Growth mechanism of surface oxide films on magnesium and magnesium alloys and nano-scale control of film structure

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The peculiar peak current appeared at around 5V of constant anodizing of magnesium decreased with increasing aluminum content of the substrates. Anodic polarization measurements show that the current peak at 5V disappears for AZ91D alloys. In addition, it disappears for all specimens when AlO_2^- ion was added in the electrolytes. This is caused by the improvement of surface passivity of magnesium and oxide protectively. Aluminum content incorporated in the film lineally increased with log of increasing concentration of aluminate ions added in the solution. However, besides the concentration of aluminate ions in the solution, the corrosion resistance evaluated by salts spray test was highest in the thick films formed at around 5V with extremely high current density. Further, it is clarified for the first time that the films formed at 5V exhibit vertical hexagonal cellular structure like the Keller model of porous anodic alumina. TEM images of the cross section of the films showed that the film was composed of two or three layers indicating the anodizing ratio of 1.9nm/V for pure Mg, 2.8 nm/V for AZ91D alloy and 4.3 nm/V for MgLiY super light alloy. They are higher than the anodizing ratio of aluminum, which is reported to be 1.4nm/V. Furthermore, we succeeded to give high corrosion resistance to MgLiY super right alloy for the first time by using phosphate solution.