Formation of Anodic Films on Mg-Al Alloys in NaOH solutions at Constant Potentials

Abstract

The formation behavior and corrosion characteristics of anodic oxide films on pure magnesium and on Mg-Al alloys were investigated, focusing on the effects of anodization potential, aluminum content, temperature, and NaOH concentration. Pure magnesium and Mg-Al alloys were anodized for 600s at 3, 10, 40, and 80V in NaOH solutions. It was found that the anodic film formed at 3 V had the best corrosion resistance, regardless of temperature, NaOH concentration, or aluminum content. An especially high current density was observed at applied potentials of 3-7 V on anodization in alkaline NaOH solutions. XRD analysis detected Mg(OH) 2 and MgO peaks in the films on the anodized specimens. The relative intensity of the Mg(OH) 2 XRD peaks decreased with increasing applied potential, while those of MgO increased. Mg(OH) 2 was generated by an active dissolution reaction with high current density at the specimen surface. Generation of Mg(OH) 2 increased with increasing temperature, while that of MgO increased with NaOH concentration. Moreover, the current density after anodization for 600 s at a constant potential decreased with increasing aluminum content in Mg-Al alloys.