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

► NiCo alloy surface oxidation is studied in two pressure regimes (5 × 10−7 mbar and 0.5 bar). ► Segregation and preferential oxidation of cobalt onto NiCo alloy surface. ► Cobalt segregation reaches a maximum in 0.5 bar O2 upon complete oxidation of Co0 to Co2+. ► Formation of CoxNi1−xO spinel oxide is favoured only at atmospheric O2 pressure oxidation.