Growth and adherence of chromia based surface scales on Ni-base alloys in high- and low-pO2 gases

The oxidation behaviour of a binary Ni–25Cr model alloy as well as ternary/quaternary NiCr-base alloys with minor additions of Mn and/or Y was studied in Ar–O2 and Ar–H2–H2O at 1000 and 1050 °C. The oxidation mechanisms were investigated using thermogravimetry and two-stage oxidation tests with 18O2- and H218O-tracers. In the case of the binary alloy it was found that chromia scales formed in Ar–H2–H2O grew faster, but showed better adherence than those formed in Ar–O2. This could be attributed to differences in oxide grain size and the resulting predominant transport processes in the chromia scale changing from nearly exclusive outward growth in Ar–O2 to mainly inward growth in Ar–H2–H2O. This change in scale microstructure and growth mechanism explains why Y-addition hardly affects the scale growth and adherence during exposure in Ar–H2–H2O. Mn additions of a few tenthʹs of a percent enhance the growth rate in the high-pO2 gas but decrease it in the low-pO2 gas. This is explained by a pO2-dependence of Mn solubility in Cr2O3 and in the Cr/Mn-spinel, in combination with segregation of Mn to the chromia grain boundaries.