Ecole Sigle

Doctorale

Renée El

Melhem Bât

Pascal Blaise

R Gourdon

M Stéphane

ÉLECTROTECHNIQUE E E A Électronique

M Gérard Scorletti
email: gerard.scorletti@ec-lyon.fr

M Philippe
email: philippe.normand@univ-lyon1.fr

Mme Sylvie

Jean-Yves Buffière
email: jean-yves.buffiere@insa-lyon.fr

Stéphanie Cauvin

M Jocelyn Bonjour

Véronique Guichard

J Y Toussaint

M Christian Montes

Keywords: Corrosion, Sol-Gel, 2D materials, EIS Corrosion, Sol-Gel, 2D matériaux, SIE

Study of the role of two dimensional nanosheets as fillers in Sol-Gel anticorrosion coatings deposited on metal surface

Acknowledgement

This thesis may never be realized without the help of the people who had supported me from the begining to the very end.

I firstly give my thanks to my parents, to my grandparents and to my dear relatives and friends in China.

I give my sincere thanks to China Scholarship Council to give me financial support. I also give my thanks to to Professor Bernard Normand for the support to the thesis scientifically, adminstratively and financially with professionalism, patience and generosity. I also thanks him for being my mentor, guiding and supporting me to be fully trained as a Ph.D with all his knowledge, experience, and ressources.

I give my special thanks to Dr. Sheng Yuan for the whole time supporting me, as my first mentor, a teacher, a friend who gave me all the introduction, guides and cooperation to the scientific world, especially during the difficult times that we have confronted together. I give my thanks to Dr. Kurt Hermange, for his cooperation and guide in my thesis, and also for being my dearest friend. I give my thanks to Dr. Dwaipayan Mallick, as one best friend of mine. All this memories and experience we've shared in France and throughout the world between us are appreciated and will be carefully protected. I give my thanks Dr. Hela bin Mensour, for her being my dearest friend, for her support, for her understanding and for every moment we've shared.

General Introduction

The economic cost caused by metal corrosion is equivalent to around 3% of the global GDP every year, which requires the intensive effort of the industry domain for researching, developing, and applying anti-corrosion solutions. 1 For the most widely used metallic structural materials such as steels and aluminum-based alloys, the chromium conversion coatings (CCC) were considered as an efficient solution. However, the toxic nature of Chromium hexavalent Cr (VI) makes CCC hazardous to the environment and the human health. 2 According to the policy of "Registration, Evaluation and Authorization of CHemicals" (REACH) launched by European Union (EU), Cr (VI) containing chemical compounds or components have been strictly restricted in EU territory. 3 The replacement of CCC pushes researchers for the exploration of new materials or different surface treatments. The later one has been investigated to replace CCC according to the applying of materials, surface reactivity, mechanical property, life duration, and of course, the environment to which the final product is exposed. 4,5 The Sol-Gel process has attracted abundant attention since its first applications from middle of twentieth century. 6 Its mild synthesis conditions, easy to obtain versatile composition, high purity of gel, and the ability of synthesizing thin-films allow it to be an interesting technique for various application environments. 7 The interest of sol-gel coating is two-fold in the industry: 1) as a primer layer to ensure good adhesion property between the top-layer and substrate; 8 2) as a barrier to prevent the penetration of the corrosive reactants to the underlying metal surface. However, its barrier property depends closely on its density that often requires a high curing temperature. Otherwise, a film derived xerogel is obtained, which is a three-dimensional network of inorganic skeleton surrounded by small compounds and solvent molecules. This structure is mesoporous or micro-porous and enables the connection of the metal to the application environment. 9 Inorganic-Organic Hybrid (IOH) Sol-Gel coatings combine the properties of organic and inorganic parts allowing to reduce porosity. They can, nevertheless, lead to delamination problems and inferior mechanical properties. 10 The addition of nano-fillers is another solution for the improvement of the barrier effect in Sol-Gel coatings. The fillers have different functionalities as inhibitors 11 or as sacrificial particles like Zn for cathodic protection. 12,13 The two-dimensional materials (2D materials) posses excellent to small species, such as water, oxygn molecules and ions. 14 This impermeability is of interest for anti-corrosion applications. 15 Being deposited directly on substrates 16 or charged as filler into coatings 17 , the hexagonal boron nitride (h-BN), due to its inertness and thermal stability, has been studied for corrosion protection applications. 18,19 Other 2D materials also showed the potential to be applied as anti-corrosion fillers to improve the barrier effect of Sol-Gel in order to allow it to replace the CCC surface treatment. 20,21 This work has been dedicated to understanding h-BN nanosheet's influence in zirconiumbased Sol-Gel coating and to find a promising approach for the anticorrosion protection on various metals.

The nanofillers were exfoliated from raw h-BN by either classic liquid-phase exfoliation. The role of the hexagonal boron nitride nanosheet morphology and concentration in different Sol-Gel matrix, the effect of various 2D materials on enhancing the barrier properties, has been discussed. The anticorrosion property of 2D nanofiller charged coatings have been analyzed using electrochemical methods. This dissertation is divided into five chapters as below:

Chapter I resumes the corrosion mechanism and the Sol-Gel process. The corrosion phenomena and the methodology of corrosion protection are described in detail. For the Sol-Gel, the key parameters on the physico-chemical properties and microstructure of the coatings are summerized. The selection of 2D materials are also explained in this chapter.

Chapter II presents the materials, the sol-gel parameters used for different coating systems, and the 2D materials exfoliation methods. The physico-chemical and structural characterization tools are detailed in the appendix part. The electrochemical analysis, including polarization and electrochemical impedance spectroscopy (EIS), are also described.

Chapter III characterized the three coating systems for different purposes.

In the first part, the h-BN nanosheet filled Zr-based single-component coatings were deposited on the 316L stainless steel surface. The existence of the passive film on stainless steel ensures good coating adhesion after "piranha" solution treatment and reduces the degradation velocity of the metal. Therefore, the deterioration of the coating system was directly correlated to the under-deposit corrosion reaction, which highlights the barrier effect of the nanofillers. In this context, the sol-gel coatings filled with different h-BN nanosheets with different concentrations of nanofillers were prepared.

In the second place, we attempted to transfer the nanocomposite coatings onto a more reactive aluminum alloy. It has been proved that the single-component matrix cannot provide sufficient protection. Moreover, the lack of coating adhesion on aluminum alloy favored the lateral propagation of the oxidation product and the coating delamination. The multicomponent sol-gel matrix based on zirconium and silicon oxide was thus used to replace the single-component one to improve the impermeability of coatings.

Until now, we had two ways to optimize the coating's barrier effect with a view to increase its lifetime. The first way consists of the adhesion enhancement, which is generally limited by the coating/ substrate materials coupling and the synthesis conditions. The other way to improve the coating's intrinsic resistance has been selected. For this purpose, the organic-inorganic hybrid coating doped with Octadecyltriethoxysilane (ODTES, -C18 alkyl group) were deposited onto carbon steel.

Chapter IV presents the electrochemical characterizations and the interpretation of the coatings developed in the third chapter. The performance against localized corrosion on 316L and against general corrosion on aluminum and carbon steel are evaluated.

Finally the results of this study and the perspectives for further investigations were given in General Conclusion and Perspectives.

I.1 Introduction

In this chapter, the corrosion mechanisms are introduced according to the types of corrosion : general corrosion, localized corrosion. The commonly used anti-corrosion methods are presented, including conversion coatings, corrosion inhibitors, sacrificial coatings, noble metal coatings, and non-metal inert coatings.

The Sol-Gel has been studied as a promising way for coating synthesis due to its mild synthesis conditions and ecofriendly properties. The hydrolysis/condensation reaction and the classification of Sol-Gel coatings are introduced. Inorganic/Organic Hybrid (IOH) Sol-Gel coatings are considered as an important category that has attracted much attention in recent years. It could increase coating thickness and flexibility compared to pure inorganic coatings by mixing organic group functionalized silane to the metal-alkoxide precursors.

The 2D materials have different physical, chemical, and mechanical properties. The selection of the hexagonal Boron nanosheet as fillers in this work is due to their inertness and impermeability.

I.2 Corrosion of metallic materials

Metals are applied in industry as structural materials. According to their different physical and mechanical properties, metallic materials such as steels, aluminum alloys, magnesium alloys, copper and copper alloys, etc., have been widely applied in different industry domains. 1 However, the common problem for metallic materials is their sensibility to oxidation and corrosion. A «NACE» report in 2016 revealed that the global cost of corrosion per year is estimated to be $2.5 trillion US dollars, which is 3.4% of the Global Domestic Production (GDP) in 2013 and could be reduced by 15 -35% with available controlling process. 2

(National Association of Corrosion Engineering (NACE))

No perfect solution exists. In order to prolong the lifetime and manage the material failure, the efficient anticorrosion strategy is eagerly demanded and to be optimized. To this end, the understanding of the mechanism of the corrosion process is mandatory.

I.2.1 Mechanism of metal corrosion

The mechanism of the corrosion is related to the electrochemical phenomenon taking place on the surface of metal. 3 Reactions are driven by the difference of potential of local heterogeneities. The part with a higher potential is called cathode, while the part with a lower potential is called anode. In metals, the electrons are moving freely inside the bulk materials. 4 The corrosion modes are presented in the next part.

I.2.2 Corrosion modes

The corrosion modes can be classified into three main categories: the generalized corrosion, the localized corrosion, and the mechanically assisted corrosion. 5

Generalized corrosion

Uniform corrosion: Uniform corrosion is also called General corrosion. It is the corrosion that happens homogeneously over the whole material surface in contact with the corrosive media. 6 General corrosion causes the mass loss of materials that can be estimated by the corrosion current density according to Faraday's law. 7 Fig. I.2 -2 shows the mechanism of general corrosion. On the bulk material, the anodes and cathodes are alternatively distributed, which depends on the metallurgical properties of the metals. Passivation: Passivation is a phenomenon that promotes the initiation and growth of a natural dense thin film on the surface of the metal due to the oxidation of the metal atoms. 9 An alloy of non passive metal could also behave passive behavior with the passif film introduced by the passsif alloy element like Cr, etc. 10 Taking stainless steel as an example, by introducing alloy elements which can form passive film such as chromium into iron-based steels, a thin layer of chromium oxide is generated on the metal surface. By electrochemical dynamic polarization curve, the passivation phenomenon of the passive metal typically presents a curve, as shown in The polarization curve of passive metal could be separated into different regions. 5 Region I: Cathodic zone, in which the applied potential is lower than the corrosion potential of the working electrode. In Region I, there is no corrosion, reduction phenomenon are privileged.

Ecorr:

The corrosion potential separate the Region I and Region II, where current density equals to 0 mA.cm -2 . At this potential ianode = -icathode.

Region II: Active zone. It is composed of 2 different parts:

Between Ecorr and Ecrit: When the potential is inferior to the potential Ecrit, the overall current density increases to prepare the metal surface for the initiation of the passive film growth.

Ecrit:

The critical potential is located at the point where the current density starts to decrease.

In the second part, the current density decreases since the passive film is initiated and grows.

Region III: Passive zone, this region corresponds to the potential field for the maintenance of passive film. The current density corresponds to Ipass, which is generated from maintaining the passive film by migration phenomenon.

Region IV: Transpassive zone, where the current increases again and keeps increasing. The applying potential is much higher than the corrosion potential of oxide compounds in the passive film, which promotes the dissolution of material.

Region V:

The reigion characterizes the oxidation of water. The oxygen evolution takes place.

Fig. I.2 -3 Typical Electrochemical Dynamic Polarization curve of a passive material 11

Localized Corrosion

Localized corrosion is defined according to the heterogeneity of the reaction. Localized corrosion does not happen on the whole material, but a localized part of the metal surface instead. 12 Once the corrosion is inhomogenous, it is a localized corrosion. Thus, the localized corrosion could be, as for a non-passive metal, an under-coating corrosion, or, as for a passive metal, a pitting corrosion.

Galvanic corrosion:

We discuss here specifically the galvanic corrosion on the metal surface, not including cathodic protection.

Different metals have different corrosion potential. When two metallic materials are getting in touched, which is called a galvanic couple, the anode metal will be corroded, while cathode metal will act as the conducting pathway for electrons. 13 Admittedly, the galvanic corrosion activity depends on the corrosion potential difference between the galvanic couple. However, the kinetic features of galvanic corrosion are also influenced by each specific couple and also by the corrosion product. 13 For example, the potential difference between zinc and steel is not the highest. However, the corrosion rate measured from this galvanic couple is much higher than the galvanic couple composed by zinc and other alloys. 14 The standard potential of some metal elements in water solution is shown in Fig. I.2 -4. Moreover, the corrosion potential of a galvanic couple will be influenced also by the characteristic of the solution.

Fig. I.2 -4 Distribution of potential in aqueous media of frequently used metals or alloys in industry 13

Pitting corrosion: Pitting corrosion happens on the passive metals. It is a localized corrosion that is initiated in the surface area of the micro-scale. 15 The passive films have different microstructures on different metals or produced by different surface treatments. The passive films isolate the corrosive media from penetrating through the substrate. However, once the passive film exhibits a defect, a break by friction or collision, or with the presence of chloride ions, the exposed metal surface will act as an anode. All passivated part acts as a large cathode. 4 Pitting corrosion is a very localized phenomenon that propagates deeply into the bulk material.

The mechanism of the chloride ions assisted pitting corrosion is briefly presented in Fig. I.2 -5a. The passive film of stainless steel consists of a thin and dense chromium oxide layer. Therefore, the corrosive species can not directly reach the metal surface. According to the literature, the chloride ions slowly penetrate into the passive film and cause the breakdown of the film, see Fig. I.2 -5b. 15 The exposed surface will thus initiate the pitting, as shown in Fig.

I.2 -5c.

The pitting corrosion propagates rapidly by an autocatalytic phenomenon. 15

Fig. I.2 -5 Mechanism of break down of passive film by the presence of Cl -and introduced pitting corrosion 31

Crevice corrosion: Crevice corrosion is the corrosion where the corrosive electrolyte are accessible to the substrate in a confined space, which always happen under coating or around junctions. 31 Lacking oxygen at the confined space will provoke an evolution of pH, which could promote corrosion. This mechanism differs from the pitting corrosion in the initiation step while the propagation step of both phenomena are the same.

Selective corrosion:

The selective corrosion is also called dealloying or selective leaching. 16 One classical example is the dezincification of brass containing Cu 70%-Zn 30%, from which the oxidation of Zn element is preferred. 17 The selective corrosion causes a decrease in the mechanical property of the alloy.

Nevertheless, dealloying could be used as a method to create nano-porous structure due to the selective elimination of participants in a solid solution material. 16 As shown in Fig. I.2 -6, by applying dealloying and re-dealloying process, the Zn phase are all removed from the Copper alloy matrix, which leaves a highly porous microstructure. 18 Meanwhile, this precipitation leaves the chromium depletion region as a region with very low content of Cr, since all the chromium are diffused into the chromium carbide precipitation region. Thus, the grain contains higher content of Cr than the Chromium depletion region, which creates a galvanic couple between the Chromium depletion region and the grain. As anode, Chromium depletion region will firstly be corroded, and the propagation of corrosion will preferentially be in the direction of the grain boundary. 19

Fig. I.2 -7 Mechanism of intergranular corrosion 18

Mechanically Assisted Corrosion

As mentioned above, once the passive film is broken due to a mechanical reason, the localized corrosion will be initiated and propagated. This corrosion is called mechanically assisted corrosion. 14

Stress corrosion cracking

Erosion / abrasion / Cavitation corrosion:

The direct mechanical erosion caused by flowing liquid, solid particles abrasion, cavitation from air bubbles could be assumed as erosion/abrasion/cavitation corrosion. These procedures include the loss of passive film by dissolution or by mechanical removal and pH changes due to geometrical modification of the solid/liquid interface. It causes the acceleration of corrosion as well as the initiation of new corrosion. 21 Tribocorrosion: Tribocorrosion is the phenomenon when a wear takes place between the metallic surface and the applying interface. 22 It is a complex phenomenon which includes several corrosion types mentioned above: corrosion fatigue, abrasion, erosion, cavitation etc. The corrosive environment, the possible particles size, nature of coating and metal substrate, the fatigue cycle characteristics as well as the stress distribution all influence the tribocorrosion. 23 Hydrogen Embrittlement: The embrittlement is caused by the presence of hydrogen, which decreases the toughness or ductility of metal. The presence of hydrogen atoms will react and produce hydrogen and increase the internal pressure of the material. The tendency of a material to allow the penetration of hydrogen or to trap the hydrogen atoms by its inner defect may influence the severity of embrittlement of the material.

I.2.3 Anticorrosion methods

As shown in Fig.

Passivation alloying or surface modification

Though carbon steel is of good mechanical properties, it behaves very low anticorrosion property. The introduction of passive film on alloying surfaces is an effective method. The elements like chromium, nickel, and cobalt, are introduced into alloys for this reason. 24,25 Stainless steel was thus developed and can be classified as martensite stainless steel, austenite stainless steel, and duplex stainless steel according to the different alloy compositions and heat treatment. 14 However, the pitting corrosion resistance plays an essential role in the use of stainless steel in chloride ions containing environment. Thus, the protection against Cl -contamination for stainless is eagerly demanded by the industry. 1 However, the change of composition for existing material may cause a consequently increase in the cost in the industry domain. 26 Thus, the "local" surface modification is a more costworthy method. The most efficient surface modification for stainless steel is chromium conversion coatings (CCC). However, Cr(VI) is hazardous for the environment and human. Its utilization has been forbidden according to the « REACH » rule launched by the European Union. 27 The alternative surface modification methods have been developed, such as the carburizing 28 , the nitriding 29 , the laser surface treatment 30 , the nano-crystal surface modification 31 , etc. The improvement of corrosion resistance of the modified materials have been proved in the litterature.

The effect of surface modification varies according to the composition after surface treatment. Taking nitriding as an example, Lavigne et al. reported that the passive film in stainless steel bearing a high nitrogen content shows a better crystallization rate but a lower corrosion resistance comparing to the low content containing sample. 29 The trivalent chromium conversion coating was firstly proposed by Matzdorf et al. and patented by the US Navy in the year 2000. 32 It has been developed for the adjustment of different pH conditions one year later by Sheasby et al. 33 The Cr(III) compound shows lower toxicity compared with the Cr(VI) species. 34 Rare earth elements, especially Lanthanide elements, shows potential in anticorrosion applications. The cerium-based conversion coatings have been developed on Aluminum alloys 35 , 316L 36 , etc. These coatings showed a good anticorrosion effect due to its inhibition property of cerium. 37 Transition metal oxyanions based conversion coatings like MnO4 2-, MoO4 2-, V 5+ /V 4+ oxides behave similar properties to CCC and have much lower toxicity. 38 For instance, the zirconium-based 39 and Zirconium/Titanium -based conversion coatings have been deposited on aluminum alloys by modifying the adhesion property with the addition of hexafluorometallate complexes (with H2ZrF6, H2TiF6 for example). These coatings showed improvement of corrosion resistance in 3.5wt% NaCl solution. 11

Corrosion inhibitors

Corrosion inhibitor is a kind of chemical compound that can directly limit the corrosion reaction by different mechanisms. It stops the metal dissolution by producing a complex compound or a precipitation on the metal surface to inhibit the electrochemical reaction between the corrosive media and the metal surface. The inhibitors can be either directly added into the liquid environment or charged into protective coatings. Many families of inhibitors have been explored and applied for different conditions in terms of pH, saline, and other working conditions. 40 Inhibitors are mainly classified as:

-the inorganic complexes composed by Mo, Mn, Co, Ni, etc., which shows less toxicity than Cr(VI) based inhibitors; 41,42,43 -the organic molecules like benzotriazole and its derivatives, amino acid etc.; 44 -The "green inhibitors" are natural products such as Vernonia Amydalina for steels, as well as the extract of Hisbiscus subdariffa for Aluminum and Zinc etc. 45 Another "green inhibitor" branch is the rare earth elements including cerium and other lanthanide elements due to its low toxicity. [START_REF] Bethencourt | Lanthanide Compounds as Environmentally-Friendly Corrosion Inhibitors of Aluminium Alloys: A Review[END_REF] The inhibition mechanisms are different according to the nature of the inhibitor and the substrate. For example, cerium salt precipitates and recovers the substrate connective area.

It is one of the most used inorganic inhibitors and has been applied to different substrates like copper, steels, aluminum alloys, etc. [START_REF] Bethencourt | Lanthanide Compounds as Environmentally-Friendly Corrosion Inhibitors of Aluminium Alloys: A Review[END_REF] The inhibitors based on cerium derivatives were directly introduced into solution with the form of cerium salts (e.g. CeCl3) or charged into cerium rich coatings. [START_REF] Bethencourt | Lanthanide Compounds as Environmentally-Friendly Corrosion Inhibitors of Aluminium Alloys: A Review[END_REF] The organic inhibitors, on the contrary, will react with the substrate and finally compose a chemical compound to block the corroded area. 3 Benzotriazole was applied on aluminum alloys and copper applications. 3,[START_REF] Finšgar | Inhibition of Copper Corrosion by 1,2,3-Benzotriazole: A Review[END_REF] It has been charged into Titania nano-containers for the protection of Al7075 in 3.5wt% NaCl solution. 19 However, both inorganic and organic inhibitors showed toxicities to the human body. [START_REF] Mittal | Cerium Oxide Nanoparticles Induced Toxicity in Human Lung Cells: Role of ROS Mediated DNA Damage and Apoptosis[END_REF][START_REF] Alotaibi | Benzotriazoles in the Aquatic Environment: A Review of Their Occurrence, Toxicity, Degradation and Analysis[END_REF]

Cathodic protection or sacrificial coating

As shown in Fig. I.2 -1b, if the protected metal is connected to another metal with a lower potential, it will be protected by cathodic protection. [START_REF] Angst | A Critical Review of the Science and Engineering of Cathodic Protection of Steel in Soil and Concrete[END_REF] The cathodic protection is considered as an efficient and secure way to protect pipelines or other underground components and has already been widely applied in industry.

By different deposition technologies such as spray coating [START_REF] Hassani-Gangaraj | Critical Review of Corrosion Protection by Cold Spray Coatings[END_REF] , hot painting [START_REF] Edavan | Corrosion Resistance of Painted Zinc Alloy Coated Steels[END_REF] , and electrodeposition [START_REF] Gavrila | Corrosion Behaviour of Zinc-Nickel Coatings, Electrodeposited on Steel[END_REF] , the anode metal could be deposited on the metal surface or charged into other coatings. Zn, Sn/Mn, Zn/Ni coatings can be deposited on steels and showed satisfied anticorrosion properties. [START_REF] Gavrila | Corrosion Behaviour of Zinc-Nickel Coatings, Electrodeposited on Steel[END_REF][START_REF] Gong | Electrodeposition of Sacrificial TinÁ/Manganese Alloy Coatings[END_REF]

Noble metal coating or non-metallic barrier coating

Different from other methods, the barrier coatings simply use the barrier effect to totally reduce the contact between the metal surface and corrosive media. This method works on the whole 1 st , 2 nd and 3 rd parts of the corrosion. Barrier coatings do not have any interaction with the electrolyte. Nevertheless, if the corrosive media penetrates through the porosity or defects to the protected metal, the later behaves as anode and be corroded even more rapidly.

Non-metallic barrier coatings have different categories. The Zn-silicate coating allows over critical concentration pigment volume. Zn-silicate and Zn-epoxy are applied as primers, but the cracks may occur if the thickness is too high. [START_REF] Sørensen | Anticorrosive Coatings: A Review[END_REF] Organic coatings are applied as top layers for many applications. Epoxy/resin [START_REF] Montemor | Functional and Smart Coatings for Corrosion Protection: A Review of Recent Advances[END_REF] , thermoplastic polymer [START_REF] Vaidya | Processing of Fibre Reinforced Thermoplastic Composites[END_REF] , thermoset polymer [START_REF] Perrin | Polyaniline Thermoset Blends and Composites[END_REF] , conducting polymers [START_REF] Twite | Review of Alternatives to Chromate for Corrosion Protection of Aluminum Aerospace Alloys[END_REF] were developed for corrosion protection. However, the main shortcoming for organic coatings is related to the low adhesion and delamination during service. [START_REF] Olajire | Recent Advances on Organic Coating System Technologies for Corrosion Protection of Offshore Metallic Structures[END_REF] Taking Epoxy coating as an example, different fillers such as silica particles [START_REF] Wang | Combined Effects of Silica Filler and Its Interface in Epoxy Resin[END_REF] , nanotubes [START_REF] Díez-Pascual | Nanoindentation in Polymer Nanocomposites[END_REF] , etc., were introduced to improve its mechanical and barrier properties.

Sol-Gel is a suitable way to elaborate inorganic and organic-inorganic hybrid coating. It is of the mild synthesis conditions (low temperature), the easy to control composition and the microstructure. [START_REF] Zheludkevich | Sol-Gel Coatings for Corrosion Protection of Metals[END_REF] The low toxicity makes it one of the promising candidates for the replacement of CCC. 3 The inorganic silicon-based Sol-Gel coatings have already been applied as a several first layers on the substrate to introduce suitable adhesion property for primer coatings onside. [START_REF] Zheludkevich | Sol-Gel Coatings for Corrosion Protection of Metals[END_REF] Nevertheless, the porous nature of Sol-Gel coating allows the penetration of the corrosive media and limits the applications. 3 Hybrid coatings with different organic ratios have drawn attention, which allows overpassing the shortcoming of both inorganic and organic coatings. [START_REF] Amiri | Hybrid Nanocomposite Coating by Sol-Gel Method: A Review[END_REF] In order to overcome the porous nature of Sol-Gel and take advantage of its non-toxic nature, the 2D nano-fillers will be charged into the Sol-Gel coatings in this work. Further exploration with hybrid coatings are done to deeply discover the role of 2D fillers inside the hybrid and inorganic coatings. Thus, from next part, the Sol-Gel process and both the inorganic and hybrid coatings will be detailed.

I.3 Sol-gel process

Sol-Gel is a chemical process that allows the production of solid materials from precursors through a sequence of hydrolysis reactions and condensation reactions. [START_REF] Liang | Facile Formation of Superhydrophobic Silica-Based Surface on Aluminum Substrate with Tetraethylorthosilicate and Vinyltriethoxysilane as Co-Precursor and Its Corrosion Resistant Performance in Corrosive NaCl Aqueous Solution[END_REF] Sol-Gel process has mild synthesis conditions, such as low temperature, low toxicity allowing the various applications, as shown in Fig. I.3 -1. 3 By using silicon-based precursors and/or transition metal (Zr, Ti, Al, Ce...) alkoxide based precursors, glasses and solid ceramic materials can be obtained after a curing treatment. [START_REF] Wang | Sol-Gel Coatings on Metals for Corrosion Protection[END_REF] Moreover, the addition of different chelating additives like chelating agent, acidic or basic catalyst, amino-containing functional group and so on, is a common method to modify the microstructure and the properties of the final product. [START_REF] Figueira | Hybrid Sol-Gel Coatings: Smart and Green Materials for Corrosion Mitigation[END_REF] Sol-gel technique has been applied for monoliths synthesis thanks to its high mass transfer efficiency. [START_REF] Siouffi | Silica Gel-Based Monoliths Prepared by the Sol-Gel Method: Facts and Figures[END_REF] Sol-gel can potentially replace the solid-phase microextraction for fibers fabrication. [START_REF] Kumar | A Review on Development of Solid Phase Microextraction Fibers by Sol-Gel Methods and Their Applications[END_REF] Sol-gel can also be used for fuel cell membrane elaboration with well-controlled porosity. [START_REF] Peighambardoust | Review of the Proton Exchange Membranes for Fuel Cell Applications[END_REF] The derived solid spheres, with different shapes such as hollowed spheres, mesoporous spheres, hierarchically multiporous spheres and hierarchically porous spheres, have been applied in the biological domain for the drug delivery and the catalysts introducing to a target system. [START_REF] Du | Spherical Silica Micro/Nanomaterials with Hierarchical Structures: Synthesis and Applications[END_REF] Sol-gel derived coatings have been proved efficient against wear, oxidation, and corrosion on the metal surfaces thanks to the high hardness, the chemical stability, the suitable adhesion property, the thermal toughness, and its low thermal conductivity. 72

Fig. I.3 -1 The applications of Sol-Gel process 73

The Sol-Gel anticorrosion coating is a promising solution for the tremendous demands in replacing hazardous Cr(VI) containing coatings since the beginning of the new century. [START_REF] Tallman | Electroactive Conducting Polymers for Corrosion Control: Part 1. General Introduction and a Review of Non-Ferrous Metals[END_REF] Wang et al. have reviewed the recent researches on Sol-Gel coatings for the anticorrosion application. [START_REF] Wang | Sol-Gel Coatings on Metals for Corrosion Protection[END_REF] Among all the choices, the Si-based precursors have been the mostly explored for the last decades. The principal shortcoming of these coatings is their limited barrier effect due to the avoidable defects and pores in the matrix. 3 The electrolyte and the corrosive species penetrate through these weak parts of the coating. In order to overcome this issue, the fillers like ZrO2, Al2O3, TiO2 were incorporated in sol-gel coating that showed improved corrosion resistance on 316L, carbon steels, aluminum alloys substrates. [START_REF] Balamurugan | Electrochemical and Structural Characterisation of Zirconia Reinforced Hydroxyapatite Bioceramic Sol-Gel Coatings on Surgical Grade 316L SS for Biomedical Applications[END_REF][START_REF] Hosseini | Self-Healing Corrosion Protection by Nanostructure Sol-Gel Impregnated with Propargyl Alcohol[END_REF][START_REF] Liu | Improving the Formation and Protective Properties of Silane Films by the Combined Use of Electrodeposition and Nanoparticles Incorporation[END_REF][START_REF] Boccaccini | Ceramic Coatings on Carbon and Metallic Fibres by Electrophoretic Deposition[END_REF] Besides, instead of pure inorganic or organic matrix, the use of organic inorganic hybrid (OIH) material allows the combination of the properties from different precursors and prevents defect formation. [START_REF] Balgude | Sol-Gel Derived Hybrid Coatings as an Environment Friendly Surface Treatment for Corrosion Protection of Metals and Their Alloys[END_REF][START_REF] Schottner | Hybrid Sol-Gel-Derived Polymers: Applications of Multifunctional Materials[END_REF] As reviewed by Figueira et al., researchers have proposed the methods for the improvement of anticorrosion properties for aluminum alloys, copper alloys and magnesium alloys. [START_REF] Figueira | Organic-Inorganic Hybrid Sol-Gel Coatings for Metal Corrosion Protection: A Review of Recent Progress[END_REF] The principle reaction and the reaction parameters will be discussed in the next section.

