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

• the present invention relates to the art of coating aluminum and alloys thereof and concerns an improved solution which accelerates the rate of formation of adherent coatings thereon, which coatings improve their corrosion resistance and are suitable as a base for organic finishes such as paints, lacquers and the like.
• This invention also concerns an improved highspeed, highly efllcient process for coating aluminum and alloys thereof, which is easily controllable to produce an adherent coating of the desired appearance and thickness in a minimum of time.
• the solutions of this invention are aqueous acidic solutions of the type which contain the hexavalent chromium and fluoride ions.
• the improvement which characterizes this invention is based on the discovery that the addition to such solutions of certain quantities of the selenium or tellurium ion, or mixtures thereof, greatly increases the rate of coating formation and efficiency of coatings which have improved corrosion resistance and utility as a base for organic finishes.
• aqueous acidic chromic acid solutions containing the fluoride ion for use in coating aluminum and its alloys are known to form a coating simultaneously with the dissolving of a portion of the aluminum substrate, which dissolving aluminum remains in the bath and preferentially complexes with the fluoride ion to continuously reduce the free fluoride in the solution that is necessary to effect a coating reaction.
• the resulting coating is of limited thickness and with continued use the bath tends to load up with dissolved aluminum, or other reaction products, and reduces the coating ability of the solution unless other steps are taken to control the build-up of such reaction products.
• United States Patent 2,796,370 discloses that such a solution can be improved by incorporating therein a ferricyanide or ferr-ocyanide compound. It has been observed in operating such solutions containing a ferricyanide that they are temperature sensitive and when the bath temperature exceeds about 130 R, either the entire bath temperature or a localized portion thereof, that there is a reaction between chromic acid and the ferricyanide which causes the pH of the bath to rise numerically and the resulting coating weight and coating efliciency to be substantially reduced. Moreover, the coating obtained under these conditions has less corrosion resistance and utility as a base for organic finishes than when produced from a lower temperature bath, and the effect on the bath from the presence of the reaction product can sometimes be overcome by replenishment but in severe cases may require dumping of the bath.
• the primary object of this invention to provide an improved solution for forming adherent corrosion resistant coatings on the surfaces of aluminum and aluminum alloys.
• Another object of this invention is to provide a method for treating aluminum and its alloys to produce on their surface an adherent coating especially suitable as a base for organic finishes which is characterized by improved efliciency and rate of coating formation and which enables operation at temperatures above about F. without detrimentally affecting the quantity or quality of the resulting coating.
• an aqueous acidic solution including the hexavalent chromium ion, the. fluoride ion, and at least one anion selected from the group consisting of the selenium anions and the tellurium anions, in proportions which are capable of forming a coating on the surface of aluminum and its alloys.
• compositions of this invention may also include mineral acids such as sulfuric, hydrochloric or nitric and proportions of cations including sodium, ammonium, potassium, ferric, zinc, aluminum and trivalent chromium ions, so long as they are not present in quantities which more than offset the accelerating elfect of the selenium or tellurium anion which is present.
• mineral acids such as sulfuric, hydrochloric or nitric and proportions of cations including sodium, ammonium, potassium, ferric, zinc, aluminum and trivalent chromium ions, so long as they are not present in quantities which more than offset the accelerating elfect of the selenium or tellurium anion which is present.
• the hexavalent chromium ion is preferably introduced in the solutions as a chromium compound and may be supplied as chromic acid or one or more of the water soluble salts thereof such as the alkali metal chromates or dichromates or as an admixture of chromic acid and its salts.
• the fluoride ion may be supplied by any fluorine-containing compound which is capable of ionizing in the aqueous acidic solutions of this invention to provide the fluoride ion, including hydrofluoric acid and its salts, fluosilicic acid and its salts and fluoboric acid and its salts.
• the alkali metal salts of hydrofluoric, fluoboric and fluosilicic acids are the preferred forms of salts for this purpose but other metallic salts, including the ammonium and barium salts may be employed.
