GB1572263A - Application of porcelain enamel coatings to ferrous workpieces - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THE APPLICATION OF POR CELAIN ENAMEL COATINGS TO FERROUS WORKPIECES (71) We EAGLE-PICHER INDUSTRIES, INC., of 580 Walnut Street, Cincinatti, Ohio 45201, United States of America, a corporation organised and existing under the laws of the State of Ohio, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:: This invention is in the field of applying porcelain enamel coatings and, more specifically, is directed to a method of pre-treating a ferrous surface for the reception of a porcelain enamel coating, the pre-treating being done by means of a low-melting glass composition in fritted form, which is capable of reacting with the underlying ferrous surface to provide a means for bonding the enamel coating to the underlying surface.

Ever since sheet steel has been provided with porcelain enamel coatings, it has been considered necessary to subject the steel to a varied series of plating and preparation steps in order to improve the adhesion and the appearance of the porcelain enamel. The only exception to this rule has been in connection with very thin hollow ware of a very inexpensive nature where surface preparation was kept at a minimum. In any quality work, however, it was always considered necessary to prepare the surface by chemical and mechanical procedures before the porcelain enamel frit was applied. A typical series of treatment steps for a steel sheet involved first repeatedly dipping the sheet in an alkaline cleaner at temperatures ranging from about 140 to 212"F (60 to 1000C).This was followed by a warm rinse with water at about 1200 to 1400F (49 to 60"C). Following the warm rinse, there was usuaily a cold rinse with water at room temperature. Then came a pickling step with which the washed material was treated with a solution of 5 to 10% by weight sulphuric acid at temperatures ranging from about 150" to 1 600F (66 to 71 0C). Following the pickling step, the material was again rinsed with water containing some small amount of sulphuric acid at room temperature.Following the cold rinsing step, a flash nickel deposition took place using temperatures of 1300 to 1800F (60 to 820C) to deposit 0.02 to 0.6 grams of nickel per square foot. Following the nickel deposition, the steel was again cold rinsed with water and a small amount of sulphuric acid to prevent formation of ferric iron. Finally, the surface was neutralized with agents such as sodium carbonate and borax at temperatures of 1200 to 1300F (49 to 540 C). The equipment required for this extensive heat treatment necessitated substantial capital investment and the time and labour involved provided a substantial portion of the cost of the enameled product.

There have been a few disclosures in the prior art which sought to avoid the extensive and time-consuming pickling and nickel coating steps in enameling steel but to our knowledge they had little or no success commercially. One such disclosure appears in the expired Zimmerman U.S. Patent No. 2,828,218 which described a frit which was applied as a ground coat to a metallic surface at an application weight of 1/8 to 3/8 ounces per square foot dry weight. The frit composition included materials such as flint, feldspar, dehydrated borax, soda ash, sodium nitrate, fluorspar, calcspar and red iron oxide. A conventional porcelain enamel cover coat was applied over the fired ground coat.

An improvement to this type of process is found in U.S. Patent No. 2,786,782 issued to Zimmerman et al. In this patent, Zimmerman et al. suggested adding black iron oxide (magnetic) and zinc oxide to frit compositions in order to improve the adherence. This material was applied in the conventional manner with mill additions as a ground coat, and after firing of the ground coat, a conventional porcelain enamel frit was applied and fired.

Typically, the ground coat of this patent had from 5 to 13% magnetite, 2 to 7% zinc oxide, 35 to 42% silica, 20 to 25% boric oxide, 11 to 22% of one or more alkali metal oxides, 4 to 6% calcium oxide and 3 to 5% alumina.

A third reference which refers to the possibility of eliminating surface preparation such as etching or metal plating is U.S. Patent No. 2,864,721 issued to King et al. This disclosure deals with a ground coat produced by milling a slip directly from raw batch enameling materials, without fritting, and applying the slip to the metallic article at a dry weight of 8 to 17 grams per square foot. The preferred ground coat included from 1 to 15% magnetite, 0 to 9% zinc oxide, 30 to 45% silica, 17 to 28% boric oxide, 11 to 22% of an alkali metal oxide, 3 to 8% calcium oxide, and 0 to 7% alumina.

