Role of sulphur in carburization, carbide formation and metal dusting of iron

The kinetics and mechanisms of reactions on iron in CH4-H-2 and CO-H-2-H2O mixtures have been studied, also in the presence of H2S, to understand carburization, iron carbide formation, metal dusting and carbon deposition. Rate equations and mechanisms of the carburization in CH4-H-2 and CO-H-2-H2O have been elucidated. Both reactions are retarded by adsorbed sulphur, the rates becoming inversely proportional to the sulphur activity a(s) similar to p(H2S)/p(H-2) with increasing a(s). At carbon activities a(C) > 1, cementite growth can be started in both gas mixtures on iron but the decomposition of this unstable carbide gives rise to a corrosion process called 'metal dusting', i.e. a disintegration of iron and steels to a dust of metal particles and graphitic carbon. This disintegration can be prevented by adsorbed sulphur, which hinders the nucleation of graphite. The stabilizing effect of sulphur on cementite and higher carbides such as Hagg carbide allows fundamental studies to be carried out on their thermodynamics, non-stoichiometry and diffusional growth mechanisms, and the presence of sulphur will allow iron carbide production in a process of great present interest-the direct reduction of iron ores in carburizing gas mixtures. Kinetic studies in the system Fe-C-S are supplemented by AES and low-energy electron diffraction investigations on the adsorption and segregation of sulphur on iron and cementite. Copyright (C) 2002 John Wiley Sons, Ltd.