Effect of Fe on corrosion of Ni-20Cr and Ni-30Cr alloys in wet CO2 gas at 650 and 700 °C

Model alloys Ni-(20, 30)Cr (wt%) with (0, 1, 5, 15%) Fe were reacted with Ar-20CO(2)-20H(2)O gas at 650 and 700 degrees C. Samples were electropolished to avoid sub-surface cold work effects on alloy diffusion. All 20Cr alloys developed non-protective scales. However, Fe alloying enhanced the corrosion resistance of Ni-30Cr by promoting the formation of a continuous chromia band or non-uniform chromia scale. Wagner's theory is shown to explain these observations. Intergranular Cr-rich carbide formation was favoured by higher alloy Fe levels.