Influence of low level iron impurities on the initial interaction of water vapour with polycrystalline magnesium surfaces

Abstract

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 secondary ion mass spectrometry (SIMS). The initial adsorption and subsequent reaction of the water molecules with the iron-containing specimens was 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 be a clear function of iron concentration. This was likely due to the fact that the solubility limit of iron in magnesium ( ~ 55 wt ppm) was exceeded in each specimen, suggesting that iron in magnesium 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 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.