Surface degradation of three Cr-containing cobalt-based alloys exposed to pure water vapour at 900°C

Three cast cobalt-based alloys containing or not chromium carbides and tantalum carbides were exposed at 900°C to 100% water vapour for 46 or 100 h. The oxidised samples were characterised by XRD, SEM and EDS. Chromia was the main oxide formed in all cases. Spinel oxide appeared on the carbide-free alloy and NiO was also detected on the TaC-containing alloy. Chromia was the single oxide formed on the alloy containing chromium carbides. The scales were thin and the subsurfaces and the bulks were slightly affected by oxidation or thermal exposure. All alloys demonstrated remarkable oxidation resistance but oxide spallation, enhanced maybe by the low thickness of the scales, by the porosities induced by chromia volatilisation, as well as by the CrTaO4 complex oxide formed at the interface in the case of the TaC-containing alloy, is a critical problem to solve. The Co-10Ni-30Cr-0.5C was the best of the three alloys.