Forming oxide films on electrical steel

VARIOUS METHODS have been devised for producing an insulating film on the surface of electrical steel laminations for the purpose of minimizing interlaminar eddy currents when the core comprising the laminations carries an alternating magnetic flux. The formation of an insulating oxide film on the surface of the laminations appears to be advantageous from a production standpoint because it does not necessitate the handling of individual laminations, which is required when varnishes or the like are applied, and because the oxidation process can be combined with a strain annealing operation which is undertaken when the optimum magnetic properties of the magnetic material are desired. If the oxide is formed in an air atmosphere at low temperatures, as where it is desired to insulate the laminations only and not to strain-anneal them, then the characteristics of the product are undesirably variable in that sometimes red ferric oxide is formed while at other times blue-black magnetic oxide is formed. Ferric oxide, common red rust, is undesirable because its thickness causes poor space factor in the core and because it is not tightly adherent. When properly produced, the magnetic oxide (Fe3O4) is tightly adherent, thin, and provides satisfactory interlaminar resistance.