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
  • C23C22/78—Pretreatment of the material to be coated

Definitions

• This invention relates to th coatings on zinc surfaces an said surfaces such as ai s amels, lacquers, etc.
• an said surfaces such as ai s amels, lacquers, etc.
• it has become customary to subject the sheet or strip to a treatment usually at the mill, to protect the surface and prevent the formation of white corrosion products in the interim between production of the sheet and the ultimate finishing of its surface in its intended end use.
• a variety of procditferent compositions have been proposed as satisfactory for use in such processes.
• the mill treatment process involves the use of an aqueous solution contalning chromic acid in combination with any of a number of other activator ingredients, such as chlorides, sulfates, fluorides, phosphates, complex fluorides borates, etc., to form a protective coating on the surfacei
• activator ingredients such as chlorides, sulfates, fluorides, phosphates, complex fluorides borates, etc.
• It .18 therefore the primary object of this invention to provide an Improved process for forming adherent cor- IOSlOIl resistant phosphate coatings on zinc and zinc alloy surfaces.
• Another object of this invention is to provide an improved process for conditioning the surfaces of zinc, electroplated zinc, galvanized zinc, hot dip zinc, electro-galvanlzed zinc and zinc alloy surfaces, to receive a phos phate coating.
• a further object of this invention is to provide solutrons for and a process of conditioning zinc and zinc alloy surfaces which have been coated with a mill treat- 7 ment coating so that the surface is rendered receptive to an adherent phosphate coating, and forming phosphate coatings on such pre-conditioned surfaces.
• any zinc or zinc alloy surface is rendered more receptive to the formation of an adherent phosphate coating thereon if it is preliminarily contacted with an aqueous alkaline solution modified to contain an effective amount of an oxidizing agent.
• the process of this invention is beneficial in pre-conditioning the surfaces of pure zinc, electroplated zinc, galvanized substrates such as iron, steel, etc., hot dip zinc surfaces including hot dip zinc containing small quantities of alloying ingredients such as aluminum, etc., and zinc alloys per se or electrodeposited, galvanized or hot dip zinc alloy surfaces. While benefit is obtained on any such zinc-containing surface irrespective of the presence thereon of a hexavalent chromium-containing coating, the greatest benefit Which results from the use of the process of this invention is obtained when the surface has been treated with a corrosion-inhibiting solution, usually containing hexavalent chromium.
• aqueous alkaline solutions which are effective for the purpose of this invention are effective because of the presence therein of the oxidizing agent.
• Numerous tests have demonstrated that unmodified aqueous alkaline solutions, regardless of degree of alkalinity and method of use including application at boiling temperature, are incapable of conditioning azinc surface to receive a phosphate coating Where that zinc surface is one which has been given a metallic chromate-containing corrosioninhibiting coating at the mill. It has been. found that because of the varied nature of the coatings applied at the mill the surfaces vary in degree of difficulty of conversion to a phosphate coating receptive condition, and the process should be varied accordingly in order to obtain the best results.
• the aqueous alkaline solution must contain a certain minimum content of oxidizing agent.
• the minimum content which is effective is an amount of oxidizing agent having the oxidizing ability of about 0.05 gram per liter of the permanganate ion in an aqueous solution having a pH numerically above 7.
• oxidizing agents have been found to be satisfactory for this purpose, including both organic and inorganic types. Somewhat better results: have been obtained from strong, powerful inorganic oxidizing agents and they are preferred. Good results have been obtained from the use of one or more ions selected from the group consisting of the permanganates, perborates, peroxides,
• chlorates, bromates, iodates, perchlorates, nitrites and hypochlorites and of this group permanganate is preferred.
• These ions are conveniently introduced into the aqueous alkaline solution in the form of a metallic salt, and preferably as the alkali metal salt. While the peroxides are also effective, they are somewhat less desirable because they are difiicult to control at the desired concentration in continuous operations.
• suitable organic oxidizing agents include picric acid, sodium metanitro-benzene sulfonate, o-nitro-chlorobenzene -p-sulfonic acid and m-nitrobenzoic acid, with picric acid giving the best results in a short processing time.
