US 2819207 A
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United States Patent PROCESS FOR ENAMELING STEEL No Drawing. Application June 22, 1953 Serial No. 363,390
4 Claims. (Cl. 204-38) The present invention relates generally to the protective coating art and is more particularly concerned with a novel method of producing tenaciously adhering, uniform and continuous vitreous enamel coatings on ferrous metal surfaces.
In accordance with the prevailing commercial practice prior to this invention and prior to the invention disclosed and claimed in co-pending application Serial No. 247,466, filed September 20, 1951, in the names of Irving P. Whitehouse, Fred E. Kendall and Paul Golar, and now Patent No. 2,748,066, porcelain enameling of hot or cold rolled ferrous metal articles, such as steel sheets and strips, involved as an essential operation the provision of bonding or ground coats of enamel. Ground coat enamels contain one or more adherence-promoting materials selected from the group comprising the oxides of antimony, molybdenum, cobalt, nickel and manganese, and they are typically but not invariably of dark color and are completely opaque. Thus, if an enamel finish of light or different color is required, a second enamel coat, known in the art as a finish coat, must be applied. In cases where a white finish is to be produced, two such finish coats are usually required. In any event, however, the additional enamel coats are produced separately and at substantial expense. Consequently, there have been numerous determined efforts made by others to bond finish coat enamels, containing no appreciable quantity of dark-colored, adherence-promoting oxide, directly to ferrous metal surfaces. To the best of my knowledge, however, these efforts have all been commercially unsuccessful in that the enamel coats produced were unsatisfactory because of inconsistent bonding to the metal surfaces and because the surfaces of these coats were often blistered or otherwise defective.
By virtue of my present invention, it is now possible to produce commercially a tenaciously adhering, uniform and continuous finish enamel coating on a ferrous metal surface produced by either hot or cold rolling, without first providing a ground coat of any type on said surfaces. Furthermore, this invention enables the employement of enamels which are substantially free from adherencepromoting oxides in manufacture of articles bearing only a single enamel coat.
My invention is predicated upon my discovery that by preparing the surface of a hot or cold rolled ferrous metal article in a certain critical manner, consistently good bonding of enamel to the article may be accomplished Without the provision of a preliminary ground coat of enamel and without employment of any adhesivepromoting oxide as a constituent of the enamel slip. Furthermore, in accordance with this invention, it is not necessary to etch or skin or otherwise remove a portion of the surface of the metal article in the course of preparing the article for application of the cover coat of enamel thereto. Assuming that the article to be enameled is clean and free of substantially all loose particles and foreign matter including oil, grease and the like, the im- Patented Jan. 7,
portant and critical thing about the process of this invention is the provision of a finely-divided coat of cobalt or nickel on the cleaned surface by means of an electrodeposition procedure. This nickel or cobalt is, in effect, burned on and the thus deposited nickel or cobalt is in finely-divided or spongy form providing a relatively large surface area compared to conventional dense depositions of these metals when they are plated under less severe circumstances than set out herebelow.
The application of the enamel coat follows the formation of this burned on nickel or cobalt coating on the surface of the article which has been previously cleaned of foreign matter. Preferably, this non-ferrous metal coating is applied to the article immediately or shortly after the cleaning operation has been completed. Also, the enamel coating is preferably applied to the nickel or cobalt coat immediately or shortly after the formation of that coat.
It has been thought by many investigators that a rough steel surface was necessary to obtain enamel cover coat adherence. These ideas were not without basis since a sandblasted or highly etched surface plus a coating of nickel gave good enamel adherence to steel. This invention will show that, if nickel or cobalt is deposited in a finely-divided form, good enamel adherence will be consistently obtained with cover coat enamels on steel even if the steel surface was previously smooth or polished. It is necessary that the burned nickel or cobalt possess a very large surface area per weight of nickel or cobalt deposited. The more surface area per weight of nickel or cobalt, i. e., the more finely-divided the nickel or cobalt, the better the enamel adherence. In support of this hypothesis, I have observed that if some mechanical process such as rolling or burnishing is performed on the finely-divided or burned nickel or cobalt coat deposited in accordance with this invention, enamel cover coat adherence is sharply reduced; and if burnishiug or rolling is severe enough, all adherence qualities are lost even though actually no nickel or cobalt is removed from the base article.
