Oxidation properties of transition metals

Abstract

The transition metals when exposed to oxygen at low and intermediate temperatures form thin, protective oxide films of up to some thousands of Angstroms in thickness. At higher temperatures, thick (> 1/z) oxide scales are generally formed. The kinetics of the high-temperature process are well established, whilst those of the low-temperature reaction are a matter of contention in the literature. Our discussion reflects this difference by incorporating a detailed consideration of lowtemperature kinetics. Oxidation kinetics are, of course, determined by the reaction mechanism. The mechanism is in turn determined by the properties of the solid oxide reaction product. It should therefore be possible to predict the kinetics of an oxidation reaction from the appropriate properties of the solid oxide or, conversely, to determine those properties from kinetic measurements. This discussion therefore commences with a review of the relevant structural, thermodynamic and transport properties of transition metal oxides. A confrontation of theory and experiment for both the lowand hightemperature r6gimes then follows, and both the successes and the inadequacies of oxidation theories are pointed out. Finally, the model employed for the oxide product is extended to include line defects and internal surfaces, and the importance of these parameters on transition metal oxidation is assessed.