Effect of H₂S on formation and decomposition of Fe₃C and Fe₅C₂ under metal dusting conditions

The high temperature corrosion process metal dusting leads to the formation and decomposition of metastable iron carbides at the surface of iron samples. A small amount of H 2 S in the carburising atmosphere causes the adsorption of sulphur onto the sample surface, which decreases the carbon transfer rate and retards or suppresses the start of metal dusting. The extent of retardation of metal dusting depends on temperature, carbon activity and H 2 S content. The higher the carbon activity the higher the H 2 S content required for suppression of metal dusting. At very high carbon activities a second iron carbide, Fe 5 C 2 (Hägg carbide), forms on the cementite surface. The carburisation experiments were conducted at 500°C using CO‐H 2 ‐H 2 O‐H 2 S gas mixtures. The microstructural investigations show that both metastable carbides decompose during metal dusting.