The Sulphidation Behaviour of Vanadium, Niobium and Molybdenum

Abstract

Sulphidation of the Group V and VI refractory metals vanadium, niobium and molybdenum has been studied at temperatures of 750, 850 and 950°C for exposure periods of up to 240 hours in hydrogenhydrogen sulphide gas mixtures such that pS₂ = 10⁻⁶ atm (l0⁻¹Pa). Recorded weight gains were low, or very low compared with those reported during sulphidation of common metals such as iron, cobalt, nickel or even chromium. Parabolic reaction kinetics were observed with all three metals at each temperature. Vanadium corroded at a distinctly fasterrate than either niobium or molybdenum, which both scaled at very low rates. Parabolic rate constants (g²/cm⁴/s) at 850°C were:1.5 × 10⁻⁹ (V), 3.9 × 10⁻¹² (Nb) and 7. 6 × 10^v12 (MO). Reactionrates increased with increase in temperature — calculated activation energies were: 138.6(v), 156.2(Nb) and 99.2(Mo) kJ/mole. Very thin scales were formed on niobium, and especially molybdenum. The much thicker scales produced on vanadium were duplexin nature. Scaling constants (cm²/s × 10⁻¹¹) at 850°C were: 44.5(V), 0.32(Nb) and 0.11 (Mo). The significance of these observations is considered.