Control of polymorphism in Al2O3 scale formed by oxidation of alumina-forming alloys

The selective oxidation of specific components in alumina-forming alloy such as CoNiCrAlY under precisely regulated oxygen partial pressures (PO2PO2) can be used to control polymorphism in Al2O3 scale formed on the alloy. Dense, smooth α-Al2O3 scale was formed rapidly by treatment at 1323 K under a thermodynamically determined PO2PO2, where both aluminum and chromium in the alloy were oxidized and elements such as cobalt and nickel were not oxidized. By contrast, under a higher PO2PO2 all the components in the alloy were oxidized, the transformation was obviously retarded, and (Co,Ni)(Al,Cr)2O4 was produced.