Corrosion behavior of AZ91 Mg alloy anodized by low-energy micro-arc oxidation: Effect of aluminates and silicates

    Corrosion behaviour of AZ91 Mg alloy anodized by low-energy micro-arc oxidation: Effect of aluminates and silicates D. Veys-Renaux, C.-E. Barchiche, E. Rocca PII: DOI: Reference:

S0257-8972(14)00353-3 doi: 10.1016/j.surfcoat.2014.04.031 SCT 19351

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

5 February 2014 17 April 2014 18 April 2014

Please cite this article as: D. Veys-Renaux, C.-E. Barchiche, E. Rocca, Corrosion behaviour of AZ91 Mg alloy anodized by low-energy micro-arc oxidation: Effect of aluminates and silicates, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.04.031

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ACCEPTED MANUSCRIPT Corrosion behaviour of AZ91 Mg alloy anodized by low-energy micro-arc oxidation: effect of aluminates and silicates D. Veys-Renaux, C.-E. Barchiche, E. Rocca.*

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Institut Jean Lamour-UMR CNRS 7198, Université de Lorraine, BP 70239, Vandoeuvre-lès-

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Nancy, 54506, France.

*Corresponding author: [email protected]

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Abstract :

AZ91 Mg alloy was anodized by micro-arc oxidation under a low constant current density (10

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mA cm-2) in an electrolytic bath containing KOH 3 M + KF 0.5 M + Na3PO4.12H2O 0.25 M. The effect of the anodizing process duration and of the presence of aluminate (NaAlO2 0.2 M)

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or silicate (Na2SiO3.9H2O 0.2 M) as additives were investigated. In terms of corrosion resistance, electrochemical methods (namely potentiodynamic scans and chronoamperometric

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measurements) corroborate the results of salt spray test and show that the resistance to pitting corrosion of the treated pieces is not systematically improved by a thicker anodized layer.

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Actually, the composition of the protective coating is the key factor: the best resistance is obtained in the presence of silicate which plays the role of self-healing agents in corrosive conditions, whereas the incorporation of aluminate in the oxide has a very weak effect on the corrosion resistance of the treated alloy.

Keywords:

magnesium alloy, micro-arc oxidation, corrosion, electrochemistry, salt spray

test.

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ACCEPTED MANUSCRIPT 1. Introduction In the framework of the reduction of energy consumption in the aircraft industry, the lightening of the structures is necessary and the use of magnesium has become of major

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interest. To fight against the high sensitivity to corrosion of magnesium (especially pitting corrosion [1]), a first way consists in alloying it (with aluminum and zinc for example) in

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order to reinforce the naturally formed passivation layer. However, the resistance enhancement is not sufficient and a surface treatment is generally necessary. Chemical

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conversion processes such as phosphatation [2-3] and chromatation [4] are commonly used as well as electrochemical conversion processes (anodizing) leading to more protective coatings

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(commercialized as HAE and DOW17 [5]). Micro-arc oxidation (MAO) or plasma electrolytic oxidation (PEO) appears as an interesting alternative solution [6] since it consists

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of a single-step anodizing process bringing both corrosion resistance [7-8] and wear resistance

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[9-10] to the coated materials. The main difference with a classical anodizing process is the

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high voltage of the treatment: the dielectric breakdown of the interface is reached and sparking electric discharges form at the metal/electrolyte interface. The resulting plasma contains both elements from the alloy and from the bath. The very high temperature reached

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at the interface induces the growth of a dry “ceramic-like” coating. Its morphology and its microstructure depend on the electrical parameters [11] and the electrolyte composition [1220]. Despite numerous publications devoted to the effect of additives such as silicates or aluminates, results are often difficult to compare because electrolytes are used in various electrical anodizing conditions.

However, many publications point out that silicate-type

additives induce the formation of forsterite Mg2SiO4 and aluminate-type additives induce the formation of the MgAl2O4 spinel compound. Moreover, the Mg2SiO4-enriched oxide seems to enhance the corrosion resistance on Mg alloys, and MgAl2O4 enriched oxide provokes a better wear resistance of the coatings. In all cases, the coating growth rate is increased by the presence of oxoanions as aluminates, silicates or phosphates in alkaline electrolytes. 2

ACCEPTED MANUSCRIPT This paper focuses on the corrosion resistance of AZ91 Mg alloy protected by MAO under a low-current galvanostatic regime (10 mA cm-2) in a classical phosphate/fluoride containing electrolytic bath. The treatment duration and the effect of additives (aluminates, silicates) in

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the electrolytic bath are considered. Most of the authors investigate the protective properties

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of coatings by electrochemical methods carried out in immersion in corrosive media.

