Morphology and Growth Mechanism of Sulfide Subscale in Superalloys

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The sulfidation behavior of Ni-Cr based alloys including Waspaloy has been investigated in the temperature range of 1173K to 1273K in a molten sodium sulfate. Optical microscopy, X-ray diffraction measurements and electron probe microanalysis on the sulfide scales formed by high-temperature sulfidation indicate that the scale consist of an external scale on the, alloy surface and a subscale in the alloy interior by selective sulfidation of elements having a large affinity of sulfurAt all the temperatures the subscales in a Ni-Cr alloy and Waspaloy grew according to a parabolic rate law, and the apparent activation energies far their processes were estimatedAlmost of all the previous papers concerning with subscale have been on the basis of the wo-dimensional observation on the cross-sections of sulfidized specimens. However, it is difficult to estimate exactly the three-dimensional morphology development of the subscale because its morphology is peculiar. In order to examine directly the three-dimensional morphology, we have developed a metal matrix dissolution, technique for the sulfidized Ni-Cr based alloys without dissolving the metallic sulfide phases constituting the subscale by using an organic solvent system solution. The organic solvent system solution, we developed, consists of bromine, cetylpyridinium bromide and acetonitrile. The three-dimensional morphology of the intergranular and intragranular sulfides foamed in the Ni-Cr based alloys suggests clearly the difference of each growth mechanism. The newly developed dissolution technique using an organic solvents system proved useful