XPS characterization of oxide films formed in cobalt-based alloys during corrosion tests at high temperature

Hard-facing cobalt-based alloys, type Stellite-6, are widely used for wear resistance in components of nuclear reactor coolant circuits. In this work, high-temperature (290 °C) corrosion tests have been performed under simulated pressurized-water reactor (PWR) primary heat transport circuit conditions at two different pHs. The corrosion resistance of this alloy was evaluated by weight-loss measurements in long-term tests. Surface analysis of the samples, using x-ray-photoelectron spectroscopy (XPS) and subsequent composition–depth profiles, was performed in order to characterize the oxide films (composition, thickness, etc.) formed during exposure at high temperature. Quantitatively different oxide film thicknesses were detected in the two conditions studied, but only slight qualitative differences were observed in the composition of the oxide layers. In both conditions the results seem to show that the corrosion mechanism of this alloy is characterized by preferential dissolution of cobalt and the formation of a protective chromium oxide film.