Nickel base alloys Haynes 230 and Inconel 617 are of interest for gas cooled reactors. At high temperature in impure helium, they generally form surface chromium-rich oxides. However above a critical temperature called TA, the scales are not stable anymore and the chromia destruction comes with a production of carbon monoxide. Reactivity tests on model alloys, with and without carbon, prove that chromia is reduced by the carbon from the alloy. TA vs P(CO) curves were also plotted for the two commercial alloys based on the experimental determination of TA in various atmospheres with increasing partial pressures of carbon monoxide. Unexpectedly, both materials exhibit an almost identical behavior although a basic equilibrium approach suggests that the chromia scale would be reduced in different conditions due to the thermodynamic particularity of the interfacial alloy/scale system.