The high-temperature corrosion of Fe–28Al and Fe–18Al–10Nb in a H2/H2S/H2O gas mixture

The corrosion behavior of Fe–28Al and Fe–18Al–10Nb (at.%) was investigated over the temperature range of 700–900°C in a H2/H2S/H2O gas mixture with varying sulfur partial pressures of 10−2–103 Pa and oxygen partial pressures of 10−19–10−15 Pa. In general, the corrosion kinetics followed the parabolic rate law although two-stage kinetics were noted in some cases. The steady-state parabolic rate constants increased with increasing temperature. Based on the equivalent addition (28 at.%)of alloying elements, it was found that Fe–18Al–10Nb exhibited a better corrosion resistance, being 2.1–3.9 orders of magnitude lower than Fe–28Al(depending on temperature). The scales formed on Fe–28Al consisted of mostly FeS, -Al2O3, and minor FeAl2S4, while the scales formed on Fe–18Al–10Nb consisted of mostly α-Al2O3, Nb3S4, Nb2O5, and minor FexNbS2 (FeNb2S4/FeNb3S6) at T ≤ 800°C and of α-Al2O3, Nb3S4, and FexNbS2 at 900°C. The formation of Al2O3 and Nb3S4 resulted in the significant reduction of corrosion rates.