Corrosion behavior of three iron-based model alloys in reducing atmospheres containing HCl and H2S at 600 °C

The corrosion of three Fe–xCr alloys (x = 8, 12, 18 wt.%) was examined at 600 °C in reducing atmospheres containing HCl and H2S with two different H2S contents and compared with the behavior of the same materials in H2S–free H2–CO2 and H2–HCl–CO2 mixtures producing similar oxygen and chlorine pressures. Exposure to the low-H2S gas mixture had only a reduced effect on the behavior of Fe–8Cr and Fe–18Cr, but increased significantly the corrosion rate of Fe–12Cr. Increasing the H2S level accelerated the corrosion of all the alloys, but particularly that of Fe–18Cr. In both cases only minor amounts of sulfur and chlorine were present close to the alloy/scale interface. An increase of the Cr content reduced the corrosion rate in both H2S gas mixtures, especially in the range 8–12 wt.% Cr, due to a larger volume fraction of Cr2O3 in the scale. The results have been discussed on the basis of the thermodynamic stability diagrams of the Fe–O–Cl–S and Cr–O–Cl–S systems.