Hot CO₂ gas is surprisingly corrosive to chromia forming alloys as a result of internal carburisation, which decreases the ability of an alloy to form a protective Cr-rich oxide scale. Comparison of Fe- and Ni- base alloys reacted with Ar-20CO₂ shows that the latter can provide superior performance, by virtue of their better resistance to carburisation. Experiments with model Fe-Cr and Ni-Cr alloys at 650^oC show that addition of water vapour to the gas is detrimental to the performance of Fe-based alloys: chromia grains are refined, leading to enhanced diffusion and accelerated scale growth; large chromia platelets grow from the scale, generating stress and leading to premature failure; the nucleation and growth of Fe-rich nodules leads to breakaway corrosion. Nickel-base alloys of marginal chromium content undergo internal oxidation in dry CO₂, accompanied by buckling of surface regions in wet CO₂.

Addition of 0.5% SO₂ to Ar-20CO₂ is mildly detrimental to the performance of Fe-Cr alloys, causing a slight acceleration in the nucleation and growth of Fe-rich oxide nodules. The addition of 20% water vapour to Ar-20CO2 is also detrimental, promoting Fe-rich oxide growth and breakaway corrosion. However, the simultaneous addition of H₂O and SO₂ to Ar-20CO₂ leads to complete passivation of the alloy surface by a slow growing Cr₂O₃ scale. These effects are discussed in terms of grain boundary segregation and diffusion of impurity species within the chromium oxide.