reproducibility purposes.

The distribution and thickness of TiCC on all phases of ZnAlMg material was evaluated by nano Auger Electron Spectroscopy (AES) with electron microscopy, using a PHI 170 setup.

The primary electron beam voltage was 20 kV and the beam current was 1 nA. A CMA analyzer with an incident angle of 30° with respect to the surface normal was used and experiments were performed at a pressure of 2,3.10 -6 Pa. Depth profiles were carried out in a sequential mode with an Ar + ion beam sputtering the surface at a voltage of 2 kV and a current of 1 µA. The ion beam sputtered a 4 mm² surface at an angle of 60 ° with respect to the surface normal. The TiCC thickness was defined at 50 % of the inflection zone of TiCC markers (Ti, P, Mn and F), The binding energy scale was calibrated on the neutral carbon set at 284.8 eV. Regarding reproducibility, the majority of quantitative and qualitative measurements were replicated at least 5 times.

Results

Depth profiles distribution of TiCC by GD-OES

Erosion profiles of TiCC on ZnAlMg

To detect the TiCC and evaluate the average depth distribution on ZnAlMg material after alkaline degreasing followed by immersion in the TiCC bath at 60 °C for 30 sec, GD-OES profiles were performed showing the distribution of TiCC tracers within the CC thickness as visible on Figure 29. % of the inflection zone of the most in-depth tracer, Ti. Regarding the distribution of the tracers visible in the figure, Ti is located at the interface with the ZnAlMg substrate while P is concentrated in the outer layer. The presence of Mn is detected in the center of the layer. The P enrichment in the outer layer of the CC is coherent with the results communicated for an industrial HDG substrate [12].

From the distribution of TiCC tracers within the CC thickness, it is possible to formulate some hypothesis about the composition of the TiCC. On one hand, it seems unlikely to have a Tix(PO4)y precipitate because the maximums of P and Ti do not appear at the same time.

However, it is in accordance with the formation of TiO2 on the surface.

The juxtaposition of P and Mn signals in the GD-OES profile is consistent with the previously reported Mnx(PO4)y presence in the TiCC bath [18].

Surface distribution of TiCC by SEM-EDS

A typical example of TiCC distribution on ZnAlMg after immersion in TiCC bath at 60 °C for 30 sec is present in Figure 30. Tracers used to detect the presence of TiCC on the surface are P, Ti, Mn and F. Although the surface rugosity of the skin-passed ZnAlMg material can interfere quantitative analysis of TiCC tracers, some observations can be made. It is worth noticing that, during the erosion process, a variation of sputtering rate from one phase to the other was observed, which is illustrated by SEM images after different erosion times by AES as presented in Figure 32. After a theoretical depth sputtering of 300 nm, one can observe that Zn-Zn2Mg phase is fully eroded. The Zn dendrites are also highly eroded as the level difference between ZnAl and Zn dendrites shows while the ZnAl one seems to be less impacted by the Ar + beam. This selectively low erosion rate is in line with the thickest oxidized layer present on the ZnAl phase. Indeed, the erosion of oxides is known to be slower than the erosion of corresponding metals.

Differences in erosion rates of different phases composing ZnAlMg coating by Ar + was also verified by GD-OES erosion and following profilometry described in the next section.

Chapter 5: Impact of phases in ZnAlMg coatings on the distribution of Ti-based conversion coatings 96

Erosion rates variation between Zn, ZnAl and Zn2Mg by GD-OES

To evaluate the impact of the erosion process on ZnAlMg phases, erosion profiles of the pure phases simulating the ZnAlMg microstructure were carried out by GD-OES at 0.1 keV. The Figure 33 shows the calculated sputtering rates after 3 min of erosion from the crater depth measured with a profilometer. The calculated sputtering rates show that the Zn phase simulating Zn dendrites exhibits the highest sputtering rate, close to the one of the Zn2Mg intermetallic phase. The ZnAl phase has the lowest sputtering rate, at least twice slower than for Zn and Zn2Mg. The difference in sputtering rate between all pure phases is to be kept in mind when evaluating the thickness of the TiCC from sputtering profiles. However, it is important to keep in mind that the sputtering rates calculated by GD-OES cannot be directly applied to evaluate selective sputtering during AES profiling. Indeed, the beam energy is 2 keV for AES while it is 0.1 keV for GD-OES and the relative sputtering rate will not evolve linearly as a function of the beam energy.

TiCC chemical speciation and Ti concentration on pure phases with XPS

To characterize the chemical speciation and the total concentration of Ti (at. %) on ZnAlMg and each pure model phase simulating ZnAlMg, the chemical forms of Ti, P, Mn and F were determined by X-Ray Photoelectron Spectroscopy on model pure phases and ZnAlMg after Chapter 5: Impact of phases in ZnAlMg coatings on the distribution of Ti-based conversion coatings 97 alkaline degreasing, immersion in the TiCC bath at 60 °C for 30 sec, surface rinsing and drying.

The measured atomic concentration of Ti on model phases are available in Table 12. To ensure a suitable XPS signal intensity for the chemical composition identification, an additional analysis was made on ZnAlMg substrate immersed in TiCC bath at 60 °C for 1 min instead of 30 °C after the alkaline degreasing. For Zn2Mg, a charge compensation was necessary because all peaks were shifted towards higher energy binding. Spectra of (a) Ti 2p, (b) P 2s, (c) Mn 2p and (d) F 1s obtained on this sample are displayed in Figure 34. From Figure 34, one can notice that the binding energy for P 2s and Mn 2p regions look similar for ZnAlMg, Zn and ZnAl substrates. In F 1s region, ZnAlMg and ZnAl show a similar peak.

However, the Zn peak is shifted towards lower binding energies.

For Ti 2p region, a peak fitting was carried out. The type of bonding involving Ti evidences the presence of TiO2 at 458.9 eV on all the substrates. The formation of TiO2 is consistent with the fact that in GD-OES profiles, the Ti is located at the metal/CC interface and not bound with P. This is also consistent with the observation that anodic titanium oxide films obtained under galvanostatic conditions in phosphate buffer consist of TiO2 while phosphates are incorporated only at the external surface [19]. In the case of ZnAl and Zn2Mg phases, additional components containing Ti were detected. For Zn2Mg phase, the highest binding energy seems to correspond to the presence of a mix between Ti-F (e.g. TiF4) and Ti-O-F compounds [20]. This is a reasonable assumption considering that Ti is present initially as TiF6 2-. For both substrates, the existence of mixed compounds can be interpreted by a pollution or by incomplete transformation of TiF6 2-into TiO2 (reaction 5-4).

The Mn 2p peaks are of lower intensity on ZnAl phase compared to the other two. It could be due to the lower Mn concentration on that specific phase but it can also be linked to the position of the element inside the TiCC layer. Indeed, as suggested by the GD-OES analysis, Mn is not located near the surface but more in depth. The ZnAl phase exhibits the thickest TiCC layer and it can be expected that quantities of P and Mn should also be the highest. Hence, a big fraction of Mn can be hindered from the XPS analysis. No metallic Zn was detected on any phase confirming that the X-ray beam did not penetrate the sample up to the substrate.

The P 2s peak measured at 191 eV can be associated with the presence of Mn3(PO4)2 but also Zn3(PO4)2 as their difference in binding energy is only 0.1 eV apart [21], [22].

In the Mn 2p region spectra, the satellite peak at 647.7 eV is typical of Mn II (MnO). The Mn 2p3 peak can be attributed to both Mn II or Mn III [23]. The binding energy of Mn 2p3 is in good agreement with the presence of MnF2 [24] or MnHPO4, particularly on Zn2Mg [25].

In the F 1s area, peaks measured on all substrates exhibit a specific binding energy, indicating that several F-based products were probably formed during the conversion reaction. The F 1s peak is extremely large. To understand this, it is important to keep in mind that the binding energies reported for MgF2 vary in a large range comprised between 684.2 eV and 686.1 eV [26]- [29]. A reference at 686 eV was selected to illustrate it [29] as it fits well the peak measured on Zn2Mg.

The presence of F-doped ZnO, ZnOx-(F2-x), could explain the width of the F 1s peak and is consistent with the peak measured on the Zn phase, shifting towards lower binding energies.

The F concentration in the TiCC is in large excess. According to the literature and Hydra-Medusa simulations, the other possible F-containing products, except for MgF2 [29] are soluble and their presence after the surface was rinsed can be only due to their retention in the porosities of the oxide.

Finally, it is interesting to notice that concentrations evolution of Ti and F are similar from one phase to another. A similar observation can be made about Mn and P. This can be attributed to the fact that the CC bath contains H2TiF6 and Mn3(PO4)4 in its formulation [18].

The complete list of the compounds considered for the analysis is given in Appendix 7.

Chapter 5: Impact of phases in ZnAlMg coatings on the distribution of Ti-based conversion coatings 100

Electrochemical behavior of pure phases in degreasing and TiCC baths

For better understanding of the reactivity of all ZnAlMg phases during the TiCC procedure, the open circuit potential of model phases relevant to the ZnAlMg composition was recorded in the degreasing bath after 2 min at 55 °C as visible in Table 13. Zn2Mg. The anodic behavior of ZnAl and ZnAlMg can be attributed to the higher concentration of Al in both materials, Al being highly anodic in highly alkaline environments [30].

Additional OCP measurement were carried out in (a) TiCC bath at 60 °C for 1 min after 2 min of alkaline degreasing and (b) in 5 wt. % Na2SO4 solution with the same pH as TiCC but without ions which could precipitate as shown in Figure 35. ZnAl is more anodic than other phases with a potential varying from -1,01 V/SCE to -1,07

V/SCE after 1 min of immersion in the TiCC bath.

A similar measurement was carried out in 5 wt. % Na2SO4 pH 2.6 at 40 °C. For Zn phase, the potential remains in the same range as what was measured in TiCC bath after alkaline degreasing. On the contrary, Zn2Mg is more anodic in 5 wt. % Na2SO4 pH 2.6 compared to the TiCC bath at the same pH. The OCP decreased in order Zn>ZnAl>Zn2Mg.

From the OCP evolution values, a strong galvanic coupling between Zn dendrites and ZnAl eutectoid is not expected. The galvanic coupling between different phases cannot hence explain A similar observation can be made for the coupling of the cathodic pure Al and the anodic Zn phase although a lower current density ranging between 25 and 30 µA.cm -² was measured, probably due to the low reactivity of pure Al observed in 5 wt. % Na2SO4 pH 2.6.

Discussion

TiCC chemical composition on all ZnAlMg phases

Using the Hydra-Medusa software calculation, the predominant species formed by each ion during the TiCC are shown in Figure 37. (e) for Mn species alone: Mn(OH)2, MnOH + , Mn(OH)3 -, Mn(OH)4 2-, Mn2(OH)3 + , Mn2OH3 + ; (f) for P species alones: H3PO4, H2PO4 -, HPO4 2-;

(g) for F species alone: HF, H2F2, HF2 -; (h) mixed species: Zn3(PO4)2.4H2O, Mn3(PO4)2, MnHPO4, ZnF2, ZnF + , AlF2 + , AlF3, AlF4 -, AlF5 2-, AlF6 3-, MgAlO4, MgF2, MgF + , MnF + , MnF2, AlPO4, TiF6 2-.

Simulations show that Zn and Mn tend to react with the phosphate groups present in the bath.

The preferential formation of hopeite (Zn3(PO4)2) is in accordance with the XPS analyses and depth profiles by AES and GD-OES. However, it is possible to assume that the substitution of Mn by Zn leads to the formation of a mixed phosphate compound ZnxMn3-x(PO4)2 during the TiCC precipitation which should also be considered.

Regarding Mn 2+ ions, the precipitation of MnHPO4 is described as the only Mn-based compound to be formed on the surface for all pH conditions. This result was also consistent with the XPS analyses and the binding energy reported for MnHPO4 in the literature [25].

The titanium contained in the conversion coating is precipitating only as TiO2 for all pH at this concentration which is also consistent with the surface characterization. Various studies focusing on the precipitation mechanisms of TiCC on HDG or Al alloys have highlighted the formation of TiO2 [4], [6], [11], [32] as well as MnxTiy(PO4)z although it is unclear if P is involved in MnxTiy(PO4)z or MnxZny(PO4)z.

Finally, the Mg 2+ ions dissolved in the TiCC bath form insoluble MgF2. Indeed, its solubility is 8 times lower than the one of ZnF2 or MnF2, all of which are possible formed precipitates from XPS analyses [33]. This hypothesis is not ruled out by XPS analyses results and is consistent with SEM-EDS observations.

Taking into the composition of each phase of ZnAlMg, the formation mechanism of TiCC can be summed up as follows in Figure 38:

Distribution of TiCC on ZnAlMg phases

From the results presented here above, the TiCC distribution is not homogeneous on ZnAlMg phases where the surface coverage decreases in order: ZnAl > Zn-Zn2Mg > Zn dendrites. A TiCC precipitation mechanism on all ZnAlMg phases is suggested in Figure 39. From the contact between ZnAlMg and the TiCC bath, anodic and cathodic areas are locally formed where the substrate will get dissolved in anodic zones (Zn2Mg lamellas and Zn-rich areas contained in ZnAl) while a pH increase occurs in cathodic zones.

After a prolongated contact of the substrate with the CC bath, the CC precipitation is extended from cathodic areas to more anodic areas of binary phases and then to monophase Zn dendrites with a lower reactivity towards the TiCC bath. The CC is self-extinguished when the local pH is not high enough anymore to enable more CC to precipitate due to the isolation of the substrate from the TiCC bath.

Electrochemical measurements indicate that a galvanic coupling is probable within ZnAlMg material that could explain the heterogeneous TiCC distribution on ZnAlMg.

Potential differences between ZnAlMg phases are not expected to induce a strong and predominant galvanic coupling between phases. Nevertheless, a galvanic coupling inside each binary phase can be strong enough and enhance the reactivity as the distance between the anodic and cathodic zones will be very small. It can hence play an important role in the TiCC precipitation kinetics. The local pH increase is expected to extend quickly from cathodic zones to anodic ones when they are closely spaced.

The dissolution of anodic areas leads to a local surface enrichment of the cathodic phase hence a faster alkalization of the media on Zn lamellas and Al-rich areas present in ZnAl respectively and a locally enhanced TiCC precipitation kinetics. This way, anodic and cathodic phases size could act as an accelerating factor regarding precipitation kinetics.

Finally, the lowest TiCC concentration measured on Zn dendrites contained in ZnAlMg could be explained by the phase homogeneity in the center of dendrites. The coupling within Zn/Zn2Mg or ZnAl induces a dissolution and a more homogeneous pH increase at the interface resulting in a faster precipitation kinetics and an overall higher TiCC thickness on the surface.

One might also consider that a conversion coating concentration gradient between the center of the Zn phase and the phase edges in direct contact with the neighboring ZnAl and Zn/Zn2Mg phases might exist. However, no EDS or AES measurements were carried out to confirm nor infirm this hypothesis.

In the case of ZnAl, an internal coupling was also evidenced from coupling pure Al with Zn phase. To explain why the concentration of TiCC is the highest on this phase, it is important to remember that the microstructure of this phase is the finest between all the phases relevant for ZnAlMg coatings which can enhance the TiCC precipitation kinetics on ZnAl.

Conclusion

The distribution and chemical composition of TiCC on the multiphase ZnAlMg substrate was evaluated and the role of individual phases for TiCC distribution was approached with additional study of model pure phases corresponding to ZnAlMg.

 A heterogeneous distribution of TiCC on ZnAlMg material after alkaline degreasing and immersion in the conversion bath at 60°C for 30 sec was demonstrated by EDS, AES and XPS measurements.