• the alkalinity of the solution may be derived from the use of any of the common alkaline salts such as the alkali metal hydroxides, carbonates, phosphates, borates, silicates, polyphosphates, pyrophosphates, mixtures thereof and the like.
• Weak alkaline solutions that is, solutions having a pH in the range of 7 to 9 give satisfactory results but require more time at a given temperature than stronger alkaline solutions and solutions having a pH numerically above 9 are preferred.
• an amount of permanganate ion in the range of about 0.2 g./l. to about 30 g./l. is adequate and provides the best overall results.
• Similar results are obtained by increasing the concentration of oxidizing agent when the concentration of the alkaline material is decreased.
• a solution containing 0.1 gram per liter sodium hydroxide was effective to condition mill treated zinc strip when the concentration of potassium permanganate was increased to 10 grams per liter and the solution was applied at 170 F. for two minutes.
• aqueous solution containing 7-10 grams per liter of sodium hydroxide mill treated zinc is conditioned to receive an adherent zinc phosphate coating when the solution contains 0.12 gram per liter of potassium permanganate and is applied for one minute at 170 F.
• Substantially equivalent oxidizing ability is obtained when the permanganate is replaced by 7.5 grams per liter of sodium bromate, grams per liter of picric acid, and 2.5 grams per liter of sodium meta nitrobenzene sulfonate.
• a solution containing 50 grams per liter sodium hydroxide and grams per liter of sodium chlorate or 10 grams per liter of sodium nitrite also had approximately equivalent oxidizing ability.
• Hydrogen peroxide is more effective than the chlorates or nitrites and can be used in a quantity of 2 to 3 times the permanganate concentrations.
• the iodates, perborates, perchlorates and hypochlorites are slightly stronger than the nitrites but are weaker than the bromates and are satisfactory at concentrations intermediate those given for bromates and nitrites.
• the upper limit of oxidizing agent is not critical and no harm apparently results from the use of concentrations up to the solubility limit although no improvement is obtained from the excess over that required to effect conditioning, as above explained.
• the oxidizing agent-containing cleaning solutions of this invention may also include conventional surface active agents of the non ionic, anionic or cationic types which are stable in the solutions, if desired.
• the presence of such a surface active agent has been found to be advantageous, particularly in cases in which the surface to be coated is slightly soiled with finger-prints, oils, greases or the like.
• the complete process of this invention comprises the steps of applying the above defined solutions of this invention to the surface to be conditioned by spraying,
• the contact time may vary from about /2 minute to 2 minutes in the ordinary case, but longer contact time is not detrimental to the improved receptivity for phosphate coatings which is imparted to the surface and contact times up to 30 minutes may be employed.
• the surface to be treated, if dirty, or otherwise contaminated, is preferably preliminarily cleaned with conventional cleaners to remove grease, finger-prints and the like before the application of the conditioning solutions of this invention. After removal of the surface from the conditioning solution, it is rinsed and thereafter may be immediately contacted with an aqueous acidic phosphate coating solution.
• any of the phosphate coating solutions conventionally employed to form phosphate coatings on zinc surfaces may be used, such as for example, those disclosed in U.S. Patents 2,813,812 and 2,835,617.
• the preconditioned surface may be subjected to another conventional conditioning step such as that described in U.S. Patents 2,310,239 or 2,874,081.
• This treatment comprises contacting the surface with an aqueous solution of disodium phosphate, sodium tri-polyphosphate or tetrasodium pyrophosphate and a titanium containing compound such as titanyl sulfate. After Withdrawal from this solution, it is ready .for contact with the phosphate coating solution.
• the surface may be subjected to a conventional aqueous dilute chromic acid rinse, if desired.
• a conventional aqueous dilute chromic acid rinse if desired.
• Example I An aqueous solution was prepared containing 11.2 grams per liter of sodium metasilicate, 11.2 grams per liter of tetra sodium pyrophosphate and 10.1 grams per liter of potassium permanganate. The solution was placed in a conventional machine for spray-washing of metal parts and the temperature of the solution was raised to and maintained between about 160 F. and F.
• the adhesion test comprised manually drawing a knife blade across the surface of the coating and comparing the resistance to dislodgment of the coating with no dislodgment being rated and the difficulty of dislodgment being evaluated numerically from 1-5, with being extremely easy dislodgment or peeling.
