Low Pressure Oxidation of Manganese in Pure Oxygen at 550 °C and 650 °C

Abstract

AbstractOxidation of manganese in pure oxygen has been studied at 550 °C and 650 °C under different pressures. In all cases we observed the formation of two oxide layers of different thickness and composition. The outer layer undergoes blistering and spalling after a time depending on the experimental conditions as shown also by a change in the kinetic curves. However, the first stage of oxidation follows always a parabolic law, with a rate constant depending on the oxygen pressure. This effect is attributed to a control of the oxide growth by the diffusion through the outer Mn3O4 layer.To explain the observed dependence of the parabolic constant on the oxygen pressure, the oxide is assumed to be an electronic conductor with prevailing cation diffusion. Wagner's treatment of the parabolic oxidation is modified to take into account the presence of two kinds of cations in Mn3O4. The final expression of Kt shows a reasonable agreement with the experimental results, if the diffusion is attributed to the neutral cation vacancies; this mechanism is also justified by the relatively low working temperatures.