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 principal object of our invention is the provision of a method and solution for the treatment of aluminum surfaces whereby they may be rendered highly passive and corrosion resistant without discoloring or in-any way marring the surface and without using cumbersome electrolytic equipment.
• Another object of the invention is to provide a process of the character described which is so simple and effective that it can be used by relatively unskilled personnel and under a-wide variety of conditions-4n other words, .a process which is relatively uncritical as to the manner and means by which it is carriedout.
• Still another object ofthe invention is the provision of a process for effecting the foregoing objectives without the necessity of removing from .the surface of the aluminum any pre-formed naturally occurring oxide film which may be present.
• fluorine-bearing compound we mean a complex fluoride such as fluosilicic acid, fluoboric acid, fluozirconic acid, fluostannic acid,fiuotitanic acid and p rates Patent f 2,825,697 r Ce Patented :Mar. e4, 1958 the soluble saltstofany of :them. 'By the term .hexavalent chromium, we mean chromic acid (CrO or its soluble salts such as ammonium dichromate, potassium dichromate, sodium chromate, etc.
• the amounts of these ingredients present .in the solution is important. Of the fluorine-bearing compound there should be present in the bathbetween 0.1 and 1.0 gram per liter calculated as-fluorine. At concentrations of less than 0.1 gram per liter there is a marked falling off in the-elfect produced by the solution. At concentrations greater than -1 gram perliter there is atendency to produce a visible coating upon the aluminum.
• the quantity of hexavalent chromium in .the bath there should be from 0.4 gram to-4/gramsper liter calculated as CrO Atconcentrations ofless than 0.4 gram very little .effect is produced and when the concentration exceeds 4 grams .per liter there is ,a tendency ,for the solution to produce a visiblecoating on the aluminum.
• the pH of the treating solution is also important.
• the fluorine-bearingcompound and ;the hemvalent chromium should be present in the solution in such proportions and amounts as to yield a pH of from 2.0 to 4.0. At pHs below 2.0 there is a tendency for the solution to produce visible coatings on the aluminum and at pHs above 4.0 the surface does .not appear to become particularly passive.
• a pH as specified .it may be adjusted,,if too high, by the addition of a suitable .quantity .of some mineralacid such as nitric. acid ,or hydrochloricacid. on the other hand, if .thepH is too .low, it can be raised ,by proper additions ofan alkali such as sodium hydroxide, ammonium hydroxide, etc.
• the manner in .whichthe solution is broughtinto contact with the surface is also capableof considerbale variation .withoutin any way hindering the results to be ob tained.
• the article to be treated maybe immersed in the bath or the solution may be applied to the surface as a spray or it may be wiped with a cloth dampened with the solution or by any other means which insures through wetting of the surface.
• the solution is applied by some wiping expedient it is allowed to dry on the surface but regardless of these various factors we have learned by experience that best results are generally obtained if the following rule of thumb method of preparing the treating bath is employed.
• the concentration of fluorine-bearing compound When the concentration of fluorine-bearing compound is relatively low then the concentration of hexavalent chromium should also be relatively low and, conversely, when the concentration of fluorine-bearing compound is high the concentration of hexavalent chromium also should be relatively high.
• Another general rule which it is useful to observe is that where the solution is used hot we prefer a more dilute solution and, conversely, where the solution is to be applied at a relatively cool temperature,
• Example 1 Grams Chromic acid 0.46 Potassium zirconium fluoride 0.29 Water, to make 1 liter.
• Example 2 Grams Chromic acid 2 Potassium titanium fluoride 1.4 Water, to make 1 liter.
• Example 1 The material of Example 1 is ideally adapted to treat aluminum in an immersion process and we have secured excellent results by immersion for from one to two minutes with the temperature of the bath maintained at approximately 160 F. Following this, the article is removed, excess liquid is rinsed otf with ordinary tap water and the surface dried.
• the surface to be treated should be relatively clean although it is not necessary to remove any natural light oxide film which may be present unless such film is of a nature which is aesthetically unappealing or unsatisfactory, it being understood in this connection that the present invention is primarily concerned with a passivating procedure which leaves the surface substantially colorless or unchanged.
• naturally occurring oxide films are not deleterious, per se, which, of course, is a distinct advantage over other procedures heretofore employed which first require the removal of any natural oxide film before the treatment is applied.
• the amount of passivation and corrosion resistance secured by our invention seems to be markedly enhanced in situations where a naturally occurring oxide film is left on the surface at the time that our solution is applied thereto.
• Example 2 The material of Example 2 is ideally adapted for application to the surface by means of brush, rags, etc. and we have found that by using a material such as exemplified by Example 2, exceedingly good passivation is achieved if the solution is permitted to act upon the surface for from one to two minutes at room temperature although no harm is done if the material is permitted to dry upon the surface should this be desirable or convenient. However, if the solution is permitted to dry on the surface the aluminum may be stained somewhat due to the hexavalent chromium powder which will be found lying loose on the surface.
• Spraying procedures for applying the solution may also be employed but in such procedures as well as in the foregoing procedures length of time and the temperature of the bath do not seem to be too critical although from the standpoint of economy in cost reasonably short 4 periods of exposure to the effects of the solution and relatively low temperatures are to be preferred.
• the invisible passivation which we secure is obtained by utilizing what may be described as an activating solution which definition is employed because of the peculiar properties imparted to the aluminum surface by the process which we have developed.
• an activating solution which definition is employed because of the peculiar properties imparted to the aluminum surface by the process which we have developed.
• a previously deoxidized aluminum surface is treated according to our invention it will exhibit, immediately after the treatment, an extremely low electrical surface contact resistance. This bespeaks an activated surface toward oxidation.
• the electric contact resistance of a surface treated in accordance with our invention rapidly increases after a short time lapse and at a much greater rate than it does on a deoxidized but otherwise untreated aluminum surface.
• the method of increasing the corrosion resistance of an aluminum surface without changing its appearance which consists in treating the surface with an aqueous acid solution consisting essentially of hexavalent chromium and a soluble complex fluorine-bearing compound, the amount of hexavalent chromium being from 0.4 gram to 4 grams per liter calculated as CrO and the amount of fluorine-bearing compound being from 0.1 gram to 1.0 gram per liter calculated as fluorine and the pH of the solution being from 2.0 to 4.0.
• hexavalent chromium is chosen from the group consisting of chromic acid (Cr0 and its soluble salts and the fluorine-bearing compound is chosen from the group which consists of fluosilicic acid, fluoboric acid, fiuozirconic acid, fluostannic acid, fiuotitanic acid and the soluble salts thereof.

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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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Cited By (16)

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Citations (10)

• 0
  • BE BE538540D patent/BE538540A/xx unknown
• 1954
  • 1954-08-05 US US448164A patent/US2825697A/en not_active Expired - Lifetime
• 1955
  • 1955-01-21 DE DEA21949A patent/DE973261C/de not_active Expired
  • 1955-02-17 GB GB4812/55A patent/GB767888A/en not_active Expired
  • 1955-02-28 FR FR1119649D patent/FR1119649A/fr not_active Expired

Patent Citations (10)

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