SULFIDATION OF SOME MCRALYX-TYPE ALLOYS

The sulphidation behavior of a series of MCrAlYX-type alloys, where M is Co or Co and Fe, and X is V, Nb, Mo, or W, added in combination at various levels, has been studied at 750-degrees-C in an atmosphere of p(S2) approximately 10(-1) Pa and p(O2) approximately 10(-18) Pa. Sulphidation kinetics followed a parabolic rate law for each alloy over prolonged periods of exposure (up to 240 hr). Inclusion of Mo in the alloy containing both V and Nb, and particularly the combined additions of Mo and W, greatly enhanced sulphidation resistance. Partial replacement of Co by Fe also provided superior sulphidation resistance. The scales formed on the Co-based alloys consisted of three subscale layers-an outer layer of Co9S8 and Co3S4, a mid-layer of Cr3S4 and an inner layer of chromium and refractory-metal sulphides. In the case of (Fe, Co)-based alloys, the scales were duplex in nature with an outer layer of Fe3S4, Co9S8 and Cr3S4 and a compact inner layer consisting of sulphides of chromium and refractory metals. For both categories of alloys, the inner layer of refractory-metal sulphides was decorated by an alumina film which inhibited the outward migration of the base elements.