• the impact test involves forming about a A deep depression in the coated surface by pressure from a /2 diameter ball, with the pressure being supplied by dropping a weight or hammer on the ball positioned on the surface.
• the height of the weight drop and the weight itself which is dropped are varied to produce the approximate A depression in the particular gauge metal being used.
• 24 gauge metal for example, a 3 /2 lb. weight dropped 12" forms the desired impression.
• the internal surface of the concavity and the convex surface of the impression are inspected for coating condition and the condition is reported as flaking or fracture measured in eighths of inches from the crown of the impression.
• the mill treated hexavalent chromium-containing coated panels had a, rating of 1 on the adhesion test.
• the panels which were free of mill treatment coating also rated 1 on the adhesion test.
• the control samples were compared and found to have an adhesion rating of 5, both for the mill treated panel and the panel which was untreated at the mill.
• the bend test showed that the mill treated panel was flaked for A3" and fractured for Ms from the small end of the mandrel, while the untreated panel was flaked for /8 and fractured for 7s.
• the non mill-treated control sample was flaked for a length of 27s, while the milltreated panel coating was peeled from the surface for a length of 4".
• the painted panels were diagonally scratched from corner to corner and subjected to a 5% salt spray test and inspected after 476 hours.
• the salt spray results are reported in terms of 16ths of an inch creepage of the corrosion from the scratch mark as a range extending from the minimum creepage to the maximum creepage.
• the non mill-treated panels had a rating of 0-2, while the milltreated panels had a rating of 0-1.5.
• the non mill-treated control sample was 40% peeled after 72 hours and 100% peeled after 140 hours, at which time it was withdrawn from the test.
• Example II An aqueous solution was prepared containing 0.1 gram per liter of sodium hydroxide and 10 grams per liter of potassium permanganate. The solution had a pH of approximately 13. With the solution at a temperature of 168 F.-l70 F., mill-treated hexavalent chromiumcontaining coated zinc panels, Armco Zincgrip, were immersed in the bath for 1 minute, rinsed, phosphate coated, water rinsed and dried. The phosphate coating was found to be insufficiently adherent. Additional panels were processed in the same solution, in the same manner for two minutes and the resulting phosphate coating was found to be satisfactorily adherent.
• Example III An aqueous solution was prepared containing 1 gram per liter of sodium hydroxide and 10 grams per liter of potassium permanganate. This solution had a pH of approximately 13. Mill-treated hexavalent chromiumcontaining coated zinc panels were immersed in the solution at 168 F.-l70 F. for 1 minute, withdrawn, rinsed, phosphate-coated, water rinsed and dried. The phosphate coating was found to be tight and satisfactorily adherent.
• Example IV An aqueous solution was prepared containing 2.5 grams per liter of sodium hydroxide and 0.25 gram per liter of potassium permanganate. The zinc panels were treated in this solution for /2 minute, in accordance with the procedure of the above examples, and the coating was insufficiently adherent, but when the treating time was increased to 1 minute an adherent phosphate coating was obtained.
• Zinc panels of the type above identified processed in this solution in accordance with the above specified steps were coated with phosphate coatings having conventional appearance and moderate adherence, but they were less adherent than the coatings obtained in 1 minute in the solution containing 0.25 gram per liter of potassium permanganate.
• Example VI A series of aqueous solutions were prepared containing 2.5 grams per liter of sodium hydroxide and sodium nitrite in concentrations of 10 grams per liter, grams per liter, grams per liter, grams per liter and 60 grams per liter. None of these solutions satisfactorily conditioned mill-treated hexavalent chromium containing coated zinc panels to a phosphate coating receptive condition.
• Example VII A series of aqueous solutions were prepared containing 2.5 grams per liter of sodium hydroxide and sodium chlorate concentrations of 2.5 grams per liter, 12.5 grams per liter, 22.5 grams per liter, and 60 grams per liter. Using the processing conditions specified in Example I, and a contact time of 1 minute, no satisfactory adherent phosphate coatings were produced. When the sodium hydroxide concentration was increased to 50 grams per liter in the solution containing 10 grams per liter of sodium chlorate, satisfactory adherent phosphate coatings were produced on surfaces immersed in the solution for 1 minute.