In recent times, the technique of applying a ground coat and a cover coat of porcelain enamel by means of electrostatic deposition has become more popular. Particular emphasis has been placed upon providing a two coat-one fire system for steels such as cold rolled steel. Examples of such current practice will be found in publications such as "Rationalization in the Enamelling Industry With Electrostatic Dry Procedures", a paper given at the International Congress in Vitreous Enamelling, in October, 1975; an article in "Iron Age" for December 15, 1975 entitled "Enamelers Place Hopes in Dry Frit Spraying", pages 49 to 52; and the article entitled "Powder: A Shot in the Arm for Porcelain Enameling" appearing in Industrial Finishing, January 1976, pages 29 to 31. In each of these articles, however, the two coat-one fire system was applied to regularly pickled workpieces.

The present invention provides an improved method for applying high quality porcelain enamel to a ferrous workpiece, particularly to workpieces composed of steels such as cold rolled steel which have heretofore been enameled only with great difficulty. The method of the present invention makes it unnecessary to emply either a pickling step or a flash nickel coating or surface roughening in preparing the metal surface for enameling.

In accordance with the present invention, a method is provided of applying a porcelain enamel coating to a ferrous workpiece having a clean, unpickled, nickel-free surface, which comprises electrostatically depositing on the workpiece surface a first coating comprising a particulate low-melting fritted glass composition, such composition being capable of reacting with the underlying workpiece surface at a firing temperature so as to etch the surface, electrostatically depositing a second coating comprising a porcelain enamel frit upon the first coating and firing the thus-coated workpiece at a temperature sufficient at least to cause the glass composition to react with and so etch the underlying workpiece surface and thereafter at a temperature sufficient to fuse the porcelain enamel frit into an enamel coating.

In carrying out the method of the invention, the fritted glass composition constitutes a reactive coat, which is first deposited electrostatically on the surface of the workpiece This reactive coat is typically classified as a "soft" glass, that is, it is highly fluid and so has a low viscosity at temperatures below the firing temperature for the cover coat. Typically, the soft glass constituting the first coating used in accordance with the present invention has such a low viscosity even at a temperature of 1,200"F (648"C) or lower. This fluidity at relatively low temperatures, coupled with the chemical nature of the glass, permits the reactive coat to actually etch and react with the underlying ferrous surface before the overlying porcelain enamel cover coat melts.

Preferably, the first coating is applied to a thickness of 0.5 to 2 mils (12 to 51 microns).

The most preferred applied thickness of the reactive or first coating is in the range from 1 to 1.5 mils (25 to 38 microns). To aid in the electrostatic deposition of the first coating, it preferably comprises particles which are encapsulated to provide an electrical resistivity in the range from 1012 to 10 ohm centimetres. An alkoxysilane makes a preferred encapsulation material.

The application of the reactive or first coating is followed by the application of a second coating in the form of porcelain enamel frit or cover coat, also by electrostatic deposition.

Preferably, this second or cover coating also is encapsulated to provide an electrical resistivity for the particles in the aforementioned range. Then, the thus-coated workpiece is fired at a temperature which is at least sufficient to cause the first coating, i.e. the soft glass or active coating, to react with the underlying workpiece surface by etching it and the firing is then continued at a temperature sufficient to cause the procelain enamel frit to fuse into the desired cover coat. Typical firing temperatures range from 1300 to 16000F (704 to 871"C) and preferably from 1400 to 15000F (760" to 816"C).

The preferred fritted reactive glass compositions used in carrying out this invention, which are described in detail below, are disclosed and claimed in our copending Application No. 31480/77 (Specification No. 1572264); such glass compositions have a matrix of the following melted composition by weight SiO2 ... ... 16%- 45So F2 ... ... ... 1.7%- 12.1% Na20 to K20 or both Na20 and K20 ... 10%- 25% B203 ... ... 10%- 26% CaO or BaO or both CaO and BaO ... 2%-20%, the composition also containing 1% to 12% by weight of CuO or 0.5% to 1.6% by weight of NiO or both CuO and NiO in the specified amounts.

