THE INITIAL INTERACTION OF WATER-VAPOR WITH MG-AL ALLOY SURFACES AT ROOM-TEMPERATURE

The room temperature interaction of water vapour with two polycrystalline Mg-Al alloy surfaces, Mg-3.Owt%Al and Mg-8.5wt%Al, has been examined in detail using Auger electron spectroscopy (AES) and X-ray photoclectron spectroscopy (XPS). By using calibrated doses of deuterated water vapour, three stages of early oxide growth were recognised: dissociative chemisorption during low exposures (up to almost-equal-to 0.5 langmuirs (L)); oxide nucleation and island growth during intermediate exposures (approximately 0.5-3.0 L); and slow, diffusion-controlled bulk oxide thickening after coalescence of the oxide islands. AES lineshape analysis indicated that water vapour dissociates on Mg-Al surfaces, leading to the simultaneous adsorption of oxygen atoms on both Mg and Al atoms present on the surface. Modelling results suggested that the rate of oxide nucleation and growth was enhanced on Mg-Al surfaces compared to on pure Mg surfaces, particularly at higher Al contents. In addition, it was found that magnesium oxide islands were preferentially nucleated at Mg-rich regions and subsequently grew to cover the mixed Mg-Al-O chemisorbed precursor layer. At longer exposures, XPS indicated that Al3+ ions were incorporated into the growing oxide film, resulting in an increased activation energy for ion movement and a pronounced decrease in the rate of bulk thickening. The surface concentration of aluminum was observed to decrease slowly during the bulk thickening regime. The results have been interpreted based on the Cabrera-Mott theory of low temperature oxidation.