The quantity of precipitated TiCC, which can be considered as a measure of precipitation kinetics, decreases in order: binary ZnAl > binary Zn/Zn2Mg > Zn  The improved CC precipitation kinetics on binary phases was explained by a galvanic coupling inside these phases (Zn/Zn2Mg and Zn/Al), contributing to stronger reactivity and more rapid pH increase on these phases. The fineness of the microstructure distribution is expected to play a key role for the local pH distribution and hence homogeneity of the deposition. It is expected that the galvanic coupling inside binary phases prevails over the coupling between the phases and Zn dendrites.

Introduction

In the pre-painted and automotive industries, nanometric TiCC and ZrCC are promising conversion coatings for providing corrosion resistance and paint adherence to painted cold rolled steel, HDG, Al alloys etc [1]- [9].

Simultaneously, monophase HDG coated steels are being replaced by multiphase ZnAlMg ones exhibiting an improved corrosion resistance, thus allowing a coating thickness reduction [10]- [14].

The use of ZrCC on multiphase ZnAlMg materials as well as aluminium alloys containing intermetallic particles has been reported to lead to a heterogeneous conversion coating distribution due to the spatially located anodes and cathodes on the surface [5], [6], [8], [15].

The chapter 5 of this manuscript focusing on the distribution and chemical composition of TiCC on ZnAlMg confirmed this hypothesis highlighting that monophase Zn dendrites are the least covered ZnAlMg phase from the absence of galvanic coupling inside the phase.

One of the possible solutions to homogenize the conversion coating distribution is to induce the deposition/cementation of more noble elements such as Cu or Fe [16]- [21] during conversion coating procedure, in order to create local galvanic coupling between the Cu/Fe particles and the ZnAlMg phase. This is expected to accelerate the precipitation kinetics leading to a more homogeneous coating thickness on the overall surface. Indeed, it was observed by Cerezo et al.

that the cementation of Cu particles during the ZrCC procedure on an AA1050 surface promoted the formation of the ZrCC [8]. This observation was also confirmed by Lostak et al.

although it was reported that the addition of Cu 2+ to the conversion bath led to a thicker but less homogeneously distributed ZrCC on HDG steel compared to Fe 3+ , especially for short immersion times in the conversion coating bath. Regarding the corrosion resistance of samples treated with the Cu/Fe-modified conversion bath, they reported that the OCP was shifted to more noble values by the addition of Cu 2+ /Fe 3+ [16]. Adhikari et al. noted an enhanced delamination resistance of samples treated with ZrCC containing Cu particles compared to those not containing Cu particles [17].

In the automotive industry, ferric and cupric ions are already part of the formulation of some conversion coatings, including Fe(NO3)3.9H2O or Cu(NO3)3.9H2O [16]. During the immersion of a HDG substrate in the conversion coating bath, the first step consists in the deposition of noble metallic (Fe or Cu) particles on the substrate [16]:

3Zn + 2Fe → 3Zn + 2Fe (6-1)
Or

Zn + Cu → Zn + Cu (6-2)
Simultaneously, anodic and cathodic reactions occur on the HDG surface creating a substrate dissolution followed by local alkalization, leading to the CC precipitation as titanium oxide [4]- [6], [8], [22]- [25]:

1. Anodic dissolution:

Zn → Zn + 2e (6-3)
2. Cathodic reduction leading to pH increase:

O + 4H + 4e → 2H O (6-4)
2H + 2e → H (6-5)
3. TiCC precipitation (pH range= 2.5 -8.5 [26]):

TiF + 4OH → TiO . H O + 6F (6-6)
In this chapter, the use of Cu 2+ ions at the pretreatment step is considered as a possible solution of the problem of inhomogeneous TiCC precipitation on multiphase substrates.

The objective of the work is to evaluate if the pretreatment resulting in the cementation of Cu particles can lead to a homogenization of TiCC distribution on all phases of ZnAlMg and if the stability of painted ZnAlMg treated TiCC after this modified pretreatment is improved compared to the painted ZnAlMg with conventional TiCC.

Firstly, the distribution and chemical form of Cu particles after the degreasing step as well as the TiCC were evaluated on all phases of ZnAlMg using surface analysis techniques. Then, a model epoxy paint was deposited on ZnAlMg coated steels with TiCC prepared after standard alkaline cleaning and after Cu-modified procedure and the stability of formed systems during immersion and polarization in a 0.1 M NaCl solution was compared. Local Electrochemical Impedance Mapping (LEIM) was used to detect degradation, identify the delamination front position and the delamination front propagation.

The applied interest of the study is that Cu 2+ -modified alkaline degreasing bath has never been associated with a TiCC dedicated to the pre-painted market as part of a 2-step procedure. In Chapter 6: Insight into the formation and degradation mechanisms of TiCC on multiphase ZnAlMg substrates using Cu 2+ modified alkaline degreasing baths 117 addition, the precipitation of Cu cathodes aiming at homogenizing the conversion coating distribution has never been reported on multiphase ZnAlMg in the literature.

Experimental

Materials

Two types of materials were tested including model ZnAlMg multiphase coatings and individual pure phases.

Model ZnAlMg materials

ZnAlMg coated steel with a thickness of 500 µm were supplied by ArcelorMittal in the skinpassed state with a roughness of 0.95 ± 0.03 µm. ZnAlMg coatings were produced by hot dip process with thickness around 10 µm. The selected composition contained Al ≤ 5 wt. % and Mg < 3 wt. %. Phase composition and details about microstructure of the materials are summed up inTable 14. Further details about ZnAlMg coatings microstructure is available in Appendix 2.

Samples with a size of 100 mm×200 mm were cut from the supplied substrate and subjected to a petroleum ether solvent cleaning process to ensure complete removal of any oils or surface contamination.

Pure phases

For the delamination studies, samples were covered with 10 µm of epoxy using spin-coater (Spin150, The Netherlands) and dried at air overnight. Scratches up to the steel (length: 15 mm) were made manually with a 1 mm Sikken blade and scratches depth were controlled using a Surtronic S 128 portable device.

For mechanistic purposes, Zn, ZnAl and Zn2Mg model phases with compositions relevant to the ZnAlMg coating were synthetized. Depending on their composition, alloys were weighed in masses and heated at 500 °C. Liquid mixtures were then filtered through ceramic filters consisting of 61 wt. % Al2O3 and 39 wt. % SiO2 (10 pores per square inch porosity). The composition, crystallinity, grain size and orientation and hardness of all synthetized phases were Chapter 6: Insight into the formation and degradation mechanisms of TiCC on multiphase ZnAlMg substrates using Cu 2+ modified alkaline degreasing baths 118 characterized using atomic absorption spectroscopy, SEM, EBSD, XRD and nanoindentation techniques. The composition of the obtained phases is summarized in Table 14. Complementary information regarding pure phases microstructure is gathered in Appendix 3. In addition, Al (99.999 % purity) supplied by GoodFellow was used. It should be noted that the studied ZnAlMg material and ZnAl phase are multiphase while Zn and Zn2Mg are monophase. Samples were polished with SiC polishing discs (up to 4000 grit). After polishing, all samples were rinsed using ethanol and dried with compressed air. Prior to immersion, the reacting area (A= 1 cm²) of the pure phases was defined using insulator tapes containing precut portholes (PortHoles ™ Electrochemical Sample Masks).

TiCC solutions

Solutions used to prepare TiCC on pure phases and model ZnAlMg material are listed in Table 15. For convenience, the commercial TiCC remained unchanged. The Cu addition was carried out in the degreasing bath prior to the conversion coating step. A new formulation was created based on the necessity to keep Cu 2+ ions soluble in an alkaline media. To prevent Cu from precipitating in the alkaline degreasing bath (pH = 12.5), 2 types of chelating agents were used, sodium gluconate (C6H11NaO7) and sodium citrate (C6H5Na3O7) in large excess compared to Chapter 6: Insight into the formation and degradation mechanisms of TiCC on multiphase ZnAlMg substrates using Cu 2+ modified alkaline degreasing baths 119 the Cu concentration (0.026 M) [27], [28]. EDTA was not a suitable choice as it can complex Cu only in acidic and neutral pH [29].

Table 15: Main components and their concentration in solutions used for the TiCC procedure

TiCC procedure

The following general procedure was used to precipitate TiCC on pure phases: All baths were used maximum 12 h after mixing and renewed every 24 h.

Local Electrochemical Impedance Spectroscopy and Mapping

Local Electrochemical Impedance Spectroscopy (LEIS) and Local Electrochemical Impedance Mapping (LEIM) were performed using a potentiostat (Solartron ModuLab XM ECS, Princeton Applied Research). Samples were epoxy-coated (polymer thickness: 10 µm) with a spin-coater (Spin150, The Netherlands) and dried at air overnight. An artificial down to steel scratch (scratch dimension: 15 mm x 1 mm x 100 µm) was manually made on samples before they were mounted as working electrodes (WE) at the bottom of a 3D printed electrochemical cell. The counter-electrode was a platinum wire and the reference one was an Ag/AgCl sat. electrode

Electrochemical measurements

Open Circuit Potential (OCP) measurements were performed using a Biologic VSP potentiostat. A three-electrode cell was used with a saturated calomel electrode (SCE) as the reference electrode, a platinum wire as the counter electrode and the sample as the working electrode.

A Pt electrode was used to carry out imposed potential polarization experiments in the Cu 2+modified alkaline degreasing bath at 55 °C in order to determine the precipitation potential of Cu particles. After an OCP measurement for 5 min, potentials ranging between -0.8 and -1.4 V vs. SCE were applied for 30 min to the sample. For all measurements, freshly prepared electrolyte was stirred at 200 rpm. After imposed potential polarizations, the Cu mass gain was measured with an OHAUS Explorer EX1103 precision scale (sensitivity: 0.001 g). Two

replicates were made for this experiment.

Surface characterization

The distribution and thickness of TiCC on all phases of ZnAlMg material was evaluated by Auger Electron Spectroscopy (AES), using a PHI 170 setup. The primary electron beam voltage was 20 kV and the beam current was 1 nA. A CMA analyzer with an incident angle of 30° with respect to the surface normal was used and experiments were performed at a pressure of 2,3.10 - 6 Pa. Depth profiles were carried out in a sequential mode with an Ar + ion beam sputtering the surface at a voltage of 2 kV and a current of 1 µA. The ion beam sputtered a 4 mm² surface at an angle of 60 ° with respect to the surface normal. The interface between the CC and the substrate was defined as the inflection point in the Ti profiles or for complex profiles as a point From Figure 40, it can be noticed that the rate of Cu cementation is not strongly influenced by the deposition potential in the range of potentials relevant for ZnAlMg phases which is coherent with Cu 2+ ions diffusion limited kinetics. The type of chelating agents selected for the degreasing bath formulation appears hence to be a convenient choice for the rest of the study.

Cu distribution and chemical composition on ZnAlMg

Typical AES mapping of the ZnAlMg surface after immersion in the Cu 2+ -modified alkaline degreasing bath at 55 °C for 2 min is shown in Figure 41. After determining that Cu particles were precipitated on every phase of the ZnAlMg material, a complementary analysis to evaluate its chemical form on the surface was carried out with the AES technique. In this case, the XPS analysis did not provide sufficient information regarding Cu chemical form as Cu° and Cu I exhibit binding energies closer than 0.2 eV for both Cu2p 3/2 and Cu2p 1/2 peaks [31]. Auger peaks of both species exhibit different energies but they are interfered by the Zn auger peak which makes the identification of Cu chemical form difficult using XPS.

However, using AES and the peak at a kinetic energy of 776 eV, it was possible to differentiate contributions coming from the metallic and oxidized Cu chemical form, although no further identification between CuO/Cu2O would be possible using this technique. the sputtering process, highlighting that Cu is present in its oxidized form on the surface and its metallic one, closer to the ZnAlMg coating. This is coherent with the results of Sarfraz et al.

[18] but also Khun who both worked with a Cu 2+ -modified ZrCC baths [20]. For short immersion times, the formation was CuO was reported to be favorable while at long immersion times (180 sec), an increasing Cu2O concentration and a decreasing CuO one was noted. The kinetic energy variation of metal Cu from 776 to 771 eV is due to the working mode used, either direct or derivative. Indeed, the theoretical kinetic energy peak value is 776 eV when working with the derivative mode. However, the data treatment was carried out on peaks displayed with the direct mode where the peak value corresponds to half of the peak height, hence 771 eV. As visible on Figure 43, profiles of Ti, P, Mn and F on the 3 ZnAlMg phases show that the TiCC thickness is comparable for all phases with an average thickness of 15 nm. On the overall, there is an improved coverage of Zn dendrites with the Cu 2+ -modified alkaline cleaner compared to the unmodified one where a thickness of 5 nm was measured (Chapter 5). On the other hand, thinner conversion coating layers are measured on both binary phases. However, this can be explained by the relatively simple nature of the formulation which was not optimized and does not contain any additives etc. No optimization of the elements concentrations was performed to increase the Cu concentration on the surface in this study. In general, the enhanced conversion coating homogeneity from using Cu noble particles is confirmed.

The presence of Cu underneath the TiCC was also detected with AES profiles and the highest concentration seems to be measured on the binary ZnAl phase compared to the other 2 phases. A similar conversion coating chemical composition was detected in all the points selected for measurements except in the F 1s region (Figure 44 d). Generally, the peaks position for all studied elements is similar to the positions found on samples treated with an unmodified alkaline cleaner prior to the TiCC precipitation described in Chapter 5.

In the Ti 2p region, a peak fitting highlighted the presence of TiO2 on the surface.

The determination of the type of phosphate precipitate created during the conversion reaction was performed using the P 2s region spectra as the P 2p region is interfered by Zn 3s.

In the P 2s region, the peak measured at 191 eV could be attributed to Mn3(PO4)2 but also Zn3(PO4)2 as they are 0.1 eV apart [32]- [33].

In the Mn 2p region spectra, the satellite peak at 647.7 eV is typical of the presence of MnO.

The Mn 2p3 peak can be attributed to the presence of Mn II or Mn III [34]. It was also noticed that the binding energy of Mn 2p3 is in good agreement with the presence of MnHPO4 on ZnAlMg [35].

For the F 1s region, a distinction must be made between both peaks. Indeed, for the Point 6, a lower binding energy was measured which highlights the presence of compounds such as ZnOxF(2-x) [36].

On the other hand, on the Point 4, the higher binding energy suggests the formation of MgF2 considering its binding energy and solubility product. [37]. On the Point 4, the intensity and peak width are larger compared to the Point 6. It can be assumed that both MgF2 and ZnOxF(2-

x) are present in that sample surface area.

The list of all the compounds with a binding energy matching the peaks detected in the P 2s, Mn 2p and F 1s regions are gathered in Appendix 8.

Surface reactivity of epoxy coated samples by LEIM

The under-paint reactivity was studied by LEIM after separated OCP-anodic polarization, OCPcathodic polarization cycles and a combination of both of them and following immersion. Two epoxy polymer coated ZnAlMg samples immersed in the TiCC bath at 60 °C for 30 sec after degreasing in a standard unmodified alkaline cleaner and Cu 2+ -modified alkaline one respectively were compared. Both samples were scratched prior to the experiment and tested in 5 cycles of alternating anodic and cathodic polarization and overnight immersion. The purpose of this experiment was to compare the delamination resistance of both samples when immersed in a 0.1 M NaCl solution after alternating cathodic and anodic polarizations. The Figure 45 shows the maps of the values of the gradient of the admittance modulus obtained on these samples. From this experiment, it was highlighted that the addition of Cu 2+ ions to the degreasing bath leads to an enhanced delamination resistance of the epoxy-coated ZnAlMg sample.