I.3.1 Sol-Gel reactions

This study will be limited to the alkoxide method in which the "Sol" represents the colloidal solution containing the undissolved particles of nanometric size (1-100 nm). Along the evolution, the so-obtained "Gel" is a three-dimensional (3D) diphasic system that has a solid network mixed with a liquid phase and small molecules. [START_REF] Soler-Illia | Mesoporous Hybrid Thin Films: The Physics and Chemistry Beneath[END_REF] Sol-Gel process consists of two main reaction steps, one is the hydrolysis reaction, and the other one is the condensation reaction. Their reaction are shown as below: [START_REF] Collignon | Cahier Technologique Sol-Gel -PDF[END_REF] The hydrolysis reactions: On the other hand, the condensation has two different kinds of reaction: 1) alcoholic and 2) non-alcoholic, as shown above. The condensation process connects two precursor molecules to each other and forms a 3D skeleton.

The hydrolysis and condensation are two competing processes that should be carefully controlled to obtain expected microstructure and properties. Indeed, the hydrolysis and condensation reaction velocity can be manipulated by different parameters. 83 The metal alkoxide solution can be directly used to form nanoparticles on SiO2, TiO2, ZrO2, etc. [START_REF] Caruso | Sol-Gel Nanocoating: An Approach to the Preparation of Structured Materials[END_REF] The Sol-Gel coating can be directly synthesized by nanoparticles by spraying or brushing. [START_REF] Shen | Corrosion Protection of 316 L Stainless Steel by a TiO2 Nanoparticle Coating Prepared by Sol-Gel Method[END_REF] The Sol can be deposited on metal surface by dip-coating, spin-coating, electrodeposition and other methods to forge Xerogel film. [START_REF] Brinker | Review of Sol-Gel Thin Film Formation[END_REF] A heat treatment of the xerogel film is classically required to remove the residual liquid phase (the solvent) accompanied with a densification and a significant amount of shrinkage. [START_REF] Siouffi | Silica Gel-Based Monoliths Prepared by the Sol-Gel Method: Facts and Figures[END_REF] Finally, a relatively dense solid coating is obtained. Moreover, by supercritical evaporation like CO2 assisted evaporation, the aerogel can be prepared. 87

I.3.2 The main parameters of Sol-Gel process

The main parameters to control Sol-Gel process are the precursor nature, the solvent, the pH of Sol, the addition of chelating agent, the aging time and aging temperature, the evaporation, the cure temperature, and the deposition method, etc. The well control of these parameters allows to tune the composition, microstructure as well as the associated physico-chemical properties of the Sol-Gel product.

The Precursor

Sol-Gel precursors are mostly Si or transition metal-based alkoxide. Treated at high temperature (> 400°C), pure inorganic material can be obtained. On the contrary, the precursors can be chemically modified with organic groups, for which the curing temperature is usually limited (< 200°C). [START_REF] Zheludkevich | Sol-Gel Coatings for Corrosion Protection of Metals[END_REF] Otherwise, the organic compound will decompose. This part will mainly discuss the properties of the inorganic precursor, while the role of organic containing precursors will be discussed in the latter part.

The precursors play a primordial role in the microstructure and the final properties of the Sol-Gel coatings. The order of reactivity of different precursors is given by: Si(OR)4 <<< Sn(OR)4 < Ti(OR)4 < Zr(OR)4 < Ce(OR)4, according to the center atom. [START_REF] Zheludkevich | Sol-Gel Coatings for Corrosion Protection of Metals[END_REF] The reactivity of the precursor based on the same transition metal is also influenced by the organic groups. For example, zirconium acetate hydroxyl(ZAH), zirconium propoxide (ZPO), zirconium n-butoxide(ZBO), zirconium tetra-butoxide (ZTB), the molecules are presented in Fig. I.3 -3. The coordination number of ZAH, ZPO, ZBO and ZTB molecules is 4, meaning four available sites for another molecule to connect. The interaction is generally ensured by forming a Zr-O-Zr covalent bonding. The presence of acetate groups reduces the reactivity of these sites, which makes the ZAH stabler than the other three precursors. Comparing ZPO and ZBO, the length of Zr-O bonding (varying in Å level) decides the reactivity of hydrolysis and condensation of each precursor. As a consequence, ZPO is more active than ZBO. [START_REF] Peter | EXAFS Study of Zirconium Alkoxides as Precursors in the Sol-Gel Process: II. The Influence of the Chemical Modification[END_REF] Thus, there is a difference in the reactivity between different Zr based precursors : ZAH <<< ZPO < ZBO. This difference can be observed on other precursor pairs, ZTB and ZBO for example.

As reviewed, the silica coatings have been reported to be efficient in the protection of metals against oxidation and corrosion in acidic conditions. [START_REF] Wang | Sol-Gel Coatings on Metals for Corrosion Protection[END_REF] Vasconcelos has reported SiO2 sol-gel coating deposited on 304L stainless steel showing enhanced corrosion resistance in 1 N H2SO4 solution and 3.5% NaCl solution. [START_REF] Vasconcelos | Corrosion Resistance of Stainless Steel Coated with Sol-Gel Silica[END_REF] Thim et al. reported the silica thin film showing better corrosion protection over aluminum alloy in 0.5 mol/L NaCl solution. [START_REF] Thim | Sol±gel Silica ®lm Preparation from Aqueous Solutions for Corrosion Protection[END_REF] However, the low thermal expansion coefficient causes delamination limiting the application of Si-based precursors for corrosion resistance of metallic components. [START_REF] Mackenzie | Physical Properties of Sol-Gel Coatings[END_REF] The aluminum-based coating has low conductivity, excellent wear resistance. The similar nature to passive film on aluminum alloys make it largely applicated and been studied for improvement of corrosion resistance. [START_REF] Twite | Review of Alternatives to Chromate for Corrosion Protection of Aluminum Aerospace Alloys[END_REF] The titanium-based precursors are chemically stable and UV-sensitive. They generally have low conductivity and high thermal resistance. [START_REF] Twite | Review of Alternatives to Chromate for Corrosion Protection of Aluminum Aerospace Alloys[END_REF] The biological compatibility makes titanium-based material interesting in medical implants for biomedical components. [START_REF] Velten | Preparation of TiO2 Layers on Cp-Ti and Ti6Al4V by Thermal and Anodic Oxidation and by Sol-Gel Coating Techniques and Their Characterization[END_REF] The cerium precursors, which show similar physical and mechanical properties to Ti-based precursors without UV sensibility and with an inhibiting nature to corrosion, are used as inhibitor as well as inert anticorrosion coatings. [START_REF] Figueira | Organic-Inorganic Hybrid Sol-Gel Coatings for Metal Corrosion Protection: A Review of Recent Progress[END_REF] The zirconium-based precursors derived zirconia coating shows excellent chemical stability and biological compatibility. 2 Furthermore, the comparable thermal expansion of zirconia can largely reduce the number of defects during heat treatment. Thanks to these advantages, the zirconia coating has been used on steels under high application temperature by plural researchers. [START_REF] Zheludkevich | Sol-Gel Coatings for Corrosion Protection of Metals[END_REF][START_REF] Izumi | Sol-Gel-Derived Coatings on Steel Sheets[END_REF] Compared with using single-component precursors, the multi-component precursor derived Sol-Gel inorganic coatings allow to combine the properties of each component. 3 Li et al. reported the improvement of durability for solar cells with the deposition of TiO2-SiO2 coating and ZrO2-SiO2 coating comparing to pure SiO2 surface. [START_REF] Li | Preparation, and Durability of TiO2/SiO2 and ZrO2/SiO2 Double-Layer Antireflective Coatings in Crystalline Silicon Solar Modules[END_REF] As reported by Mumjitha et al., the TiO2-SiO2 coatings showed superior wear resistance, corrosion resistance and cell adhesion with less cytotoxicity than the pure TiO2 coating. [START_REF] Mumjitha | Fabrication of TiO2-SiO2 Bioceramic Coatings on Ti Alloy and Its Synergetic Effect on Biocompatibility and Corrosion Resistance[END_REF] Other researchers have shown that multicomponent coatings Al2O3-SiO2, TiO2-SiO2, ZrO2-TiO2 coatings have improved anticorrosion properties on stainless steels in NaCl solution thanks to the less crack tendency. [START_REF] Zheludkevich | Sol-Gel Coatings for Corrosion Protection of Metals[END_REF][START_REF] Liu | Preparation and Photocatalytic Properties of La-Doped TiO2/ZrTiO4 Heterogeneous Thin Films[END_REF][START_REF] Bu | Corrosion Behavior of ZrO2-TiO2 Nanocomposite Thin Films Coating on Stainless Steel through Sol-Gel Method[END_REF]

Fig. I.3 -3 Four different Zirconium based precursors

The solvent

The polarity, caused by the distribution of dipolar moment in a molecule, predominate the intermolecular interactions. [START_REF] Mahltig | Hydrophobic Silica Sol Coatings on Textiles-the Influence of Solvent and Sol Concentration[END_REF] The molecules with similar polarity will attract each other while the dissimilar polarity does repulse. The solvents are of different polarities, such as water and ethanol are more polar than solvent like isopropanol, n-butanol etc. [START_REF] Hristian | Solvents and Solvent Effects in Organic Chemistry[END_REF] The difference in polarity is reported to be able to change the hydrolysis reaction of Sol-Gel colloidal particle size. [START_REF] Malay | Effects of Solvent on TEOS Hydrolysis Kinetics and Silica Particle Size under Basic Conditions[END_REF] Moreover, the water molecule participates in the hydrolysis reaction, which will also influence the final structure of the Sol-Gel coating.

The pH

The pH can catalyze the hydrolysis reaction, which makes the pH-control essential for the preparation of stable Sol. In acidic conditions, the cations are attracted by oxygen atoms that accelerate the hydrolysis reaction releasing -OR groups. In basic conditions, the hydroxide OH - is attracted by the metal atom inside the metal alkoxide molecule favoring the formation of M-O-M bonds, which makes the condensation reaction privileged. [START_REF] Collignon | Cahier Technologique Sol-Gel -PDF[END_REF] As hydrolysis and condensation are influenced by pH, the macromolecules will show different structures. In acidic conditions, the long chains, often called ''spaghetti'' shape, are formed, resulting in a dense structure and more critical internal stress in the solid phase, as shown in Of note, the pH of the Sol is also limited by the sensitivity of the metal substrate. The deposition of the coating on metal that is sensitive to an acidic or alkaline environment will cause a weak coating/substrate interface. ) 83,101 102 The chelating agent

The chelating agents react with the alcoholic site on the precursor molecule and form a chelation complex. As shown in Fig. I.3 -5, the chelating agent will block the active site of metal alkoxide precursor. [START_REF] Yu | Effect of Chelating Agent Acetylacetone on Corrosion Protection Properties of Silane-Zirconium Sol-Gel Coatings[END_REF] Pappet et al. reported, by studying zirconium alkoxide based Sol-Gel, with appropriate content of chelating added into the solvent, the gelating reaction could be stabilized comparing to the instant gelation with low content of chelating agent.

Too much chelating agents may slow down the hydrolysis and condensation, the formation of colloidal structure could take years, which is also not desired. [START_REF] Papet | Transparent Monolithic Zirconia Gels: Effects of Acetylacetone Content on Gelation[END_REF] The concentration of chelating agent influence the reaction rate of hydrolysis and condensation, ended up in a microstructure modified according to the different content of chelating agent. [START_REF] Papet | Transparent Monolithic Zirconia Gels: Effects of Acetylacetone Content on Gelation[END_REF] Various molecules can be used as chelating agents such as carboxyl acid, -diketone, amines, ketoester, etc. 105 The aging time and aging temperature

The aging of the Sol-Gel will influence the colloid size as well as the final product microstructure. The aged Sol is able to lead to a more porous structure. Liu et al. have proved that the formation of hydroxyapatite sol-gel coating is highly dependent on the aging time to have sufficient apatite structure. [START_REF] Liu | Structural Evolution of Sol-Gel-Derived Hydroxyapatite[END_REF] Chou et al. have reported that the aged Sol deposited coating will behave a decreased corrosion resistance comparing to the original Sol deposited coating. [START_REF] Chou | Organic-Inorganic Hybrid Coatings for Corrosion Protection[END_REF] Ginter et al. have reported that the aging time will have a crucial influence on microstructure of synthesized zeolite Y particles by SiO2 and Al2O3 Sols. [START_REF] Ginter | A Physicochemical Study of the Aging of Colloidal Silica Gels Used in Zeolite Y Synthesis[END_REF] The aging temperature as well as pH condition are also reported to be influencial for the microstructure final product from the same report. Bosc et al. reported that a small change of aging temperature will lead to a great drop of stability for titania Sol. 109 The evaporation and curing temperature

During the drying in room temperature or curing in oven, the evaporation of the solvent molecule produces pores inside of the coating. [START_REF] Collignon | Cahier Technologique Sol-Gel -PDF[END_REF] The slow evaporation reduces the chance of cracking and big pores, while internal stress is also lower. [START_REF] Sakka | Sol-Gel Technology as Representative Processing for Nanomaterials: Case Studies on the Starting Solution[END_REF] The curing time and temperature will both play an important role in the final microstructure of the coating. [START_REF] Lampke | Cost-efficient Conversion Coatings for Corrosion Protection Prepared by the Sol-gel Process[END_REF] High temperature post-treatment could be applied in the case that the crystallization of the coating matrix are desired. [START_REF] Nouri | A Comparative Study of Heat Treatment Temperature Influence on the Thickness of Zirconia Sol-Gel Thin Films by Three Different Techniques: SWE, SEM and AFM[END_REF]

Deposition methods

Different deposition methods could be applied to prepare sol-gel coatings such as dipcoating [START_REF] Mohd Yusoff | Dipcoating of Poly (ε-Caprolactone)/Hydroxyapatite Composite Coating on Ti6Al4V for Enhanced Corrosion Protection[END_REF] , spin-coating 114 and electrophoretic deposition, spray coating, cathodic plasma deposition, Electrodeposition, and etc. 115 ,116 It has been reported in detail that the most widely applied deposition method are: dip coating and spin coating, respectively. [START_REF] Brinker | Review of Sol-Gel Thin Film Formation[END_REF][START_REF] Bornside | Spin Coating: One-dimensional Model[END_REF] The electrodeposition was lately reviewed to be able to synthesize thick and defect-free coatings. [START_REF] Zheludkevich | Sol-Gel Coatings for Corrosion Protection of Metals[END_REF] This work will mainly explain the principle of the dip-coating process. As the figure of dip coating shown in Fig. where h is the thickness of the coating, η is the viscosity of the Sol, νo is the dragging speed, σ is the surface tension of the Sol, ρ is the density of the Sol, and g is the gravity factor. Thin coating can be obtained after the drying step with the evaporation of the solvent. Since the coating shrinks during the solvent evaporation and the heat treatment, tensile stress may be induced between the substrate and the coating. , the inorganic coatings have mesopores and macro-porous formed during the evaporation process, as mentioned above. These pores allow the penetration of electrolyte if the coatings are in amorphous phase. A heat treatment is often required to densify the sol-gel film but making the coating prone to cracks at the same time. [START_REF] Wang | Sol-Gel Coatings on Metals for Corrosion Protection[END_REF] As shown from Fig. I.4 -1d and e, For organic coatings, the barrier effect reduces over time because of the electrolyte penetration in the coating. Once the electrolyte reaches the metal surface, under-paint corrosion takes place that prompts the delamination of the coating. [START_REF] Grundmeier | Corrosion Protection by Organic Coatings: Electrochemical Mechanism and Novel Methods of Investigation[END_REF] The hybrid coatings, as shown in Fig. I.4 -1c, allows it to combine the properties of the inorganic and organic parts. In a general way, the addition of inorganic components can improve the coating's mechanical properties while the organic component brings better flexibility. [START_REF] Ha Lee | Origin from the Presentation of Preparation of Spherical Silica-coated Ceria Nanoparticles by Sol-Gel Method[END_REF]. However, the decomposition of organic compound limits the temperature of heat treatment for hybrid coating. Generally, hybrid materials can be categorized into two classes. [START_REF] Figueira | Organic-Inorganic Hybrid Sol-Gel Coatings for Metal Corrosion Protection: A Review of Recent Progress[END_REF] For class I, in the hybrid materials, the organic group and the inorganic groups are connected with weak force (Van der Waals, Hydrogen force), while non chemical bonding forms between each other. In Class II, on the contrary, the chemical bondings are established. The hybrid coating's properties depend on the organic/inorganic ratio. As shown in Fig. I.4 -3, the left end of the composition axis corresponds to the pure inorganic materials such as glasses or ceramics, while the right end of the axis corresponds to the pure organic materials such as the thermal plastic or epoxy. Different examples of hybrid coatings are noted in the axis of the organic compound percentage in the coating. Indicating the hybrid coatings with more organic participation like Silicone containing epoxy [START_REF] Jin | Synthesis and Application of Epoxy Resins: A Review[END_REF] and Polydimethylsiloxane [START_REF] Eduok | Recent Developments and Applications of Protective Silicone Coatings: A Review of PDMS Functional Materials[END_REF] ; more inorganic participation such as MTMS or PTMS derived coatings; [START_REF] Figueira | Organic-Inorganic Hybrid Sol-Gel Coatings for Metal Corrosion Protection: A Review of Recent Progress[END_REF] and coatings with both participation of inorganic and organic parts like SiO-PVP coating [START_REF] Li | Efficiency Enhancement of Cu(In,Ga)Se2 Solar Cells by Applying SiO2-PEG/PVP Antireflection Coatings[END_REF] as well as Epoxy coating modified with GPTMS. 124 The sol-gel coatings are mostly synthesized with organic silane (> 90%). Unlike other precursors, the organic silanes are the silicon precursors with one or plural organic functional groups (alkyl chains…). [START_REF] Figueira | Organic-Inorganic Hybrid Sol-Gel Coatings for Metal Corrosion Protection: A Review of Recent Progress[END_REF] The organic silane can be represented commonly as:

(RO)x-Si-Ry' (x+y=4)
where the R' represents a functional group, x represents the number of alcohol groups connected to the central Si atom, y represents the number of organic groups. It has x+y equaling to the coordinate number the central metal atom (x + y = 4 in the case of Si). For example, Phemytrimethoxysilane (PTMS) is derived from Tetraethoxysilane (TEOS) with one Benzene cycle. The Octadecyltriethoxysilane (OTDES) has an alkyl chain (C18).

I.5 Sol-Gel coatings for corrosion protection

As discussed in the corrosion part, the Sol-Gel coatings provide mainly the barrier effect to prevent the contact of corrosive species with the underlying metal surface. Several mechanisms can be used to promote the barrier effect:

-Blocking of penetration of electrolyte. [START_REF] Figueroa | Hydrophobic Surface Treatments for Improving the Corrosion Resistance of Anodized AA2024-T3 Alloys[END_REF] -Prolonging the penetration path of corrosive media. [START_REF] Nielsen | Models for the Permeability of Filled Polymer Systems[END_REF] -Inhibition effect with penetrating corrosive media by fillers inside the coating. 59

I.5.1 Physical properties influencing Sol-Gel coatings for corrosion protection

Morphology -Thickness

The barrier layer plays a vital role in blocking electrolytes from getting in touch with the substrate Once the coating is saturated by the electrolyte, the coating can prolong the penetration path of corrosive media and reduce the exposed domain on the metal surface to the corrosive media. For crack-free coating, the thicker the coating is, the better barrier effect the coating behaves.

Other than the Landau-Levich showed in Eq. I -1, the coating thickness can also be influenced by thermal treatment since the coating shrinks during the solvent evaporation. During the heat treatment, the stress is induced by the different thermal expansion ratio between the metal and the coating, limiting thickness acquired by heat treatment.

Sol-Gel inorganic coating or hybrid coating with a high inorganic ratio is difficult to achieve a high thickness due to the inner stress. Wang et al. have reviewed and reported that a crackfree inorganic Sol-Gel coating is limited to 2 µm, for which 200 nm to 1 µm thickness is frequently observed. [START_REF] Wang | Sol-Gel Coatings on Metals for Corrosion Protection[END_REF] In order to increase the coating's thickness, the addition of organic groups is considered as one of the most efficient ways to modify the flexibility of the coating. By introducing organic groups in precursor as GPTMS or PTMS, the coating thickness can reach 10 µm as reviewed by Wang et al. [START_REF] Wang | Sol-Gel Coatings on Metals for Corrosion Protection[END_REF] Other authors had reported the preparation of the 50 µm hybrid coating using a multi-layer process.

Fillers in Sol-Gel coatings are also reported to be able to increase the thickness. [START_REF] Liu | Improving the Formation and Protective Properties of Silane Films by the Combined Use of Electrodeposition and Nanoparticles Incorporation[END_REF] Castro et al. have synthesized Sol-Gel hybrid coating with TEOS and MTES doped with Si-nanoparticle fillers. [START_REF] Castro | Coatings Produced by Electrophoretic Deposition from Nano-Particulate Silica Sol-Gel Suspensions[END_REF] The thickness and the life-time of the coating in 0.5M NaCl solution are increased coherently. Hammer et al. also reported that the carbon nanotube could increase 500 nm thickness of the siloxane-PMMA based hybrid coating. [START_REF] Hammer | Carbon Nanotube-Reinforced Siloxane-PMMA Hybrid Coatings with High Corrosion Resistance[END_REF] In order to increase the thick crack-free coatings, some new methods have been developed. For example, the Layer by Layer deposition (LbL) consists in depositing layers with opposite charge on the substrate in sequence. [START_REF] Decher | Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites[END_REF] The so-prepared coatings have also been applied for corrosion protection applications. 130

Morphology -Porosity

During the evaporation procedure after deposition, the removal of the solvent molecules will leave the pores in the coating. As already been shown in Fig. 2 -4 in earlier part, the size of the pores will be influenced by the colloidal size in the Sol, the assemblage of colloid, and the shape of the clusters. [START_REF] Collignon | Cahier Technologique Sol-Gel -PDF[END_REF] In order to modify the porosity, several common methods can be applied, such as:

-from molecule-level, modifying the microstructure by using a catalyst process (acid, basic, UV…), adding chelating agent, or using multiple precursors or organic silanes 131 , -mixing flexible organic component; -controlling the evaporation velocity, for Xero-gel; [START_REF] Brinker | Evaporation-Induced Self-Assembly: Nanostructures Made Easy[END_REF][START_REF] Grosso | Highly Porous TiO 2 Anatase Optical Thin Films with Cubic Mesostructure Stabilized at 700 °C[END_REF] -the phase transfer by heat treatment [START_REF] Nouri | A Comparative Study of Heat Treatment Temperature Influence on the Thickness of Zirconia Sol-Gel Thin Films by Three Different Techniques: SWE, SEM and AFM[END_REF] or multi-phase stabilization; [START_REF] Ryu | Corrosion Protection Performance of YSZ Coating on AA7075 Aluminum Alloy Prepared by Aerosol Deposition[END_REF] -adding fillers; [START_REF] Zheludkevich | Corrosion Protective Properties of Nanostructured Sol-Gel Hybrid Coatings to AA2024-T3[END_REF] -applying the multi-layer deposition to cover the upper layer pores. [START_REF] Park | Microstructure Effects in Multidipped Tin Oxide Films[END_REF] Specifically, the organic groups can be added to increase the density of Sol-Gel coating. Subasri et al. have proposed that the methyl group in the TEOS and MTES based Sol-Gel hybrid coating has important porosity. [START_REF] Subasri | Effect of Plasma Surface Treatment and Heat Treatment Ambience on Mechanical and Corrosion Protection Properties of Hybrid Sol-Gel Coatings on Aluminum[END_REF] Instead of that, N-alkylamide group containing precursors can control the porosity in the hybrid coatings. [START_REF] Wen | Organic/Inorganic Hybrid Network Materials by the Sol-Gel Approach[END_REF]

Adhesion

The adhesion property depicts the ability of the coating to connect with the substrate. The adhesion can be strong or weak according to the bonding nature, like a covalent bond or Van 40 der Waals force, etc. A good adhesion of the coating on the substrate prevents the diffusion of the corrosive reactant laterally between the coating and substrate. Therefore, the delamination of the coating during service is limited, assuring a longer life-time. As shown in Fig. I.5 -1a, the Sol-Gel coating can form Si-O-Me covalent bonds with the metal surface. Inorganic Sol-Gel coating principally behaves a good adhesion property, while for organic coating the delamination is a main issue for the failure of the deposited system. [START_REF] Zheludkevich | Sol-Gel Coatings for Corrosion Protection of Metals[END_REF] Moreover, the addition of organic compounds in the hybrid coating decrease the adhesion property of Sol-Gel, which explains the frequent delamination phenomenon in organic coatings and hybrid coatings with epoxy-based precursors. [START_REF] Li | Corrosion Performance of Epoxy Coatings Modified by Nanoparticulate SiO2[END_REF] To enhance the coating/ substrate adhesion, surface treatment is usually required to metal surface. As shown in Fig. I.5 -1c, after immersed in phosphate acid, the Mg surface is activated with phosphate acid groups, which opens the opportunity for the silica matrix to establish the covalent bonding between phosphate ions. [START_REF] Hu | Recent Progress in Corrosion Protection of Magnesium Alloys by Organic Coatings[END_REF] Piranha solution, Kroll solution, NaOH, Plasma treatment have been proved to be efficient to active the pure titanium surface. [START_REF] Casagrande | Pretreatment Effect of the Pure Titanium Surface on Hybrid Coating Adhesion Based on Tetraethoxysilane and Methyltriethoxysilane[END_REF][START_REF] Fu | Sol-Gel Derived TiO2 Coating on Plasma Nitrided 316L Stainless Steel[END_REF]

Fig. I.5 -1 a) The scheme of covalent bond by Sol-Gel coating deposited on metal surface b) The role of Sol-Gel coating in a multi-coating corrosion protection system b) The surface modification by Phosphate acid on Mg surface for better adhesion properties

Hydrophobicity

Hydrophobicity depicts the ability of a surface to repulse water, which is the result of surface tension at the liquid-solid interface. Fig. I.5 -2a illustrates a solid/ air/ liquid ternary system with the relative surface tensions, where L refers to the liquid phase, G refers to the gas phase, and S refers to the solid phase, respectively. Thus LG, SL, and SG correspond to the surface tension between Liquid-Gas, Solid-Liquid, and Solid-Gas, respectively.

Statistical Contact Angle (SCA) is the most widely applied method to evaluate the hydrophobicity of a surface. Wenzel et al has proposed that a roughness will influence the contact angle while Cassie and Baxter have proposed Cassie's law, indicating that the contact angle can quantify the wettablitliy of the surface. [START_REF] Cassie | Wettability of Porous Surfaces[END_REF] As observed in Fig. I.5 -2, the  is the socalled contact angle, which has been used to characterize the surface hydrophobicity. Using pure water drop, when  is lower than 90°, the surface is hydrophilic, as in Fig. I.5 -2c. When  is higher than 90°, the surface is hydrophobic, see Hydrophobic surfaces show a lower penetration rate of the electrolyte due to the lower affinity of water. A highly hydrophobic surface is favored to promote adequate corrosion resistance. However, a long-duration exposure to a humid environment may lead to a severe decrease of the hydrophobicity. [START_REF] Vazirinasab | Application of Superhydrophobic Coatings as a Corrosion Barrier: A Review[END_REF] The hydrophobicity for Sol-Gel coatings could be obtained by introducing non-polar organic groups modified silane with methyl groups [START_REF] Rivero | Hydrophobic and Corrosion Behavior of Sol-Gel Hybrid Coatings Based on the Combination of TiO2 NPs and Fluorinated Chains for Aluminum Alloys Protection[END_REF] , fluorine groups, long alkyl chain (with precursor like Octyltriethoxysilane (OTES, R3Si+C8) [START_REF] Jothi | Synergistic Effect of Silane Modified Nanocomposites for Active Corrosion Protection[END_REF] , Decyltriethoxisilane (DTES, R3Si+C10) [START_REF] Dalod | Functionalized TiO 2 Nanoparticles by Single-Step Hydrothermal Synthesis: The Role of the Silane Coupling Agents[END_REF] , Octadecyltriethoxysilane (ODTES, R3Si+C18) [START_REF] Qin | Achieving Excellent Anti-Corrosion and Tribological Performance by Tailoring the Surface Morphology and Chemical Composition of Aluminum Alloys[END_REF] ). Hydrophobicity can also be introduced by adding fillers such as nano-particles [START_REF] Liang | Facile Formation of Superhydrophobic Silica-Based Surface on Aluminum Substrate with Tetraethylorthosilicate and Vinyltriethoxysilane as Co-Precursor and Its Corrosion Resistant Performance in Corrosive NaCl Aqueous Solution[END_REF] , nano clay 150 , 2D nanosheets 151 , nanotubes [START_REF] Oh | Efficient Synthesis of Pt Nanoparticles Supported on Hydrophobic Graphitized Carbon Nanofibers for Electrocatalysts Using Noncovalent Functionalization[END_REF] , etc.

I.5.2 Application of Sol-Gel coatings for corrosion protection

As a new method to replace CCC coatings, Sol-Gel coating has been largely explored in recent decades. For corrosion protection, according to the different requirements for applications and application environments, Sol-Gel coatings should be designed differently.

Inorganic coatings

With the porous nature and low thickness as fact, inorganic Sol-Gel coatings exhibit low corrosion resistance in principle. The xerogel inorganic coatings are treated in different ways: high-temperature heat treatment (over 600 °C, ceramic coatings), medium temperature treated coatings (200°C to 600°C) and low temperature cured coatings.