• the selenium or tellurium ion accelerator may be supplied in the solutions of this invention by adding to the aqueous chromic acid solution containing the fluoride ion any ionizable selenium or tellurium compound 01' selenium or tellurium compound oxidizable by chromic acid to a selenite, selenate, tellurite or tellurate, such as, for example, selenium dioxide, selenious acid, selenic acid or the alkali metal salts thereof, the sodium, potassium or ammonium salts of telluric and tellurous acids.
• aqueous chromic acid solution containing the fluoride ion any ionizable selenium or tellurium compound 01' selenium or tellurium compound oxidizable by chromic acid to a selenite, selenate, tellurite or tellurate, such as, for example, selenium dioxide, selenious acid, selenic acid or the alkali metal salt
• compositions of this invention include the hexavalent chromium ion, the fluoride ion and at least one accelerator anion in the quantities, or relative proportions which are set forth below in Formulation I, expressed in percent weight/ volume, therein, and throughout this specification and claims, unless otherwise indicated.
• the hexavalent chromium ion and the selenium anion concentrations are provided in terms of the quantities of Cr0 and Se0 respectively, which are present in the bath.
• the fluoride ion concentration represents the free or active fluoride ion which is present in the solution and which is available for effecting the coating-forming reaction with the aluminum and aluminum alloy substrate
• active fluoride ion represents the fluoride ion which is present in an amount in excess of that which is required to complex all of the aluminum ion which is present in the solution.
• Solution component percent w./v.
• Solution component percent w./v.
• the fluoride ion may be introduced in the form of hydrofluoric acid or its salts, and when supplied in this form, the coating solution preferably contains about 0.1 to about 3 grams of hydrofluoric acid per liter of solution.
• the coating solution preferably contains about 0.1 to about 7 grams of fluosilicic acid per liter.
• the coating solution preferably contains about 0.5 to about grams of fluobaric acid per liter.
• coating e'fliciency refers to the ratio of metal loss in milligrams per sq. ft. of surface divided by the coating weight in milligrams per sq. ft. The lower numbers indicate a higher efliciency since the coating is formed while dissolving a smaller quantity of aluminum from the surface, and this smaller quantity of dissolved aluminum decreases the difficulty of maintaining the coating solution in optimum coating forming condition.
• the coating solution As the coating solution is continued in operation, there is a gradual tendency for the aluminum and trivalent chromium ion concentrations to increase, although the quantity of these ions lost by drag-out ultimately equals the quantity of them being supplied as products of the coating reaction. It is satisfactory to offset the increasing concentration of aluminum and at least a portion of the trivalent chromium ion by replenishing the operating solution with a fluorine-bearing compound to insure an active fluoride ion concentration, as above 4 defined, within the numerical range above set forth in Formulations I or II.
• the solutions of this invention may be operated in conjunction with an ion exchange unit of the type and by the procedures generally described in Roy A. I-Ialversen, United States Patent 2,967,791, issued Jan. 10, 1961.
• the coating solutions of this invention are acidic and may contain acids in addition to chromic acid.
• the solutions may satisfactorily have a pH in the range of about 0.5 to about 2.5 and preferably about 1 to about 2.0.
• the pH range refers to measurements taken by using an electrical pH meter employing a glass electrode and a calomel electrode by immersing the electrodes in fresh portions of the solution and observing the indicated value until uniform readings are obtained in 30 seconds-1 minute time interval.
• the glass electrode is maintained in optimum condition by taking care to immediately rinse the electrode upon its removal from the solution being tested in a solution of five normal hydrochloric acid by inserting it in that solution for two to three minutes and thereafter rinsing the electrode in pure water. Between measurements the electrode is kept immersed in pure water and prior to use is checked against a standard buffer.
• nitric acid tends to increase the rate of coating formation and to increase the coating efficiency of an operating solution of this invention. Such improvement has been observed with concentrations of nitric acid of between about 0.05% and about 3% and is preferred for use where the fastest rate of coating formation is desired. At the lowest operating temperatures it is most advantageous to include the nitrate ion in the operating solution. When operating the solutions of this invention at temperatures between about F. and F. which are free of nitrate, it is possible to obtain coatings having a weight between 15 and 25 milligrams per sq. ft. in times as short as about 2 seconds, and when nitrate is added to these solutions, similar coating weights can be obtained in about 1 second of surface contact.
