Initial Oxidation of Pure Iron and the Effect of the Allotropic Transformation

Abstract

A volumetric method has been used to study the kinetics of the initial oxidation of oure iron at two pressures of dry oxygen (viz. _p O₂ = 7·6 and 29 cm mercury) in the range 800–1000° in which the first allotropic transformation point of iron occurs. It was found that the oxidation of both body-centred cubic (b.c.c.) and face-centred cubic (f.c.c.)–iron at _p O₂ = 7·6 cm and of only b.c.c. at _p O₂ = 29 cm follows a modified parabolic law; a true parabolic law was found to operate in the oxidation of f.c.c.–iron only at _p O₂ = 29 cm. An anomaly, which is obtained in the Arrhenius plot at the allotropic change point of iron (∼911°), is attributed to the effect of phase-boundary reactions on the oxidation kinetics. It would appear from the concept of vacancy distribution in iron, that for a modified parabolic law, it is the metal/oxide interfacial reaction in f.c.c.–iron and the oxide/oxygeninterfacial reaction in b.c.c.–iron that governs the phase-boundary controlled reaction rate.