US 3454483 A
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United States Patent 01 ifice 3,454,483 Patented July 8, 1969 ELECTRODEPOSITION PROCESS WITH PRETREAT- MENT IN ZINC PHOSPHATE SOLUTION CON- TAINING FLUORIDE Dennis B. Freeman, Harrow, England, assignor to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed Apr. 29, 1965, Se r. No. 451,963
Claims priority, application Great Britain, Apr. 30, 1964,
18,043/ 64 Int. Cl. C23b 13/00; C23f 17/00; B01k 5/02 US. Cl. 204181 16 Claims ABSTRACT OF THE DISCLOSURE An improved process for electrophoretically painting metal surfaces wherein the metal to be painted 1s first coated with an aqueous acidic zinc phosphate solution which contains at least 0.1 grams per liter of fluoride ions. The thus-coated metal surface is then rinsed with a dilute aqueous solution containing hexavalent chromium and the thus-rinsed surface 'is then electrophoretically painted.
This invention relates to an improved process for painting metal surfaces and more particularly, it relates to an improved method for painting metal surfaces whereln the paint is applied by electrophoretic methods.
In the treatment of various metal surfaces, 1t 1s frequently the practice to provide on the metal surface a protective coating which serves as a base for a subsequently applied paint. Frequently, such palnt-base coatings are phosphate coatings, such as zinc phosphate coatings or the like. After the application of such phosphate paint-base coatings, it is often the practice to apply a dilute aqueous solution containing hexavalent chrom um ions to the phosphate coating. Such chromate solutions enhance the paint coating which is subsequently app lied to the metal surface. These phosphate paint base coatlngs and chromate rinse solutions and their methods of application are well-known to those in the art. I
Recently, a great deal of Work has been done 1n developing processes whereby water-thinned paint are applied to metal surfaces by electrophoretic methods, WhlCh methods frequently involve the phenomenon of electroosmosis and electrolysis as well as electrophoresis. It has been found that phosphate paint-base coatings, and particularly zinc phosphate coatings, which have been applied prior to the use of conventional painting techniques, are also beneficial when used prior to the electrophoretic application of paint. In addition to the usual advantages which are obtained by using the zinc phosphate materials as paint-base coatings, it has been found that the Zinc phosphate coating has a higher resistance than the untreated metal surface. Thus, when paint is applied by electrophoresis, improved paint coverage on the surface is obtained. The use of a chromate rinse over the phosphate coating, prior to the electrophoretic application of the paint, has also been found to be advantageous in the same manner as when such a rinse is used prior to the application of paint by conventional techniques.
Although the overall adhesion and quality of the paint coating is generally improved by the use of a chromate rinse over the zinc phosphate coatings, in some instances it has been found that when such rinse is used, minute depressions in the paint film applied by electrophoretic application, are formed. In many instances, these depressions are actually holes which go completely through the paint film to the substrate and these defects have been found to be particularly serious when the metal surface has been force dried after the application of the chromate rinse. Often, these depressions or holes have been sufficiently bad that it has been necessary to completely omit the chromate rinse after the application of the zinc phosphate coating. This is not, however, a satisfactory solution to the problem in that by eliminating the rinse, the advantages obtained by its use are also lost.
It is, therefore, an object of the present invention to provide an improved method for treating metal surfaces to provide a paint-base coating thereon prior to the application of a paint by electrophoresis.
Another object of the present invention is to provide a pretreatment for metal surfaces, using a chromate rinse, prior to the application of a paint to the metal surface utilizing electrophoretic painting techniques.
A further object of the present invention is to provide an improved process for painting metallic surfaces, whereby the paint is applied using electrophoretic methods.
These and other objects of the present invention will become apparent to those skilled in the art from the description of the invention which follows.
Pursuant to the above objects, the present invention includes a process for treating metal surfaces which comprises contacting the metal surface to be treated with an acidic zinc phosphate solution containing at least 0.1 gram per liter of simple fluoride, calculated as F maintaining the solution in contact with the metal surface for a period sufficient to form a zinc phosphate containing coating on the metal, contacting the thus-coated surface with a hexavalent chromium containing solutions and, thereafter, electrophoretically applying a paint to the thustreated metal surface. By thus utilizing a zinc phosphate solution which contains at least 0.1 gram per liter of simple fluoride to provide the paint-base coating on the metal surface, the phosphate coating obtained may be rinsed with a hexavalent chromium containing solution and then painted using electrophoretic painting techniques, without the formation of depressions or holes in the paint film.
