HIGH-TEMPERATURE-SULFIDATION BEHAVIOR OF FE-MO-MN-AL ALLOYS

The sulfidation behavior of multiphase, iron-based alloys containing u p to 24 a/o molybdenum, up to 163 a/o manganese, and up to 24 a/o alum inum was examined in flowing H2/H2S gases, corresponding to a sulfur p artial pressure of 4 Pa, at 800-degrees-C. An accelerated sulfidation rate was almost invariably observed on the quaternary alloys, but slow linear kinetics were found for Fe-22Mo-17Al. This behavior is due to the different products of the preferentially-attacked ferrite phase. I f FeAl2S4 formed over the ferrite phase, the sulfur-incorporation rate into the scale was slowed down and accordingly the alloys had excelle nt protection, whereas formation of a MnS + FeS + MoS2 mixture led to poor protection or breakdown of a protective scale. The nature of the ferrite reaction products was determined by the ferrite composition, w hich can vary widely. The molybdenum-rich R-phase and AlMo3 reacted wi th sulfur slowly. When a protective preferential-sulfidation zone form ed, the unreacted intermetallic phases provided a mechanical framework for FeAl2S4.