Nature of Surface Film on Matrix Phase of Mg Alloy AZ80 Formed in Water

The nature of the surface film formed on the α-Mg matrix phase of AZ80 in pure H2O was investigated using scanning transmission electron microscopy–energy-dispersive spectroscopy (STEM-EDS). Thin foil cross sections of the film/substrate interface were prepared using focused ion beam (FIB) milling. The film formed consists of a diffuse bilayer structure. A relatively thick, more-hydrated outer layer is found to form on a thinner, less-hydrated inner layer. Al is found to be enriched in both layers relative to the substrate, but is enriched to a higher level within the outer layer. No evidence of an inner aluminum oxide (Al2O3)-rich skeletal layer is observed. The EDS analyses suggest that the more-hydrated outer layer is comprised of double Mg-Al hydroxide-type compound, which is formed as a corrosion product.