Anh Son Nguyen

Nadine Pébère
email: nadine.pebere@ensiacet.fr

A local electrochemical impedance study of the self-healing properties of waterborne coatings on 2024 aluminium alloy

Keywords:

In the present work the processes involved in the degradation of a coated 2024 aluminium alloy from an artificial scratch were investigated by local electrochemical impedance spectroscopy (LEIS). The aim was to assess the self healing process linked to the release of inhibitive pigments from the coatings. A comparison is provided for the local behaviour of a chromated coating, currently used in the aeronautic industry, and a Cr free system. Local impedance diagrams were obtained above the scratched areas for different exposure times to the electrolyte (1 mM NaCl). For the chromated sample, the impedance values were high and increased with time, clearly showing a significant inhibition in the scratch. In contrast, for the Cr free coating, impedance values were lower and in several areas, corrosion products appeared in the scratch. In these corroded zones, after several days of immersion, the diagrams have similar shape as that observed on bare 2024 aluminium alloy. Mappings, performed at 3 Hz, for different exposure times to the electrolyte also clearly showed the self healing process in the presence of chromates or the development of the corrosion for the unchromated coating. At 1 kHz, similar mappings were observed for the two systems providing support to the fact that the coatings presented good adhesion to the substrate. It must be emphasized that LEIS gives a fast response on the efficacy of the inhibitive pigments and thus, complements the information obtained from the conventional EIS technique.

Introduction

The organic coatings currently used in the aeronautic industry present very high corrosion protection. However, since the beginning of the 1990s, the high toxicity associated with hexavalent chromium, used as inhibitive pigment, has imposed restrictions on their use in industrial applications. Today, the development of new Cr free healing systems remains an important challenge because, on the one hand, the new protective systems must match the environmental regulations and on the other hand, their performances, particularly in the long term, must be preserved. During the last decades, numerous works were devoted to the development and to the assessment of new materials for corrosion protection of metallic structures including various methodologies for the incorporation of "green" corrosion inhib itors in the coatings to provide an active corrosion protection [START_REF]Handbook of smart Coatings for Material Protection[END_REF][START_REF]Smart Composite Coatings and Membranes, Transport, Structural, Environmental and Energy Applications[END_REF].

Starting from these considerations, we developed a wide study to obtain a better knowledge of degradation mechanisms as a function of exposure time of chromate free coatings for corrosion protection of 2024 aluminium alloy (AA2024). In a first part, commercial water based coatings formulated with either stron tium chromate (SrCrO 4 ) or a mixture of Cr(VI) free pigments have been compared in an impedance study [START_REF] Nguyen | Impedance analysis of the distributed resistivity of coatings in dry and wet conditions[END_REF][START_REF] Nguyen | Impedance study of the influence of chromates on the properties of waterborne coatings deposited on 2024 aluminium alloy[END_REF]. These previous works have been focussed on the use of different models for analysing the conventional impedance data. In the present work, local electro chemical impedance measurements were performed in the presence of an artificial defect to characterize the processes involved in the degradation of the coated AA2024 and to evaluate the healing efficiency.

