Carburization and Metal Dusting on Iron

Abstract

Fundamental studies have been conducted on the kinetics and mechanisms of reactions of iron CH4-H2 and CO-H2-H2O mixtures, also in the presence of H2S, to understand carburization, iron carbide formation, metal dusting and carbon deposition. The rate equations and mechanisms of the surface reaction in carburization of iron are presented, the carburization in CH4-H2 is much slower than in CO-H2. Both reactions are retarded by adsorbed sulfur, the carburization rate becomes inversely proportional to the sulfur activity aSpH2S/pH2 with increasing aS. At carbon activities aC>1 cementite growth can be started in both gas mixtures on iron, however, the decomposition of this unstable carbide gives rise to a corrosion process 'metal dusting', i.e. the material disintegrates to a dust of metal particles and graphitic carbon. This disintegration can be prevented by adsorbed sulfur which hinders the nucleation of graphite. The stabilizing effect of sulfur on cementite and higher carbides such as Hägg carbide, allows fundamental studies about their thermodynamics, non-stoichiometry and diffusional growth mechanisms, on the other hand the presence of sulfur will allow iron carbide production in the reduction of iron ores in carburizing gas mixtures.