Oxide Formation on Magnesium Single Crystals. I. Kinetics of Growth

The kinetics of oxide film formation on the basal and prismatic planes of magnesium single crystals were investigated in an ultra-high vacuum microbalance system. A parabolic rate law describes the oxide film growth in an average thickness range of 50–1000 Å when formed at 400°C and 2.5 Torr oxygen pressure. The parabolic rate constant varies approximately as the one-sixth power of the oxygen pressure. If the rate constant is expressed in terms of the Arrhenius expression, exp (−Eact/RT), the experimental activation energy for oxidation of high-purity samples is Eact=39±3 kcal/mole. These three results are consistent with the Wagner—Hauffe theory of oxide growth by diffusion of lattice defects. The activation energy for oxidation can be shown to be the sum of the energy for extrinsic diffusion of cation vacancies in the oxide and one-third of the energy required to create a vacancy at the oxide—oxygen interface. Furthermore, the experimental parabolic rate constant can be calculated approximately by consideration of the associated interfacial defect equation. The kinetics of oxidation for crystals containing impurities of various valencies substantiate the hypothesis that the defect nature of the oxide is determined primarily by the formation of cation vacancies at the oxide—oxygen interface. It is postulated that vacancy formation at the interface is related to an oxygen adsorption—dissociation process occurring at selected sites on the oxide surface.