During the last decades, it has been demonstrated that local electrochemical impedance spectroscopy (LEIS) is a powerful tool to investigate corrosion processes on bare metal surfaces [5 14] or on coated alloys [ 15 28]. For instance, local impedance diagrams obtained on an AZ91 magnesium alloy have shown that it was possible to show the contribution of each phase constituting the alloy in its corrosion behaviour [START_REF] Galicia | Local and global electrochemical impedances applied to the corrosion behaviour of an AZ91 magnesium alloy[END_REF]. The LEIS technique was also used to investigate galvanic couplings, such as aluminium/copper [START_REF] Jorcin | Galvanic coupling between pure copper and pure aluminum: experimental approach and mathematical model[END_REF][START_REF] Blanc | The origin of the complex character of the Ohmic impedance[END_REF] aluminium/magnesium [START_REF] Lacroix | Localized approach to galvanic coupling in an aluminum-magnesium system[END_REF] or zinc/carbon steel [START_REF] Mouanga | Galvanic corrosion between zinc and carbon steel investigated by local electrochemical impedance spectroscopy[END_REF]. The Al/Cu couple was first designed by Jorcin et al. [START_REF] Jorcin | Galvanic coupling between pure copper and pure aluminum: experimental approach and mathematical model[END_REF] to study corrosion phenomena associated with copper rich intermetallic particles in aluminium alloys. Local impedance measurements were also performed on the Al/Cu couple to analyse the role of different inhibitors on the galvanic coupling [START_REF] Boisier | Corrosion inhibition of 2024 aluminium alloy by sodium decanoate[END_REF][START_REF] Marcelin | Synergistic effect between 8-hydroxyquinoline and benzotriazole for the corrosion protection of 2024 aluminium alloy: A local electrochemical impedance approach[END_REF]. For example, it was shown that the galvanic coupling was strongly limited by mixing 8 hydroxyquinoline and benzotriazole which was not the case when the inhibitors were used separately [START_REF] Marcelin | Synergistic effect between 8-hydroxyquinoline and benzotriazole for the corrosion protection of 2024 aluminium alloy: A local electrochemical impedance approach[END_REF]. Inductive loops, consistently observed on the local impedance diagrams during galvanic couplings, were attributed to the local ohmic impedance [START_REF] Blanc | The origin of the complex character of the Ohmic impedance[END_REF]. Several works were dedicated to the use of the LEIS technique to investigate the degradation of organic coatings [ 15 21]. As an example, Philippe et al. [START_REF] Philippe | Investigating localized degradation of organic coatings comparison of electrochemical impedance spectroscopy with local electrochemical impedance spectroscopy[END_REF] investigated polymer coated galvanized steel. They showed that the global impedance measurements provided surface averaged responses correspond ing to both the polymer properties and its defects, whereas LEIS allowed coating defects to be isolated. Jorcin et al. [START_REF] Jorcin | Delaminated areas beneath organic coating: a local electrochemical impedance approach[END_REF] have investigated the initiation and propagation of delamination at the steel/epoxy vinyl coating interface from an artificial defect after aging in a salt spray chamber. LEI mappings, performed at 5 kHz, revealed weak zones with poor adherence to the metal substrate around the scratches. The mappings, obtained for various exposure times to the salt spray, allowed the delamination surface area to be quantified. Macedo et al. [START_REF] Macedo | Contribution to a better understanding of different behaviour patterns observed with organic coatings evaluated by electrochemical impedance spectroscopy[END_REF] have used both global and local impedances to investigate different coatings (epoxy, alkyd and polyurethane) on a carbon steel. They showed that the anomalous increase in resistance with immersion time of the alkyd paint was linked to the nature of the polymeric network and not to phenomena occurring at the metal/paint interface. Mouanga et al. [START_REF] Mouanga | EIS and LEIS investigation of aging low carbon steel with Zn-Ni coating[END_REF] have investigated a Zn Ni + chromate conversion coating deposited on carbon steel. In the presence of a large scratch (2 mm), they showed by LEIS that, after 21 days of immersion in the corrosive solution, the impedance modulus measured at 1 Hz was higher at the centre of the defect than on the coating, showing the high protection afforded by the chromate conversion coating. Recently, LEIS was used to investigate self healing ability of various inhibitors, such as 8 HQ [ 26 28], cerium acetylacetonate, or sodium molybdate [START_REF] Kartsonakis | Multifunctional epoxy coatings combining a mixture of traps and inhibitor loaded nanocontainers for corrosion protection of AA2024-T3[END_REF] encapsulated in reservoirs and added to epoxy coatings applied on AA2024. LEIS was used in mapping mode only, at a frequency of 10 Hz, with a scanned area including a defect. The measurements carried out for different immersion times provided information on the kinetics of local inhibition by the encapsulated inhibitors in the damaged areas.

The aim of the present work is to highlight the great potential of the local impedance technique for the characterization of the self healing processes of organic coatings artificially damaged and to propose a methodology for investigating coatings containing environmentally friendly inhibitors. For this purpose, the two water based systems (chromated (CC) and unchromated (NCC)) previously characterized by conventional EIS were used [START_REF] Nguyen | Impedance analysis of the distributed resistivity of coatings in dry and wet conditions[END_REF][START_REF] Nguyen | Impedance study of the influence of chromates on the properties of waterborne coatings deposited on 2024 aluminium alloy[END_REF]. First, local impedance measurements were performed over the scratches for various exposure times to the corrosive solution. Then, local mappings were obtained above and around the scratch at different frequencies (1 kHz, 30 Hz and 3 Hz) and for different immersion times. For the chromated system only, the influence of the width of the scratch was investigated. The results obtained with the conventional EIS [START_REF] Nguyen | Impedance analysis of the distributed resistivity of coatings in dry and wet conditions[END_REF][START_REF] Nguyen | Impedance study of the influence of chromates on the properties of waterborne coatings deposited on 2024 aluminium alloy[END_REF] were also briefly recalled to emphasize the complementarity of global and local impedance measurements to assess the corrosion protection performance of organic coatings.

