Initial stages of oxidation of near-stoichiometric titanium carbide at low oxygen pressures

A novel approach to the oxidation mechanism of near-stoichiometric TiC is presented. It is confirmed by consideration of solid-state chemical kinetics model and electron microscopy observations in parallel. At low oxygen pressures and moderate temperatures the initial step of the process is connected with the dissolution of oxygen and subsequent decomposition of oxygen-oversaturated oxycarbide, which ultimately results in the nucleation of oxide phase, in particular anatase, belike stabilised by residual carbon. An anatase-rutile transformation is concurrent with deeper carbon burn-off in the oxide scale, which sinters at higher temperatures. This mechanism shifts the process to a gas diffusion regime, governed by the scale permeability, but determined by solid-state diffusion that is reflected in the kinetics, as further temperature increase is accompanied by a decrease of the oxidation rate, so in general the process is characterised by the negative value of apparent activation energy.