Kinetics and Morphological Development of the Oxide Scale on Iron at 800°C in Oxygen at 2.5 × 10 − 3 to 3.0 × 10 − 1 Torr Pressure

Abstract

A thermogravimetric technique was employed to investigate the oxidation of iron in oxygen over the pressure range . The oxidation curves exhibited distinct intervals of linear kinetics followed by transitions to intervals of parabolic kinetics during exposures extending to 125 min. Linear kinetics governed the growth of uniformly thick wustite scales; the linear rate constants showed a proportional dependence on oxygen pressure due to reaction control by a phase boundary reaction involving nondissociative adsorption of oxygen. The sticking coefficient of oxygen on wustite was equal to 0.067. Parabolic kinetics governed growth of wustite‐magnetite scales containing magnetite as outermost layers. The values of the parabolic rate constants were independent of oxygen pressure since scale growth was directly dependent on the iron vacancy gradient in wustite established by the oxygen activities at the and interfaces.