The concentration gradient of iron-ion-vacancies in wüstite scaling films and the mechanism of oxidation of iron

Abstract

The concentration gradient of iron-ion-vacancies in Wüstite scaling films is measured electrochemically. The films were obtained by oxidation of pure iron in oxygen between 650° and 1000°C. The concentration gradient of vacancies is constant over the whole Wüstite film. According to Wagner's theory of scaling a constant concentration gradient of mobile defects is to be expected under two conditions : (1) the diffusion coefficient of the defects is independent of concentration, and (2) the thermodynamic activity of the defects is proportional to concentration (or the nth power of concentration). It follows from measurements of Himmel, Mehl, and Birchenall that both conditions are almost fulfilled in the case of the diffusion of iron-ion-vacancies in Wüstite between 800° and 1000°C. From the measured gradients and velocities of oxidation of the iron specimen the diffusion coefficient of the mobile defects are calculated. At higher temperatures these values are somewhat larger than the diffusion coefficients of iron-ion-vacancies obtained from measurements of self diffusion of iron in Wüstite. Marker experiments show that this difference may be due to the participation of oxygen in the transport of matter through the Wüstite. Deviations from the parabolic law of oxidation of iron are thought to be caused by the formation of cracks in the iron-Wüstite boundary. © 1958.