Mechanism of chemical conversion coating film growth on magnesium and magnesium alloys

The structure and the composition of surface films formed by chemical conversion coating in Dow7 on pure magnesium and magnesium die cast AZ91D have been studied by XRD, XPS, SEM and TEM combined with ultramicrotomy to clarify the growth mechanism. The films are amorphous containing no definite crystallites detectable by XRD. The major constituents of the films are MgF2 and MgOx(OH)(y). The content of NaMgF3 increases very much with increasing finishing time. In addition, small amounts of Cr2O3 and NH4+, whose contents also increase with finishing time, are found. In the film formed on AZ91D, a substantial amount of Al, presumably present as spinel (MgAl2O4) or as it's hydroxide, and small amounts of FeOx(OH)(y) and Mn4+ are found in addition to above film components on pure magnesium. The film surface of AZ91D shows a granular structure with each granule corresponding to a single grain of the substrate. Cylindrical porous cell structure of chemical conversion coating films on magnesium, which is formed by anodic reaction, has been confirmed for the first time by direct TEM observation. Film growth proceeds mainly by the formation of MgF2 and MgOx(OH)(y) at film/metal interface by anodic reaction and subsequent film dissolution followed by precipitation of NaMgF3, Cr2O3 and NH4+ in the film. The porous film is composed of cell 4 colonies in the size of sub-microns having central mother pores, which are branching into fine pores.