US2484242A - Coating ferrous metal sheets with an insulating film - Google Patents

Description

Oct. 11, 1949.

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J. NAGEL ET AL COATING FERROUS METAL SHEETS WITH AN INSULATING FILM Filed April 3, 1946 INVENTORS fiv/z j; A/aya/ and an l/marl Patented Oct. 11, 1949 COATING FERROUS METAL SHEETS WITH AN INSULATING FILM Fritz J. Nagel, Homewood, and Clifford C. Horstman, Sharpsville, Pa., assignors to westinge house Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application April 3, 1946, Serial No. 659,415

4 Claims.

This invention relates to insulation, and in particular to tenaciously adherent films possessing high ohmic resistance for magnetic material.

In preparing magnetic materials, such, for example, as silicon iron, for the building of magnetic cores therefrom, it is necessary that the laminations be provided with electrical insulation between one another in order to minimize eddy currents and to achieve low losses. The insulation should preferably be an extremely thin film to provide for a high space factor. In many cases, it is further necessary that the insulating material applied to laminations should withstand the elevated temperatures which are encountered in strain annealing cores after assembly in order to improve their efficiency. Temperatures encountered in strain annealing occasionally reach 1200 C. and are rarely less than 600 C. In addition, it is highly desirable that the insulation for the magnetic sheet material be capable of withstanding bending, scraping and other mechanical abuse or chemical treatment which may be encountered in forming the magnetic material to shape and assembling cores therefrom. Magnetic material is usually cut or punched after the insulating material is applied thereto, and thereafter the cut or punched magnetic material may be bent or wound in order to produce a predetermined core structure. Additionally, the assembled cores may be subjected to machining or grinding, and, in some cases, etching with acids in order to remove burrs and the like.

It has been proposed heretofore to treat the surfaces of ferrous magnetic laminations with phosphoric acid in order to produce an insulating iron phosphate film or coating on the surfaces of the laminations. Extensive experience with phosphoric acid treated ferrous magnetic material of this kind has shown that the insulation resistance of the films is relatively low, often of the order of 1 to 2 ohms per square centimeter when tested by simple contact pressure alone. When the test contacts are applied with a force of 50 pounds per square inch upon the phosphoric acid treated material and twisted, the insulation resistance frequently drops to values of the order of 0.15 to 0.5 ohm per square centimeter. Often the phosphate film is disrupted during the twisting and the resistance is zero. For numerous applications, the latter resistance values are too low to be satisfactory. The lack of adherence and abrasion resistance as evidenced by decrease of resistance upon twisting of the contacts as well as visual observation of the breaking up of the film indicates that the prior iron phosphate insulation is not satisfactory for many uses. Considerable difiiculty has been encountered in actual experience with this type of insulation.

It has been suggested that the addition of powdered inert refractory solids, such, for example, as magnesia, alumina, talc, or silica to the phosphoric acid might improve the insulating film on ferrous laminations treated therewith. However, tests of the modified insulating film so produced showed that the increase in resistance is relatively meager. Tests indicate that resistance values as low as from 1 to 5 ohms per square centimeter are obtained with untwisted contacts, and the resistance values drop to about the same values as the plain iron phosphate films, when the contacts under pressure as above described are twisted. The film therefore still has low strength and adherence.

The object of this invention is to provide a tenaciously adherent insulating film possessing high ohmic resistance on ferrous magnetic material.

A further object of the invention is to provide for producing an adherent film having high ohmic resistance on ferrous magnetic material by treating the material with a composition comprising phosphoric acid and an organic ester of silicic acid.

A further object of the invention is to provide a coating composition of phosphoric acid and a silicon compound capable of decomposing to produce microscropically fine silica for reaction with the phosphoric acid products when applied to ferrous material.

