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. Sulp hidation kinetics followed a parabolic rate law for each alloy over pr olonged periods of exposure (up to 240 hr). Inclusion of Mo in the all oy containing both V and Nb, and particularly the combined additions o f Mo and W, greatly enhanced sulphidation resistance. Partial replacem ent of Co by Fe also provided superior sulphidation resistance. The sc ales 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 l ayer of chromium and refractory-metal sulphides. In the case of (Fe, C o)-based alloys, the scales were duplex in nature with an outer layer of Fe3S4, Co9S8 and Cr3S4 and a compact inner layer consisting of sulp hides of chromium and refractory metals. For both categories of alloys , the inner layer of refractory-metal sulphides was decorated by an al umina film which inhibited the outward migration of the base elements.