On the oxidation of iron in CO₂ + CO mixtures. III: Coupled linear parabolic kinetics

Abstract

High-purity iron has been oxidized at 1000–1200° C in CO₂ and in CO₂ + CO with different compositions and at different total gas pressures (0.1–1 atm.). The experimental work has comprised thermogravimetric reaction rate measurements and characterization of the wüstite scales by metallography and x-ray diffraction. The overall results have been analyzed in terms of a classical model for coupled linear/parabolic kinetics, where it is assumed that the surface of growing wüstite scales has exactly the same defect structure and defect concentrations as that of bulk wüstite equilibrated in the same gaseous atmospheres. Important discrepancies are found between the predicted and the experimentally observed reaction behavior. Thus, both the linear and parabolic rate constants are found to be dependent on the partial pressure of CO₂ and the total gas pressure of the CO₂ + CO gas mixtures, and furthermore, the reaction in CO₂ + CO is slower than in O₂ and in H₂O + H₂ with the same oxygen activity. In order to explain the experimental results, it is suggested that CO and CO₂ molecules interact with the wüstite surface and thereby affect the defect structure and defect concentrations in a thin surface layer, and that this, in turn, affects both the linear and parabolic reaction rates.