EFFECT OF SULFUR ON THE STABILITY OF CEMENTITE

Cementite Fe3C 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-H2O-H2S mixtures at temperatures between 400-700 degrees 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-H2O and other strongly carburizing atmospheres can be suppressed by the presence of some H2S. The range of H2S/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 Fe3C by reduction of ores in methane, is possible only by the presence of some sulfur.