More specifically, the practice of the method of the present invention, a metal surface to be treated is contacted with an aqueous acidic coating solution containing zinc phosphate and at least 0.1 gram per liter of a simple fluoride. Various metal surfaces may be treated by the method of the present invention, such as ferrous metal surfaces, i.e., iron or steel, including various alloys Wherein the predominant component is iron, zinc or zinc alloy surfaces, wherein the predominant component is zinc, including zinc coated ferrous surfaces, e.g., hot dipped galvanized iron or steel surfaces, and aluminum. The coating solutions are maintained in contact with the metal surface to be treated for a period'of time suflicient to effect the formation of the desired zinc phosphate containing coating on the metal surface. Contact times of about /2 to about 3 minutes are typical although in some instances either lesser or greater contact times may be used, depending upon the amount of coating which it is desired to form on the surface.
' As has been indicated hereinabove, the phosphate coating solutions used contain at least 0.1 gram per liter of a simple fluoride, calculated as F. The maximum :amount of the simple fluoride present has not been found to be critical, amounts up to the maximum solubility of the fluoride used in the phosphate solution being suitable, with amounts up to about 3-4 grams/ liter being typical. Various simple fluorides may be used, provided the cation of the fluoride is not detrimental to the metal surface treated, the phosphate coating produced or the subsequently applied paint coating. Exemplary of the fluorides which may be used are the alkali metal fluorides, i.e., the fluorides of sodium, potassium, lithium, cesium, or rubidium, zinc fluorides, hydrofluoric acid, and the like. Of these, the preferred source of fluoride is sodium fluoride or hydrofluoric acid and hereinafter, primary reference will be made to the use of these materials. This is not, however, to be taken as a limitation as to the fluoride materials which may be used, but only as exemplary of these materials.
In many instances, it has been found to be advantageous also to include in the zinc phosphate coating solution a complex fluoride, in addition to the simple fluoride. Here again, various complex fluorides may be used, provided the cation of the complex fluoride is not detrimental to the metal substrate, the phosphate coating produced or the paint coating subsequently applied. Exemplary of these complex fluorides which may be included in the composition are fluosilicates, fluoborates, fluotitanates, and the like. These may be added as the corresponding acid, the alkali metal salt, the zinc salt, or the like. Desirably, the complex fluoride added is a fluosilicate, preferably added as fluosilicic acid or sodium fluosilicate. Although varying quantities of the complex fluoride may be added to the phosphate coating solution, up to the maximum solubility of these fluorides in the solution, the preferred ratio of complex fluoride, expressed as SiF to simple fluoride, expressed as F, is :1. This ratio of complex fluoride to simple fluoride in the solution is a preferred ratio and is not to be taken as a limitation of the amount of complex fluoride in the solution, since amounts from 0% of the complex fluoride up to its maximum solubility in the phosphate solution may be used.
The zinc phosphate coating solutions are aqueous acidic solutions containing phosphate ions and zinc ions, generally in amounts at least suflicient to form the dihydrogen phosphate with the phosphate ions, and are well-known to those in the art. Typically, such coating solutions may contain from about 0.5 to 2.5% by weight P0 ions although greater and lesser amounts of the phosphate ions may be contained in the solution in many instances. In addition to the simple fluoride and the complex fluoride, these solutions may also contain nickel, typically in amounts within the range of about 0.01 to about 0.4% by weight, calculated as Ni, copper ions, typically in amounts within the range of about 0.0003 to about 0.0005 by weight, and oxidizing ions such as nitrate and/or nitrite ions, the nitrate ions typically being present in amounts within the range of about 0.2 to about 1% by weight and the nitrite ions typically being present in amounts within the range of about 0.002 to about 0.01% by weight. Compositions of this type are exemplified by those described in U.S. Patent 2,835,617 and 2,591,479. These zinc phosphate coating solutions, are, however, merely exemplary of those which may be used as other aqueous acidic zinc phosphate solutions may also be used, provided they contain at least 0.1 gram per liter of the simple fluoride, as has been set forth hereinabove.
