Sulfidation of pure and chromium-modified beta-NiAl intermetallic compound - mechanism studies

The studies on sulfidation mechanism of pure and chromium modified [MATH]-NiAl intermetallic compound were carried out in He/S2 gas mixtures at high sulfur pressure (about 2 x 103 Pa). Gold markers were deposited on the surface of specimens prior to sulfidation run by a PVD process. The distribution of gold marker after sulfidation indicated that the outer scale layer, composed of sulfospinels, NiAl2S4 and/or CrAl2S4, and nickel sulfide, grows by an outward diffusion of cations. This is accompanied by an inward penetration of sulfur resulting in the formation of internal sulfides (Al2S3 and CrAl2S4). The inward transport of sulfur may be due to dissociation of the sulfospinel phases at the scale/alloy interface. Secondary formation of fine-grained sulfospinel phases was observed in the vicinity of the scale/alloy interface. Another 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 β-NiAl, NiAl-4Cr and NiAl-10Cr. The formation of a stratified scale on NiAl-20Cr is explained by periodic detachment of the scale layers and their reproduction on the alloy surface due to inward penetration of sulfur