Effects of 7·5 vol.-% water vapour on oxidation of nickel based alloy between 900 and 1100°C

Abstract

A Ni based SY 625 alloy was oxidised at 900, 1000 and 1100°C under dry and wet conditions. Water vapour has little effect on the oxidation rate and scale composition. At 900 and 1000°C, the outer scale is composed of Cr₂O₃, and a continuous NbNi₄–Ni₃Mo subscale is found at the oxide/alloy interface. At 1100°C, the scale is composed of an outer chromia scale and an internal CrNbO₄ subscale. Nevertheless, the oxide scale morphology differs between dry and wet conditions. Under dry conditions, the oxide scale appears to be compact, and chromia pegs are observed at the internal interface. The oxide scales formed under wet conditions show that porosities spread inside the scale, and the chromia grain size is smaller. At 1100°C, some scale spallation is observed under dry and wet conditions probably due to the molybdenum oxidation, leading to MoO₃ evaporation and void accumulation at the internal interface.