The effect of the reactions at the scale-gas and scale-metal. Interfaces on the kinetics of oxidation of pure metals

Abstract

During an initial transient stage the high-temperature kinetics of corrosion of pure metals by a single oxidant, such as oxygen, is in principle affected by the reactions occurring at the interface between the oxide and the gas and/or the oxide and the metal. These effects are examined using far rate laws of the surface reactions forms already proposed in the literature. It is shown that, neglecting the effects due to the space charge, the transient state may be described by means of an instantaneous parabolic rate constant which is equal to zero at the start of the process and increases gradually with time tending to the steady-state value corresponding to the conditions of equilibrium at the two interfaces. The kinetic behaviour and all the relevant parameters during the transient state may be calculated by giving appropriate values to the factors related to the two interface reactions. These calculations have been carried out both for the case of p-type and n-type semiconducting oxides, taking NiO and ZnO as simple examples. The results show that the actual kinetics change gradually from an initial linear-like behaviour to a final parabolic stage which is reached when the conditions of equilibrium are very nearly established at the two oxide interfaces. Furthermore, the most-effective kinetic control is due to the surface reaction which would occur more slowly in the absence of kinetic limitations by the other surface reaction.