Hiroshi Yakuwa

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

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.

Sulfidation Properties of Ni–20Cr and Ni–13.5Co–20Cr Alloys at 873 K under Low Sulfur Pressures in H2S-H2 Atmospheres

The sulfidation properties of Ni–20Cr and Ni–13.5Co–20Cralloys as well as a nickel-base superalloy AISI685 were investigated at873 K in H2S–H2 gas mixtures with sulfur partial pressures of10−4, 10−5.5, and 10−7Pa by mass-gain measurements,electron-probe microanalysis (EPMA), and X-ray diffraction (XRD)analysis. Sulfidation obeyed the parabolic rate law, and the parabolic rateconstants decreased in the order Ni–20Cr, Ni–13.5Co–20Cr,and AISI685 at each sulfur pressure. With decreasing sulfur pressure, therate constants first decreased slowly and then rapidly at a 10−7 Pasulfur pressure. At both 10−4 and 10−5.5 Pa sulfur pressures,Ni–20Cr formed a surface scale with a duplex structure of inner(Cr3S4) and outer (Ni3S2) layers, while Ni–13.5Co–20Cr formeda triplex structure of inner (Cr3S4), intermediate(Ni,Co)Cr2S4, and outer[Ni3S2+(Co,Ni)9S8] layers. Thesurface scale formed on AISI685 was verycomplex, comprising at least four layers, a fibrous (Co,Ni)9S8 top surface,outer [Ni3S2+(Co,Ni)9S8], and intermediate [(Cr,Ti)3S4] layers, as well asan inner layer containing Cr, Ti, Mo, Al, and S. At the 10−7 Pa sulfurpressure, which is lower than the dissociation pressure of Ni3S2, bothNi–20Cr and Ni–13.5Co–20Cr formed a surface scale ofCr3S4 covered by a thin NiCr2S4 layer, accompanied by copious internalsulfidation of Cr3S4 and/or CrS. On AISI685 there was a surface scale of(Cr,Ti)3S4 accompanied by the usual internal sulfidation. It is discussedthat diffusion of cations in the inner Cr3S4 layer is the rate-determiningstep for the growth of the multilayer structures. At the 10−7 Pasulfur pressure, diffusion of Cr and S contribute to form a thin surfacescale and internal sulfidation, respectively.

Effect of Titanium and Aluminum Contents on the Grain-Boundary Sulfidation of a Nickel-Based Superalloy

Abstract The effect of grain-boundary carbides on grain-boundary sulfidation was investigated on a Ni-based superalloy (Ni-20%Cr-13.5%Co-4%Mo-1.5%Ti-3%Al), which was developed for the fluid catalyt...

Sulfidation-Resistant Nickel-Based Superalloy for Fluid Catalytic Cracking Gas Expander Turbine

Abstract A Ni-based superalloy with excellent sulfidation resistance, high-temperature tensile properties, and hot workability was developed as a material for the rotors of fluid catalytic cracking...