Effect of solution pH on stress corrosion and electrochemical behaviour of aluminum alloy with micro-arc oxidation coating

In order to investigate the effect of solution pH on stress corrosion and electrochemical behavior of aluminum alloy with micro-arc oxidation (MAO) coating, the constant load test and in situ electrochemical impedance spectroscopy (EIS) had been carried out on 7050-T6 aluminum alloys (AA7050-T6) with MAO coating in 3.5 wt% NaCl solution at pH 3, pH 7 and pH 13. EIS was used to evaluate corrosion process of MAO coating and the corresponding equivalent circuit model was established. The results of EIS and mechanical properties of MAO specimens after constant load test exhibited that pH have a significant effect on the corrosion resistance and plasticity of the aluminum alloy with MAO coating. The corrosion resistance was as follows: pH 7 > pH 3 > pH 13. The loss of plasticity was as follows: pH 7 < pH 3 < pH 13. The EIS and the corroded surface morphology demonstrated that the corrosion damage was dominated by pitting corrosion in 3.5%wt NaCl solution at pH 3 and 7. General corrosion was dominant in 3.5%wt NaCl solution at pH 13. In 3.5 wt% NaCl solution at pH 3 and 7, the corrosion resistance decreased at first and then increased at the beginning of corrosion. In the middle stage, the corrosion resistance decreased at first and then stabilized. The corrosion resistance decreased again in the final stage of corrosion. In 3.5 wt% NaCl solution at pH 13, the corrosion resistance decreased at the beginning of corrosion and then increased, and the MAO coating exfoliated completely in the middle stage of corrosion. (paper)