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However, non-electrochemical accelerated corrosion tests have the advantage to reveal more clearly the corrosion behavior of pieces in service, despite their long duration and the

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difficulty of results interpretation. Consequently, only few papers were devoted to the study of salt spray tests performed on anodized magnesium alloys [21-22]. Recently, S. Yagi et al.

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have observed a self-repairing effect of some compounds on scratched anodized ACM522 magnesium alloy [21] exposed to salt spray tests during short a short period (168 h). In the

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present work, the corrosion behavior of coated samples, especially the pitting corrosion

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resistance, is comparatively studied by electrochemical methods (polarization curves,

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chronoamperometry) and by 500 h exposure to salt spray test. The results are explained in the light of metallographic analyses performed before and after the corrosion tests.

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2. Material and methods 2.1 Micro-arc oxidation process Magnesium alloy used is AZ91D. Plates (thickness 1.2 cm) were prepared by high pressure die casting with the following composition: 8.3-9.7 wt.% Al, 0.15 wt.% Mn, 0.35-1 wt.% Zn, 0.1 wt.% Si, 0.005 wt.% Fe, 0.03 wt.% Cu, 0.002 wt.% Ni and Mg for the rest. Prior to MAO treatment, all the samples were abraded with SiC paper up to 1200 grit, etched in 650 g L-1 phosphoric acid for 10 s, rinsed with deionised water for 30 s, immersed in a 100 g L-1 NaOH solution during 10 s, and finally rinsed with deionised water for 30 s. Anodizing process was performed by using a Micronics Generator System (Type MX 300 V10 A) in a two-electrode cell [23]. The sample was set as an anode (active surface: 32 cm2) 3

ACCEPTED MANUSCRIPT and two titanium cathodes were positioned on both sides, to ensure a homogeneous coating over the entire surface. A constant current density of 10 mA cm-2 was applied in a reference electrolytic bath containing 3 M KOH, 0.5 M KF, 0.25 M Na3PO4.12H2O (noted further “no

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additives” bath), with a measured pH equal to 13.4. Two kinds of additives to the reference

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bath were separately studied: silicates 0.2 M (Na2SiO3.9H2O) and aluminates 0.2 M

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(NaAlO2). A short anodizing duration (10 min) and a long one (1 h) were compared. The morphology and the composition of the resulting coatings were investigated by scanning

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electron microscopy (SEM Hitachi S-4800, Secondary Electron Mode for surface analyses and Backscattered Electron Mode for cross-section analyses) coupled with energy dispersive

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X-Ray spectroscopy (EDX) (acceleration voltage : 15 kV, Al2O3 and SiO2 and MgO used as

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2.2 Corrosion resistance

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standard compounds).

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The corrosion resistance of the coated materials was first evaluated by electrochemical methods carried out in a reference corrosive solution, noted “ASTM water” further in the text (ASTM standard water according to D1384-87), which contains : Na2SO4 148 mg L-1,

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NaHCO3 138 mg L-1 and NaCl 165 mg L-1 (pH=8.3 ; σ = 650 µS cm-1). The set-up was made of a three-electrode cell connected to a potentiostat (EGG PAR 273A) and a Frequency Response Analyzer (Solartron 1255) driven by a computer equipped with the SoftCorr PAR software for data acquisition and analyses. The sample is mounted as a circular working electrode with 3 cm2 of active area, facing to a disk of platinum (1 cm2) used as a counter electrode [24]. The reference electrode was a KCl-saturated calomel electrode (SCE, E = +0.242 V/SHE). In the results part, all the potentials are given versus this reference. The following experimental sequence was performed for all treated samples (and for an untreated one):

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ACCEPTED MANUSCRIPT i) Measurement of the corrosion potential (Ecorr) (or open-circuit potential) during 24 hours in ASTM corrosive water. ii) Measurement of the anodic and cathodic polarization curves separately after 24 hours

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immersion in ASTM water with a scan rate of 1 mV s-1. These curves were not ohmic-

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drop compensated. Nevertheless, the distance between the reference electrode and the

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working one was kept constant (1 cm) thanks to fixed positions in the electrochemical cell.

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Besides the potentiodynamic experiments, chronoamperometric measurements were performed at Ecorr+100 mV after 24 hours immersion in order to evaluate the resistance to

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pitting corrosion of the treated samples.