Experimental

The coating samples used and the global and local impedance measurement protocols are presented in this section.

Coating samples

Both CC and NCC consisted of a two component water based paint, manufactured by Mapaero SAS, Pamiers, France, using a polyaminoamide (Versamid 1 type) as base and a bisphenol A epoxy polymer as hardener, titanium oxide (12 wt. %), talc (11 wt. %) and silica (1 wt. %). In the CC, SrCrO 4 (16 wt. %) was added as inhibitive pigment, whereas in the NCC, SrCrO 4 was replaced by a mixture of zinc oxide and a phosphosilicate (10 wt. %). The coatings were deposited onto 2024 T3 aluminium alloy plates. The chemical composition in weight percent of the alloy was: Cu: 4.90; Mg: 1.31; Mn: 0.56; Si: 0.08; Fe: 0.26; Zn: 0.10; Ti: 0.01 and Al to balance. The specimens consisted of 125 mm  80 mm  1 mm plates machined from a rolled plate. Before painting, the samples were degreased at 60 C (pH = 9) for 15 min, rinsed twice with distilled water, then etched in an acid bath at 52 C for 10 min, and rinsed again with distilled water. The liquid paints were applied by air spraying. After curing at 60 C, the coatings were 20 AE 2 mm thick.

For most of the experiments, scratches were done manually using a cutting knife. Three scratches of 30 mm length were performed on each sample. Each scratch was observed with an optical microscope before the electrochemical measurements to check the width of the scratch and to verify that the scratch reached the substrate. The scribe has a U shape profile and was approximately 110 mm width and 100 mm depth (Fig. 1a). Some measurements were also performed, for the CC sample only, on a wider scratch (1 mm width and 200 mm depth) by using a scribing machine (Fig. 1b). The scratch size remains a relevant parameter to investigate self healing processes because mass transport of the inhibitive pigments from the coating (release step) to the freshly exposed substrate is a determining step [START_REF] Oltra | Influence of mass transport on the competition between corrosion and passivation by inhibitor release after coating breakdown[END_REF], particularly when the inhibitor release is slow. For the tested chromated system, the amount of chromate release from the intact coating was previously determined by UV vis spectroscopy [START_REF] Nguyen | Impedance study of the influence of chromates on the properties of waterborne coatings deposited on 2024 aluminium alloy[END_REF]. It can be assumed that the leaching rate of chromate in the solution would be lower in the presence of a scratch than for the intact coating due to the chromates interaction with the AA2024 substrate ( corrosion inhibition in the scratch). Moreover, the release of chromates should be dependent on the scratch size. This point was not investigated in the present study. For the Cr(VI) free pigments, it was checked that they are little soluble in the NaCI solution.

The corrosive medium was prepared from deionized water by adding NaCI (reagent grade). The concentration for the conven tional impedance was 0.5 M (0.01 S cm-1 ) and 1 mM (100 µ.S cm- 1 ) for the local impedance. By decreasing the electrolyte conductivity, accurate measurements at low frequencies can be achieved [START_REF] Huang | Local electrochemical impedance spectroscopy: a review and some recent developments[END_REF] which is necessary to investigate the self healing processes. In the present study, a good accuracy was obtained for frequency up to 1 Hz.

El ectrochemical impedance measurements

Global impedance measurements were performed for intact coatings in a classical three electrode cell, realized by fixing a cylindrical Plexiglas tube on top of the coated sample. The working electrode was a 24 cm 2 portion of the sample. A saturated calomel electrode (SCE) and a Pt sheet were used as reference and counter electrode, respectively. A Biologie VSP apparatus was used to measure the impedance of the coated samples, for exposure times ranging from 2 hours to ca. 1 month. Measurements were performed under potentiostatic conditions, at the open circuit potential, with a 30mV peak to peak sinusoidal perturbation. Frequency was swept downwards from 10 5 Hz to 0.01 Hz, recording 8 points per decade.

