Abstract
AZ31 Mg alloy was anodized with a pulse potential between the transpassive (10 VAg/AgCl) and active regions (−1.35 VAg/AgCl) in 2 M NaOH aqueous solution at 303 K. Optimal conditions for the pulse anodizing were a duty ratio of 91%, a frequency of 0.09 Hz, and an anodizing time of 600 s. Pulse anodizing caused a remarkable 11-fold decrease in the surface porosity and a 1.6-fold increase in the film thickness from those obtained under a constant potential of 10 VAg/AgCl. Furthermore, an Al-enriched crystalline oxide layer was formed on the outer surface of MgO, which improves the corrosion resistance of the Mg alloy in neutral solutions. In consequence, the pitting potential Epit increased to −1.36 VAg/AgCl and the corrosion current density icorr decreased to 60 ± 10 μA cm−2, resulting in an approximately 3-fold decrease in the corroded area after anodic breakdown and salt spray tests.
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