The investigation of the corrosion resistance was supplemented by a non-electrochemical

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method, namely a salt spray test (SST) performed in accordance with the ASTM B117

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standard method in a climatic chamber, model S450 Ascott [25] (NaCl solution: 50 g L-1,

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temperature: 35°C). After 500 hours of exposure to the salt spray test, the samples were cut and analyzed in cross-section by SEM (Hitachi S-4800).

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3. Results

3.1 MAO process and coatings characterization The MAO process under such low-current galvanostatic conditions consists in four steps (Fig 1). In 10 seconds, the voltage reaches 20 V through a linear increase, very likely thanks to the presence of fluorides in the electrolyte, reinforcing the electrochemical interface resistance, as shown by a previous study [26]. Then, from 20 V, a second step of voltage rise lasts 30 seconds to 1 minute and is interrupted when the breakdown voltage of the dielectric interface has been reached leading to the occurrence of sparking electric discharges. The voltage evolution during this step, as well as the value of the breakdown voltage, is influenced by the presence and the nature of additives 5

ACCEPTED MANUSCRIPT in the electrolytic bath. Actually this step corresponds to the growth of an insulating layer in which all the elements from the electrolyte can be incorporated and consequently modify its dielectric properties [27-28]. Silicates and aluminates reinforce the resisting properties of this

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first anodized layer (grown by a classical anodizing regime) so that the breakdown voltage is

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reached faster in presence of these additives in the electrolytic bath.

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The third step named micro-arc regime is visually determined by the apparition of the first sparks, and begins at 47 V in the silicate-containing bath, 53 V in the reference electrolytic

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bath and 55 V in the presence of aluminates. At the beginning of the micro-arc oxidation regime, sparks are fine and the voltage rises slowly.

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A fourth step of anodizing starts after 10 minutes of treatment and is characterized by very energetic electric discharges. From an electrochemical point of view, this last high-voltage

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regime is marked by instability of the voltage value until 1 hour of treatment.

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The thickness of the grown coatings, revealed by the cross-sections of the treated samples,

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depends both on the anodizing duration and on the electrolytic composition (Fig 2 and Fig.3). From around 5 µm after 10 min of anodizing (Fig. 3a), the thickness of the coatings increases under the effect of plasma discharges and varies in the range of 10 to 20 µm after 1 h of

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anodizing (Fig. 2 a, c, e).

A very thin and adhesive inner layer (submicronic scale),is present in all cases. This inner layer is formed under a classical anodic oxidation mechanism, before the micro-arc regime [28]. Aluminates added to the reference electrolytic bath obviously reduce the coating growth rate whereas silicates increase it and modify the roughness of the film. Actually, a typical island-like morphology is observed at the surface of all samples, but the islands are wider and thicker in the silicate-containing electrolyte (Fig 2 b, d, f). The semi-quantitative estimation of elemental compositions by EDX spectroscopy shows the incorporation of the additives in the coatings. Coatings formed in the silicate-containing electrolyte contain 2 to 3 wt.% Si. Coatings formed in the aluminate-containing electrolyte 6

ACCEPTED MANUSCRIPT contain 5 to 6 wt.% Al whereas coatings formed in the other electrolytes contain 1.5 to 2.5 wt.% Al, arisingfrom the oxidation of the Al-rich intermetallic phases dispersed in the AZ91 alloy. No significant difference in the elemental compositions of coatings could be noted

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between short and long anodizing duration.

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3.2 Electrochemical behavior

The electrochemical behavior of the samples, investigated by corrosion potential potentiodynamic

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measurements,

4b, Fig. 5b, Fig. 6b) anodizing.

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Corrosion potential measurements

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performed in ASTM water, is described after 10 min (Fig. 4a, Fig. 5a, Fig. 6a) and 1 h (Fig.

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Corrosion potential measurements show a similar behavior of the untreated and short-time

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anodized samples during the ten first hours of immersion in corrosive water, characterized by a quite rapid increase of the values, corresponding to the passivation of the surface (Fig. 4a). Then, in the case of the untreated sample, the corrosion potentials oscillate around an average

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value, corresponding to the initiation of a pitting process, whereas they brutally fall after 12 to 15 hours of immersion for the short-time anodized samples, meaning a break in the passivation phenomenon. After 1 hour of anodizing (Fig 4b), the corrosion potentials of the different treated samples evolve quite differently. Whereas the corrosion potential of the sample anodized without additive stabilizes after 3 hours at a relative low value (-1.6 V vs. SCE), the sample anodized with aluminates shows an increase in the corrosion potential values followed by a progressive decrease, indicating a reduction in the passivation efficiency. Regarding the sample anodized in the silicate-containing electrolytic bath, the corrosion potential is erratic during the first 5

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ACCEPTED MANUSCRIPT hours of immersion. Then, the trend is clearly a continuous increase of the corrosion potential values, which seems to reveal a progressive transformation of the coating in ASTM water.

