A comprehensive investigation of the sulfidation behavior of binary Co-Mo alloys

The sulfidation behavior of Co-Mo alloys containing up to 40 wt.% Mo was studied over the temperature range 600–900°C in both 10^–2 and 10^–4 atm. sulfur vapor. All of the alloys were two-phase, with the alloys containing up to 30Mo consisting of Co₃Mo plus solid-solution Co, and the Co-40Mo alloy consisting of the two intermetallic compounds, Co₃Mo and Co₇Mo₆. The sulfide scales which formed were duplex, with an outer layer of cobalt sulfide and a complex, heterophasic inner layer whose phases were both composition- and temperature-dependent. The parabolic rate constant for the sulfidation kinetics decreased with increasing Mo content at all temperatures investigated. Three activation energies, all different from that of pure Co, were observed. Furthermore, Co-30Mo exhibited a kinetics inversion between 800 and 850°C. This inversion was largely the result of the formation of an innermost layer of Co_1.62Mo₆S₈ at the high temperatures. Specifically, the presence of this sulfide in the inner scale caused a significant decrease in the growth rate of the outer layer of cobalt sulfide. In fact, formation of a more compact, innermost layer of Co_1.62Mo₆S₈ at 900°C compared to that at 850°C resulted in a negative activation energy for the growth of the cobalt sulfide in this temperature range. The variation in the activation energies was due to both the duplex nature of the scales which formed and the phase constitution of the inner scale. A simple model has been developed to explain the changes in the activation energies. At 800°C the sulfidation rate of the Co-Mo alloys was essentially the same at the two sulfur pressures studied. The predominant phase in the inner layer of Co-10Mo and Co-20Mo was CoMoS₃, while for Co-30Mo and Co-40Mo it was MoS₂. However, in the case of the latter alloys, Co_1.62Mo₃S₄ formed in the region of the alloy/scale interface at temperatures 850°C and above. Although the MoS₂, which had formed on Co-40Mo, appeared to be a continuous layer, it was in fact found to be relatively nonprotective. Platinummarker experiments revealed the position of the original metal surface to be the interface between the inner and outer scales.