Mathematical modelling of diffusion-driven oxidation in metals

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This industrially-sponsored project is aimed at providing a predictive quantitative description of the formation of a protective oxide layer on a metal surface. Oxidation in pure oxygen, pure moisture, and mixtures of the two (e.g. humid air) are of interest. The kinetic properties of these oxidation reactions have been studied extensively in the past, and a large number of models describing the rates of reaction have been proposed. However, with just a few notable exceptions, these models have been empirical by their nature rather than mathematical descriptions of plausible underlying physical mechanisms. Oxidation in the absence of moisture is dominated by migration of oxygen anions through the oxide substrate towards the metal, while in the presence of moisture it is likely that ionic diffusion of hydroxyl anions is the rate limiting step. The overall process is thus one of combined temperature- and time-dependent chemical and ionic diffusion within a growing substrate of variable composition. It is vital that the dominant physical mechanisms are well modelled to provide both qualitative understanding and a quantitative predictive capability for long-term material ageing.