Abstract
The kinetics and mechanism of sulfide scale growth on pure molybdenum have been investigated in an elemental sulfur 
atmosphere and in an 
gas mixture over the temperature range of 700°–950°C. Below about 800°C, linear kinetics were observed, and at higher temperatures the kinetics were parabolic. The surface orientation of the metal affected the sulfidation mode and rate, and the 
scale showed a strong epitaxial relationship with the metal at all temperatures. For the low temperature linear kinetics, a sulfide crystal nucleation step is believed to be rate limiting. In the high temperature parabolic regime, the diffusion of sulfur ions through the scale is the rate‐limiting step. After the scale reaches some critical thickness, 
crystals form beneath the 
scale, which slows the reaction somewhat. The parabolic rate constants for sulfidation in 
gas mixtures at high temperatures were about one order of magnitude higher than those in elemental sulfur at the same sulfur activity.