Effect of alloying additions and presence of water vapour on short-term air oxidation behaviour of cast Ni-base superalloys

Ni-14Cr-9Co (wt.%) based alloys with additions of Al, Ti and Ta were exposed in dry and wet (+20 vol.%H2O) air at 950 degrees C for up to 100 h and characterized with GD-OES and SEM/EDX. Ni-14Cr-9Co-alloy exhibited initially Cr2O3-scale formation followed by breakaway Ni-oxidation. 3%Al addition to Ni-14Cr-9Co stabilized the Cr2O3-scale and largely suppressed Ni-oxide formation. Ni-14Cr-9Co-3Al-5Ti formed a fast growing Cr2O3-scale attributed to p-type Ti-doping, but Ni-oxide formation was completely inhibited. 5%Ta addition to Ni-14Cr-9Co3Al promoted formation of a thin alumina scale. Addition of H2O to the test gas increased growth of Ni-rich oxides and shortened time to breakaway for the Al-free alloy.