Effect of sulfur on the stability of cementite

Abstract

Cementite Fe 3 C is an unstable carbide, which should decompose to iron and graphite. It was possible to grow cementite on iron samples in flowing CO‐H 2 ‐H 2 O‐H 2 S mixtures at temperatures between 400–700°C. The cementite is stabilized by adsorbed sulfur, preventing graphite deposition, which would initiate metal dusting. Thus, this corrosion phenomenon, occurring in CO‐H 2 ‐H 2 O and other strongly carburizing atmospheres can be suppressed by the presence of some H 2 S. The range of H 2 S/H 2 ratios and temperatures in which iron is inert against metal dusting, corresponds to the range where a monolayer of sulfur is adsorbed in iron. It may be supposed that the iron carbide process, i.e. the production of Fe 3 C by reduction of ores in methane, is possible only by the presence of some sulfur.