Briefly stated, my invention comprises the steps of removing substantially all loose particles of foreign matter from the surface of a ferrous metal article which is to receive an enamel coat, and electrodepositing on the thus cleaned surface a coat of finely-divided metal selected from the group consisting of nickel and cobalt, this coat being of critical thickness in excess of about 0.4 gram per square foot of ferrous metal surface. To the thus prepared surface of the article, a porcelain enamel slip is applied directly and the slip is fired suitably under conventional circumstances to produce an enamel coating.
Prior to the application of the nickel or cobalt coat to the surface of the metal article to be enameled, the metal article is cleaned and freed from grease and adhering foreign matter by any suitable means such as alkaline reagents, electrolytic methods and organic solvents and in any case this cleaning is sufficiently complete so that the metal surface displays no water breaks.
While the pickling step is not essential to the successful practice of this invention and to obtain consistently the results and advantages of this invention, a light pickling operation may, if desired, be carried out to remove rust spots and traces of oxide which remain on the surface following the cleaning operation. A mineral acid solution of conventional composition may be used to accomplish this rust and oxide removal, as those skilled in the art will understand. However, if the metal article to be enameled is clean and there are no visible signs of oxidation on the surface to receive the coat, this pickling step is preferably omitted and there is no sacrifice of I enameling adhesion or enamel surface quality as a result.
Following the cleaning operation or the pickling operation, whichever immediately precedes the electrodeposition step of this invention, the metal article to be enameled is thoroughly washed and rinsed free of adhering solution with or without a preliminary acid or alkaline neutralizing step, as desired.
The thus prepared article is next treated as promptly as convenient and preferably immediately to produce on the clean surface a coating of nickel or cobalt of critical thickness in excess of about 0.4 gram per square foot of ferrous metal surface. This plating operation is carried out in a bath of suitable composition as described hereinafter and is effected by electrodeposition at a relatively high current density, the conditions of the plating opera tion being adjusted to assure formation of a finely-divided nickel or cobalt coat possessing a very large surface area per weight of nickel or cobalt deposited. The period of contact of the metal article with the plating under electrodeposition conditions will depend upon the various factors governing the rate of deposition of the nickel or cobalt on the metal article surface. Thus, the temperature of the plating bath, the conductivity of this bath, the current density and similar factors interplay to determine deposition rate as well as the character of the nickel or cobalt deposition and accordingly these factors are to be taken into consideration and adjusted or allowed for in carrying out this electrodeposition operation.
The thickness of the nickel or cobalt coat is critical only as to a minimum, the results and advantages of this invention being obtainable consistently where such coats of medium to relatively heavy grade are provided. Thus, for instance, with coating. weights as high as 2.0 grams per square foot, good enamel adherence is obtained. However, since no advantage is gained from the heavier coating weights, I prefer to use coatings in the lighter end of the range.
Following the application of the desired nickel or cobalt coat to the article to be enameled, the thus plated surface of the article is washed or rinsed to remove adhering plating solution and dried preparatory to receiving an enamel slip which is suitably applied in any conventional manner and then is fired to provide the enamel cover for the article, surface.
Some typical plating baths giving the desired type of nickel or cobalt coatings in accordance with this invention having the following compositions:
Example I Grams per liter Nickel sulfate l Ammonium sulfate 100 Example II Nickel sulfate Ammonium chloride 100 Ammonium sulfate 200 Example III Nickel sulfate 10 Ammonium chloride 100 Ammonium sulfate 200 Example IV Nickel sulfate 10 Ammonium chloride 200 Example V Nickel sulfate 10 Sodium sulfate 500 Ammonium chloride 50 Example VI Cobalt chloride l0 Ammonium chloride 200 Example VII Nickel sulfate l0 Ammonium sulfate 200 Example VIII Nickel sulfate 10 Ammonium chloride 100 Example IX Nickel sulfate 10 Ammonium sulfate Example X Cobalt sulfate 1O Ammonium chloride 200 These plating baths include, in addition to the foregoing constituents, water making up the baths in the ratio to the salt ingredients as stated above.
