The corrosion of Co-Nb alloys in reducing/oxidizing-sulfidizing gases at 600–800°C

Abstract

The corrosion of the two pure metals and of two alloys containing 15 and 30 wt% Nb has been studied at 600–800°C in H 2-H 2S-CO 2 gas mixtures providing 10 −8 atm S 2 at all temperatures and 10 −24 atm O 2 at 600°C and 10 −20 atm O 2 at 700 and 800°C. The corrosion kinetics were rather complex, being sometimes parabolic and in other cases nearly linear. Under a constant temperature the addition of niobium generally reduced the corrosion rate, except at 700°C when pure cobalt corroded more slowly than the two alloys. The corrosion rates for the same material decreased with an increase in temperature under the same sulfur pressure. Except at 800°C under 10 −8 atm S 2, which is below the dissociation pressure of cobalt sulfide, the scales presented an outer layer of pure cobalt sulfide and an inner layer of complex composition containing a mixture of double sulfide, niobium oxide and in some cases of unreacted metallic cobalt particles. The addition of niobium was generally beneficial, the effect increasing with its concentration in the alloy, but the corrosion rates of the alloys were still much higher than that of pure niobium, mainly as a result of the lack of formation of a protective layer of niobium sulfide. The corrosion behavior is examined with special reference to the consequences of the low solubility of niobium in cobalt and to the relation between the microstructure of the alloys and the scales.