The single figure of the accompanying drawing illustrates schematically one form of continuous coating apparatus which may be employed to practise the method of the present invention.

In the Figure reference numeral 10 has been applied generally to a powdered frit source which contains the particulate low-melting fritted glass composition forming the first or reactive coating. A vacuum pump 11 propels the particulate material into a fluidizing chamber 12 wherein the particles are suspended in a stream of air. The fluidized particle stream is then passed by means of a conduit 13 to a manifold 14 disposed in a treating chamber generally indicated at reference numeral 15. The chamber 15 is provided with a pair of opposed slots 16 and 17 through which an overhead conveyor 18 extends. The conveyor 18 is arranged to deliver individual plates 19 to the treating vessel 15, the depths of the slots 16 and 17 being sufficiently long to accomplish this purpose.

Within the treating chamber 15 a plate 19 is subjected to spraying by means of a plurality of electrostatic spray guns 20 and 21 which are fed with a fluidized suspension of particles from the manifold 14 through feed lines 22 and 23. The guns 20 and 21 are also provided with compressed air through suitable inlet lines (not shown). Electrostatic spray guns 20 and 21 are commercially available articles available, for example, from the DeVilbiss Company of Toledo, Ohio. Generally speaking, such guns include a charged electrode past which the fluidized powder particles issue and become charged. An opposite charge is provided on the plate 19, typically by having the conveyor 18 at ground potential. The electrostatic spray guns 20 and 21 can be continuously operated or intermittently operated as desired.To prevent powder spray from leaving the treating chamber 15, there can be provided a pair of air seals 24 and 25 which provide curtains of air across the slots 16 and 17, the curtains terminating in collectors 26 and 27, respectively. Any powder trapped in the curtains is recirculated by means of conduits 28 and 29 to the fluidizing chamber 12 or elsewhere.

Excess powder which appears in the chamber 15 settles by gravity onto an inclined bottom 30 of the treating chamber 15 and is recycled to the fluidizing chamber 12 by means of a conduit 31.

The figure in the drawing illustrates a continuous electrostatic application system for applying the reactive coat to a plate 19. Such a plate need only be cleaned, using for instance conventional alkaline cleaners, rinsed and dried. No pickling or flash nickel coating is required. It will be understood, of course, that a similar system can be used for applying the second or cover coating of porcelain enamel frit electrostatically.

The only pretreatment required for the ferrous workpiece in accordance with the present invention is a simple cleaning, typically with an alkaline-type cleaner, followed by rinsing and drying. Neither acid pickling nor nickel flash coating is required to secure adequate adhesion of the porcelain enamel layer onto the workpiece.

The nature of the reactive coating has a great deal to do with the improved results obtained according to the present invention. The reactive coating is characterized as a very soft glass, i.e., one that has a low viscosity (in comparison with ordinary glasses) at relatively low temperatures of 1200"F or less. The chemical nature of the soft glass also, we believe, renders it very reactive toward the ferrous surface so that it actually chews or attacks the metal, creating a strong anchoring bond to which the fused porcelain enamel cover coat can readily adhere.

The reactive coating must melt and flow out before the porcelain enamel cover coat melts or else specks of the coating will be visible in the cover coat. To determine the proper relationship between the viscosities of the molten reactive coating and the molten cover coat, we used the standard fusion flow test. The test details are given in ASTM Test Method C374-70 "Fusion Flow of Porcelain Enamel Frits (Flow Button Methods)". This publication is also incorporated herein by reference. We used 3.0 gram frit samples and made the tests at 827"C (1520"F). We found that the flow button formed from the reactive coating should be at least twice as long as that for the cover coat, and can be as long as 20 times.

Preferably, the flow from the reactive coating button should be from 3 to 10 times the flow measured for the cover coat button.