As reported, high-temperature treatment may cause cracks in the coating. [START_REF] Vallet-Regí | Localized Corrosion of 316L Stainless Steel with SiO2-CaO Films Obtained by Means of Sol-Gel Treatment[END_REF] However, as mentioned before, Nouri et al. have proved that Zr-based ceramic coatings could behave adequate corrosion resistance due to similar thermal expansion rate to the substrate. [START_REF] Nouri | A Comparative Study of Heat Treatment Temperature Influence on the Thickness of Zirconia Sol-Gel Thin Films by Three Different Techniques: SWE, SEM and AFM[END_REF] Coatings treated with medium temperatures are proved to show efficiency in corrosion protection. Atik et al. have reported the influence of different temperatures on corrosion of ZrO2 deposited 316L immersed in NaCl 30g/L solution. [START_REF] Atik | Sol-Gel TiO2-SiO2 Films as Protective Coatings against Corrosion of 316L Stainless Steel in H2SO4 Solutions[END_REF] The ZrO2-based coatings treated over 450°C showed were denser and its EIS curves showed a second time constant at middle frequency relating to porosity of the dense film, other than the charge transfer phenomenon from low frequency. [START_REF] Delimaneto | Electrochemical Studies of the Corrosion of 316l Stainless-Steel Coated with Sol-Gel Zro2 Films[END_REF] To overcome the short comings of the inorganic Sol-Gel coating for corrosion protection, multi-precursor are employed to decrease porosity and increase thickness, with relatively low heat treatment temperature. TiO2-SiO2 for stainless steel in H2SO4 condition [START_REF] Atik | Sol-Gel TiO2-SiO2 Films as Protective Coatings against Corrosion of 316L Stainless Steel in H2SO4 Solutions[END_REF] , SiO2-CaO-P2O for stainless steel in Ca enriched SBF condition, [START_REF] Galliano | Sol-Gel Coatings on 316L Steel for Clinical Applications[END_REF] SiO2-ZrO2 on carbon steel 157 in 3.5% NaCl solution were successfully realized. These different coating systems showed improved corrosion resistance compared with the bare substrate, which is evidenced by a nobler Ecorr and lower corrosion current density.

Hybrid Sol-Gel coatings

By using organic silane or siloxane, scientists succeeded in synthesizing hybrid Sol-Gel coatings, which overcomes the shortcomings of inorganic sol-gel coatings.

The statistic study of the class II coatings developed between 2001 -2013 is shown in Fig. I.5 -3. Hybrid coating has been mainly applied to Aluminium alloys, and steels. The application of hybrid coating on Zn and Zn alloy, Cu and Cu alloy and Mg and Mg alloys are very limited by the surface reactivity of these metals. [START_REF] Figueira | Organic-Inorganic Hybrid Sol-Gel Coatings for Metal Corrosion Protection: A Review of Recent Progress[END_REF] For Iron-based alloys, TEOS and GPTMS have been employed for over 70% of researches. [START_REF] Figueira | Organic-Inorganic Hybrid Sol-Gel Coatings for Metal Corrosion Protection: A Review of Recent Progress[END_REF] TEOS offers a mild synthesis condition and could be used with accompany of other precursor for function groups. Among different hybrid precursors, GPTMS is the most widely applied precursor. GPTMS has a glycidoxy group which can act as a binder enabling the connection between inorganic and organic groups inside the Sol-Gel matrix. Meanwhile, more flexibility and less porosity could be obtained. [START_REF] Figueira | Organic-Inorganic Hybrid Sol-Gel Coatings for Metal Corrosion Protection: A Review of Recent Progress[END_REF] For aluminum and its alloys, GPTMS was the most used precursor, which shows 72% among publications of hybrid coating from 2001 -2013 according to a statistic analysis performed by Figueira et al. 81

Composite Sol-Gel coatings with additives

In order to improve the production of Sol-Gel coatings, the barrier effect, self-healing effect or sacrificial effect, etc., are introduced into Sol-Gel coatings. As mentioned in the first part of the chapter, metal oxide, lanthanide metals, natural product, nano-container incorporated inhibitors, etc., can be introduced as inhibitors into Sol-Gel matrix. Zinc particles are introduced as sacrifice agents that provide a self-healing effect for the coating.

Allongside the inhibiting fillers, Liu et al. charged inert silica nano-particles into decyltrimethoxysilane (DTMS) coatings and showed improvement of anti-corrosion property and enhancement of the thickness of the coating. [START_REF] Liu | Improving the Formation and Protective Properties of Silane Films by the Combined Use of Electrodeposition and Nanoparticles Incorporation[END_REF] Kim et al. have incorporated silica nanoparticles with different sizes into TEOS and MTES hybrid coatings on 304 SS. It seems that the sample with 10nm and 50nm particles together gives a suitable anti-corrosion property, while the smaller particle showed a better effect. [START_REF] Kim | Corrosion Protection Enhancement Effect by Mixed Silica Nanoparticles of Different Sizes Incorporated in a Sol-Gel Silica Film[END_REF] ZrO2 and CeO2 nano-particles were also charged into TEOS and tetraocthylorthosilicate (TEOCS) derived coating on AA2024, among which ZrO2-CeO2 coating system revealed the best protection performance. [START_REF] Gonzalez | A Silanol-Based Nanocomposite Coating for Protection of AA-2024 Aluminium Alloy[END_REF] The barrier effect of Sol-Gel coating can be improved by adding nanofillers that provide prolongation of the penetration path for active reactants. This mechanism is the so-called "zigzag path" theory proposed by Nielson et al. in the first place. [START_REF] Nielsen | Models for the Permeability of Filled Polymer Systems[END_REF] The 2D nanomaterial, like graphene and h-BN nanosheet, exhibit impermeability to molecules as small as helium due to their sp 2 electronic structure. [START_REF] Hu | Proton Transport through One-Atom-Thick Crystals[END_REF] Though not yet applied in hybrid coating, hexagonal boron nitride (h-BN) and Molybdenum disulfide (MoS2) had been applied in polymer coatings and showed improvement of the anti-corrosion property. [START_REF] Husain | Marine Corrosion Protective Coatings of Hexagonal Boron Nitride Thin Films on Stainless Steel[END_REF][START_REF] Zhao | Epoxy Coating Modified by 2D MoS2/SDBS: Fabrication, Anticorrosion Behaviour and Inhibition Mechanism[END_REF]

I.6 2D materials

Graphene and the compounds with similar layered structures, such as hexagonal Boron Nitride nanosheet (BNNS) and Molybdenum disulfide (MoS2) nanosheet, are defined as twodimensional (2D) materials due to their specific atomic-level thickness. [START_REF] Gupta | Recent Development in 2D Materials beyond Graphene[END_REF] Classified with different microstructures, as shown in Fig. I.6 -1, there are many more categories of 2D materials in recent years. [START_REF] Novoselov | Two-Dimensional Gas of Massless Dirac Fermions in Graphene[END_REF] The h-BN is called white graphene due to its similar atomic layered sp 2 structure and insulating property. Transition Metal Dichalcogenides (TMDS), with MoS2 as an example, have the closest structure other than h-BN to graphene. The newly explored complex or organic 2D materials have also been studied by researchers like MXenes (transition metal carbide, nitride or carbon nitride such as Ti2C, Mo2N, Ti3CN) and Covalent Organic Frameworks (COFs). 166

Fig. I.6 -1 The categories of 2D materials 166

2D materials have received abundant attention from applications like micro/nano-electronic devices [START_REF] Radisavljevic | Single-Layer MoS2 Transistors[END_REF] or lubricants [START_REF] Filleter | Friction and Dissipation in Epitaxial Graphene Films[END_REF] by applying their quantum size effect, high surface to volume ratio [START_REF] Xu | Graphene-Like Two-Dimensional Materials[END_REF] and weak Van de Waals force between layers. With h-BN as an example, it is a semiconductor (bandgap 5.2eV-5.9eV) [START_REF] Kovtyukhova | Reversible Intercalation of Hexagonal Boron Nitride with Bronsted Acids[END_REF][START_REF] Cassabois | Hexagonal Boron Nitride Is an Indirect Bandgap Semiconductor[END_REF][START_REF] Wang | Electronic Transport of Encapsulated Graphene and WSe2 Devices Fabricated by Pick-up of Prepatterned BNNS[END_REF] , and presents chemical inertness and high stability under high temperature and pressure. [START_REF] Zhang | Friction and Wear Behavior of Laser Cladding Ni/BNNS Self-Lubricating Composite Coating[END_REF] Its sp 2 structure allows it to be impermeabe to small molecules as small as hydrogen, making it a promising barrier material for steel protection against corrosion phenomena. [START_REF] Hu | Proton Transport through One-Atom-Thick Crystals[END_REF] Graphene also presented impermeability to oxidation and have been tried to protect Cu and Cu/Ni alloys from oxidation. [START_REF] Chen | Oxidation Resistance of Graphene-Coated Cu and Cu/Ni Alloy[END_REF] MoS2, on the other hand, also proved oxidation resisitivity by its heavy reactivity to oxide and stopping the oxide from penetrating its thin layer. [START_REF] Sen | Monolayers of MoS 2 as an Oxidation Protective Nanocoating Material[END_REF] For anticorrosion applications, h-BN has been widely added into polymer matrix to increase coating's barrier effect by torturing the penetration path of the ions and molecules. Cui et al reported over the influence of h-BN content in an epoxy organic coating, in which an optimized h-BN content of anticorrosion property has been proposed. [START_REF] Cui | Anticorrosive performance of waterborne epoxy coatings containing waterdispersible hexagonal boron nitride (BNNS) nanosheets[END_REF] Hussain et al. have discussed the result about the h-BN charged PVA coatings and proposed a comprehensible role of barrier property of BNNS by EIS analysis. [START_REF] Husain | Marine Corrosion Protective Coatings of Hexagonal Boron Nitride Thin Films on Stainless Steel[END_REF] Other researches have been done with BNNS charged in resin [START_REF] Tabkhpaz | Development of nanocomposite coatings with improved mechanical, thermal, and corrosion protection properties[END_REF] , polyimide [START_REF] Huang | Anti-corrosion characteristics of polyimide/h-boron nitride composite films with different polymer configurations[END_REF] and PMMA 181 organic coatings. Comparing to graphene, [START_REF] Chang | Room-Temperature Cured Hydrophobic Epoxy/Graphene Composites as Corrosion Inhibitor for Cold-Rolled Steel[END_REF] graphene oxide is more applied to epoxy coatings for an improvement of the anticorrosion properties, due to its more functional group on the sheet structure allowing it to integrate better in the coating matrix. [START_REF] Ramezanzadeh | Enhancement of Barrier and Corrosion Protection Performance of an Epoxy Coating through Wet Transfer of Amino Functionalized Graphene Oxide[END_REF][START_REF] Pourhashem | Exploring Corrosion Protection Properties of Solvent Based Epoxy-Graphene Oxide Nanocomposite Coatings on Mild Steel[END_REF][START_REF] Yu | Fabrication of Graphene Oxide-Alumina Hybrids to Reinforce the Anti-Corrosion Performance of Composite Epoxy Coatings[END_REF] Bing et al. has charged Graphene oxide in ZrOx/GPTMS coating on AA2024/T3 for improvement of corrosion resistance. [START_REF] Xue | Corrosion Protection of AA2024-T3 by Sol-Gel Film Modified with Graphene Oxide[END_REF] MoS2 has also been charged into composite coating for an improvement of anticorrosion properties. [START_REF] Hu | Electroless Ni-P-(Nano-MoS 2 ) Composite Coatings and Their Corrosion Properties[END_REF][START_REF] Zhao | Epoxy Coating Modified by 2D MoS2/SDBS: Fabrication, Anticorrosion Behaviour and Inhibition Mechanism[END_REF] However, few 2D materials had been integrated into inorganic coatings, from which Graphene oxide has been tried in integration with some ceramic coatings for mechanical properties such as reinforcement, adsorption, etc. [START_REF] Shin | Fabrication and Properties of Reduced Graphene Oxide Reinforced Yttria-Stabilized Zirconia Composite Ceramics[END_REF][START_REF] Li | Dielectric and Electromagnetic Wave Absorption Properties of Reduced Graphene Oxide/Barium Aluminosilicate Glass-Ceramic Composites[END_REF] . Facing the demand of improving the barrier effect of inorganic Sol-Gel coating and hybrid coatings, 2D materials shows a promising potential for achieving this aim. Some attempt have been done for charging GO 191 , h-BN 192 in hybrid coatings.

I.6.1 Preparation methods

Typically, the 2D materials can be synthsized from Chemical Vapor Deposition method (CVD) or CVD-derived molecule deposition techniques. The "bottom-up" method allows to synthesize mono or few layered large crystal of 2D materials with high purity. [START_REF] Kim | Synthesis of Hexagonal Boron Nitride Heterostructures for 2D van Der Waals Electronics[END_REF] Graphene, h-BN, TMDs, Metal oxide all have been synthesized by CVD method. [START_REF] Cai | Chemical Vapor Deposition Growth and Applications of Two-Dimensional Materials and Their Heterostructures[END_REF] Otherwise, it is also possible to obtain them through the exfoliation process from the raw materials (bulk crystal) like mechanical cleavage, liquid exfoliation, ion-intercalation assisted exfoliation, chemical vapor deposition method. [START_REF] Zhang | Ultrathin Two-Dimensional Nanomaterials[END_REF] Other methods have also been developed largely in recent years.

The mechanical cleavage are the original method for the synthesis of 2D materials, as Geim et Novosolov had developed the method to acquire the layered graphene from graphite using scotch on graphite bulk materials. [START_REF] Novoselov | Two-Dimensional Gas of Massless Dirac Fermions in Graphene[END_REF] The ultrasonic-assisted liquid exfoliation allows a larger yield however this method highly depends on the stability between the 2D materials and the solution. Kim et al has studied using pure water to effectuate exfoliation of different 2D materials. [START_REF] Kim | Direct Exfoliation and Dispersion of Two-Dimensional Materials in Pure Water via Temperature Control[END_REF] Though both the yield and stability are low, the ''green'' way showed capability to enable 2D materials to be exfoliated in water efficiently, and especially at 60 °C. Nicolosi et al has developed ultrasonic assisted liquid exfoliation with different solvent, and the organic solvents proved to be very effective for the improvement of content of 2D materials. [START_REF] Coleman | Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials[END_REF] The addition of ionic species allows us to break the Van de Waals force by the ions intercalation between the atomic layers. Moreover, the adsorption of ions stabilize 2D materials in the solvent. 197

I.6.2 Selection of 2D materials

Generally, the protection effect of 2D materials can be achieved in two ways. At first, the deposition of the film of the 2D materials via CVD or PVD procedure result in a water repellency effect. The reduction of substrate surface energy of the substrate is caused either by chemical functionalization or creating micro/nanoscale roughness forming a superhydrophobic surface. [START_REF] Pakdel | Boron Nitride Nanosheet Coatings with Controllable Water Repellency[END_REF] On the other hand, 2D materials have been incorporated into polymer and/or ceramic matrix mitigate passage of oxygen and moisture to the prevented surface. [START_REF] Prasai | Graphene: Corrosion-Inhibiting Coating[END_REF] However, graphene is well known as a superconductor that can potentially lead to galvanic corrosion coupling with the underlying metal. Keeping this issue in mind, h-BN nanosheet (BNNS), which have a similar impermeability, could be a better choice as a protective barrier because they are more thermally and chemically stable than graphene and, 200 more importantly, do not cause galvanic corrosion due to their electrical insulation. 201,[START_REF] Huang | Anti-Corrosion Characteristics of Polyimide/h-Boron Nitride Composite Films with Different Polymer Configurations[END_REF][START_REF] Yi | Boron Nitride Nanosheets as Oxygen-Atom Corrosion Protective Coatings[END_REF] Otherwise, the graphene oxide derived by electrochemically expanded graphite as well as MoS2 have also been interested in the anticorrosion applications.

I.7 Conlusion

In this chapter, the necessity of protection for metals against corrosion has been reviewed. Among various anticorrosion methods, Sol-Gel coating has been proved to be eco-friendly and has mild synthesis conditions for various inorganic or OIH coating. However, the mesoporous or microporous structure nature of Sol-Gel coating needs to be improved to achieve a high barrier effect.

The modified Sol-Gel matrix with organic silane could increase the flexibility as well as the thickness of the coating but in detriment to the adhesion property. The inert fillers act as a barrier that seals the pores and prolongs the penetration pathway, as reported by Nielson et al. [START_REF] Nielsen | Models for the Permeability of Filled Polymer Systems[END_REF] Both methods have been proved to be efficient in different application conditions.

Among different nanofillers, the 2D nanosheets show promising properties due to its impermeable structure to small molecules. The microstructure, physical and chemical properties of different 2D materials have been discussed. Notably, the hexagonal Boron Nitride nanosheet has been selected as the fillers due to its low electrical conductivity and excellent impermeability to small molecules and ions.

II.1 Introduction

In this chapter, the raw materials, the preparation of materials, and the experimental procedure are presented.

As discussed in Chapter I, the metals suffer different corrosion phenomena according to their reactivity and surface conditions. Notably, in a highly-concentrated NaCl solution, the stainless steel (316L) exhibits pitting corrosion, while the aluminum alloy (AA2017) and carbon steel (XC10) suffer general corrosion without protective coatings. The Sol-Gel process for three different coating system are detailed, which are respectively:

1) Zirconium oxide single-component coating on 316L

2) Zirconium oxide-Silicon oxide multi-component coating on Aluminium alloy (AA2017) 3) Organic-inorganic hybrid coating on carbon steel (XC90)

The exfoliation methods are also presented. The h-BN, electrochemically expanded graphene, and MoS2 powders were exfoliated in pure water, n-butanol, or isopropanol. The exfoliation and centrifuge parameters are given, and the names of the associated samples are listed. Finally, the electrochemical characterization techniques employed in this works are reviewed, including the applied three-electrodes electrochemical cell, open circuit potential test, potentiodynamic polarization, Electrochemical Impedance Spectroscopy (EIS) test.

II.2 Metal Substrates

In order to study the effect of 2D materials in Sol-Gel coating for barrier property enhancement, a substrate with an adequate surface reactivity is required. The selection of the substrate is based on the stability of the coating/ substrate interface.

II.2.1 Stainless steel

Stainless steel, which contains over a moderate amount of chromium as an alloying element, stands very well the general corrosion. 1 It has a higher tensile strength and better elongation property but lower yield strength than carbon steel. 2 Austenite stainless steels are well explored in industry domains, especially for the steels revealed from American Iron and Steel Institute (AISI): the AISI 316, AISI 304, and AISI 2205. The thin and dense oxide film on the metal surface is called passive film that can stop direct contact of the environment with the underlying metal. This property allows stainless steel to serve for marine field, nuclear plants, high-temperature plants, etc. However, stainless steel is prone to pitting corrosion, which is usually unpredictable and very dangerous for the structures due to the unexpected failure. Chloride ions (Cl -) promote the pitting corrosion rate. 3 The stainless steel 316L, which is an Austenite alloy, allows the investigation of the evolution of the Sol-Gel matrix due to its inertness to corrosive environment, which reduces the complexity of the observed system.

Stainless steel 316L is purchased from Goodfellow (the weight percentage: C 0.03%, Cr 18.00%, Ni 10.00%, Mo 3.00%, Mg 2.00%, Si 0.75%, P 0.045%, S 0.03%, N 0.10%). The substrate was cut into 50 x 25 x 3 mm 2 samples and grounded up to 1200 mesh using SiC paper from PRESI.

Before dip coating, samples were rinsed with distilled water, ethanol (96%, Ethanol for analysis, ChimiePlus), and acetone (Acetone for analysis, Chimieplus).

In addition, 316L were immersed in the "Piranha" solution with a ratio of H2O2 (35%, Chimieplus) to H2SO4 (96%, Chimie-plus) equals to 1: 2 for 15min. This treatment cleaned the surface up and favored the formation of -OH polar groups, which is necessary for the Sol wettability on the passivation film.

II.2.2 Aluminum alloy

Aluminum and its alloys are interesting for their lightweight in various transportation applications. Pure aluminum exhibits good corrosion resistance due to the existence of a homogeneous Al2O3 oxide layer. However, the aluminum alloys inclines to localized corrosion due to the presence of intermetallic compounds. 4 The distinct phase inside the aluminum alloys and the alloying element Mg, Cu, etc., form galvanic couples, which weakens the resistance of the alloy. 5 Unlike 316L, the passive film of Aluminum alloys is less inert from a chemical and electrochemical point of view. In the same electrochemical system, the evolution of the coated sample is much faster than on 316L, so that the endurance of the coating system can be evaluated.

Aluminum alloy 2017-T4 is purchased from Goodfellow (the weight percentage: Cr 0.05%, Cu 4%, Fe 0.4%, Mg 0.5%, Mn 0.5%, Si 0.4%, Zn 0.2%, compensate by aluminum). The substrate was also cut into 50 x 25 x 3 mm 2 samples and grounded up to 1200 mesh. Samples were rinsed with distilled water, ethanol, and acetone, and no surface pretreatment was performed since Aluminum Alloy behaves good surface reactivity to Sol-Gel coatings.

II.2.3 Carbon steel

Carbon steel is the most frequently applied metallic material in the world. 6 The modifiable property and the low fabrication cost allow the carbon steel to be applied in construction, wire, marine field, and many other industrial domains. 7 However, they are prone to general corrosion even in atmosphere conditions, which increases the difficulty of maintenance and limits the lifetime of the products. 8 To ensure a satisfying protection performance on carbon steel, the coatings are required to be dense or thick, when it is not dense enough, which presents good adhesion as well as strong barrier properties to limit the connection between the environment and the substrate. Moreover, the synthesis condition must be mild, for which acidic conditions or strong basic conditions should be avoided.

Carbon steel XC90 is purchased from Goodfellow (C 0.80 -0.93 %, Mn 0.30 -0.65%, Si 0.15 -0.35, P 0.03 -0.035 %, S 0.035 -0.05 %, compensate by iron).The substrate was similarly cut into 50 x 25 x 3 mm 2 samples and grounded up to 1200 mesh, rinsed with ethanol and acetone before dipping. No pretreatment to the surface was performed since carbon steels are sensitive to the environment.

II.3 Preparation of Sol-Gel procedure

In order to isolate the role of BNNS, it requires the synthesis procedure to be designed attentively. In the following part, the selection of Sol-Gel materials as well as the synthesis procedure will be presented in three different categories: the single-component coating, the multi-component coating, and the organic-inorganic hybrid coating.

II.3.1 ZrO single-component coating on 316L

The first system is dedicated to observing the evolution of Sol-Gel coating on a metal substrate.

The effect of the 2D nanofillers are expected to be primitively observed. Thus, a stable coating/ substrate interface is necessary to avoid the delamination. Within this context, the stainless steel 316L is selected as the substrate due to the presence of the stable passive film. The Zirconium Oxide has a similar thermal expansion rate to 316L, allowing the prospective study for future applications in the domain like medical implants, etc. 9

The inorganic coating was prepared using Zirconium acetate hydroxide (Aldrich Chemistry) (ZrO) as a precursor. The Sol-Gel procedure for ZrO Sol is illustrated in Fig.

II.3 -1.
The coatings prepared with this composition will be denoted ZrO in this work. We added 3mg zirconium acetate hydroxide in 50mL ethanol and 2.5 mL acetone. Using acetic acid, the pH of the solution was maintained at 3.2 to obtain low Zetapotential and a stable solution. 10 The ZrO Sol was mixed with pure water with the volume ratio of ZrO : water = 3 : 1 before the dip-coating process. For nanofiller charged samples, the 2D materials were directly exfoliated in water.

Fig. II.3 -1 Scheme of preparation procedure for ZrO Sol

II.3.2 ZrO-SiO multi-component coating on Aluminum alloy

To accelerate the electrochemical reaction, we attempted to transfer the coating to a more active metal AA2017, which is more sensitive in NaCl solution than 316L. A multi-component Sol-Gel coating based on ZrO-SiO was used instead of the ZrO single-component coating system to reduce the coating porosity.

Zirconium n-butoxide (ZrBTO, 80% w/w in 1-Butanol, Sigma-Aldrich) was selected as the Zirconium resource. At first, ZrBTO was dissolved into pure isopropanol (2-propanol for The molar ratio between n(Zr) : n(Si) was maintained at 1 : 2, while the molar ratio between GPTMS and ODTES (Octadecyltriethoxysilane, 98%, n-isomer 85% min, Alfa Aesar) was maintained as 1 : 3. The ODTES was added drop by drop to the mixture. The formation of the Micelles was observed at room temperature after the ODTES addition. Heated to 60°C, the Sol became transparent. Therefore, the dip-coating process was also performed at 60°C. The pH of the Sol was maintained with ammonium hydroxide to pH = 9. 1 Temp: temperature 2 Drying: the duration between each dipping process. 3 Curing: the heat treatment for the evolution of the Sol-Gel matrix in oven.

II.5. The exfoliation of 2D materials

In order to adapt to the Sol-Gel synthesis condition, the classic liquid-phase exfoliation method was performed in this work. The solvents are selected according to the Sol-Gel procedure. Fig. II.5 -1 shows the ultrasonic machine (FB11201, capacity 2.75L, Voltage 115V) with a homemade cooling system.

The raw materials were put in the solvent and directly exfoliated in the ultrasonic bath and then centrifuged with corresponding parameters. Particularly, for graphene, the graphite was preliminerly expended using electrochemical way. The expanded graphite was treated in the ultrasonic bath to obtained the few-layered nanosheet (GENS). The electrochemical method for expanded graphite is introduced in the appendix.

The exfoliation parameter as well as the appellation are listed in Table . 2 -2.

63

II.6 Electrochemical characterization techniques

The electrochemical techniques allow us to quantitatively explore the relationship between electrode and its behavior under open-circuit conditions or applied potential (like a linearly increased or sinusoidal potential).

The electrochemical cell used for this work is a classical three-electrode system as illustrated in

II.6.1 Open circuit potential

The Open Circuit Potential (OCP) is the potential when a metal is immersed in the electrolyte in a natural state. OCP is often used to find the corrosion potential of a system.

II.6.2 Electrochemical Dynamic Polarization

When the immersed metal is submitted to linear potential, reactive steps can soon be observed. At corrosion potential, the anode and the cathode current densities are equal. This current density is called icorr , which is the current observed from both the anode side and the cathode side, which can be calculated by the Bulter-Volmer kinetic law in Eq. 2 -1:

12 𝒊 𝒕𝒐𝒕𝒂𝒍 = 𝒊 𝒄𝒐𝒓𝒓 [𝒆𝒙𝒑 ( 𝜶𝒏𝑭 𝑹𝑻 𝜼) -𝒆𝒙𝒑 ( -(𝟏 -𝜶)𝒏𝑭 𝑹𝑻 𝜼)] Eq. 2 -1
In which the itotal is the total current density,  is the transfer coefficient, η is the overpotential between the applied potential and corrosion potential (OCP in most of the case), F is the faraday constant, R is the gas constant and T is the temperature.

Parameters of Polarization

The dynamic polarization was acquired by linear scanning voltammetry (LSV) from Gamry Ref600 potentiostat after 24 hours of immersion in NaCl 30g/mL solution. The scanning potential range was fixed from -50 mV vs. OCP until 1000 mV vs. SCE, scanning rate 0.167mV/min.

II.6.3 Electrochemical Impedance Spectroscopy

The electrochemical impedance is the impeding phenomenon when an AC current is applied.

An electrochemical impedance can be described by a complex number depending on the phase of the sinusoidal signals. The electrochemical impedance is given by: 13

𝒁(𝝎) = 𝑽(𝝎) 𝒊(𝝎) Eq. 2 -2
As a complex number, it could be decomposed into a real part and an imaginary part as shown in:

𝒁(𝝎) = 𝑹𝒆(𝝎) + 𝒋𝑰𝒎(𝝎) Eq. 2 -3
where Re is the magnitude of the real part of impedance, the Im is the magnitude of the imaginary part of Impedance and j is the imaginary unit. If we plot the Re value and Im value as x-axis and y-axis respectively, the Nyquist curve is got as shown in

Equivalent circuit and numerical fittings

EIS gives the global impedance response of the electrochemical system. If these responses correspond to the validated physical and chemical phenomenon, the EIS can be fitted by an equivalent electric circuit (EEC) proposed by Randles using numerical tools. 12 Randles assumed that the electrochemical system working electrode is composed of resistors and capacitors. Later reports have proposed Constant Phase Element (CPE) to describe the imperfect capacitors. When coatings are deposited on the metals, supplementary R/C or R/Q couples should be considered in the EEC. These couples are introduced due to the impedance introduced by the coatings or other surface modifications. 15 The impedance of the CPE is given by the formula: 16 𝑍 𝐶𝑃𝐸 = 1 𝑄(𝑗𝜔) 𝛼

Eq. 2 -4

The Q value is attributed to the surface current density distribution. The Q value can be evaluated by: 17

𝑄 = | sin( 𝜋 2 • 𝛼) • (-1) 𝑍 • (2𝜋𝑓(𝑡)) 𝛼 | Eq. 2 -5

Parameters used for EIS tests

The EIS tests were carried out in this work using Gamry Ref600 potentiostat. The frequency varied from 100 kHz to 3 mHz, while the amplitude of the scanning potential was fixed at 15 mV vs. OCP.

III.1 Introduction

As discussed in the above chapter, this work has been separated into three Sol-Gel coated metal systems. Thus, the characterization in this chapter is based on the three coating systems.

For each coated system, the Sol-Gel solution was evaluated with Dynamic Light Scattering Spectroscopy (DLS) to understand the colloidal size as well as the stability within three days. Then, the h-BN exfoliated solution was characterized by TEM to observe the morphology of the nanosheets. The BNNS concentrations for three different raw h-BN were estimated by UV-Visible spectroscopy. Finally, the surface morphology and water contact angle of as-deposited coatings were discussed. Raman spectroscopy and IR spectra were employed to characterize the existence of 2D nanofillers. reported that with controlled exfoliation and centrifuge procedure, the dimension and shape of 2D nanosheets could be tuned. 1 In this work, since the applied exfoliation method was mild in an ultrasonic bath with a one-time centrifuge, the morphology of final exfoliated nanosheets mainly depends on its raw powder morphology. Furthermore, the lateral size of the nanofiller influences the anticorrosion behaviors of sol-gel coatings. . This process is accompanied by a moderate carbon oxidation at the edge and in the atomic plan. The oxygen content of the GE is measured by EDS to be 9 at%. Therefore instead of pristine graphene, we have prepared the oxide graphene for the reason that the electrical conductivity is significantly reduced. 4 The so-prepared GE can be easily dispersed in aqueous media with a relatively high concentration than pristine graphene that is insoluble.

