Origin of the selective Cr oxidation in CoCr alloy surfaces

CoCr alloy surfaces are known to oxidize with the formation of stable Cr2O3 protecting films. However, the inherent mechanisms initiating the selective oxidation of one component, commonly observed in most metal alloys exposed to an oxidizing environment, are still largely unknown. Here we present a study of the early oxidation stages of a Co0.67Cr0.33(0001) alloy surface by means of first-principles molecular dynamics based on density-functional theory. We observe the initial formation of a cobalt-rich amorphous oxide matrix, which crucially acts as the promoter of a selective chromium oxidation already at coverages below 2 ML. In this process, the outward diffusion of Cr atoms leads to vacancy formation beneath the surface and enables the diffusion of oxygen atoms into inner atomic layers. This mechanism differs from the oxidation of either pure Co or Cr surfaces, revealing that selective oxidation within CoCr is triggered by a cooperative interaction between the components.