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 3V 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.