Other objects of the invention will, in part, be

obvious and will, in part, appear hereinafter For a better understanding of the nature and objects of the invention, reference should be had to the followin detailed description and drawing, in which:

Figure 1 is a schematic view of a process for producing insulated magnetic strips and building a magnetic core in accordance with this invention;

Fig. 2 is a greatly enlarged fragmentary crosssectional view of the insulated lamination of this invention applied thereto; and

Fig. 3 is a view in elevation, partly broken, of an insulation tester.

According to the present invention, insulating films of extraordinary ohmic insulation resistance which are tenaciously adherent and resistant to abrasion are produced on ferrous materials by applying to and chemically reacting therewith a solution having as its essential ingredients phosphoric acid and a silicon compound capable on decomposition of producing microscopically fine silica. No insulating material known approaches either the resistance or adherence of the film so produced.

The insulating film is produced by applying to ferrous material, such, for example, as silicon iron laminations, a coating of a solution composed of a volatile solvent carrying substantial amounts of phosphoric acid and a silicon compound capable of liberating microscopically fine silica, such, for example, as tetraethyl silicate, and heat-treating the ferrous material and applied coating at a temperature of from 300 C. to 950 C. for a short period of time. A complex chemical reaction apparently takes place during the heat treatment. The phosphoric acid is believed to react with the ferrous metal to produce iron phosphates. Simultaneously, the tetraethyl silicate decomposes to produce microscopically fine silica. There is substantial evidence indicating that a chemical reaction between the iron phosphate (and perhaps the phosphoric acid) and the finely divided silica occurs. Either a true chemical action takes place or else secondary valence forces bonding the finely divided silica and iron phosphate are established, since unusual adherence and strength and other characteristics in the resulting film are secured not otherwise attainable. While the foregoing facts appear to support our belief that a chemical reaction occurs, it is to be understood that this is only explanatory and we are not to be held to this theory.

In preparing the coating composition for application to the ferrous material, silicon compounds capable of producing on decomposition finely divided silica are employed. Organic esters of silicic acid are particularly desirable for this purpose. Typical esters are tetraethyl silicate, tetrabutyl silicate, methyl silicate, gylceryl silicate, diglycol silicate and ethylene glycol silicate, and condensed silicates such as hexa-ethyl sili cate, and mixtures of any of these. In the subsequent examples and discussion, tetraethyl silicate will be employed as exemplary of other silicon compounds as well.

Sufllcient water should be present with the tetraethyl silicate or other hydrolyzable silicates to produce a decomposition of the ester to silicic acid which may then be readily converted to silica, in the form of silica gel, or the like.

Ortho-phosphoric acid has been found the most satisfactory substance for use in the composition. Any strength phosphoric acid above about may be used. In practice 50% to 85% phosphoric acid is employed. In some cases, it has been found possible to substitute chromic acid up to an amount where it does not exceed 35% of the weight of the phosphoric acid. Furthermore, slight amounts of alkaline dihydrogen phosphates, such, for example, as monosodium dihydrogen phosphate, may be substituted for a small portion of the phosphoric acid. Considerations of cost and ease of preparation will render the ortho-phosphoric acid alone preferable in most instances.

In preparing the composition, the phosphoric 1 acid and tetraethyl silicate are preferably made into a solution.. For this purpose it has been found that either water or a simple aliphatic alcohol may be employed as a solvent with good results. However, the best results were obtained when a mixture of water and the alcohol were used as the solvent for the phosphoric acid and the silicon compound. Examples of simple aliphatic alcohols suitable for the practice of this invention are ethanol, propanol, isopropanol,

. ble that the simple alcohol be present in an amount equal to at least about half the weight of the water. It will be appreciated that alcoholic solutions having substantial amounts of water, such, for example, as 70% ethanol or 60% isopropanol may be used alone without the addition of water. Denatured ethanol is satisfactory for use.

In preparing a solution, good results have been obtained by first dissolving the tetraethyl silicate in the simple alcohol in the following manner: 50 parts by weight of tetraethyl silicate and 30 parts by weight of 95% ethanol are mixed and permitted to stand about 12 hours and then 10 parts of water are added. The tetraethyl silicate solution so produced dissolves quite readily in the remainder of the solvent along with the phosphoric acid. Hereafter this solution will be designated in the examples as a tetraethyl silicate solution.