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Polarization curves

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After 24 h of immersion in ASTM water, consistently with the previous measurements, the

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corrosion potentials of the samples treated during 10 min and of the untreated one are confined in the range -1.5 to -1.4 V vs SCE (Fig. 5a). The cathodic behavior of all anodized

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samples is similar, with a slight inhibition of the water reduction process thanks to the coating, in comparison with the untreated sample. Regarding the anodic side of the

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polarization curves, the current density of the untreated alloy shows a quick increase above the corrosion potential, characteristic of an active state with dissolution of the alloy in

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oxidizing conditions. On the other hand, all treated samples exhibit a passive state with a

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reduced constant current density (about 1 µA cm-2). The value of the passivation current

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density seems to be independent of the composition of the treatment bath, but the length of the passivation plateau is extended in the case of the aluminate-containing electrolyte. Long anodizing treatment (Fig. 5b) modifies more the cathodic part of the polarization curves

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than short anodizing process. Actually, the kinetic of water reduction is significantly decreased, especially for the sample treated in the silicate-containing bath. Regarding the anodic side, the current density values on the passivation plateau are in the same range for long time anodized samples than for short time anodized samples (1 µA cm-2). The increase of anodizing time has no effect on the passivation plateau for the sample anodized in the reference bath. Nevertheless, in presence of aluminates, the passivation plateau has completely disappeared. And the passive behavior has been greatly improved for the sample anodized for a long time in the silicate containing electrolyte, as indicated by the large passivation plateau on Fig. 5b.

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ACCEPTED MANUSCRIPT Chronoamperometric measurements Chronoamperometric measurements, performed under +100 mV polarization relative to the corrosion potential after 24 hours immersion in ASTM water, enable to compare the

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resistance to localized corrosion of the samples anodized during 10 min (Fig. 6a) and during

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1 h (Fig. 6b) with an untreated one.

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By decreasing the current density by two orders of magnitude during this test, the short treatments greatly enhance the resistance against localized corrosion, identically whatever the

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chemical composition of the electrolytic bath. Surprisingly, longer anodizing durations damage this gain for the samples treated in the reference electrolytic bath and in presence of

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added aluminates. In these cases, the measured current density is only 3 to 4 times lower compared to this measured on the uncoated sample. Nevertheless, in the specific case of

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silicate-containing electrolyte, the behavior is improved by the extension of the anodizing

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process.

3.3 Characterization after salt spray test As shown in Fig. 7a, after 500 h of exposure to the salt spray test, the untreated plate is

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completely corroded: a thick corrosion products layer (between 200 and 300 µm) is observed over the entire surface, corresponding to a generalized corrosion. Visually, white zones on the Fig. 7a mainly correspond to the accumulation of corrosion products of magnesium and black zones are related to the presence holes or deep pits on the surface of plate. Locally, large size “corrosion points” (or pits) and/or corrosion product aggregates can be observed (Fig. 8a). From visual observations (Fig. 7), whatever the duration of the anodizing process and the composition of the electrolyte, the corrosion behavior is largely improved by all the treatments. After short anodizing treatments (Fig. 7b, d and f), the corrosion phenomena are significantly reduced for all samples but have not completely disappeared. Only dark zones, distributed 9

ACCEPTED MANUSCRIPT over the entire surface, can be macroscopically observed, corresponding to the developments of some large pits, in which the anodized layer and the alloy have been dissolved by the run off of NaCl solution in salt spray chamber. Microscopic cross-sections reveal moreover that

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the thickness of the anodized layer is not sufficient to avoid the expansion of pits during this

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accelerated corrosion test (Fig. 8b). In fact, the numerous cracks and thin zones in the 10 min-

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anodized layer are responsible for the formation of pits. Moreover the thin anodized layer can be easily transformed or incorporated into corrosion products of the treated samples in these

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corrosion conditions.

