The Oxidation of Four Co-20Ni-xCr-yAl (x=8,15 wt.%; y=3,5 wt.%) Alloys Under 1 atm O2 at 800 °C and 900 °C

The oxidation of four Co-20Ni-xCr-yAl (x = 8,15 wt.%; y = 3,5 wt.%) alloys under 1 atm O-2 for 24 h was studied at 800 and 900 degrees C. All the alloys formed complex scales, irregular both in thickness and composition. The formation of external alumina scales was more favored at 800 degrees C than at 900 degrees C and for larger chromium and aluminum contents, but was never complete. The simultaneous presence of chromium and aluminum is beneficial for increasing the oxidation resistance of these alloys with respect to ternary Co-20Ni-xCr and Co-20Ni-yAl alloys of similar composition. However, an Al content of 5 wt.% is not yet sufficient to form continuous alumina scales even when coupled with 15 wt.% Cr. The partial substitution of cobalt by nickel has a beneficial effect on the ability of the quaternary Co-20Ni-xCr-yAl alloys to form protective external alumina scales in comparison with the ternary Co-Cr-Al alloys with similar chromium and aluminum contents.