Effect of allotropic changes in iron and cobalt on their initial reaction with oxygen.

Isothermal kinetic studies of the initial oxidation of pure iron and cobalt have been made, using the volumetric and the manometric methods respectively. Results have been obtained for iron at two different pressures of oxygen, (p°2 =7.6 and 29 cm. Hg, absolute), with the oxide-thickness ranging between 10 and 70mu for the period during which the studies have been made, The oxidation of both body centred- and face centred-cubic iron at p°2 = 7.6 cm. and body centred-cubic iron at p°2 = 29 cm. follows a modified parabolic rate law. A true parabolic rate law applies only to the oxidation of face centred-cubic iron at p°2 = 29 cm. The rate of oxidation of iron, however, decreases with increasing p°2-values, though in the case of cobalt the Og rate is independent of p values, A direct-logarithmic rate law is obeyed in the oxidation of both hexagonal close-packed and face centred-cubic structure of cobalt and the oxide-0 thickness varies from 400 to 5000 A. The Arrhenius plots show, in all cases, an anomaly at the transformation temperature (namely, ~911 °C and 386°C for the oxidation of iron and cobalt respectively). Such an anomaly is believed to be due to the effect of phase-boundary reactions on the oxidation kinetics. In the case of iron, it has been shown from the concept of vacancy distribution that, for a modified parabolic rate law, of the two phase-boundary reactions that at the metal/oxide interface is rate controlling in face centred-cubic whilst in body centred cubic iron it is the oxide/gas interfacial reaction. In the case of cobalt, the mechanism of oxidation has been proposed to be controlled by the combined processes of electron emission at the metal/oxide interface and its availability at the oxide/gas interface