Effect of Nb on the high-temperature sulfidation behavior of cobalt

The sulfidation behavior of Co-Nb alloys containing up to 30wt.% Nb was studied in sulfur vapor at a pressure of 0.01 atm in the temperature range of 600–700°C. Increasing niobium content decreased the sulfidation rate, following the parabolic rate law. An activation energy of 25.6 kcal/mole was obtained for Co-10Nb, Co-20Nb, and Co-25Nb, while a value of 20.5 kcal/mole was found for Co-30Nb. All were two-phase alloys, consisting of solid solution -Co and the intermetallic compound, NbCo₃. The two-phase alloys formed a rather thick outer layer of cobalt sulfides and a heterophasic inner layer that was complex. The inner layer always contained the mixed sulfide CoNb₂S₄ which, depending on the alloy composition, coexisted with cobalt sulfide, NbS₂, and / or NbCo₃ particles. Short-time sulfidations showed that the solid solution initially sulfidized rapidly to form nodules of cobalt sulfide, whereas the NbCo₃ phase formed a thin protective layer of NbS₂. The nodules grew laterally until they coalesced into the continuous, outer thick layer, while the NbS₂ completely or partially reacted with the cobalt sulfide to form CoNb₂S₄. Platinum markers were always found at the interface between the inner and outer scales, the location of the original metal surface.