On the “coke” growth in carburizing and sulfidizing atmospheres upon High temperature corrosion of iron and nickel base alloys

Abstract

Abstract The graphite deposition from carbonaceous atmospheres can initiate a catastrophic deterioration of alloys in high temperature corrosion. The graphite nucleation and growth is catalyzed by metal surfaces and affected by the presence of sulfur. Gravimetric studies have been performed on the carbon transfer from CH 4 H 2 or CH 4 H 2 H 2 S atmospheres to iron, nickel or ironnickel alloys at 1000°C. The carbon activities were a c = 1 (equilibrium with graphite), a c = 5 or a c = 10; the sulfur pressure was in a range where the metal surfaces are nearly saturated with adsorbed sulfur. The carburization, i.e. the transfer of C into solid solution is retarded in the presence of sulfur since surface sites are blocked for the methane decomposition. In the sulfur‐free environment graphite layers grow with their basal planes parallel to the metal surface – for nickel an epitaxial growth occurs which is extremely slow. In the presence of sulfur the graphite can only nucleate in small islets which grow to irregular nodules. This results in a retardation by sulfur of the graphitisation on iron, whereas the growth of graphite on nickel is accelerated by sulfur. The transition between these ways of graphitisation behaviour was studied for FeNi.