The chemistry of the reactive coating can vary widely as long as it has the low viscosity and chemical nature required to react with the underlying metal surface.

The reactive coating may be based upon a borosilicate glass matrix having the following composition in its melted form: Ingredient Broad Range Preferred Range SiO2 16-45 % by weight 19-39 % by weight F2 1.7-12.1" " 3.4-8.6" Na2O or K20 10-25 " " 17-23 or both B203 10-26 " " 17-26 CaOorBaO 2-20 " " 11-17 or both The above oxides and other elements form the base composition which gives a glassy frit to hold the bond promoter and other oxides. The preferred bond promoter oxides are cupric oxide (CuO) and nickel oxide (NiO), either or both of which can be used. Preferably, the cupric oxide is added in amounts of 1 to12% of the melted composition and, most preferably, from 2 to 9%by weight.The nickel oxide is preferably used in amounts of from 0.5 to 1.6%by weight of the melting composition, and most preferably from 0.9 to 1.3% by weight.

Certain other oxides are desirably added but are not essential in the sense that the bond promoting oxides can work without the added oxides. These added oxides are materials such as cobalt oxide (CoO) in amounts of from 0 to 1% and preferably from 0.5 to 0.9% of the melted composition. Another oxide which can be added is manganese oxide (MnO) in amounts of from 0 to 5% by weight of the melted composition, and preferably from 0 to 1 % by weight.

Still other oxides can be added for various purposes, including improvement of bond or adjustment of the flow rate. These miscellaneous oxides are given in the following table: Oxide Broad Range Preferred Range ZnO 0-5 %by weight 2-4 % by weight A1203 0-6 " " 0-2 P2O5 0-5 " " 0-2 TiO2 0-5 " " 0-1 Li2O 0-2.2 " " 0-1.5 While many different oxides can be used in the reactive coating, as noted above, there are several oxides which should not be used because they tend to destroy the bond when added to the reactive coating. These oxides are iron oxide, antimony oxide and molybdenum oxide. The effect of iron oxide is just the contrary to its effect in the compositions described in the Zimmerman U.S. Patent Nos. 2,786,782 and 2,828,218, and Burnham et al. U.S.

Patent No. 2,864,721 who teach the use of iron oxide as an adherence metal oxide in conventional ground coat frits. When used in the compositions of the present invention, iron oxides give no bond and provide pits in the coating.

The reactive coating can be smelted in the conventional manner for the frits. Typically, we use an 1800"F (982"C) rotary smelter with a 30 minute residence time.

Specific reactive coating compositions have been made up from the following raw batch compositions.

