The Initial Interaction Of Water Vapour With Magnesium And Magnesium Alloy Surfaces

The initial interaction of water vapour with polycrystalline magnesium and magnesium alloy surfaces has been quantitatively followed from the earliest stage using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The fitting of AES data to theoretical models suggested that a three-stage oxidation behaviour was operative on pure Mg surfaces: (1) dissociative chemisorption during doses up to {dollar}{lcub}\approx{rcub}0.7{dollar} langmuir (L) (1L = {dollar}1.3\times10\sp{lcub}-4{rcub}{dollar} Pa{dollar}\cdot{dollar}s); (2) oxide nucleation and growth which is complete by {dollar}{lcub}\approx{rcub}5{dollar}L at an average island height of four monolayers; and (3) bulk thickening after coalescence of the oxide islands. The bulk thickening stage was adequately described by a logarithmic-type growth law and was found to be controlled by the movement of metal cations from the metal/oxide interface to the oxide/gas interface. Hydrogen (deuterium) was determined by nuclear reaction analysis (NRA) to be present in the film only in small relative amounts.;The effect of Ar{dollar}\sp+{dollar} ion bombardment on the interaction of water with pure Mg surfaces was systematically studied by AES. The effect was most pronounced for the oxide nucleation and growth stage. The dissociative adsorption and bulk thickening stages were only weakly affected by prior ion bombardment. The results have been interpreted based on the assumption of competition between the effects of radiation defects (vacancies, vacancy clusters, dislocation loops) and implanted argon atoms on the oxidation process.;Water interactions with Mg-Al alloy surfaces (Mg-3.0Al, Mg-8.5Al) resulted in enhanced oxide nucleation and growth at low exposures as compared to pure Mg. At longer exposures, Al{dollar}\sp{lcub}3+{rcub}{dollar} cations were incorporated into the growing oxide, resulting in a pronounced decrease in the rate of bulk oxide thickening.;Water adsorption and oxide growth on dilute Mg-Fe alloy surfaces (120 to 700 wt ppm) was shown to be retarded during the very early stages. The growth rate at longer exposures was unaffected by the iron content. Segregation of iron into the oxide film was, however, detected by static secondary ion mass spectrometry (SSIMS). Finally, the saturated oxide films formed on Fe-containing magnesium specimens were shown by XPS to be more defective than those formed on pure magnesium