First-principles investigation of surface properties and adsorption of oxygen on Ni-22Cr and the role of molybdenum

Nickel-based alloys are extensively utilized in high-temperature applications where corrosion resistance and mechanical strength are important. The formation of an oxide-layer on these surfaces is associated with corrosion resistance but the early stages of this process are not well understood and difficult to model. The studies in the available literature are currently limited to introducing an element such as Cr to the top layer of the nickel surface slab. While this approach has yielded some useful insights, it is not entirely accurate. In this work, we used the special quasi-random structure (SQS) method to first build reliable representations of the Ni-Cr alloy. Next, several surface slabs were cut from these alloys and oxygen adsorption at 0.25 ML coverage was investigated. It was generally observed that Cr made the oxygen adsorption process more thermodynamically favorable and the nature of the interaction was partially electrostatic. We then introduced Mo substitutions to the top layer of the surface to investigate the effect of low levels of Mo on the oxygen adsorption. It was revealed that Mo helped further stabilize the adsorbed configuration. A correlation was observed between the net charge transferred and the thermodynamic stability of the metal-oxygen system and the stability of the adsorbate on the alloy surface was mainly dependent on the local environment of the adsorbate. The subsurface atoms for the cases considered in this study provided only minor contribution to the stability of the alloy-oxygen system and hence utilizing the doping approach on the top layer may be appropriate for studying oxygen adsorption on this particular alloy surface to a good approximation but this issue requires further investigation. Finally, the workfunctions of the surfaces and the effects of oxygen adsorption on the workfunctions were also characterized and the implications of the results for corrosion resistance were discussed.