Classifications

• • 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
  • C23C22/07—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 phosphates
  • C23C22/08—Orthophosphates
  • C23C22/20—Orthophosphates containing aluminium cations

Definitions

• the present invention relates to the art of forming adherent, integrally bound coatings on metallic surfaces which are useful as protection for said surfaces against corrosion, as a base for paint and to protect the surfaces during metal deformation operations and the like. More particularly, this invention relates to an improved method for forming modified phosphate coatings on the surfaces of commercial metals which are susceptible to atmospheric corrosion particularly the surfaces of irons and alloys thereof.
• phosphate coatings there are two main types of solutions which are in use.
• the first of these solutions employs the dihydrogen phosphate of a heavy metal, particularly zinc and manganese and these aqueous acidic solutions form a coating in which the metal of the solution forms an integral part of the coating on the metal surface, that is, the coating includes zinc or manganese.
• phosphate coatings are crystalline in nature and are fairly heavy coatings having a thickness on the order of about grams per square meter. The crystalline nature of such coatings may be revealed either by looking at it with the naked eye or under a microscope at a magnification of up to about 1200 diameters.
• the size of the crystals has an important influence on the absorption qualities on the coating and its ability to form a tight adherent base for organic finishes, such as paints, lacquers, or the like, and it is additionally known that the crystalline size of the particles within such a coating may be refined by addition to the solution of oxidizing agents or other additives such as the condensed phosphates or the like, and by the addition of such additives, the coating weight can be reduced to within the range of about 1.5 to about 3 grams per meter and with a concurrent reduction in the size of the crystals in the coating.
• the second type of process makes use of the dihydrogen phosphates of the alkaline earth metals or of ammonium phosphates or organic phosphates.
• Solutions of the alkaline earth metals or ammonium or organic phosphates are aqueous acidic solutions which form on a metal surface, for example, iron, a coating which has a much finer particle size than that which was obtained from zinc and manganese dihydrogen phosphate solutions.
• the particle size of such coatings is reduced to such a degree that they are considered amorphous-like coatings because an examination of them under a microscope at a magnification up to about 1800 diameters does not reveal a true crystalline structure.
• Such coatings are ordinarily thinner and fall into the range of 0.5 to about 0.8 grams per square meter.
• Coatings from the dihydrogen phosphates of zinc and manganese provide good protection against corrosion and are particularly good in aiding in the cold forming of metal surfaces.
• such heavy zinc or manganese phosphate coatings do not provide as good a base for paint as the coatings of the second type.
• the second type of coating solution produces a coating which provides improved adherence for paints, lacquers, enamels, or the like, but such coating is far less satisfactory as far as resistance to corrosion or as an aid in the deformation of the metal coated with that light weight amorphous-like coating.
• the present invention provides a solution and a method for forming coatings which combines the advantages of the above two types of coatings and avoids their disadvantages.
• this invention it has been found that the addition of aluminum ions to an aqueous acidic solution of zinc or manganese phosphate produces a refinement of the crystals and a reduction in their size which places them in the category of the second type of coatings, namely, amorphous-like coatings.
• the incorporation of aluminum ions in the aqueous dihydrogen zinc and manganese phosphate solutions of this invention has been found to cause a substantial reduction in the thickness or weight of the coating which is formed relative to the coating which would be formed by an otherwise identical solution except that it contains no aluminum, and the coatings of this invention have the Weight in the range between about 0.8 and 1.6 grams/sq. meter or otherwise stated, about to about 150 milligrams per square foot, and the preferred coatings have a weight in the range of about 1 gram to 1.3 grams/ sq. meter or to milligrams per square foot.
• Such amorphous-like coatings have been found to be particularly satisfactory in resisting corrosion and to be substantially improved as a base for paint, lacquer, or the like, and moreover, to provide good protection to the metal surface during formation operations such as forming, rolling, stamping and the like.
• the basic solution which is improved by incorporation of the aluminum ion thereby may be any of the heretofore known aqueous acidic zinc or manganese phosphate solutions.
• Such solutions typically contain about 0.5 to about 2.5% P0 sufiicient zinc or manganese to form the dihydrogen phosphate and has a total acid in the range of about 10 to about 50 points.
• the total acid expressed in points refers to the number of ml. of N/10 sodium hydroxide which is required to neutralize a ten ml. sample of the operating solution to a phenolphthalein end point.
• the phosphate coating solution should contain an oxidizing agent which is capable of accelerating the rate of coating formation, or metal attack, so that a uniform coating can be obtained in a reasonably short period of time, for example, about 1-5 minutes.
