A method for long-term sulfidation of metal at low sulfur pressures and its application to sulfidation of an Fe-20 at.% Al alloy at 1023 K

Abstract

A method of long-term Sulfidation that involves generating sulfur vapor from dissociation of Ag₂S or from transformation of NiS to Ni₃S_2+x is described. Sulfidation tests of iron samples showed the approach to be essentially valid. At 1023 K, Ag₂S dissociation of Ag₂S gives rise to$p_{s_2 } = 7.9 \times 10^{ - 6}$ atm and, in the presence of an inert carrier gas, efficient transport of sulfur to the metal was effected. Liberated silver grew in the form of discrete whiskers, hence did not interfere with continued dissociation of Ag₂S. Equilibration of NiS with Ni₃S_2+x occurs at$p_{s_2 } = 1.4 \times 10^{ - 4}$ atm at 1023 K but, with continued loss of sulfur to the metal, the nonstoichiometric Ni₃S_2±x predominates and the ambient$p_{s_2 }$ is no longer constant. Thus for long-term studies, Sulfidation with NiS/Ni₃S_2+x mixtures is suitable only for metals that sulfidize slowly or at rates independent of the ambient$p_{s_2 }$. Using mainly the NiS-Ni₃S_2+x mixture as sulfur source, sulfidation tests up to 72 h at 1023 K were conducted on an Fe-20 at.% Al alloy. Sulfidation was apparently insensitive to the ambient$p_{s_2 }$and formation of a three-layer FeS/(Fe, Al)₃S₄/Al₂S₃ scale with internal precipitation zone proceeded at similar rates, and a relationship involving coupled diffusion is proposed. At longer exposure times, transformation of FeS to (Fe, Al)₃S₄ became extensive. Mechanical failure of the scale, followed by rapid healing beneath the original precipitation, also occured after extended reaction periods (>50 hr).