Reference should be had to the following examples, in which all parts are by weight, for the preparation and use of typical compositions in accordance with the invention.

The water and ethanol were admixed and the phosphoric acid was then poured into the solvent mixture with stirring and finally the tetraethyl silicate solution was added. The solution was applied to silicon iron laminations as a thin coating, and when heat treated at a temperature of 450 C. for two minutes in a muille type furnace a hard, durable film was produced. When tested with the tester shown in Fig. 3 of the drawing, the insulating resistance was infinity. The insulation resistance did not decrease appreciably even though the contacts were twisted through an angle of while under a pressure of 50 pounds per square inch.

To show that a chemical reaction has taken place between the finely divided silica and the iron phosphate in the film, a test was made on the same silicon iron by applying a phosphoric acid solution similar to Example I, but having silica flour equal to the silica present in the ethyl silicate; the film so produced has less than 2 ohms median resistance with untwisted contacts. On twisting the film was disrupted and the resistance fell to about zero.

Referring to Fig. 1 of the drawing, there is illustrated a schematic diagram of a suitable process for applying to ferrous magnetic material the composition of Example I, and building a wound core from the resulting insulated material. Strip silicon iron Hi from a supply coil 8 is coated in a tank 12 containing the tetraethyl silicate-phosphoric acid solution H such as that of Example I. It will be understood that the composition H may be applied in other ways as by spraying.

flowing and the like. The strip I initially passes over a guide roll IO, thence into the solution where it passes under a submerged roller l8 and then passes over two rubber squeeze rolls and 22 maintained under a selected pressure for predeterminin the amount of the solution l4 to be rfatlained on the surfaces of the magnetic mater The strip l0 coated with the solution is then heat-treated in the furnace 24 at a temperature of from about 300 C. to 950 C. for a brief period of time. It is preferred to maintain a relatively low oxidizing or inert atmosphere in the furnace in order to prevent undue oxidation and the formation of rust. In an oxidizing atmosphere, such as air, heating should not be conducted for longer than five minutes at a temperature of 550 C. or more than two minutes at 600 C. Good results have been obtained in an open muille furnace by heating to a temperature of 600 C. for one minute. In a furnace provided with a relatively nonoxidizin or an inert atmosphere, for example nitrogen gas, the coatings may be heattreated at temperatures of 950 C. for 30 seconds as the upper limit and ten minutes or longer at temperatures of 300 C. with excellent results.

The heat treatment applied to the strip l0 coated with the solution l4 should be carried out in the light of the subsequent processing of the insulated strip. If the insulated strip is to be reannealed, for example, as is required to eliminate strains in wound cores produced therefrom, the heat treatment in furnace 24 is preferably conducted at temperatures of the same order as the required subsequent reannealing temperatures. In actual practice the reannealing temperatures for wound cores range from about 650 C. to 1000 C. and accordingly the heat-treating of the strip carrying the phosphoric acid and ester of silicic acid solutions should be conducted at temperatures approximating the reannealing temperature of cores prepared therefrom.

During the heat treatment in the furnace 24, the water and alcohol solvent of the solution i4 evaporates, the phosphoric acid reacts with the ferrous metal while the tetraethyl silicate decomposes simultaneously, whereby microscopically fine silica is precipitated during the reaction involving the phosphoric acid. The smoothest films are secured when the heat treatment is not too rapid so that vapors are not evolved at too great a rate. A refractory, homogeneous film characterized by good adherence and high electrical insulation is quickly produced.

