Oxidation of nickel-based alloys in dry and water vapour containing air

Ni-based alloys are widely used in power generation and their oxidation behaviour in the uncoated state is of interest, especially the impact of water vapour in the air on the formation of a protective underlying alumina scale. High-temperature X-ray diffraction was applied to investigate in situ the oxide scale formation in the initial state on the alumina formers CM247DS and CMSX4 in comparison to the chromia formers IN792 and SCA425+. Post-oxidation analysis was performed by field emission scanning electron microscopy. The samples were oxidised for 100 h at 950 degrees C in dry air and in air with 20% relative moisture in the high temperature device on the X-ray diffractometer. In dry air, CM247 and CMSX-4 form alpha-Al2O3 from the beginning simultaneously with spinels and nickel oxide. The alumina scale underlies the spinels which spall partially on cooling. When adding water vapour, the same oxides were formed simultaneously resulting in a comparable oxide scale. IN792 forms in dry air mainly NiAl2O4 and transitionally CrO2 under laid by an alumina scale. With water vapour, Cr2O3 forms and the underlying alumina scale shows a non coherent dendritic structure. SCA425+ forms in dry air Cr2O3 and Cr containing mixed oxides and with water vapour a more coherent alumina scale than IN792.