Interestingly, even if the surface darkened around the scratch after anodic polarization (visible because the polymer was transparent), the under-paint reactivity was detected by LEIM only after a combination of both, anodic and cathodic polarizations and the following overnight immersion. This indicates the importance of both, anodic and cathodic reactions for the degradation of the interface in case of TiCC. The binary ZnAl phase was largely covered with particles. Zn dendrites exhibited a partial coverage while Zn2Mg lamellas showed very little amount of Cu on its surface. A possible explanation for this singularity comes from the necessity of an electron exchange from the metal substrate to Cu 2+ ions to initiate the Cu precipitation.

During the alkaline degreasing step, the ZnAlMg phases have a different reactivity in alkaline conditions. The Al-containing phase ZnAl is easily dissolved at pH 12.5 which produces electrons available for Cu 2+ ions to precipitate [38]. A similar statement can be made about Zn dendrites since Zn is sensitive to alkaline conditions, although in a less pronounced way than Al [38]. Zn2Mg can form Mg hydroxides in alkaline media, limiting the Cu 2+ reduction on its surface. From all these observations, one can conclude that Cu precipitation is triggered by the surface etching.

It was confirmed by Cerezo et al. as they noted that the nucleation of Cu particles was preferentially occurring on and around intermetallics contained in the microstructure of Al alloys which were preferentially getting dissolved during the alkaline surface etching [19].

Effect of Cu cementation on TiCC precipitation mechanism

The TiCC thickness seems to be affected by the coverage rate of Cu particles on ZnAlMg surface after the degreasing step. Indeed, the improved homogeneity of the conversion coating is based on the precipitation of noble cathodes on ZnAlMg. However, a complete coverage of the ZnAlMg surface is not aimed as it would inhibit the galvanic coupling between Cu and ZnAlMg phases. In this case, ZnAl phases are highly covered with Cu particles which isolate the phase from the surrounding TiCC solution during the conversion coating precipitation. A higher conversion coating thickness could have been achieved if a lower concentration of Cu was to be cemented on the phase surface. On the other hand, the precipitation of Cu on Zn dendrites was partial which favors the formation of a galvanic coupling between the Cu particles and the Zn dendrites and accelerate the precipitation kinetics of the conversion coating.

Based on the results observed on Figure 43, a new TiCC precipitation mechanism is suggested here after in Figure 46, taking into consideration the Cu precipitation after the degreasing step. One can believe that the TiCC homogeneity lies in the distance separating the Cu particles.

Since they are much smaller than ZnAlMg phases themselves, they are closely spaced, creating many local alkalization spots on the surface. Knowing that the TiCC formation will start at cathodes before extending itself to anodes, the little distance between anodes and cathodes will induce a much faster surface coverage by the TiCC.

Conclusion

In this chapter, a solution was proposed to obtain a homogeneous TiCC distribution on multiphase ZnAlMg substrate. This solution included a pretreatment resulting in the cementation of Cu particles on ZnAlMg. The impact of this pretreatment on the homogenization of TiCC distribution was investigated.

Even if the solution was inspired from CC formulations used in the automotive industry, where Further tests aiming at optimizing the Cu concentration to be precipitated on the substrate surface would be interesting to perform. Indeed, since the TiCC homogeneity is closely linked with the distance separating cathodes and anodes on the surface, it is expected that a full coverage of the ZnAlMg surface is not to be achieved at the risk of obtaining an even surface For PCC, the formation mechanism is described here after:

Anodic dissolution :

Me → Me + ne (1)
Cathodic reduction leading to pH increase:

NO + 2H → NO + H O (2)
Or

NO + 8H + 6e → NH + H O (3)
PCC precipitation in pH range from 4.7 to 8.5:

2PO + 3Zn + H O → Zn (PO ) . nH O (4)
Homogeneous PCC distribution covering all the phases despite a significant variation in the density of activator sites on different phases was attributed to the effect of PCC grains size, which is higher than the lateral dimensions of the constituents of binary phases and zinc dendrites. This way, during the PCC precipitation, grains initially formed on cathodic areas grow until they are in contact with each other and fully cover the substrate. The distribution of cathodes and anodes in ZnAlMg microstructure is fine enough compared to the PCC grains dimension to enable a full coverage of the surface as schematically represented in Figure 47 . For TiCC, the CC formation mechanism can be summarized as follows:

-Anodic dissolution:

Me → Me + ne (1)
-Cathodic reduction leading to pH increase:

O + 4H + 4e → 2H O (4)
and

2H + 2e → H (5)
-TiCC precipitation (pH range= 2.5 -8.5):

TiF + 4OH → TiO . H O + 6F (6)
The presence of galvanic coupling inside binary phases can enhance the phase reactivity in both, anodic and cathodic processes, and improve coating deposition kinetics. Experiments on model pure phases confirmed the possibility of coupling inside binary phases as illustrated in More detailed specification of the TiCC components on different ZnAlMg phases and the methods permitting this specification are shown in Figure 49. The galvanic coupling between Cu particles and the substrate results in more rapid and more homogeneous pH increase in vicinity of the surface and hence in a more homogeneously distributed CC during the TiCC procedure as schematically displayed in Figure 52. Nano-AES confirmed that a well detectable homogeneous layer of TiCC can be formed on all phases of ZnAlMg substrate after Cu-containing degreasing (Figure 53), while after a standard alkaline degreasing, Zn dendrites were not covered at this temperature.

Effect of microstructure on the degradation mechanism of painted ZnAlMg

The importance of the microstructure on the degradation of painted ZnAlMg coatings is evidenced from the fact that painted ZnAlMg coated steel can develop blisters in outdoor exposure if a defect in the paint and coating goes down to the steel substrate while no blisters are formed in similar conditions on HDG steels. Moreover, the blisters are absent if the scratch does not go to the steel, which can be interpreted as a key role of anodic processes in blistering observed on painted ZnAlMg with artificial defect down to steel.

The SKP mapping demonstrated the existence of interconnected paths between the visually isolated blisters. The path is characterized by lower than the surrounding matrix Volta potential values. SEM observations of cross sections revealed that the path contains corroded ZnAlMg coating under intact paint and that the initial lack of adherence can originate from these corroded areas. Similar to corrosion of unpainted ZnAlMg, it seems that the underpaint corrosion of these coatings starts with selective corrosion of Zn2Mg phase.

Two hypotheses explaining how the microstructure can affect the stability of the metal/polymer interface were formulated.

First, considering that galvanostatic anodic polarization is much more harmful for delamination in the case of a multiphase ZnAlMg coated steel than for a monophase HDG steel, the galvanic coupling between phases of the coating can enhance under paint reactivity and anodic undermining, giving easier access for water and oxygen to the cathodic areas.

Another hypothesis explaining the role of the multiphase microstructure can be related to the non homogeneous conversion coating on multiphase substrate resulting in a local lack of adherence. The latter can contribute to easy access of water and oxygen to the ZnAlMg substrate but also to a localization of the corrosion process. This hypothesis seems to be confirmed by the surface analysis (chapter 5) and by the fact that after the pretreatment enabling a homogeneous TiCC distribution, the stability of the interface metal/polymer in painted ZnAlMg seems to be significantly improved (chapter 6).

The exact role of the multiphase microstructure for the performance of painted ZnAlMg coated steel still needs to be understood. Indeed, it cannot be ruled out that the starting point of the corrosion propagation under the paint could be a combination of the initial inhomogeneity of the conversion coating, its low chemical stability and the ZnAlMg substrate corrosion, but the initial distribution of conversion coating seems to be a very important factor.

Finally, another pretreatment, also using the principle of metal cementation in order to homogenize TiCC distribution between ZnAlMg phases was proposed for industrial application and accelerated tests made in ArcelorMittal using this solution demonstrated significantly improved resistance to blister formation for the painted samples prepared with this new pretreatment.

Perspectives

Through the work about the mechanisms of TiCC formation, it was understood that the inhomogeneity of TiCC deposition is related to the inhomogeneity of the distribution of cathodic and anodic zones and a physico-chemical solution in which the surface activation can be achieved by a cementation of noble metal was proposed. This solution stays, however, at the level of laboratory tests and both, industrial application of the surface treatment procedure and a deep understanding of the improved blistering resistance should be made.

For the process application, it would be very interesting to optimize the formulation of the cleaner to enable an optimum distribution of Cu cathodes as a function of the substrate and to accelerate the deposition kinetics of the TiCC, therefore increasing the CC thickness.

To improve the understanding of blistering mechanisms, first of all more data are necessary concerning the stability of TiCC and the underlying substrate during the degradation of painted systems. Even if the SEM and SKP results demonstrated the presence of anodic dissolution under the paint and the presence of corroded "routes" between the blisters, the spatial resolution of SKP and the complexity of sample preparation for high resolution SEM were unable to elucidate the exact nature of the phases responsible for these routes. From the corrosion mechanisms of ZnAlMg alloys, one can expect that the binary and ternary eutectics containing Zn2Mg should behave as the most anodic phases and dissolve preferentially, however a direct proof in the case of under paint corrosion is still missing. Moreover, in the present PhD, only the role of the substrate microstructure was considered. At the same time, the presence of soluble contamination trapped under the paint during the coil coating process due to the absence of rinsing step still needs to be addressed.

Deeper understanding of degradation mechanisms at the ZnAlMg/TiCC/polymer interface and the interpretation of all the results of local EIS measurements at different steps of ageing are also necessary.

Considering the improved adhesion after accelerated electrochemical corrosion testing of samples for which the surface preparation included Cu cementation pretreatment, it can be, firstly, attributed to a more homogeneous and thicker CC layer which improved initial adhesion limiting water diffusion between the substrate and the paint. However, it is not sufficient to explain the absence of locally propagating selective dissolution of anodic phases. One of the possible explanations is that a contact of all phases with homogeneously distributed cemented Cu metal forces a homogeneous dissolution around the scratch and formation of corrosion products blocking further corrosion propagation. The fact that adhesion is improved on the Zn dendrites limits the electrolyte access to their surface and the local pH increase once the electrolyte arrives. Another aspect of this work that needs to be extended is the evaluation of the corrosion resistance of painted ZnAlMg coated steels treated with the Cu 2+ -modified alkaline cleaner in long term natural exposure to determine and quantify the gained corrosion resistance. The solution is under testing at the moment and a definite answer will be available in a few years. Actuellement, les aciers galvanisés (HDG) sont remplacés par des revêtements multiphasés de ZnAlMg dans les marchés du prélaqué et de l'automobile. Grâce à l'augmentation de la concentration en Al et l'ajout de Mg, une augmentation de la résistance à la corrosion est possible, réduisant l'épaisseur du revêtement sacrificiel et les coûts de production tout en allongeant la durée de vie des revêtements ZnAlMg. Pour assurer l'adhérence de la peinture sur les substrats ZnAlMg, des couches de conversion (CC), parmi lesquels des CC épaisses de phosphates (PCC) et des CC fines de Ti (TiCC), sont appliquées pour assurer l'adhérence de la peinture et améliorer la résistance à la corrosion des revêtements.

Objectifs :

Dans le contexte décrit précédemment, l'objectif de la thèse est de comprendre l'origine du phénomène de cloquage et en particulier :

-Les mécanismes de corrosion entraînant la formation de cloques sur les revêtements peints de ZnAlMg traités avec les TiCC.

-La différence de résistance à la corrosion entre les revêtements ZnAlMg peints traités avec les TiCC comparés aux PCC et en particulier, le rôle de l'activateur à base de Ti sur la formation de la PCC sur revêtements multiphasés de ZnAlMg.

-L'impact de la microstructure du ZnAlMg sur les mécanismes de formation des TiCC.

-La modification de la distribution du TiCC sur les phases de ZnAlMg par cémentation d'un métal noble.

Méthodologie :

Pour atteindre ces objectifs, la sonde de Kelvin, a révélé la présence de chemins de corrosion sous peinture liants les cloques depuis la rayure à l'acier jusqu'aux cloques les plus distantes. Des immersions de substrats ZnAlMg avec différentes microstructures et des phases pures simulant la microstructure des ZnAlMg dans des bains de TiCC ou de PCC ont été réalisées à différentes étapes de la procédure de conversion. Dans le cas de la PCC, l'impact du niveau de fraîcheur sur la formation de la PCC a aussi été étudiée. Les phases pures identifiées dans la microstructure des revêtements ont été synthétisées en partenariat avec l'Université Chapitre 4 (« Impact de la microstructure multiphasée des substrats ZnAlMg la distribution de l'activateur à base de Ti et la PCC ») se focalise sur la distribution de la PCC à différentes étapes de la procédure de conversion et en fonction de la fraîcheur de l'activateur.

Le rôle de l'activateur utilisé avant la PCC sur la formation de grains cristallins et couvrants sus les phases de ZnAlMg a été mis en avant. Une expérience de chronopotentiométrie du revêtement de ZnAlMg peint et traité avec le TiCC au contact de NaCl a été réalisée pour caractériser la propagation de la corrosion au sein du revêtement (Figure 6). L'impact de la concentration et la forme chimique de l'activateur à différentes étapes de la procédure de phosphatation est présentée sur la Figure 10.

Impact des phases du revêtement ZnAlMg sur la distribution du TiCC

Pour comprendre l'origine du développement des cloques sur les revêtements ZnAlMg peints, une étude de la formation du TiCC sur le revêtement ZnAlMg par immersion du substrat dans le bain de TiCC suivi d'un rinçage de la surface a été réalisé.

Au cours de la réaction de conversion de TiCC sur un substrat de Zn, la formation de la CC se déroule en 2 étapes. La mise en contact du substrat avec le bain de TiCC entraîne l'apparition de zones anodiques et cathodiques à la surface du revêtement. Aux anodes ont lieu la dissolution du métal tandis qu'aux cathodes, la réduction de l'oxygène est la réaction majoritaire. Une augmentation du pH est observée qui entraîne la précipitation du TiCC dans un 2 nd temps. Le mécanisme de précipitation de TiCC sur l'acier galvanisé est décrit ci-après :

-Dissolution anodique:

Zn → Zn + 2e (1-4) .

Conclusions

L'étude du rôle de la microstructure des revêtements ZnAlMg sur le développement de cloques sur des ZnAlMg traités avec TiCC avant application de peinture a été vérifiée dans cette thèse.

La présence de différentes phases dans le revêtement a un impact ou cours de la formation de la CC sur le ZnAlMg mais aussi pendant le processus de dégradation du revêtement peint. Des tests de corrosion accélérés ont permis de confirmer le gain de résistance à la corrosion des échantillons ayant subi une cémentation de Cu à la surface.

Figure 1 : 7 Figure 2 : 8 Figure 3 :

 17283 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 4 :

 4 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 5 :

 5 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 6 :

 6 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 7 :

 7 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 8 :

 8 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 9 :

 9 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 10 :

 10 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 11 :

 11 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 12 :

 12 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 13 :

 13 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 14 :

 14 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 15 :

 15 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 16 :

 16 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 17 :

 17 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 18 :

 18 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 20 :

 20 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 21 :

 21 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 22 :

 22 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 23 :

 23 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 24 :

 24 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 26 :

 26 Figure 1: Scheme of the Hot Dip Galvanizing process [10] ...................................................... 7 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12]) ........................................................................................................................ 8 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market ..................Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling .....................................................................................................................Figure 5: SEM photo of an electrogalvanized steel after 180 sec of phosphating in a ZnMnNi PCC bath [36] ...........................................................................................................................Figure 6: Germination of PCC crystals on sites with adsorbed activator [37] .........................Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment ..................................................................................................................Figure 8: Anodic delamination mechanism..............................................................................Figure 9: Cathodic delamination mechanism ...........................................................................Figure 10: Scheme of the cathodic blistering mechanism ........................................................Figure 11: Auger electron emission principle in AES .............................................................Figure 12: Principle of X-ray photoelectron spectroscopy [7] .................................................Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle ............Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram .....Figure 15:Scheme of the SKP principle ...................................................................................Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment ..............................................................................................................................Figure 17: SEM images of cross sections of painted ZnAlMg with blisters ............................Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential. .......... Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7. ..................................................................................................................................................Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH7 ............................................................Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C .....................................................................Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch ..........................................................................................................................Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg ................................................Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn ......................................... Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C.................................................................Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) ...................................

