Thermal stability of surface oxides on nickel alloys (NiCr and NiCrMo) investigated by XPS and ToF-SIMS

Thermal stability of surface oxides on Ni-20Cr and Ni-20Cr-10Mo alloys, including air formed native oxides and passive films formed by electrochemical passivation, was investigated by stepwise heat treatments in ultra-high vacuum up to 300 °C. The modifications of the surface oxides were analyzed by XPS and in situ ToF-SIMS. The analytical surface characterization at the nanoscale revealed modifications of the protective surface oxides that depend on temperature and nature of the alloy. A loss of Ni(OH)2 in the outer hydroxide layer and a decrease of film thickness are observed on both alloys above 100 °C. At 200 °C, Ni hydroxide in the outer layer is no longer stable while MoO3 decreases and MoO2 increases in the oxide film. At 300 °C no native oxide film remains at the surface of Ni-20Cr alloy, whereas a thin Cr oxide film partially covering the surface remains on passivated Ni-20Cr and for both native and passivated surfaces of Ni-20Cr-10Mo alloy. It is shown that the thermal stability of the surface oxide is significantly improved by electrochemical passivation on both alloys and is further enhanced by the presence of Mo in the oxide on the Mo-containing alloy. Such findings are useful for the development of corrosion protection strategies.