Effects of water vapour on the oxidation of a nickel-base 625 alloy between 900 and 1,100 °C

The effect of water vapour was studied on a nickel-based SY 625 alloy oxidized at 900, 1000 and 1100 A degrees C under dry and wet conditions. It appears that H2O has little effect on the oxidation rate and scale composition after 48 h. The outer scale is composed of chromia Cr2O3. At 900 and 1,000 A degrees C, NbNi4 and Ni3Mo intermetallics are found at the oxide/alloy interface. At 1,100 A degrees C, the scale is composed of an outer chromia scale and an internal CrNbO4 subscale. At this temperature the oxide scale morphology differs between dry and wet conditions. Under dry conditions the oxide scale appears to be compact but the external part of the scale partially spalled of during cooling. The oxide scales formed under wet conditions show porosities spread inside the scale and the chromia grain size is smaller. At 1,100 A degrees C scale spallation is observed under dry conditions due void accumulation in the middle part of the scale. Under wet conditions the uniform distribution of the porosities inside the scale leads to a better scale adherence.