Figure 27 : 70 Figure 28 : 71 Figure 29 :

 2770287129 Figure 27: PCC grains density and morphology on different materials as indicated after immersion in the PCC bath for 12 sec at 60 °C using: (1) no activation, (2) aged activation and (3) fresh activation (see text for detail) .................................................................................... 70 Figure 28: Surface coverage evolution as a function of immersion time, ZnAlMg substrate composition, bath temperature and use of activator (as indicated) .......................................... 71 Figure 29: Distribution of P, Ti, Mn and F by GD-OES profile on ZnAlMg after alkaline degreasing followed by immersion in TiCC bath at 60 °C for 30 sec ..................................... 91 Figure 30: (a) SEM image of ZnAlMg coating (as indicated) after immersion in TiCC at 60 °C for 30 sec and EDS distribution of P, Ti, Mn and F on different phases and (b) SEM image of ZnAlMg and EDs mapping of Mg and F distribution after immersion in TiCC at 60 °C for 30 sec ............................................................................................................................................. 93 Figure 31: AES profiles of different phases of ZnAlMg after immersion in TiCC bath at 60 °C for 30 sec after standard cleaning ............................................................................................. 94 Figure 32: SEM images of ZnAlMg surface after alkaline degreasing and immersion in TiCC bath at 60 °C for 30 sec : (a) before sputtering, (b) after theoretical 70 nm of depth sputtering and (c) after theoretical 300 nm of depth sputtering ................................................................ 95 Figure 33: Mean sputtering rates of untreated and polished (4000 SiC) Zn, ZnAl and Zn2Mg .................................................................................................................................................. 96 Figure 34: XPS spectra windows of TiCC tracers on pure phases and multiphase ZnAlMg substrates as indicated after standard degreasing and immersion in the TiCC bath at 60 °C for 30 sec ........................................................................................................................................ 98 Figure 35: Open Circuit Potentials (OCP) of pure phases in (a) TiCC bath pH 2.6, T=60 °C after 2 min of alkaline degreasing and (b) 5 wt. % Na2SO4 pH 2.6, T=60 °C ....................... 101 Figure 36: Potential evolution and current density of (a) Zn/Al coupling and (b) Zn/Zn2Mg coupling after respectively 30 min of immersion and 2 h of phase coupling in 5 wt. % Na2SO4 pH 2,6 at 25 °C ....................................................................................................................... 102 Figure37: Predominant equilibrium species for 113 mM Zn 2+ + 57 mM Al 3+ + 57 mM Mg 2+ + 112 mM PO4 3-+ 66 mM Ti 4+ + 88 mM Mn 2+ + 420 mM F -using the Hydra-Medusa software and associated database of equilibrium constants at 25 °C. Complexes used in the simulation include (a) for Zn species alone: ZnO (cr), Zn(OH)2, Zn(OH)3 -, Zn(OH)4 2-, Zn4(OH)4 4+ , ZnOH + , Zn2(OH)6 2-, ZnOH 3+ ; (b) for Al species alone: Al(OH)3, AlOOH, Al(OH)4 -, AlOH2 + , Al13O4(OH)247+ , Al2(OH)2 4+ , Al3(OH)4 5+ , AlOH2 + ; (c) for Mg species alone: MgO, Mg(OH)2, MgOH + , Mg4(OH)4 4+ ; (d) for Ti species alone: TiO2, TiO(OH)2, TiO(OH)3 -, TiOOH + ; (e) for Mn species alone: Mn(OH)2, MnOH + , Mn(OH)3 -, Mn(OH)4 2-, Mn2(OH)3 + , Mn2OH3 + ; (f) for P species alones: H3PO4, H2PO4 -, HPO4 2-; (g) for F species alone: HF, H2F2, HF2 -; (h) mixed species: Zn3(PO4)2.4H2O, Mn3(PO4)2, MnHPO4, ZnF2, ZnF + , AlF2 + , AlF3, AlF4 -, AlF5 2-, AlF6 3- , MgAlO4, MgF2, MgF + , MnF + , MnF2, AlPO4, TiF6 2-............................................................ 103 Figure 38: Schematic diagram of the TiCC precipitation on ZnAlMg .................................. 105 Figure 39: Scheme of the TiCC precipitation mechanism on ZnAlMg ................................. 106 Figure 40: a) Measured OCP of different phases in Cu-modified alkaline cleaner and b) Cu mass gain measured on the Pt electrode as a function of the imposed potential in the Cumodified alkaline degreasing bath at 55 °C ........................................................................... 123 Figure 41: AES mapping of ZnAlMg surface after immersion in Cu alkaline cleaning at 55 °C for 2 min ................................................................................................................................. 124 Figure 42: AES deconvolution of Cu auger peak of ZnAlMg after Cu alkaline degreasing for 2 min at 55 °C ............................................................................................................................ 125

Figure 43 :

 43 Figure 27: PCC grains density and morphology on different materials as indicated after immersion in the PCC bath for 12 sec at 60 °C using: (1) no activation, (2) aged activation and (3) fresh activation (see text for detail) .................................................................................... 70 Figure 28: Surface coverage evolution as a function of immersion time, ZnAlMg substrate composition, bath temperature and use of activator (as indicated) .......................................... 71 Figure 29: Distribution of P, Ti, Mn and F by GD-OES profile on ZnAlMg after alkaline degreasing followed by immersion in TiCC bath at 60 °C for 30 sec ..................................... 91 Figure 30: (a) SEM image of ZnAlMg coating (as indicated) after immersion in TiCC at 60 °C for 30 sec and EDS distribution of P, Ti, Mn and F on different phases and (b) SEM image of ZnAlMg and EDs mapping of Mg and F distribution after immersion in TiCC at 60 °C for 30 sec ............................................................................................................................................. 93 Figure 31: AES profiles of different phases of ZnAlMg after immersion in TiCC bath at 60 °C for 30 sec after standard cleaning ............................................................................................. 94 Figure 32: SEM images of ZnAlMg surface after alkaline degreasing and immersion in TiCC bath at 60 °C for 30 sec : (a) before sputtering, (b) after theoretical 70 nm of depth sputtering and (c) after theoretical 300 nm of depth sputtering ................................................................ 95 Figure 33: Mean sputtering rates of untreated and polished (4000 SiC) Zn, ZnAl and Zn2Mg .................................................................................................................................................. 96 Figure 34: XPS spectra windows of TiCC tracers on pure phases and multiphase ZnAlMg substrates as indicated after standard degreasing and immersion in the TiCC bath at 60 °C for 30 sec ........................................................................................................................................ 98 Figure 35: Open Circuit Potentials (OCP) of pure phases in (a) TiCC bath pH 2.6, T=60 °C after 2 min of alkaline degreasing and (b) 5 wt. % Na2SO4 pH 2.6, T=60 °C ....................... 101 Figure 36: Potential evolution and current density of (a) Zn/Al coupling and (b) Zn/Zn2Mg coupling after respectively 30 min of immersion and 2 h of phase coupling in 5 wt. % Na2SO4 pH 2,6 at 25 °C ....................................................................................................................... 102 Figure37: Predominant equilibrium species for 113 mM Zn 2+ + 57 mM Al 3+ + 57 mM Mg 2+ + 112 mM PO4 3-+ 66 mM Ti 4+ + 88 mM Mn 2+ + 420 mM F -using the Hydra-Medusa software and associated database of equilibrium constants at 25 °C. Complexes used in the simulation include (a) for Zn species alone: ZnO (cr), Zn(OH)2, Zn(OH)3 -, Zn(OH)4 2-, Zn4(OH)4 4+ , ZnOH + , Zn2(OH)6 2-, ZnOH 3+ ; (b) for Al species alone: Al(OH)3, AlOOH, Al(OH)4 -, AlOH2 + , Al13O4(OH)247+ , Al2(OH)2 4+ , Al3(OH)4 5+ , AlOH2 + ; (c) for Mg species alone: MgO, Mg(OH)2, MgOH + , Mg4(OH)4 4+ ; (d) for Ti species alone: TiO2, TiO(OH)2, TiO(OH)3 -, TiOOH + ; (e) for Mn species alone: Mn(OH)2, MnOH + , Mn(OH)3 -, Mn(OH)4 2-, Mn2(OH)3 + , Mn2OH3 + ; (f) for P species alones: H3PO4, H2PO4 -, HPO4 2-; (g) for F species alone: HF, H2F2, HF2 -; (h) mixed species: Zn3(PO4)2.4H2O, Mn3(PO4)2, MnHPO4, ZnF2, ZnF + , AlF2 + , AlF3, AlF4 -, AlF5 2-, AlF6 3- , MgAlO4, MgF2, MgF + , MnF + , MnF2, AlPO4, TiF6 2-............................................................ 103 Figure 38: Schematic diagram of the TiCC precipitation on ZnAlMg .................................. 105 Figure 39: Scheme of the TiCC precipitation mechanism on ZnAlMg ................................. 106 Figure 40: a) Measured OCP of different phases in Cu-modified alkaline cleaner and b) Cu mass gain measured on the Pt electrode as a function of the imposed potential in the Cumodified alkaline degreasing bath at 55 °C ........................................................................... 123 Figure 41: AES mapping of ZnAlMg surface after immersion in Cu alkaline cleaning at 55 °C for 2 min ................................................................................................................................. 124 Figure 42: AES deconvolution of Cu auger peak of ZnAlMg after Cu alkaline degreasing for 2 min at 55 °C ............................................................................................................................ 125

  weight and molar composition and supplier .................................................. 34 Table 4: Composition of pure phases and ZnAlMg ................................................................. 44 Table 5: Fraction of the surface (%) covered by Zn dendrites, binary phases Zn-Zn2Mg and Zn-Al, and ternary eutectic Zn-Al-Zn2Mg determined from the SEM images of top surface of coatings A, B and C and the typical linear size (µm) of phases ............................................... 60 Table 6: Model phases relevant for ZnAlMg coatings: their chemical composition (from AAS), phase crystallography (from XRD) and microstructure (from SEM) as well as open circuit potential (OCP) in PCC bath at 60 °C after 12 sec and 100 sec of immersion ........................ 61 Table 7: Main components and their concentration in solutions used for the bication (Zn-Ni) phosphating procedure ............................................................................................................. 61 Table 8: XPS analysis results of the atomic concentration of Ti measured on each phase (at. % Zn/ at. % Ti) ............................................................................................................................. 66 Table 9: Composition of ZnAlMg and pure phases relevant for ZnAlMg coating .................. 86 Table 10: Main components and their concentration in solutions used for the TiCC procedure .................................................................................................................................................. 87 Table 11: Elemental concentrations for the Hydra-Medusa software simulation of the precipitated species in TiCC..................................................................................................... 88 Table 12: Atomic concentration of Ti measured on model phases (at. % Ti) .......................... 97 Table 13: Open circuit potential (OCP) of pure phases after 2 min in the degreasing bath at 55 °C ............................................................................................................................................ 100 Table 14: Composition of ZnAlMg and pure phases corresponding to ZnAlMg .................. 118 Table 15: Main components and their concentration in solutions used for the TiCC procedure ................................................................................................................................................ 119 Table 16: XPS parameters used for Ti 2p peak deconvolution .............................................. 153 Table

%

  of Al and 0.5-6 wt. % of Mg HDG steel: Hot Deep Galvanized steel CC: Conversion Coating REACH: Registration, Evaluation, Authorization and Restriction of Chemicals CrVI CC: Hexavalent chromium conversion coating (use restricted by REACH legislation) TiCC: Ti-based conversion coating dedicated to the pre-painted market PCC: Phosphate ZnNi-based conversion coating requiring a Ti-activation step prior to the conversion coating procedure OC: Organic Coating XPS: X-ray Photoelectron Spectroscopy AES: Auger Electron Spectroscopy TFA: Target Factor Analysis PCA: Principal Component Analysis FEG SEM: Field Emission Gun Scanning Electron Microscopy EDS: Energy Dispersive Spectroscopy GD-OES: Glow Discharge Optical Emission Spectroscopy SKP-FM: Scanning Kelvin Probe Force Microscopy OCP: Open Circuit Potential SHE: Standard Hydrogen Electrode SCE: Saturated Calomel Electrode DiP: Differential Interferometry Profiles EIS: Electrochemical Impedance Spectroscopy LEIM: Local Electrochemical Impedance Spectroscopy CPD: Contact Potential Difference TEM: Transmission Electron Microscopy wt. %: Weight Percent at. %: Atomic Percent BZS: Basic Zinc Salts LDH: Layered Double Hydroxide Introduction ZnAlMg coatings are multiphase sacrificial coatings replacing monophase HDG Zn-0,2

  distribution on ZnAlMg phases by means of noble metal cementation and the efficiency of the proposed solution was demonstrated. The manuscript is organized in four parts and includes seven chapters. The first part includes this introduction which develops the context and the scientific questions that were answered during the PhD and the first chapter which summarizes the state of the art relevant to the work. The composition, microstructure and corrosion resistance of ZnAlMg sacrificial coatings are reviewed as well as the existing knowledge of the conversion coatings formation mechanisms for TiCC and PCC on Zn-based substrates and the degradation mechanisms of painted Zn-based coated steel, described in the literature, leading to delamination and blistering. The literature review enabled the selection of the pure phases simulating the phases contained in the ZnAlMg microstructure. The second part, Materials and Methods, covers the materials and the electrochemical setup used in this work and introduces the main techniques used throughout the PhD, namely surface analysis techniques used to characterize the CC distribution and chemical composition and local electrochemical techniques for the detection of under paint corrosion such as delamination or blistering. The third part presents the most important results obtained during this PhD. This part consists of 4 chapters formatted as publications prepared for submission to different journals. Chapter 3 ("Comprehension of the degradation mechanisms TiCC-treated and painted ZnAlMg coated steels") approaches the impact of ZnAlMg microstructure in the development and propagation of delamination and blisters. Corrosion mechanisms of painted ZnAlMg coated steel are discussed. Chapter 4 ("On the effect of multiphase microstructure of ZnAlMg substrate on the Ti-based activation and phosphate conversion coating distribution") focuses on the PCC distribution on ZnAlMg at different CC procedure steps and as a function of the activator freshness. The role of the Ti-based activator used prior to the phosphatation process in the formation of covering and crystalline grains on all ZnAlMg phases was exhibited. The inhomogeneity of the interaction of different ZnAlMg constituents with Ti-containing activator was shown and the importance of the micrometric grain size of PCC crystals for the formation of a homogeneous layer was demonstrated. Chapter 5 ("Impact of phases in ZnAlMg coatings on the distribution of Ti-based conversion coatings") develops the origin of the heterogeneous distribution of TiCC on ZnAlMg substrates. The electrochemical behavior of pure phases contained in the ZnAlMg microstructure confirmed the existence of a galvanic coupling inside phases and inhomogeneous TiCC distribution assessed by surface analysis techniques correlated with the expected from galvanic coupling distribution of cathodic zones. A schematic mechanism of TiCC formation on multiphase ZnAlMg substrate was proposed and the role of the ratio between the microstructure inhomogeneity scale and the CC element size was highlighted. Chapter 6 ("Insight into the formation and degradation mechanisms of TiCC on multiphase ZnAlMg using Cu 2+ -modified alkaline degreasing baths") presents the solution selected to homogenize the TiCC distribution on ZnAlMg coatings, which is based on the cementation of metallic Cu during the degreasing step, allowing a finer cathodes/anodes distribution. The uniform chemical composition and the homogeneous thickness distribution of TiCC on ZnAlMg after the new pretreatment was confirmed by surface analysis techniques and better resistance in cosmetic corrosion test compared to a conventional procedure was confirmed by local electrochemical impedance mapping.

