Structure and growth mechanism of anodic films formed on magnesium die cast AZ91D

The structure and the growth mechanism of anodic oxide films grown on magnesium die cast AZ91D have been studied by electron microscopy combined with an ultramicrotomy technique. The anodic films are highly uneven in thickness. This may have been caused by film destruction resulting from breakdown accompanied by gas evolution, and also by uneven anodic reaction owing to the presence of cavities and compositional irregularities at the grain boundaries. Porous cell structure with dispersed MgF₂ and NaMgF₃ crystalline particles is found in the films. It is assumed that anodic film growth proceeds by the formation of MgF₂ and magnesium oxyhydroxide at the metal/film interface and the dissolution of the film at the pore bases. The crystallization of MgF₂ and the formation of NaMgF₃ subsequently proceed in the porous layer.