The sulfidation behavior of several commercial ferritic and austenitic steels

Abstract

The sulfidation behavior of C-steel, 1Cr-0,5Mosteel, 12Cr-1Mo-0.25V steel, 18Cr-10Ni-Ti steel, thebinary alloys Fe-20Cr, Fe-25Cr, Fe-30Cr, and pure Cr wasinvestigated between 400 and 700°C in a94Ar-5H2-1H2S gas mixture. All steels sulfidize according tocomplex kinetics which, after a period with decreasingrate, can be approximated by a linear rate law. Thescale of the three ferritic steels consists of two layers, an outer outward-growing one of FeSwith traces of dissolved Cr and an inner, inward-growingone, which contains in addition to Fe the alloyingelements Cr and Mn. Most of the outer FeS layer is separated from the inner layer and can be splitinto several partial layers, the number increasing withincreasing sulfidation time and temperature. The scaleon the austenitic 18Cr-10Ni-Ti steel differs insofar as that of the ferritic steels as theouter FeS layer contains some Ni and that a third layerof the spinel FeCr2S4 is formedbetween the outer and the inner layer. This intermediatelayer is responsible for the lower sulfidation rate of this materialcompared with that of the ferritic steels. The scale ofthe binary Fe-Cr alloys is similar to that of theaustenitic steel. From AE-measurements it can be deduced that the separation of the outer FeSlayer occurs during isothermal sulfidation and isaccompanied by an increase in the AE event rate. Theseparation is a consequence of the formation and growth of pores in the region close to the inner/outerlayer interface and the development of compressivegrowth stresses in the outer FeS layer. While detachmentof the FeS layer on the ferritic steels was already observed at 400°C, the austenitic steelshowed a similar separation of the FeS layer only at600°C. The detached FeS layer is obviously rathergas tight. Differences in the sulfur partial pressure ofthe bulk gas and the gas in the cavity between theinner and separated outer layer lead to a reduction ofFeS at the inner surface of the detached FeS layer. TheFe ions and electrons, produced by this reaction, diffuse outward, forming new FeS on the outerFeS surface. This process not only shifts the detachedFeS layer continuously away from the core of thespecimen but offers also the possibility of healing cracks in the separated FeS layer. This scaledetachment does not stop scale growth. After scaleseparation the total sulfidation reaction consists of atleast seven partial reactions: phase-boundary reaction at the outer surface, diffusion of iron ionsand electrons outwards in the detached FeS layer,formation of H2S at the inner surface of thedetached layer, gas diffusion in the cavity, formationof FeS on top of the porous inner layer, gas diffusionin the channels of the porous inner layer, FeS formationat the metal/scale interface. When the new FeS layer ontop of the porous inner layer exceeds a critical thickness, the detachment of the FeSlayer from the inner porous layer repeats. This processcan take place several times, leading to an outer FeSpartial scale, split into several layers, which are separated by relatively large cavities andkept together only locally by FeS bridges. The overallreaction rate is controlled by the phase-boundaryreaction at the outermost FeS surface.