Figure 1 :

 1 Figure1: Scheme of the Hot Dip Galvanizing process[10]

Figure 2 :

 2 Figure 2: Scheme of the cross-section of ZnAlMg coated steel. Zn-Zn2Mg binary phase is composed of bright Zn lamellas and dark Zn2Mg ones while the ternary one includes Zn, Zn2Mg and an Al mesh. Zn-Al binary phase is composed of bright Zn areas and dark Al-rich ones (inspired by [12])

2. 1 .

 1 Main layers present in painted ZnAlMg coated steelA typical structure of painted ZnAlMg coated steels is illustrated in Figure3(layers thickness is not representative).

Figure 3 :

 3 Figure 3: Structure of a painted ZnAlMg coated steel for the pre-painted market

Figure 4 :

 4 Figure 4: Schematic representation of anodic and cathodic reactions distribution during galvanic coupling

Figure 6 :

 6 Figure6: Germination of PCC crystals on sites with adsorbed activator[37]

Figure 7 :

 7 Figure 7 : Blistering around the down to steel defect after 22 months of natural exposure in a marine environment

Figure 8 :

 8 Figure 8: Anodic delamination mechanism

Figure 9 :

 9 Figure 9: Cathodic delamination mechanism

Figure 10 :

 10 Figure 10: Scheme of the cathodic blistering mechanism Water, oxygen and cations diffuse through the paint in absence of a defect. Under the blister, cathodic and anodic regions are well defined. The metal dissolution takes place at the center of the blister. The cathodic regions are located in the outer part of the blister where the oxygen is more easily renewed [60]-[61].

Chapter 1 :

 1 State of the ArtChapter 2: Materials and MethodsFrom the literature review displayed in Chapter I -State of the art, different techniques were employed to understand the degradation mechanisms of painted ZnAlMg coated steels and the formation mechanisms of conversion coatings on ZnAlMg. On one hand, degradation mechanisms of conversion coatings were investigated using SKP, LEIM and current imposed dissolution. On the other hand, the comprehension of conversion coatings precipitation mechanisms was carried out through 2 complementary approaches. First, the distribution of conversion coating elements on ZnAlMg phases was determined using Scanning Electronic Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The overall coating thickness on ZnAlMg was measured using GD-OES (Glow-Discharge Optical Emission Spectroscopy) while the conversion coating thickness on each ZnAlMg phases was measured with Auger Electron Spectroscopy (AES). The GD-OES technique was also used to evaluate the position of the TiCC elements as a function of depth. In complement, Open Circuit Potential (OCP) measurements and anodic/cathodic polarizations were performed on pure phases simulating ZnAlMg microstructure to confirm the TiCC precipitation mechanism on the ZnAlMg surface.Second, The TiCC composition and concentration after immersion of the model ZnAlMg coating and pure phases in the TiCC bath were studied by X-ray Photoelectron Spectroscopy (XPS).

Figure 11 :

 11 Figure 11: Auger electron emission principle in AES Auger electrons are collected by an electron energy analyzer which measures the kinetic energy of the emitted Auger electrons based on the following equation:

Figure 12 :

 12 Figure12: Principle of X-ray photoelectron spectroscopy[7]

Figure 13 :Chapter 2 :

 132 Figure 13: Scheme of the Glow Discharge Optical Emission Spectroscopy principle

  : Scanning Kelvin Probe (SKP) and local Electrochemical Impedance Mapping (LEIM) LEIM is a local electrochemical technique used in this work to appreciate local surface reactivity's evolution impacted by corrosion and corrosion products formation. Local Electrochemical Impedance Spectroscopy (LEIS) and Local Electrochemical Impedance Mapping (LEIM)[9],[10] set up is composed of a potentiostat coupled with a lock-in amplifier.A three-electrode configuration is used with a metallic sample as working electrode, a Ag/AgCl electrode as reference and a platinum wire as counter electrode. Impedance measurements are carried out with a homemade local probe composed of two Ag wires of 250 µm diameter in a glass capillary (sealed in two joined capillaries with the distance between the two Ag wires centers d = 1.3 mm). The probe was positioned at 150 ± 25 µm above the WE and moved with a 3-axis positioning system driven by a motion encoder.

Figure 14 :

 14 Figure 14: Local Electrochemical Impedance Spectroscopy (LEIS) explanatory diagram

Figure 15 :Chapter 3 :

 153 Figure 15:Scheme of the SKP principle The probe tip and the sample surface near one another form a capacitor. The potential difference between the 2 metals, called the Volta Potential, induces the creation of an electric field from the electrons flow.During the measurement, the probe tip vibrates at a specific frequency, generating an alternative current (to maintain the electric field between the 2 metal surfaces). Simultaneously, a DC voltage is applied to counter balance the Volta Potential and allow electrons to flow in both directions. The CPD is found when the applied DC voltage gives a nullified AC current.

Chapter 3 :

 3 Comprehension of the degradation mechanisms of TiCC-treated and painted ZnAlMg coated steels 43 Delamination from the cut edges and/or blistering randomly distributed on the surface are known phenomena in the event of outdoor exposure of coil coated materials[1]-[5]. Blisters are local defects characterized by the coating bulging and resulting from a weakness at the metal/polymer interface or between polymer layers[2]. This phenomenon has already been addressed in the literature, and numerous degradation mechanisms including chemical, electrochemical (anodic or cathodic undermining), osmotic, mechanical etc. were proposed[6]-[9].

Chapter 3 :

 3 Comprehension of the degradation mechanisms of TiCC-treated and painted ZnAlMg coated steels For mechanistic purposes, Zn, ZnAl (21.4 wt. % of Al) and Zn2Mg model phases with compositions matching the different phases contained in ZnAlMg coatings were prepared.

Figure 16 :

 16 Figure 16: Photo of ZnAlMg material after 24 months of outdoor exposure in marine environment

Figure 17 :

 17 Figure 17: SEM images of cross sections of painted ZnAlMg with blisters

Figure 18 :

 18 Figure 18: SKP mapping of painted and corroded ZnAlMg material with a down to steel scratch after 24 months of outdoor corrosion testing. The left figure shows optical image, the middle figure shows the topography and the right figure shows the values of Volta potential.

Chapter 3 :

 3 figure. The potential mapping is shown on the right side and the corresponding scale is at the bottom of the image.

Figure 19 :

 19 Figure 19: General view of painted HDG and ZnAlMg materials with an artificial defect down to steel after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH 7.

Figure 20 :

 20 Figure 20: SEM-EDS mapping of ZnAlMg material after 72 h of anodic galvanostatic polarization in 1 wt. % NaCl solution with initial pH 7

Chapter 3 :

 3 Comprehension of the degradation mechanisms of TiCC-treated and painted ZnAlMg coated steels 51 21 also confirm that the Zn2Mg lamellas contained in the binary Zn-Zn2Mg phase are more anodic than the Zn dendrites and the binary ZnAl phase.

Figure 21 :

 21 Figure 21: Anodic and cathodic polarization curves of pure phases relevant for ZnAlMg coating in 5 wt. % NaCl solution at a pH 7 and T=25 °C

Figure 22 :

 22 Figure 22: Scheme of the degradation mechanism of painted ZnAlMg material from the down to steel scratch

Chapter 4 :

 4 On the effect of multiphase microstructure of ZnAlMg substrate on the Ti-based activation and phosphate conversion coating distribution Authors: Perrine Tanguy a,b, 1 , Christian Allély b , Diana Dragoe c , Vaclav Sefl d and Jan Stoulil d and Polina Volovitch a a Chimie ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie Paris (IRCP), F-75005 Paris, France b Global Research and Development, ArcelorMittal Research Maizières, France c Institut de Chimie Moléculaire et des Matériaux d'Orsay, CNRS, Université Paris-Sud 11, 91405 Orsay Cedex, France d University of Chemistry and Technology Technicka 5, Prague, Czech Republic Abstract: The paper verifies the impact of multiphase microstructure of ZnAlMg coatings on the distribution of bi-cation phosphate conversion coating (PCC). Three ZnAlMg coatings and three pure phases (Zn, Zn2Mg and ZnAl) relevant for these coatings were characterized after Ti-based activation and after complete PCC procedure. Highly alloyed ZnAl and Zn2Mg were more sensitive to the ageing of activator bath than Zn. X-ray photoelectron spectroscopy demonstrated that Ti concentration on the surface after Ti-containing activation step decreased in order Zn>Zn2Mg>>ZnAl and that Ti-O group is chemically bound to the Zn surface while for the alloyed phases mainly TiO2 was formed. Scanning electron microscopy demonstrated that for the coatings with a fine microstructure, the complete covering by PCC was achieved in standard conditions despite the differences in the activation state of different phases and different kinetics of PCC formation. This was explained by the hypothesis that the variance in the reactivity of different phases which constituent the coating can be leveled by the phosphate crystal size, exceeding the substrate microstructure inhomogeneity.

Table 7 : 62 2. 2 . 1 .--

 76221 Main components and their concentration in solutions used for the bication (Zn-Ni) phosphating procedure Chapter 4: On the effect of multiphase microstructure of ZnAlMg substrate on the Ti-based activation and phosphate conversion coating distribution Bication Zn-Ni phosphating procedure The exposed area (A= 1 cm²) of the samples was defined using insulating tapes containing precut portholes (PortHoles ™ Electrochemical Sample Masks). The following procedure was used to precipitate phosphate conversion layers on pure phases and model ZnAlMg materials: Alkaline degreasing in BONDERITE C-AK 75 at 55 °C for 30 sec -Osmosis water rinsing at 50 °C and at ambient temperature -Immersion in surface activation bath (Bonderite M-AC 50CF) at room temperature for 5 sec Immersion in bication Zn-Ni PCC bath (BONDERITE M-AD 40110 + BONDERITE M-ZN 1421-A) at 40 °C or 60 °C with immersion time between 0 and 40 sec.

(

  FEG-SEM) on a JEOL 7800F device at an accelerating voltage of 15kV. Samples were observed not later than 4 h after the phosphating procedure. Several dozens of pictures of the sample surface were taken to evaluate the phosphating homogeneity.The titanium-based compound contained in the activator after immersion of the model ZnAlMg coatings and pure phases in the activator bath were studied by X-ray Photoelectron Chapter 4: On the effect of multiphase microstructure of ZnAlMg substrate on the Ti-based activation and phosphate conversion coating distribution 63 Spectroscopy (XPS). The XPS spectrometer (K Alpha, Thermo Fisher Scientific, base pressure in the low 10 -9 mbar range) was equipped with a monochromatic aluminium source (Al Kα, 1486.68 eV). A spot size of 400 µm corresponding to an irradiated zone of approximately 1 mm 2 was used. The hemispherical analyzer was operated at 0° take off angle in the Constant Analyzer Energy (CAE) mode, with a pass energy of 200 eV and an energy step of 1 eV for the acquisition of wide scans, while narrow scan spectra centered on the Ti 2p region were recorded at a pass energy of 100 eV and an energy step of 0.1 eV due to the very low Ti signal concentration on the surface. Charge compensation was achieved by means of a "dual beam" flood gun, using low energy electrons (5 eV) and argon ions. Samples were fixed on the support using adhesive conducting tape.The recorded spectra were processed by means of the Avantage software, using a peak-fitting routine with Shirley background and symmetrical 70 %-30 % mixed Gaussian-Lorentzian peak shapes. The atomic ratios were evaluated following normalizations of the peak areas with the Scofield sensitivity factors.

Figure 23

 23 Figure 23 shows the XPS spectra recorded on (a) as-received activator, (b) dissolved and solvent evaporated at 35 °C for 50 h activator and (c) ZnAl, (d) Zn and (e) Zn2Mg phases after

Figure 23 :

 23 Figure 23: Comparison of Ti 2p XPS spectra of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) ZnAl, (d) Zn and (e) Zn2Mg Even without peaks deconvolution, it can be seen from Figure 23, that Ti 2p 3/2 and Ti 2p 1/2 peaks are larger on model phases compared to as-received activator and dissolved + solvent evaporated activator, indicating that titanium is involved in several types of chemical bonding on model phases. Binding energies of Ti 2p 3/2 and Ti 2p 1/2 peaks for activator coated Zn2Mg exhibit the initial Ti bonding amongst other while Zn and ZnAl show newly created bonds for the titanium.

Figure 24 :

 24 Figure 24: Ti 2p XPS spectra and respective curve-fittings of: (a) as-received activator, (b) dissolved and solvent evaporated activator, (c) Zn2Mg and (d) Zn Curve-fitting for the (a) as-received activator and (b) dissolved and solvent evaporated activator show the presence of Na4TiO(PO4)2 which is in accordance with the initial composition of the activator. An additional doublet at 457.7 eV (Ti2p3/2) and 463.3 eV (Ti2p1/2) could also be present as clearly indicates the spectrum in Fig. 24-d. This doublet is shifted comparing to both, Na4TiO(PO4)2 and TiO2 (which could be formed by hydrolysis of the initial solution) and was attributed to the formation of titanate-like compound, indicated as XTiO3 because the shift direction compared to TiO2 and binding energies are consistent with titanates (ZnTiO3 or Na2TiO3) [35]-[38].

Figure 25 :

 25 Figure 25: Evolution of open circuit potential of pure phases relevant for ZnAlMg coatings as a function of immersion time in PCC bath at 60 °C After 12 sec of immersion in the PCC bath, all phases exhibit OCP value close to the potential of pure zinc in these conditions (E° = -1.0 V/SCE for [Zn 2+ ] = 1 M). In the early stages of the PCC formation, the measured potential can be attributed to the potential of the model phase substrate, mainly composed of zinc in all 3 cases.

3. 3 . 1 .Figure 26 and

 3126 Figure 26 and Figure 27 illustrate typical morphologies of PCC crystals formed on the coatings (Figure 26) and pure phases (Figure 27) at different PCC treatment conditions -with and without activation step, varied temperature and treatment time.

Figure 26 :

 26 Figure 26: PCC grains density and morphology on ZnAlMg surfaces as a function of the use of a fresh activator, bath temperature and immersion time (as indicated) To evaluate the distribution of PCC at early stages of crystallization, some experiments were made at 40 °C instead of 60 °C. At 40 °C, a complete coverage of the surface of compositions B and C is achieved after 32 sec (Figure 26) while at 60°C the complete coverage of the composition C is achieved at 12 sec (composition B was not tested). The composition A exhibits a partial coverage after 32 sec at 40 °C but is completely covered at 60 °C after 12 sec. The slower deposition kinetics of the material A will be considered in the discussion section. PCC grains look homogeneously distributed on all ZnAlMg phases at different treatment times whether the immersion in the PCC bath was carried out at 40 or 60 °C.

Chapter 4 :

 4 On the effect of multiphase microstructure of ZnAlMg substrate on the Ti-based activation and phosphate conversion coating distribution 70 phase as the grain size is on average 10 times larger than each phase of the microstructure. A comparable observation can be made for composition B as the surface locally contains Zn dendrites with a smaller size than PCC grains. No preferential precipitation of phosphate grains on a specific phase was observed on composition C. Generally, the grain size is unchanged from one ZnAlMg composition to the other.

Figure 27 :

 27 Figure 27: PCC grains density and morphology on different materials as indicated after immersion in the PCC bath for 12 sec at 60 °C using: (1) no activation, (2) aged activation and (3) fresh activation (see text for detail)

Chapter 4 : 71 3. 3 . 2 .

 47132 On the effect of multiphase microstructure of ZnAlMg substrate on the Ti-based activation and phosphate conversion coating distribution Surface coverage evolution with time and T°C of PCC treatment From the analysis of a large number of SEM photos of PCC on all ZnAlMg materials after different times of PCC treatment (see Figure26for typical examples), the surface coverage by PCC at different immersion times was determined. As without activator, incomplete coverage at short immersion times does not reveal any preferential PCC deposition, but in contrast with the phosphating w/o the activator, grains are smaller (photos 5, 7, 10 and 11). Grains are more or less spherical at the initial steps of the PCC formation while they exhibit a more angular shape once the surface full coverage is completed.

