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US 3,940,291 AGranted
Insulative coatings for electrical steels
AssigneeArmco Steel Corporation
InventorJames D. Evans
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AssigneeArmco Steel Corporation
InventorJames D. Evans
Estimating the term…
Insulative coatings for electrical steels having improved resistance to deterioration during stress relief anneals or other heat treatments carried on in reducing atmospheres. They may be produced by coating electrical steel with an aqueous solution of phosphoric acid, magnesium oxide and one or more additional mineral acids present in an amount sufficient to adjust the pH level of the solution to from about 0.5 to about 3.0. The mole ratio Mg/PO.sub.4 should be from about 0.6 to about 1.5.
A process of providing an insulative coating on oriented silicon steel sheet-gauge stock for electrical uses comprising the steps of applying to said stock a coating composition comprising an aqueous solution having a Mg/PO.sub.4 mole ratio of from about 0.6 to about 1.5, said solution consisting essentially of an aqueous solution of phosphoric acid, magnesia, and at least one substance chosen from the class consisting of sulfuric acid, nitric acid and magnesium salts thereof, said substance being present in an amount sufficient for said solution to have a pH within a range of from about 0.5 to about 3.0, drying said coated stock and subjecting said coated stock to a heat treatment to form said insulative coating.
A process of providing an insulative coating on oriented silicon sheet-gauge stock for electrical uses comprising the steps of applying to said stock a coating composition comprising an aqueous solution having an Mg/PO.sub.4 mole ratio of from about 0.6 to about 1.5, said solution consisting essentially of an aqueous solution of compounds chosen from the class consisting of mono-basic magnesium phosphate, di-basic magnesium phosphate, tri-basic magnesium phosphate, magnesia, and phosphoric acid, plus at least one substance chosen from the class consisting of sulfuric acid, nitric acid and magnesium salts thereof, said substance being present in an amount sufficient for said solution to have a pH within a range of from about 0.5 to about 3.0, drying said coated stock and subjecting said coated stock to a heat treatment to form said insulative coating.
Dependencies are inferred from the claim text.
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From the US filing history. Foreign family members aren’t listed.
In the form of sheets: with insulating coating
Characterised by the process: for obtaining burned-in conversion coatings
Coating starting from inorganic powder
Magnets
Term ended 24 Feb 1993 (estimated)
USPTO file history isn’t available for this patent, so only indexed dates are shown.
1. Field of the Invention
The invention relates to improved insulative coatings for electrical steels, and more particularly to phosphate based insulative coatings resistant to deterioration during heat treatments in reducing atmospheres.
2. Description of the Prior Art
FIGS. 1 and 2 comprise graphs plotting the change in resistivity during stress relief annealing against the molar ratio Mg/PO.sub.4 in the coating solution.
The coatings of the present invention are particularly suitable for use with oriented silicon steels of the cube-on-edge variety, but may be used with silicon steels of other orientations. When used with cube-on-edge oriented silicon steels, the coatings may be applied either on the bare metal (the mill glass base coating having been removed), or over a mill glass which formed during the high temperature anneal. In the practice of the present invention the coating solution should have a mole ratio Mg/PO.sub.4 in the range of from about 0.6 to about 1.5 and preferably in the range of from about 0.7 to about 1.3. The pH level of the final solution should fall within the range of from about 0.5 to about 3.0 and preferably within the range of from about 1 to about 2.
When in the prior art, as exemplified by the above mentioned U.S. Pat. No. 2,501,846, magnesia was added to an aqueous solution of phosphoric acid in quantities to be wholly dissolved by the acid, a solution resulted having a pH of approximately 3 or less and a Mg++ to PO.sub.4 .sup.-.sup.-.sup.- mole ratio of about .5 or less. This is true because about .455 (or, in round numbers, about .5) is the theoretical maximum mole ratio achievable in such a solution. If, to obtain a higher mole ratio, one were simply to add more magnesia, the mole ratio of the system would go up but the mole ratio of the solution would not. While the pH of the solution would go up, the mole ratio would not due to formation of Mg H PO.sub.4 which is insoluble in water when the pH is above about 4 or 5.
There are two primary ways in which the mole ratio and pH values required by the present invention may be achieved. First, to an aqueous solution of magnesia and phosphoric acid and additional quantity of magnesia may be added together with one or more mineral acids such as sulfuric acid or nitric acid. The mineral acids are preferably volatile at a relatively low temperature.