Oxidation of low carbon steel in moist atmospheres

In metal coating processes, steel oxidation can take place in direct fired furnaces when unexpected stoppages occur. Leakage ofair into the furnace chamber results in oxidising atmospheres containing H2 or O2 and H2O. When the temperature is higher than570°C, a stable wüstite can form and rapid oxide scale formation results.Experiments were conducted on low carbon steel in wet nitrogen gas atmospheres in the temperature range 800 to 1200°C. Allmeasured oxidation kinetic rate constants were orders of magnitude smaller than theoretical predictions. The slow rates arepartially due to the unbuffered nature of the gas.Well-controlled oxidation experiments were carried out on the low carbon steel in a series of N2-H2-H2O gas atmospheres attemperatures of 800 to 1200°C, where wüstite is the only stable iron oxide product.With fixed pO2 and temperature, according to Wagner's solid-state diffusion theory the parabolic rate constant should be fixed.However, kp values were much smaller than predicted, and kp increased with increasing pH2O. It is proposed that the mobile speciein solid-state diffusion is hydroxyl, and kp is proportional to pH2O0.5.With fixed pH2O and temperature, kp increased with increasing pO2. At 900°C, kp is proportional to pO20.4, but is proportional to pO2 at985°C. Measured kp was much smaller than the predicted results. However, the dependence of kp on pO2 was close to solid-statediffusion based theoretical prediction at both temperatures.Pure iron always reacts much faster than low carbon steel. Oxidation rate constants for iron were in good agreement withtheoretical predictions. The slow steel kinetics could be due to its manganese content, leading to formation of an (FeMn)O solidsolution. The mechanism is not clear.At the same temperature, kp values of low carbon steel oxidation were affected by both pH2O and pO2. The relationship is describedby kp=αpO2"1/n +βpH2O"1/2, where n is a temperature dependent parameter, α and β are related to vacancy and hydroxylpermeability. The different values of α and β for the two materials account for the large differences in oxidation behaviour betweenpure iron and low carbon steel observed in reducing gas atmospheres.When low carbon steel and pure iron were oxidized in O2-H2O gas atmospheres, multi-layer scales developed. Low carbon steeland pure iron had similar kp values, always in reasonable agreement with Wagner's solid-state diffusion predictions