Contact of these coating solutions with the metal surfaces to be treated may be carried out in any convenient manner, as for example by spraying, immersion, flowing, or the like. In many instances, application of the solutions by spraying is preferred so that primary reference hereinafter will be made to the use of spraying techniques. Desirably, the solutions are applied hot, temperatures within the range of about 40 to 80 degrees centigrade being typical, with temperatures within the range of about 45 to about 70 degrees being preferred. After the desired contact of the surfaces to be treated and the coating solution has been efiected, the metal surfaces are preferably rinsed with Water so as to remove any of the acidic coating solution which may remain on the surface. Preferably, a hot water rinse is used, with water temperatures within the range of about 50 to about 80 degrees centigrade being typical. As with the application of the phosphate coating solution, various contacting techniques may be used, with rinsing by spraying being preferred.
After the hot water rinse, the phosphate coated surfaces are then rinsed in a hexavalent chromium containing solution. These hexavalent chromium containing rinsing solutions are preferably aqueous solutions containing a source of hexavalent chromium, calculated as CrO typically in an amount within the range of about 0.03 to about 1% by weight of the solution, and preferably in an amount within the range of about 0.07 to about 0.5% by weight of the solution. Various water-soluble or Water dispersable sources of hexavalent chromium may be used in formulating the rinsing solution, provided the anions and cations introduced with the hexavalent chromium do not have a detrimental affect on either the solution itself, the coated surfaces treated or the subsequently applied paint coating. Exemplary of hexavalent chromium materials which may be used are chromic acid, the alkali metal and ammonium chromates, the alkali metal and ammonium dichromates, the heavy metal chromates and dichromates, such as those of zinc, calcium, chromium, Fe+ magnesium and aluminum, and the like. The rinsing solution may be applied to the coated metal surfaces using various techniques, including immersion, flooding, spraying and the like, with spraying techniques being preferred. Generally, it is preferred that the aqueous hexavalent chromium containing rinse solution is maintained at an elevated temperature while it is in contact with the phosphate coated metal surface to be treated. Temperatures within the range of about 35 to 60 degrees centigrade and contact times of up to about 60 seconds are typical. If desired, these hexavalent chromium containing rinsing solutions may also contain phosphate ions, preferably as ortho phosphoric acid. Where phosphoric acid is included in the rinse composition, amounts up to about 1% by weight of the rinsing solution are typical. It will, of course, be appreciated with regard to the concentration of the hexavalent chromium and phosphate ions in the rinse solution as well as the temperatures and contact times, that values both greater and lesser than those which have been indicated hereinabove, may be used in some instances, the specific ranges which have been given being merely exemplary of those which may be used.
Following the application of the hexavalent chromium containing rinse solutions, the treated metal surfaces are, preferably again rinsed with water so as to remove any of the acidic rinse solution which may remain on the surface. Thereafter, a water-thinned paint is applied to the treated metal surfaces by electrophoresis. In the electrophoretic painting process, the coated metal surface to be painted may be either the anode or the cathode, depending upon the characteristics of the paint which is used. The improvements obtained in using the method of the present invention have been found to be particularly striking when the coated metal surface is the anode, so that this method of operation is generally preferred.
The electrophoretic application of the paint may be carried out in various ways, as are known to those in the art. Typically, the coating solutions utilized are dilute aqueous solutions, having a solids content within the range of about 3 to 15 percent solids. As has been indicated, the metal to be coated is preferably the anode and the voltages used are typically within the range of about 50 to 1000 volts (direct current). Typical current densities used and coating times required are, respectively, from about 0.1 to 7 amperes per square foot and from about 10 seconds to about 2 minutes. Normally, the coating solution is at substantially room temperature, i.e., about degrees centigrade, but elevated temperatures, e.g., 30 to 40 degrees centigrade or even higher, may be used if desired. The paint applied using these techniques are Water-thinned resin paints which are, typically aqueous solutions based on synthetic resins such as alkyd resins, acrylic polymers, melamine resins, and the like. These aqueous resin solutions generally have a pH of about 9 and the solvent used is either water or an aqueous alcoholic mixture.