As an example of the high adherence and mechanical strength of the insulating film, the subsequent building of a wound core from the insulated strip I0 is illustrated. Building of wound cores is believed to bethe most severe test of the properties of integral insulation on laminations. The insulated-strip l0 passes through the guide rolls 28 to a winding machine where it is wound on a rectangular mandrel 28 into a continuous rectangular or wound core 30. A pressure roll 22 is applied to assure the formation of a tight core. It will be found that the strip may be bent about the mandrel without the insulating film being loosened, even though the radii at the corners of the mandrel 28 may be 4; inch or less. The film does not powder or otherwise separate from the strip during the winding operation.

The wound core 30 is then subjected to a strain anneal in an annealing oven 34 at temperatures of from 700 C. to 950 C. without the insulating film on the laminations softening or fusing and sticking the laminations to one another. This characteristic is quite important since fused insulation will give rise to strains when the core is cooled to room temperature. Insulating films produced by applying only phosphoric acid or phosphoric acid and an inert filler such as talc often results in adhesions being formed between laminations during the strain annealing, and it is necessary to apply physical force to separate the laminations. Application of the physical force is not only time-consuming and uneconomical, but may result in some damage to the core. The atmosphere in the strain annealing oven is preferably one containing hydrogen.

In order to fill solidly the spaces between the turns, the core 30 after strain annealing is then impregnated in the tank 36 containing an insulating resin 38. Thereafter the resin impregnated core 30 is baked in the oven 40 to harden and polymerize the resin. While the core may be used in this condition, for many applications it must be cut, the resin bonded core 30 is then mounted in a jig 42 and severed into predetermined sections by a cutter 44. It will be apparent that the resin bond strength cannot be better than the film adherence. It has been found that abrasive bonded wheels, metal cutting saws, milling cutters, and other machine tools may be employed in machining the core 30 to any desired shape and size without disruption of the insulating film of this invention. Thereafter the cut core 30 is mounted on a jig 46 for grinding the cut faces thereof by the grinding wheel 48 to render them smooth and plane. The ground core may be subjected to acid etching in order to remove any burrs that may be present on the machined surfaces due to the grinding and machining operations.

The building of wound core sections is believed to be the severest practical test of the adherence, heat and mechanical resistance, and durability of an insulating film on magnetic laminations known to the art today. Thus the insulating film on the laminations must withstand extremely elevated temperatures and mechanical and chemical reagents without losing its adherence to the ferrous metal or deteriorating unduly in insulation resistance. In wound cores using oriented silicon iron, a minimum of one ohm per square centimeter is the least acceptable resistance, and preferably the resistance should be higher: 5 ohms per square centimeter and better. The insulated laminations produced as here disclosed have surpassed this requirement, Tests of completed cores show that losses are extraordinarily low for laminations carrying the film produced from the phosphoric acid and silicon compound of this invention. Losses of considerably less than .1 watt per pound of 13 mil silicon iron at 15,000 gauss with 60 cycles alternating current are regularly obtained. Furthermore, high space factors of 96% to 99% are secured. v

The insulated strip l0 produced bythe heattreating furnace 24 obviously can be made into any predetermined type of lamination by punching or cutting,vor otherwise machining to shape.

The insulating film will withstand punching operation such as encountered in producing L- shaped or other shaped laminations for transformer cores and the like, ormotor and generator rotor and stator laminationsin which sheets are punched into complex shapes. The insulating ed for vertical movement by cation of a predetermined tracted for the passing of has from a value of 0.1 ohm By applying the tester to a sheet ofmag- 7 film will withstand the application and heat treatment required for employing the usual baking varnishes and other insulating materials employed in building motors, generators, transformers and other electricalapparatus.

A particularly desirable application of this invention is for preferentially oriented silicon iron containing from 2% to 5% silicon-usually about 3.25% silicon. The preferentially oriented 'silicon iron is extremely sensitive to strains and the insulating films here disclosed cooperate successfully therewith to produce substantially strain free insulated laminations. Not only strips but wires, bars, straps and the like can be treated.