In formulating compositions of the type described above for use in accordance with this invention, there are certain facts which must be kept in mind such as the nickel or cobalt concentration of the plating bath. A principal object, in accordance with this invention, is to burn the plated nickel or cobalt and thereby produce a finelydivided spongy coating on the base article. If the nickel or cobalt concentration in the bath runs relatively high, a more or less smooth metallic-appearing coat is formed on the base article instead of a deposit of the type required by this invention having a smutty, black, nonmetallic appearance. Where the concentration of nickel or cobalt is sufficiently high in the plating bath that the tendency is to produce the undesirable metallic-appearing deposits, satisfactory results may consistently be obtained by plating at an increased voltage. However, since it is desirable to maintain voltage in the plating bath relatively low to conserve power and it is also economical to limit nickel or cobalt concentration in the plating baths, this matter of the amounts of cobalt and nickel salts used in compounding these baths is an important practical factor in the commercial practice of this invention.
As the foregoing examples indicate, I have found that an ammonium salt is necessary in the plating solutions used in accordance with this invention. However, some of the ammonium salt may be replaced with another conducting salt such as sodium or potassium chloride. The ammonium salts and the sodium and potassium salts em ployed in preparing the baths for use in this invention serve to increase the conductivity of the baths and thus enable burning of the cobalt and nickel coatings more thoroughly and successfully, particularly when these conducting salts are maintained in fairly high concentration in the baths.
The plating operation of this invention may satisfactorily be carried out over a wide range of temperature but the optimum temperature is in the range between room temperature and boiling point temperature of the solution with the preference being the upper end of the range, as at about 200 F. At the higher temperatures of the plating bath, the nickel or cobalt coatings burn more readily and accordingly less plating time is necessary in order to deposit the same amount of metal on the base article.
Direct current voltages in the order between 6 and 15 volts have been found suitable for producing a burned" deposit of nickel or cobalt in accordance with this invention. Due consideration must be given the nickel or cobalt concentration when evaluating voltages and plating time. If the nickel or cobalt concentration is low, i. e., in the order of 5 grams per liter of NiSO -6H O or 5 gramsper liter of CoCl -6H O, the lower voltage would give burned deposits, but a longer plating time is required in order to plate a sufficient amount of nickel or eelaaw.
cobalt. Conversely, if the nickel or cobalt concentration is i i. e., in the order of 15 grams per liter of NiSO -6H or 15 grams per liter of CoCl -6H O, then a higher voltage would have to be employed, but the plating time would be decreased. Assuming the mean value, i. e., grams per liter of NiSO -6H O or 10 grams per liter of CoCl -6H O, which is between the arbitrary limits set above, then the voltage employed in this plating operation will preferably be in the neighborhood of 10 to 12 volts.
The spacing of the anode and cathode is an important economic fact and, in accordance with my preference, these electrodes should be relatively close together, a spacing of an inch between them being quite satisfactory. The anodes may be of gas carbon, lead or soluble nickel or cobalt or any suitable combination of these types. The soluble and insoluble anodes may be so proportioned that the metal concentration in the bath remains substantially unchanged during the plating operation.
The plating time will suitably vary from between about 10 to 45 seconds and will depend upon the amount of nickel to be deposited and the desired fineness or particle size of the resulting metal coats.
For the purpose of illustration and not by way of limitation, the applicant offers the following example of the process of this invention as it has actually been successfully carried out to assist those skilled in the art in gaining a better understanding of this invention.
Example XI A 20-gauge steel sheet was immersed in a suitable vat containing a hot alkaline electrode cleaning solution, the sheet being made the anode in a direct current circuit in which the current density was maintained at 50 amperes per square foot of sheet surface immersed in said solution. The cleaner solution was maintained between 160 and 180 F., heat being applied to the cleaner vessel in any suitable manner to compensate for heat loss. Dirt, grease and oil adhering to the sheet and foreign particles on the sheet surface were substantially entirely removed by this electrical cleaning treatment and on removal from the cleaning solution the sheet was thoroughly rinsed with water, freeing it from adhering alkaline cleaner. The sheet, which was chemically clean after this rinse, was subjected to an alkaline dip to inhibit rusting. The alkaline dip was a weak aqueous soda ash solution containing about 0.5% sodium carbonate. The sheet, on emerging from this clip, was scrubbed once again with clean water in order to remove the thin film of soda ash adhering to its surface and then was transferred to a plating bath where it was immersed for a period of 15 seconds as the cathode in an electric circuit including a sheet lead anode spaced approximately one inch from the steel sheet. The plating bath contained 10 grams per liter of nickel sulphate and 120 grams per liter of ammonia sulphate and had a pH of about 5.5. The temperature of the plating bath on immersion of the cleaned ferrous metal sheet therein was about 200 F. and the voltage was about 9.5 volts during the plating operation, while the current density was about 550 amperes per square foot of cathode surface throughout the plating period.
