MICROPROCESSES OF METAL DUSTING ON IRON

In strongly carburizing atmospheres iron and steels are subject to 'metal dusting', a catastrophic carburization, leading to disintegration into a dust of carbon and metal particles. TEM investigations of iron foils in different states of this corrosion process have clearly shown now that unstable cementite is an intermediate in the reaction. The carbide is formed at the iron surface after supersaturation (a(C) > 1), its disintegration is started by graphite deposition on its surface (a(C) = 1). The carbon atoms from Fe3C disintegration are attached to the graphite planes which are vertically oriented to the cementite surface, the iron atoms diffuse through the graphite and agglomerate to small particles (similar to 20 nm) which act as catalysts for further carbon deposition from the atmosphere, till they are densely covered with graphite. The TEM investigations are in good agreement with preceding studies concerning kinetics and mechanisms of metal dusting.