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

随着基体铝含量的增加，在5V左右出现了特殊的阳极氧化峰值电流。阳极极化测量结果表明，AZ91D合金在5V处电流峰值消失。此外，在电解液中加入AlO_2^-离子后，所有试样均消失。这是由于镁和氧化物的表面活性得到了保护性的提高。随着溶液中铝酸盐离子浓度的增加，膜中铝的含量呈线性增加。然而，除了溶液中铝酸盐离子的浓度外，盐雾试验评价的耐腐蚀性在5V左右形成的厚膜中最高，电流密度极高。此外，首次阐明了在5V下形成的薄膜呈现类似多孔阳极氧化铝Keller模型的垂直六边形细胞结构。薄膜的透射电镜（TEM）显示，薄膜由两层或三层组成，阳极氧化率为纯Mg为1.9nm/V， AZ91D合金为2.8 nm/V， MgLiY超轻合金为4.3 nm/V。它们高于铝的阳极氧化率（报道为1.4nm/V）。此外，我们还首次成功地利用磷酸盐溶液使MgLiY超右合金具有高耐蚀性。

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.