In carrying out the method of the present invention, the metal surface to be treated, such as a ferrous metal surface, is first cleaned. Although any conventional metal cleaning composition and method may be used, in many instances it has been found desirable to utilize a cleaning solution containing an alkali metal silicate and alkali metal metaborate, such as that disclosed in British patent specification No. 932,970. Exemplary of such a composition is one having the following composition:
Components: Parts by weight Sodium metasilicate 616 Sodium metaborate 300 Titanium activator containing 1.5% titanium and prepared in accordance with US. Patent Such cleaning compositions may be applied in various ways, such as by immersion, flowing, spraying, or the like, with spraying techniques being preferred.
After the cleaning of the metal surface has been completed, it may, if desired, be rinsed with water to remove any of the alkaline cleaning solution which may remain on the surface. The cleaned metal surface is then contacted, preferably by spraying, with an aqueous acidic zinc phosphate solution containing at least 0.1 gram per liter of a simple fluoride, until the desired zinc phosphate coating is formed on the metal surface. The coated metal may then be rinsed in water, followed by a rinse with dilute chromic acid or a dilute solution of chromic acid and phosphoric acid. If no intermediate operations or inspection are required, the rinsed metal surface may be passed directly through the painting tank, wherein a paint coating is applied -by electrophoresis, without any intermediate drying. In this case, however, it may be desirable to water rinse the metal surface after the chromate rinse so as to avoid contamination of the paint. In the electrophoretic application of the paint, the paint tank itself may serve as the cathode while the parts to be coated, as the anode, may be introduced into the tank by means of an energized conveyor. After the application of the paint, the painted surfaces may then be subjected to whatever heating is necessary to effect the necessary drying and/or curing of the paint lfim.
In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. These examples are, however, merely exemplary of the process of the present invention and are not to be taken as a limitation thereof. In these examples, unless otherwise indicated, temperatures are given in degrees centigrade.
Example 1 Ferrous metal panels were cleaned using the compositions and procedures as set forth in British patent specification No. 932,970. Following the cleaning, the panels were rinsed twice in cold water. The panels were then phosphate-coated by spraying with a solution containing 1.8 grams per liter N 2.3 grams per liter zinc, 5.9 grams per liter PO 0.3 grams per liter nickel, 0.18 grams per liter sodium and 0.07 grams per liter N0 1.2 grams per liter Na SiF and 0.3 grams per liter NaF. This solution was at a temperature of about 50 degrees centigrade and the solution was sprayed on the panels for about 1 minute. Following the application of the phosphate coating, the panels were water rinsed and then sprayed for 30 seconds with an aqueous solution containing 0.09 grams per liter chromic acid and 0.06 grams per liter phosphoric acid, which solution was at a temperature of about 40 degrees centigrade. These panels were then dried in an oven at a temperature of about 120 degrees centigrade for 5 minutes. The thusdried panels were then painted by electrophoresis with a water-based primer paint. The paint was an aqueous composition of a high acid number, above 50, phenol modified alkyd resin containing added hydroxyl substituents, which composition contained about 10% solids. The paint had a pH of about 7.3 and was at a temperature of about .20 degrees Centigrade. With the panels as the anode, a direct current was applied for about two minutes at a voltage of about 160 volts to deposit the paint film. During this time, the current density, which was initially about 5 amps/square foot, dropped to about 0.5 amp/square foot. The resulting paint coating was smooth, showed no evidence of depressions or pin-holes and gave excellent results when tested in the 5% salt spray, humidity, knife blade adhesion and standard deformation tests.
The procedure of the proceeding examples is repeated using a zinc phosphate solution containing hydrofluoric acid and fluosilicic acid in place of the respective sodium salt, and using a chromate rinse containing about 0.13 grams per liter chromic acid, to obtain substantially the same results.
Example 2 By way of comparison, the procedure of Example 1 was repeated with the exception that the zinc phosphate coating solution used contained no sodium fluoride or sodium fluosilicate. The paint coating obtained on the panels which were coated by this process was found to have numerous pin-holes, many of which extended through the paint film to the substrate.
Example 3 The procedure of Example 1 was repeated with the exception that the zinc phosphate coating solution used contained about 0.3 gram per liter of hydrofluoric acid instead of the sodium fluorsilicate and sodium fluoride. The painted panels obtained by using this procedure were found to be substantially the same as those obtained in Example 1, showing no evidence of pin-holing or other defects in the film.
