On the oxidation of iron in CO₂+CO gas mixtures: I. Scale morphology and reaction kinetics

Abstract

High-purity iron has been oxidized in CO₂ and CO₂+CO gas mixtures and different total gas pressures (0.1–1 atm.) at 1000–1200°C. Reaction rates and kinetics have been studied by thermogravimetry and reacted specimens have been characterized by electron microscopy and x-ray diffraction. The time dependence of the growth of wüstite may generally be described by an S-shaped curve. The scale texture of intially formed wüstite is related to that of the underlying γ-iron; however, as the scale grows, the texture gradually changes to a final one in which the (001) plane of wüstite is parallel to the iron substrate. The oxidation rate increases with this change in texture. It is concluded that the oxidation during these initial stages is essentially controlled by a surface reaction, and that the increasing reaction rate reflects an increased number of reaction sites at the surface during the change in texture. As the scale grows in thickness, the reaction rates go through a maximum, and are followed by a decreasing, parabolic-like behavior, reflecting that iron diffusion through the scales becomes increasingly important. Cold-worked iron oxidizes faster than annealed iron during the initial oxidation stages. An overall model is presented that relates the scale morphology to the detailed reaction kinetics under different reaction conditions.