The thermal oxidation of pure iron has been studied at 350-500°C in the oxygen pressure range 0.1 to 10-5 Torr. The oxidation of Fe-9% Cr alloy, self- and chromium-implanted iron was also studied at 400°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 using optical and electron microscopy, secondary ion mass spectrometry and various x-ray techniques. Ion implantation was performed at 300 kV and theoretical (LSS) calculations of ion distributions were made. Initially iron oxidised rapidly to form magnetite, whilst a subsequent reduction in rate was accompanied by formation of haematite crystals on the specimen surface Oxidation mechanisms are elucidated using the pressure and temperature dependence of oxidation rate, and with the aid of a novel mathematical model. Haematite formation is slower at lower pressures, and this oxide is not observed below 10 -3 Torr 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 implantatation 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. The benefits of chromium were short-lived because the implanted chromium readily dissolved in the growing oxide, and the initial high concentrations of chromium near the metal-oxide interface were not maintained. The behaviour of chromium implanted material is compared with 'breakaway oxidation' of conventional iron-chromium alloys.
|Date of Award||1981|