Carburization Kinetics of Solid Iron at 1523 K via the CO Gas Isotope Exchange Technique

The carburization kinetics on the surface of solid iron was investigated at 1523 K. By using the CO gas isotope exchange technique, the pure chemical reaction rate was measured excluding the influence of solid-phase mass transfer of carbon in iron. The kinetics of the gas-phase mass transfer was determined via theoretical calculations. The surface reaction rate with carbon monoxide and hydrogen gases was of the order of 10⁻⁵ mole/cm² s atm that was at least one order higher than the diffusion rate of carbon in iron. In addition, the surface reaction rate was measured at varying sulfur concentrations to elucidate the effect of surface-active sulfur on the carburization kinetics. A linear relationship was observed between the surface reaction rate and the reciprocal of the sulfur activity in solid iron. The steps involved in the surface reaction with carbon monoxide were investigated based on a simple site-blockage model and Langmuir-type ideal adsorption isotherms. The adsorption of carbon monoxide at low sulfur concentration (< 50 ppm) on the iron surface was identified as the rate-limiting step of the surface reaction for carburization.