SULFIDATION OF PURE AND CHROMIUM-MODIFIED BETA-NIAL INTERMETALLIC COMPOUND - MECHANISM STUDIES

The studies on sulfidation mechanism of pure and chromium modified bet a-NiAl intermetallic compound were carried out in He/S2 gas mixtures a t high sulfur pressure (about 2 x 10(3) Pa). Gold markers were deposit ed on the surface of specimens prior to sulfidation run by a PVD proce ss. The distribution of gold marker after sulfidation indicated that t he outer scale layer, composed of sulfospinels, NiAl2S4 and/or CrAl2S4 , and nickel sulfide, grows by an outward diffusion of cations. This i s accompanied by an inward penetration of sulfur resulting in the form ation of internal sulfides (Al2S3 and CrAl2S4). The inward transport o f sulfur may be due to dissociation of the sulfospinel phases at the s cale/alloy interface. Secondary formation of fine-grained sulfospinel phases was observed in the vicinity of the scale/alloy interface. Anot her secondary reaction takes place between the sulfospinels and nickel sulfide nodules present on the scale surface, greatly contributing to the overall growth of the sulfospinel layer on beta-NiAl, NiAl-4Cr an d NiAl-10Cr. The formation of a stratified scale on NiAl-20Cr is expla ined by periodic detachment of the scale layers and their reproduction on the alloy surface due to inward penetration of sulfur.