III.2 Characterization of raw materials

III.2.1 Characterization of 2D powders

Characterization of ZrO single-component Sol-Gel solution

The BNNS was firstly studied and charged into ZrO coatings to probe its behavior in an inert system. The influence on the barrier effect of the Sol-Gel coatings of morphology as well as the concentration of BNNS were studied and discussed in sequence.

The photo of ZrO Sol is shown in The band at around 208 -210 nm is the characteristic peak h-BN for its E2g mode. 6 As mentioned in Chapter II, the extinction coefficient has been calculated by weighting and UVmeasurement at 300 nm in order to decrease the influence of diffraction, giving 2924 mg/ml/m. This calculated value is coherent with the value reported by other authors. 7 By applying the Beer-Lambert law, the concentrations of different BNNS were estimated to be 15 g/mL that is in good agreement with the reported value. 8 Table . 3 -1 lists the concentrations for BN-A, BN-B, and BN-C without a remarkable difference in concentrations.

However, the Zeta Potential of the colloidal solution of BNNS is different (see Table . 3 -1). Among them, BN-A shows the lowest value of -30.1 mV, meaning the best stability. Between BN-B and BN-C, the BNNS solution with the bigger size showed a higher stability than the smaller ones. ZrO_Ref shows a static water contact angle of 67°, which means a hydrophile surface due to the content of the polar organic groups in the xerogel coating. It is surprising to observe a significant difference in contact angles among the ZrO_BNA, ZrO_BNB, and ZrO_BNC samples. This result is reproducible, where the difference is most likely due to the different BNNS fillers.

Fig. III.3 -6 SEM Image and water contact angle of a) and c) ZrO_Ref, b) and d) ZrO_BNA, e) and g) ZrO_BNB and f) and h) ZrO_BNC

The FTIR/ATR is used to verify the composition of the as-deposited BNNS charged ZrO coatings.

From the FTIR/ATR curve shown in Fig. III.3 -7, all three coatings with BNNS filler showed a peak at 1350 cm -1 , which is related to shifted B-N stretching. This is not observed on the ZAH_Ref. 9 This phenomenon proves the existence of BNNS inside the charged coatings. We can observe from all the spectra that signals at wavenumber of 1570 cm -1 (the spherical mark)

and 1440 cm -1 (the triangle mark) are attributed to the acetate (-CO2 groups) anti-symmetrical and symmetrical stretching vibration. 10,11 All four solutions showed similar stability according to the similar zeta potential and can stay stable for only 3 to 4 days before the agglomeration. The re-exfoliation ultrasonic treatment before the synthesis of the coating could help the agglomerated nanosheets to be redispersed into the solution. All coatings show defect-free and homogeneous surfaces, while ZrO_BN170 and ZrO_BN130 have contact angles higher than 90°, indicating a surface hydrophobicity. This hydrophobicity is absent in ZrO_BN90 and ZrO_BN60, while these two coatings showed a slightly higher contact angle than the ZAH_Ref. It is thus understood that the high concentration of BNNS charged ZrO coating shows an increased contact angle until the coating becomes hydrophobic. The coating thickness of ZrO-SiO multi-component coating is less than 3 m as measured from cracked regions. Its thicker than ZrO single-component coating due to the introduction of GPTMS groups, which increases the flexibility of the coating and allows larger thickness without cracks.

Fig. III.3 -7 The FTIR/ATR spectroscopy of ZrO deposited coatings with or without three BNNSs fillers

Characterization of as-exfoliated solution of BN-C with different concentration

Characterization of ZrO coatings charged with BN-C with different concentrations

Characterization of as-exfoliated BNNS isopropanol solution

The UV-Vis curve of BN-C in isopropanol is shown in

III.3 -13 UV-Visible spectroscopy of BN-C exfoliated in isopropanol with inset table showing the parameters of the solution

Characterization of ZrO-SiO multi-component coatings

The The DLS curve shows the colloidal size distribution of ZrO-SiO/C18 hybrid Sol-Gel solution after synthesis. The ZrO-SiO/C18 hybrid Sol-Gel solution is composed of coloidals of an average size of around 400 nm as principle colloidals and around 3 nm as minority ones. This means that the basic condition as well as the charge of ODTES led to a larger colloidal size in the Sol-Gel solution comparing to the ZrO-SiO Sol.

Sol-gel coating Substrate

Butanol is low. The concentration of BNNS in n-Butanol is between the value of BNNS in isopropanol and BNNS in water solution.

III.3.4 Characterization of Expanded Graphene nanosheets synthesized coatings

The expanded graphene was exfoliated into water and n-Butanol pure solutions with the same exfoliation and centrifuge parameter the BNC_Sol170. To probe the role of electrochemically expanded graphite derived nanosheets (GENS), the characterizations of both coating systems are performed.

Characterization of GENS charged ZAH coating

The GENS is firstly introduced into ZrO coating and deposited on 316L to probe its behavior in an inert system.

The TEM image of GENS and its diffusion pattern are shown in for GENS exfoliated from GE in water are coherent to the result reported elsewhere. 3 The zeta potential (inset table) is around -34.7 mV. However, the concentration of GENS is only 90 g/mL, which is relatively low comparing to other 2D fillers.

Characterization of GENS charged ZrO-SiO/C18 coating

The GENS is introduced into ZrO-SiO/C18 and deposited on carbon steel to probe its behavior in an active system. To sum up, MoS2 showed a high capability of being exfoliated in pure solvent, indicating an active surface energy of the nanosheets. )and A1g mode (around 408 cm -1 ) shown in ZrO-SiO_MoS2 curve. 17 The LA represents for the longitudinal acoustic mode and the LA influenced raman shifts (A1g-LA at around 179 cm -1 , 2LA at around 466 cm -1 and A1g+LA at around 645 cm -1 ) also fits the characteristic of MoS2 as reported. 18

III.4 Conlusion

The three different Sol-Gel systems were successfully deposited on the corresponding substrates. The 2D materials filled coatings show defect-free and homogeneous surface. The physico-chemical and microstructure characterizations from the raw materials to the colloidal solution until the final coatings are detailed.

For Zirconium oxide single-component coating on 316L, the Sol-Gel matrix prepared shows similar morphology. Different surface hydrophobicity can be observed with the different h-BN derived BNNS and the concentration of BNNS. In order to reduce the effect of the surface tension, the electrochemical tests require a 24h immersion until the stable state.

On Aluminum alloy (AA2017), the single-component coating was firstly transferred with success. Furthermore, a ZrO-SiO multicomponent coating was synthesized to enhance the density of the Sol-Gel matrix as well as the nanocomposite coating's performance against corrosion.

For the further reinforcement of the coating barrier effect, the alkyl group (-C18) was doped in the Sol, which induces a hydrophobic surface and water repellency is the pores. This coating was deposited on carbon steel (XC90) Raman Shift (cm -1 )

IV.1 Introduction

The For ZrO coating on 316L, the bare substrate and ZrO coated samples were firstly characterized by OCP and Polarizations in 24 hours of immersion and by EIS during 120 hours of immersion.

The influence of the BNNS nanofiller is evaluated by the comparison with the bare substrate and unfilled coating. Electrochemical Equivalent Circuit (EEC) and numerical fittings are employed to establish the correlation between the electrochemical behavior and the mechanism of reactions. To understand the effect of BNNS morphology, the ZrO coating charged with BNNS exfoliated from BN-A, BN-B, and BN-C were studied. In addition, the influence of BNNS concentration on the ZrO coating's barrier effect was investigated as well.

For this purpose, the BN-C derived BNNS was selected and added into the ZrO matrix with different concentrations.

Hereafter, we attempted to transfer the BNNS filled ZrO coating on aluminum alloy. However, the ZrO single component coating showed a similar behavior to the AA2017 bare substrate, indicating an insufficient barrier effect on AA2017 substrate. The study of ZrO singlecomponent coating will thus be limited on 316L substrate for the observation of the role of 2D materials in inorganic coating and the evolution of the coating.

To study the role of 2D materials in corrosion reduction, the ZrO-SiO multi-component Sol-Gel coating was used, which allows the combination of the mechanical properties of both components. The polarization and EIS curves after one week of immersion were compared to evaluate the effect of BNNS on corrosion protection. Since corrosion and delamination were observed, an enhancement of coating matrix is required to search for a coating/BNNS couple with strong and efficient barrier effect.

Finally, the alkyl group (ODTES, -C18) was doped into the multi-component matrix on carbon steel XC90 to modify its hydrophobicity. The polarization and EIS were performed to characterize the electrochemical behavior of bare and coated carbon steel, with or without BNNS fillers.

The other 2D materials, GENS and MDNS were charged in ZrO/316L to observed their function in inorganic coating, evolution of coatings and to compare with BNNS. Then, they were charged in ZrO-SiO/C18/carbon steel system to understand their influence on the efficiency of corrosion protection.

IV.2 Study of BNNS in ZrO single-component coating on 316L

The discussion starts with the study of the ZrO single-component coating in NaCl 30g/L solution by controlling parameters of BNNS fillers, size and concentration. The evolution of barrier effect of BNNS charged coating with varying parameters have been explored..

IV.2.1 Electrochemical behavior of 316L bare substrate

The electrochemical behavior of 316L has been measured to establish a reference in terms of the electrochemical behavior of the coated system. The fitting parameters of 316L bare substrate after 16 hours and 120 hours of immersion are shown in Table 4 -1. It is observed that the resistance Rfilm decreases to half of the value, meaning that the barrier property of the film decreases over time which could be explain by more and more chloride in the film. The Rct also decrease, indicating lower corrosion resistance over time. Thus, all three evolutions reveals that the passivation film exhibit a degradation in NaCl solution and justify to enhance the resistance of the system. 7

IV.2.2 Effect of ZrO single-component coating on 316L

The electrochemical behavior of zirconium acetate hydroxide derived coating without fillers (ZrO_Ref) has been compared with bare 316L and further nanocomposite coating. It constitute the reference of the electrochemical behavior of coated systems before improvement.

The electrochemical characterization follows the same protocol. First of all, the OCP is characterized and pursued with polarisation curve and impedance spectroscopy.

The OCP evolution of ZrO_Ref in NaCl 30g/L for 120 hours is shown in Fig. The polarization for ZrO_Ref has been performed in the same conditions as for the substrate.

After 24 hours of immersion to obtain a stable electrochemical system, the polarization curves of ZrO_Ref and bare 316L substrate for comparison were plotted in From Table 4 -2, both resistance Rpore and Rct are just slightly decreased indicating a slow reaction rate. The generation of passive film may cover the pitting with this low reaction rate. 9 The dl close to 0.75 indicating a diffusion phenomena over the metal surface.

Fig. IV.2 -10 EEC for ZrO_Ref long term duration of EIS test Table 4 -2 Fitting parameters of impedance curves of ZrO_Ref at 16 hours and 120 hours

IV.2.3 Influence of BNNS charged in ZrO/316L

It is understood that the ZrO_Ref coating is protective by reducing the exposure of the metal surface. Nevertheless, the porous microstructure of the ZrO Sol-Gel matrix is the main drawback of the barrier effect offered by the coating.

To overcome this issue, a heat treatment is necessary to obtain a dense ceramic structure, as mentioned in Chapter I. However, the high-temperature heat treatment is not achievable for all systems since it can cause unexpected modifications in the substrates (like the inner stress, the change of crystallinity, and the carburizing, etc.). By introducing fillers in xerogel film, on the contrary, it is possible to bring inhibition function or change the distortion/ tortuosity of the pores in the coating to seal porosity or to delay water up-take.

Among different nanofillers, the 2D nanosheets are promising choices. The excellent impermeability in nano-metric thickness can efficiently prolong the penetration path in the coating. 10,11,12 As mentioned in previous chapters, the influence of 2D nanosheets on the Moreover, the current density of ZrO_BN increases slightly from a low potential to 1.1V. The current density confirms a very low reactivity of the BN coated system which is maintained under the anodic potential range.

At the early stage of the anodic part the current density reaches by ZrO_BN is, however higher than ZrO_Ref coating. This phenomenon can be explained by that the weaker barrier effect of ZrO_Ref coating will cause a higher oxidation rate on the metal surface. Thus, the accumulation of the oxidation product within pores of the coating lowers the current density. However, once the potential reaches the value where the metastable pitting corrosion happens on the substrate, around 0.5 V vs SCE, the current density surpasses the current density of ZrO_BN. The less sensitive behavior of ZrO_BN is related to its improved barrier property. 4 -3. With the addition of BNNS in the sol-gel coating, the Rpore of the coating increases to 2.08 x 10 6 Ohm.cm 2 , which is twice of the value of ZrO_Ref (9.76 x 10 5 ohm.cm 2 .). The Rct increase to 3.38 x 10 6 Ohm.cm 2 that is 20 times higher than the pure sol-gel coating. The high Rfilm is generally attributed to the barrier effect of the non conductive matrix and the tortured connection pathway for the ions exchange induced by impermeable BNNS. Meanwhile, the high Rct indicates low reaction rates at the metal surface. The value dl indicates diffusion phenomena in the pores.

IV.2.4 Influence of morphology of BNNS on ZrO/316L

In the last part, it was understood that the BNNS improves the barrier effect of ZrO coating. However, the enhancement needs to be optimized. As an inert nanofiller, the physical properties of BNNS, especially the size of the atomic layers, may play an essential role in the improvement. 13 Thus, the three raws BN powders with different average diameters developed in chapter II were selected to perform this optimization study.

The previous ZrO_BN is of an average diameter of 4.5 µm. The other two BN powders have a larger size of 7 µm and a smaller size of 1 µm. The nanocomposite coating samples are named according to the size of the raw BN powders. For further discussion, ZrO_BNA denote to the one that has been discussed before with the intermediate size (4.5 m) ZrO_BNB is the one with smallest BN (1 m), ZrO_BNC is the one charged with the biggest BN (7 m). The coating limits the reactivity of the substrate significantly. Nevertheless, some differences in the behavior can be discussed.

In the case of ZrO_BNB, the nanosheet is of a smaller size than BNA, while its polarization curve is rather similar to ZrO_Ref. It seems that the smaller sized BNNS cannot influence the barrier properties. The polarization curve of ZrO_BNC shows a higher Ecorr than ZrO_BNA and a lower current density, which is one order of magnitude lower at the beginning part of the anodic curve. The lower current densities are obtained from the biggest BNNS size that seems to promote the corrosion resistance of the coated system.

IV.2 -18 SEM image of the surface morphology obtained with a) ZrO_BNB and b) zoom on the corrosion product of ZrO_BNB , c) ZrO_BNC and d) zoom on corrosion product of ZrO_BNC

Based on the behavior of these nanocomposite coatings, we can define the critical nanofiller size dc as the minimum size to improve coating's barrier property. The influence of BNNS according to its different sizes can be described as: The arrows indicating the penetrating corrosive reactant are mostly representing chloride ions and oxygen molecules. It is important to note that the critical dimension dc is not an absolute value but depends closely on the mesopores of the sol-gel matrix. Meanwhile, the doping content is another vital parameter that can influence the dc value for a coating system. It should be noted that the effect of the nanofiller is also limited by its dispersion in the Sol. For the BNNS with a much bigger size than dc, an agglomeration may take place and limits the homogeneity of the nanocomposite. Therefore, an optimal size exists with which the coating would perform the best corrosion protection behavior.

The Nyquist plots of EIS obtained at OCP of BNNS charged coating after 24 hours of immersion in 30 g/ml NaCl solution are compared in Hereafter, ZrO_BNC coating was chosen for long-duration EIS analysis to study the nanocomposite coating aging in NaCl solution. angle of -85°, which is higher than the one of 316L at 120h. Meanwhile, the low frequency time constant shifts to a lower frequency due to an increased corrosion resistance.

To better understand the aging of ZrO_BNC, the numerical fitting of EEC was performed and discussed with the EIS measurement extended to fifty days (1200h).

Fig. IV.2 -21 a) Nyquist plots and b) Bode plots of ZrO_BNC long-duration tests and 316L at 120 hours as reference (Black arrow noting the evolution direction over time of the data of ZrO_BNC)

The EIS results are fitted by the numerical tool using the same EEC with two time constants as for the ZrO_Ref shown in Meanwhile, for pure ZrO_Ref coating, the Rct value is at first higher than that of the bare substrate and then converges to the curve of 316L after 72h of immersion. This similarity can be explained by that the pure sol-gel coating cannot cause a sufficient influence on metal surface reactivity. Since the xerogel coating is mesopores, the susceptible spots (inclusion, defects…) in the passive film can not be covered and are exposed to the electrolyte. On the other hand, the Rfilm or Rpore value mainly characterizes the barrier effect of the coating. The slight decrease of the Rpore in ZrO_Ref is due to the deterioration of the coating matrix or the adsorption of the ions through the coating. 16 The Sol-Gel coating shows, in general, good stability and inertness.

However, the evolution of ZrO_BNC is quite different compared with both the bare substrate and the ZrO_Ref. Firstly, the value of charge transfer resistance Rct at the end of 120h is six times higher than the substrate and the ZrO_Ref. The higher Rct value involves a lower metal surface reactivity since less metal surface under deposit is exposed to the electrolyte. In the second place, the Rpore of ZrO_BNC is higher than ZrO_Ref. This enhancement of the barrier property is classically explained by the "tortuous pathway" mechanism that involves a prolonged diffusion period within the coating. 17 Upon 24h, the resistance Rpore increases, which should be attributed to the sealing effect of the oxidation product in the already established penetration pathway. the CPE exponent dl of the double layer increased from 0.51 to 0.85, which reveals a transition from a diffusion to a capacitive behavior at the bottom of the pores. This transition is also associated with the accumulation of the oxidation product that seals the coating pores.

Specifically, after 400h of immersion, the dl value increased from 0.51 to 0.7, indicating that the electrochemical system became a kinetic limiting process.

Besides, it can be observed that the resistance in the coating pores Rpore kept increasing until 1200 hours of immersion, reaching 4.73 x10 6 Ohm.cm 2 that is four times higher than the initial resistance and six times higher than the stabilized Rpore of ZrO_Ref. The increase of the Rpore is with a similar reason of the increasing trend within 120 hours. The Rct slightly increased, which is almost 10 times higher than the Rct of 316L substrate and ZrO_Ref, indicating as well an increased corrosion resistance over time. To sum up, the aging behavior of BNNS filler charged ZrO coating with optimized BNNS size will seal the established penetration path over time by the synergy effect of the BNNS barrier and the oxide product. The coated system behaved a robust barrier effect with a capacitive double layer and an increasing resistance (Rpore + Rct) that is stabilized after 1200h of immersion in 3,5w% NaCl solution. This overall resistance of ZrO_BNC at 1200 hours of immersion is 6.5 x10 6 Ohm.cm 2 that is 20 times higher than the stabilized value 316L at 120 hours of immersion.

IV.2.5 Influence of concentration of BNNS on ZrO/316L

Except for BNNS's morphology, its concentration also plays a vital role in the performance of nanocomposite coating. 19 The concentration of 15 g/mL has been reported to be a stable concentration for BNNS in aqueous solution at room temperature. 20 When the BNNS concentration becomes higher than this value, the aggregation may take place and cause unexpected defect or crack in the inorganic matrix. That is why most researchers have used aqueous solutions with various surfactants 21 , appropriate ions 22 , or chemically modified BNNS 23,24 to improve the homogeneity and limits these risks. Thus, it is interesting to explore the influence of high concentration BNNS (> 15 µg/mL) in zirconium-based sol-gel coating on its barrier effect.

To this end, the centrifuge time of BN-C in aqueous media is reduced to 40min, 30min, 20min, and 10min to prepare the higher concentrated colloidal solutions. The so-obtained solution concentrations are 60 g/mL, 90 g/mL, 130 µg/mL, and 170 g/mL. The corresponding coatings are named ZrO_BN60, ZrO_BN90, ZrO_BN130, and ZrO_BN170, respectively. The exfoliated BNNS solutions were added to the Sol with the same preparation parameters used for ZrO_BNC then deposited on 316L substrate, see Chapter II -II.6.1. The electrochemical behavior of the samples with different concentrations will be compared.

The OCP values of the coatings are listed in Table 4 -4. ZrO_BN130 and ZrO_BN170 samples have similar OCP values to that of the references and to that of the ZrO_BNC coatings. On the contrary, ZrO_BN60 and ZrO_BN90 display both low OCP values, which are even lower than the ZrO_Ref. To further understand these behaviors, the EIS tests were performed.

1 0 -2 1 0 -1 1 0 0 1 0 1 1 0 2 1 0 3 1 0 4 1 0 5

1 0 -2 1 0 -1 1 0 0 1 0 1 1 0 2 1 0 3 1 0 4 1 0 5 It is logical that the aggregation of the BNNS reduces the nanocomposite coating's resistance in the pores since more defects will be formed. 25,26 With the concentration increase, the Rct kept increase due to less corrosion, which indicates the predominant influence of BNNS has switched from defects to barrier effect. Considering ZrO_BN170, the impermeable film had been formed, no charge transfer were observable.

The capacitive behavior of the coating also evolves according to the increase of concentration.

The coat stays stable for all samples, indicating a similar passive behavior of all samples. On the contrary, the dl increased with the increase of concentration of BNNS from 0.54 to 0.82 until a barrier film is observed for ZrO_BN170. This variation trend evident a transition from a kinetic limit process to a capacitive process with the increased BNNS concentration. the BNNS are all distributed in coating in parallel. Fundamentally, dip-coating is a microscale capillary filtration process. 27 In the withdrawing direction, the substrate surface was firstly wetted by the dispersion solution. The nanofiller is driven by the capillary force along the 28 Çamurlu et al. noted that the agglomerated BNNS might enrich at the surface of the coating. 23 Consequently, after the drying step, a concentration gradient of BNNS inside the coating, decreasing from the outer side to the inner side, will be forged. Given the coating thickness of three-layers is 400nm, as shown in Fig. IV.2 -28b, each dip contributed less than 150 nm in liquid-phase if we took the shrinkage into account. Since the single-dip thickness is much smaller than BNNS lateral size, these 2D materials were spread out by the capillary force and orientated in parallel with the coating surface and formed an impermeable film.

Fig. IV.2 -28 a) Scheme of the distribution of BNNS during dip-coating b) SEM image of the cross-section of as-dipped coating

Based on the observation, it can be summarized that the influence of the BNNS concentration on the coating's barrier effect, which can be divided into three phases, see -I) When the BNNS concentration is low, the dispersion is well-controlled, leading to a high resistance coating system.

-II) When the concentration of BNNS increases, the effect is twofold: 1) the prolongation of the penetration pathway; 2) the aggregation inducing defects, as illustrated in Fig. IV.2 -29a. The compromise of these two effects makes ZrO_BNC an even better candidate. Meanwhile, the electrochemical system turns more and more capacitive at low frequency.

-III) With the continually increasing BNNS, an impermeable film could be forged and allows almost no penetration to happen, as illustrated in Fig. IV.2 -29b. The Faradic behavior is thus no more detectable by EIS. To sum up, the ZrO_BN170 shows excellent capacitive behavior at low frequencies regime, which characterizes a powerful barrier performer under stationary conditions. However, the degradation of this impermeability of the coating started after 3 weeks of immersion. The impermeability provided by BNNS with over critical concentration displayed still a capacitive behavior at low frequency at the end of 8 weeks of immersion. An extra time constant at high frequency is observed starting from 3 weeks of immersion while the semicircle loop of this time constant decreases rapidly in 5 weeks. This is possibly due to the interacton of ZrO coating and the water.

IV.2.6 Conclusion

The study of the influence of the morphology as well as the concentration in ZrO coatings on 316L immersed in NaCl 30g/L solution have been explored and discussed.

The morphology plays an important role in the enhancement of barrier effect by BNNS fillers in the ZrO inorganic coating, by which there is a critical size, 7 m of average size, for an effective improvement of barrier effect.

The unfunctionalized BNNS showed two influences to the coating matrix: the barrier effect and the introduction of defects.With low concentration, when the size of the BNNS is over critical size, an enhancement of barrier effect can be acquired. When the concentration is high, the introduction of defects predominates and the coating shows decreased barrier effect. With increasing of concentration of BNNS, the influence of defects decreases and the influence of barrier effect increases. After a critical concentration, a total impermeable layer can be observed.

The coating synthesized with the study of morphology and concentration influence of BNNS showed long term stability.

IV.3 Study of BNNS in ZrO-SiO multi-component coating on AA2017

Generally, the protective coating's performance leans on its intrinsic physicochemical properties such as its surface tension, density, elemental composite, etc. Nevertheless, the life-time and the endurance of the coating are also associated with the coating/ substrate interface properties. In this part, we attempted to transfer the coating to a more sensitive metal and to evaluate the nanocomposite coating's performance.

Aluminum alloys have a passive film based on a dense Alumina layer, which is less stable than the passive film on 316L. 31 The galvanic couple inside the intermetallic particles distributed in the metal matrix makes it sensitive to localized corrosion, especially with the presence of chloride ions. 32 This makes Aluminum alloy behave a much higher reactivity in NaCl solution compared with 316L. By understanding the evolution of corrosion reaction as well as the regeneration of oxide film on the aluminum alloy, the barrier effect of BNNS could be probed.

IV.3.1 Electrochemical behavior of AA2017 bare substrate

The electrochemical behavior of bare AA2017 substrate has been studied firstly. The potentiodynamic polarization curve of AA substrate is shown in Fig. IV.3 -1. The corrosion potential is located at -0.77 V vs. SCE. The current density steeply increased on the anodic side, indicating an active corrosion that is coherent to the literature report. 33 It is to note that no passive state is observed due to the high concentration of Cl -ions. Indeed, the porous inorganic coating enables the penetration of the corrosive molecules so that the connected aluminum surface undergoes oxidation reaction under deposit. The lateral propagation of the reaction weakens the coating/ substrate adhesion until the total coating de-adhesion. 127

There are two ways to overcome this issue: 1) the reinforcement of the adhesion to prevent the crack propagation at the interface after the corrosion takes place; 2) increase the coating's barrier effect that restricts the penetration from top to bottom. The first way has been widely considered in many literatures reports to the coating and substrate material coupling. Here, we essentially focus on the second way to optimize the coating's intrinsic properties. To this end, the multicomponent hybrid Sol-Gel coating is a more appropriate choice since it allows the combination of the advantage of different units in the coating.

IV.3.3 Effect of ZrO-SiO multi-component coating on AA2017

The electrochemical behavior of pure ZrO-SiO multi-component coating deposited on AA2017 has been studied, named ZrO-SiO_Ref, see Chapiter II -II.3.2 and II.5.1 for the preparation parameters.

The potentiodynamic polarization curve of the AA2017 substrate and of the ZrO-SiO_Ref after 24 hours of immersion are illustrated in Fig. IV.3 -4. The Ecorr of ZrO-SiO_Ref lower than the substrate, indicating an earlier initial potential for the anodic reaction to come to pass. This phenomenon has also been observed by Xue et al. 35 Although the coating did not behave passive state, the current density showed two orders of magnitude lower compared to AA2017 substrate. The deposited coating reduced the exposure surface as well as blocks the penetration of corrosive molecules. It is well known that 2000 series of AA are sensitive to localized corrosion introduced by intermetallic compound, 32 at the grain boundary, and the metal crystalline. The polarization curve is a combination of these two corrosion phenomena. 36 Overall, the reference coating deposited sample showed an improved corrosion resistance with a significantly decreased reactivity of the metal surface. 129 water penetration rate within coating. 38 At middle frequency, one time constant can be observed between the frequency of 10 1 -10 2 Hz. This time constant correlates to the influence of the oxide film on the AA2017 substrate. 39,40 At low frequency, the increase of phase angle indicates the increased corrosion resistance due to the generation of oxide products, see red arrow on the left in At the frequency's regime between 10 to 10 4 Hz, a flat stage can be observed, where the coated sample displays exclusively resistive behavior. 41 A flat stage has been observed from ZrO-SiO_Ref, as shown in Fig. IV.3 -5a. The imperfect flat stage of ZrO-SiO_Ref indicates the existence of penetration of ions, which is due to the overlap of the time constant connected to the coating and the oxide layer. The perfect flat stage of ZrO-SiO_BN represents a total resistive behavior, indicating that the electrolyte are blocked from penetrating the coating at higher frequency than the flat stage due to the sealing of pores by BNNS fillers. 42 The higher impedance value of the flat stage of ZrO-SiO_BN is due to the sealing of the pores by BNNS. Thus, the BNNS charge coating showed a better barrier effect than the reference coating due to the enhancement provided by BNNS.

Comparing to the ZrO-SiO_Ref showing an impedance degradation from 1.5 x 10 4 to 2 x 10 2 Ohm.cm 2 of the in one week, the ZrO-SiO_BN showed a slower degradation rate, from 6 x 10 4 Ohm.cm 2 to 9 x 10 2 Ohm.cm 2 in one week. This depicts that the BNNS charged coating has better stability than the coating without charge. The deterioration of the coating of ZrO-SiO_BN can also be observed from the Phase curve shown in This is due to the increased quantity of corrosion product which is similar to the trend observed from ZrO-SiO_Ref.

IV.3.5 Conclusion

The AA2017 substrate, after immersed in NaCl 30g/L solution, will be severely corroded due to the corrosive environment with chloride ions. The coating without filler showed a barrier effect. However, severe corrosion phenomenon can still be observed.

The BNNS filler has been proved to be able to enhance the barrier effect by limiting the migration of ions. However, the corrosion causes delamination, indicating an insufficient corrosion protection behavior by the coating/BNNS system. The addition of BNNS may also decreases the adhesion property of the coating and accelerate the delamination.

In order to search for a coating/BNNS couple with strong and efficient barrier effect which protects the metal surface, an enhancement of coating matrix will be deposited on an even more sensitive surface to highlight the barrier effect of the BNNS enhanced coating.

