Coated ferrous substrates comprising an amorphous magnesia-silica complex

A method of producing a separator and electrical insulating coating on magnetic ferrous material which comprises applying a coating composition comprising MgO, Mg(OH).sub.2 or mixtures thereof, at least one boron compound and at least one amorphous magnesia-silica complex containing from about 0.001 to 2.0 percent by weight of an alkali metal oxide or hydroxide, wherein the mole-ratio of the MgO:SiO.sub.2 is from about 1:25 to 14:1, said magnesia-silica complex being amorphous as indicated by its X-ray powder diffraction pattern and exhibiting the following differential thermal behavior characteristics:

• an endothermic peak at about 250.degree.C., an exothermic peak at about 820.degree.C. and at about 980.degree.C., to preoxidized magnetic ferrous material and annealing said material at an elevated temperature from about 950.degree.-1500.degree.C. for from about 2 to 50 hours in a neutral or reducing atmosphere and wherein the boron compound is present at from about 0.01 to 30 parts by weight and the magnesia-silica complex is present from about 2 to about 200 parts by weight per 100 parts by weight of MgO.