Adsorption and segregation of sulphur, and its influence on the carburization and nitrogenation of iron and steel

Abstract
The surface segregation of sulphur on iron was investigated by A.e.s. on iron polycrystals in the temperature range 700-1100 °C. Strong surface segregation occurs even at low bulk concentrations of 2—20 ug/g. The enthalpy of segregation for S(dissolved) = S (adsorbed) is about —165 kJ/mole S. By l.e.e.d. studies at 750 °C, ordered adsorption structures were observed on iron single crystal surfaces. In the segregation studies the equilibrium sulphur activity, a s , was determined by the concentration of dissolved sulphur. The results were correlated with the surface reaction kinetics in the carburization and nitrogenation of iron in the presence of adsorbed sulphur. These studies were performed in a flow apparatus at atmospheric pressure, the sulphur activity being established by the H 2 S/H 2 ratio in the gas atmosphere. The effect of adsorbed sulphur was studied for the reactions CH 4 (g) = C (dissolved) + 2H 2 (g) and N 2 (g) = 2N(dissolved) by a resistance relaxation method on thin iron foils at 800—900 °C. Even at bulk concentrations of about 0.1 ug/g S, the surface reactions were strongly retarded. Carbon and nitrogen transfer can only occur on reaction sites free of adsorbed sulphur. It can be seen from the kinetics that two sites are needed for the dissociation of N 2 but one site is sufficient for the decomposition of CH 4 . From the reaction kinetics, the adsorption isotherm for sulphur adsorption and segregation at 850 °C could be derived.