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

AZ91 Mg alloy was anodized by micro-arc oxidation under a low constant current density (10 mA cm− 2) in an electrolytic bath containing KOH 3M + KF 0.5M + Na3PO4⋅ 12H2O 0.25 M. The effects of the anodizing process duration and the presence of aluminate (NaAlO2 0.2 M) or silicate (Na2SiO3⋅ 9H2O 0.2 M) as additives were investigated. In terms of corrosion resistance, electrochemical methods (namely, potentiodynamic scans and chronoamperometric 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. 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.