EFFECT OF AL ON THE HIGH-TEMPERATURE SULFIDATION OF FE-30NB

Fe-30Nb-Al alloys containing up to 9.1 wt.% Al were sulfidized at 0.01 atm sulfur vapor over the temperature range of 600-900-degrees-C. The sulfidation kinetics followed the parabolic rate law for all the alloys at all temperatures. The parabolic rate constants decreased with increasing Al content. Extremely slow sulfidation rates, even slower than that of pure Nb at low temperatures, were observed for alloys containing high Al (> 4.8 wt.%). Duplex sulfide scales formed on alloys containing small amounts of Al. The outer layers were compact FeS, while the inner layers were a double sulfide, Fe(x)Nb2S4, containing partially sulfidized intermetallic islands. Very thin scales formed on the alloys containing high Al, but the nature of the scales is unknown. The intercalation of Al into the Nb-sulfides and the associated charge transfer induced a blockage of the transport of iron through the sulfide as well as a greater incorporation of Nb into the scale.