For the local electrochemical impedance measurements, a five electrode configuration was used: the scratched coated samples as working electrode, a carbon composite (Goodfellow) circular grid was used as a counter electrode, the saturated calomel electrode as a reference electrode and the bi electrode with the two platinum probes. LEIS measurements were carried out with a PAR Madel 370 Scanning Electrochemical Workstation that consisted of a 370 scanning contrai unit , a Solartron 1287 potentiostat, a Solartron 1255 B frequency response analyser, a high impedance input amplifier using a differential electrometer, and a probe scanning assembly driven in the x, y, and z directions. LEIS measurements were performed under potentiostatic regulation, at the open circuit potential. Local impedance spectra were acquired at various locations over the scratches to verify the reproducibility. At least five measurements were performed in each scratch. The distance of the bi electrode to the working electrode was carefully controlled and the position was the same for ail the experiments (200 µ.m ± 50 µ.m ). It was checked that the distance between the probe and the substrate (100 µ.m to 300 µ.m) little atfected the local impedance results. For the commercial device, the distance of the probe to the working electrode is not the main parameter because this distance is much lower than the distance between the two platinum probes (2 mm) and thus, the size of the probe (electrode dimension and inter electrode distance) is the most relevant factor which contrai the accuracy of the measurements and particularly limits the spatial resolution of the technique. The error on the impedance or admittance linked to the probe distance can be considered small and thus CC and NCC results can be directly compared.

The applied potential amplitude was 100 mV and the frequen cies ranged from 65 kHz to 1 Hz with 8 points per decade. For the LEI mappings (LEIM), the probe was stepped across a designed area of the sample. In the present study, the analysed partwas an area of 16 x 16 mm, above and around the scratch. The step size was 500 µ.m in the x and 500 µ.m in the y directions. The local mappings (2D or 3D) consisted in plotting the admittance rather than the impedance because corrosion sites are easily observable with peaks instead of valleys [START_REF] Wittmann | The Detection and Mapping of Defects in Organic Coatings Using Local Electrochemical Impedance Methods[END_REF]. A relatively good reproducibility of the results was observed in the present work and the results shown for a single coating were typical of other nominally identical coatings.

Scanning electron microscopy (SEM) observa tions

SEM analyses were carried out on a LEO 435VP instrument equipped with an electron dispersive X ray (EDX) spectrometer. The accelerating voltage was 15 kV. Observations and analysis of the corrosion products formed in the scratch of the NCC sample after one month exposure to the NaCI 1 mM were perforrned

Results and discussion

Gl obal impedance

The results from the conventional impedance for the two coatings (CC and NCC) without defects were already published [START_REF] Nguyen | Impedance analysis of the distributed resistivity of coatings in dry and wet conditions[END_REF][START_REF] Nguyen | Impedance study of the influence of chromates on the properties of waterborne coatings deposited on 2024 aluminium alloy[END_REF] and are brietly recalled here for a better understandingof the systems in the absence of defects. Fig. 2 compares the impedance diagrams of the CC and NCC coatings after 1 day and 1 month immersion in 0.5 M Na Cl solution. For bath coatings the impedance diagrams are little modified with exposure to the electrolyte. The data have been analysed with a mode) assuming an exponential variation of the coating resistivity along the coating thickness [START_REF] Nguyen | Impedance analysis of the distributed resistivity of coatings in dry and wet conditions[END_REF][START_REF] Nguyen | Impedance study of the influence of chromates on the properties of waterborne coatings deposited on 2024 aluminium alloy[END_REF]. The analysis confirmed that penetration of water and ions occurs on different time scales (31 35). The former process is faster and affects permittivity more strongly than resistivity; the latter is slower and affects almost exclusively resistivity. Fig. 3 shows the variation of the impedance modulus at f=10 mHz (Z10mHz) as a function of immersion time in 0.5 M Na Cl for CC and NCC. The two curves are close. lt can be seen that, at the beginning ofimmersion, the impedance modulus is the same for the two systems and then, for bath samples, it decreases when the immersion time increases. The impedance modulus is always lower for CC than for NCC which was ascribed to the leaching of chromates, and was confirmed by chemical analysis of the solution (4). The curves in Fig. 3 are close to the curves obtained from the mode! showing the immersion time dependence of the zero frequency limit of the impedance (Z 10 ,a 1 ) of CC and NCC (Fig. 12 in reference ( 4)). lt is interesting to recall that although the global impedances of CC and NCC coatings were rather similar, the data analysis have shown that R po re values were ten times larger for CC than for NCC and the Young impedance was initially larger but decreased at a higher rate for CC than for NCC to become ca. 10 times lower after one month (4). Thus, it can be concluded that the global impedance diagrams mainly account for the barrier properties of the coatings. The ditference between the two coatings has been attributed to chromate leaching. Finally, after 1 month immersion, the barrier properties of bath coatings remained high and the AA2024 substrate was not corroded Thus, independently of the type of pigments in the formulation, the paint durability cannot be evaluated from the global impedance results only.

