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 °C under dry and wet conditions. It appears that H2O has little effect on the oxidation rate and scalecomposition after 48 h. The outer scaleis composed of chromiaCr2O3. At 900 and 1,000 °C, NbNi4 and Ni3Mo intermetallics are found at the oxide/alloy interface. At 1,100 °C, the scaleis composed of an outer chromia scaleand an internal CrNbO4 subscale. At this temperature the oxide scalemorphology differs between dry and wet conditions. Under dry conditions the oxide scaleappears to be compact but the external part of the scalepartially spalledof during cooling. The oxide scalesformed under wet conditions show porosities spread inside the scaleand the chromiagrain size is smaller. At 1,100 °C scale spallationis 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 scaleleads to a better scaleadherence.