After 1 hour of anodizing treatment, the composition of the electrolytic bath seems to make a

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real difference in terms of corrosion resistance between the different plates. In the cases of plates treated during 1 h in the reference bath and in the presence of aluminates, the visual

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aspect of anodized plates are quasi-similar to the one anodized during 10 min : localized dark

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pits appear on the surface (Fig. 7c and e), which correspond to the typical metallographic

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cross section displayed on Fig. 8b. In opposite, the plate anodized in the presence of silicates exhibits a real high corrosion resistance after 500 h of salt spray test: only a few pits of small size are macroscopically observed (Fig. 7g). As can be noted on the metallographic

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observations of Fig. 8c, the thick silicate-containing anodized layer is covered by an outer layer of corrosion products, in which EDX spectroscopy analysis reveals enrichment in silicon. In this case, the silicon-containing anodized layer seems to be partially transformed into silicon-enriched corrosion products reinforcing the protection.

4. Discussion The combination of electrochemical and non-electrochemical tests reveals that the corrosion resistance of AZ91 Mg alloy is improved by all kind of treatments but not systematically enhanced by an increase of the treatment duration. From an electrochemical point of view, a thicker anodized layer (induced by a longer anodizing treatment) results in an inhibition of the 10

ACCEPTED MANUSCRIPT cathodic reaction due to a slowing down of the diffusion process of electroactive species through the coating. Regarding the anodic dissolution of the alloy, electrochemical measurements performed after

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short anodizing indicate an improvement of corrosion resistance of AZ91 Mg alloy with both

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the apparition of a passivation plateau and a decrease of the current density measured during

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anodic chronoamperometric experiments. This enhancement of the corrosion resistance is corroborated by the observations after salt spray exposure whatever the composition of the

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electrolyte. Nevertheless, the coating thickness remains thin so that the formation of pits cannot be completely inhibited.

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After a longer anodizing treatment (1 h of anodizing), the coatings are thicker, but the key parameter seems to be the composition of the anodized oxide, that mainly depends on the

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electrolyte composition. Samples anodized for 1 hour in the bath without and with aluminates

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do not exhibit a better corrosion resistance than samples anodized for 10 minutes despite a

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thicker coating. Localized corrosion form occurring on long-time anodized pieces could be due to the macro-cracks created by the high energetic electric discharges across the thick oxide layer. In the specific case of long anodizing treatment with silicates, electrochemical

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results and observations after salt spray test exposure clearly indicate a significant improvement in the corrosion resistance regarding short anodizing treatment. The passivation plateau is lengthened, the chronoamperometric current is lowered, and the number of pits is hugely reduced after exposure to salt spray test. However, SEM observations do not show a significant improvement of density of the oxide layer (or reduction of number of holes or cracks) formed in the silicate-containing electrolyte. Actually, high energetic discharges observed during long treatment under galvanostatic conditions lead also to the formation of cracked coatings but induce the formation of forsterite Mg2SiO4, thanks to the high temperature reached at the interface, as shown by previous studies [28]. According to the SEM analysis after salt spray test, the presence of an outer layer enriched in silicon suggests a 11

ACCEPTED MANUSCRIPT mechanism of sealing of the porosities during the corrosion process. Indeed, magnesium silicate compounds are very insoluble and can form in contact with water voluminous compounds of serpentine type and brucite, as follows [29]:

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2 Mg2SiO4 + 3H2O  Mg3Si2O5(OH)4 + Mg(OH)2.

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This mechanism may explain the good corrosion behaviour of silicate-enriched anodized layer

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on AZ91 alloy.

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5. Conclusion

Magnesium alloys can be easily anodized under a galvanostatic mode with a low current

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density (10 mA cm-2). With a same current density, microanalysis and microscopic observations reveal that the growth rate, the morphology and the composition of the coatings

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depend on the composition of the electrolyte, especially the presence of aluminates or

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silicates.

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Regarding the corrosion resistance, electrochemical methods combined with a nonelectrochemical test show that the corrosion behavior of anodized magnesium alloy is not systematically improved by a thicker anodized layer, because the coating remains cracked and

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porous whatever the anodizing time. Nevertheless, the presence of silicates as additives in the anodized layer clearly enhances the corrosion resistance, silicon-containing corrosion products playing the role of self-healing agents of porosities in corrosive conditions.

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ACCEPTED MANUSCRIPT References [1] J.E. Gray, B. Luan, J. Alloy. Compd. 336 (2002) 88. [2] L. Kouisni, M. Azzi, M. Zertoubi, F. Dalard, S. Maximovitch, Surf. Coat. Technol. 185

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[25] ASTM B117-11 standard in “ASTM Volume 03.02 Corrosion of Metals; Wear and

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ACCEPTED MANUSCRIPT List of figure captions

Figure 1: Voltage-time curves of micro-arc process performed on AZ91 samples with a

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constant current density (10 mA cm-2) in a KOH/KF/Na3PO4 based electrolyte.