• an oxidizing agent which is capable of accelerating the rate of coating formation, or metal attack, so that a uniform coating can be obtained in a reasonably short period of time, for example, about 1-5 minutes.
• nitrite is the preferred accelerating agent and may be present in an amount from about 0.002% to about 0.015%.
• Suitable oxidizing agents include about 0.03 to 0.05% bromate, 0.02%0.l5% sulfite, 0.05.25% sodium meta nitrobenzene sulfonate, 0.05-0.25 nitro methane and 0.050.25 nitro guanidine, 0.0*l%4% nitrate, 0.54% chlorate, 0.0 l%-1% sodium ethylene diamine tetraacetate, or mixtures thereof, and particularly mixtures of nitrate and nitrite.
• the aluminum ions may be introduced into the solution in the form of any aluminum salt or compound that is soluble in the aqueous acidic solution of zinc phosphate and which will not destroy the oxidizing agent or detrimentally affect the coating-forming ability of the solution.
• Typical examples of aluminum compounds which are suitable for this purpose include aluminum acetate, aluminum sulfate, aluminum phosphate, aluminum carbonate, aluminum fluoborate and aluminum hydroxide. It is preferred to introduce the aluminum into the solution in a form which adds anions already present in the solution, for example, phosphates, and it is desirable to avoid anything other than minor quantities of chloride, chromate or arsenate anions.
• the aluminum ion should be present in the solution in a proportion of about 0.2 gram to about grams per liter and preferably in a concentration in the range of about 0.04 gram to about 1 gram per liter.
• the stability and equilibrium of the solution, during operation, is best assured when the weight ratio of aluminum to zinc is between 1 to 7 and 1 to 40 and preferably is between 1 to 20 and 1 to 30.
• the solutions of this invention are suitable to form coatings over a wide range of temperatures and will form coatings at all temperatures between normal room temperature and the boiling point of the solution, but are preferably operated in the range of about 70 F. to 120 F.
• the solutions may be applied by brushing, dipping, spraying or atomizing or the part to be coated may be immersed in the solution maintained in the above given temperature range.
• the coating is preferably rinsed in a dilute aqueous chromic acid solution, or one which contains the hexavalent chromium ion, and a suitable concentration for such a solution is one which contains the equivalent of about 2 to about 4 ounces of CrO per 100 gallons of water.
• Example 1 An aqueous solution was prepared by adding per liter of water:
• this composition contains 15.1 grams per liter P0 21.7 grams per liter N0 8.6 grams per liter Zn and 0.3 gram per liter aluminum.
• the solution had a total acidity of 30 points and a pH of 2.3.
• Another bath was prepared, identical to the above, except that it contained no aluminum and panels coated in this solution under conditions identical to those specified above were found to be coated with a coating in the range of 67 grams per square meter and the particles of that coating were found to be typical large grain crystals.
• Panels from the aluminum containing solution were painted in conventional fashion, and subjected to salt spray corrosion and accelerated humidity tests, and in all cases were found to be at least equivalent to similarly painted panels coated with zinc phosphate produced from solutions containing no aluminum.
• the painted panels were subjected to the conventional scratch adhesion test, impact test, and bend test and in all cases the results were satisfactory.
• Example II An aqueous solution was prepared to contain:
• This solution has a total acidity of 30 points and a pH of 2.2.
• Example III A solution was prepared containing:
• This soltuion had a pH of 2.8 and a total acidity of 16 points.
• the solution was applied to cold rolled steel panels by pressure spraying at about 45 C. and an inspection of the coating showed it to have a general appearance similar to that obtained in Example I and a coating weight of 1.0 grams per square meter.
• a process for coating the surface of a metal which is susceptible to atmospheric corrosion which comprises applying to said surface an aqueous acidic solution consisting essentially of a phosphate selected from the group consisting of zinc and manganese phosphates, the P0 content of said solution being in the range of .5 to about 2.5%, and at least one oxidizing agent selected from the group consisting of 0.002% to 0.015% nitrite, 0.03 to 0.05% bromate, 0.02%0.15% sulfite, 0.050.25% sodium metal nitrobenzene sulfonate, 0.050.25% nitro methane, 0.050.25% nitro guanidine, 0.01%4% nitrate, 0.5- 4% chlorate and 0.01%-1% sodium ethylene diamine tetra-acetate, and the aluminum ion in an amount in the range of about 0.02 to about 5 grams per liter, said solution having a temperature in the range of room temperature to the boiling point of said solution.

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• Chemical Treatment Of Metals (AREA)

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Citations (6)

• 0
  • DE DENDAT1287890D patent/DE1287890B/de active Pending
• 1961
  • 1961-03-23 FR FR856583A patent/FR1293013A/fr not_active Expired
• 1962
  • 1962-01-22 GB GB2282/62A patent/GB943169A/en not_active Expired
  • 1962-01-22 CH CH75762A patent/CH398249A/fr unknown
  • 1962-01-22 BE BE612937A patent/BE612937A/fr unknown
  • 1962-02-02 ES ES0274253A patent/ES274253A1/es not_active Expired
  • 1962-03-12 US US179205A patent/US3146133A/en not_active Expired - Lifetime

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