Figure 28 :

 28 Figure 28: Surface coverage evolution as a function of immersion time, ZnAlMg substrate composition, bath temperature and use of activator (as indicated) Without activator, the PCC kinetics is very slow. PCC treatment at 40 °C instead of 60 °C increases the time necessary for a complete coverage of the coatings B and C to 32 sec instead of 12 sec at 60 °C, while the composition A is still uncovered even after 32 sec. However, although different ZnAlMg materials and pure phases demonstrated differences in PCC grain morphology and PCC layer growth kinetics, a complete coverage by the PCC can be achieved on all multiphase coatings after 12 sec of immersion at 60 °C.

Chapter 4 :

 4 On the effect of multiphase microstructure of ZnAlMg substrate on the Ti-based activation and phosphate conversion coating distribution 74 the activator with different ZnAlMg constituents and a formation of the homogeneous PCC layer with similar morphology on the coatings.

Figure 7 :

 7 Figure 7: Scheme of the Ti-activator and PCC distributions explaining homogeneous precipitation

Chapter 5 :

 5 During activation step, a variation of types of bonding on all 3 model phases were highlighted by XPS Highest Ti concentration: Zn > Zn2Mg > ZnAl Zn: Formation of XTiO3 (ZnTiO3 etc) favored compared to TiO2 Zn2Mg: Formation of TiO2 favored compared to XTiO3 (ZnTiO3 etc) Fast nucleation kinetics + grain shape favored on Zn by high Ti concentration associated to the formation of XTiO3 on the surface In general, the slower interaction of Zn2Mg with the activator does not seem to be critical during the PCC formation step, probably due to the relatively small size binary eutectic Zn2Mg phases in studied ZnAlMg materials. On the uneven distribution of Ti-based conversion coating on multiphase ZnAlMg coating Authors: Perrine Tanguy a,b, 1 , Christian Allély b , Diana Dragoe c , Michel Lahaye d , Vaclav Sefl e and Jan Stoulil e and Polina Volovitch a a Chimie ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie Paris (IRCP), F-75005 Paris, France b Global Research and Development, ArcelorMittal Maizières Research, France c Institut de Chimie Moléculaire et des Matériaux d'Orsay, CNRS, Université Paris-Sud 11, 91405 Orsay Cedex, France d PLateforme Aquitaine CAractérisation de MATériaux Placamat, CNRS, Université de Bordeaux, 33600 Pessac e University of Chemistry and Technology Technicka 5, Prague, Czech Republic Corresponding author: Polina Volovitch Tel.: (+33) 1 44 27 20 74 Fax: (+33) 1 46 34 07 53 Email: polina.volovitch@chimieparistech.psl.eu Abstract: The chemical composition, speciation and spatial distribution of Ti-based conversion coating (TiCC) on multiphase ZnAlMg coating on steel was studied by SEM-EDS, XPS, nano AES, GD-OES. A ZnAlMg coating containing Zn dendrites, binary eutectoid Zn-Al and binary eutectic Zn2Mg-Zn as well as three pure phases (Zn, ZnAl and Zn2Mg) simulating the ZnAlMg microstructure were characterized after alkaline degreasing and TiCC procedure. XPS demonstrated the presence of TiO2, Mn-modified hopeite, F-doped ZnO on all phases and MgF2 on Mg-containing substrates. Electron microscopy and nano-AES spectroscopy and mapping demonstrated a heterogeneous TiCC distribution on the multiphase substrate, binary eutectoid ZnAl and binary eutectic Zn2Mg phases being covered with thicker CC than monophase Zn dendrites. Galvanic coupling inside binary eutectic phases was proposed to be responsible for the enhanced TiCC precipitation kinetics on these phases because of the closely spaced anodes and cathodes on ZnAl and Zn2Mg compared to Zn dendrites. Electrochemical tests of model phases consolidated this hypothesis.

2. 3 .-

 3 TiCC procedure 2.3.1. Bication Zn-Ni phosphating procedure The following general procedure was used to precipitate Ti-based conversion layers on pure phases: Alkaline degreasing at 55 °C for 2 min -Osmosis water rinsing at 50 °C and at ambient temperature -Immersion in TiCC bath at 60 °C -Osmosis water rinsing at room temperature -Drying in hot air stream For ZnAlMg coating, the alkaline degreasing was performed by spraying on both faces of the sample for 12 sec. All baths were used maximum 12 h after mixing and renewed every 24 h. The immersion of the ZnAlMg substrate in the TiCC bath followed by an osmosis water rinsing enabled the conversion of insoluble CC elements on ZnAlMg without having to consider the possible impact of the application procedure on the TiCC on ZnAlMg phases and minimized Chapter 5: Impact of phases in ZnAlMg coatings on the distribution of Ti-based conversion coatings 88 the effect of unreacted and/or soluble products. Three replicates were performed to control the reproducibility.

Chapter 5 :

 5 Impact of phases in ZnAlMg coatings on the distribution of Ti-based conversion coatings 90 to avoid the figure overload hence a less effective readability. A minimum of 2 replicates were made to control the reproducibility.The chemistry of the deposited layer and the average concentrations after immersion of the model ZnAlMg coating and pure phases in the TiCC bath were studied by X-ray Photoelectron Spectroscopy (XPS) using K Alpha, Thermo Fisher Scientific spectrometer, (base pressure in the low 10 -9 mbar range) equipped with a monochromatic aluminium source (Al Kα, 1486.68 eV). A spot size of 400 µm corresponding to an irradiated zone of approximately 1 mm 2 was used. The hemispherical analyzer was operated at 0 ° take off angle in the Constant Analyzer Energy (CAE) mode, with a pass energy of 200 eV and an energy step of 1 eV for the acquisition of wide scans, while narrow scan spectra centered on the Ti 2p region were recorded at a pass energy of 100 eV and an energy step of 0.1 eV. Charge compensation was achieved by means of a "dual beam" flood gun, using low energy electrons (5 eV) and argon ions. The determination of the type of phosphate precipitates created during the conversion reaction was performed using the P 2s region as the P 2p region is interfered by Zn 3s. Samples were fixed on the support using adhesive conducting tape.The recorded spectra were processed by means of the Avantage software, using a peak-fitting routine with Shirley background and symmetrical 70 % -30 % mixed Gaussian-Lorentzian peak shapes. The atomic ratios were evaluated following normalizations of the peak areas with the Scofield sensitivity factors.

Figure 29 :

 29 Figure 29: Distribution of P, Ti, Mn and F by GD-OES profile on ZnAlMg after alkaline degreasing followed by immersion in TiCC bath at 60 °C for 30 sec The figure presents the intensity of different elemental signals as a function of the sputtering time and not the eroded depth because the average thickness of TiCC evaluated from DiP was in the range of 10 nm and the exact calibration of erosion rate for such a thin layer on the top surface with significant rugosity is impossible. Black arrows in the figure designate the maximum intensity of each component of TiCC. All tracers do not reach their maximum intensity at the same time. This way, the CC/substrate interface was defined as a position at 50

Figure 30 : 3 . 3 .

 3033 Figure 30: (a) SEM image of ZnAlMg coating (as indicated) after immersion in TiCC at 60 °C for 30 sec and EDS distribution of P, Ti, Mn and F on different phases and (b) SEM image of ZnAlMg and EDS mapping of Mg and F distribution after immersion in TiCC at 60 °C for 30 secFirst of all, the preferential precipitation of TiCC tracers was observed on Al-rich phase at the expense of Zn dendrites in Figure30-a. While the highest concentrations of P, Ti and Mn are observed on the Al-rich phase, the fraction of P and Ti were near the detection limit of the method on Zn dendrites. Only a small concentration of P and Mn were measured on Zn-Zn2Mg binary eutectic, but no Ti. The highest concentrations of F were measured on AlZn and Zn-Zn2Mg. The similarity of Mg and F distribution patterns was confirmed through an EDS

Figure 31 :

 31 Figure 31: AES profiles of different phases of ZnAlMg after immersion in TiCC bath at 60 °C for 30 sec after standard cleaning Figure 31 presents typical AES depth profiles obtained on different phases of ZnAlMg coating.The thinnest TiCC is detected on Zn dendrites (< 10 nm). In fact, the signal of TiCC tracers

Figure 32 :

 32 Figure 32: SEM images of ZnAlMg surface after alkaline degreasing and immersion in TiCC bath at 60 °C for 30 sec : (a) before sputtering, (b) after theoretical 70 nm of depth sputtering and (c) after theoretical 300 nm of depth sputtering

Figure 33 :

 33 Figure 33: Mean sputtering rates of untreated and polished (4000 SiC) Zn, ZnAl and Zn2Mg

Figure 34 :

 34 Figure 34: XPS spectra windows of TiCC tracers on pure phases and multiphase ZnAlMg substrates as indicated after standard degreasing and immersion in the TiCC bath at 60 °C for 30 sec

Figure 35 :

 35 Figure 35: Open Circuit Potentials (OCP) of pure phases in (a) TiCC bath pH 2.6, T=60 °C after 2 min of alkaline degreasing and (b) 5 wt. % Na2SO4 pH 2.6, T=60 °C

Chapter 5 :

 5 Impact of phases in ZnAlMg coatings on the distribution of Ti-based conversion coatings 102 the better coverage of binary phases and their reactivity should be approached considering the possibility of galvanic coupling inside the phase.To simulate the reactivity of binary phases (ZnAl and Zn/Zn2Mg), galvanic coupling experiments were made. The results of galvanic coupling of Zn with Zn2Mg are shown in Figure36a. For ZnAl eutectoid[31], the behavior was simulated by the galvanic coupling of the Zn phase with pure Al, as shown in Figure36 b. It is expected that all tendencies observed from the coupling will be aggravated compared to the actual intensity of the intra-phase coupling occurring within ZnAl phase.

Figure 36 :

 36 Figure 36: Potential evolution and current density of (a) Zn/Al coupling and (b) Zn/Zn2Mg coupling after respectively 30 min of immersion and 2 h of phase coupling in 5 wt. % Na2SO4 pH 2,6 at 25 °C When coupling Zn with Zn2Mg, the Zn phase plays the role of cathode while Zn2Mg becomes the anode and current density reaches 100 µA.cm -2 . Although, a low potential variation exists between Zn and Zn2Mg (≈ 70 mV/SCE), the high current density measured indicates that a non-

Figure 37 :

 37 Figure 37: Predominant equilibrium species for 113 mM Zn 2+ + 57 mM Al 3+ + 57 mM Mg 2+ + 112 mM PO4 3-+ 66mM Ti 4+ + 88 mM Mn 2+ + 420 mM F -using the Hydra-Medusa software and associated database of

Figure 38 :

 38 Figure 38: Schematic diagram of the TiCC precipitation on ZnAlMg

Figure 39 :

 39 Figure 39: Scheme of the TiCC precipitation mechanism on ZnAlMg



  The TiCC chemical composition from XPS, AES, GD-OES, EDS and Hydra-Medusa software precipitation simulations enabled the identification of the following products:  TiO2 (sometimes doped with F). No phosphates of Ti was detected  Mn3-xZnx(PO4)2 0 < x < 3 (with possibility of some MnHPO4 and MnxOy  MgF2 and some F-doped ZnO Chapter 6: Improvement of TiCC homogeneity on multiphase ZnAlMg substrate using surface activation by Cu 2+ modified alkaline degreasing Authors: Perrine Tanguy a,b, 1 , Christian Allély b , Diana Dragoe c , Michel Lahaye d , Anna Krasnova a and Polina Volovitch a a Chimie ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie Paris (IRCP), F-75005 Paris, France b Global Research and Development, ArcelorMittal Maizières Research, France c Institut de Chimie Moléculaire et des Matériaux d'Orsay, CNRS, Université Paris-Sud 11, 91405 Orsay Cedex, France d PLateforme Aquitaine CAractérisation de MATériaux Placamat, CNRS, Université de Bordeaux, 33600 Pessac Corresponding author: Polina Volovitch Tel.: (+33) 1 44 27 20 74 Fax: (+33) 1 46 34 07 53 Email: polina.volovitch@chimieparistech.psl.eu Abstract: In the previous chapter, the inhomogeneous coverage of multiphase ZnAlMg substrates by Ti-conversion coating (TiCC) was ascribed to a variation of local reactivity between phases. In order to overcome this problem and activate the whole surface of the multiphase substrate, a Cu 2+ -modified alkaline pretreatment is proposed and its effect on the chemical speciation and spatial distribution of TiCC on the multiphase substrate is described in the present chapter. After the degreasing step, EDS and AES confirmed the presence of metallic Cu nanoparticles on all phases of ZnAlMg. Following the TiCC deposition, AES revealed a similar TiCC thickness on all ZnAlMg phases. X-ray Photoelectron Spectroscopy (XPS) confirmed the homogeneity of the TiCC chemistry on the ZnAlMg surface after using the Cu 2+modified alkaline cleaner. The origin of the TiCC improved homogeneity was proposed to be in the cathodic nature of the cemented metal versus all phases of ZnAlMg, which induced a finer and homogeneous distribution of local cathodes and anodes, accelerated and homogenized the local pH increase necessary for the TiCC precipitation. Local Electrochemical Impedance Mappings evidenced the enhanced corrosion resistance of TiCC coated and painted ZnAlMg coated steel pretreated with the Cu 2+ -modified cleaner compared to the sample prepared with a conventional alkaline cleaner.

1 .

 1 Immersion in Cu 2+ -modified alkaline degreasing bath at 55 °C for 30 sec or 2 min 2. Osmosis water rinsing at 50 °C and at ambient temperature 3. Immersion in the TiCC bath at 60 °C for 30 sec 4. Osmosis water rinsing at room temperature 5. Drying in hot air stream

at 50 %Chapter 6 :

 506 height of the maximal signal, A peak with a kinetic energy of 776 eV was selected to follow Cu evolution because the main auger peaks of Cu are interfered by Zn. For the differentiation of the Cu forms contained in the Cu peak, the Target Factor Analysis (TFA) methods was used. First, a Principal Component Analysis was performed to identify the two most differentiated shapes of the Cu peak and define them as metallic Cu and oxidized Cu forms. Using the 2 peaks, a deconvolution was made as a function of the erosion depth to determine the concentration of each form of the Cu element in the erosion profile. A minimum of 2 replicates were made to control the reproducibility. Insight into the formation and degradation mechanisms of TiCC on multiphase ZnAlMg substrates using Cu 2+ modified alkaline degreasing baths 122The TiCC composition and concentration after immersion of the model ZnAlMg coating and pure phases in the TiCC bath were studied by X-ray Photoelectron Spectroscopy (XPS). The XPS spectrometer used (K Alpha, Thermo Fisher Scientific, base pressure in the low 10 -9 mbar range) was equipped with a monochromatic aluminium source (Al Kα, 1486.68 eV). A spot size of 400 µm corresponding to an irradiated zone of approximately 1 mm 2 was used. The hemispherical analyzer was operated at 0° take off angle in the Constant Analyzer Energy (CAE) mode, with a pass energy of 200 eV and an energy step of 1 eV for the acquisition of wide scans, while narrow scan spectra centered on the Ti 2p region were recorded at a pass energy of 100 eV and an energy step of 0.1 eV. Charge compensation was achieved by means of a "dual beam" flood gun, using low energy electrons (5 eV) and argon ions. Samples were fixed on the support using adhesive conducting tape.The recorded spectra were processed by means of the Avantage software, using a peak-fitting routine with Shirley background and symmetrical 70 %-30 % mixed Gaussian-Lorentzian peak shapes. The atomic ratios were valuated following normalizations of the peak areas with the Scofield sensitivity factors.The binding energy scale was calibrated on the neutral carbon set at 284.8 eV. Regarding reproducibility, the majority of quantitative and qualitative measurements were replicated at least 5 times.