IV.4 Study of BNNS fillers in Organic-Inorganic hybrid coating on carbon steel

As discussed in above part, the modification of ZrO-SiO multicomponent coating will be done by adding a hydrophobic modification to the coating with the precursor Octadecyltetraethoxysilane (ODTES). The BNNS will be charged in this coating to study the influence of barrier effect on a metal surface which is even more sensitive than AA2017.

This substrate could enable the probe of role of BNNS under a sensitive corrosion environment.

The carbon steel has a much more sensitive surface to a corrosive environment. In 30g/L NaCl solution, once the substrate was reached by the electrolyte, generalized corrosion takes place.

IV.4.1 Electrochemical behavior of carbon steel bare substrate

The electrochemical behavior of bare carbon steel XC90 has been studied in NaCl 30g/L solution. The OCP value close to 24 hours of immersion shown in Table 4 -6 is similar to the value reported by researchers. In the second place, the electrochemical behavior of the carbon steel deposited by ZrO-SiO/C18 coating without adding of BNNS, named as ZrO-SiO/C18_Ref, has been studied. The OCP value of CS_Ref at 24 hours of immersion in NaCl 30g/L solution has been presented in Table . 4 -1. It is observed that the OCP of ZrO-SiO/C18_Ref is slightly higher than carbon steel substrate.

Table. 4 -1 OCP value of ZrO-SiO/C18_Ref at 24 hours of immersion in NaCl 30g/L soultion

The polarization curve of ZrO-SiO/C18_Ref superposed with carbon steel is presented in Fig.

IV.4 -3. The Ecorr of ZrO-SiO/C18_Ref is much higher than that of the carbon steel substrate.

The current density of ZrO-SiO/C18_Ref on the anodic side is over three orders of magnitude lower than carbon steel, indicating much lower corrosion rate compared to the bare substrate since the coating efficiently limits the connective metal area to the electrolyte. To sum up, the ZrO-SiO/C18 coating without charge of fillers provide a better corrosion protection than ZrO-SiO multicomponent system coating. The ionic migration is largely blocked due to the higher hydrophobicity level of ZrO-SiO/C18, which 87.27° and 10° higher than the one of ZrO-SiO. ODTES has successfully increased the barrier property of the coating. 139 Nevertheless, a diffusive process was observed which means the corrosion reaction rate is rather important. The delamination of the coating can be observed after the polarization.

IV.4.3 Influence of BNNS charged ZrO-SiO/C18/carbon steel

The BNNS charger exfoliated with a high concentration method has been charged in ZrO-SiO/C18 hybrid coating, named as ZrO-SiO/C18_BN. The electrochemical behavior has been studied and shown below to understand the property of the coating as well as the role of BNNS in protecting the carbon steel substrate.

The OCP value of ZrO-SiO/C18_BN is shown in Table . 4 -2. The OCP value is 19 mV. This value is significantly higher compared to both the reference coating and bare substrate. The The Ecorr of ZrO-SiO/C18_BN is similar to the OCP value and is much higher than both reference samples, indicating a remarkably reduced surface reaction due to the presence of the coating. The current density of ZrO-SiO/C18_BN is 2.5 orders of magnitude lower than the one of ZrO-SiO/C18_Ref and 6 orders of magnitude lower than the bare substrate. The nobler Ecorr and low current density together infers an excellent corrosion resistance of the coating system. The time constant at high frequency indicates the strong and stable barrier property from the coating to the migration of ions with a phase angle close to 90° over time. 45 The middle frequency stayed stable, together analysis with the impedance curve, the charge transfer reaction happened but is with a low density due to the high impedance value.

IV.4.4 Conclusion

The carbon steel submits active corrosion in NaCl 30g/L. The ZrO-SiO/C18 coated sample, ZrO-SiO/C18_Ref, showed a barrier property with corrosion happening under coating, meaning the adding of ODTES successfully improved the barrier property of ZrO-SiO coating. After the potentiodynamic polarization, the delamination of the coating is observed.

The ZrO-SiO/C18 coating charged with BNNS, ZrO-SiO/C18_BN, showed significant enhancement of barrier effect comparing to ZrO-SiO/C18_Ref. After potentiodynamic polarization, no modification can be observed from the coating. The charge transfer reaction happened but with a very low rate. The coating showed a stable barrier property over 20 hours of immersion, but a degradation phenomenon can be observed.

The modification of ODTES to ZrO-SiO multicomponent coating successfully put out a coating matrix to probe the enhancement of barrier property of BNNS. For futher studies, the function of other 2D materials could be explored with the two matrix that have been established by the above study. (ZrO and ZrO-SiO/C18)

IV.5 The influence of other 2D materials on Sol-Gel coatings

Having understood the role of BNNS on Sol-Gel coatings by effect of adding 2D materials, it is interesting to explore the influence of other 2D materials on Sol-Gel coatings. With these studies, not only the role of 2D materials in Sol-Gel could be verified, but also the differences between 2D materials, and the properties influencing these roles can be probed.

In this aim, the electrochemically expanded graphite derived nanosheets (GENS) and MoS2 nanosheets (MDNS) are charged in two different Sol-Gel matrix which have been applied for the study of BNNS.

Both 2D nanofillers are firstly charged in ZrO single-component coatings deposited on 316L to observed their function in inorganic coating as well as the evolution of coated matrix in long term duration.

The nanofillers are then charged in ZrO-SiO/C18 OIH hybrid coating on carbon steel to compare their influence to the effiency of coating's barrier effect.

IV.5.1 The influence of other 2D materials on ZrO/316L

The GENS was introduced into ZrO coating to study its function in the inorganic coating's anticorrosion properties as well as the evolution of the coating. The GENS was exfoliated and centrifuged with high concentration preparation method in water (See Chapter II -II.5.1), which is the same as for the ZrO_BN170. The concentration of GENS after exfoliation is 90 g/mL that is between the concentration of ZrO_BNC and ZrO_BN170. It should be mentioned that the graphene derived by electrochemical expansion has 9% of oxygen content (see Chapter III) due to the oxidation of carbon under high potential. The so-prepared GE is polar and can be more easily dispersed into the sol-gel matrix. This leads to a higher limit of stable concentration for GENS (90 g/mL) than BNNS (g/mL). The GENS charged coating was firstly deposited on 316L substrate. The sample is named ZrO_GE. The coating showed almost the same Ecorr indicating the surface condition is similar to 316L_Sub, which is lower than ZrO_Ref. This is possibly due to a penetrable coating for corrosive molecules. The current density is one order of magnitude lower than the substrate indicating that the coating reduces the exposure surface of the coating and reduces the corrosion intensity. However, at the beginning of anodic region until 0.65 V vs. SCE, the current density of ZrO_MoS2 is higher than the current density of ZrO_Ref. Indicating a worse corrosion resistance than ZrO_Ref. At higher frequency, the current density of ZrO_MoS2 is similar to ZrO_Ref. ZrO_MoS2 behaved passive state while no pitting were been observed as it has shown up over 0.68V vs. SCE for 316 substrate. The coating showed a limited barrier effect for the bare substrate but can effectively stop the pitting at voltage lower than 1.2 V vs. SCE. particles are MoS2 agglomerated particles, the frequent presence of these particles with holes proves that it might have connections to the extra time constant. To sum up, the ZrO_MoS2 coating showed similar behavior to 316L_Sub observed from polarization curve. However, it showed less corrosion intensity and no pitting potential. It behaved a low function in offering tortured pathway. An extra time constant can be observed at high frequency, which could possibly be the cause for the low corrosion resistance behavior provided by ZrO_MoS2 coating.

IV.5.2 The influence of other 2D materials on ZrO-SiO/C18/carbon steel

The GENS has been charged in ZrO-SiO/C18 coating to probe the role of GENS in a hydrophobic hybrid coating as well as to explore the role of enhancement of blocking property of isolating coatings for protection of carbon steel. GENS was exfoliated and centrifuged with the same method as for BNNS in n-Butanol. The GENS n-Butanol solution behaved an extra high concentration, 570 g/mL but an extra low stability. The coating was named ZrO-SiO/C18_GE. The open circuit potential of ZrO-SiO/C18_GE at 24 hours of immersion in NaCl solution is shown in Table . 4 -3. The OCP value is 7.9 mV/SCE which is slightly nobler than the one of CS_BN.

Table. 4 -3 OCP value of ZrO-SiO/C18_GE at 24 hours of immersion in NaCl 30g/L solution

The polarization curve of ZrO-SiO/C18_GE superposed with reference samples after 24 hours immersed in NaCl 30g/L. The polarization curve of ZrO-SiO/C18_GE showed a much nobler Ecorr than both reference samples, and a current density which is three times lower than ZrO-SiO/C18_Ref and more than six times lower than carbon steel substrate. Comparing to ZrO-SiO/C18_BN, the coating showed an even nobler Ecorr and a lower current density.

The GENS has been charged in ZrO-SiO/C18 coating to probe the role of GENS in a hydrophobic hybrid coating as well as to explore the role of enhancement of blocking property of isolating coatings for protection of carbon steel. GENS was exfoliated and centrifuged with the same method as for BNNS in n-Butanol. The GENS n-Butanol solution behaved an extra high concentration, 570 g/mL but an extra low stability. The coating was named ZrO-SiO/C18_GE. Ohm.cm 2 and decreased to 2 x 10 7 Ohm.cm 2 after 20 hours of immersion. The corrosion happened under coating, but the corrosion rate is really low, which is similar to the case of coating ZrO-SiO/C18_BN. This protection is better than the active protection by passive film from the coatings deposited 316L. Thus, it could be a perspective that the active protection fillers can be introduced for the coating matrix.

The phase curve showed clearly two time constants which are of the same characters as ZrO-SiO/C18_BN. The time constant at high frequency indicates the coating providing the barrier effect to block the migration of ions. At middle frequency, a time constant between 10 Hz to 10 3 Hz is observed, indicating the oxide film. It showed a larger phase angle (over -70°) than ZrO-SiO/C18_BN, which is lower than -60°. From the Phase curves, a degradation behavior is observed, which is faster than the degradation of ZrO-SiO/C18_BN. To prove the roles of MDNS in hydrophobic hybrid coating and explore possible applications, the MDNS was exfoliated with the same method as to BNNS and GENS in n-Butanol. The concentration was the highest among three different 2D fillers, 680 mg/mL, but the stability was rather low. The sample was named ZrO-SiO/C18_MoS2.

The OCP value of ZrO-SiO/C18_MoS2 at 24 hours of immersion in NaCl 30g/L solution is shown in Table . 4 -4. This value is slightly higher than the value of ZrO-SiO/C18_Ref and ZrO-SiO/C18_Sub. The SEM image of surface morphology of ZrO-SiO/C18_MoS2 after polarization is shown in Fig.

IV.5 -15. A large area of delamination is observed while corrosion products is seen on the metal surface. This proves that the surface modification due to corrosion and the delamination of the coating could be distributed all around the metal surface. ZrO-SiO/C18_MoS2 -515

than ZrO-SiO/C18_Ref and decreases over time. Different from ZrO-SiO/C18_Ref, no diffusion phenomenon can be observed. To sum up, the MDNS charge ZrO-SiO/C18 coating showed similar behavior to ZrO-SiO/C18_Ref with less heavy corrosion reaction on the metal surface. The ZrO-SiO/C18_MoS2 can offer limited enhancement of blocking property of isolating coating.

IV.5.3 Conclusion

Both GENS charged coatings showed an improved corrosion protection behavior comparing to the reference coatings. The ZrO_GE provided tortured pathway, but no total capacitive behavior can be observed. The ZrO-SiO/C18_GE coating showed enhancement of barrier effect of the coating. The two coatings showed even higher enhancement property than the corresponding coatings charged by BNNS.

The MDNS charged ZrO coating showed inferior corrosion resistance than ZrO_Ref and similar corrosion behavior than 316L substrate. However, the impedance response of ZrO_MoS2 is even lower than 316L_Sub with an extra time constant existing at high frequency showing extra phenomenon on introducing the corrosion reactions. The MDNS charge ZrO-SiO/C18 could not offer much enhancement of barrier effect to the coating, which showed a similar behavior comparing to ZrO-SiO/C18_Ref.

BNNS and GENS can both introduce strong barrier effect to Sol-Gel coatings, while BNNS showed distribution behavior than GENS in water solution. To achieve a stronger tortured pathway, the well distributed GENS is a good choice. For a stronger impermeability by totally blocking the ionic migration, the BNNS could be a candidate. For hybrid coatings, though GENS showed lower stability and agglomeration, its high concentration also showed a strong distribution behavior than BNNS. Both coating showed similar properties in the hybrid coating.

However, MDNS, due to its active reactivity, cannot propose good barrier properties to Sol-Gel thin coatings. Perspective could be done to explore the role of MoS2 nanosheets in thick coatings.

IV.6 Conclusion

In this chapter, we studied the role of the 2D nanosheets in enhancing barrier effects in different coating systems.

Zirconium oxide single-component coating on 316L: Firstly, it has been proved that the morphology of BNNS plays an important role in corrosion resistance. A critical diameter exists, 7 m in average size, for the BNNS to exhibit its barrier effect in the coating. The BNNS exfoliated from large-sized BN powder showed a prolonged penetration pathway to the coatings. The coating showed a continuous increase of barrier effect in 1200 hours of immersion because of a synergy effect between the barrier effect of BNNS and the sealing of the corroded sites by oxide product.

The concentration of BNNS in the Sol-Gel coating is another essential parameter. The suspension of BNNS in aqueous media can stay stable at a concentration lower than 15 g/mL. Beyond this concentration value, the nanosheet agglomeration occurs spontaneously, leading to the formation of unexpected defects with a critical size that is more influential than the pores. As a consequence, the coating's corrosion resistance decreased. With the continuously increased BNNS content to 170 µg/ml, the coating exhibited a total capacitive behavior. This variation is explained by the orientation of BNNS and the formation of a highly concentrated layer driven by dip-coating capillary force.

Zirconium oxide -Silicon oxide multi-component coating on Aluminium alloy: the multicomponent sol-gel coating exhibited better corrosion resistance than the ZrO coating. The BNNS charged coating enhanced the coating barrier effect. Nevertheless, the corrosion caused delamination. The addition of BNNS decreased the adhesion of coating, which in return accelerate the delamination.

Organic-inorganic hybrid coating on carbon steel (XC90): The addition of hydrophobic alkyl group (ODTES) in the ZrO-SiO Sol allows to change the surface tension and the water repellency in the pore. The coating without BNNS limited the corrosion on carbon steel. For the coating charged with BNNS, the corrosion were prohibited and delamination weren't observed.

General Conclusion and Perspectives

Sol-Gel coatings have been considered as a promising candidate for the replacement of Chromium Conversion Coatings (CCC) for corrosion protection of metals. Neverthelss, its porosity nature (intermolecular, interparticular, intercluster pores) constitute connected corrosion pathway within the coating. Heat treatment or adding of fillers have been applied by researchers to improve the barrier effect of Sol-Gel coatings. Among them, graphene-like two-dimensional (2D) materials have got attention due to their impermeability to molecules. The 2D hexagonal Boron Nitride is interesting as a filler for corrosion protection coatings for its chemical inertness and large bandgap (5.2eV-5.9eV).

This thesis aims to study the influence of 2D materials to Sol-Gel coatings deposited on metals for corrosion protection applications. The hexagonal Boron Nitride nanosheets (BNNS) was mainly studied while electrochemically expanded graphite exfoliated nanosheets (GENS) and Molybdenum disulfide exfoliated nanosheets (MDNS) were exploited afterward. The study was performed by charging 2D nanofillers into Sol-Gel matrix on metal with different reactivities in corrosive environment. The influence of 2D materials were investigated with the help of electrochemical characterization (Open Circuit Potential, Potentiodynamic Polarization, Electrochemical Impedance Spectroscopy) in NaCl 30g/L.

The 2D materials have been proved to be able to seal the pores in the coating and enhance the barrier effect of the coating. The enhancement is due to a synergy effect provided by both Sol-Gel matrix and nanofiller. We proposed an approach to syntheize an 2D fillers charged organic inorganic hybrid coating which shows an excellent corrosion protection to the substrate metal.

Firstly, Zirconium oxide based (ZrO) single-component coating has been deposited on stainless steel 316L. The 316L substrate behaves inertness to general corrosion and is prone to pitting corrosion. The porous Sol-Gel inorganic coating allows the penetration of electrolyte to the stable interface of 316L which allows the role of 2D fillers during the evolution of ZrO/316L system to be observed.

By using three BN powders with three different average diameters, a critical lateral size (dc) at 7 m was proposed for the BN powder to provide evident barrier effect to the coating. If the average size is inferior to this onset value, the resistance reinforcement to the Sol-Gel will be minor. This study was performed with the stable suspension of BNNS in aqueous media with a concentration inferior to 15 g/mL.

The influence of BNNS concentration superior to stable concentration has been studied. When the concentration gets higher than stable concentration, the influence of BNNS becomes twofold: 1)The formation of defects caused by aggregation of BNNS 2) The sealing effect to pores. The coating with increased concentration of BNNS (60 g/mL) firstly showed even lower protection the coating without fillers. With the concentration kept increasing (130 g/mL), the sealing effect predominates and the barrier effect is enhanced. After the concentration surpasses 170 g/mL, a total capacitive behavior can be observed which can be explained by the formation of an impermeable film of BNNS.

166

The stable interface of 316L hinders influence of 2D materials during the corrosion process. Thus, in order to probe the barrier effect contributed to BNNS fillers, the ZrO singlecomponent coating was directly transported to AA2017 for its higher reactivity to corrosion. Neverthelss, severe corrosion was observed, which is due to the weak barrier effect combined between ZrO porous coating and oxide film of AA2017. It was thus concluded that the ZrO/316L can be used for probing of role of 2D fillers as well as evolution of coating, but a Sol-Gel matrix with less porosity is required.

Secondly, ZrO -SiO multi-component coating deposited on AA2017 with Zirconium n-Butoxide / (3-Glycidyloxypropyl) trimethoxysilane (GPTMS) as precursor, has been synthesized. The reduction of corrosion due to the barrier effect of BNNS has been proved comparing to the coating without filler. However, delamination was observed due to corrosion.

To search for a coating/BNNS couple with efficient corrosion protection to metal surface, further enhancement of the Sol-Gel matrix were required by either increasing the adhesion to slow down the delamination or by modifying the Sol-Gel matrix for a stronger barrier effect Thirdly, to further enchance Sol-Gel matrix for a stronger barrier effect, an Organic Inorganic Hybrid (OIH) Coating is deposited on carbon steel XC90 by incorperating Octadecyltetraethoxysilane (ODTES) to ZrO -SiO multicomponent coating. Therefore, the hydrophobic alkyl group (-C18) changes the polarity of the ZrO-SiO coating to hydrophobic and increases both its flexibility and thickness. ZrO-SiO/C18 coating showed enhanced corrosion protection than ZrO-SiO coating by allowing limited corrosion to take place on the sensitive carbon steel surface. The introduction of BNNS successfully prohibited corrosion and delamination.

Finally, the study of GENS and MDNS were performed on ZrO/316L and ZrO-SiO/C18/carbon steel systems to compare with BNNS under different reactivity condition. GENS showed stronger enhancement than BNNS in both cases while MDNS showed inadequate behavior and even lowers the barrier effect of the coating. Specifically, the impermeable film was not observed by GENS nor MDNS charged in ZrO/316L, which is possibly due to the BNNS nature of inertness and low compability to Sol-Gel matrix.

The ZrO-SiO/C18 coating charged with BNNS or GENS fillers could be well applied for corrosion protection applications.

Considering all the conclusions above, future researches on the following points may allow an enhancement of the coating performance.

1. The bonding between BNNS and Sol-Gel matrix: functionalization of the BNNS could improve the bond between the BNNS and the Sol-Gel matrix. Meanwhile, by functionalization, the BNNS concentration can also be augmented. 2. The optimization of the highly concentrated BNNS film: In order to understand the mechanism of the formation of the film, BNNS with different functional groups could be applied to increase the integrity of the BNNS to the coating. 3. The optimization of influence of 2D materials other than BNNS: The optimization study of the influence of the concentration of GENS fillers to the barrier effect of ZrO and ZrO-SiO/C18 coatings in NaCl 30g/L solution.

4. The identification of critical size (dc) from exfoliated BNNS: The first part study have given the existence of critical size for BN powders. To understand the influence of BNNS in detail, the dc from exfoliated nanosheets could be explored with optimization study by specific preparation method as well as characterization with microscopies. In Eq.2, C is the concentration of charger and A is the intensity of the signal; l is the length of interaction path of light and  is the extinction coefficient. Obviously, the extinction coefficient should be decided before the calculation. The  can be defined by measuring the angle of the slope between the concentration and Intensity from UV-Vis, as shown in Fig. A -2. Every machine and every target specie may differ the Every target species will have a character wavelength for which at that wavelength the intensity of UV-Vis will be extra high.

Table of Figures

Figures from Chapter I

Figures from Chapter II

Figures from Chapter III

Figures from Chapter IV

A.1.3 Abbreviations and Acronyms of characterization tools

To avoid the influence of diffraction of extra high intentsity at the character wavelength. the calculation of content of the target species are perfomed with the intensity at a wavelength which has been proved to be less affected by the diffractions. With h-BN for example, the wavelength is selected at 300 nm. 1

Fig. A -2 mesurement of extinction coefficient by weighting and measuring with UV-Vis

In this work, the measurement of concentration of hexagonal Boron Nitride (h-BN) with different morphology exfoliated solution are measured by UV-Vis Spectroscopy, and for the measurement of h-BN with high concentraton method, with different solvent as well as the Graphene oxide and MoS2 in different solvent, the concentration will be measured directly by weighting with the balance. The absorption extinction coefficient of h-BN is measured as 2924 mg/ml/mm at wavelength of 300 nm. This calculated value is coherent to the value reported from other authors, proving that this value as acceptable. 2

A.2.4 Raman spectroscopy

As shown in Fig. A -3, when there is a proton introduced to a materials, the electron will be scattered as shown from the scheme. The elastic scattering is called Rayleigh Scattering. The inelastic scattering is called Stokes Raman Scattering and is of very low possibility comparing to elastic scattering. This inelastic scaterring has been applied for the identification of elements and bonds in material science and is call Raman Scattering Spectroscopy.

Fig. A -3 Scheme of energy level for the explaination of Rayleigh scattering and Stokes Raman Scattering

The Raman Spectroscopy is employed confined laser to increase the chance of Raman scattering and calculate the phonon scattering from the energy loss of the detected output light. It is widely applied for the identification of the elements or molecules by detecting phonons representing the characteristic bonds.

The Raman Spectroscopy employed in this work is HORIBA, Aramis with the laser using the wavelength 514nm. Samples of ZrO-SiO/C18 coatings were directly measured in the Raman Spectroscopy after synthesis.

A.2.5 Fourier Transform Infrared Spectroscopy/ Attenuated Total Reflectance

The Fourier Transform Infrared Spectroscopy (FTIR) is following the similar principle as UV-Vis but using light at the wavelength range of Infrared. The absorption of the light will coordinate to certain bonds inside materials and is applied for the identification of bonds and existence of materials. Fourier Transform Infrared Spectroscopy/ Attenuated Total Reflectance (FTIR/ATR) is the FTIR technology applying with total reflectance phenomenon to rapidly detect the near surface region instead of bulk materials. The resolution depth is mostly under 2 microns.

For the ATR mode, as shown in Fig. A -4., since the ATR crystal has a higher refractive index than the sample, the total reflectance phenomenon will happen. Thus, by calculating the energy lost from pre-defined light path, the identification of thin materials can be achieved. In this work, the ZEISS Supra55 SEM was employed for the morphology of the different samples before and after the corrosion. The higher the electron energy is, the deeper the electron can reach on the surface of the material. With the modification of voltage of the introduced electron beam, the surface morphology with different depth can be characterized. With a coating of 300 nm for example, if the voltage is chosen as 20 kV, the metal surface under coating will be observed. If the voltage is 2kV or less, the surface morphology of the coating will be observed while the morphology under coating cannot be.

The SEM used in this work is with a ZEISS Supra55 from Clym platform from INSA de Lyon, France. The voltage used are 20 KV for deep detection and 2KV for near surface detection. The inert and stable samples were firstly cleaned with ethanol in ultrason for 15 min and rinsed with distilled water. The unstable samples were cleaned with compression air gun before the measurement.

A.2.9 Transmission Electron Microscopy

Transmission Electron Microscopy (TEM) is a technique using emitted electron to transmit the sample material and detecting the transmitted electrons. In order to transmit the material, the thickness of the materials need to be penetrable by electrons (< 100nm).

As shown in The Transmission Electron Micrographs were obtained with a JOEL-2010 microscope operating at 200 kV from the Clym plateform form INSA de Lyon. The 2D materials samples were exfoliated in different solvent and the 2D materials containing solvent were dropped on to the carbon network containing copper filter and dried for a whole night before being transported for the TEM measurement.

Appendix.3 Preparation of Electrochemically expanded graphene

Electrochemically Expanded Grapheite (GE) synthesized from Graphite foil bought from Fischers, USA. The Graphite foil has been exfoliated with a potential of 20 V applied in Na2SO4 1 mol/L solution. The as-exfoliated GE were filtered and rinsed with water and target solvent for 3 times each. Finally, the rinsed GE have been exfoliated in water, n-Butanol with the concentration of 30 mg/mL (with undried solution directly after the synthesis of GE) for 60 hours and centrifuged for 10 min with 2000 rad/min. The powder, exfoliated water solution and exfoliated n-Butanol solution of GE are named as GE powder, GE water and GE n-But respectively. The coatings charged with GE fillers will be noted with a tail noting GE at the end.

After the exfoliation and centrifuge, the 2D materials suspension were stored and been reexfoliated for 1 hour before added into Sol-Gel solutions.

I

  give my thanks to Mrs. Ying Huo, Mr. Zhiheng Zhang, and Dr. Elizavieta Lapuskana for being my dearest friend and for all the experience that we have shared in these years. I give my thanks to Dr. Quentin Lemarie, Mr.Philippe Martinet, Mrs. Imen Ferjani for the friendship as well as the sharings and supportings. I give my thanks to Dr. Benoit Ter-Ovanessian, Dr. Sabrina Marcelin for their scientific support , helps and hard works during my thesis. I give my thanks to Professor Marion Frégonèse, Dr.José Bolivar for guiding me with patience and consideration. I give my thanks to all the crews in CorrIS group, especially for Mrs. Yasmina Salhi, and for the crews in the Laboratory of MATEIS, for the support and cooperation. I give my special thanks to Mr.Long LIANG for supporting me thoughout the whole time, and sharing so much experience and precious moments that I may never forget in my life. Thank you Yu SHI IV.4.2 Effect of Organic-Inorganic hybrid coating on carbon steel ................................. IV.4.3 Influence of BNNS charged ZrO-SiO/C18/carbon steel ........................................ IV.4.4 Conclusion ............................................................................................................. IV.5 The influence of other 2D materials on Sol-Gel coatings............................................ IV.5.1 The influence of other 2D materials on ZrO/316L ................................................ IV.5.2 The influence of other 2D materials on ZrO-SiO/C18/carbon steel ..................... IV.5.3 Conclusion ............................................................................................................. IV.6 Conclusion ................................................................................................................... General Conclusion and Perspectives .................................................................................... Table of Figures ...................................................................................................................... List of Tables ........................................................................................................................... Appendix.................................................................................................................................

1 For 2 3 4 Fig. I. 2 -

 12342 Fig. I.2 -1a illustrates the different steps of the corrosion reaction, in which the oxidation & dissolution of bulk material, reduction, conducted circuit are denoted from 1 to 3. Corrosion protection can be realized by either preventing these three main processes or by increasing the relative potential of the protected electrode with a sacrificial metal, as shown from No.4 in Fig. I.2 -1b.4 The corrosion modes are presented in the next part.

Fig. I. 2 - 1

 21 Fig. I.2 -1 Schemes of a) mechanism of corrosion and b) mechanism of cathodic protection

Fig. I. 2 - 2

 22 Fig. I.2 -2 Scheme of mechanism of general corrosion from a) side view b) top view8

  Fig. I.2 -3.

Fig. I. 2 - 6

 26 Fig. I.2 -6 SEM images of porous structure synthesized by dealloying-redealloying method done by Zhu et al with different magnifications a) 40 b) 300 c) 3000 d) 30,000 16 Intergranular corrosion: The intergranular corrosion happens when the crystallized materials has a difference in corrosion potential between the inner part of the grain and the grain boundary. With stainless steel for example, as shown in Fig. I.2 -7, the chromium carbide precipitation in the middle of the grain boundary causes a rich Cr region.18 Meanwhile, this precipitation leaves the chromium depletion region as a region with very low content of Cr, since all the chromium are diffused into the chromium carbide precipitation region. Thus, the grain contains higher content of Cr than the Chromium depletion region, which creates a galvanic couple between the Chromium depletion region and the grain. As anode, Chromium depletion region will firstly be corroded, and the propagation of corrosion will preferentially be in the direction of the grain boundary.19

:

  Stress corrosion cracking (SCC) is caused by the synergy effect of the pitting corrosion or other localized corrosion and the stress distribution at the corroded site. As the scheme of pitting corrosion shown in Fig.I.2 -8, once there is a pitting or crevice corrosion, the stress field around the opened site may further break the repassivated area and promote the corrosion to propagate. The stress corrosion cracking can behave intergranular corrosion or transgranular corrosion, which is depending on the microstructure of the materials.20

Fig. I. 2 - 8

 28 Fig. I.2 -8 Scheme of mechanism of pitting corrosion 20Corrosion fatigue: Corrosion fatigue is the corrosion assisted fatigue phenomenon. All materials have fatigue limits, and the existence of corrosion like SCC or pitting will decrease the fatigue life-time of a material.

