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
  • C23C22/60—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 alkaline aqueous solutions with pH greater than 8
  • C23C22/66—Treatment of aluminium or alloys based thereon

Definitions

• ABSTRACT is for a process for providing an oxide coating on aluminum which process uses chemical means only while avoiding the use of an electric current as is required in an anodizing process. Absent conventional steps such as water rinses and the like, the process comprises cleaning, including desmutting of an aluminum part to the extent necessary, and treating with an aqueous alkaline solution of a ferricyanide compound for a time sufficient to oxidize the aluminum part.
• the treatment with the ferricyanide solution is considered to be a chemical oxidizing step similar to the formation of an oxide film electrically in an anodizing process, but lower in cost and having other advantages.
• the oxidized aluminum surface is corrosion and heat resistant, acts as a base for various coating and is readily dyed to produce brilliant colors.
• anodizing is one of the most widely used methods for oxidizing aluminum, it has drawbacks, one of which being the high cost of the process. This is due to the requirement for special equipment associated with passing a current through a solution. Thus, electrodes, a rectifier and special lead lined or stainless steel tanks are required. In addition, the anodizing process requires low temperatures i.e., from about 70 to 75F for soft anodizing and from to F for hard anodizing. Since formation of the oxide coating is an exothermic reaction, special refrigeration equipment is required to operate the process. Further, it is necessary to air oxidize the solution during the anodizing process.
• the subject invention provides materials and a process for oxidizing aluminum to provide corrosion and heat resistant coatings which are readily dyed and receptive to various finishes.
• the process comprises cleaning and desmutting an aluminum part to the extent required and contacting the aluminum part with an aqueous alkaline oxidizing solution of a ferricyanide compound for a time sufficient to oxidize the surface of the aluminum part.
• the oxidized aluminum part may then be dyed with a suitable dye to effect a desired colored coating or coated with any other suitable finish such as paint, a lacquer or the like.
• the critical step is considered to be the treatment with the aqueous alkaline ferricyanide solution which is considered to be a chemical oxidizing step similar to the formation of the oxide film'electrically in an anodizing process.
• the process of the subject invention overcomes the disadvantages noted above with respect both to anodizing and to chemical treatment in accordance with US. Pat. No. 2,796,371.
• anodizing expensive electrical equipment and special tanks are not required as the treatment process is completely chemi cal in nature.
• the process of this invention is an improvement thereover in that the oxide coatings are harder and possess better corrosion and heat resistant properties.
• the color imparted is more uniform, brighter and substantially more desirable than those of the aforesaid chemical process.
• the part Prior to treating an aluminum part with the oxidizing formulation of this invention, the part is preferably cleaned to the extent necessary. Cleaning can be a combination of steps comprising solvent degreasing, preferably with a hydrocarbon solvent such as benzene to remove grease and oil or by soak cleaning. Thereafter, the part may be etched with a mild acid or alkaline cleaner to remove dirt, oxides and other contamination but not so as to form visible surface irregularities. Finally, if necessary, the part may be treated with a conventional desmutter to remove any residues that may be left on the surface of the part. Such a desmutter could comprise a mixture of dilute nitric and hydrofluoric acid. Intermediate between each of the aforesaid steps would be a water rinse. The entire process of cleaning is conventional in the art and not considered to be a part of this invention.
• the next step in the process comprises formation of an oxide film chemically, which film is considered to be similar in function to the oxide film formed electrochemically in the conventional anodizing process.
• the chemical oxidant used is an aqueous, alkaline solution of a ferricyanide compound, preferably an alkali or alkaline earth ferricyanide, though ferricyanide compounds having cations other than alkali and alkaline earth metal cations are suitable provided such cations do not deposit on the aluminum part being treated.
• concentration of the ferricyanide compound in solution is not critical, from 0.01 moles per liter to saturation being a satisfactory range. However, at the low concentration of 0.01 moles per liter, the process is impractical because of the long treatment time required.
• a preferred range for the ferricyanide compound is from about 0.1 to 0.5 moles per liter and most preferably, from about 0.20 to 0.30 moles per liter.
• the oxidizing solution is an aqueous, alkaline solution with pH adjusted to between about and 13, preferably between 1 1.2 and 12.0 and more preferably between 11.5 and 11.8.
• a pH adjustor is preferably used such as a hydroxide, a carbonate or a tribasic phosphate.
• the alkali and. alkaline earth metal hydroxides and carbonates are suitable, potassium carbonate, sodium hydroxide and trisodium phsophate being the most preferred materials.
• the amount of pH adjustor used is that amount that gives the desired pH. Using potassium carbonate as an example, from about 0.1 to about 0.5 moles per liter of solution provides the desired pH.
• an inhibitor for aluminum in the oxidizing solution is also desirable to include an inhibitor for aluminum in the oxidizing solution to prevent darkening of the oxide coating and to produce brighter color if the aluminum part is subsequently contacted with a dye during the processing sequence.
• a suitable inhibitor is a tribasic phosphate such as tribasic sodium phsophate and tribasic potassium phosphate.
