Sulfidation behavior of rhenium and cobalt-rhenium alloys

The sulfidation behavior of Re and three Go-Re alloys, 15, 30, and 45 w/o, was studied over the temperature range 700-800 degrees C at sulfur pressure s of 10(-4) and 10(-2) atm. The kinetics of sulfidation followed the parabo lic rate law and the activation energies for all alloys were similar to tha t of pure cobalt. A positive rate dependency on sulfur pressure was observe d and Pt markers were located at the metal-scale interface, both observatio ns clearly suggesting that outward cation diffusion through a P-type sulfid e scale occurred. Two dominant sulfides, Co9S8 and ReS2, formed. Weight gai ns decreased for a given set of conditions with increasing rhenium content. An order of magnitude decrease in the sulfidation rate occurred as the rhe nium content increased from 15 to 45 w/o. Preferential sulfidation of cobal t initially occurred causing a rhenium-enriched zone to form in the substra te beneath the cobalt-sulfide scale. Steep Re-concentration gradients devel oped, the zone depth increasing with rhenium content. Significant sulfur di ffusion into the substrate also occurred with greater sulfur penetration ta king place as rite rhenium content increased Sulfides formed at all tempera tures were the same on the three alloys, but the scales were denser on the higher-rhenium alloys. The initial sulfide to form was Co3S4, but, subseque ntly, Co9S8 became the dominant sulfide, forming beneath the outer Co3S4 la yer.. ReS2 formed at lower cobalt levels. Pure Re was also studied, the sul fidation rare being about 10(4) times slower than that of cobalt. The decre asing rate of sulfidation with increasing Re content is attributed primaril y to stower cobalt diffusion outward through the Re-enriched substrate, a p henomenon similar to that observed by C. Wagner for the oxidation of Ni-Pt alloys.