Oxidation of low carbon steel in multicomponent gases .1. Reaction mechanisms during isothermal oxidation

The article describes rates of oxidation of low carbon steel in various nitrogen-based atmospheres of O-2, CO2, and H2O in the temperature range 800 degrees C to 1150 degrees C. In characterizing the oxidation process, the weight gains of the samples per unit surface area vs time data were analyzed. Reaction rates during oxidation in the binary atmospheres of CO2-N-2 and H2O-N-2 followed a linear rate law and were found to be proportional to the partial pressures of CO2 or H2O. These rates were controlled by rate of reactions at the oxide surface and were highly dependent on oxidation temperature. The activation energies of the phase boundary reactions obtained were approximately 274 and 264 kJ/mole, for oxidation in CO2 and H2O atmospheres, respectively. Oxidation in gases containing free oxygen showed that the main oxidizing agent was the free oxygen and that additions of CO2 and H2O had little effect on the magnitude of the initial oxidation rates. Experiments for oxidation in multicomponent gases showed that the overall oxidation rates were the additions of rates resulting from oxidation with the individual gaseous species O-2, CO2, and H2O. Oxidation in these atmospheres exhibited an initial linear rate law which gradually transformed into a parabolic. Examination of scale microstructure after 1 hour of oxidation showed that, for oxidation in carbon dioxide and water vapor atmospheres, only wustite was present, while in atmospheres containing free oxygen, all three iron oxides, wustite, magnetite, and hematite, were present.