'Other ferrous material, containing any of the usual alloying elements, aluminum, nickel, cobalt and the like, capable of reacting with phosphoric acid may be treated with the composition of this v invention to produce the highly desirable electrical insulation disclosed. Sheet silicon iron having up to 7% silicon, from '2 mils to mils in thickess is easily treated with the composition. The ferrous material may be thereafter formed to shape by winding, bending or otherwise mechanically deforming without anyserious impairment of the insulating film.

. Reference should be had to Fig. 2 of the drawing showing 'a greatly enlarged cross-sectional view of the sheet magnetic material It with the adherent integral insulating film 50 on the surfaces thereof. The film i0 is composed of iron phosphate chemically reacted with the microscopically fine silica produced by the simultaneous decomposition of the tetraethyl silicate as described hereinbefore. In most cases, the thickness of the film 50 is approximately 2% of the thickness of the sheet l0. However, the thickness of the film 50 may be varied considerably from this ratio to suit individual requirements. On highly satisfactory insulated 13 "mil sheet silicon iron, the film thickness has been measured as approximately 0.25 mil. However, in many cases the thickness has been varied 0.5 mil for 13-mil thick sheets and in all cases, the insulation resistance and adherence of the film has been at a high level.

Referring to Fig. 3 of the drawing, there is 11- iustrated an insulation tester 60, known as the v c-clamp spot tester, which has been found to be extremely useful in evaluating'the properties magnetic material. The

of insulating films on a U-shaped frame 02 protester n is composed of vided witha lower stationary contact member 64 movable contact 86 guidand an upper relatively the shaft 80 passing 8 netic steel, 9, reading is obtained on the measuring instrument 84 when the contacts simply rest under the pressure of spring ll against the insulating'film, if'any thereon. By twisting the frame 02 with respect to the sheet being tested, the insulating film between the contacts 64 and 60 is subjected to relatively severe abrasion, and the resistance recorded on the instrument .4 will generally drop considerably. No refractory material known on laminations has been able to withstand one twist of approximately-90 without a substantial drop in insulation resistance. Brittle or nonadherent films will be disrupted completely and the resistance will be zero. The insulatingmaterial of this invention will almost invariably withstand three or four successive twists without the resistance dropping below the range of 10 to 100 ohms per square centimeter, and usually the insulationresistanee remains at approximately infinity. Therefore it will be obvious that the nature of the insulation is entirely different, both in properties and performance, over any magnetic sheet insulating material known heretofore.

We have prepared numerous compositions in accordance with this invention, and have been able to produce therewith on magnetic material insulating films having the unexpected properties set forth. ,The applied compositions have been varied within the range of 100 parts of solvent, 5 to parts by weight of phosphoric acid, and from 1 to parts by weight of tetraethyl silicate or its equivalent. Since the organic silicates vary a silicon dioxide equal to from 1% to 100% of the 1 weight of the phosphoric way, the compositions comprise 30 partsbyweight' from 0.1 to

through the upper part of the frame 02. A spring 1| encircling the shaft 6| provides for the applipressure of 50 pounds per'square inch at the face of the contact. A manually operated lever 12 connected to the shaft 08 enables the upper contact 06' to be rea sheet between the contacts 84 and 66. An insulated conductor."

connects the movable contact 68 in circuit with" an insulated bushing 16 while a second insulated conductor 18 connects the stationary contact ll to a second insulated bushing 80. Two conductors 02 connect the bushings 16 and 80 to a meter device 84 containing a source of low voltage current for measuring the resistance between the contacts 64 and 86. -The measuring device 84 is capable of determining'ohmic resistance of coatper square centimeter up to infinity on .a scale.

acid. Stated another of phosphoric acid, an organic ester of silicic acid in an amount sufiicient to produce on decomposition silicon dioxide-in an amount equaltofrom 1 to 100% of the weight of the phosphoric acid, and from 50 to 400 parts by weight of a solvent for the phosphoric acid and the silicic 'acid ester, the solvent containing sufiicient water to hydrolyze the silicic acidester. Outstanding results are obtained by'employing suflicient organic silicate ester to provide silica equal to from 4% to 20% of the weight of the phosphoric acid.

