The structure and the formation mechanism of surface oxide films grown on magnesium die cast AZ91D have been studied. The substrate surface after chemical etching shows grains of Mg solid solution matrix of several μm in size with Mg17Al12 compound at grain boundaries, large and small voids, and cavities between the grains. The film surface after chemical conversion coating has a granular structure, with each granule corresponding to a single grain. Direct cross sectional observation revealed that the film was formed by anodic reaction and had a porous cell structure which was separated from the substrate by a thin barrier layer at the film/metal interface. The porous film is composed of cell colonies in the sub-micron range that have branched fine pores, with central holes (mother pores) sized approximately 50nm. Square shaped holes about 300nm in size observed on the film surface are believed to be cathodic sites.
