The corrosion behavior of Fe-Al alloys in H₂/H₂S/H₂O atmospheres at 700-900°C

The corrosion behavior of five Fe-Al binary alloys containing up to 40 at.% Al was studied over the temperature range of 700-900°C in a H₂/H₂S/H₂O mixture with varying sulfur partial pressures of 70^-7-10^-5 atm. and oxygen partial pressures of 10^-24-10^-20 atm. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants decreased with increasing Al content. The scales formed on Fe-5 and -10 at.% Al were duplex, consisting of an outer layer of iron sulfide (FeS or Fe_1-xS) and an inner complex scale of FeAl₂S₄ and FeS. Alloys having intermediate Al contents (Fe-18 and-28 at.% Al) formed scales that consisted of mostly iron sulfide and A1₂O₃ as well as minor a amount of FeAl₂S₄. The amount of A1₂O₃ increased with increasing Al content. The Fe-40 at.% Al formed only Al₂O₃ at 700°C, while most Al₂O₃ and some FeS were detected at T≥800°C. The formation of Al₂O₃ was responsible for the reduction of the corrosion rates. © 1997 Plenum Publishing Corporation.