Sulfidation Behavior of Rhenium and Cobalt-Rhenium Alloys

The sulfidation behavior of Re and three Co-Realloys, 15, 30, and 45 w/o, was studied over thetemperature range 700-800°C at sulfurpressures of 10-4 and 10-2 atm. The kinetics of sulfidation followed theparabolic rate law and the activation energies for allalloys were similar to that of pure cobalt. A positiverate dependency on sulfur pressure was observed and Pt markers were located at the metal-scaleinterface, both observations clearly suggesting thatoutward cation diffusion through a P-type sulfide scaleoccurred. Two dominant sulfides,Co9S8 and ReS2, formed. Weight gainsdecreased for a given set of conditions with increasingrhenium content. An order of magnitude decrease in thesulfidation rate occurred as the rhenium content increased from 15 to 45 w/o. Preferentialsulfidation of cobalt initially occurred, causing arhenium-enriched zone to form in the substrate beneaththe cobalt-sulfide scale. Steep Re-concentrationgradients developed, the zone depth increasing withrhenium content. Significant sulfur diffusion into thesubstrate also occurred, with greater sulfur penetrationtaking place as the rhenium content increased. Sulfides formed at all temperatures were the same on thethree alloys, but the scales were denser on thehigher-rhenium alloys. The initial sulfide to form wasCo3S4, but, subsequently,Co9S8 became the dominant sulfide, forming beneaththe outer Co3S4 layer.ReS2 formed at lower cobalt levels. Pure Rewas also studied, the sulfidation rate being about104 times slower than that of cobalt. Thedecreasing rate of sulfidation with increasing Recontent is attributed primarily to slower cobaltdiffusion outward through the Reenriched substrate, aphenomenon similar to that observed by C. Wagner for theoxidation of Ni-Pt alloys.