Raw Batch Compositions RAW MATERIAL A B C D E F G H I J K L M N % % % % % % % % % % % % % % Silica 20.7 20.7 16.0 14.6 20.9 25.7 20.2 20.4 16.0 15.9 20.0 17.8 20.9 15.6 Feldspar - - - - - - - - 21.1 - - - - Fluorspar 3.1 3.1 3.1 11.0 5.9 10.9 10.9 11.0 7.5 3.1 3.1 3.1 3.1 3.1 Soda Ash 0.5 3.1 2.2 3.9 1.2 6.7 4.4 4.7 13.9 2.3 5.1 - 2.3 Sodium Nitrate 0.8 0.8 0.8 0.9 0.9 0.9 0.9 0.9 1.0 0.8 0.8 0.9 0.8 0.8 Cryolite 6.6 - - - - 6.8 6.8 6.8 - - 6.6 - - Zinc Oxide 2.9 2.9 2.9 3.0 3.1 3.0 3.0 3.0 - 2.9 2.9 2.9 2.9 2.9 Calcium Carbonate 9.5 9.4 9.4 - 2.4 - - - - 9.4 9.5 9.5 9.4 9.5 Barium Carbonate 6.9 6.9 6.9 7.1 - 7.1 7.1 7.1 9.6 6.9 6.9 6.9 6.9 6.9 Potassium Carbonate 4.9 4.8 4.9 5.0 5.1 5.0 5.0 5.0 - 4.9 4.2 4.9 4.9 4.9 Sodium Silico Fluoride 7.7 13.4 13.4 6.8 11.8 0.8 0.8 0.8 0.5 13.4 7.6 13.6 13.4 13.5 Anhydrous Borax 26.0 25.6 25.7 33.4 34.3 22.7 26.8 26.8 24.0 25.7 26.1 22.9 25.8 30.4 Cobalt Oxide 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.5 0.7 0.7 0.7 0.7 0.7 Manganese Dioxide 0.8 0.8 0.8 0.8 0.8 0.8 4.4 0.8 1.0 0.8 0.8 0.8 - 0.8 Lithium Carbonate 3.2 3.2 3.2 3.4 3.4 3.3 3.3 3.4 - 3.2 - 3.2 3.2 3.2 Sodium Phosphate 1.0 - 1.0 - - 1.0 1.0 1.0 - 1.0 1.0 5.9 1.0 1.0 Nickel Oxide 1.1 1.1 1.1 1.2 1.2 1.0 1.0 1.2 1.3 1.1 1.1 1.1 1.1 1.7 Anhydrous Boric Acid - - - - - - - - - - - 2.2 - 1.4 Titanium Dioxide (Rutüc) - - - - - - - 2.7 - - - - - Copper Oxide 3.6 3.5 7.9 8.2 8.3 3.6 3.7 3.7 3.6 7.9 3.6 3.6 3.6 3.6 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 These raw batch compositions resulted in the following melted composition:: Melted Compositions OXIDE A B C D E F G H I J K L M N % % % % % % % % % % % % % % SiO2 26.9 29.4 23.8 19.4 27.4 30.0 23.8 24.0 24.0 38.2 28.8 26.0 25.8 27.6 F2 7.3 7.3 7.3 7.3 7.3 7.3 7.3 7.3 3.4 7.3 7.3 7.3 7.3 7.3 Na2O 16.6 16.6 16.6 16.6 16.6 16.6 16.6 16.6 10.7 16.6 19.6 16.6 16.6 16.6 ZnO 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 - 3.3 3.3 3.3 3.3 3.3 CaO 8.6 8.5 8.5 8.5 4.5 8.5 8.5 8.5 6.3 8.5 8.5 8.5 8.5 8.5 BaO 6.0 6.0 6.0 6.0 2.0 6.0 6.0 6.0 8.8 6.0 6.0 6.0 6.0 6.0 K2O 3.7 3.7 3.7 3.7 3.7 3.7 3.7 3.7 2.7 3.7 3.2 3.7 3.7 3.7 Al2O3 1.8 - - - - 1.8 1.8 1.8 4.4 - 1.8 - 1.8 CoO 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.6 0.7 0.7 0.7 0.7 0.7 MnO 0.6 0.6 0.6 0.6 0.6 0.6 3.6 0.6 0.9 0.6 0.6 0.6 - 0.6 Li2O 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 - 1.5 - 1.5 1.5 1.5 P2O5 0.6 - 0.6 - - 0.6 0.6 0.6 - 0.6 0.6 3.6 0.6 0.6 NiO 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.6 1.3 1.3 1.3 1.3 1.3 B2O3 20.1 20.1 20.1 25.1 25.1 17.1 20.1 10.1 19.7 20.1 20.1 20.1 20.1 20.1 CuO 4.1 4.1 9.1 9.1 9.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 9.1 TiO - - - - - - - - - - - - - 103.1 103.1 103.1 103.1 103.1 103.1 103.1 103.1 103.1 103.1 103.1 103.1 103.1 103.1 Minus O For F2 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 One of the features of the reactive coating of the present invention which distinguishes it from the materials heretofore used in ground coats is the hydrophilic nature of the glass.

Indeed, the glass is sufficiently soluble in water so that it could not be effectively used in conjunction with mill additions in forming an aqueous slip for application to a metal surface as a conventional ground coat.