Local impedance

Variation o/Ecorr versus exposure time in 1 mM NaCl solution

Fig. 4 shows the variation of the corrosion potential (E corr l for the CC and NCC scratched samples as a function of the exposure time to 1 mM NaCI solution. lt can be seen that, from the beginning of immersion, the values are significantly ditferent for the two samples but they remain relatively constant when the immersion time increases. For CC, Ecorr is about 035 V more cathodic than for the NCC. For the intact coatings, E rorr was also more negative for CC than for NCC of about 0.1 V (4) . The shift of Ecorr in the cathodic direction accounts for the inhibitive action of chromates on the corrosion of the AA2024 (36 41 ). lndependent polarisation curves carried out on bare AA2024 in the presence of chromates also showed a shift of E rorr as well as a decrease of bath the anodic and -0.4 cathodic current densities (not shown). Here, the significant shift of Ecorr for the CC sample confirmed the inhibitive action of chromates which stro ngly limited the oxygen reduction reaction on the intermetallic particles of the AA2024 an d improves the oxide film formation. For the wider scratch (1 mm), E rorr was about -0.6V/SCE instead of -0.9V/SCE for the narrow scratch. This ditference would be linked to the chromates content in the scratch and would suggest that the specific action of chromates on the cathodic sites (intermetallic particles) and/or anodic sites (alu minium matrix) would be dependent on their concentration. For the CC sample, the scratches remained bright even after several months of immersion. This indicates that the release of chromates from the coating was enough to prevent corrosion.

> ? •9 -�• .. ... -�-........... •O --0.6 w -0.8 vi f î î î•••••••••••••f > . . . ... ... 0 -1.0 w • •□• •CC • -0-•NCC -1.

Influence of the immersion time

To verify the reproducibility of the measurements, several local impedance diagrams were acquired at various locations over the scratches. As an example, six ditferent measurements obtained on CC and NCC samples are shown in Fig. S. For the CC sample, the diagrams are almost superimposed accounting for a good reproducibility (Fig. Sa andb). lnstead, for the NCC the diagrams are not strictly identical. As it will be discussed later, corrosion appeared in the scratches but only in some areas and thus, a larger variability of the impedance response was observed (Fig. Sc andSd). Ali the diagrams (CC and NCC) presented the same shape in the high frequency range (f> 1 kHz) with the presenoe of an additional time constant attributed to local Ohmic impedance [START_REF] Blanc | The origin of the complex character of the Ohmic impedance[END_REF][START_REF] Huang | Local electrochemical impedance spectroscopy: a review and some recent developments[END_REF]. The high frequency part of the diagrams will not be discussed for the remainder of the study. For CC, some points at low frequency, around 1 Hz, are scattered (Fig. Sb). This indicates that, in our configuration, the highest value of the local impedance which can be accurately measured is around 10 6 n cm 2 (30). Fig. 6 shows the impedance diagrams (Nyquist and Bode representations) for CC obtained over the scratch for ditferent immersion times. A capacitive behaviour with high impedance values can be observed independently of the immersion time. Moreover, the impedance modulus increases until 336 h of immersion and then stabilizes. These observations confirmed the corrosion inhibition of AA2024 in the scratch and the progressive formation of a protective layer. For the NCC, two ditferent zones were observed, particularly visible after 24 h of immersion: one with a black deposit and another one with white corrosion products. SEM observations and EDX analysis were performed in the scratch of the NCC sample after one month of exposure to the 1 mM NaCI solution (Fig. 7). The EDX spectrum of the black deposit (Fig. 7a) revealed the presence of Si, S and Zn which would indicate the presence of zinc oxide and phospho silicate, incorporated in the coating to replace chromates. In the corroded area ( Fig. 7 9 for comparison. In the presence of the black deposit, the impedance modulus slightly increases between 24h and 504h immersion (from 1 x10 5 to Sx 10 5 !l cm 2 ) which would be indicative of a protective action of the deposit. On the corroded are a, the shape of the diagram after 24 h of immersion is similar to that obtained on the black deposit for the same immersion time except a slightly lower impedance modulus. After 504h of immersion, the impedance modulus on the corroded zone decreases (from Gx 10 4 to 2x 10 4 !l cm 2 ) and the diagram is close to that obtained on the bare AA2024, showing that the substrate was not protected in this area.

