Growth and adherence of chromia based surface scales on Ni-base alloys in high- and low- pO 2 gases

Abstract

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–O 2 and Ar–H 2–H 2O at 1000 and 1050 °C. The oxidation mechanisms were investigated using thermogravimetry and two-stage oxidation tests with 18O 2- and H 2 18O-tracers. In the case of the binary alloy it was found that chromia scales formed in Ar–H 2–H 2O grew faster, but showed better adherence than those formed in Ar–O 2. 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–O 2 to mainly inward growth in Ar–H 2–H 2O. This change in scale microstructure and growth mechanism explains why Y-addition hardly affects the scale growth and adherence during exposure in Ar–H 2–H 2O. Mn additions of a few tenth's of a percent enhance the growth rate in the high- pO 2 gas but decrease it in the low- pO 2 gas. This is explained by a pO 2-dependence of Mn solubility in Cr 2O 3 and in the Cr/Mn-spinel, in combination with segregation of Mn to the chromia grain boundaries.