The reactive coating is applied in fritted form. Since the composition is highly susceptible to water, it is recommended that the frit be produced by roll quenching instead of by quenching under water.

The dry particles of frit making up the reactive coating are applied by electrostatic deposition. This is a technique which has received considerable attention in recent years.

For a background of electrostatic spraying techniques, reference is invited to the article by M. L. Pouilly in the October, 1953 issue of "Finish" magazine, pages 71 to 73. The Pouilly article refers to the use of high voltage packs in electrostatic spraying for porcelain enamel which are capable of producing 80,000 to 140,000 volts, and currents from 1 to 10 milliamperes.

It is also highly desirable that both the reactive coating and the porcelain enamel should be finely divided for deposition by electrostatic coating. Generally, the frit in each case is ground in a ball mill to a retention of 0.2 to 2% on a 200 mesh screen (3 to 12% on a 325 mesh screen).

Since the reactive coat and the cover coat are both applied by electrostatic deposition, it is important that the resistivity of the porcelain enamel powder be controlled for spraying efficiency and also for adherence to the metal. Consequently, it is highly desirable to encapsulate the electrostatically sprayed particles in a synthetic resin in order to increase the resistivity of the particles to the range of 1012 to 10 ohm centimeters. To accomplish this, we preferably make use of the techniques described and claimed in the Nedeljkovic U.S. Patent No. 3,930,062, issued December 30, 1975 and assigned to the same assignee as the present application.

Briefly stated, the Nedeljkovic patent describes a method of pretreating borosilicate glass powders to reduce their caking tendencies and to bring their resistivities up to where they can be successfully sprayed by electrostatic deposition techniques. The anti-caking capabilities of the compositions are improved by mixing the frit in particulate form with an alkoxysilane having the formula: RnSi-(OCH3)4 n where R is a methyl or phenyl group, or both, and n is 1 or 2.

These materials are desirably combined with the frit in an amount of about 0.2 to 0.8% by weight of the mixture. Apparently the alkoxysilane reacts with water present to form a silanole which then presumably reacts with the hydroxyl groups in the glass to improve the electrical resistivity.

The dry adherence of the frit to the substrate can be improved by treating the frit with an adhesion promoter consisting of silazane having the following formula: R3-Si-NH-Si-R3 where R is hydrogen, an alkyl radical, an aryl radical, or a combination of alkyl and aryl radicals.

These materials are preferably added in amounts of 0.05 to 0.5 % by weight.

Additional benefits are derived in terms of improving the deposition rate, when the mixture being sprayed also includes a chlorosilane having the formula: Rn-Si-Clcn where R is an alkyl or aryl radical, or both, and n is 1, 2 or 3.

The reactive coating is preferably applied to the ferrous surface to a thickness in the range of 0.5 to 2 mils 612 to 51 microns). Most preferably, the coating thickness is 1 to 1.5 mils (25 to 38 microns).

After application of the reactive coating, a conventional porcelain enamel frit can be applied dry over the precoated surface, also by means of electrostatic deposition. In order to increase the electrical resistivity of the porcelain enamel frit to increase its adhesion tendencies, this frit is also treated by encapsulation with a synthetic resin such as polyethylene, or with the alkoxy silane compounds referred to in the aforementioned Nedeljkovic patent. In any event, the electrical resistivity of the porcelain enamel frit is modified until it is in the range of 10 t2 to 1016 ohm centimeter.

Following application of the cover coat, the coated article is then fired in the normal way at temperatures ranging from 1300 to 1600"F (704 to 8710C) and preferably from 1400 to 1500"F(760" to 8160C).