In applying the compositions to silicon iron, ood results have been position has been applied in the amount of from 1 to 10 gallons per ton of 13-mil thick sheets of silicon iron. tained with the application of three ton of the composition of Example I.

The following examples list various satisfactory formulations of the composition with which good results were obtained when applied to ferrous material.

Example I! a Parts 95% ethanol 80 Water 120 Phosphoric acid 60 Tetraethyl orthosilicate solution --e 20 trample III 95% ethanol 50 Distilled water H3P04 i. 80

'75v Ethyl silicate solution 30 obtained when the com- Excellent results have been ob- Example 17 Parts 95% ethanol 50 Distilled water 50 HaPO4 (85%) 60 (210: Ethyl silicate solution so Example V Parts 95% ethanol 100 HsPO4 80 Ethyl silicate solution 70 Example V1 Parts 95% ethanol 50 Water 50 11:104 l Ethyl silicate solution. 30 C313: 1 NaHzPO4 10 Example VII Parts Isopropyl alcohol (99%) 50 Water 50 Ethyl silicate solution 30 Phosphoric acid '10 Example VIII Parts 95% ethanol 100 Fhosphoric acid, 85% so Ethylene glycol silicate 3o Example 1X Parts 95% ethanol 1Q!) Phosphoric acid, 85% l0 Ethylene glycol silicate so 'Za'a'mpZe X Parts Esopropanol, 99% iii!) Phosphoric acid, 85% 80 Ethylene glycol silicate 20 Example X11 Parts t"5% etha 50 Water 50 Phosphoric acid, 85% 7c Ethylene glycol silicate solution 30 Example XII Parts 95% ethanol 100 Phosphoric acid, 85% 70 Ethylene glycol silicate 30 ClzOa 5 Example XIII Parts Distilled water 100 Phosphoric acid, 85% '10 Ethylene glycol silicate 30 In the above examples, it has been found that ethylene glycol silicate produces somewhat smoother films than the tetraethyl silicate. In all cases the insulating films had high resistance and adhered well to the magnetic sheets.

We have found that the inclusion of solid fillers in the compositions of this invention is not particularly beneficial. For example, finely pulverized magnesium oxide, aluminum oxide or silica flour appeared to reduce the insulation resistance and adherence of the films. While small 10 rated if desired for certain purposes, not advised.

While it has been proposed to employ an organic silicate. such as tetraethyl silicate, alone for producing a siliceous coating on surfaces of magnetic laminations, tests of such materials with the tester of Fig. 3 of the drawing indicate that the ohmic resistance and the adherence are relatively poor. The decomposition of ethyl silicate, alone, or with finely powdered aluminum oxide filler for instance, produces a very thin film that lacks both ohmic resistance and adhesion. The ohmic resistance at best is much less than one ohm per square centimeter and the twisting of the c-clamp tester disrupts the film and the resistance drops to zero. Therefore, it is totally unexpected that the combination of their use is phosphoric acid with an organic esterof $111010 acid will produce an insulated coating having such extraordinary adherence and ohmic resistance as disclosed herein. Numerous other advantages obtained by the combinations set forth are obtained that are not available with prior art insulating coatings from magnetic material.

Since certain changes in carrying out the above process and certain modifications in the article which embody the invention may be made without departing from its scope, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. In the process of treating ferrous metal sheets to provide on the surfaces thereof a tenaciously adherent film possessing high ohmic resistance, the steps comprising applying to the sheets a coating composition composed of 30 parts by weight of phosphoric acid, an organic ester of silicic acid in an amount sufilcient on decomposition to produce silicon dioxide equal to from 1% to of the weight of the phosphoric acid, and about from 50 to 400 parts of a solvent for the phosphoric acid and organic ester of silicic acid, the solvent composed of water and a simple compatible aliphatic alcohol, the alcohol being at least half the weight of the water, the water being present in an amount suflicient to hydrolyze the organic ester, the composition applied in an amount of from 1 to 10 gallons per ton of 13 mil thick sheet, and heat-treating the ferrous metal sheets and applied composition at a temperature of from about 300 C. to 950 C. to react the phosphoric acid with the ferrous metal, to liberate microscopically fine silica by decomposition of the organic ester of silicic acid, and to volatilize the solvent, the reaction product "of the phosphoric acid and the ferrous metal and the fine silica producing the adherent insulating coating.

