Effect of surface roughness on the development of protective Al2O3 on Fe–10Al (at.%) alloys containing 0–10 at.% Cr

The effect of alloy surface roughness, achieved by different degrees of surface polishing, on the development of protective alumina layer on Fe– 10 at.% Al alloys containing 0, 5, and 10 at.% Cr was investigated during oxidation at 1000 8C in 0.1 MPa oxygen. For alloys that are not strong Al2O3 formers (Fe–10Al and Fe–5Cr–10Al), the rougher surfaces increased Fe incorporation into the overall surface layer. On the Fe–10Al, more iron oxides were formed in a uniform layer of mixed aluminum- and iron-oxides since the layer was thicker. On the Fe–5Cr–10Al, more iron-rich nodules developed on an otherwise thin Al2O3 surface layer. These nodules nucleated preferentially along surface scratch marks but not on alloy grain boundaries. For the strong Al2O3-forming Fe–10Cr–10Al alloy, protective alumina surface layers were observed regardless of the surface roughness. These results indicate that the formation of a protective Al2O3 layer on Fe–Cr–Al surfaces is not dictated by Al diffusion to the surface. More cold-worked surfaces caused an enhanced Fe diffusion, hence produced more Fe-rich oxides during the early stage of oxidation.