  Preliminary verification of Cu deposition from the modified cleaner by potentiostatic cathodic polarization Imposed cathodic potentials experiments were performed on a Pt electrode immersed in the Cu 2+ -modified alkaline bath to evaluate the mass of electrodeposited Cu on the Pt surface. In addition, it enabled testing if the chelating agents, sodium gluconate and sodium citrate were not too strong and would not prevent a proper Cu deposition. The tested potentials were chosen to represent the potentials, which can be reached by all the ZnAlMg phases in the Cu 2+ -modified alkaline bath (table (a) in Figure 40) and confirm if the potential gap between Cu and ZnAlMg phases enables the Cu 2+ ions reduction. The measured OCP values and Cu mass gains measured with a scale as a function of the imposed potential are displayed in Figure 40.

Figure 40 :

 40 Figure 40: a) Measured OCP of different phases in Cu-modified alkaline cleaner and b) Cu mass gain measured on the Pt electrode as a function of the imposed potential in the Cu-modified alkaline degreasing bath at 55 °C

Chapter 6 :Figure 41 :

 641 Figure 41: AES mapping of ZnAlMg surface after immersion in Cu alkaline cleaning at 55 °C for 2 min

Figure 42 :

 42 Figure 42: AES deconvolution of Cu auger peak of ZnAlMg after Cu alkaline degreasing for 2 min at 55 °C

Chapter 6 : 3 . 3 .

 633 Insight into the formation and degradation mechanisms of TiCC on multiphase ZnAlMg substrates using Cu 2+ modified alkaline degreasing baths 126 TiCC thickness distribution on ZnAlMg phases measured by AES To evaluate if the cementation of Cu particles can lead to a homogenization of the TiCC distribution on all phases of ZnAlMg, AES profiles were performed on all ZnAlMg phases after its immersion in a TiCC bath at 60 °C for 30 sec following a Cu 2+ -modified alkaline cleaning.

Figure 43 :

 43 Figure 43: AES profile of ZnAlMg after immersion in TiCC bath at 60 °C for 30sec after Cu alkaline cleaning

Chapter 6 :

 6 Insight into the formation and degradation mechanisms of TiCC on multiphase ZnAlMg substrates using Cu 2+ modified alkaline degreasing baths 127 This is consistent with the results displayed on Figure41where a high concentration of white Cu particles is precipitated on the ZnAl phase.

3. 4 .

 4 TiCC chemical speciation on ZnAlMg after Cu 2+ -modified alkaline degreasing To evaluate the chemical composition of the TiCC on ZnAlMg when using a Cu 2+ -modified alkaline cleaner at 55 °C for 2 min prior to the immersion in the TiCC bath at 60 °C for 30 sec, a XPS analysis was performed. Spectra of (a) Ti 2p, (b) P 2s, (c) Mn 2p and (d) F 1s are shown in Figure 44.

Figure 44 :

 44 Figure 44: a) Ti 2p, b) P 2s, c) Mn 2p and d) F 1s XPS window spectra of ZnAlMg after Cu 2+ -modified alkaline degreasing and immersion in the TiCC bath at 60 °C for 30 sec

Figure 45 :

 45 Figure 45: Gradient of the admittance modulus for ZnAlMg degreased with the standard cleaner: (a) after 2 h of immersion, (b) after 24 h of immersion and for ZnAlMg degreased with the Cu 2+ modified cleaner: (c) after 2 h of immersion with alternating, (d) after 31 h of immersion

Chapter 6 : 4 . Discussion 4 . 1 .

 6441 Insight into the formation and degradation mechanisms of TiCC on multiphase ZnAlMg substrates using Cu 2+ modified alkaline degreasing baths 130 Cu precipitation mechanism on ZnAlMg phases Results in Figure 41 displayed an uneven distribution of Cu particles over all ZnAlMg phases.

Figure 46 :

 46 Figure 46: Scheme of the TiCC precipitation mechanism on ZnAlMg after Cu2+-modified alkaline degreasing procedure

Cu- 7 )Chapter 6 :

 76 2+ ions are incorporated to the CC baths to precipitate and create local and homogeneously distributed cathodes on metal surface, the main difference lies in the fact that Cu is not added to the conversion coating bath formulation but to the alkaline cleaner. The Cu precipitation occurs as a distinct step, prior to the TiCC precipitation.The results demonstrated that the formulation of a Cu 2+ -modified alkaline cleaner lead to the cementation of metallic Cu nanoparticles on all ZnAlMg phases. The Cu precipitation is triggered by the surface etching hence the electron exchange as represented here after:Zn + Cu → Zn + Cu(6Local concentration of deposited Cu decreased in order: binary ZnAl > Zn > Zn2Mg intermetallics The cementation of Cu induces a new distribution of cathodes and anodes on ZnAlMg, more closely spaced. Many local alkalization spots are created on the surface leading to a more homogenized TiCC distribution on ZnAlMg phases. Insight into the formation and degradation mechanisms of TiCC on multiphase ZnAlMg substrates using Cu 2+ modified alkaline degreasing baths 133 The TiCC chemical composition remains unchanged on the overall for ZnAlMg samples treated with the Cu 2+ -modified alkaline cleaner compared to those degreased with the unmodified one and TiCC contains: TiO2, ZnxMn3-x(PO4)2, MnH(PO4)2, MnxOy, and MgF2, Preliminary tests of the interface stability of polymer-coated ZnAlMg materials treated with the Cu 2+ -modified alkaline cleaner showed that the addition of Cu 2+ ions to the alkaline cleaning bath led to slower degradation of the system compared to the systems prepared with standard cleaner.

.and perspectives 1 .

 1 potential and an inhibited galvanic coupling. Local alkalization is not initiated hence limiting the TiCC precipitation mechanism. Deeper understanding of degradation mechanisms at the ZnAlMg/TiCC/polymer interface and the interpretation of all the results of local EIS measurements at different steps of ageing are also necessary. Conclusion Effect of microstructure on the formation of CC The importance of the microstructure for conversion coating formation mechanisms and following performance was demonstrated. The hypothesis proposed during the PhD states that the microstructure effects become critical for the CC distribution if typical microstructure elements of the substrate, which can become local anodic sites during the CC formation, are significantly bigger than the elementary structural constituents of the formed conversion coating. This hypothesis was verified at different steps of PCC and TiCC procedures and a solution to improve the homogeneity of the TiCC distribution on the multiphase ZnAlMg substrate via cementation of metallic Cu was proposed. This way, corrosion mechanisms of painted ZnAlMg coated steel as well as the precipitation mechanism of TiCC and PCC on multiphase ZnAlMg coatings were understood. Combining surface characterization of ZnAlMg coatings and pure phases, simulating ZnAlMg materials, at different steps of the CC procedure, it was confirmed that nanometric Ticontaining surface pretreatment (activator for PCC) and TiCC are heterogeneously distributed on ZnAlMg phases while micrometric PCC forms a homogeneous layer. The heterogeneous CC distribution was consistent with the worsened blistering resistance of TiCC-treated ZnAlMg surfaces compared to PCC-treated samples ones.

Figure 47 :

 47 Figure 47: Scheme of the Ti-activator and PCC distributions explaining homogeneous precipitation

Figure 48 :

 48 Figure 48: OCP of individual phases and current density measured for galvanic coupling of (a) Zn/Al and (b) Zn/Zn2Mg phases relevant for binary eutectics. The measurement was made after 30 min of immersion of uncoupled phases in 5 wt. % Na2SO4 solution with initial pH 2,6 at 25 °C (OCP) followed by 2 h of phase coupling (galvanic current values).

Figure 49 :

 49 Figure 49: Schematic representation of formation of different CC species during reaction of Zn, ZnAl and Zn2Mg phases in TiCC bath

Figure 50 : 7 ) 9 )

 5079 Figure 50: Schematic representation of inhomogeneous TiCC formation on the multiphase substrateTo overcome the influence of the microstructure/CC elements size ratio, it was proposed to achieve a Cu cementation on ZnAlMg phases during the degreasing step prior to the TiCC procedure. The cementation reaction can be described as follows:

Figure 51 :

 51 Figure 51: AES mapping of ZnAlMg surface after immersion in Cu alkaline cleaning at 55 °C for 2 min

Figure 52 :

 52 Figure 52: Scheme of the TiCC precipitation mechanism on ZnAlMg after Cu2+-modified alkaline degreasing procedure

Figure 53 :

 53 Figure 53: AES profile of ZnAlMg after immersion in TiCC bath at 60 °C for 30 sec after Cu alkaline cleaning

  Des premiers tests de corrosion extérieurs ont montré qu'en présence d'une rayure à l'acier (tests industriels d'évaluation de la résistance à la corrosion cosmétique), des cloques peuvent se former sur des ZnAlMg peints avec des TiCC mais pas avec des PCC. Dans le même temps, les revêtements ZnAlMg traités avec les PCC ainsi que les aciers galvanisés traités avec les TiCC ne présentent pas de cloques. Le cloquage apparaît donc comme le résultat d'un manque d'adhérence entre le substrat ZnAlMg et le TiCC. Une des raisons expliquant le cloquage sur ZnAlMg est la présence de multiples phases (Zn(Al)-Zn2Mg-Zn) qui peuvent créer un couplage galvanique entre les phases durant la précipitation de la CC. Il est admis que la réaction de précipitation des CC se déroule selon un mécanisme en 2 étapes : (1) Dissolution du métal dans les zones anodiques associé à une alcalinisation dans les zones cathodiques due aux réactions cathodiques et (2) Précipitation de la CC quand le pH de précipitation est atteint. Un substrat multiphasé pourrait entraîner une Résumé 165 distribution hétérogène des cathodes et anodes et changer la vitesse de nucléation et de croissance de la CC sur ces phases ainsi que les propriétés de la CC.

  technologique de Prague afin de les tester et de comprendre leur contribution individuelle aux mécanismes de formation des CC et de leur dégradation. La microscopie électronique à balayage et l'analyse dispersive en énergie ont permis d'observer la morphologie et distribution des grains de PCC sur les phases des revêtements ZnAlMg et phases pures. La spectroscopie de photoélectrons a permis de caractériser la concentration et la chimie de TiCC et de l'activateur à base de Ti nécessaire à la procédure de PCC sur les phases pures correspondant au ZnAlMg. La spectroscopie d'électrons Auger a permis l'évaluation de l'épaisseur de TiCC précipitée sur chaque phase du ZnAlMg. Des expériences de couplage galvanique entre phases pures ont été menées pour explorer les couplages galvaniques possibles dans les phases binaires et évaluer leur impact au moment de la formation des CC et distribution sur toutes les phases de ZnAlMg. La spectroscopie d'impédance électrochimique locale et la cartographie Résumé 166 d'impédance électrochimique locale ont été utilisées pour déterminer le gain de résistance à la délamination et formation de cloques des revêtements ZnAlMg traités avec un dégraissant alcalin modifié au Cu permettant une précipitation plus homogène de TiCC sur ZnAlMg. Structure : Cette thèse est organisée en 4 parties et 7 chapitres. La première partie est composée du chapitre d'introduction, de « l'état de l'art » (chapitre 1). La deuxième partie comprend la méthodologie expérimentale (chapitre 2) décrivant succinctement les techniques utilisées. La troisième partie comprend quatre chapitres de résultats sous formes de publications et qui se focalisent sur les thématiques suivantes : Chapitre 3 (« Compréhension des mécanismes de dégradation des revêtements ZnAlMg peints et traités aux TiCC ») abord l'impact de la microstructure des substrats ZnAlMg dans le développement et la propagation de la délamination et des cloques sous peinture.

Chapitre 5 ( 1 .

 51 « Impact des phases du revêtement ZnAlMg sur la distribution du TiCC ») met en évidence l'origine de la distribution hétérogène du TiCC sur le ZnAlMg. Chapitre 6 (« Aperçu des mécanismes de formation et dégradation des TiCC sur ZnAlMg après utilisation d'un dégraissant alcalin contenant des ions Cu 2+ ») présente une solution choisie pour homogénéiser la distribution du TiCC sur les revêtements ZnAlMg qui est basée sur la cémentation de Cu pendant le dégraissage, permettant une distribution des cathodes et anodes plus fines. La dernière partie est constituée du chapitre 7 « Conclusions et perspectives ». Ce résumé en français reprend la même structure que la version en anglais en proposant à chaque fois une synthèse des conclusions majeures des différents chapitres du manuscrit. État de l'art Résumé 167 Historiquement, les revêtements sacrificiels de l'acier utilisés dans le marché du bâtiment sont à base de zinc (Zn-0.2 wt. % Al) mais de nouveaux revêtements, ZnAlMg, sont en passe de les remplacer. Ils sont obtenus en utilisant le procédé de galvanisation (Hot Dip en anglais). La bande d'acier est immergée dans un bain de zinc, aluminium et magnésium liquide à des teneurs comprises entre 1 et 11 wt. % d'Al et jusqu'à 3 wt. % de Mg selon la composition visée. Après refroidissement du revêtement, quatre phases solides peuvent se trouver dans la microstructure, à savoir une phase riche en Zn, une phase riche en Al, l'intermétallique Zn2Mg et Zn11Mg2. Tandis que Zn11Mg2 est rarement présent dans la microstructure, les autres phases peuvent former des dendrites de Zn, des phases binaires eutectiques Zn-Al et Zn-Zn2Mg et/ou un ternaire eutectique Zn-Al-Zn2Mg selon la composition. Dans le cas des alliages à faible teneur en Mg, l'eutectique ternaire est considéré comme un eutectique binaire Zn-Al.

Figure 1 :

 1 Figure 1: Schéma de la coupe d'un revêtement ZnAlMg. Le binaire Zn-Zn2Mg est composé de lamelles de Zn (claires) et de lamelles de Zn2Mg (foncées) tandis que le ternaire comprend Zn, Zn2Mg et une fine phase d'Al. Le binaire ZnAl est composé de zones riches en Zn (claires) et zones riches en al (foncées) De nombreux tests de corrosion accélérés ou expositions naturelles (Test VDA, brouillard salin, scab test etc) ont permis de montrer que les revêtements de ZnAlMg possèdent une résistance à la corrosion améliorée comparé aux aciers galvanisés classiques. Il est communément admis que cela est en grande partie dû à la formation de produits de corrosion compact qui agissent comme une barrière physique à la réduction de l'oxygène sur la surface du ZnAlMg. Une réduction par deux de l'épaisseur du revêtement sacrificiel est possible tout en maintenant une résistance à la corrosion équivalente à celle des aciers galvanisés. Pour les revêtements ZnAlMg, l'épaisseur standard est de 10 µm tandis qu'elle est de 20 µm pour l'acier galvanisé.

Figure 2 :

 2 Figure 2: Structure d'un revêtement ZnAlMg peint Différentes couches de peintures peuvent être appliquées selon les propriétés physiques, chimiques, électrochimiques etc désirées. L'épaisseur totale de peinture appliquée sur le revêtement est comprise entre 20 et 250 µm. L'adhérence de la peinture et la résistance à la corrosion du revêtement peint sont assurées par la présence d'une couche de conversion (CC) entre le revêtement ZnAlMg et la peinture. Ce sont des fines couches d'oxydes insolubles et inorganiques dont l'épaisseur varie entre 10 nm et 1 µm selon la nature de la CC utilisée. Ils agissent comme une barrière physique et insoluble limitant la réduction de l'oxygène sur la surface du ZnAlMg et ainsi la propagation de la corrosion. La formation des CC est de nature électrochimique et suit un mécanisme général commun à toutes les formulations existantes. Avant l'application de la CC, un dégraissage alcalin est d'abord réalisé afin d'éliminer les traces d'huiles, d'oxydes etc résiduels à la surface de l'échantillon. La formation de la CC a lieu en 2 étapes. Tout d'abord, des zones anodiques et cathodiques sont définies au contact du traitement de conversion. Aux anodes, le métal est dissout tandis qu'aux cathodes, la réduction de l'oxygène et des protons sont les réactions majoritaires comme visible sur la figure 3.

