The effects of ion implantation on the thermal oxidation of iron and iron-chromium alloys

The thermal oxidation of pure iron has been studied at 350 to 500 deg C in the oxygen pressure range 0.1 to 10-5Torr. The oxidation of Fe-9% Cr alloy, self- and chromium-implanted iron was also studied at 400 deg C and 0.1 Torr. A novel oxidation apparatus based on a solid-state oxygen electrolyte was developed, which measured oxidation rates and was sensitive to transient oxidation effects. Oxidised and non-oxidised specimens were examined. Initially iron oxidised rapidly to form magnetite, whilst a subsequent reduction in rate was accompanied by formation of haematite crystals on the specimen surface. Haematite formation is slower at lower pressures, and this oxide is not observed below 10-3Torr due to continuous reduction by iron. Fe-9% Cr alloy oxidised parabolically at one tenth of the rate of pure iron, forming a protective M2O3 oxide containing chromium adjacent to the metal. Self-implanted iron oxidised parabolically, whereas chromium implantation produced an initial low oxidation rate which increased markedly once the shallow implanted layer had been consumed. Radiation damage caused by implantation promoted more rapid oxidation, particularly with high ion doses; smooth fine-grained oxides were formed, containing an increased number of short-circuit diffusion paths and a high proportion of spinel. (author)