The corrosion behavior of Fe-Al alloys in H-2/H2S/H2O atmospheres at 700-900 degrees 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 degrees C in a H-2/H2S/H2O mixture with varying sulfur partial pressures of 10(-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 Fe1-xS) and an inner complex scale of FeAl2S4 and FeS. Alloys having intermediate AI contents (Fe-18 and -28 at.% Al) formed scales that consisted of mostly iron sulfide and Al2O3 as well as minor a amount of FeAl2S4. The amount of Al2O3 increased with increasing Al content. The Fe-40 at.% Al formed only Al2O3 at 700 degrees C, while most Al2O3 and some FeS were detected at T greater than or equal to 800 degrees C. The formation of Al2O3 was responsible for the reduction of the corrosion rates.