EXAMPLE Pieces of cold rolled steel having thicknesses between 18 and 22 gauge (0.079 to 0.127 cm) were coated with a reactive fritted glass composition having the following melted analysis: SiO2 26.8%by weight F2 7.3%" Na2O 16.6%" ZnO 3.3%" CaO 8.5% " " BaO 6.2%" K20 3.7%" A1203 1.8%" CoO 0.7% " " MnO 0.6% " " Li2O 1.5%" P205 0.6%" NiO 1.3%" B203 20.1%" CuO 4.1%" " 103.1 Minus 0 for F2 3.1 100.0 The coating was applied to a thickness of about 1 mil (25 microns) by means of a DeVilbiss electrostatic spray gun used for the application of porcelain enamel coatings.The surface of the cold rolled steel had merely been washed, rinsed and dried prior to application of the coating. Next, a commercial cover coat (Chi-Vit 14350) was milled to a fineness of 0.5 retention on a 200 mesh screen and it was then screened using a 60 mesh screen.

Approximately 0.5% of methyl trimethoxy silane and 0.2% of hexamethyl disilazane were added to the powder. This material was electrostatically sprayed over the precoated part and then fired at 14500F (788"C) for 3-1/2 minutes. The resulting porcelain enamel coating exhibited good adherence to the substrate and was of good quality.

The advantages of the method of the present invention are numerous. For one, it eliminates the pickling step, the nickel plating tanks, associated rinse tanks and milling equipment. Furthermore, it allows the use of non-premium steels in one-fire enameling process.

This, in turn, provides savings in floor space, capital equipment cost, energy, labour and materials, as well as less water pollution. The quality of the finished part is comparable to that obtained by current porcelain enameling processes.

WHAT WE CLAIM IS: 1. A method of applying a porcelain enamel coating to a ferrous workpiece having a clean, unpickled, nickel-free surface, which comprises electrostatically depositing on the workpiece surface a first coating comprising a particulate low-melting fritted glass composition, such composition being capable of reacting with the underlying workpiece surface at a firing temperature so as to etch the surface, electrostatically depositing a second coating comprising a porcelain enamel frit upon the first coating and firing the thus-coated workpiece at a temperature sufficient at least to cause the glass composition to react with and so etch the underlying workpiece surface and thereafter at a temperature sufficient to fuse the porcelain enamel frit into an enamel coating.

2. A method according to claim 1, in which the first coating is applied to a thickness of 0.5 to 2 mils (12 to 51 microns).

3. A method according to claim 2, in which the first coating is applied to a thickness of 1 to 5 mils (25 to 38 microns).

4. A method according to any preceding claim, in which either or both of the first coating and the second coating comprise particles which are encapsulated to provide an electrical resistivity in the range from 1012 to 10 ohm centimetres.

5. A method according to claim 4, in which the particles are encapsulated in an alkoxysilane.

6. A method according to any preceding claim, in which the coated workpiece is fired at a temperature sufficient to cause both the first coating to react with the underlying surface and the frit to fuse into the enamel coating in one firing step.

7. A method according to any preceding claim, in which the workpiece has a surface of cold-rolled steel.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. EXAMPLE Pieces of cold rolled steel having thicknesses between 18 and 22 gauge (0.079 to 0.127 cm) were coated with a reactive fritted glass composition having the following melted analysis: SiO2 26.8%by weight F2 7.3%" Na2O 16.6%" ZnO 3.3%" CaO 8.5% " " BaO 6.2%" K20 3.7%" A1203 1.8%" CoO 0.7% " " MnO 0.6% " " Li2O 1.5%" P205 0.6%" NiO 1.3%" B203 20.1%" CuO 4.1%" " 103.1 Minus 0 for F2 3.1 100.0 The coating was applied to a thickness of about 1 mil (25 microns) by means of a DeVilbiss electrostatic spray gun used for the application of porcelain enamel coatings.The surface of the cold rolled steel had merely been washed, rinsed and dried prior to application of the coating. Next, a commercial cover coat (Chi-Vit 14350) was milled to a fineness of 0.5 retention on a 200 mesh screen and it was then screened using a 60 mesh screen. Approximately 0.5% of methyl trimethoxy silane and 0.2% of hexamethyl disilazane were added to the powder. This material was electrostatically sprayed over the precoated part and then fired at 14500F (788"C) for 3-1/2 minutes. The resulting porcelain enamel coating exhibited good adherence to the substrate and was of good quality. The advantages of the method of the present invention are numerous. For one, it eliminates the pickling step, the nickel plating tanks, associated rinse tanks and milling equipment. Furthermore, it allows the use of non-premium steels in one-fire enameling process. This, in turn, provides savings in floor space, capital equipment cost, energy, labour and materials, as well as less water pollution. The quality of the finished part is comparable to that obtained by current porcelain enameling processes. WHAT WE CLAIM IS:

