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

Abstract
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.