• pH of the operating solution has an important effect on the active fluoride content of a solution containing any given quantity of total fluoride.
• the fluoride which is present tends to form a greater proportion of aluminum fluoride complexes which are richer in fluoride than AlF such as AlF AlF and AlF
• This shift in equilibrium of the various aluminum fluoride complexes which are present removes fluoride from the solution which would otherwise be available for accelerating the attack of the substrate and coating formation.
• the solutions of this invention may be applied to the aluminum or aluminum alloy surface to be coated by dipping, brushing or spraying the solution on the surface, after conventional cleaning procedures have been employed to free the surface of oil, grease, oxide or the like.
• the coatings of this invention vary in appearance from irridescent to light-gold through yellow to brown as the coating weight increases. The color is also affected by the selenium anion concentration, for example, concentrations of selenium dioxide up to about 0.3% produce coatings that are yellow to 'brown as the coatings vary in coating weight from about 28 to 88 mg./sq. ft.; between 0.3% and 1% of SeO at coating weights of about 70 mg./ sq.
• the color is faded from the normal yellow to brown to only a faint yellow haze; between 1% and about 5% the coatings are reddish in color, approaching ,a deep brick red color, at coating weights between 47 and 58 mg./sq. ft.
• Example I A solution was prepared containing 0.5% CrO 0.35% fluoride, as hydrofluoric acid, 0.15% aluminum, 0.017% selenium dioxide.
• the selenium dioxide used was a commercial grade having the analysis 69% minimum selenium, 0.02% 0.03% tellurium, 0.01% -0.03% iron, 0.003%-0.01% copper and 1%4% water.
• the solution had a pH of 1.9 and a free acid of points.
• points refers to the number of ml. of 0.1 normal sodium hydroxide solution, which is required to titrate a 10 ml. sample of the solution to a brom cresol green end point.
• a solution was prepared containing 0.5 CrO 0.41% fluoride, as hydrofluoric acid, 0.25% nitrate ion, as HNO 0.2% aluminum and 0.05% SeO
• the solution had a pH of 1.6 and a free acid of 11 points.
• Coatings were formed on aluminum panels cleaned and processed as described in Example I at a plurality of temperatures, the coating weights and metal loss determined, as set forth in tabular form below.
• Example II has a lower ratio of fluoride to aluminum than Example I, and while this lower active fluoride produces somewhat less coating weight at comparable temperatures, the coating efficiency is substantially improved relative to Example I.
• Example III A aqueous solution was prepared containing 0.5% CrO 1.19% fluoride, as hydrofluoric acid, 0.54% aluminum, 0.35% CI, 1.3% N0 and 0.035% SeO The solution had a pH of 1.59 and a free acid of 9.5 points.
• Example IV A aqueous solution was prepared containing 0.35%
• Another portion was modified to contain 0.033% SeO and under the same conditions produced an 88 mg./ sq. ft. coating at a coating efiiciency of 0.22.
• Another portion was modified to contain 0.2% Se0 and operating under the same conditions produced a coating weight of 88 mg./ sq. ft. at a coating efliciency of 0.16, the coating having a characteristic yellow brown color.
• the SeO concentration was increased to 0.3% and under the same operating conditions a coating of 75 mg./sq. ft. was formed in 15 seconds but the coating was substantially colorless, with only a faint hint of yellow detectable by visual inspection.
• To additional portions of the solution quantities of Se0 were added to provide concentrations of 0.5 and 1% of SeO and the resultant coatings were approximately 70 mg./ sq. ft, and substantially colorless and comparable to those coatings produced from the bath containing 0.3% SeO
• Another portion of the bath was modified to contain 2% SeO and under similar application conditions produced a coating of 58 mg./sq. ft. at a coating efliciency of 0.20, which coating was reddish in color.
• Another portion of the bath was modified to contain 3% SeO and under the same coating conditions produced a coating having a weight of 56 mg./sq ft. at a coating efiiciency of 0.24, which coating had a color approaching brick red.