From the local impedance diagrams obtained for both CC and NCC, three parameters were graphically extracted: the impedance modulus at low frequency (Z1Hz), the parameters ex and Q, associated to a constant phase element (CPE) which account for the non ideal behaviour of the systems. For CC, the ex and Q values were graphically determined with a good accuracy in a large frequency range [START_REF] Orazem | Enhanced graphical representation of electrochemical impedance data[END_REF]. For the NCC, on the corroded areas, the parameters ex and Q cannot be accurately measured and, for this reason, are not reported in Fig. 10 which shows the variation of the parameters as a function of immersion time in 1 mM Na Cl solution. For CC, the values of ZiHz increase during the first days of immersion, until 72 h and then, remain stable around 2 x 10 6 !l cm 2 for longer immersion times (Fig. 10a). For the NCC, the impedance modulus are lower than for the CC. For short immersion times, whether in presence of the black deposit or the corrosion products in the scratch, the values of ZiHz are comparable but ten times lower than those obtained over the scratch for CC. When the immersion time increases, white the impedance modulus remains constant above the black deposit, it progressively decreases on the corroded area. For the NCC, these results can be explained by the fact that, at the beginning of immersion, the black deposit was formed ait along the scratch and the impedance modulus was the same independently of the analysed area. When the immersion time increases, the corrosion starts at some locations and the modulus decreased. The self healing process or the corrosion growth can be also observed on the variation of the CPE parameters. For the CC, the parameter ex is independent of the immersion time and is around 0.9 (Fig. 10b). The values ofQare low (10-1 n-1 cm-2 s "' ) and characterize a film etfect rather than a double layer capacitance. The parameter Qslightly decreases when the immersion time increases (Fig. 10c). This behaviour, in

Mappings of the scratched samples

When using the local impedance technique, it is interesting to map the sample surface around the scratched area. The choice of the frequency to perform the mapping is a relevant parameter. Fig. 11 shows the admittance modulus over the scratch for CC and NCC after 504 h of exposure to 1 mM NaCI. The cmves obtained after 24 h of immersion for the bare AA2024 and the intact coating are reported for comparison in Fig. 11. ln the HF range (f> 1 kHz), the curves account for the ohmic drop and for the current and potential distributions [START_REF] Blanc | The origin of the complex character of the Ohmic impedance[END_REF][START_REF] Huang | Local electrochemical impedance spectroscopy: a review and some recent developments[END_REF]. Around 1 kHz, the admittance modulus on the scratched samples ( CC and NCC) are relatively close but when the frequency decreases, clear differences can be observed between the samples. Particularly at low frequency (f = 1 Hz), the admittance value measured above the corroded area for NCC is close to that obtained for the bare AA2024 but in contrast, the admittance value obtained above the scratch for the CC sample is of the same order of magnitude to that obtained for the intact coating. This figure underlined that, depending on the frequency used to map the surface, different phenomena might be observed Fig. 12 presents some mappings [START_REF] Huang | Local electrochemical impedance spectroscopy: a review and some recent developments[END_REF] performed over the scratched area for three frequencies (1 kHz, 30 Hz and 3 Hz) for the CC and NCC samples after 1 month of exposure to 1 mM NaCI. At 1 kHz, the mappings for the two systems are identical and the scratch is dearly visible (Fig. 12a andd). lt can be emphasized that, even after one month of immersion in the electrolyte, there is no admittance step observable on the edge of the scratch, which could be attributed to coating delamination [START_REF] Jorcin | Delaminated areas beneath organic coating: a local electrochemical impedance approach[END_REF]. This result would indicate that the adhesion of the coatings is high, independently of the types of pigments used in the formulation. The good adhesion would be mainly linked to the chemical composition of the coating and to the surface pre treatment. At 30 Hz and 3 Hz, the mappings show that the admittance values in the scratch for CC (Fig. 12b andc) are lower than those measured for NCC (Fig. 12e andf). For NCC, the corroded and non corroded zones can be easily observed, particularly at 3 Hz, where the two zones are well separated (two peaks can be seen on the mapping which correspond to the areas with corrosion products (Fig. 12f)). At 3 Hz and for CC, the admittance values in the scratch are of the same order of magnitude than those measured above the coating and the scratch is no longer clearly visible on the mapping (Fig. 12c). From these data, it can be concluded, as expected, that a low frequency is appropriate to show the self healing process, in the present case, linked to the release of chromates from the coating. As example, mappings perforrned at 3 Hz for CC and NCC are reported in Fig. 13 for short immersion times. After 24 h of immersion, the admittance of CC sample is significantly lower than that of the NCC, showing that the self healing by the chromates is fast and effective. For CC, the admittance progressively decreases when the immersion time increases, showing the improvement of the corrosion resistance in the scratch, in agreement with the local impedance diagrams (Figs. 6 and10). After 168 h of immersion, the scratch becomes almost invisible at this admittance scale (Fig. 13c). For NCC, a decrease of the admittance can be noticed between 24 h and 48 h of immersion (Fig. 13d ande) which can be explained by the presence of the dari< deposit in the scratch (Fig. 7a). After 168 h of immersion (Fig. 13f), an admittance peak is observed in the mapping, linked to the appearance of localized corrosion ( Fig. 7b ). Finally , it is interesting to investigate the self healing process for the CC sample on a wider scratch. Fig. 14 shows the 2D mappings obtained above a scratch of 1 mm width. The mappings are compared to those obtained for the scratch of 110 µ.m width. After 3 h of immersion, the difference in the size of the two scratches are clearly seen. ln the middle of the large scratch, the admittance modulus is higher than that measured on the edge (Fig. 14d). However, on the edge, the admittance values are similar to those observed over the smalt scratch (Fig. 14a). For the smalt scratch, the mapping indicates that chromate release was sufficient to induce the self healing in the whole scratch but for the large scratch, the chromates have not yet reached the middle of the scratch which remains active. After 72 h of immersion, the self healing process is clearly visible for the smalt scratch, character ized by the decrease of the admittance in the scratch (Fig. 14b ). ln the middle of the large scratch, some areas always present high admittance values (Fig. 14e). The interesting result is that, after 168 h of immersion, the self healing process is also clearly seen on the large scratch (Fig. 14f). The strong decrease of the admittance confirms the inhibitive action of chromates and the formation of a prot ective layer in the scratch. After 168 h of immersion, the difference in size between the two scratches is no longer discemible on the mappings (Fig. 14c andd). These results showed that the inhibition of the corrosion of AA2024 is controlted by the release and the mass transport of chromated species from the paint to the centre of the scratch; it appeared that the chromate concentration is sufficient to protect the AA2024 surface. This analysis is in agreement with the fact that at the beginning of exposure to the electrolyte, the cathodic reaction on the intermetallic particles is limited by the presence of chromates in the scratch (shift of E corr in the cathodic direction (Fig. 4)). Then, for longer exposure time, the release of chromates in the scratch progressively increased which improved the anodic passivation of the alloy surface (Fig. 6).