No difficulty was experienced in controlling bath temperature through a number of repetitions of the plating operation and cathode efficiencies in terms of nickel deposition approximated The pH range of the plating bath is not critical since satisfactory burned can be obtained whether the pH is less than one or greater than eight. However, if the pH of the bath substantially exceeds six, some ammonia will be lost and the ammonium sulphate concentration will be changed undesirably. Consequently, it is preferable to keep the pH range of the bath between about 2.5 and about six.
Having thus described the present invention so that those skilled in the art may be able to understand and practice the same, I state that what I desire to secure by Letters Patent is defined in what is claimed.
What is claimed is:
l. The method of producing a tenaciously adhering, substantially uniform and continuous porcelain enamel coat on a ferrous metal article which comprises the steps of removing substantially all loose particles and foreign matter from the surface of said article, electrodepositing on the thus cleaned surface a substantially uniform thickness coat of spongy burned metal selected from the group consisting of nickel and cobalt and of weight in excess of about 0.4 grams per square foot, applying porcelain enamel slip directly to the resulting electroplate surface, and firing said slip and producing said enamel coat, said electrodeposition step being conducted to produce a metal coat of the finely-divided and spongy character of the metal coat produced by immersing a clean ferfous metal body as a cathode approximately one inch from a lead anode in an aqueous solution containing about grams per liter of ammonium sulfate and about 10 grams per liter of a substance selected from the group consisting of nickel sulfate and cobalt sulfate at about 200 F. for between about 5 and about 15 seconds while imposing upon the electrodes a ten-volt current.
2. The method of producing a tenaciously adhering, substantially uniform and continuous porcelain enamel coat on a ferrous metal article which comprises the steps of removing substantially all loose particles and foreign matter from the surface of said article, electrodepositing on the thus cleaned surface a substantially uniform thickness coat of spongy burned nickel and of weight in excess of about 0.4 grams per square foot, applying porcelain enamel slip directly to the resulting electroplate surface, and firing said slip and producing said enamel coat, said electrodeposition step being conducted to produce a metal coat of the finely-divided and spongy character of the metal coat produced by immersing a clean ferrous metal body as a cathode approximately one inch from a lead anode in an aqueous solution containing about 100 grams per liter of ammonium sulfate and about 10 grams per liter of nickel sulfate at about 200 F. for between about 5 and about 15 seconds while imposing upon the electrodes a ten-volt current.
3. The method of producing a tenaciously adhering, substantially uniform and continuous porcelain enamel coat on a ferrous metal article which comprises the steps of removing substantially all loose particles and foreign mater from the surface of said article, electrodepositing on the thus cleaned surface a substantially uniform thickness coat of spongy burned cobalt and of weight in excess of about 0.4 grams per square foot, applying porcelain enamel slip directly to the resulting electroplate surface, and firing said slip and producing said enamel coat, said electrodeposition step being conducted to produce a metal coat of the finely-divided and spongy character of the melal coat produced by immersing a clean ferrous metal body as a cathode approximately one inch from a lead anode in an aqueous solution containing about 100 grams per liter of ammonium sulfate and about 10 grams per liter of cobalt sulfate at about 200 F. for between about 5 and about 15 seconds while imposing upon the electrodes a ten-volt current.
4. The method of producing a tenaciously adhering, substantially uniform and continuous, porcelain enamel coat on a ferrous metal article which comprises the steps of applying a porcelain enamel slip directly to a metal coat on the surface of said ferrous article, which coat is spongy and burned, is substantially uniform in thickness, weighs in excess of about 0.4 grams per square foot and is selected from the group consisting of nickel and cobalt, and firing said enamel slip and converting it into an enamel coat.
(References on following page) References Cited in thc file of this patcnt FOREIGN PATENTS UNITED STATES PATENTS 674,490 Great Britain "June 25, 1952 McGohan Dec. 14, 1937 OTHER REFERENCES giggg et a1 5g 6 Electra-deposition of Metals, Langbein and Byannt, um Henry Carey Brim-o and Co., Inc, New York (1924), Mamn May 1949 page 348 Copy in Div 56 Chester June 6, 1950 Whitehouse et a1. May 29, 1956