Hot corrosion of the Ni-20%Cr alloy and waspaloy immersed in Na ₂So₄

The nickel-based superalloy of Waspaloy and the Ni-20%Cr alloy were hot-corroded in the molten Na₂SO₄ under the flowing air for up to 720 ks at 1273 to 1473K to clarify the sulfidation/oxidation behavior by means of XRD, OM, SEM, EPMA and TEM. They formed a subscale which was composed of intergranular and intragranular compounds in the alloy interior together with an external scale on the alloy. The kinetic of the subscale formations was followed by a parabolic rate law. The external scale and subscale formed on the Ni-20%Cr alloy consisted Of Cr₂O₃ and Cr₂S₃, Cr₅S₆ and a small amount Of Cr₂O₃, respectively. Those formed on Waspaloy consisted of (Cr "middot; Co "middot; Ti)₂O₃ and TiS ₂, Cr₂S₃, Cr₅S₆, Al ₂O₃ and (Cr"middot;Co"middot;Ti) ₂O₃, respectively. In order to observe directly the three dimensional morphology of the subscale, a metal matrix was dissolved using an organic solution that does not dissolve the metallic sulfides or oxides which constitute the subscale. The organic solution developed in this investigation consisted of bromine, cetylpyridinium bromide and acetonitrile. The SEM images of the three-dimensional subscale extracted from the corroded alloys with the organic solution showed that the intergranular compounds formed thin walls of sulfides and oxides along grain boundaries and an intragranular sulfide formed isolated needles in the alloy grain interior near the external scale.