• Example VIII An aqueous solution was prepared containing 2.5 grams per liter sodium hydroxide and 2.5 grams per liter sodium bromate. Another solution wa prepared containing 7.5 sodium hydroxide and 7 .5 sodium bromate. Zinc panels of the above specified type immersed for 1 minute in the solution containing 2.5 grams per liter of sodium bromate failed to produce satisfactory adherent phosphate coatings, whereas, adherent phosphate coatings were produced after the use of 7.5 grams per liter of sodium bromate for 1' minute prior to phosphate coating.
• Example IX An aqueous solution was prepared containing 10 grams per liter of caustic soda and 2.5 grams per liter of sodiurn meta nitrobenzene sulfonate. Zinc panels immersed for 1 minute in this solution at 168 F.170 F. Were coated with an adherent phosphate coating when subjected to the phosphate coating and chromic acid rinsing steps as specified above in Example I.
• Example An aqueous solution was prepared containing 10 grains per liter of sodium hydroxide and 3.75 grams per liter of picric acid. Zinc panels immersed for 5 minutes in this solution at 168 F.-170 F. were coated with an adherent phosphate coating When subjected to the phosphate coating and chromic acid rinsing steps as specified above in Example I.
• a method for conditioning zinc and zinc alloy surfaces to receive a phosphate coating which comprises the from the presence therein of about 0.2 gram per liter to about 3 0 grams per liter of the permanganate ion.
• a method for conditioning zinc and zinc alloy surfaces to receive a phosphate coating which comprises the steps of applying to said surface an aqueous alkaline solution having a pH of about 9 to about 14 and containing an alkalinity equivalent to that produced by the presence therein of at least about 0.1 gram per liter of sodium hydroxide, said solution having dissolved therein an oxidizing agent, said solution having at least the oxidizing ability equivalent to that obtained from the presence therein of about 0.05 gram per liter of permanganate ion.
• said oxidizing agent is selected from the group consisting of the permanganates, perborates, chlorates, perchlorates, peroxides, bromates, iodates, nitrites and hypochlorites.
• a method for conditioning zinc and zinc alloys having on the surface thereof a corrosion inhibiting coating comprising the steps of applying to said surface an aqueous alkaline solution having at least the alkalinity equivalent to that resulting from the presence therein of about 0.1 gram per liter of an alkali metal hydroxide, and containing an oxidizing agent in an amount to give to said solution at least the oxidizing ability equivalent to that derived from the presence therein of about 0.2 gram per liter to about 30 grams per liter of the permanganate ion.
• a method for forming an adherent phosphate coatmg on zinc and zinc alloy surfaces which comprises the steps of applying to said surfaces an aqueous alkaline solution containing an oxidizing agent, said solution having at least the oxidizing ability equivalent to that of a solution having dissolved therein at least about 0.05 gram per liter of a permanganate ion, and thereafter Contacting the said surface with an aqueous acidic phosphate coating solution so as to form thereon an adherent phosphate coating.
• a method for forming an adherent phosphate coating on zinc and zinc alloy surfaces, which surfaces are coated with a corrosion inhibiting hexavalent chromiumcontaining coating which comprises the steps of applying to said surface an aqueous alkaline solution having at least the alkalinity equivalent to that resulting from the presence therein of about 0.1 gram per liter of an alkali metal hydroxide and containing dissolved therein an oxidizing agent in an amount sutficient to produce at least the oxidizing ability equivalent to that of a solution having dissolved therein about 0.2 gram per liter to about 30 grams per liter of the permanganate ion, and thereafter contacting said surface with an aqueous acidic phosphate coating solution so as to form thereon an adherent phosphate coating.

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• 1960
  • 1960-12-14 US US75660A patent/US3046165A/en not_active Expired - Lifetime
• 1961
  • 1961-10-17 FR FR876219A patent/FR1303904A/fr not_active Expired
  • 1961-10-23 BE BE609483A patent/BE609483A/fr unknown
  • 1961-12-08 DE DEM51123A patent/DE1200641B/de active Pending
  • 1961-12-13 GB GB44641/61A patent/GB949734A/en not_active Expired

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