Abstract
The sulfidation behavior of multiphase, iron-based alloys containing up to 24 a/o molybdenum, up to 16.3 a/o manganese, and up to 24 a/o aluminum was examined in flowing H 2 /H 2 S gases, corresponding to a sulfur partial pressure of 4 Pa, at 800° 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. If FeAl2S4 formed over the ferrite phase, the sulfur-incorporation rate into the scale was slowed down and accordingly the alloys had excellent 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, which can vary widely. The molybdenum-rich R-phase and AlMo3 reacted with sulfur slowly. When a protective preferential-sulfidation zone formed, the unreacted intermetallic phases provided a mechanical framework for FeAl2S4.
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