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Internal oxidation often occurs in electrical steels during decarburization anneals of sufficiently long time and high oxygen potential, resulting in deterioration of magnetic properties. Relationships between subscale thickness, anneal time, and effective oxygen potential of the anneal atmosphere were established for a Si-Al-bearing electrical steel. These kinetics are described by a Rhines diffusion equation modified to accommodate the oxygen potential. Electron microprobe and Auger spectroscopy confirmed the significance of solute element diffusion. Permeability at 1.5 T was found to decrease about 50 units for each micron of subscale thickness, while core loss remained essentially constant. SEM observations of the fine structure were used to estimate that the coercivity of the subscale is sufficiently high that this region is barely magnetized. Thus, the induction of the clean interior metal must be higher than the measured average value, resulting in reduced apparent permeability. Good agreement with observation was obtained.

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