Kinetics of initial oxide growth on Fe-Cr alloys and the role of vacancies in film breakdown

Abstract

The transition from slow oxidation of Fe—Cr alloys covered with a protective film of chromium-rich α-M₂O₃ to the faster rate characteristic of the formation of thicker duplex oxide scales (M₂O₃) is described. The morphology of localized film breakdown together with discrete, sequential changes in the oxidation rate constant indicate that film failure is associated with the condensation into voids at the metal—oxide interface of vacancies injected into the metal during the oxidation process; the vacancy fraction prior to condensation is estimated to be ∼ 10-2. Since metal freshly exposed to gaseous oxidant is depleted in chromium as a result of the formation of the initial oxide film, growth of crystals of spinel oxide is now favoured. The number of spinel nuclei remains essentially constant after the initial period but they increase in size relatively quickly to cover the entire surface with a duplex scale. A model for the process is developed which accords well with kinetic data.