ON THE MECHANISM OF COBALT SULFIDATION

The kinetics and mechanism of cobalt sulphidation has been studied as a. function of temperature (773-1023 K) and sulphur pressure (1-10(4) Pa) by means of the thermogravimetric, SEM and X-ray techniques, as we ll as using the inert marker and radio-tracer methods. It has been sho wn that the sulphidation process is diffusion controlled, the rate det ermining step being the outward volume diffusion of cations. In accord ance with the phase diagram of Co-S system the sulphide scale on cobal t is heterogeneous. At the sulphur pressures higher than the dissociat ion pressure of CoS2 phase the sulphidation fate is pressure independe nt and at the lower pressures it increases along with growing pressure according to the theoretical predictions. Over the whole temperature and pressure range studied the rate of cobalt sulphidation is more tha n three orders of magnitude higher than the oxidation rate of this met al. This significant difference results mainly from the very high defe ct concentration in Co1-yS and Co9S8 sulphides, participating in compa rable amounts in the scale formation on cobalt at elevated temperature s.