The structure and the composition of surface films formed by chemical conve
rsion coating in Dow7 on pure magnesium and magnesium die cast AZ91D have b
een studied by XRD, XPS, SEM and TEM combined with ultramicrotomy to clarif
y the growth mechanism. The films are amorphous containing no definite crys
tallites detectable by XRD. The major constituents of the films are MgF2 an
d MgOx(OH)(y). The content of NaMgF3 increases very much with increasing fi
nishing 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 additi
on to above film components on pure magnesium. The film surface of AZ91D sh
ows 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 co
nfirmed 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 o
f 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.