Oxidation of a cobalt chromium alloy: An x-ray photoemission spectroscopy study

Abstract

The oxidation of a Co86Cr14alloy surface was studied by x-ray photoemission spectroscopy. Following ion sputtering at 25 °C, the unoxidized alloy surface composition was nearly the same as the bulk composition. Vacuum annealing of the surface caused Cr enrichment at the surface. This is discussed in the context of the expected behavior at thermodynamic equilibrium. Oxidations from 25 to 400 cC were done under controlled exposure conditions using dry oxygen at 10–7 Torr and under atmospheric conditions. A layered mixture of oxides was formed at the surface and was rich in Co oxides near the surface and either rich in Cr oxide or at the bulk Cr/Co ratio in the inner oxide layers, depending on the oxidation conditions. The observed oxide structure is proposed to be a consequence of the competition between the preference for Cr to oxidize because of its higher oxidation potential relative to Co and the relative supply of Co available for oxidation compared to the supply of Cr due to the different diffusivities of Co and Cr in the oxide structure. The results indicate that Co has a much higher mobility than Cr in this temperature range, as is the case above 800 °C. The oxide growth appears to be dominated by the high oxidation potential of Cr prior to the formation of a complete oxide layer and, subsequently, is dominated by the high Co diffusivity, under all conditions examined. © 1988, American Vacuum Society. All rights reserved.