THE SULFIDATION OF CO-NB ALLOYS AT 600-800-DEGREES-C IN H2-H2S MIXTURES UNDER 10-8ATM S2

The sulfidation behavior of Co-Nb alloys containing 15 and 30 wt% Nb was studied at 600-800-degrees-C in H-2-H2S mixtures providing a sulfur pressure of 10(-8) atm but also 10(-7) atm at 800-degrees-C. The alloys formed duplex scales with an outer layer of pure cobalt sulfide and an inner complex layer where both Co and Nb were present, except at 800-degrees-C under 10(-8) atm S2 when the sulfur pressure was below the dissociation of cobalt sulfide and only double sulfide formed. The corrosion kinetics were generally complex and intermediate between linear and parabolic. Niobium additions reduced the sulfidation rate with respect to pure cobalt, but the low corrosion rate of pure niobium was never attained, even when the sulfur pressure was below the dissociation of cobalt sulfide. The lack of exclusive sulfidation of niobium is attributed to the formation of double Co-Nb sulfides through which cobalt can diffuse outwards, even though less rapidly than through pure cobalt sulfide.