Sulphidation of Fe-Nb,Co-Nb and Ni-Nb Alloys in H_2-H_2S at 700℃

The corrosion of alloys based on Fe,Co and Ni containing 15 and 30 wt-% Nb was studied at 700℃in H_2-H_2S mixtures providing 10~(-8) atm S_2.The Fe and Co-based alloys formed duplex scales withan outer layer of pure base-metal sulphide and an inner layer containing both metals.The Nb addi-tions were not able to produce exclusive Nb sulphidation but yielded a significant decrease of thecorrosion rates for both types of alloys with respect to the pure metals even though they were stillmuch higher than that of pure Nb.The Ni-base alloys corroded rapidly forming a single complexscale layer,mainly due to the appearance of a liquid Ni-S solution.