Influence of Steel Structure on the Oxide Film on Blight Annealed Sheets

The oxide film on cold rolled, steel sheets (bright annealed) has been investigated by cathodic reduction and electron diffraction. The oxide film on the sheets defurnaced into air at 150°C from annealing furnace mainly consists of fine crystallines of cubic oxide (γ-Fe2O3+Fe3O4) accompanied with crystallines of MnO. The film thickness varies from 23 Å to 129 Å according to cold reduction and annealing temperature, but the species of steels has little effect on the thickness. The growth of film is highly dependent on the grain size of steel, this fact suggests that the oxidation is affected by heterogeneity in the film owing to the existence of grain boundary. Although the preferred orientation of grains slightly affects the film thickness, the growth of (111) preferred orientation makes the oxide film unstable, so it increases the rate of rusting. Cementite also increases the rusting by forming the defects in the oxide film.