Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D13/00—Electrophoretic coating characterised by the process
  • C25D13/20—Pretreatment
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
  • C23C22/05—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions
  • C23C22/06—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6
  • C23C22/34—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides
  • C23C22/36—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides containing also phosphates
  • C23C22/362—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using aqueous solutions using aqueous acidic solutions with pH less than 6 containing fluorides or complex fluorides containing also phosphates containing also zinc cations

Definitions

• 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.
• 1t 1s frequently the practice to provide on the metal surface a protective coating which serves as a base for a subsequently applied paint.
• palnt-base coatings are phosphate coatings, such as zinc phosphate coatings or the like.
• phosphate paint-base coatings it is often the practice to apply a dilute aqueous solution containing hexavalent chrom um ions to the phosphate coating.
• chromate solutions enhance the paint coating which is subsequently app lied to the metal surface.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• a complex fluoride in addition to the simple fluoride.
• 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.
• the complex fluoride added is a fluosilicate, preferably added as fluosilicic acid or sodium fluosilicate.
• 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.
• 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.
• 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.
• 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.
• 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.
• the metal surfaces are preferably rinsed with Water so as to remove any of the acidic coating solution which may remain on the surface.
• a hot water rinse is used, with water temperatures within the range of about 50 to about 80 degrees centigrade being typical.
• various contacting techniques may be used, with rinsing by spraying being preferred.
• 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.
• 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.
• 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.
• 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.
• a water-thinned paint is applied to the treated metal surfaces by electrophoresis.
• 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 coating solutions utilized are dilute aqueous solutions, having a solids content within the range of about 3 to 15 percent solids.
• 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.
• 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.
• 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.
• the metal surface to be treated such as a ferrous metal surface
• a ferrous metal surface is first cleaned.
• 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:
• 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.
• the metal surface 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.
• 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.
• 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.
• 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.
• 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.
• 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.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrochemistry (AREA)
• General Chemical & Material Sciences (AREA)
• Mechanical Engineering (AREA)
• Chemical Treatment Of Metals (AREA)
• Application Of Or Painting With Fluid Materials (AREA)

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Citations (14)

• 1964
  • 1964-04-30 GB GB18043/64A patent/GB1040806A/en not_active Expired
• 1965
  • 1965-04-03 DE DE1571080A patent/DE1571080C3/de not_active Expired
  • 1965-04-07 AT AT316465A patent/AT272037B/de active
  • 1965-04-08 BE BE662277D patent/BE662277A/xx unknown
  • 1965-04-26 SE SE5425/65A patent/SE308049B/xx unknown
  • 1965-04-27 NL NL656505338A patent/NL149550B/xx not_active IP Right Cessation
  • 1965-04-28 ES ES0312338A patent/ES312338A1/es not_active Expired
  • 1965-04-29 CH CH605965A patent/CH437961A/fr unknown
  • 1965-04-29 US US451963A patent/US3454483A/en not_active Expired - Lifetime

Patent Citations (14)

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