Surface oxidation of NiCo alloy: A comparative X-ray photoelectron spectroscopy study in a wide pressure range

[Display omitted] ► NiCo alloy surface oxidation is studied in two pressure regimes (5×10−7mbar and 0.5bar). ► Segregation and preferential oxidation of cobalt onto NiCo alloy surface. ► Cobalt segregation reaches a maximum in 0.5bar O2 upon complete oxidation of Co0 to Co2+. ► Formation of CoxNi1−xO spinel oxide is favoured only at atmospheric O2 pressure oxidation. Oxidation of NiCo alloy has been studied under two pressure regimes, 5×10−10 and 5×10−1bar, by X-ray photoelectron spectroscopy (XPS). The aim of this work is to investigate the synergetic effect between the two alloy components during the initial stages of oxidation. The results showed that at low oxygen pressure, segregation and preferential oxidation of cobalt takes place, while oxidation of nickel is largely suppressed. The species dominating the surface is CoO but small amount of metallic cobalt still remains even after prolonged oxidation at 670K. At 0.5bar O2 pressure, alloy oxidation was found to be temperature depended. From 420K to 520K, cobalt is completely transformed to CoO and the Ni:Co atomic ratio at the surface approaches a minimum, similar to the observations at low pressure regime. However, at higher temperatures (from 520K to 720K), nickel is re-segregated on the surface, in the expense of cobalt, while CoO is further oxidized to Co3O4. At this temperature range formation of mixed Ni–Co–O spinel-like oxides is probable as supported by the characteristic modifications of the Ni 2p3/2 photoelectron peak and the increase of the Ni:Co atomic ratio.