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Conclusions

Local electrochemical impedance spectroscopy was used to assess the self healing process linked to the release of inhibitive pigments from the coatings. From the local impedance diagrams obtained above the scratched areas for different exposure times to the electrolyte, the self healing process was clearly observed for the chromated system. The self healing can be also observed by the mappings above the scratch at a frequency of 3 Hz. From LEIS measurements a better understanding of local corrosion inhibition mechanisms was obtained. It is interesting to point out that the mapping at 1 kHz provided support to the fact that the two systems presented good adhesion to the substrate. Finally, it can be emphasized that global and local impedances are complementary techniques to assess corrosion protection performance of organic coatings. The present work proposed a methodology (with chromates as reference) which can be used to characterize the self healing processes of coatings containing environmentally friendly inhibitors.

Fig. 1 .

 1 Fig. 1. Optical micrographs of the cross-section of the two types of artificial defects: (a) cutter scribing and (b) machine scribing.

Fig. 2 .

 2 Fig. 2. Global impedance response in Bode representation for the AA2024 coated samples (CC and NCC)obtained after a day and a month immersion in 0.SM NaCI.

8 �Fig. 3 .

 83 Fig. 3. lmpedance modulus at 10 mHz (Z 1 omHz) as a function of immersion time in 0.5 M NaCI for the M2024 samples protected by CC and NCC.

Fig. 4 .

 4 Fig. 4. Corrosion potential (E.:.rr) as a fonction of immersion time in 1 mM NaCI for the scratched coated samples (small scratch). Error bars account for experiments perforrned on two samples and five scratches.

