KINETICS AND MECHANISM OF HIGH-TEMPERATURE SULFIDATION OF AN FE-23,4CR-18.6AL ALLOY IN H2S-H2 ATMOSPHERES

Sulfidation of an Fe-23.4Cr-18.6Al (at. %) alloy was investigated in H2S-H2 atmospheres, 10(-6) less-than-or-equal-to P(s2) less-than-or-equal-to 10(2) Pa, at 973 K. It was found over this pressure range that sulfidation after an early transient period followed the parabolic rate law, being diffusion controlled. An investigation was carried out of the scales formed during early transient sulfidation over the sulfur pressure range p(s2) = 10(-6)-10(2) Pa. Fully developed scales were multilayered consisting of an inner compact layer of equiaxed grains, an intermediate layer of equiaxed and columnar grains exhibiting a small degree of porosity, and an outer porous layer of distinct plates and needles. The grains of the inner and intermediate layers contained quarternary sulfide phases. The following phases were identified: spinels (CrFe)Al2S4 and (FeAl)Cr2S4, hexagonal (FeCr)Al2S4, (CrAlFe)2S3, and (CrAlFe)5S6. The plates and needles were composed of hexagonal (FeCr)Al2S4 and (CrAlFe)2S3 at p(s2) = 10(-6) and 10(-5) Pa from which pyrrhotite, FeS, grew at p(s2) greater-than-or-equal-to 10(-4) Pa. The alloy exhibited a small degree of internal sulfidation; the precipitates were Al2S3 doped with Cr and Fe. Marker measurements indicated that growth of the inner sulfide layer was supported by inward migration of sulfur. The morphological development of the scales at 973 K was similar to that obtained at 1073 and 1173 K.