While there have been described various embodiments of the invention the compositions and methods described are not intended to be understood as limiting the scope of the invention, as it is realized that changes therewithin are possible and it is intended that each element recited in any of the following claims is intended to be understood as referring to all equivalent elements for accomplishing the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.
What is claimed is:
1. In a process for painting a metal surface wherein the surface is coated with a phosphate coating, the coating rinsed with a dilute solution containing hexavalent chromium and, thereafter, paint is applied to the rinsed surface by means of electrophoresis, the improvement which comprises forming the phosphate coating on the surface by contacting the surface with an aqueous acidic zinc phosphate solution which contains at least 0.1 grams per liter of a simple fluoride, calculated as F".
2. The process as claimed in claim 1 wherein the zinc phosphate coating solution also contains a complex fluoride.
3. The process as claimed in claim 2 wherein the ratio of complex fluoride, calculated as SiF to simple fluoride calculated as F- is about 5:1.
4. The process as claimed in claim 3 wherein the complex fluoride and the simple fluoride are added to the zinc phosphate solution as sodium fluosilicate and sodium fluoride respectively.
5. A process for treating metal surfaces which comprises contacting the metal surface to be treated with an aqueous acidic zinc phosphate solution containing at least 0.1 grams per liter of a simple fluoride, calculated as F, maintaining the phosphate solution in contact with the metal surface for a period sufiicient to form a zinc phosphate containing coating on the metal surface, rinsing the thus-coated surface with a hexavalent chromium containing solution and, thereafter, electrophoretically applying paint to the thus-treated metal surface.
6. The method as claimed in claim wherein the zinc phosphate solution also contains a complex fluoride.
7. The method as claimed in claim 6 wherein the ratio of complex fluoride, calculated as SiF to simple fluoride, calculated as F, is about 5:1.
8. The process as claimed in claim 7 wherein the complex fluoride and the simple fluoride are added to the zinc phosphate solution as sodium fiuosilicate and sodium fluoride, respectively.
9. The process as claimed in claim 8 wherein the zinc phosphate coating solution also contains at least 0.01 gram per liter of nickel, calculated as Ni.
10. The process as claimed in claim 5 wherein the coated metal surface is the anode during the electrophoretic application of the paint.
11. The process as claimed in claim 7 wherein the coated metal surface is the anode during the electrophoretic application of the paint.
12. The process as claimed in claim 9 wherein the coated metal surface is the anode during the electrophoretic application of the paint.
13. The process as claimed in claim 12 wherein prior to contacting the metal surface to be treated with the zinc phosphate coating solution, the surface is cleaned by contacting it with an aqueous cleaning solution'containing an alkali metal silicate and an alkali metal metaborate.
14. An article having a painted metal surface formed in accordance with the process of claim 5, which surface is characterized by having thereon a paint film which is substantially free of holes.
15. An article having a painted metal surface formed in accordance with the process of claim 9 which surface is characterized by having thereon a paint film which is substantially free of holes.
16. An article having a painted metal surface formed in accordance with the process of claim 13, which surface is characterized by having thereon a paint film which is substantially free of holes.
References Cited UNITED STATES PATENTS 2,312,855 3/1943 Thompson 148-615 2,477,841 8/1949 Ward 1486.l5 2,665,231 l/l954 Amundsen et al. 1486.15 2,935,423 5/1960 Kapfer et al. 1486.15 3,060,066 10/1962 Ross et al. 1486.15 3,261,723 7/1966 Craig 148-615 3,364,081 1/1968 Forsbcrg 1486.15 2,438,013 3/ 1948 Tanner 204 2,631,951 3/1953 Chester 204-35 2,648,625 8/1953 Buser 204--35 2,825,682 3/1958 Missel et al. 204-382 3,106,484 10/1963 Miller 20435 3,230,162 1/1966 Gilchrist 204-181 FOREIGN PATENTS 768,443 2/ 1957 Great Britain.
JOHN H. MACK, Primary Examiner.
E. ZAGARELLA, JR., Assistant Examiner.
. U.S. Cl. X.R. 2043
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