THE SULFIDATION BEHAVIOR OF SEVERAL COMMERCIAL FERRITIC AND AUSTENITIC STEELS

The sulfidation behavior of C-steel, 1Cr-0.5Mo steel, 12Cr-1Mo-0.25 V steel, 18Cr-10Ni-Ti steel, the binary alloys Fe-20Cr, Fe-25Cr, Fe-30Cr , and pure Cr was investigated between 400 and 700 degrees C in a 94Ar -5H(2)-1H(2)S gas mixture. All steels sulfidize according to complex k inetics which, after a period with decreasing rate, can be approximate d by a linear rate law. The scale of the three ferritic steels consist s of two layers, an outer outward-growing one of FeS with traces of di ssolved Cr and an inner, inward-growing one, which contains in additio n to Fe the alloying elements Cr and Mn. Most of the outer FeS layer i s separated from the inner layer and can be split into several partial layers, the number increasing with increasing sulfidation time and te mperature. The scale on the austenitic 18Cr-10Ni-Ti steel differs inso far as that of the ferritic steels as the outer FeS layer contains som e Ni and that a third layer of the spinel FeCr2S4 is formed between th e outer and the inner layer. This intermediate layer is responsible fo r the lower sulfidation rate of this material compared with that of th e ferritic steels. The scale of the binary Fe-Cr alloys is similar to that of the austenitic steel. From AE-measuremnts it can be deduced th at the separation of the outer FeS layer occurs during isothermal sulf idation and is accompanied by an increase in the AE event rate. The se paration is a consequence of the formation and growth of pores in the region close to the inner/outer layer interface and the development of compressive growth stresses in the outer FeS layer. While detachment of the FeS layer on the ferritic steels Iras already observed at 400 d egrees C, the austenitic steel showed a similar separation of the FeS layer only, at 600 degrees C. The detached FeS layer is obviously rath er gas tight. Differences in the sulfur partial pressure of the bulk g as and the gas in the cavity between the inner and separated outer lay er lead to a reduction of FeS at the inner surface of the detached FeS layer. The Fe ions and electrons, produced by this reaction, diffuse outward, forming new FeS on the outer FeS surface. This process not on ly shifts the detached FeS layer continuously away from the core of th e specimen but offers also the possibility of healing cracks in the se parated FeS layer. This scale detachment does not stop scale growth. A fter scale separation the total sulfidation reaction consists of at le ast seven partial reactions. phase-boundary reaction at the outer surf ace, diffusion of iron ions and electrons outwards in the detached FeS layer, formation of H2S at the inner surface of the detached layer, g as diffusion in the cavity, formation of FeS on top of the porous inne r layer, gas diffusion in the channels of the porous inner layer, FeS formation at the metal/ scale interface. When the new FeS layer on top of the porous inner layer exceeds a critical thickness, the detachmen t of the FeS layer from the inner porous layer repeats. This process c an take place several times, leading to an outer FeS partial scale, sp lit into several layers, which are separated by relatively large cavit ies and kept together only locally by FeS bridges. The overall reactio n rate is controlled by the phase-boundary reaction at the outermost F eS surface.