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

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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.