2. In the process of treating ferrous metal sheets to provide on the surfaces thereof a tenaciously adherent film possessing high ohmic resistance, the steps comprising applying to the surfaces of the sheets a coating composition composed of from 5 to 60 parts by weight of phosphoric acid, from about 1 to 65 parts by weight of the tetraethyl silicate, and 100 parts by weight of a solvent, the solvent composed of water and a compatible simple aliphatic alcohol, the alcohol being at least half the weight of the water, the water being present in an amount sufflcient to hydrolyze the tetraethyl silicate, the composition being applied in an amount of from 1 to 10 amounts of solid refractories may be inc rpo gallons per ton of 13 mil thick sheets, and heattreating the ferrous metal sheets and applied composition at a temperature of from 300 C. to 950 c.to react the phosphoric acid with the ferrous metal, to liberate microscopically fine silica by decomposition of the tetraethyl silicate, and to volatilize the solvent, the reaction product of the phosphoric acid and the ferrous metal and the fine silica producing the adherent insulating coating. T

3. In the process of treating ferrous metal sheets to provide on the surfaces thereof a tenaciously adherent fllm possessing high ohmic resistance, the steps comprising applying to the surfaces of the sheets a coating composition composed of from 5 to 60 parts by weight of phosphoric acid, from about 1 to 65 parts by weight of the ethylene glycol silicate, and 100 parts by weight of a solvent, the solvent composed of water and a, compatible simple aliphatic alcohol, the alcohol being at least half the weight of the water, the water being present in an amount sumcient to hydrolyze the ethylene glycol silicate, the composition being applied in an amount of from 1 to gallons per ton of 13 mil thick sheets,'and heat-treating the ferrous metal sheets and applied composition at a temperature of from 300 C. to 950 C. to react the phosphoric acid with the ferrous metal, to liberate microscopically fine silica by decomposition of the ethylene glycol sillcate, and to volatilize the solvent, the reaction product of the phosphoric acid and the ferrous metal and the fine silica producing the adherent insulating coating.

4. In the process of providing a tenaciously adherent film possessing high ohmic resistance on a ferrous metal sheet, the steps comprising applying to the surfaces of the sheet a coating composition composed of Etc 60 parts by weight of a organic ester, but not mixture of phosphoric acid and chromic acid, the chromic acid not exceeding 35% 01 the weight of the phosphoric acid, from 1 to parts by weight of an organic ester of silicic acid, parts by weight of a solvent composed of water and a compatible simple aliphatic alcohol, the water being present in an amount sufliclent to hydrolyze the exceeding half the weight of the solvent, and heat-treating the sheet and the applied coating composition to provide for simultaneously reacting the phosphoric and chromic acids with the ferrous metal and liberating microscopically fine silica from the organic ester of silicic acid.

FRITZ J. NAGEL.

CLIFFORD C. HORSTMAN.

REFERENCES CITED The following references are of record in the file of this .patent:

UNITED STATES PATENTS Number Name Date 1,750,270 Jones Mar. 11, 1930 1,889,654 Griessbach et a]. Nov. 29, 1932 2,144,425 Cook Jan. 17, 1939. 2,161,319 Schamberger June 6, 1939 2,413,949 Broverman Jan. 7, 1947 FOREIGN PATENTS Number Country Date 679,083 France Jan. 5, 1930 570,990 Germany Feb. 22, 1933 OTHER REFERENCES King: "Paint, Varnish, Lacquer, Enamel and Colour Manufacture," May 1931, pages 52 to 55.

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