Figure 3 :

 3 Figure 3:Schéma des réactions anodiques et cathodiques pendant le couplage galvanique L'augmentation de pH est l'élément déclencheur de la precipitation de la CC. Dès que le pH seuil de précipitation de la CC est atteint, la formation de la couche d'oxydes a lieu, isolant le revêtement ZnAlMg de l'air ambiant.

Figure 4 :

 4 Figure 4 : Cloquage visible autour de la rayure à l'acier après 22 mois de test de corrosion en milieu naturel (marin)

Figure 5 :

 5 Figure 5: Cartographie SKP d'un revêtement ZnAlMg peint présentant un défaut à l'acier et après exposition en milieu naturel pendant 24 mois La cartographie du potentiel de surface du revêtement ZnAlMg sous la peinture mesurée par sonde de Kelvin sur un échantillon cloqué permet de mettre en évidence les zones corrodées sous les cloques (Figure 5). Une observation plus fine de ces zones permet de réaliser que les zones corrodées sont plus étendues que les cloques elles-mêmes et lient plusieurs cloques entre elles. Dans certains cas, un chemin de corrosion partant de la rayure à l'acier jusqu'aux cloques distantes est visible.

Figure 6 :

 6 Figure 6: Cartographie MEB-EDS d'un revêtement ZnAlMg peint après 72 heures de chronopotentiométrie à 5 µA dans une solution de NaCl 1 wt. % pH 7Au niveau du front de délamination, la cartographie de la microstructure montre la présence de corrosion à un stade peu avancée dans la microstructure. L'oxydation de la phase riche en Mg, Zn-Zn2Mg semble indiquer le caractère anodique de cette phase qui pourrait corroder préférentiellement au contact de NaCl.

Figure 7 :

 7 Figure 7: Courbes de polarisations anodiques et cathodiques des phases pures représentant le ZnAlMg dans une solution de NaCl 5 wt. % pH 7 T=25 °C

Figure 8 :

 8 Figure 8: Spectre XPS Ti 2p et déconvolution : (a) activateur à l'état initial, (b) activateur après dissolution et évaporation du solvant, (c) Zn2Mg et (d) Zn Après l'étape d'activation, la concentration et la forme chimique du Ti varient selon le substrat utilisé. Sur les phases pures représentant le ZnAlMg, le Ti est présent en plus forte concentration sur la phase de Zn que celle de Zn2Mg, la concentration en Ti mesuré étant trop faible sur ZnAl pour permettre une déconvolution du pic de Ti 2p. Sur la phase Zn2Mg, la présence de TiO2 comme composant majoritaire a été mise en évidence tandis que sur Zn, c'est un composé de type XTiO3 qui a été mesuré. Les énergies de liaisons mesurées favorisent la présence d'un TiO2 lié chimiquement au substrat (ZnTiO3).

Figure 9 :

 9 Figure 9: Morphologie et distribution des grains de PCC sur différents substrats de ZnAlMg en fonction de la présence de l'activateur, température du bain de phosphatation et temps d'immersion (voir figure) L'absence d'activateur avant la phosphatation conduit à la formation d'amas de grains larges d'environ 10 nm permettant un recouvrement limité du revêtement. En présence d'activateur, une phosphatation à 60 °C pendant 12 sec permet la formation de petits grains d'1-2 µm, cristallins et couvrant 100 % de la surface. Aux stades intermédiaires de recouvrement du ZnAlMg, des grains de PCC peuvent être observés sur toutes les phases du revêtement, indiquant la distribution homogène de la CC sur la surface.

(1- 5 )--

 5 Réduction cathodique entraînant une alcalinisation : Précipitation du TiCC: TiF + 4OH → TiO . H O + 6F

Figure 10 :

 10 Figure 10: Profils d'érosion par spectroscopie auger des phases de ZnAlMg après immersion dans le bain de TiCC à 60 °C pendant 30 sec après un dégraissage alcalin La mesure de l'épaisseur du TiCC sur chaque phase du revêtement ZnAlMg (Figure 10) montre une hétérogénéité de distribution du TiCC sur les phases. Les phases alliées ZnAl et Zn-Zn2Mg

Figure 11 :

 11 Figure 11: Evolution du potential et de la densité de courant de (a) Zn/Al et (b) Zn/Zn2Mg après 30 min d'immersion puis 2 h de couplage dans Na2SO4 5 wt. % pH 2,6 à 25 °C

Figure 12 :

 12 Figure 12: Profils d'érosion par spectroscopie auger des phases de ZnAlMg après immersion dans le bain de TiCC à 60 °C pendant 30 sec après un dégraissage alcalin contenant des ions Cu2+ La cémentation de Cu permet une distribution des anodes et cathodes plus fine ce qui permet un recouvrement plus rapide des zones anodiques et une distribution de TiCC plus homogène

Figure 13 :

 13 Figure 13: Gradient du module d'admittance de ZnAlMg revêtu de polymère modèle sur le TiCC après dégraissage alcalin standard: (a) après 2 h d'immersion, (b) après 24 h d'immersion et pour ZnAlMg traité avec le dégraissant aux ions Cu2+: (c) après 2 h d'immersion, (d) après 31 h d'immersion Après 31 h d'immersion dans la solution de NaCl à 0.1 M, le front de délamination est à peine modifié pour l'échantillon traité avec le dégraissant au Cu alors que celui traité avec le dégraissant alcalin standard a subi une augmentation de la zone délaminée de 40 % par rapport à la largeur initiale.

  En combinant des caractérisations de surface sur ZnAlMg et phases pures à différentes étapes de la réaction de conversion de PCC et TiCC, une compréhension des différences de résistance à la corrosion des 2 CC a été permise. En effet, la distribution de TiCC sur les différentes phases de ZnAlMg est hétérogène alors que dans le cas de la PCC, la distribution est homogène sur Résumé toutes les phases. Pour expliquer cette différence de distribution, le ratio entre la microstructure et la taille des éléments de la CC a été avancé.Pour le PCC, l'activateur à base de Ti possède une réactivité différente face aux phases de ZnAlMg en termes de composés formés et de concentration mesurée. La distribution homogène était attribuée à la taille des grains de PCC qui sont d'une taille similaire à la microstructure.Pendant l'étape de conversion, les grains grandissent jusqu'à être en contact les uns avec les autres et recouvrir la surface. La distribution des anodes et cathodes dans la microstructure est assez fine comparé à la dimension des grains pour permettre un recouvrement complet de la surface.

Figure14:Figure 15 :

 15 Figure14: Schéma de la distribution de l'activateur au Ti et de la PCC permettant une précipitation homogène Pour le TiCC, la taille des éléments de la CC sont plus petits ce qui s'est révélé critique pendant l'étape de conversion. En effet, le couplage galvanique au sein des phases alliées entraîne une distribution bien définie des anodes et cathodes à la surface qui ont un impact sur la formation de TiCC. La cinétique de précipitation est plus rapide sur les phases binaires grâce à a distribution fine des zones anodiques et cathodiques. A l'inverse, les dendrites de Zn monophasées sont moins bien couvertes que les phases alliées.

Figure 16 :

 16 Figure 16: Schéma du mécanisme de précipitation de TiCC sur ZnAlMg après dégraissage alcalin contenant des ions Cu2+ La microstructure de ZnAlMg joue aussi un rôle favorisant l'apparition de cloques. D'un côté, la précipitation hétérogène de TiCC sur les phases de ZnAlMg peut entraîner des manques de CC localement sur la surface et ainsi des zones de moindre résistance à la corrosion. D'un autre côté, en présence d'un défaut à l'acier, la propagation de la corrosion est facilitée par la présence de différentes phases et en particulier de la phase anodique et continue de Zn2Mg-Zn.

Figure 17 :

 17 Figure 17: Schéma du mécanisme de dégradation des revêtements ZnAlMg peints en présence d'un défaut à l'acier

Table 1 :

 1 List of the ZnAlMg coatings compositions commercialized by the main European and Japanese steel companies [2]-[9] ................................................................................................ 6

Table 2 :

 2 Standard electrochemical potential (V vs SHE) in 1M NaCl, 25 °C, 1 atm [65] ...... 19

Table 3 :

 3 Phases

Table 1 :

 1 List of the ZnAlMg coatings compositions commercialized by the main European and Japanese steel companies[2]-[9]

	Company	Commercial name	Commercialization year	wt.% Zn wt.% Al wt.% Mg
	Nisshin Steel	ZAM	1998	91.0	6.0	3.0
	Nippon Steel Corp.	Super Dyma	2002	86.0	11.0	3.0
	Tata Steel	Magizinc	2007	97.0	1.5	1.5
	Voestalpine	Corrender	2007	96.0	2.0	2.0
	Thyssen Krupp	ZM Ecoprotect	2008	98.0	1.0	1.0
	ArcelorMittal	Magnelis	2010	93.3	3.7	3.0

  State of the Art delayed by the presence of Mg 2+ ions while Al 3+ ones promoted the formation of stable LDH even in very alkaline conditions. A non-oxidized Al skeleton was noted to keep corrosion products compactness.

Thierry et al. studied the behavior of ZnAlMg coated steels in natural exposure and proposed that the dissolution of Al promotes the formation of different types of compact LDH corrosion products, lowering the corrosion rate of ZnAlMg

[8]

. The Mg was explained to induce a lower surface pH on cathodes and the formation of magnesium hydroxycarbonates and possibly magnesium hydroxychlorides on anodes.

Finally, Salgueiro et al. observed the formation of corrosion inhibiting Basic Zinc Salts (BZS)

and LDH products on ZnAlMg surfaces under cathodic polarization in various electrolytes

[16]

which correlated with lower cathodic currents. The transformation of BZS into ZnO was Chapter 1 :

Table 2

 2

	: Standard electrochemical potential (V vs. SHE) in 1M NaCl, 25 °C, 1 atm [65]
		Electrochemical potential standard (V vs. SHE)
	Zn ↔ Zn 2+ + 2e -	-0.76 ± 0.01
	Fe ↔ Fe 2+ + 2e -	-0.44 ± 0.01

Table 3 :

 3 Phases weight and molar composition and supplier

	Phases	Composition / wt. %	Supplier
	Al	≈ 100	GoodFellow
	Zn(Al)	-	UCT Prague
	Zn2Mg	84(Zn) 16(Mg)	UCT Prague

Table 4 :

 4 Composition of pure phases and ZnAlMg

Table 5 :

 5 Fraction of the surface (%) covered by Zn dendrites, binary phases Zn-Zn2Mg and Zn-Al, and ternary eutectic Zn-Al-Zn2Mg determined from the SEM images of top surface of coatings A, B and C and the typical linear size (µm) of phases

Table 6 :

 6 Model phases relevant for ZnAlMg coatings: their chemical composition (from AAS), phase crystallography (from XRD) and microstructure (from SEM) as well as open circuit potential (OCP) in PCC bath at 60 °C after 12 sec and 100 sec of immersion

Table 8 :

 8

XPS analysis results of the atomic concentration of Ti measured on each phase (at. % Zn/ at. % Ti)

Table 9 :

 9 Composition of ZnAlMg and pure phases relevant for ZnAlMg coating

Table 12 :

 12 Atomic concentration of Ti measured on model phases (at. % Ti)XPS confirmed the highest amount of Ti on ZnAl and the lowest on the Zn phase. This observation is consistent with EDS and AES results.

	Label	% 𝑎𝑡. 𝑇𝑖 % 𝑎𝑡. 𝑇𝑖 + % 𝑎𝑡. 𝑃 + % 𝑎𝑡. 𝑀𝑛 + % 𝑎𝑡. 𝐹 + % 𝑎𝑡. 𝑍𝑛
	Zn	7.2 ± 0.8
	ZnAl	14.3 ± 1.5
	Zn2Mg	10.8 ± 1.0

Table 13 :

 13 Open circuit potential (OCP) of pure phases after 2 min in the degreasing bath at 55 °C

	Label	OCP in degreasing bath after 2 min at 55 °C / V vs. SCE
	Zn2Mg	-1.43 ± 0.02
	ZnAl	-1.63 ± 0.02
	Zn	-1.41 ± 0.02

From the degreasing step, ZnAl and ZnAlMg exhibit a much lower potential than Zn and

Table 14 :

 14 Composition of ZnAlMg and pure phases corresponding to ZnAlMg

Table 17 :

 17 Binding energies of compounds with energies fitting pure phases and ZnAlMg peaks after standard degreasing and immersion in the TiCC bath « Impact de la microstructure des revêtements ZnAlMg sur les mécanismes de formation et la performance des couches de conversion sans Cr »

	Par Perrine TANGUY
	Thèse de doctorat en Chimie Physique et Chimie Analytique
	R SUM
	Introduction
	Contexte :

Acknowledgments

where a 0.1 s per sample integration period with 20 mV peak-to-peak potential perturbation was applied. The electrochemical cell was filled up with a 0.1 M NaCl solution where samples were immersed 1 h before the beginning of the measurement to saturate the polymer coating with water, allowing the ionic flow through the polymer. A homemade set-up using a Solartron potentiostat was used. Measurements were done with a homemade local probe composed of two Ag wires of 250 µm diameter in a glass capillary (sealed in two joined capillaries with the distance between the two Ag wires centers d = 1.3 mm). The probe was positioned at 150 ± 25 µm above the WE and moved with a 3-axis positioning system (UTM25, Newport) driven by a motion encoder (MM4005, Newport). The data treatment was made with a homemade software.

All impedance maps were measured on 8x3 cm² areas with steps of 200 µm along both X-and Y-directions which required about 15 minutes for each map.

Each step included global and local impedance measurements in two points above the polymer (A) and the scratch (B) with frequencies ranging from 100 kHz to 1 Hz and a LEIM mapping at a constant frequency of 500 Hz for the sample treated with the unmodified cleaner. For the one degreased with the Cu 2+ -modified alkaline one, 4 mappings were performed at 500 Hz while the last one was made at 300 Hz to avoid an inductivity phenomenon.

Measurements of global and local impedances were performed at the OCP for 1 h to study their evolution. First, a LEIM analysis was performed at OCP followed by global and local impedance measurements in points A and B (in the same conditions). Then, an anodic polarization of the sample at -0.7 V vs Ag/AgCl saturated reference electrode was applied before performing the LEIM analysis, global and local impedances measurements in points A and B. This alternating OCP-anodic polarization procedure was repeated 3 times for the reference sample degreased using an unmodified alkaline degreasing and 5 times for samples degreased with the Cu 2+ -modified alkaline degreasing until the darkened area around the artificial defect under the polymer became approximately twice wider than the initial defect. In a second step, a cathodic polarization (-1.5 V vs Ag/AgCl saturated) and anodic one (-0.6 V vs Ag/AgCl saturated) were applied alternately for both samples. An additional mapping was performed on the sample treated with the unmodified alkaline cleaner due to the delamination propagation.

The typical output used so far in the literature to report on LEIM experiments is mapping of the local impedance, z(r), or the local admittance, z -1 (r), at a fixed frequency. In the present work, the gradient of the local admittance is used to present the results [30]. Appendix 5 (Chapter 4) The Table 17 shows the different compounds fitting the peaks measured by XPS on pure phases and ZnAlMg material and displayed in Figure 57.

Appendix 8 (Chapter 6) The Table 18 shows the different compounds fitting the peaks measured by XPS on pure phases and ZnAlMg material and displayed in Figure 58.

Appendix 8 162

Expérimental