1. A method of applying a porcelain enamel coating to a ferrous workpiece having a clean, unpickled, nickel-free surface, which comprises electrostatically depositing on the workpiece surface a first coating comprising a particulate low-melting fritted glass composition, such composition being capable of reacting with the underlying workpiece surface at a firing temperature so as to etch the surface, electrostatically depositing a second coating comprising a porcelain enamel frit upon the first coating and firing the thus-coated workpiece at a temperature sufficient at least to cause the glass composition to react with and so etch the underlying workpiece surface and thereafter at a temperature sufficient to fuse the porcelain enamel frit into an enamel coating.

2. A method according to claim 1, in which the first coating is applied to a thickness of 0.5 to 2 mils (12 to 51 microns).

3. A method according to claim 2, in which the first coating is applied to a thickness of 1 to 5 mils (25 to 38 microns).

4. A method according to any preceding claim, in which either or both of the first coating and the second coating comprise particles which are encapsulated to provide an electrical resistivity in the range from 1012 to 10 ohm centimetres.

5. A method according to claim 4, in which the particles are encapsulated in an alkoxysilane.

6. A method according to any preceding claim, in which the coated workpiece is fired at a temperature sufficient to cause both the first coating to react with the underlying surface and the frit to fuse into the enamel coating in one firing step.

7. A method according to any preceding claim, in which the workpiece has a surface of cold-rolled steel.

8. A method according to any preceding claim, in which the first coating has the follow

ing melted composition by weight: SiO... ... ... 16%-45% F2 ... ... ... 1.7% -12.1% Na2O to K2O or both Na2O and K2O ... 10%-25% B2O3... ... ... 10%-26% CaO or BaO or both CaO and BaO ... 2%-20% the composition also containing 1% to 12% by weight of CuO or 0.5% to 1.6% by weight NiO or both CuO and NiO in the specified amounts.

9. A method according to claim 8, in which the melted composition of the first coating comprises, by weight: SiO2... ... ... 19%-39% F2 ... ... ... 3.4%-8.6% Na2O or K2O or both Na2O and K2O .. 17%-23% B2O3... ... ... 17%-26% CaO or BaO orbothCaOandBaO ... 11%-17%

10. A method according to claim 8 or 9, in which the composition contains, by weight, 2% to 9% of CuO or 0.9% to 1.3% of NiO or both CuO and NiO in the specified amounts.

11. A method according to claim 8, 9 or 10, in which the composition also includes, by weight, of the melted composition either or both of: CoO ... ... 0 to 1% NmO ... ... 0 to 5%

12. A method according to claim 11, in which 0.5% to 0.9% of CoO and/or 0 to 1% of MnO, by weight of the melted composition, are present.

13. A method according to any of claims 8 to 12, in which the composition also includes, by weight, one or more of the following oxides: ZnO .. . 0-5% Al2O3 .. . 0-6% P2O5 .. ... 0-5% TiO2 .. ... 0-5% Li2O ... ... 0-2.2%

14. A method according to any preceding claim, in which the first coating fuses into a low viscosity glass when subjected to a temperature not in excess of 1200 F (649 C).

15. A method according to any preceding claim, in which the first coating has a fusion flow (ASTM C374-70) at least twice as high as that of the second coating.

16. A method of applying a porcelain enamel coating to a ferrous workpiece according to claim 1, substantially as herein described with reference to the accompanying drawings.

17. An article having a porcelain enamel coating, when made by a method according to any preceding claim.