Analysis on the microstructure and growth mechanism of surface oxide films on magnesium and magnesium alloys

镁在空气中自然形成的薄膜薄而致密，并且具有无定形结构。在潮湿空气中，由于水通过初始层进入，在金属和初始层之间形成水合层。在水中形成的膜包含具有片状形态的附加顶层，其通过微溶镁的再沉积形成。随着合金中铝含量的增加，各层均发生脱水并富集氧化铝，厚度减小。这些变化是显著的，尤其是当合金的铝含量增加到4wt%以上时，该阈值的特征还在于耐腐蚀性的显著改善。该转变对应于氧化物膜的最内层中35wt%的Al。MgAl合金中加入稀土元素后，耐蚀性进一步提高。这归因于显著的脱水，导致氧化物的稳定性和钝化性增加。对于纯镁阳极氧化膜的生长，通过直接截面观察证实了类似于Keller模型的圆柱形孔结构和阻挡层。据推测，阳极膜的生长主要通过在金属/膜界面处形成MgF 2和Mgx+y/20 x（OH）y以及在孔基底处膜的溶解来进行。MgF_2的晶化和NaMgF_3的形成在多孔层中同时进行。在镁压铸AZ 91 D上生长的膜中也发现了类似的孔结构;然而，膜的厚度高度不均匀。结果表明，化学转化膜是通过阳极反应形成的，具有多孔的细胞结构。尺寸约为300 nm的方形孔和晶界被认为是阴极位点。

The naturally formed film on magnesium in air is thin and dense, and it has an amorphous structure. In humid air, a hydrated layr forms between the metal and the initial layr as a result of water ingress through the initial layr. The film formed in water contains an additional top layr with platelet-like morphology, formed by re-deposition of sparingly soluble magnesium. With increasing aluminum content of the alloys, all layrs became dehydrated and enriched in aluminum oxide, and they decrease in thickness. These changes are significant especially as the aluminum content of the alloy is increased above 4 wt %, a threshold characterized also by a significant improvement in the corrosion resistance. This transition corresponds to 35 wt % of Al in the innermost layr of the oxide film. Alloying of the MgAl alloys with rare earth elements causes further improvement of the corrosion resistance. This is attributed to a significant dehydration, causing increased stability and passivity of the oxide. For the anodic film growth on pure magnesium, the cylindrical pore structure and barrier layr which are similar to the Keller's model of anodic alumina are confirmed by direct cross-sectional observation. It is assumed that anodic film growth proceeds mainly by the formation of MgF2 and Mgx+y/20x (OH) y at the metal/film interface and the dissolution of the film at pore bases. The crystallization of MgF2 and the formation of NaMgF3 simultaneously proceed in the porous layr. A similar pore structure is also found in the film grown on magnesium die cast AZ91D ; however, the film is highly uneven in thickness. It was revealed that the film after chemical conversion coating was formed by anodic reaction and had a porous cell structure. Square shaped holes about 300 nm in size and grain boundaries are believed to be cathodic sites.