  Fig. S. Local impedance diagrams (Bode coordinates) obtained over the scratches after 24h immersion in 1 mM NaCI: (a, b) CC and (c. d) NCC -results of six independent measurements are sbown on the figures: (a, c) modulus and (b, d) phase angle.

  b ), the Al, Cu and Mg peaks, characterizing the Al 2 CuMg particles, can clearly be seen and the Si, S and Zn peaks are not observed, in agreement with the development of the corrosion. The local impedance diagrams recorded above these two areas display different behaviours (Fig. S ). The diagrams are character ized by a single capacitive loop but the impedance values are about ten times lower on the corroded zone. Over the black deposit, the impedance slightly increases with immersion time (Fig. Sa) but over the corroded area, the impedance decreases (Fig. Sb). Fig. 9 compares the local impedance diagrams in Bode coordinates obtained for the two distinct areas of the NCC sample after 24 h and 504 h of immersion. The diagram of the bare AA2024 is reported in Fig.

4 Fig. 6 .(Fig. 7 . 4 Fig. 8 .

 46748 Fig. 6. Local impedance di�rams above the scratch for CC as a function of immersion time in 1 mM NaO: (a) Nyquist, (b) impedance modulus and (c) phase angle.

0Fig. 9 .

 9 Fig. 9. Local impedance di�rams (Bode represenration) obtained for NCCafter 24 h and 504 h of immersion in 1 mM NaCI at two dilferent locations above the scratch: non-<:orroded zone (open symbol) and corroded zone (solid symbol).

Fig. 10 .

 10 Fig. 10. Variation as a fonction of immersion time in 1 mM Na Cl of dilferent parameters extracted from the local impedance diagrams for CC and NCC: ( a) impedance modulus at 1 Hz (Z1ttz) and (b, c) CPE parameters (a and Q). Error bars account for experiments performed on two samples and five scratches.

o 4 Fig. 11 .

 411 Fig. 11. local admittance modulus obtained over the scratches for CC and NCC after 504 h of immersion in 1 mM NaCI. Comparison with the local admittance modulus of the bare AA2024 and of the intact coating.

Fig. 12 .

 12 Fig. 12. Local admittance mappings performed at 1 kHz, 30 Hz and 3 Hz over the scratch for: (a, b, c) CC and (d, e, f) NCC after a month immersion in 1 mM NaCl.

Fig. 13 .

 13 Fig. 13. Local admittance mappings performed at 3 Hz over the scratch after 24 h, 48 h and 168 h of immersion in 1 mM NaCl for: (a, b, c) CC and (d, e, f) NCC.

Fig. 14 .

 14 Fig. 14. Local admittance mappings (2D) performed at 3 Hz over the scratch after 3 h, 72 h and 168 h of immersion in 1 mM NaCl for the CC sample: (a, b, c) small scratch and (d, e, f) large scratch.

Acknowledgments

The authors gratefully acknowledge the company MAPAERO (Pamiers, France) for the preparation of the coated samples and more particularly Pauline Côte and Pierre Jean Lathière for fruitful discussion on coating technologies. The PhD thesis of Nguyen Anh Son was prepared in the framework of the associated international laboratory "Functional Composite Materials (FOCOMAT)" between France and Vietnam.


Arnold Blaisonneau

Julie Maury

Antoine Armandine

Les Landes

Théophile Guillon

Hydromechanical modelling of the hydraulic stimulation of a fault zone as deep geothermal target

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Hydromechanical modelling of the hydraulic stimulation of a fault zone as deep geothermal target

Arnold Blaisonneau, Julie Maury, Antoine Armandine Les Landes, Théophile Guillon

In the framework of the H2020-DEEPEGS project the main technological challenge addressed is to optimise the well architecture and stimulation methods for different geological contexts in order to demonstrate the economic and technical viability of the deep geothermal exploitation. One of the two demonstrators, the VDH French doublet, targets a fault zone in the granitic basement as deep geothermal system in a rifting context (Upper Rhine Graben in eastern France).

This paper aims to conceptualize a fault zone to represent an equivalent hydromechanical behaviour. The objective is to keep the major features of the fault zone while simplifying secondary features that will make the model too complex. The question of how to realize such a conceptualization depending on the knowledge of tectonic context, lithology, hydraulic behaviour of each part of the fault zone and existing data will be addressed. This conceptual model will be used to model hydromechanical processes acting during the hydraulic stimulation of a well in such specific fractured context. Thanks to a DFN numerical model, built on the base of the conceptualization step, the efficiency of hydraulic stimulation will be tested with respect to the well trajectory. The results of the simulations will be analysed to get a qualitative response of the stimulated fracture network (most stimulated fractures and an estimated gain of their hydraulic aperture) around the open hole of the well and an improvement of the global well productivity enhancement.