The effect of sulphur on the carburization of three Fe-19Ni-13Cr alloys with various Al additions at 900 °C in oxygen-contaminated CH₄-H₂-H₂S atmospheres

Highlights: • The extent of carburization decreased with an increase of sulfur pressure. • Al additions to the Fe-19Ni-13Cr alloys were beneficial to reduce carbon attack. • A thin CrS scale blocked carbon attack for low Al alloys under high P(S₂). • Thermodynamic diagram was used to describe corrosion by multi-oxidant gases. • High P(S₂) and O₂ traces blocked carbon attack for 6 at.% Al alloy by forming continuous Al₂O₃ scales. - Abstract: The sulphur effect on the carburization of three Fe-19Ni-13Cr alloys with various Al additions was studied at 900 °C in four CH₄-H₂-H₂S gases providing sulphur pressures from 0 to 4.8 × 10⁻¹¹ atm. The corrosion rates, decreasing with increasing sulphur pressure, were slower for Fe-19Ni-13Cr-6Al than for the other alloys under each gas mixture. The carburization of Fe-19Ni-13Cr-6Al was blocked in the two H₂S-richer gas mixtures by the growth of external alumina scales. The carbon attack was also prevented on Fe-19Ni-13Cr and Fe-19Ni-13Cr-2Al in the gas richest in H₂S by the formation of thin continuous CrS scales.