Microprocesses of coke formation in metal dusting

The microprocess of coke formation during metal dusting on iron in a carbur izing atmosphere with medium and extremely high carbon activities as well a s the influence of sulphur have been studied down to the nanometer scale us ing high resolution electron microscopy (HREM) and analytical electron micr oscopic techniques (AEM). While for medium carbon activities the metal dust ing proceeds via a formation, disintegration and further decomposition of a metastable carbide Fe3C into Fe and C, the additional formation of the car bide Fe5C2 and the stabilization of carbides in the coke region have been o bserved for extremely high carbon activities. If sulphur is present in the atmosphere metal dusting takes place solely in the S-free surface areas. Fu rthermore, sulphur deposited from the atmosphere will suppress the nucleati on of graphite in the coke. In addition, the results reveal that, irrespective of the degree of the car bon activity, there is a fundamental initial reaction micromechanism of met al dusting characterized by a vertically oriented deposition of graphite la ttice planes with respect to the original surface of the substrate and with free ends affecting the decomposition of the carbides and thus forming a c oke of carbon and iran, or of carbide particles, depending on the carbon ac tivity.