  Fig. I.2 -9 illustrates the scheme of how the Zinc-rich primers works and the self-healing mechanism of the zinc-rich primer coatings. Zinc particles are released to the damaged zone on the metal surface. As zinc is priorly corroded, the corrosion product will mask the connective zone.[START_REF] Sørensen | Anticorrosive Coatings: A Review[END_REF]

Fig. I. 2 - 9

 29 Fig. I.2 -9 Mechanism of Zn particles charged sacrificial coating[START_REF] Sørensen | Anticorrosive Coatings: A Review[END_REF]

M(OR) 4 + 5 M 6 M 8 9 (

 45689 H2O  M(OR)3OH + ROH React.(OR)3OH + H2O  M(OR)2(OH)2 + ROH React.(OR)2(OH)2 + H2O  MOR(OH)3 + ROH React.7 MOR(OH)3 + H2O  M(OH)4 + ROH React.The condensation reactions: (OR)3MOR + HOM(OR)3  (OR)3M-O-M(OR)3 + HOR React.OR)3MOH + HOM(OR)3  (OR)3M-O-M(OR)3 + H2O React.10 where M represents the center atoms like Si or a transition metal atom, R represents the alcoholic organic group. The scheme of the Sol-Gel process can be shown as Fig. I.3 -2. The hydrolysis reaction consists of the replacement of the alcohol group in a precursor molecule by a hydroxide group.

Fig. I. 3 - 2

 32 Fig. I.3 -2 Scheme of a) -c) Sol-Gel reaction as well as d) -k) solidification procedure for applicationsFig. I.3 -2d to k shows the scheme of solidification and shaping process of Sol-Gel technique.The metal alkoxide solution can be directly used to form nanoparticles on SiO2, TiO2, ZrO2, etc.[START_REF] Caruso | Sol-Gel Nanocoating: An Approach to the Preparation of Structured Materials[END_REF] The Sol-Gel coating can be directly synthesized by nanoparticles by spraying or brushing.[START_REF] Shen | Corrosion Protection of 316 L Stainless Steel by a TiO2 Nanoparticle Coating Prepared by Sol-Gel Method[END_REF] The Sol can be deposited on metal surface by dip-coating, spin-coating, electrodeposition and other methods to forge Xerogel film.[START_REF] Brinker | Review of Sol-Gel Thin Film Formation[END_REF] A heat treatment of the xerogel film is classically required to remove the residual liquid phase (the solvent) accompanied with a densification and a significant amount of shrinkage.[START_REF] Siouffi | Silica Gel-Based Monoliths Prepared by the Sol-Gel Method: Facts and Figures[END_REF] Finally, a relatively dense solid coating

Fig. I. 3 -

 3 4a shows the velocity of hydrolysis and condensation reaction in Sol as the functions of the pH value. It is clearly shown that both hydrolysis and condensation reaction has the lowest rate when the pH is close to 7, that is why catalyst (acidic or basic) are frequently used to accelerate the reaction.

  Fig. I.3 -4b. On the contrary, a basic condition leads to the formation of spherical colloid. The so-prepared solid phase is ''piled particles'' that have more pores but less internal stress. The relationship between the colloid size and the pH can be roughly illustrated as Fig. I.3 -4c.

Fig. I. 3 - 4

 34 Fig. I.3 -4 Influence of pH to the microstructure a) the figure of the reaction speed of hydrolysis and condensation influenced by pH b) particles size influence by pH c) the shape of the final product influenced by pH (reform from the work of Pandey etel.)83,101 102

Fig. I. 3 - 5

 35 Fig. I.3 -5 Mechanism of chelating agent blocking precursor active sites for Zr-based precursor and diketone[START_REF] Yu | Effect of Chelating Agent Acetylacetone on Corrosion Protection Properties of Silane-Zirconium Sol-Gel Coatings[END_REF]

Fig. I. 3 - 6

 36 Fig. I.3 -6 The scheme of a) dip coating b) spin coating and c) electrochemical deposition

3

 3

Fig. I. 4 - 1

 41 Fig. I.4 -1 Scheme of the barrier effect of a) and b) inorganic coating, c) hybrid coating and d) and e) organic coating

Fig. I. 4 - 2

 42 Fig. I.4 -2 Incorporation of Inorganic and organic compound by the use of GPTMS[START_REF] Figueira | Organic-Inorganic Hybrid Sol-Gel Coatings for Metal Corrosion Protection: A Review of Recent Progress[END_REF]

Fig. I. 4 - 3

 43 Fig. I.4 -3 Classification of hybrid coatings with the x-axes showing the ratio of organic compound in the coatingsI.4.2 Organic functionalized silane precursors

  Fig. I.4 -4 lists the frequently used Si-based precursors.

Fig. I. 4 - 4

 44 Fig. I.4 -4 Names and structures of the most frequently used inorganic Si-based precursors; Frequently used hybrid organicinorganic Si based precursors and the Si-based precursor used in this work

Fig. I. 5 -

 5 2d. Specifically, when  is over 150°, the surface is so-called superhydrophobic and allows a strong water-repellency. The superhydrophobic surface usually has small vacancies on the surface, which trapped air bubbles forming a superficial composite, as shown in Fig.I.5 -2e.

Fig. I. 5 - 2

 52 Fig. I.5 -2 The Scheme of a) surface tension, b) Superhydrophobic coating and the images of contact angle for c) hydrophilic surface, d) hydrophobic surface and e) superhydrophobic surface

Fig. I. 5 - 3

 53 Fig. I.5 -3 The pie chart of the distribution of the applicated materials of class II coatings[START_REF] Figueira | Organic-Inorganic Hybrid Sol-Gel Coatings for Metal Corrosion Protection: A Review of Recent Progress[END_REF]

  To reduce the reactivity of ZrBTO, a chelating agent acetylacetone (AcAc, ≥ 99.5%, Sigma-Aldrich) was introduced immediately and stirred until a homogenous solution was obtained. The chelation reaction between ZrBTO and AcAc is shown in Fig. II.3 -2. Then (3glycidyloxypropyl)trimenthoxysilane (GPTMS, ≥98%, Sigma-Aldrich) was added to this mixture at 60°C. We slowly introduced water to initialize the hydrolysis reaction. With Nitric Acid (1 mol/L), the solution pH was tuned to pH = 4. The scheme of preparation procedure for ZrO-SiO Sol is shown in Fig. II.3 -3.

Fig. II. 3 - 2

 32 Fig. II.3 -2 Scheme of the AcAc blocking active sites on Zirconium n-Butoxide and the precursors of ZrO-SiO

Fig. II. 3 - 4

 34 Fig. II.3 -4 Scheme of preparation procedure for ZrO-SiO/C18 Sol II.4. Protocols of dip-coating Fig. II.4 -1 shows the dipping machine with a homemade heat container connected with a thermostat. The dipping machine is Xdip-SV1 from Apex Instruments. The dipping parameters are listed in Table.2 -1.

Fig. II. 4 - 1

 41 Fig. II.4 -1 Dip-coating machine connected with heat container and heat generator

4 SS

 4

Fig. II. 5 - 1

 51 Fig. II.5 -1 Image of a) Ultrasonic machine for exfoliation b) Centrifuge machine

  Fig. II.6 -1. The Working Electrode (WE) is the sample for the electrochemical test. The Reference Electrode (RE) is characterized by its potential vs. Standard Hydrogen Electrode (SHE). The RE used in this work is the Saturated Calomel Electrode (SCE) with a standard potential of +0.244 V vs. SHE, as indicated in Figure Fig. II.6 -2. Meanwhile, the auxiliary electrode, often called the counter electrode (CE), connects the working electrode, the Alternating current (AC) or Direct Current (DC) potential source, and the electrolyte to establish a connected electrical circuit. It consists of inert and electronic conductive materials like graphite that we used in this study.

Fig. II. 6 - 1

 61 Fig. II.6 -1 Scheme and image of the electrochemical cell

  Fig. II.6 -3. From the Nyquist curve, the impedance magnitude of the point(R0, I0) is shown as |Z|. The slope angle between the imaginary magnitude |I0| and R0 is the phase angle, noted . As it is shown from Nyquist curve, every point has an impedance value from one specific frequency. If we plot the impedance magnitude |Z| and phase angle  as a function of the frequency, the Bode curves are obtained as shown in Fig. II.6 -4 a and b respectively.

66

 66

Fig

  Fig. II.6 -3 Nyquist curve from Electrochemical Impedance Spectroscopy

Fig. II. 6 -

 6 Fig. II.6 -5a shows the simplest Randles model for metals. In this model, the Rs is the resistance from the electrolyte of the immersed system, and Rct is the resistance from the charge transfer reaction on the metal surface. The two resistance parameters are shown in the Nyquist curve in Fig. II.6 -3.Cdl is the double layer resistance at the metal surface caused by the charged transfer procedure. Many reports show that the Cdl is not always accurate since the surfaces are not homogeneous.14 Thus Constant Phase Element (CPE, Qdl) is proposed to introduce a coefficient  associated with the morphological variation and describe the imperfect state of

Fig. II. 6 - 5

 65 Fig. II.6 -5 Electrochemical Equivalent Circuit of a) Randles model b) CPE modified Randles model

  Raw h-BN: The morphology and size distribution of BN-A, BN-B, and BN-C powders and the statistic analysis are shown in Fig. III.2 -1. It is observed that BN-A has an average lateral size of 4.5 m. The BN-B shows a size peaked at 1 m, while BN-C has a size peaked at 7 m. Lin et al.

2

 2

Fig

  Fig. III.2 -1 Morphology of raw BN powders of a) BN-A b) BN-B, and c) BN-C The photo of as-purchased graphite foil and the SEM image of expanded graphite (GE) synthesized with the method proposed by Dhakate et al. are shown in Fig. III.2 -3a and b,

Fig

  Fig. III.2 -2 a) The photo of Graphite foil before electrochemical expanding b) and after The MoS2 powder employed in this work is shown in Fig. III.2 -3b. The particle size of MoS2 is around 1-2 µm in diameter for the lateral size.

Fig. III. 2 - 3

 23 Fig. III.2 -3 Morphology of MoS2 by SEM III.2.2 Characterization of metal substrates The photos and SEM images of polished substrates are shown in Fig. III.2 -4. After polishing, the surfaces of all bare substrates show continuous polish traces. The different hardness of the metals leads to rather different roughness.

Fig. III. 2 - 4

 24 Fig. III.2 -4 Photos and SEM images of as-polished metal substates of a) and b) 316L, c) and d) AA2017, e) and f) carbon Steel XC90 bare substrates III.3 Characterization of the Sol-Gel systems III.3.1 Characterization of BNNS charged ZrO coatings on 316L

  Fig. III.3 -1a. The Sol-Gel solution is transparent with a light white color. The as-exfoliated solution was prepared with a low concentration of BN-A, BN-B, and BN-C, cf. chapter II -II. 5, are shown in Fig. III.3 -1b, where no apparent sediment can be observed. The colloidal solutions prepared with a high concentration named BNC_Sol60, BNC_Sol90, BNC_Sol130, and BNC_Sol170 are shown in Fig. III.3 -1c. The BNC_Sol90, BNC_Sol170 are not more transparent. Fig. III.3 -1d showed the solution of ZrO Sol mixed with BNNS/water solution with a ratio of 3 : 1. To disaggregate and to ensure a good homogeneity, a 10 min ultrasonic post-treatment was systematically performed.

Fig

  Fig. III.3 -1 Photo of a) ZrO Sol-Gel solution, b) BN-A, BN-B, and BN-C as exfoliated water solution with low concentration preparation method and c) BNC_Sol60, BNC_Sol90, BNC_Sol130 and BNC_Sol170 as-exfoliated water solution with high concentration preparation method d) the mixed ZrO and BNNS/water solution with ratio 3: 1The DLS curves obtained with the water injected ZrO sol after stewing for one day, two days, and three days are plotted in Fig.III.3 -2. It is observed two particle sizes peaked at around 7 nm and 50 nm, respectively. The small one characterized the single colloid size, while the one at 50 nm indicates the agglomeration occurring. The distribution of colloidal size remains stable over three days, and even less agglomeration can be observed at the end. It should be noted that the matrix of zirconium oxide coating will not have a significant influence on the coating anticorrosion behavior.5

Fig. III. 3 - 2

 32 Fig. III.3 -2 The DLS spectroscopy of three days evolution of ZrO Sol-Gel solutionThe zone of cracks shown in Fig.III.3 -3a to 3c, for ZrO_Ref and ZrO_MoS2 and ZrO_BN, respectively, depict that the ZrO single-component coating is having a thickness less than 300 nm. Large particles of 2D fillers can be observed.

Fig

  Fig. III.3 -3 Zone of cracks of a) ZrO single-component coating with BNNS fillers (shown in rectangles)Characterization of as-exfoliated solution of three different BNNS aqueous solutionThe state of h-BN flakes was probed by TEM step-by-step during the exfoliation. Fig. III.3 -4a shows the TEM images corresponding to a thin single-crystallite of h-BN. The selective area electron diffraction (SAED) pattern exhibits bright hexagonal dots (inset), which are representative of single h-BN crystals. The Moire patterns in Fig. III.3 -4b indicate misorientation between h-BN layers induced by the exfoliation process around the c-axis. In the final colloidal solution, the BNNS has a two-dimensional structure with a thickness of a few atomic layers approaching to that of the graphene, as shown in Fig. III.3 -4c.

FigFig

  Fig. III.3 -4 the TEM image for different exfoliation states: a) single crystal, b) misorientation induced by the exfoliation treatment, and c) few-layered BNNS. The inset is the corresponding selected area electron diffraction.

Fig

  Fig. III.3 -5 The UV-Vis spectroscopy of BN-A, BN-B, BN-C as-exfoliated solution Table. 3 -1 Zeta potential and concentration of as-exfoliated solution from BN-A, BN-B and BN-C

Fig

  Fig. III.3 -8 shows the UV-Vis spectra of BN-C with different centrifuge time of 10 min, 20 min, 30 min, and 40min, which have the concentration measured by weighting and drying in the oven and the zeta potential of the four samples varying from 60 to 170 g/mL (see Table.3 -2). The concentration of the BNNS decreases with the centrifuge time. According to the concentration, the BNNS solutions are named BNC_Sol170, BNC_Sol130, BNC_Sol90, and BNC_Sol60, respectively. The spectra were obtained with the solutions diluted five times to avoid the saturation of the detector.

  Fig. III.3 -8 shows the UV-Vis spectra of BN-C with different centrifuge time of 10 min, 20 min, 30 min, and 40min, which have the concentration measured by weighting and drying in the oven and the zeta potential of the four samples varying from 60 to 170 g/mL (see Table.3 -2). The concentration of the BNNS decreases with the centrifuge time. According to the concentration, the BNNS solutions are named BNC_Sol170, BNC_Sol130, BNC_Sol90, and BNC_Sol60, respectively. The spectra were obtained with the solutions diluted five times to avoid the saturation of the detector.

Fig

  Fig. III.3 -8 The UV-Vis spectroscopy of BNC_Sol170, BNC_Sol130, BNC_Sol90 and BNC_Sol60 as-exfoliated solution

Fig

  Fig. III.3 -9shows the SEM image and statical contact angle on the ZrO coating charged with BN-C with different concentrations. All coatings show defect-free and homogeneous surfaces, while ZrO_BN170 and ZrO_BN130 have contact angles higher than 90°, indicating a surface hydrophobicity. This hydrophobicity is absent in ZrO_BN90 and ZrO_BN60, while these two coatings showed a slightly higher contact angle than the ZAH_Ref. It is thus understood that the high concentration of BNNS charged ZrO coating shows an increased contact angle until the coating becomes hydrophobic.

Fig

  Fig. III.3 -9 SEM Image and water contact angle of a) and c) ZrO_BN170, b) and d) ZrO_BN130, e) and g) ZrO_BN90 and f) and h) ZrO_BN60 III.3.2 Characterization of BNNS charged ZrO-SiO coatings on AA2017 Characterization of ZrO-SiO multi-component Sol-Gel solution The ZrO-SiO multicomponent coating has been synthesized and deposited on AA2017 to further study the role of BNNS inside the Sol-Gel coatings. Fig. III.3 -10 shows the image of

Fig. III. 3 -

 3 Fig. III.3 -10 Image of a) As-prepared Sol-Gel solution of ZrO-SiO and b) BNNS solution exfoliated in isopropanol DLS has been performed for Zirconium n-Butoxide + GPTMS suolution, as shown in Fig. III.3 -11.The size of the colloid is around 3 nm, and a small amount of agglomeration has an average size of 50 nm -60 nm. The agglomeration phenomenon is due to the hydrolysis process is favored in an acid solution, leading to a final structure of Sol-Gel solution with longer chains.12

FigFig

  Fig. III.3 -11 DLS of ZrO-SiO Sol-Gel solution after synthesis The chemical bonds in ZrO-SiO Sol-Gel coating are characterized with FT-IR spectra shown in Fig. III.3 -12. The two typical bands at 1591 and 1529cm -1 are the characteristic signals of Zr/ AcAc ligand.13 The bands at 1377 and 1279 cm-1 are attributed to the stretching vibration of C-O-Zr in this ligand.14

  Fig. III.3 -13, in which the inset table shows the concentration of BNNSs in isopropanol as well as the zeta potential of the colloidal solution. By weighting measurement, BNNS exfoliated in isopropanol showed a concentration around 112 g/mL. The absolute value of zeta potential is low, indicating the low stability of the colloidal solution.

Fig.

  Fig. III.3 -13 UV-Visible spectroscopy of BN-C exfoliated in isopropanol with inset table showing the parameters of the solution

Fig

  Fig. III.3 -14b, the coating surface is rougher. Meanwhile, the BNNS charged coating shows a bigger contact angle than the reference coating.

Fig. III. 3 -

 3 Fig. III.3 -14 SEM Image and water contact angle of a) and c) ZrO-SiO_Ref and b) and d) ZrO-SiO_BN III.3.3 Characterization of BNNS charged ZrO-SiO/C18 coatings on carbon steelCharacterization of ZrO-SiO/C18 organic inorganic hybrid Sol-Gel solutionThe ZrO-SiO/C18 hybrid Sol-Gel solution has been synthesized to study the role of BNNS in providing a protecting behavior for active surfaces. The ODTES with -C18 alkyl group has been introduced to increase the hydrophobicity of the coating on the coating surface and within the pores. The photo of ZrO-SiO/C18 hybrid solution is shown in Fig. III.3 -15a. The micelle is formed due to the presence of surfactant ODTES. After a heat-treated at 55°C, this issue can be resolved, and the solution became transparent. The BNNS n-Butanol solution is shown in Fig. III.3 -15b.

Fig. III. 3 -

 3 Fig. III.3 -16 DLS of ZrO-SiO/C18 hybrid Sol-Gel solution after synthesisThe thickness of ZrO-SiO/C18 organic inorganic hybrid coating can be measured from the SEM image shown in Fig.III.3 -17. The thickness of the reference coating is over 3 m. The higher thickness than ZrO-SiO multi-component coating is principally due to the introduction of C18 alkyle group from ODTES which allows a higher flexibility of the coating.

Fig. III. 3 -

 3 Fig. III.3 -17 Corroded region of ZrO-SiO/C18_Ref by a) SEM and b) Confocal Optical Microscopy Characterization of as-Exfoliated BNNS solution in n-Butanol The UV-Vis spectra of BN-C in n-Butanol is shown in Fig. III.3 -18, and the inset table shows the parameters of the solution. The characteristic peaks of h-BN can be observed at 210 nm, indicating the presence of BNNS. The Zeta potential test shows that the stability of BNNS in n-

Fig. III. 3 -

 3 Fig. III.3 -18 UV-Visible spectroscopy of BN-C exfoliated in n-Butanol with inset table showing the parameters of solutionThe surface morphology and photos of contact angle of ZrO-SiO_Ref and ZrO-SiO_BN are shown in Fig.III.3 -19. Both coatings showed a crack-free and homogeneous surface. The ZrO-SiO_Ref showed a contact angle of 87.27°, which is higher than the multi-component coating without ODTES. This is due to the introduction of ODTES with its apolar C-18 chain. Meanwhile, the BNNS charged coating showed a slightly increased hydrophobicity.

Fig. III. 3 -

 3 Fig. III.3 -19 SEM Image and watercontact angle of a) and c) ZrO-SiO/C18_Ref and b) and d) ZrO-SiO/C18_BN

  Fig. III.3 -20. It is shown that GENS presented a-few-layered structure, while the diffraction pattern can clearly show its hexagonal crystal structure.

Fig. III. 3 -

 3 Fig. III.3 -20 The a) TEM image of GENS exfoliated in water and b) the diffusion patternFig. III.3 -21 shows the UV-Vis of GENS exfoliated water solution. The peak at around 260 nmfor GENS exfoliated from GE in water are coherent to the result reported elsewhere.3 The zeta potential (inset table) is around -34.7 mV. However, the concentration of GENS is only 90 g/mL, which is relatively low comparing to other 2D fillers.

Fig. III. 3 -

 3 Fig. III.3 -21 UV-Visible spectroscopy of GE exfoliated in water with inset table showing the parameters of the solutionThe surface morphology and photos of the contact angle of ZrO_GE are shown in Fig. III.3 -22.The surface showed a homogeneous and defect-free morphology while the contact angle is 93.07° indicating a hydrophobic surface.

Fig. III. 3 -

 3 Fig. III.3 -22 a) SEM Image and b) water contact angle of ZrO_GE

  Vis spectroscopy of GE exfoliated in n-Butanol is shown in Fig. III.3 -23. The same characteristic peak at 260 nm of graphene oxide has been observed, proving the existence of GENS.

Fig

  Fig. III.3 -23 UV-Visible spectroscopy of GE exfoliated in n-Butanol with inset table showing the parameters of solutionThe SEM image of surface morphology as well as the photo of the contact angle of ZrO-SiO/C18_GE, is shown in Fig.III.3 -24. The coatings showed an inhomogeneous surface which is possibly due to the agglomeration of the high concentration GENS. It is also possibly due to the micelles of the ODTES due to the presence of ODTES. The surface showed a high contact angle around 92°, while this contact angle is slightly higher than the reference coating ZrO-SiO/C18_Ref.

Fig. III. 3 -

 3 Fig. III.3 -24 a) SEM Image and b) water contact angle of ZrO-SiO/C18_GE The Raman spectra of ZrO_SiO/C18_Ref and ZrO_SiO/C18_GE on carbon steels are shown in Fig. III.3 -25. The presence of GE are proved by the G-band at 1594 cm -1 and D-band at 1328

Fig. III. 3 -

 3 Fig. III.3 -27 a) SEM Image and b) water contact angle of ZrO_MoS2 Characterization of MDNS charged ZrO-SiO/C18 coating Fig. III.3 -28 shows the UV-Vis spectroscopy of MoS2 exfoliated in n-Butanol, a similar shaped curve can be observed as from the MoS2 exfoliated in water, indicating the presence of the

Fig. III. 3 -

 3 Fig. III.3 -28 UV-Visible spectroscopy of MoS2 exfoliated in n-Butanol with inset table showing the parameters of the solutionThe SEM image of surface morphology and photo of contact angle of ZrO-SiO/C18_MoS2 are shown in Fig.III.3 -29. The surface is defect-free and homogeneous, while the contact angle is slightly lower than the reference sample. This is possibly due to the high surface energy of MoS2 may give polarity to the coating surface and reduce the hydrophobicity of the coating.

Fig. III. 3 - 93 The

 393 Fig. III.3 -29 SEM Image and water contact angle of ZrO-SiO/C18_MoS2

Fig. III. 3 -

 3 Fig. III.3 -30 Raman shift of all ZrO-SiO/C18_Ref and ZrO-SiO/C18_MoS2 deposited carbon steel samples

  electrochemical characterizations were performed to understand the role of 2D materials on barrier effect in different coating systems. The electrochemical test performed in this work includes Open Circuit Potential (OCP) test, Potentiodynamic Polarization, electrochemical impedance spectroscopy (EIS) tests. All electrochemical tests were performed in NaCl 30g/L solution. The flow chart of the study is shown in Fig. IV.1 -1.

Fig. IV. 1 - 1

 11 Fig. IV.1 -1 Scheme of the research topics based on different systems

  The evolution of the open circuit potential (OCP) of bare 316L in NaCl 30 g/L for 120 hours is shown in Fig. IV.2 -1. The OCP increases from -14 mV/SCE after 16 hours of immersion to 73 mV/SCE for 120 hours of immersion. Between 40 to 60 hours, the OCP can be considered as stabilized, expressing a stable surface reactivity. 1 After 60 hours, the OCP increases again which depicts a stable evolution of the passible film with time.

Fig. IV. 2 - 1 99 Fig 100 Fig 2 )

 21991002 Fig. IV.2 -1 Open Circuit Potential evolution of 316L bare substrate in NaCl 30g/L over 120 hours (Arrow indicates the OCP value at 24 hours of immersion)Potentiodynamic polarization has been performed after 24 hours of immersion to characterize electrochemical behavior of the material. There is a specific reactivity for each potential. The polarization curves presented in Fig. IV.2 -2 inform about these reactions. As we can see, the corrosion potential Ecorr reaches 37 mV/SCE, which is consistent with the OCP value recorded at 24 hours of immersion, as presented in Fig. IV.2 -1. The passive state is confirmed by the low current density from Ecorr to 0.68 V/SCE. 2,3 After the potential of 0.68 V/SCE, the current density increases drastically due to the pitting propagation. Previously several current density transcient informed about metastable pits. 4 Fig. IV.2 -3 shows the surface morphology after the polarizations. The bare 316L is deeply corroded since the bulk dissolution of the metal can be observed. From the zoomed image, it is observed that the pitting propagates into the bulk metal.

  Fig. IV.2 -7. ZrO_Ref shows a nobler Ecorr. The barrier effect provided by zirconium oxide coating is efficient since the current density is significantly lower than the one of bare 316L substrate. No pit, neither current density increasing attest that the sol-gel coating act as a barrier. However, as shown in Fig. IV.2 -8, after the polarization, some corrosion products can still be observed on ZrO_Ref. This phenomenon can be explained by under deposit corrosion due to the porous microstructure of the sol-gel coating that allows the penetration of the corrosive electrolyte. This phenomenon of dissolution was too slow to be recorded during polarization but can appear during the storage after electrochemical measurement.

Fig. IV. 2 - 7 104 Fig. IV. 2 - 9

 2710429 Fig. IV.2 -7 Polarization curve of ZrO_Ref and 316L bare substrtate in NaCl 30g/L after 24 hours of immersion

  barrier effect in inorganic coatings has not been clearly explored yet. In this part, the understanding of the influence of BNNS on the barrier effect of ZrO coating is performed in the first place. Detailed elaboration parameters are already described in chapter 2. The BNNS with an average size of 4.5 m has been exfoliated into water solution and charged in ZrO coating.The evolution of OCP values inNaCl 30g/L for 24 hours immersion of the ZrO coating with or without charge compared to the bare 316L substrate are shown in Fig. IV.2 -11. After 6 hours of immersion, the OCP of ZrO_BN is stabilized.

Fig. IV. 2 -Fig. IV. 2 -

 22 Fig. IV.2 -11 Open Circuit Potential of ZrO_BN compared to the bare 316L substrate and ZrO_Ref for 24 hours of immersion in NaCl 30g/L solution Fig. IV.2 -12 shows the polarization curves of the bare 316L substrate, compared to the electrochemical behaviour of ZrO_Ref, and ZrO_BN. The BNNS charged coating exhibits higher Ecorr, which is consistent with the observation from OCP evolution. Both lower current density and nobler Ecorr demonstrate the improvement of the barrier effect of ZrO_BN compared with the bare 316L substrate.

Fig. IV. 2 -

 2 Fig. IV.2 -13 shows the surface morphology after the polarization. Compared with the deeply corroded bare 316L substrate, no dissolution of metal or stable pit can be observed from ZrO_BN. Nevertheless, small spots of corrosion products can be observed on the surface.

Fig. IV. 2 -

 2 Fig. IV.2 -13 SEM image of a) morphology of ZrO_BN after polarization and b) Zoomed corrosion product In parallel, the EIS tests after 24 hours of immersion in 30 g/ml NaCl solution have been performed to ZrO_BN as well. The diagram of the bare 316L substrate and ZrO_Ref are superposed (see Fig. IV.2 -14). ZrO_BN shows a clear semicircle loop with a diameter larger than both references, indicating a better corrosion resistance. Since the BNNS charged nanocomposite coating is inert at OCP, it is appropriate to use the same EEC of the ZrO_Ref and bare 316L substrate to simulate the electrochemical behavior, as shown in Fig. IV.2 -15. The numerical fitting results are listed in Table4-3. With the addition of BNNS in the sol-gel coating, the Rpore of the coating increases to 2.08 x 10 6 Ohm.cm 2 , which is twice of the value of ZrO_Ref (9.76 x 10 5 ohm.cm 2 .). The Rct increase to 3.38 x 10 6 Ohm.cm 2 that is 20 times higher than the pure sol-gel coating. The high Rfilm is generally attributed to the barrier effect of the non conductive matrix and the tortured connection pathway for the ions exchange induced by impermeable BNNS. Meanwhile, the high Rct indicates low reaction rates at the metal surface. The value dl indicates diffusion phenomena in the pores.

Fig. IV. 2 -

 2 Fig. IV.2 -14 Nyquist curve of ZrO_BN with fitting and superposed with bare 316L substrate and ZrO_Ref

  Open Circuit Potential (OCP) values of BNNS charged samples in NaCl solution (3.5 wt%) are shown in Fig. IV.2 -16. All coated samples show stabilized OCP after 6 hours of immersion. The OCP Values of all three samples are in the range of 80 mV, which could be considered as the same values. Among them, ZrO_BNA displays the lowest potential.

Fig. IV. 2 -

 2 Fig. IV.2 -16 OCP of three different BNNS charged ZrO coated samples in 24 hours of immersion in NaCl 30g/L solution From the polarization curves of the nanocomposite coatings shown in Fig. IV.2 -17, Different samples showed similar electrochemical behaviors. The coating limits the reactivity of the substrate significantly. Nevertheless, some differences in the behavior can be discussed.

  Fig. IV.2 -17 Electrochemical dynamic polarization curves in NaCl 30g/L of all BNNS charged samples Fig. IV.2 -18 shows the surface morphology of both ZrO_BNB and ZrO_BNC after the polarizations. ZrO_BNC shows much less under deposit corrosion than ZrO_BNB, and ZrO_BNA (see Fig. 4.3 -3). Both polarization curves and SEM images reveal the fact that ZrO_BNC shows the best corrosion resistance.

Fig.

