Protective and non-protective scale formation of NiCr alloys in water vapour containing high- and low-pO2 gases

The oxidation behaviour of NiCr alloys with Cr contents of 10, 20 and 25 wt.%, respectively, were studied in Ar–O2, Ar–O2–H2O and Ar–H2O mixtures. TG and SEM analysis revealed that the chromia scales formed on Ni–25Cr in the wet gases did not differ substantially from those formed in Ar–O2. For the two “borderline” alloys Ni–10Cr and Ni–20Cr, addition of water vapour to Ar–O2 hampered the formation of a protective chromia scale which, especially for Ni–10Cr, resulted in substantially increased scale growth rates compared to exposures in dry gas. Different from numerous observations described in the literature for “borderline” FeCr alloys with intermediate Cr contents of 10–20%, the corresponding NiCr alloys showed in Ar–H2O a smaller tendency for non-protective scale formation than in Ar–O2–H2O. This is caused by the decreasing growth rate of NiO with decreasing pO2 of the test gas, with the secondary effect that external chromia scale formation is promoted in low-pO2 gases such as Ar–H2O. Even if the alloy Cr content was too low to obtain external chromia scale formation, the oxidation rate in Ar–H2O would, in contrast to low-Cr FeCr alloys, be quite small due to the very slow growth rate of NiO in this low-pO2 gas.