Wagner theory and short circuit diffusion

Abstract

The growth of an oxide film on a metal by thermal oxidation is usually discussed in terms of Wagner's theory in which the oxidation rate is controlled by the transport of ions across the oxide film under the combined effects of concentration gradients and electric fields. This leads to parabolic growth kinetics with a rate constant which in most cases is controlled by the diffusive properties of the oxide. However, the expected relationship between growth rate and diffusion constants is not generally confirmed experimentally and usually the oxidation rate is many orders of magnitude greater than expected. For Ni oxidation, the fast rates are a result of rapid diffusion along grain boundaries in the oxide and when this is taken into account the oxidation rates and diffusion data can be reconciled. However, in the case of Cr oxidation even fast diffusion along grain boundaries is still apparently too slow to account for the measured oxide growth rates. Short circuit diffusion of oxygen appears to occur to a greater or lesser extent in practically all oxide films. Oxygen transport can be the process controlling film growth (SiO₂ and Al₂O₃), providing oxidant for the growth of the inner layer of duplex films (on dilute alloys), or be a minor process which may generate stress (for pure metals). In all these cases, the evidence suggests that oxygen is transported as molecules along short circuit pathways. Impurities have a significant influence on short circuit transport and can be beneficial (e.g. reactive element effect). One of their important roles here is their ability to block short circuit diffusion.

MST/930