The oxidation-sulfidation of FeNb alloys at 600–800°C in H 2-H 2S-CO 2 mixtures

Abstract

The corrosion of the two pure metals and of two Fe-Nb alloys containing 15 and 30 wt% Nb has been studied at 600–800°C in H 2-H 2S-CO 2 gas mixtures providing a sulfur pressure of 10 −8 atm and oxygen pressures of 10 −24 atm at 600°C and 10 −20 atm at 700–800°C. Both iron and the two alloys corroded according to an approximate parabolic rate law, with an instantaneous rate constant increasing with time. The rate constants for corrosion decreased with an increase in the Nb content of the alloys under constant temperature but increased with temperature for the same alloy. In all cases the rate constants were slightly lower than those for the corrosion in H 2-H 2S mixtures under the same temperature and sulfur pressure, but remained still quite high as compared to the rate of corrosion of pure niobium due to the lack of formation of a protective layer of niobium compounds. The scales were duplex, presenting an outer layer of pure FeS and an inner complex layer containing a mixture of FeS, double sulfide, niobium oxides and some double oxide. Islands of the Nb-rich Fe 2Nb phase remained partly unsulfidized in the inner scale region for the most concentrated alloy. The corrosion behaviour observed is a result of the low solubility of niobium in iron and of the two-phase nature of the alloys.