The growth and structure of oxide films on Fe. II. Oxidation of polycrystalline Fe at 240–320°C

Abstract

The influence of surface pretreatment and metal orientation on the oxidation of coarse-grained polycrystalline Fe has been studied at 240 to 320°C in 5×10⁻³ Torr O₂ using electron diffraction, electron microscopy, and Mössbauer spectroscopy to complement kinetic data. Consistent with previous studies on Fe single crystals, differences in oxidation kinetics for surfaces covered with an electropolish film from those with a similar thickness prior oxide formed by dry oxidation at room temperature are interpreted in terms of differing densities of leakage paths in the oxide layers. The more complex kinetics for electropolished polycrystalline Fe are a result of the leakage path density, the degree of oxide separation, and the extent of α-Fe₂O₃ formation varying with substrate orientation. Where adherent Fe₃O₄ layers are formed on polycrystalline and single-crystal Fe surfaces, the parabolic rate constants give an activation energy which is consistent with a previous value of 32 kcal · mole⁻¹, suggesting that at these low temperatures the transport mechanism for magnetite growth is cation diffusion via easy diffusion paths in the oxide.