Iron Carburization in CO-H2-He Gases, Part I: Experiment

Carburization of iron was studied at atmospheric pressure over the temperature range 850 degrees C to 1150 degrees C in gaseous mixtures of CO, H-2, and He. The resistance relaxation method was applied to measure the carburization rates. Experimental results show that for carburization in CO-He mixtures, the carburization rate increased proportionally with CO partial pressure with a reaction order of unity. The overall rate also increased with temperature up to approximately 960 degrees C and subsequently decreased with further increases in temperature. For carburization in CO-H-2-He mixtures, the carburization rate increased with both the CO and H-2 partial pressures under most conditions and was considerably faster than the rate in mixtures without hydrogen. Up to approximately 960 degrees C, the rate was nearly independent of temperature with H-2 present, but decreased with a further increase in temperature. The decrease in reaction rate for mixtures with and without H-2 at 960 degrees C coincides closely with the change in phase of iron from alpha to gamma. Experiments at 925 degrees C with constant PH2 and PCO indicate that CO dissociation on the iron surface is faster than oxygen removal from the surface except at high ratios of PH2/PCO, and therefore oxygen removal was generally the rate-limiting step. The rate of oxygen removal from the surface by H-2 was found to be of the same order as that by CO. At high hydrogen levels, the rate of oxygen removal exceeds dissociative absorption of CO and the latter becomes rate controlling. The present results are used to establish a numerical model for the carburization of iron, which is described in a companion paper. (C) 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 327-336, 2009