THE INFLUENCE OF SULFUR PRESSURE ON SULFIDATION BEHAVIOR OF NICOCRAL(Y) ALLOYS AT HIGH-TEMPERATURE

Sulfidation of alloy having nominal composition Ni-23Co-19Cr-12Al (wt% ) with and without the addition of 0.6% yttrium was studied at tempera tures 1073-1273 K in sulfur vapor at atmospheric pressure and in H-2/H 2S gas mixtures at sulfur pressure of 10(-3) and 10(-1.5) Pa. Sulfidat ion runs were followed thermogravimetrically. Phase and chemical compo sition of sulfide scales and scale morphologies were determined by mea ns of XRD, EDX, EPM and SEM analyses. After certain initial period sul fidation of both materials followed approximately a parabolic rate law . The estimated sulfidation rates for each alloy increased with sulfur pressure and temperature. The sulfide scales on both materials showed complex microstructures and compositions, depending on sulfidation co nditions, with several sulfide and sulfospinel phases present, such as (Ni,Co)S, (Ni,Co)3S4, (Ni,Co)Cr2S4, (Cr,Ni,Co)Al2S4 or (CrNi,Co)S and (Cr,Ni,Co)3S4. There was no evidence of yttrium segregation either to the grain boundary regions in the scale or to the alloy/scale interfa ce.Yttrium dissolved in the sulfide phases and accelerated the sulfida tion process. This behaviour was ascribed to the doping effect.