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 be included therein. It has been found that the presence of such cations, including ammonium ions, must be avoided if consistently satisfactory coatings are to be obtained, particularly where the coatings we dried upon the treated zinc surfaces without intermediate rinsing.
• 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 zinc ion (calculated as Zn), as noted hereinabove, 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/liter, the 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 lhnitation with respect to the coating ability of the solution.
• the amount of zinc present in the solution exceeds the stoichiometric 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., chloride and complex fluoride ions.
• the amount of zinc exceeds the quantity requ red to become stoichiometrically associated with all of the anions present in the coating solution; then there occurs a precipitation believed to be zinc chromate (ZnCrO 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 chloride, zinc dichromate or zinc fluosilicate.
• the anion portion resistance ASTM-B-117-61 thereof must not be in any Way detrimental to the coating reaction.
• Anions which have been found to be detrimental when added to the coating solutions of this invention include sulfate and simple fluoride. The former anion causes rapid loss of coating quality and weight with severe etching of the zinc articles being coated. The latter anion essentially completely prevents coating formation and severely etches the zinc surfaces being treated.
• hexavalent chromium ion concentration (calculated as Cr it is essential that this constituent be present in amounts ranging from 5.0 to 75 grams/liter. Where less than the minimum amount is employed, the corrosion resistance and paint adhesion qualities of the subsequently produced coatings will be found to be adversely affected. Conversely, the upper limit of hexavalent chromium is equally critical, and should not exceed 75 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.
• the amount of chloride ion (calculated as Cl) which must be employed in the freshly prepared coating solutions of this invention, must, as noted hereinabove, be within the range of 1 to 20 grams/liter. If the chloride concentration falls below 1 gram/liter, the coatings will be very low in weight, and will become invisible so that it becomes substantially impossible to distinguish coated from uncoated surfaces. If the amount of chloride ion in the coating solution as initially prepared is allowed to exceed 20 grams/liter, no useful coating will be obtained due to etching of the surfaces.
• the concentration of chloride ion may exceed 20 grams/liter so long as there are suflicient cations from the class consisting of Zinc and trivalent chromium to combine, stoichiornetrically, with the chloride ion, while simultaneously maintaining the required solution pH and ranges of other essential constituents as taught.
• one experimental bath has been found to contain upwards of 80 grams/liter of chloride ion and still produced satisfactory coating.
• nitric acid to be employed for this purpose should be not less than approximately 0.05% by weight.
• the limit is to be determined, of course, by the pH of the solution which, as above indicated, must be maintained between 0.57 and 3.0. In view of this the quantity of nitric acid which can be utilized must not be more than will lower the pH below 0.57.
• the impact test comprises dropping a inch ball upon a test panel under a given inch-pound force. The deformed surface is then examined for loose or cracked paint and ratings in inches of paint failure are reported.
• the discovery of the effect of the use of nitric acid' is especially surprising in view of applicant's subsequent'discovery that nitric acid, when used alone in place of the hydrochloric acid, fails completely to produce the tween the values of 0.57 and 3.0.
• salt spray corrosion resistance is the quality which presents the determining factor in any particular instance
• the quantity of fluotitanate should be limited to 15 grams/liter--otherwise, it is unnecessary to place an upper limit upon this ingredient because even larger quantities will produce unusually line results with respect to impact and bending resistance.
• the coating solutions of the present invention may be applied to zinc surfaces by any conventional means such 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.
• Example I An aqueous solution was prepared so as to contain:
• This solution as prepared had a pH of 1.2 and was applied to clean galvanized steel panels using a 10 second spray application at 48 C.
• the coatings produced were pale, golden brown in color, and the treated panels were subsequently painted with an acrylic paint and cured at 230 C. for three minutes. After 240 hours of salt spray testing (ASTM-B1176l) the coated and painted panels were examined and found to contain only a trace of corrosion.
• Example HI An aqueous solution was prepared containing:
• Example IV A solution was prepared containing:
• This solution was 1.5. This solution was used to treat a long succession of galvanized steel panels, with replenishment and pH adjustment using nitric acid, being effected, as required, until approximately 1000 ft. of surface area had been coated. The coating cycle was effected at 26 C. using a 30- second dip treatment to produce uniform, iridescent, golden-brown coatings on the galvanized steel. Samples of the coated galvanized panels were painted with an acrylic finish and cured at 230 C. for 3 minutes. Subsequent salt spray corrosion testing (ASTM-B-117-61) showed essentially no failure after 336 hours.
• Example V The solution of Example I was applied by means of roller coater apparatus to galvanized strip moving at a lineal speed of 20 feet/minute. Coating was effected at room temperature (24 C.), and the solution applied to the metal surface was allowed to remain thereon for 30 seconds before drying with externally applied heat. An acrylic paint was then applied to the strip and baking was effected at 232 C. Sections of the coated and painted strip were subjected to salt spray corrosion testing (ASTMB11761) and after 240 hours exposure essentially no corrosion was found on the metal surfaces.
• ASTMB11761 salt spray corrosion testing
• 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 chloride.
• the solid, concentrated formulations may in some cases require the addition of hydrochloric acid upon dissolution With water so as to provide the desired initial level of the essential chloride ions.
• Example VII This admixture is dissolved in suflicient dilute hydro chloric acid to make a solution containing by weight of the above composition having a. pH of 1.25.
• An aqueous acid solution consisting essentially of:
• the zinc ion is supplied from the group consisting of zinc oxide and the chloride, dichromate and fiuosilicate salts of zinc and wherein the chromium ion is supplied in the form of chromic trioxide (CrO 9.
• the complex fluoride radical is supplied by use of not more than 15 grams/liter of fiuotitanate (Til- 10.
• the complex fluoride radical is supplied from the group consisting of the respective acid and the zinc salt of the complex fluoride acid.
• the Zinc ion is supplied from the group consisting of zinc oxide and the chloride, dichromate and fluosilicate salts of zinc and wherein the zinc content is no greater than the quantity required to satisfy the stoichiometric amount needed to fully satisfy all of the anions present in the coating solution.
• said chloride ion concentration being no greater than that which is required, stoichiometrically, to combine with cations from the class consisting of zinc and trivalent chromium, said solution being substantially free of alkali and alkaline earth cations and being maintained at a pH of from 0.57 to 3.0 and the treatment being continued until "a chromate conversion coating is formed on the surface.
• a solid concentrate for use in preparing an aqueous acid coating solution upon dissolution in water said concentrate consisting essentially of:

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• Chemical Treatment Of Metals (AREA)

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Citations (4)

• 0
  • BE BE637718D patent/BE637718A/xx unknown
  • NL NL297691D patent/NL297691A/xx unknown
  • NL NL125963D patent/NL125963C/xx active
• 1963
  • 1963-06-04 US US285222A patent/US3130085A/en not_active Expired - Lifetime
  • 1963-08-21 GB GB33176/63A patent/GB1061435A/en not_active Expired
  • 1963-11-06 DE DE19631521656 patent/DE1521656B2/de active Pending

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