The Sulfidation of Two-Phase Fe-Cu Alloys in H2-H2S Mixtures at 500-700°C

Abstract

The sulfidation behavior of three two-phaseFe-Cu alloys containing 25, 50, and 75 wt.% copper hasbeen investigated in H2-H2Smixtures at 500-700°C under gas-phase sulfur pressures, which is significantly abovethose for the dissociation of both FeS andCu2S. In all cases, the three alloyssulfidized more slowly than both pure metals under thesame conditions. At all temperatures, Fe-25Cu showed the slowestgrowth rates, whereas Fe-50Cu sulfidized more rapidlythan the other two alloys. However, the kinetics curvesfor the three alloys tended to overlap, particularly at the higher temperatures. The scales werecomplex and contained an outer layer composed of amixture of two different Cu-Fe double sulfides,Cu5FeS4 and CuFeS2,plus an inner zone containing a mixture of metalliciron with the double sulfideCu5FeS4formed by completesulfidation of the copper-rich phase and partialsulfidation of the iron-rich phase. This region also contained large voids,possibly because of outward diffusion of metal cations,whereas the iron-rich islands were mainly unattacked.The depth of internal attack increased with increasing temperature and/or iron content. Finally,particles of almost pure copper metal, probably formedduring cooling from the reaction temperature, werepresent at the scale-subscale interface, as inclusions in the scale and as whiskers protruding out ofthe external scale surface.