  Fig. IV.2 -18 SEM image of the surface morphology obtained with a) ZrO_BNB and b) zoom on the corrosion product of ZrO_BNB , c) ZrO_BNC and d) zoom on corrosion product of ZrO_BNC

1 ) 2 )

 12 Without nanofiller, the electrolyte penetrates via sol-gel coating's interparticle and intercluster pores to the substrate. The way of penetration depends on the microstructure of the Sol-Gel matrix and its affinity to the corrosive reactants, as shown in Fig. IV.2 -19a. When the BNNS average size, d < dc, the nanofiller is too small to seal totally the coating's pores. Consequently, the electrochemical behavior of the coated system is mainly characterized by Sol-Gel matrix properties, as shown in Fig. IV.2 -19b. 3) When d ∼ dc, the BNNS with a bigger size seals the pores inside the coating and reduce the oxidation rate, as shown in Fig. IV.2 -19c. 4) When d > dc, the penetration pathway is furtherly prolongated. The migration of the material has to take the tortuous way to reach the metal surface. 14,15 At the same time, the system's resistance is predominated by the mass transfer process, as shown in Fig. IV.2 -19d.

Fig. IV. 2 -

 2 Fig. IV.2 -19 Scheme of the 2D fillers in sol-gel coating system: a) pure porous Sol-Gel matrix without filler, b) filler with size d < dc, c) filler with size d ∼ dc, d) filler with size d > dc

Fig. IV. 2 - 20 .

 220 At high frequency, all samples behave similarly. The High-frequency behavior characterizes the capacitance properties at the electrolyte/coating interface. At low frequency, ZrO_BNC shows the biggest diameter of the semicircle loop, meaning the most promising corrosion resistance which is in good agreement with the polarization. ZrO_BNC showed slightly better EIS performance than ZrO_BNA at OCP, which is coherent to the observation from the polarization curve shown in Fig.IV.2 -17.

Fig. IV. 2 -

 2 Fig. IV.2 -20 the Nyquist curve of ZrO coated samples with three different BNNS fillers after 24 hours immersion in NaCl 30g/L at OCP with a) full range of frequency b) zoomed area at high frequency The evolution of ZrO_BNC nanocomposite coating system was studied by longterm EIS analysis. The Nyquist and Bode plots of ZrO_BNC over immersion time is depicted in Fig. IV.2 -21. The first observation from Nyquist curve, shown in Fig. IV.2 -21a, reveals a constantly increasing trend of the semicircle loop over time (black arrow). This variation indicates a significant enhancement of resistance, which is unlike what happened to the 316L substrate (see Fig. 4.1 -5) and the ZrO_Ref (see Fig. 4.1 -12). For the laters, the semicircle loop was reduced or remained stable over immersion time. Fig. IV.2 -21b shows the bode curves obtained with ZrO_BNC. Two time constants can be observed, among which one stands for the resistance and capacitance of coating and film, while the other one stands for the charge transfer resistance and double layer capacitance. It can be observed that the middle frequency time constant is stable and has a maximum phase

Fig. 4 . 1 - 7 .

 417 The fitting parameters of ZrO_BNC are shown in Annexe 1 in detail. Fig. IV.2 -22 shows the evolution trend of Rfilm and Rct for the bare 316L substrate, ZrO_Ref, and ZrO_BNC during 120h of immersion. As discussed in the reference part (IV.1.1 and IV.1.2), the overall resistance (Rfilm + Rct) of 316L decreased due to the chloride ions contamination. With the deterioration in the passive film, the corrosion resistance decreases simultaneously.

18

 18

Fig. IV. 2 -

 2 Fig. IV.2 -22 The evolution of EIS fitting parameter value over 120 hours of the three samples a) Rfilm or Rpore, b) Rct

Fig. IV. 2 -

 2 Fig. IV.2 -23 The evolution of ZAH_BNC of a) coat and dl values and b) Rpore and Rct values over 1200h

  Fig. IV.2 -24a plots the Nyquist curves after 24 hours of immersion in 3.5w% NaCl solution. Compared with the Nyquist plot of ZrO_Ref shown in Fig. 4.2 -6, ZrO_BN60, and ZrO_BN90 show smaller semicircle loops. With the increase of BNNS concentration, there is a noticeable broadening in the semicircle loop. ZrO_BN170 shows a capacitive behavior and its high frequency behavior are shown as zoomed from in Fig. IV.2 -24b. With the continually increasing concentration of BNNS, as observed from Fig. IV.2 -24a, the capacitive behavior increases. Finally the Faradic process is replaced by a total capacitance behavior for ZrO_BN170, which provides a substantial impermeability.

Fig. IV. 2 -Fig. IV. 2 -

 22 Fig. IV.2 -24 Nyquist plots at OCP at 24 hours of immersion in NaCl solution of a) ZrO coatings charged with different BNNS b) Zoomed region at high frequency for ZrO_BN170 The Bode curves of coatings are superposed in Fig. IV.2 -25. The corresponding EEC of all samples are shown in Fig. IV.2 -26. We used the same EEC for ZrO_BN60, ZrO_BN90, ZrO_BN130 as for the reference samples. In Fig. IV.2 -26b, the EEC represents for ZrO_BN170 with a CPE due to its capacitive behavior from the whole coating system. The strong capacitive behavior of ZrO_BN170, represented by Qsys, blocks the detection of the signal from the coated sample, which limits the selection of the EEC. The fitting parameters of ZrO_BN60, ZrO_90 and ZrO_130 at 24 hours of immersion are given in Table 4 -5.

Fig. IV. 2 -

 2 Fig. IV.2 -26 EEC for a) ZrO_BN60, ZrO_BN90, ZrO_BN130 and b) ZrO_BN170

Fig. IV. 2 -

 2 Fig. IV.2 -27 plots the fitting parameters for different coatings with increasing BNNS concentration.ZrO_BNC has much higher Rpore and Rct values than ZrO_BN60, ZrO_BN90, ZrO_BN130. ZrO_BN60 showed a low Rpore and Rct though the concentration are much higher than the one of ZrO_BNC. It is logical that the aggregation of the BNNS reduces the nanocomposite coating's resistance in the pores since more defects will be formed.25,26 With the concentration increase, the Rct kept increase due to less corrosion, which indicates the predominant influence of BNNS has switched from defects to barrier effect. Considering ZrO_BN170, the impermeable film had been formed, no charge transfer were observable.

Fig. IV. 2 -

 2 Fig. IV.2 -27 Comparison of fitting parameters between ZrO_BNC and ZrO_BN60, ZrO_BN90, ZrO_BN130, ZrO_BN170 for a) Rpore and Rct and b) coat and dl Table 4 -5 Fitting parameters of ZrO_BN60, ZrO_BN90 and ZrO_BN130 at 24 hours of immersion

  Fig. IV.2 -27.

Fig. IV. 2 -

 2 Fig. IV.2 -29 Model of the porous structure with BNNS filler a) with increased BNNS concentration but no impermeable film b)with an impermeable film The evolution of ZrO_BN170 immersed in NaCl 3.5wt% in longterm duration has been studied. The Nyquist curves of the ZrO_BN170 from 12 hours of immersion to 8 weeks of immersion are shown in Fig. IV.2 -30. From Fig. IV.2 -30a, the Nyquist curves showed a slight decrease of the capacitive due to distributions, porosity effects or fractal geometry effects (as explained in Chapter II -II.3.5)29,30 , which indicates a decrease of capacitive behavior (red arrow). To discuss the EIS behavior in detail, the analysis of Bode curves are separated into two parts before and after one week of immersion.

Fig. IV. 2 - 2 ) 2 )

 222 Fig. IV.2 -30 Nyquist curve of ZrO_BN170 from 3 weeks of immersion till 8 weeks of immersion (With red arrow indicating the decreasing trend of slope of capacitive region) The bode curves of evolution in one week are shown in Fig. IV.2 -31. Both have shown little evolution over time, indicating the stability of the coating in one week of immersion.

Fig. IV. 3 - 1 34 - 1 Fig

 31341 Fig. IV.3 -1 Potentiodynamic Polarization curves of AA2017 substrate after immersed in NaCl 30g/L solution for 24 hoursThe Bode curves of the AA2017 substrate after 6h or 144h of immersion in NaCl 30g/L are shown in Fig. IV.3 -2. It is observed that AA2017 substrate shows an increasing impedance during immersion under OCP due to the growth of oxide products. The impedance value reaches 4 x 10 4 Ohm.cm 2 at 3 mHz at 144h of immersion. Two time constants can be observed, which are correlated to the oxide film property and charge transfer on the metal surface.34

Fig. IV. 3 - 3

 33 Fig. IV.3 -3 Bode a) Phase and b) Impedance curves of comparison between AA2017 substrate and AA/ZrO_BNC at 6 hours of immersion and 144 hours of immersion in NaCl 30g/L solution

Fig. IV. 3 - 4

 34 Fig. IV.3 -4 Potentiodynamic Polarization curves of AA2017 substrate and ZrO-SiO_Ref after 24 hours of immersion in NaCl 30g/L The bode curves of ZrO-SiO_Ref evolution after one week of immersion in NaCl 30g/L solution are shown in Fig. IV.3 -5. From Fig. IV.3 -5a, the evolution of the Bode curve showed a decrease trend at middle and high frequency (see red arrow). This trend is connected to the deterioration of the protective behavior provided by the coating and the oxide film on the substrate. 37 At high frequency, f = 10 5 Hz, the phase angle initially was -70° at 6 hours of immersion and finally decreased to -40° after one week of immersion. The direction of degradation is shown with the red arrow on top right in Fig. IV.3 -5b. The first time constant observed from high frequency is attributed to the coating's properties. Its degradation is a sign of the increase in

  Fig. IV.3 -5b.

Fig. IV. 3 - 5

 35 Fig. IV.3 -5 Bode a) Impedance curves and b) Phase curves of ZrO-SiO_Ref in 1 weeks of immersion in NaCl 30g/L solution As discussed above, the corrosion protection behavior of the coating is still low. The surface morphology of ZrO-SiO_Ref after one week of immersion has been shown in Fig. IV.3 -6. It is observed that the under deposit corrosion cause small areas of delamination and weight loss (as shown with white arrows and zoomed region).

Fig. IV. 3 - 6

 36 Fig. IV.3 -6 Surface morphology of ZrO-SiO_Ref after 1 weeks of immersion (White arrows indicating the delamination, red rectangle indicating zoomed region of corroded site) IV.3.4 Influence of BNNS charged in ZrO-SiO/AA2017 The polarization curves of the coating charged with BNNS superposed with AA2017 substrate and ZrO-SiO_Ref are shown in Fig. IV.3 -7. ZrO-SiO_BN showed a polarization curve quite similar to that of the ZrO-SiO_Ref. The current density of ZrO-SiO_BN is two orders of magnitude lower than the AA2017 substrate, evidencing the protection effect. The lower Ecorr showed from ZrO-SiO_BN may be explained by a lateral propagation of the under-deposit corrosion. Indeed, with the zeta potential of -11.3 mV indicating a low stability of BNNS as shown in Fig. III.3 -12 in Chapter III, the agglomeration of BNNS is inevitable. The presence of agglomerated BNNS will decrease the adhesion property of the coating to the substrate and cause lateral propagation of corrosion until the delamination of the coating.

Fig. IV. 3 - 7

 37 Fig. IV.3 -7 Potentiodynamic polarization curve for ZrO-SiO_BN superposed with AA2017 substrate and ZrO-SiO_Ref

  of ZrO-SiO_BN throughout two weeks of immersion are shown in Fig. IV.3 -8.

  Fig. IV.3 -8b, from the highfrequency time constant shifted to the higher frequency, which is shown by the red arrow on top of the figure. The time constant corresponding to the oxide layer showed an increasing capacitive behavior at middle frequency, as shown in the red rectangle in Fig. IV.3 -8b. This indicates the regeneration of oxide film due to the corrosion. At low frequency, the phase angle also increased, meaning decreased charge transfer reaction rate thanks to the oxide film regenerated on the metal surface, as shown by the red arrow on the left side of Fig. IV.3 -8b.

Fig. IV. 3 - 8

 38 Fig. IV.3 -8 Bode a) Impedance curves and b) Phase curves of ZrO-SiO_BN in 2 weeks of immersion in NaCl 30g/L solution According to Bode curves, after two weeks of immersion, the ZrO-SiO_BN showed similar behavior to the bare AA2017 substrate. The surface morphology of ZrO-SiO_BN after 2 weeks of immersion displays a lot of corrosion product see Fig. IV.3 -9.

133 Fig. IV. 3 - 9

 13339 Fig. IV.3 -9 The SEM image of surface morphology after 2 weeks of immersion in NaCl of ZrO-SiO_BN

  of carbon steel is shown in Fig. IV.4 -1. The Ecorr is lower than the value observed at OCP. From the anodic part, the current density increases steeply, which corresponds to a general corrosion phenomenon on the metal.

Fig. IV. 4 - 1 136 The

 41136 Fig. IV.4 -1 Polarization curve of carbon steel at 24 hours of immersion

Fig. IV. 4 - 2

 42 Fig. IV.4 -2 Bode curves a) Impedance valuee b) Phase curve of carbon steel bare substrate after 20 hours of Immersion in NaCl 30g/L solution IV.4.2 Effect of Organic-Inorganic hybrid coating on carbon steel

Fig. IV. 4 - 3

 43 Fig. IV.4 -3 Polarization curve of carbon steel and ZrO-SiO/C18_Ref at 24 hours of immersion The SEM image of the surface morphology of ZrO-SiO/C18_Ref after polarization is shown in Fig. IV.4 -4a. The coating is gravely delaminated. From Fig. IV.4 -4b, large quantity corrosion products can be observed on the delaminated region and under deposit. It seems that the propagation of the damage continues preferentially on the coating/ substrate interface and cause the coating failure.

Fig

  Fig. IV.4 -4 SEM image of a) surface morphology of ZrO-SiO/C18_Ref after polarization and b) zoomed region

Fig. IV. 4 - 5

 45 Fig. IV.4 -5 Nyquist curve of ZrO-SiO/C18_Ref in 20 hours of immersion in NaCl 30g/L solution. The Bode curves ZrO-SiO/C18_Ref are shown in Fig. IV.4 -6. From Fig. IV.4 -6a, the impedance value of coating without BNNS nanofiller at 3 mHz reduced about one order of magnitude in 20 hours. From Fig. IV.4 -6b, two time constants can be observed. One time constant is at high frequency (10 2 ~ 10 5 Hz) representing coating property. The shift of this time constant toward high frequency regime indicates the deterioration of barrier property of the coating. The other time constant is the one at low frequency around 10 -1 Hz, which characterizes the charge transfer and the capacity properties of the coating/ substrate interface. This low frequency time constant stays stable over time.

Fig. IV. 4 -

 4 Fig. IV.4 -6 a) Bode Impedance curves and b) Bode Phase curves of ZrO-SiO/C18_Ref with fitting in 20 hours of immersion inNaCl 30g/L solution.

  ZrO-SiO/C18_BN is superposed with both reference samples in Fig. IV.4 -7.

Fig. IV. 4 - 7

 47 Fig. IV.4 -7 Polarization curve of ZrO-SiO/C18_BN, ZrO-SiO/C18_Ref and carbon steel substrate at 24 hours of immersion The SEM image of the ZrO-SiO/C18_BN after the polarization is shown in Fig. IV.4 -8. There is neither obvious damage to the coating nor oxidation product that can be observed, proving the strong protecting behavior observed from the polarization curve up to 1.5 V vs. SCE. No delamination has been observed indicating the barrier effect of the matrix is largely enhanced by BNNS.

Fig. IV. 4 - 8

 48 Fig. IV.4 -8 SEM image of surface morphology of ZrO-SiO/C18_BN after polarization Nyquist curves of ZrO-SiO/C18_BN in 20 hours of immersion in NaCl 30g/L solution are plotted for a quick observation. The asymmetric semicircle loop of ZrO-SiO/C18_BN indicating the presence of more than one time constant. The semicircle loop is of large diameter at immersion of 1 hour and decreases to 2.5 x10 6 Ohm.cm 2 .

Fig. IV. 4 - 9

 49 Fig. IV.4 -9 Nyquist curves of ZrO-SiO/C18_BN during 20 hours of immersion in NaCl 30g/L solution The Bode curves of CS_BN are shown in Fig. IV.4 -10 with fitting results. The Bode impedance curve from Fig. IV.4 -10a showed the protecting behavior of the ZrO-SiO/C18_BN coating varies slightly over time, and the highest impedance value reached 10 7 Ohm.cm 2 range. From Fig. IV.4 -10b, two clear time constants are observed.The time constant at high frequency indicates the strong and stable barrier property from the coating to the migration of ions with a phase angle close to 90° over time.45

142 Fig. IV. 4 -

 1424 Fig. IV.4 -10 a) Bode Impedance curves and b) Bode Phase curves, for ZrO-SiO/C18_BN during 20 hours of immersion

The

  Open Circuit Potential (OCP) evolution of ZrO_GE during five weeks of immersion has been plotted in Fig. IV.5 -1. It is observed that the OCP was initially stable at around 65 mV vs. SCE, then stablized at 70 mV vs. SCE.

Fig. IV. 5 - 1

 51 Fig. IV.5 -1 OCP evolution of ZrO_GE during 5 weeks of immersion The EIS Bode curves are shown in Fig. IV.5 -2 with fittings, and the EEC for fitting is shown in Fig. IV.5 -3. The EEC is the same as the one of ZAH_Ref, with one Rs representing solution resistance, one Rpore/Qcoat representing the resistance, and CPE for capacitance from the pores in the coating and from the passive film,the Rct/Qdl standing for the charge transfer resistance, and CPE for double layer capacitance. From Fig. IV.5 -2a, the impedance value of ZrO_GE stayed unvaried over five weeks of immersion. The impedance value reached 4 x 10 6 Ohm.cm 2 at the lowest frequency. This value is higher than the one reached by ZrO_BNC in long term duration, see IV.2.5 -Fig. IV.2 -24.

  As shown in Fig.IV.5 -2b with the dashed line, the lowest phase angle of the middle frequency of ZrO_GE is around 87° that is 5° higher than the one observed for ZrO_BNC from Fig. IV.2 -24. This means a stronger corrosion resistance provided by the coating enhanced passive film. Moreover, the phase angle at lowest frequency decreased from -50° to -70°. This means that the time constant at low frequency shifted to a lower frequency side. This time constant represents the charge transfer process, and this shifting indicates that the initiation of charge transfer reaction is delayed. The charge transfer is more and more discouraged with the time evolving. The shifting phenomenon of low frequency time constant was also observed from the sample ZrO_BNC during long term EIS test, which is shown in Fig. IV.2 -24. However, the shifting of ZrO_GE is stronger than ZrO_BNC, indicating a more tortured pathway provided by GENS than by BNNS.

FigFig

  Fig. IV.5 -2 a) Bode Impedance curves and b) Phase curves of ZrO_GE in 5 weeks of immersion in NaCl 30g/L water solution

Fig. IV. 5 - 2 )

 52 Fig. IV.5 -4 a) Nyquist curve of ZrO_GE and b)SEM image of surface morphology of ZrO_GE after 5 weeks of immersionThe MDNS was exfoliated in water and charged in ZrO coating and deposited on 316L. The MoS2 showed a concentration of 150 mg/mL which is similar to BNNS in ZrO_BN170 but a low stability. The defect free coating showed no observable improvement in hydrophobicity comparing to ZrO_Ref. To further understand the function of MDNS in ZrO coating, the electrochemical tests were performed.The evolution of OCP values of ZrO_MoS2 immersed in NaCl 30g/L in 24 hours were shown in Fig.IV.5 -5. The OCP kept increasing over time indicating an unstable surface condition.

Fig. IV. 5 - 6 Fig

 56 Fig. IV.5 -6 Potentiodynamic polarization curves of ZrO_MoS2 superposed 316L_Sub after 24 hours of immersion in NaCl solution The Bode curves of ZrO_MoS2 coating immersed in 72 hours are shown in Fig. IV.5 -7. Observing from Bode Impedance curve in Fig. IV.5 -7a, the impedance value at highest frequency is around 10 5 Ohm.cm 2 which is slightly higher than 316L substrate, indicating a limited protection behavior. The degradation of the curves is minor, indicating a stable surface condition over 72 hours of immersion. The phase curves shown in Fig. IV.5 -7b showed one extra time constant at frequency between 10 2 Hz to 10 3 Hz, which will be discussed in later parts. At middle and low frequency, two time constants can be observed. The time constant at middle frequency represents the passive film while the time constant at low frequency represents the charge transfer and double layer capacitance.

Fig. IV. 5 - 8

 58 Fig. IV.5 -8 SEM images of surface morphology of ZrO_MoS2 after 24 hours of immersion in NaCl solutionsThe Nyquist curve and the zoomed region at high frequency are presented in Fig.IV.5 -9. The Nyquist curve showed a low semicircle loop indicating an inadequate corrosion resistance offered by the coating. At high frequency, a 45° slope can be observed at the beginning of all curves and its significance decreases over time.

Fig. IV. 5 -

 5 Fig. IV.5 -9 a) Nyquist curve and b) zoomed region at high frequency of EIS tests of ZrO_MoS2

Fig. IV. 5 -Fig

 5 Fig. IV.5 -10 Polarization curve of ZrO-SiO/C18_GE surperposed with reference samples after 24 hours immersed in NaCl 30g/L The SEM image of ZrO-SiO/C18_GE surface morphology after the polarization is shown in Fig. IV.5 -11. The surface shows no cracks or delamination or oxide product.

Fig

  Fig. IV.5 -12 Nyquist curves of ZrO-SiO/C18_GE immersed in NaCl 30g/L solution in 20 hours The Bode curves are shown in Fig. IV.5 -13. From Bode impedance curve from Fig. IV.5 -13a, the impedance value at lowest frequency almost reached 7 x 10 7Ohm.cm 2 and decreased to 2 x 10 7 Ohm.cm 2 after 20 hours of immersion. The corrosion happened under coating, but the corrosion rate is really low, which is similar to the case of coating ZrO-SiO/C18_BN. This protection is better than the active protection by passive film from the coatings deposited 316L. Thus, it could be a perspective that the active protection fillers can be introduced for the coating matrix.

Fig

  Fig. IV.5 -13 a) Bode Impedance curves and b) Phase curves of ZrO-SiO/C18_GE immersed in NaCl 30g/L solution

  curves of ZrO-SiO/C18_MoS2 is shown in Fig.IV.5 -14. The MDNS modified coating showed the same Ecorr comparing to the ZrO-SiO/C18_Ref, indicating less barrier effect offered from the 2D material to reduce the chance of the corrosive molecules to get in touch with the metal surface. The GENS had introduced barrier effect to reduce the penetration of corrosive molecules.

Fig. IV. 5 -

 5 Fig. IV.5 -14 Potentiodyanamic Polarization curve of ZrO-SiO/C18_MoS2 after 24 hours of immersion in NaCl 30g/L solution superposed with reference samples

Fig. IV. 5 -

 5 Fig. IV.5 -15 SEM image of surface morphology of ZrO-SiO/C18_MoS2 after polarization Nyquist curves of ZrO-SiO/C18_MoS2 immersed in NaCl 30g/L solution in 20 hours are shown in Fig. IV.5 -16. With a quick observation, it is understood that the semicircle loop is higher

Fig. IV. 5 -

 5 Fig. IV.5 -16 Nyquist curves of ZrO-SiO/C18_MoS2 immersed in NaCl 30g/L solution in 20 hours The Bode curves are shown in Fig. IV.5 -17. From the Bode Impedance curve from Fig. IV.5 -17a, it is observed that the impedance value at low frequency are of the same order of magnitude comparing to ZrO-SiO/C18_Ref but without the perturbation of the signals. This indicates the surface condition is more stable comparing to the ZrO-SiO/C18_Ref. However, the improvement in corrosion resistance is limited.From Bode phase curves shown in Fig.IV.5 -17b, the curves are flat at middle, indicating less heavy corrosion reaction on the metal surface with low charge transfer rate. The high frequency time constant stayed stable as well as the low frequency time constant, indicating the coating are not strongly evoluted due to less corrosion phenomenon comparing to the reference sample. The ZrO-SiO/C18_MoS2 showed a higher stability in time comparing to ZrO-SiO/C18_Ref. However, comparing to both ZrO-SiO/C18_BN and ZrO-SiO/C18_GE, ZrO-SiO/C18_MoS2 does not introduce much protective behavior though the concentration of MoS2 is the highest amongst all.

159 Fig. IV. 5 -

 1595 Fig. IV.5 -17 Bode a) Impedance curves and b) Phase curves of EIS tests of ZrO-SiO/C18_MoS2

Fig. I. 2 - 1

 21 Fig. I.2 -1 Schemes of a) mechanism of corrosion and b) mechanism of cathodic protection18 Fig. I.2 -2 Scheme of mechanism of general corrosion from a) side view b) top view ........... Fig. I.2 -3 Typical Electrochemical Dynamic Polarization curve of a passive material ........... Fig. I.2 -4 Distribution of potential in aqueous media of frequently used metals or alloys in industry 13 .................................................................................................................................. Fig. I.2 -5 Mechanism of break down of passive film by the presence of Cl -and introduced pitting corrosion 31 .................................................................................................................... Fig. I.2 -6 SEM images of porous structure synthesized by dealloying-redealloying method done by Zhu et al with different magnifications a) 40 b) 300 c) 3000 d) 30,000 16 .................. Fig. I.2 -7 Mechanism of intergranular corrosion 18 ................................................................. Fig. I.2 -8 Scheme of mechanism of pitting corrosion 20 .......................................................... Fig. I.2 -9 Mechanism of Zn particles charged sacrificial coating 55 ......................................... Fig. I.3 -1 The applications of Sol-Gel process ........................................................................ Fig. I.3 -2 Scheme of a) -c) Sol-Gel reaction as well as d) -k) solidification procedure for applications .............................................................................................................................. Fig. I.3 -3 Four different Zirconium based precursors.............................................................. Fig. I.3 -4 Influence of pH to the microstructure a) the figure of the reaction speed of hydrolysis and condensation influenced by pH b) particles size influence by pH c) the shape of the final product influenced by pH (reform from the work of Pandey et el.) 83, ................ Fig. I.3 -5 Mechanism of chelating agent blocking precursor active sites for Zr-based precursor and diketone 103 ........................................................................................................ Fig. I.3 -6 The scheme of a) dip coating b) spin coating and c) electrochemical deposition .. Fig. I.4 -1 Scheme of the barrier effect of a) and b) inorganic coating, c) hybrid coating and d) and e) organic coating .............................................................................................................. Fig. I.4 -2 Incorporation of Inorganic and organic compound by the use of GPTMS 81 ........... Fig. I.4 -3 Classification of hybrid coatings with the x-axes showing the ratio of organic compound in the coatings ........................................................................................................ Fig. I.4 -4 Names and structures of the most frequently used inorganic Si-based precursors; Frequently used hybrid organic-inorganic Si based precursors and the Si-based precursor used in this work ...................................................................................................................... Fig. I.5 -1 a) The scheme of covalent bond by Sol-Gel coating deposited on metal surface b) The role of Sol-Gel coating in a multi-coating corrosion protection system b) The surface modification by Phosphate acid on Mg surface for better adhesion properties .................... Fig. I.5 -2 The Scheme of a) surface tension, b) Superhydrophobic coating and the images of contact angle for c) hydrophilic surface, d) hydrophobic surface and e) superhydrophobic surface ...................................................................................................................................... Fig. I.5 -3 The pie chart of the distribution of the applicated materials of class II coatings 81

Fig. II. 3 - 1

 31 Fig. II.3 -1 Scheme of preparation procedure for ZrO Sol ....................................................... Fig. II.3 -2 Scheme of the AcAc blocking active sites on Zirconium n-Butoxide and the precursors of ZrO-SiO ............................................................................................................... Fig. II.3 -3 Scheme of preparation procedure for ZrO-SiO Sol ................................................ Fig. II.3 -4 Scheme of preparation procedure for ZrO-SiO/C18 Sol ......................................... Fig. II.4 -1 Dip-coating machine connected with heat container and heat generator ............ Fig. II.5 -1 Image of a) Ultrasonic machine for exfoliation b) Centrifuge machine ................. Fig. II.6 -1 Scheme and image of the electrochemical cell ...................................................... Fig. II.6 -2 Scheme of potential difference between different reference electrodes ............. Fig. II.6 -3 Nyquist curve from Electrochemical Impedance Spectroscopy ............................. Fig. II.6 -4 Bode curves a) Magnitude of Impedance b) phase angle ...................................... Fig. II.6 -5 Electrochemical Equivalent Circuit of a) Randles model b) CPE modified Randles model ........................................................................................................................................

Fig. III. 2 - 1

 21 Fig. III.2 -1 Morphology of raw BN powders of a) BN-A b) BN-B, and c) BN-C ....................... Fig. III.2 -2 a) The photo of Graphite foil before electrochemical expanding b) and after .... Fig. III.2 -3 Morphology of MoS2 by SEM ................................................................................ Fig. III.2 -4 Photos and SEM images of as-polished metal substates of a) and b) 316L, c) and d) AA2017, e) and f) carbon Steel XC90 bare substrates ......................................................... Fig. III.3 -1 Photo of a) ZrO Sol-Gel solution, b) BN-A, BN-B, and BN-C as exfoliated water solution with low concentration preparation method and c) BNC_Sol60, BNC_Sol90, BNC_Sol130 and BNC_Sol170 as-exfoliated water solution with high concentration preparation method d) the mixed ZrO and BNNS/water solution with ratio 3: 1 ................... Fig. III.3 -2 The DLS spectroscopy of three days evolution of ZrO Sol-Gel solution ................ Fig. III.3 -3 Zone of cracks of a) ZrO single-component coating with BNNS fillers (shown in rectangles) ................................................................................................................................ Fig. III.3 -4 the TEM image for different exfoliation states: a) single crystal, b) misorientation induced by the exfoliation treatment, and c) few-layered BNNS. The inset is the corresponding selected area electron diffraction. ................................................................... Fig. III.3 -5 The UV-Vis spectroscopy of BN-A, BN-B, BN-C as-exfoliated solution .................. Fig. III.3 -6 SEM Image and water contact angle of a) and c) ZrO_Ref, b) and d) ZrO_BNA, e) and g) ZrO_BNB and f) and h) ZrO_BNC ................................................................................... Fig. III.3 -7 The FTIR/ATR spectroscopy of ZrO deposited coatings with or without three BNNSs fillers ............................................................................................................................. Fig. III.3 -8 The UV-Vis spectroscopy of BNC_Sol170, BNC_Sol130, BNC_Sol90 and BNC_Sol60 as-exfoliated solution ............................................................................................................... Fig. III.3 -9 SEM Image and water contact angle of a) and c) ZrO_BN170, b) and d) ZrO_BN130, e) and g) ZrO_BN90 and f) and h) ZrO_BN60 ...................................................... Fig. III.3 -10 Image of a) As-prepared Sol-Gel solution of ZrO-SiO and b) BNNS solution exfoliated in isopropanol.......................................................................................................... Fig. III.3 -11 DLS of ZrO-SiO Sol-Gel solution after synthesis...................................................

