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
  • C23C22/34—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
  • C23C22/37—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 hexavalent chromium compounds

Definitions

• a concomitant object is the provision of an aqueous acid chromate conversion coating solution which, over long periods of use, is capable of forming consistently high quality coatings on zinc surfaces.
• Yet another object of this invention is the provision of an aqueous acid chromate conversion coating solution which will essentially completely react with zinc surfaces thereby leaving no water soluble salts thereon after drying even without the benefit of a final rinse.
• the present invention is based upon the discovery that an aqueous acid solution consisting essentially of:
• substantially no alkali or alkaline earth cations is meant such cations which might be added deliberately as in prior art coating processes: This term is not intended to, and does not, imply that the normal levels of cations found in the usual commercial water supplies must be eliminated prior to use in the coating solutions of this invention.
• the chromate conversion coating solutions of this invention must be present in the chromate conversion coating solutions of this invention in an amount of at least 3.0 grams/liter. If the minimum amount of zinc is permitted to fall below 3.0 grams/litenthe zinc surfaces being treated will be subjected to an etching action with very little, if any coating being formed thereon. So far as an upper limit of zinc ion is concerned, there is no apparent limitation with respect to the coating ability of the solution. For example, where the amount of zinc present in the solution exceeds the stoichiometr-ic quantity required for forming hexavalent chromium salts thereof, such excess amounts of zinc will then become associated with other available anions in the coating solution, i.e., sulfate and complex fluoride ions.
• Zinc When the amount of Zinc exceeds the quantity required to become stoichiometrically associated with all of the anions present in the coating solution, then there occurs a precipitation believed to be zinc chromate (ZnCr0 upon the zinc surfaces being treated.
• this salt precipitation has not been found to be detrimental to coating quality, and is undesirable only from the viewpoint of excess chemical consumption.
• this cation may be added as the oxide, or as a salt of one of the essential anions, such as for example zinc sulfate, zinc dichromate or zinc fluosilicate.
• an important jconsideration tobe observed is that whatever salt of zinc is employed, the anionportion thereof must not be in any way detrimental to the coating reaction.
• Anions which have been found to be detrimental when addedto the coating solutions of this inven- .tion include chloride and simple fluoride.
• Theformer stituent be present in amounts ranging from 5.0 to 35 grams/liter. Where less than the minimum amount is employed, the corrosion resistant qualities of the subsequently produced coatings will be found to'be adversely affected.
• the upper limit of hexavalent chromium is equally critical, and should not exceed 35 grams/liter in order to avoid the occurrence of blistering under subsequently applied painted films after exposure of the metal to humid atmospheres.
• Hexavalent chromium ion is preferably added to the coating solutions of this invention in the form of chromic trioxide (CrO
• this coating anion may be introduced as a salt of zinc if desired.
• sulfate ion (calculated as 80,) which must be employed in the freshly prepared coating solutions of this invention, must, as noted hereinabove, be within the range of' 0.05 to 27 grams/liter. If the sulfate concentration falls below 0.05 gram/liter, any coatings which will be produced, if indeed any are produced at all, will be found to be very thin and of very low weight so that nouseful coatings can be obtained. Conversely, if the amount'of sulfate ion in the coating solution as initiallyfprepared is allowed to exceed 27 grams/liter,
• the impact test comprises dropping a inchball upon a test panel under a given inch-pound force. 'The deformed surface is then examined for loose or cracked paint and ratingsin inches of paint failure are reported.
• the discovery of the effect of the useof nitric 'acid is especially surprising in'view of applicants further discovery that nitric acid, .when used alone in place of the sulfuric acid, fails completely to produce the desired I coatings upon zinc surfaces.
• more or less of this constituent can be utilized.
• the upper limit of complex fluoride radical appears to be controlled solely by the limit of solubility" of the respective radical in the acid coating solution.
• the coating solutions of the present invention may be applied to zinc surfaces by any conventional means such 5 as spraying, dipping or roller coating. Roller coating is preferred since it has been found to be particularly adaptable to commercial strip line operations and provides consistently high quality results without the need of auxiliary equipment required in dip or spray operations.
• the resulting coating may be water rinsed and air dried preparatory to the application of a siccative film.
• the chromate conversion coating may be baked after application, whether or not a water rinse was utilized.
• a preferred operation consists in employing a water rinse followed by air drying, and the subsequent application of a siccative film. The painted metal surface is then baked to cure the decorative organic finish.
• Zinc surfaces which are to be coated by the process of this invention should be given a suitable cleaning pretreatment for removal of oils, greases, etc. which are frequently present on such surfaces.
• a suitable cleaning pretreatment for removal of oils, greases, etc. which are frequently present on such surfaces.
• This solution as prepared, had a pH of 2.03 and was applied to clean, galvanized steel panels using a 10 second spray application at 48 C.
• the coatings produced were iridescent, golden-brown in color, and the panels were subsequently painted with an acrylic paint and cured at 230 C. for three minutes. After 336 hours of salt spray testing (ASTM-B-117-61) the coated and painted panels were examined and found to contain only a trace of corrosion.
• solid, concentrated formulations of the essential coating constituents for purposes of making up and replenishing the coating solutions of this invention.
• These solid concentrates must contain from 3.5 to 10% of a zinc salt of a complex fluoride radical selected from the class above described, from 40 to 65 CrO or the equivalent amount of CrO in the form of a zinc salt thereof, and the balance to be supplied by a zinc salt selected from the class consisting essentially of the oxide and the sulfate.
• the solid, concentrated formulations may in some cases require the addition of sulfuric acid upon dissolution with water so as to provide the desired initial level of the essential sulfate ions.
• Example 11 This admixture is dissolved in suflicient dilute sulfuric acid to make a solution containing 5% by weight of the above composition having a pH of 1.25.
• An aqueous acid solution consisting essentially of:
• the solution of claim 1 which also contains nitrate 3.
• the solution of claim 1 which has a pH between 1.0 and 2.0.
• the zinc ion is supplied from the group consisting of zinc oxide and the sulfate, dichromate and fluosilicate salts of zinc and where in the zinc content is no greater than the quantity required to satisfy the stoichiometric amount needed fully to satisfy all of the anions present in the coating solution.
• a solid concentrate for use in preparingan aqueous acidcoating solution upon dissolution in water consisting essentially of (a) 3.5 to 10% of azinc salt of a complex fluoride radical selectedfrom the class consisting of Til- BF and SiF (b) from. 40 to CrO and (c) the balance zinc sulfate.

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• Chemical & Material Sciences (AREA)
• General Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Chemical Treatment Of Metals (AREA)

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Citations (4)

• 0
  • NL NL127354D patent/NL127354C/xx active
  • NL NL297873D patent/NL297873A/xx unknown
  • BE BE638461D patent/BE638461A/xx unknown
• 1963
  • 1963-07-22 US US296491A patent/US3130086A/en not_active Expired - Lifetime
  • 1963-08-21 GB GB33177/63A patent/GB1061436A/en not_active Expired
  • 1963-11-06 DE DE19631521657 patent/DE1521657B2/de active Pending

Patent Citations (4)

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