HYDROGEN DOPING EFFECTS IN THE SULFIDATION OF MOLYBDENUM

The kinetics of molybdenum sulfidation have been studied in H2S/H-2 gas mixtures at 1173 K. Sulphur partial pressures were controlled by the equilibrium reaction between H2S, H-2, and S2. Pure molybdenum metal was sulfidized at a fixed p(S2) value of 133 Pa with varying H2S and H-2 partial pressures, and at fixed H2S partial pressures of 5.06 x 10(4) and 5.06 x 10(3) Pa with varying hydrogen and sulfur partial pressures. The gravimetric sulfidation kinetics were parabolic under all conditions. X-ray diffraction analysis of the reaction product scale revealed the presence of MoS2 only. The sulfide scales were of uniform thickness, had a compact morphology, and were tightly adherent to the metal. The sulfidation rates at a fixed sulfur partial pressure increased with increasing hydrogen partial pressure. At fixed p(H2S) values, the rate was almost constant at high P(H-2) values, but increased substantially as p(H-2) was decreased. Defect models for hydrogen dissolution in MoS2 are developed and compared with the experimental results. It is concluded that the sulfidation rate effects are due to hydride anion occupation of interlayer sites in MoS2.