Initial oxide-film growth on Mg-based MgAl alloys at room temperature

The initial, thermal oxidation of bare, Mg-based MgAl alloys (containing up to 7.31 at.% Al) at 304 K in the partial oxygen pressure range of 10−6 ⩽ pO2 ⩽ 10−4 Pa was investigated by angle-resolved X-ray photoelectron spectroscopy and real-time in situ spectroscopic ellipsometry. The chemical constitution of the initially grown oxide film resembles that of a MgO-type of oxide with, adjacent to the alloy/oxide interface, Al enrichments in both the oxide film and the subsurface region of the alloy substrate. The Al-to-Mg content of the oxide films is governed by the alloy composition in the subsurface region, which deviates from the bulk alloy composition due to sputter cleaning prior to oxidation. Continued oxide-film growth proceeds by the transformation of a defective surface-oxide structure into “bulk” oxide upon reacting with outwardly diffusing Mg cations under the influence of a surface-charge field. The effective rate of depletion of Mg from the alloy subsurface region is governed by the competing processes of preferential oxidation of Mg and interfacial segregation of Mg.