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-O-2 and Ar-H-2-H2O at 1000 and 1050 degrees C. The oxidation mechanisms were investigated using thermogravimetry and two-stage oxidation tests with O-18(2)- and (H2O)-O-18-tracers. In the case of the binary alloy it was found that chromia scales formed in Ar-H-2-H2O 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-H2O. 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-H2O. 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 Cr2O3 and in the Cr/Mn-spinel, in combination with segregation of Mn to the chromia grain boundaries. (c) 2007 Elsevier B.V. All rights reserved