INFLUENCE OF LOW-LEVEL IRON IMPURITIES ON THE INITIAL INTERACTION OF WATER-VAPOR WITH POLYCRYSTALLINE MAGNESIUM SURFACES

The room temperature interaction of water vapour with polycrystalline magnesium specimens deliberately contaminated with very low levels of iron (120 to 700 ppm by weight) has been examined using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and second ary ion mass spectrometry (SIMS). The initial adsorption and subsequen t reaction of the water molecules with the iron-containing specimens w as shown to be retarded during the very early stages. The influence of iron on the uptake of oxygen on magnesium surfaces was not found to b e a clear function of iron concentration. This was likely due to the f act that the solubility limit of iron in magnesium (approximately 55 w t ppm) was exceeded in each specimen, suggesting that iron in magnesiu m solid solution strongly influences the oxidation process. The oxide growth rate at longer exposures, during the bulk thickening regime of the oxidation process, was found to be unaffected by the iron content. Segregation of iron into the oxide film was, however, detected by sta tic secondary ion mass spectrometry (SSIMS). Finally, the saturated ox ide films formed on Fe-containing magnesium specimens were shown by XP S to be more ''defective'' than those formed on pure magnesium.