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Figure 2: SEM micrographs of AZ91 samples anodized during 1 h (10 mA cm-2) in a KOH/KF/Na3PO4 based electrolyte: without additives (a, b), with 0.2 M aluminates (c, d),

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with 0.2 M silicates (e, f). Cross-section (a, c, e) and surface (b, d, f) observations.

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Figure 3: SEM micrographs of AZ91 samples anodized during 10 min (10 mA cm -2) in a KOH/KF/Na3PO4 based electrolyte with 0.2 M silicates: cross-section (a) and surface (b)

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observations.

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Figure 4: Corrosion potential evolution versus immersion time in ASTM water of AZ91 samples anodized (10 mA cm−2) in a KOH/KF/Na3PO4 based electrolyte during 10 min (a),

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and 1 h (b).

Figure 5: Potentiodynamic scans performed after 24h immersion in ASTM water of Mg AZ91 samples anodized (10 mA cm−2) in a KOH/KF/Na3PO4 based electrolyte during 10 min (a), and 1 h (b).

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ACCEPTED MANUSCRIPT Figure 6: Chronoamperometric curves recorded after 24 h immersion in ASTM water at E=Ecorr+100 mV on AZ91 samples anodized (10 mA cm−2) in a KOH/KF/Na3PO4 based

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electrolyte during 10 min (a), and 1 h (b).

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Figure 7: Visual aspect after 500 h of salt spray test of AZ91 samples anodized (10 mA cm−2)

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in a KOH/KF/Na3PO4 based electrolyte : untreated (a); without additives during 10 min (b) and 1 h (c) ; with 0.2 M aluminates during 10 min (d) and 60 min (e) ; with 0.2 M silicates

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during 10 min (f) and 1 h (g).

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Figure 8: Typical cross-section observations after 500 h of salt spray test: (a) Generalized corrosion (untreated samples); (b) Extended pits (case of 10 min-anodized with silicates); (c)

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“sealed” coating (case of 1h-anodized samples with silicates).

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Figures

Figure 1: Voltage-time curves of micro-arc process performed on AZ91 samples with a

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constant current density (10 mA cm-2) in a KOH/KF/Na3PO4 based electrolyte.

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a)

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Figure 2: SEM micrographs of AZ91 samples anodized during 1 h (10 mA cm-2) in a KOH/KF/Na3PO4 based electrolyte: without additives (a, b), with 0.2 M aluminates (c, d), with 0.2 M silicates (e, f). Cross-section (a, c, e) and surface (b, d, f) observations.

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a)

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Figure 3: SEM micrographs of AZ91 samples anodized during 10 min (10 mA cm -2) in a

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KOH/KF/Na3PO4 based electrolyte with 0.2 M silicates: cross-section (a) and surface (b)

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b) Figure 4: Corrosion potential evolution versus immersion time in ASTM water of AZ91 samples anodized (10 mA cm−2) in a KOH/KF/Na3PO4 based electrolyte during 10 min (a), and 1 h (b). 20

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(b) Figure 5: Potentiodynamic scans performed after 24 h immersion in ASTM water of Mg AZ91 samples anodized (10 mA cm−2) in a KOH/KF/Na3PO4 based electrolyte during 10 min (a), and 1 h (b). 21

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b) Figure 6: Chronoamperometric curves recorded after 24 h immersion in ASTM water at E=Ecorr+100 mV on AZ91 samples anodized (10 mA cm−2) in a KOH/KF/Na3PO4 based electrolyte during 10 min (a), and 1 h (b). 22

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c)

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Figure 7: Visual aspect after 500 h of salt spray test of AZ91 samples anodized (10 mA cm−2) in a KOH/KF/Na3PO4 based electrolyte : untreated (a); without additives during 10 min (b) and 1 h

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c) Figure 8: Typical cross-section observations after 500 h of salt spray test: (a) Generalized corrosion (untreated samples); (b) Extended pits (case of 10 min-anodized with silicates); (c) “sealed” coating (case of 1h-anodized samples with silicates). 24

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Highlights

Electrochemical measurements and salt spray test provide the same ranking for materials treated in different conditions.

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The pitting corrosion resistance is not systematically enhanced in case of thicker coating.

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Silicates incorporated in the anodized layer play the role of self-healing agents in corrosive conditions.

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