The influence of additions of refractory elements on the sulphidation behaviour of cobalt-based alloys

Abstract

The sulphidation of a number of Co-Cr-Al-Y-X type experimental alloys, where X is Ti, Hf, V, Nb, Mo or W at levels of 1.5, 5 or 10%, has been studied. Experiments were conducted isothermally in a bioxidant sulphidizing/oxidizing environment ( p S 2 = 10 −6 atm) (10 −1 Pa) and p O 2 = 10 −23 atm (10 −18 Pa) at 750°C for periods of up to 240 h. Their performance was compared with that of four model alloys generic to these Co-Cr-Al-Y-K types, namely Co-5Al-1Y, Co-25-Cr, Co-25Cr-1Y, and Co-25Cr-3.5Al-1Y, and two commercial alloys, HK40(L) and Airesist 213. It has been established that generally such additions are beneficial, with a substantial reduction in material wastage. The composition and morphology of the scales is described. The significance of the results is considered in the light of theory concerning the design and development of sulphidation resistant alloys.