Formation mechanism of surface oxide layer of grain-oriented silicon steel

The surface oxide layer of grain-oriented electrical steels was investigated by scanning electron microscopy. The formation mechanism and the influence on the glass film of the surface oxide layer were analyzed by the calculation of thermodynamics and kinetics. The surface oxide layer with 2.3 μm in thickness is mainly composed of SiO₂, a small amount of FeO and Fe₂SiO₄. During the formation of surface oxide layer, the restriction factor was the diffusion of O in the oxide layer. At the initial stage of the decarburization annealing, FeO would be formed on the surface layer. SiO₂ and silicate particles rapidly nucleated, grew and formed a granular oxide layer in the subsurface. As the oxidation layer thickens, the nucleation of new particles decreases, and the growth of oxide particles would be dominant. A lamellar oxide layer was formed between the surface oxide layer and the steel matrix, and eventually formed a typical three-layer structure. During the high temperature annealing, MgO mainly reacted with SiO₂ and Fe₂SiO₄ in the surface oxide layer to form Mg₂SiO₄ and Fe₂SiO₄ would respond first, thus forming the glass film with average thickness of 4.87 μm.