Initial oxidation rate of pure iron and the effect of the curie temperature

Abstract

Kinetic studies of the early stages of the oxidation of pure iron, initiated at a clean metal surface, have been carried out at a pressure of 76 mm Hg absolute, and isothermally in the range 700–900°c, using a constant pressure-differential gas apparatus. The duration of each isothermal study has been restricted to the first two minutes after the start of the reaction, and the thickness of oxide formed is calculated to be of the order of 150 000 Å. During this time values of the instantaneous rate of reaction dW/dt, where W is the weight of oxygen consumed per unit area of the specimen and t is the time, have been obtained at intervals of a few seconds from the instant when it becomes practically possible to take a first reading. The relation between dW/dt and W is shown to be exponential, and on extrapolating the linear plot log₁₀ dW/dt against t to t=0, the initial reaction rate or reaction rate constant k, is obtained for each temperature. The plot log₁₀ k against 1/T, where T is the absolute temperature, is continuous as a cusp which rises to a sharp peak at the Curie temperature (755°c). The activation energies for oxidation in the temperature-independent ranges above and below the Curie temperature are 33 100 and 21 700 calories/mole respectively. From these results it is deduced that the rate controlling step for the observed non-parabolic oxidation is electron transfer from metal to oxide at the metal–oxide interface.