• Another portion of the solution was modified to contain 5% Se0 and under the same application conditions produced a coating having a weight of 47 mg./sq. ft. at a coating efliciency of 0.27 and the coating was brick red in color.
• Example VII An aqueous solution was prepared containing 0.90% CrO 0.15% aluminum and 0.33% fluoride, as hydrofluoric acid, the solution having a pH of 1.85 and a free acid of 3.6 points. A portion of this bath was modified to contain 0.03% TeO added as H TeO -2H O. This bath was heated to 120 F. and sprayed on clean 3003 aluminum panels, for 15 seconds, and upon inspection the surface was found to be uniformly coated with a characteristic yellow brown coating having a weight of 33 mg./sq. ft., at a coating efficiency of 0.88. Another portion of the bath was modified to contain 0.15% TeO and under the same application conditions produced a coating having a weight of 36 mg./ sq. ft. at a coating efficiency of 0.63.
• a method for forming a protective coating on the surface of aluminum and its alloys which comprises the steps of applying to said surface an aqueous acidic solution comprising the hexavale-nt chromium ion in an amount equivalent to about 0.05% to about 5% of OrO active fluoride ion in an amount of about 0.01% to about 0.7%, and at least one ion selected from the group consisting of the selenium ion and the tellurium ion in an amount equivalent to about 0.01% to about 5% of selenium dioxide.
• a method for forming a protective coating on the surface of aluminum and its alloys which comprises the steps of applying to said surface an aqueous acidic solution comprising the hexavalent chromium ion in an amount equivalent to about 0.2% to about 1.0% of CrO active fluoride ion in an amount of about 0.02% to about 0.5%, and at least one ion selected from the group consisting of the selenium ion and the tellurium ion in an amount equivalent to about 0.016% to about 0.5% of selenium dioxide.
• a method for forming a protective coating on the surface of aluminum and its alloys which comprises the step of treating said surface with an aqueous acidic solution comprising at least one chromic compound selected from the group consisting of CIO;, and the water soluble salts thereof, a bath soluble fluorine containing compound ionizable in said bath to provide the fluoride ion, and at least one accelerator ion selected from the group consisting of the selenium ion and the tellurium ion, said chromic compound being present in an amount of about 0.05% to about 5%, said fluorine compound being present in an amount in excess of that required to form AlFg with all of the aluminum ion present in said solution equivalent to about 0.1 to about 3 grams of hydrofluoric acid per liter of solution for the hydrofluoric acid and salts thereof, an amount equivalent to about 0.1'to about 7 grams of fluosilicic acid per liter of solution for the fiuosilicic acid and salts thereof, an amount equivalent to about 0.5 to about
• a method for forming a protective coating on the surface of aluminum and its alloys which comprises the step of treating said surface with an aqueous acidic solution comprising at least one chromic compound elected from the group consisting of CrO and the water soluble salts thereof, a bath soluble fluorine containing compound ionizable in said bath to provide the fluoride ion, and at least one accelerator ion selected from the group consisting of the selenium ion and the tellurium ion, said chromic compound being present in an amount of about 0.2% to about 1.0%, said fluorine compound being present in an amount in excess of that required to form AlF with all of the aluminum present in said solution equivalent to about 0.1 to about 3 grams of hydrofluoric acid per liter of solution for the hydrofluoric acid and salts thereof, an amount equivalent to about 0.1 to about 7 grams of fluosilicic acid per liter of solution for the fluosilicic acid and salts thereof, an amount equivalent to about 0.5 to about 15 grams of fluoboric acid per

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• 1963
  • 1963-07-11 US US294272A patent/US3329536A/en not_active Expired - Lifetime
• 1964
  • 1964-05-04 BE BE647491D patent/BE647491A/xx unknown
  • 1964-07-03 DE DEM61606A patent/DE1239165B/de active Pending
  • 1964-07-09 NL NL6407787A patent/NL6407787A/xx unknown
  • 1964-07-10 CH CH909564A patent/CH476858A/de not_active IP Right Cessation
  • 1964-07-10 GB GB28666/64A patent/GB1024926A/en not_active Expired
  • 1964-07-10 SE SE8507/64A patent/SE305106B/xx unknown

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