Fig. III. 3 -

 3 Fig. III.3 -12 a) FTIR spectroscopy of ZrO-SiO reference coating deposited on AA2017 ......... Fig. III.3 -13 UV-Visible spectroscopy of BN-C exfoliated in isopropanol with inset table showing the parameters of the solution .................................................................................. Fig. III.3 -14 SEM Image and water contact angle of a) and c) ZrO-SiO_Ref and b) and d) ZrO-SiO_BN ...................................................................................................................................... Fig. III.3 -15 Photo of a) ZrO-SiO/C18 hybrid Sol-Gel solution and b) BNNS exfoliated n-Butanol solution ....................................................................................................................... Fig. III.3 -16 DLS of ZrO-SiO/C18 hybrid Sol-Gel solution after synthesis ............................... Fig. III.3 -17 Corroded region of ZrO-SiO/C18_Ref by a) SEM and b) Confocal Optical Microscopy ............................................................................................................................... Fig. III.3 -18 UV-Visible spectroscopy of BN-C exfoliated in n-Butanol with inset table showing the parameters of solution ........................................................................................ Fig. III.3 -19 SEM Image and watercontact angle of a) and c) ZrO-SiO/C18_Ref and b) and d) ZrO-SiO/C18_BN ....................................................................................................................... Fig. III.3 -20 The a) TEM image of GENS exfoliated in water and b) the diffusion pattern ..... Fig. III.3 -21 UV-Visible spectroscopy of GE exfoliated in water with inset table showing the parameters of the solution ...................................................................................................... Fig. III.3 -22 a) SEM Image and b) water contact angle of ZrO_GE ......................................... Fig. III.3 -23 UV-Visible spectroscopy of GE exfoliated in n-Butanol with inset table showing the parameters of solution ...................................................................................................... Fig. III.3 -24 a) SEM Image and b) water contact angle of ZrO-SiO/C18_GE ........................... Fig. III.3 -25 Raman shift of all ZrO-SiO/C18_Ref and ZrO-SiO/C18_GE deposited carbon steel samples ..................................................................................................................................... Fig. III.3 -26 UV-Visible spectroscopy of MoS2 exfoliated in water with inset table showing the parameters of solution ...................................................................................................... Fig. III.3 -27 a) SEM Image and b) water contact angle of ZrO_MoS2 ..................................... Fig. III.3 -28 UV-Visible spectroscopy of MoS2 exfoliated in n-Butanol with inset table showing the parameters of the solution .................................................................................. Fig. III.3 -29 SEM Image and water contact angle of ZrO-SiO/C18_MoS2 ............................... Fig. III.3 -30 Raman shift of all ZrO-SiO/C18_Ref and ZrO-SiO/C18_MoS2 deposited carbon steel samples ............................................................................................................................

Fig. IV. 1 - 1

 11 Fig. IV.1 -1 Scheme of the research topics based on different systems .................................

Fig. IV. 2 -

 2 Fig. IV.2 -6 Open Circuit Potential evolution of ZrO_Ref and 316L bare substrate in NaCl 30g/L for 120 hours ................................................................................................................ Fig. IV.2 -7 Polarization curve of ZrO_Ref and 316L bare substrtate in NaCl 30g/L after 24 hours of immersion ................................................................................................................ Fig. IV.2 -8 SEM image of morphology of ZrO_Ref surface after polarization (rectangles are showing the undercoating corrosion) .................................................................................... Fig. IV.2 -9 Bode curves of ZrO_Ref over 120 hours a) Nyquist plots b) Bode plots ............. Fig. IV.2 -10 EEC for ZrO_Ref long term duration of EIS test ................................................ Fig. IV.2 -11 Open Circuit Potential of ZrO_BN compared to the bare 316L substrate and ZrO_Ref for 24 hours of immersion in NaCl 30g/L solution ................................................... Fig. IV.2 -12 Polarization curves in NaCl 30g/L of bared 316L substrate, ZrO_Ref and ZrO_BN (with the circuit pointing the early anodic region of both ZrO_BN and ZrO_Ref) ................. Fig. IV.2 -13 SEM image of a) morphology of ZrO_BN after polarization and b) Zoomed corrosion product ................................................................................................................... Fig. IV.2 -14 Nyquist curve of ZrO_BN with fitting and superposed with bare 316L substrate and ZrO_Ref ............................................................................................................................ Fig. IV.2 -15 EEC for ZrO_BN long term duration of EIS test ................................................. Fig. IV.2 -16 OCP of three different BNNS charged ZrO coated samples in 24 hours of immersion in NaCl 30g/L solution .......................................................................................... Fig. IV.2 -17 Electrochemical dynamic polarization curves in NaCl 30g/L of all BNNS charged samples ................................................................................................................................... Fig. IV.2 -18 SEM image of the surface morphology obtained with a) ZrO_BNB and b) zoom on the corrosion product of ZrO_BNB , c) ZrO_BNC and d) zoom on corrosion product of ZrO_BNC ................................................................................................................................. Fig. IV.2 -19 Scheme of the 2D fillers in sol-gel coating system: a) pure porous Sol-Gel matrix without filler, b) filler with size d < dc, c) filler with size d ∼ dc, d) filler with size d > dc ....... Fig. IV.2 -20 the Nyquist curve of ZrO coated samples with three different BNNS fillers after 24 hours immersion in NaCl 30g/L at OCP with a) full range of frequency b) zoomed area at high frequency ........................................................................................................................ Fig. IV.2 -21 a) Nyquist plots and b) Bode plots of ZrO_BNC long-duration tests and 316L at 120 hours as reference (Black arrow noting the evolution direction over time of the data of ZrO_BNC) ................................................................................................................................ Fig. IV.2 -22 The evolution of EIS fitting parameter value over 120 hours of the three samples a) Rfilm or Rpore, b) Rct .................................................................................................. Fig. IV.2 -23 The evolution of ZAH_BNC of a) coat and dl values and b) Rpore and Rct values over 1200h.............................................................................................................................. Fig. IV.2 -24 Nyquist plots at OCP at 24 hours of immersion in NaCl solution of a) ZrO coatings charged with different BNNS b) Zoomed region at high frequency for ZrO_BN170 ................................................................................................................................................ Fig. IV.2 -25 Bode curves of ZrO coatings charged with BNNS with different concentration a) Phase curve (with red arrows indicating the lowest point of ZrO_BN170), b) Impedance curve. ...................................................................................................................................... Fig. IV.2 -26 EEC for a) ZrO_BN60, ZrO_BN90, ZrO_BN130 and b) ZrO_BN170 ....................

Fig. IV. 2 -

 2 Fig. IV.2 -27 Comparison of fitting parameters between ZrO_BNC and ZrO_BN60, ZrO_BN90, ZrO_BN130, ZrO_BN170 for a) Rpore and Rct and b) coat and dl ........................................... Fig. IV.2 -28 a) Scheme of the distribution of BNNS during dip-coating b) SEM image of the cross-section of as-dipped coating ......................................................................................... Fig. IV.2 -29 Model of the porous structure with BNNS filler a) with increased BNNS concentration but no impermeable film b)with an impermeable film .................................. Fig. IV.2 -30 Nyquist curve of ZrO_BN170 from 3 weeks of immersion till 8 weeks of immersion (With red arrow indicating the decreasing trend of slope of capacitive region) Fig. IV.2 -31 Bode curves of ZrO_BN170 from 12 hours of immersion till 1 weeks of immersion a) Phase curves b) Impedance curves .................................................................. Fig. IV.2 -32 Bode curves of ZrO_BN170 at 3, 4, 5 and 8 weeks of immersion a) Impedance curves and b) Phase curves .................................................................................................... Fig. IV.3 -1 Potentiodynamic Polarization curves of AA2017 substrate after immersed in NaCl 30g/L solution for 24 hours .................................................................................................... Fig. IV.3 -2 Bode curves for AA2017 substrate during at 6 hours and 144 hours of immersion in NaCl 30g/L .......................................................................................................................... Fig. IV.3 -3 Bode a) Phase and b) Impedance curves of comparison between AA2017 substrate and AA/ZrO_BNC at 6 hours of immersion and 144 hours of immersion in NaCl 30g/L solution ......................................................................................................................... Fig. IV.3 -4 Potentiodynamic Polarization curves of AA2017 substrate and ZrO-SiO_Ref after 24 hours of immersion in NaCl 30g/L ..................................................................................... Fig. IV.3 -5 Bode a) Impedance curves and b) Phase curves of ZrO-SiO_Ref in 1 weeks of immersion in NaCl 30g/L solution .......................................................................................... Fig. IV.3 -6 Surface morphology of ZrO-SiO_Ref after 1 weeks of immersion (White arrows indicating the delamination, red rectangle indicating zoomed region of corroded site) ...... Fig. IV.3 -7 Potentiodynamic polarization curve for ZrO-SiO_BN superposed with AA2017 substrate and ZrO-SiO_Ref ..................................................................................................... Fig. IV.3 -8 Bode a) Impedance curves and b) Phase curves of ZrO-SiO_BN in 2 weeks of immersion in NaCl 30g/L solution .......................................................................................... Fig. IV.3 -9 The SEM image of surface morphology after 2 weeks of immersion in NaCl of ZrO-SiO_BN ............................................................................................................................. Fig. IV.4 -1 Polarization curve of carbon steel at 24 hours of immersion ............................. Fig. IV.4 -2 Bode curves a) Impedance valuee b) Phase curve of carbon steel bare substrate after 20 hours of Immersion in NaCl 30g/L solution .............................................................. Fig. IV.4 -3 Polarization curve of carbon steel and ZrO-SiO/C18_Ref at 24 hours of immersion ................................................................................................................................................ Fig. IV.4 -4 SEM image of a) surface morphology of ZrO-SiO/C18_Ref after polarization and b) zoomed region ................................................................................................................... Fig. IV.4 -5 Nyquist curve of ZrO-SiO/C18_Ref in 20 hours of immersion in NaCl 30g/L solution. .................................................................................................................................. Fig. IV.4 -6 a) Bode Impedance curves and b) Bode Phase curves of ZrO-SiO/C18_Ref with fitting in 20 hours of immersion in NaCl 30g/L solution. ....................................................... Fig. IV.4 -7 Polarization curve of ZrO-SiO/C18_BN, ZrO-SiO/C18_Ref and carbon steel substrate at 24 hours of immersion .......................................................................................

Fig. IV. 4 - 8

 48 Fig. IV.4 -8 SEM image of surface morphology of ZrO-SiO/C18_BN after polarization ........ Fig. IV.4 -9 Nyquist curves of ZrO-SiO/C18_BN during 20 hours of immersion in NaCl 30g/L solution ................................................................................................................................... Fig. IV.4 -10 a) Bode Impedance curves and b) Bode Phase curves, for ZrO-SiO/C18_BN during 20 hours of immersion ................................................................................................ Fig. IV.5 -1 OCP evolution of ZrO_GE during 5 weeks of immersion .................................... Fig. IV.5 -2 a) Bode Impedance curves and b) Phase curves of ZrO_GE in 5 weeks of immersion in NaCl 30g/L water solution ................................................................................ Fig. IV.5 -3 EEC for EIS curves of ZrO_GE ............................................................................... Fig. IV.5 -4 a) Nyquist curve of ZrO_GE and b)SEM image of surface morphology of ZrO_GE after 5 weeks of immersion ................................................................................................... Fig. IV.5 -5 Evolution of OCP values of ZrO_MoS2 immersed in NaCl 30g/L in 24 hours of immersion............................................................................................................................... Fig. IV.5 -6 Potentiodynamic polarization curves of ZrO_MoS2 superposed 316L_Sub after 24 hours of immersion in NaCl solution ...................................................................................... Fig. IV.5 -7 Bode a) Impedance curves and b) Phase curves of ZrO_MoS2 immersed in NaCl solution for 72 hours .............................................................................................................. Fig. IV.5 -8 SEM images of surface morphology of ZrO_MoS2 after 24 hours of immersion in NaCl solutions ......................................................................................................................... Fig. IV.5 -9 a) Nyquist curve and b) zoomed region at high frequency of EIS tests of ZrO_MoS2 ............................................................................................................................... Fig. IV.5 -10 Polarization curve of ZrO-SiO/C18_GE surperposed with reference samples after 24 hours immersed in NaCl 30g/L .......................................................................................... Fig. IV.5 -11 SEM image of surface morphology of ZrO-SiO/C18_GE after the polarization Fig. IV.5 -12 Nyquist curves of ZrO-SiO/C18_GE immersed in NaCl 30g/L solution in 20 hours ................................................................................................................................................ Fig. IV.5 -13 a) Bode Impedance curves and b) Phase curves of ZrO-SiO/C18_GE immersed in NaCl 30g/L solution ................................................................................................................ Fig. IV.5 -14 Potentiodyanamic Polarization curve of ZrO-SiO/C18_MoS2 after 24 hours of immersion in NaCl 30g/L solution superposed with reference samples ............................... Fig. IV.5 -15 SEM image of surface morphology of ZrO-SiO/C18_MoS2 after polarization .. Fig. IV.5 -16 Nyquist curves of ZrO-SiO/C18_MoS2 immersed in NaCl 30g/L solution in 20 hours ....................................................................................................................................... Fig. IV.5 -17 Bode a) Impedance curves and b) Phase curves of EIS tests of ZrO-SiO/C18_MoS2 ........................................................................................................................

3

 3

Fig. A - 4

 4 Fig. A -4 Scheme of mechanism of FT-IR/ATR4

Fig. A - 5

 5 Fig. A -5 Contact Angle measurement KRUSS EasyDrop StandardA.2.7 Optical microscopyTo roughly identify the coatings' quality and the presence of the cracks, digital optical microscopy was employed. The digital microscopy that employed in this wotk is a Hirox Digital microscope (model KH-7700), as shown in Fig.A -6. The image loss of sharpness is due to the low depth of field of the microscopy and the surface curvature. The Hirox system is controlled by a CT-7 controller allowing an auto 3D calibration system to correct the resolution loss. This correction compiles multiple images together and built a 3D surface morphology.

Fig. A - 6

 6 Fig. A -6 Digital Optical Microscopy Hirox in Laboratoire de Mateis in INSA de Lyon, France A.2.8 Scanning Electron Microscopy

Fig. A - 7

 7 Fig. A -7 Scheme of a) Mechanism of Scanning Electron Microscopy Mechanism of the interaction between b) PE and SE c) PE and BSE d) PE and Chacatristic other signals. Scanning Electron Microscopy (SEM) bombards the surface of the material using accelerated electrons under high tension. The detectors collect the responding signals, such as electrons, electromagnetic waves as X-rays, etc., as shown in Fig. A -7a. By scanning the whole sample surface and calculation with the computer software, the morphology of the target surface could be presented. The electrons which are called Primary Electrons (PE) from the electron beam will interact with the local electrons which are orbiting around the nucleus, as shown in Fig. A -7b-d. Different responses from the local atoms could be received. Fig. A -7b shows the Secondary Electrons(SE) when one electron in the orbit is baombarded out by the PE. This mode is used for detection of morphology of the surface. Fig. A -7c shows the Back Scattered Electrons (BSE) when the PE is reflected back by elastic scattering, which is frequent in scanning heavy element. This technique is frequently used to understand the distribution of elements on a surface, by giving a map of contrast showing different colors according to atomic numbers of elements. Fig. A -7d shows character X-ray emission which allows the detector to identify the bombarded element by calculating the identical energy level emitted of X-ray. This technique is call Energy Dispersive X-ray Spectroscopy (EDS or EDX).The techniques mentioned above are the three most widely applied technique for SEM. Other emissions can also be observed by SEM technique, such as Auger electron and Cathodeluminescence, etc.

  Fig. A -8a and b, through 2 different focus path, the transmitted electron from the specimen could be focused on 2 different planes. The electronic path shown in Fig. A -8a could give a Selected Area Electron Diffraction (SAED) pattern of the specimen as shown in Fig. A -8d. This pattern reflects the microstructure of the specimen. Different microstructure will give out different diffraction pattern as circles, repeating dots, etc. Fig. A -8b shows the electronic path for the TEM magnification image. The magnified image could clearly demonstrate different layers of the sample if it is thin enough, as shown in Fig. A -8c.

Fig. A - 8

 8 Fig. A -8 Scheme of TEM a) Diffraction Pattern Mode and b) Magnification Image Mode ; 5 the Image of c)Magnified image and d) Diffraction Pattern 6

  I.2 -1, the anticorrosion mechanism consists of the prevention of one or several reactions 1, 2, 3 as shown in Fig. I.2 -1a. Otherwise, the connection with an extra anode can also protect the metals which becomes cathode, see Fig. I.2 -1b. Four main anticorrosion methods are presented here. 3

Table .

 .

	Sol	Sub	Dipping temp 1	Immerse time	Pulling speed	Number of dips	Drying 2 duration/temp	Curing 3 duration/temp
	ZrO	SS 4	RT 5	10 min	0.5 mm/s	3	10min/RT	24h/120°C
	ZrO-SiO	AA 6	60°C	1 min	30 mm/s	3	10min/150°C	24h/150°C
	ZrO-SiO/C18	CS 7	60°C	1 min	30 mm/s	3	10min/150°C	24h/150°C

2 -1 Dipping parameters of the Sol-Gel coatings

Table .

 .

2 -2 All parameters of materials, exfoliation, centrifuge, Sol-Gel substrate and Sample Names

2D materials

Solvant Exfoliation*

1 Centrifuge *2 Sol-Gel Substrate Sample Name

					ZrO	316L	ZrO_Ref
	Reference samples				ZrO-SiO	AA2017	ZrO-SiO_Ref
					ZrO-SiO/C18	CS XC 90	ZrO-SiO/C18_Ref
	BN-A	water	60	90/1500	ZrO	316L	ZrO_BNA
	BN-B	water	60	90/1500	ZrO	316L	ZrO_BNB
	BN-C	water	60	90/1500	ZrO	316L	ZrO_BNC
	BN-C	water	60	40/2000	ZrO	316L	ZrO_BN60
	BN-C	water	60	30/2000	ZrO	316L	ZrO_BN90
	BN-C	water	60	20/2000	ZrO	316L	ZrO_BN130
	BN-C	water	60	10/2000	ZrO	316L	ZrO_BN170
	BN-C	water	60	10/2000	ZrO	AA2017	AA/ZrO_BN
	BN-C	Isopropanol	60	10/2000	ZrO-SiO	AA2017	ZrO-SiO_BN
	BN-C	n-Butanol	60	10/2000	ZrO-SiO/C18	CS XC 90	ZrO-SiO/C18_BN
	GE	water	60	10/2000	ZRO	316L	ZrO_GE
	GE	n-Butanol	60	10/2000	ZrO-SiO/C18	CS XC 90	ZrO-SiO_GE
	MoS2	water	60	10/2000	ZRO	316L	ZrO_MoS2
	MoS2	n-Butanol	60	10/2000	ZrO-SiO/C18	CS XC 90	ZrO-SiO/C18_MoS2

*1: The exfoliation time with unit of hours *2: The Centrifuge time/velocity with unit of min/rad.s -1

Table .

 .

3 -2 Zeta potential and concentration of as-exfoliated solution from BNC_Sol170, BNC_Sol130, BNC_Sol90 and BNC_Sol60

Table 4 -

 4 IV.2 -6. In the same figure, the evolution of the bare 316L is superposed. OCP of ZrO_Ref stays stable during 120 hours between 85 ~ 95 mV/SCE, indicating the achievement of a stable state of the zirconium oxide layer after immersion. Compared with stainless steel OCP, we can observe that the bare substarte required several hours to reach a steady state. At this stage of the analysis, the coating act as a barrier that separates the substrate from the electrolyte.

	Sample	Rs /ohm.cm 2	Rfilm /ohm.cm 2	film	Qfilm /f.s^(a-1)	Cfilm F	Rct /ohm.cm 2	dl	Qdl /f.s^(a-1)
	316L 16h	30.835	4.33E +05	0.93	9.67E-06	1.34	4.32E +05 0.56 3.59E -06
	316L 120h	26.665	2.26E +05	0.94	9.54E-06	1.71	9.87E +04 0.50 4.75E -05

1 Fitting parameters of impedance curves of 316L bare substrate at 16 hours and 120 hours Fig. IV.2 -6 Open Circuit Potential evolution of ZrO_Ref and 316L bare substrate in NaCl 30g/L for 120 hours

Table 4 -

 4 4 Open Circuit Potential values of the ZrO coatings charged BNNS with different concentration

	Samples	OCP (mV/SCE)
	ZrO_BN170	72.5
	ZrO_BN130	50.5
	ZrO_BN90	-130.5
	ZrO_BN60	-140.5

  43

Table 4 -

 4 6 Stabilized OCP value of carbon steel at 24 hours of immersion in NaCl 30g/L solution

Table . 4

 . -2 OCP value of ZrO-SiO/C18_BN at 24 hours of immersion in NaCl 30g/L solution

  3 -13 UV-Visible spectroscopy of BN-C exfoliated in isopropanol with inset table showing the parameters of the solution .................................................................................. Fig. III.3 -14 SEM Image and water contact angle of a) and c) ZrO-SiO_Ref and b) and d) ZrO-SiO_BN ...................................................................................................................................... Fig. III.3 -15 Photo of a) ZrO-SiO/C18 hybrid Sol-Gel solution and b) BNNS exfoliated n-Butanol solution ....................................................................................................................... Fig. III.3 -16 DLS of ZrO-SiO/C18 hybrid Sol-Gel solution after synthesis ............................... Fig. III.3 -17 Corroded region of ZrO-SiO/C18_Ref by a) SEM and b) Confocal Optical Microscopy ............................................................................................................................... Fig. III.3 -18 UV-Visible spectroscopy of BN-C exfoliated in n-Butanol with inset table showing the parameters of solution ........................................................................................ Fig. III.3 -19 SEM Image and watercontact angle of a) and c) ZrO-SiO/C18_Ref and b) and d) ZrO-SiO/C18_BN ....................................................................................................................... Fig. III.3 -20 The a) TEM image of GENS exfoliated in water and b) the diffusion pattern ..... Fig. III.3 -21 UV-Visible spectroscopy of GE exfoliated in water with inset table showing the parameters of the solution ...................................................................................................... Fig. III.3 -22 a) SEM Image and b) water contact angle of ZrO_GE .........................................

Fig. III.3 -23 UV-Visible spectroscopy of GE exfoliated in n-Butanol with inset table showing the parameters of solution ...................................................................................................... Fig. III.3 -24 a) SEM Image and b) water contact angle of ZrO-SiO/C18_GE ........................... Fig. III.3 -25 Raman shift of all ZrO-SiO/C18_Ref and ZrO-SiO/C18_GE deposited carbon steel samples ..................................................................................................................................... Fig. III.3 -26 UV-Visible spectroscopy of MoS2 exfoliated in water with inset table showing the parameters of solution ...................................................................................................... Fig. III.3 -27 a) SEM Image and b) water contact angle of ZrO_MoS2 ..................................... Fig. III.3 -28 UV-Visible spectroscopy of MoS2 exfoliated in n-Butanol with inset table showing the parameters of the solution .................................................................................. Fig. III.3 -29 SEM Image and water contact angle of ZrO-SiO/C18_MoS2 ............................... Fig. III.3 -30 Raman shift of all ZrO-SiO/C18_Ref and ZrO-SiO/C18_MoS2 deposited carbon steel samples ............................................................................................................................

  2 -1 Open Circuit Potential evolution of 316L bare substrate in NaCl 30g/L over 120 hours (Arrow indicates the OCP value at 24 hours of immersion) .......................................... Fig. IV.2 -2 Polarization curve of 316L bare substrate in NaCl 30g/L after 24 hours of immersion.................................................................................................................................Fig. IV.2 -6 Open Circuit Potential evolution of ZrO_Ref and 316L bare substrate in NaCl 30g/L for 120 hours ................................................................................................................ Fig. IV.2 -7 Polarization curve of ZrO_Ref and 316L bare substrtate in NaCl 30g/L after 24 hours of immersion ................................................................................................................ Fig. IV.2 -8 SEM image of morphology of ZrO_Ref surface after polarization (rectangles are showing the undercoating corrosion) .................................................................................... Fig. IV.2 -9 Bode curves of ZrO_Ref over 120 hours a) Nyquist plots b) Bode plots ............. Fig. IV.2 -10 EEC for ZrO_Ref long term duration of EIS test ................................................ Fig. IV.2 -11 Open Circuit Potential of ZrO_BN compared to the bare 316L substrate and ZrO_Ref for 24 hours of immersion in NaCl 30g/L solution ................................................... Fig. IV.2 -12 Polarization curves in NaCl 30g/L of bared 316L substrate, ZrO_Ref and ZrO_BN (with the circuit pointing the early anodic region of both ZrO_BN and ZrO_Ref) ................. Fig. IV.2 -13 SEM image of a) morphology of ZrO_BN after polarization and b) Zoomed corrosion product ................................................................................................................... Fig. IV.2 -14 Nyquist curve of ZrO_BN with fitting and superposed with bare 316L substrate and ZrO_Ref ............................................................................................................................ Fig. IV.2 -15 EEC for ZrO_BN long term duration of EIS test ................................................. Fig. IV.2 -16 OCP of three different BNNS charged ZrO coated samples in 24 hours of immersion in NaCl 30g/L solution ..

Fig. IV.2 -3 SEM image of the morphology of 316L bare substrate after the polarization ..... Fig. IV.2 -4 EIS curves of 316L bare substrate over 120 hours a) Nyquist plots and Bode plots ................................................................................................................................................ Fig. IV.2 -5 Electrochemical Equivalent Circuit of 316L bare substrate immersed in NaCl ... ........................................................................................ Fig. IV.2 -17 Electrochemical dynamic polarization curves in NaCl 30g/L of all BNNS charged samples ...................................................................................................................................

Table . A

 . -3 Abbreviations and Acronyms of characterization tools

	Abbreviations & Acronyms	Full Name
	ASE	Auger Electron Spectroscopy
	ATR	Attenuated Total Reflectance
	BSE	Back Scattered Electrons
	CE	Counter Electrode
	CVD	Chemical Vapor Deposition
	DLS	Dynamic Light Scattering Spectroscopy
	EC	Electrochemical Equivalent Circuit
	EIS	Electrochemical Impedance Spectroscopy
	FTIR	Fourier Transformed infrared spectroscopy
	LBL	Layer By Layer deposition
	LSV	Linear Scanning Voltage
	OCP	Open circuit potential
	PVD	Physics Vapor Deposition
	Raman	Raman Spectroscopy
	RE	Reference Electrode
	Redox	Reduction-Oxidation
	SCC	Stress Corrosion Cracking

SCE Saturated Calomel Electrode SE Secondary Electron SEM Scanning Electron Microscopy TEM Transmission electron Microscopy UV-Vis Ultraviolet-Visible Spectroscopy WCA Water Contact Angle WE Working Electrode

  Different atomic structured molecules may absorb the light with different intensity by introducing the light from ultraviolet region (100 nm -400 nm) to visible lights region ( 400 nm -750 nm) and near infrared region (750 nm -1400 nm), different participants inside the solution (liquid, gaseous or transparent solid) absorb different intensity lights with different wave numbers. With Beer-Lambert the content of the target specie can be calculated. The Beer-Lamber law is shown in:

	𝑪 𝒍	= 𝜶 • 𝑨	Eq. A -1

Cette thèse est accessible à l'adresse : http://theses.insa-lyon.fr/publication/2021LYSEI004/these.pdf © [Y. Shi], [2021], INSA Lyon, tous droits réservés

Chapter I. Bibliography Chapter II. Materials and Experiments

Chapter III. Elaboration of coating and Characterization

Chapter IV. Electrochemical characterization

List of Tables

 Table. 2
Appendix

Appendix.1 Abbreviations and Acronyms

A.1.1 Abbreviations and Acronyms of materials

Appendix.2 Characterization Tools

A.2.1 Introduction

In order to probe the properties of the materials employed in this work, different characterization tools are used.

The Spectroscopies are used to understand the properties of liquid solutions. UV-Visible Spectroscopy is used to identify the existence of 2D materials as well as measuring the concentration of exfoliated solutions. Dynamic light scattering (DLS) spectroscopy is used to characterize the colloidal size and the stability of Sol-Gel solution. Raman Spectroscopy and FT-IR Spectroscopy/Transmission mode and ATR mode are used to characterize the properties of as-desposited coatings.

The microscopies are also employed to characterize the morphology of the materials and coatings. According to the need of different size dimention to characterize, different microscopies are employed.

A.2.2 Dynamic Light Scattering Spectroscopy

The Dynamic Light Scattering Spectroscopy (DLS) was applied for the Sol-Gel solutions with the Zetasizer (Malvern, nano ZSP) to understand the colloidal size. By measuring the scattering phenomenon between the introduced laser and output laser, the particles size could be calculated. Different particle size may influence the intensity of scattered light. Since the DLS is sensitive to the particles size of the solution and is designed for nanoparticles, the Sol-Gel solution with 2D materials does not show reliable results

Fig. A -1 Mechanism of DLS for particles with differernt size

A.2.3 Ultraviolet-visible Spectroscopy

Ultraviolet-Visible spectroscopy (UV-Vis) is a technique using the introduced ultraviolet light and visible light to measure the content of the target species in a liquid, gaseous or even solid under specific conditions. The species could be molecules, ions, suspending particles, etc.

Appendix.4 Fitting parameters of EIS tests

A.4.1 Fitting Parameters of 316L bare substrate over 120 hours of immersion