2007 Volume 93 Issue 7 Pages 498-505
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 · Co · Ti)2O3 and TiS2, Cr2S3, Cr5S6, Al2O3 and (Cr · Co · 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.