Phase Diagram of the Ni–Cr–S System at 873 K between 10−4 and 10−8 Pa

Abstract

The phase diagram of the Ni–Cr–S system was determined at 873 Kby sulfidation of thin Ni-Cr alloy sheets (22, 50, 66.6, and 80 at.% Cr) at10−4.0, 10−5.5, 10−7.0,and 10−8.0 Pa in H2S–H2 gasmixtures. X-ray diffraction analysis identified two sulfides,NixCr3−xS4 (0<x<1.0) and (Ni,Cr)3S2,while concentration profiles were measured by electron-probe microanalysis(EPMA). The compositions of the terminal sulfides and alloys in equilibriumwere determined at sulfur pressures below 10−5.5 Pa, whereasat 10−4.0 Pa they were obtained from the Cr3S4and Ni3S2 phases at their interface. The proposedphase diagram shows that the solubility limit of Cr into Ni3S2is only a few atomic percent Cr, while the Ni content in Cr3S4decreases rapidly with decreasing sulfur pressures from 14 at.% Ni, a metalratio NNi/(NNi+NCr) of 0.32, at 10−4.0Pa to a negligible value at sulfur pressures below 10−8.0 Pa.