Phase interactions in the growth of thin oxide films on iron-chromium alloys

Abstract

The crystal structures and orientation relationships of thin oxide films formed on single crystal and polycrystalline iron-chromium alloys oxidized at 600°C in CO₂-l %CO gas are described. The oxide phase formed initially on the metal is enriched in chromium and consists either of finely polycrystalline α-M₂O₃ or of epitaxial γ-M₂O₃, depending upon crystal orientation and composition of the alloy. As the oxide film thickens and becomes progressively diluted with iron, the epitaxial γ-M₂O₃ is overlaid by either oriented M₃O₄ or oriented α-M₂O₃. The γ-M₂O₃ is so oriented that close matching of metal atoms at the metal-oxide interface occurs, while the transition γ-M₂O₃to M₃O₄ or α-M₂O₃ is associated with parallel alignment of the anions at the oxide-oxide interface. The formation of initial oxide and the oxide growth sequences which occur as the film thickens are explained by considering the maximum chromium solubility and the relative mobility of iron and chromium ions in each phase, as deduced from crystal field theory, together with interfacial and strain energy factors.