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

The room temperature interaction of water vapour with two polycrystall ine Mg-Al alloy surfaces, Mg-3.Owt%Al and Mg-8.5wt%Al, has been examin ed in detail using Auger electron spectroscopy (AES) and X-ray photocl ectron spectroscopy (XPS). By using calibrated doses of deuterated wat er vapour, three stages of early oxide growth were recognised: dissoci ative chemisorption during low exposures (up to almost-equal-to 0.5 la ngmuirs (L)); oxide nucleation and island growth during intermediate e xposures (approximately 0.5-3.0 L); and slow, diffusion-controlled bul k oxide thickening after coalescence of the oxide islands. AES linesha pe 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 co mpared to on pure Mg surfaces, particularly at higher Al contents. In addition, it was found that magnesium oxide islands were preferentiall y nucleated at Mg-rich regions and subsequently grew to cover the mixe d Mg-Al-O chemisorbed precursor layer. At longer exposures, XPS indica ted that Al3+ ions were incorporated into the growing oxide film, resu lting in an increased activation energy for ion movement and a pronoun ced decrease in the rate of bulk thickening. The surface concentration of aluminum was observed to decrease slowly during the bulk thickenin g regime. The results have been interpreted based on the Cabrera-Mott theory of low temperature oxidation.