• the amount of the tribasic phosphate is not critical, small amounts providing some benefit with larger amounts providing greater benefit. In general, the concentration may be as low as 0.01 moles per liter to sautration or even in excess of saturation.
• a preferred range for the tribasic phosphate is from 0.04 to 0.50 moles per liter.
• a preferred formulation in accordance with the invention is as follows:
• the oxidizing solution of this invention may be used over a wide temperature range, but preferably'is used below its boiling point within a temperature range of from about 70 to 100F and preferably from about 75 to 90F.
• the time of contact of an aluminum part with the oxidizing solution is likewise not critical, periods of time ranging from 1 to 60 minutes being suitable and from 5 to minutes being typical.
• those aluminum alloys containing a high copper content require a shorter treatment than those with a low copper content. It should be understood that there is a relationship between. concentration of ingredients in the oxidizing solution, temperature and time; more concentrated solutions or higher temperatures or a combination of the two resulting in shorter treatment time.
• the aluminum part is rinsed and may be coated with a finish such as those disclosed in U.S. Pat. No. 2,796,371 incorporated herein by reference, or a solution of a colorant which may be either an organic dye or even an inorganic pigment.
• a finish such as those disclosed in U.S. Pat. No. 2,796,371 incorporated herein by reference, or a solution of a colorant which may be either an organic dye or even an inorganic pigment.
• a colorant which may be either an organic dye or even an inorganic pigment.
• Many of the colorants that may be used are those conventionally usd in anodizing. Typical of such dyes are the following, set forth for purposes of example only:
• the parts treated with the alkaline solution of the ferricyanide may be colored in accordance with prior art anodizing treatment procedures.
• dye concentra tion, treatment temperature and time are conventional, temperatures of from to 150F being appropriate with treatment time ranging from about k to 30 minutes.
• the part may be sealed if desired, using the conventional sealing step of immersion of the colored part in a solution such as nickel acetate or sodium dichromate or any other conventional material in accordance with art-recognized procedures.
• a solution such as nickel acetate or sodium dichromate or any other conventional material in accordance with art-recognized procedures.
• EXAMPLE 1 An aluminum panel of No. 6061 alloy measuring 2 X 4 X 0.016 inch is degreased by soaking for five minutes in a conventional non-etching aluminum soak cleaner made up at 60 grams per liter and maintained at 150F. The panel is then removed, water rinsed, and next immersed in a conventional mild alkaline etching cleaner consisting of 55 grams of cleaner (Clepo No. 30R) dissolved in one liter of water. The cleaning bath is maintained at about 150F. The panel is removed after about one minute treatment in the bath and rinsed in cold water. The clean panel is then immersed in a 10 percent nitric acid solution to desmut the same and provide a clean surface. A treatment time of one-half minute is used.
• the clean panel is rinsed with cold water and immersed in a solution comprising of 80 grams of potassium ferricyanide, 40 grams of potassium carbonate, 40 grams of trisodium phosphate and water to one liter.
• the pH of the solution is maintained at about 12.0, the temperature of the solution is held at about 75F and immersion time is about 20 minutes. Thereafter the panel is removed and rinsed with water.
• the panel, having an oxide coating, is then dyed by immersion for two minutes in a dye bath maintained at about F consisting of 4 grams of Chromoxane Pure Blue BA (CI. 43830) in one liter of water.
• the pH of the dye is adjusted to between 6.0 and 7.0.
• the dyed panel is rinsed with water and sealed in a solution containing 50 grams of sodium dichromate dissolved in one liter of water with the pH maintained at about 5.9. The time of sealing is 15 minutes and the temperature of the sealing bath is maintained at about 210F. The panel is then rinsed with water, dried in the air, and buffed by hand. It has a uniform dark blue coloration and has good wear and corrosion resisting properties.
• EXAMPLE 3 Follwing the procedure of example 1, the following aluminum alloys were dyed: 7075, 1100, 6061, 6063, 3003, 5052, 2024, 2021, 2017 and 2014 and cast alloys E40 and 356. In all cases, the panel is uniformly colored and has good wear and corrosion resistance.
• EXAMPLE 4 The procedure of example 1 is repeated, but the trisodium phosphate is omitted from the oxidizing formulation. The results are similar to those of example 1 except that the colored surface is dark and not as brilliant.
• the present invention provides a process for oxidizing aluminum and for coloring aluminum which is low in cost; does not require electrical equipment; and can be applied to small parts such as pins, bolts and the like.
• the colorant appears to be uniformly distributed throughout the oxide coating and is adherent to the aluminum part.
• the resulting colored aluminum and aluminum alloys have a deep shade and are more attractive than coatings that have heretofore been obtainable on chemically-treated aluminum or aluminum alloy surfaces.
• a process for forming an oxide coating on aluminum and its alloys which coating is corrosion and heat resistant, is receptive to coating and readily dyed comprising the steps of cleaning the aluminum to the extent necessary and formation of said oxide coating using chemical means consisting of contacting the surface of said aluminum with an aqueous, alkaline solution of a ferricyanide salt, said solution having a pH between about and 13 and said ferricyanide salt being present in solution in an amount of at least 0.01 moles per liter of solution.
• ferricyanide solution contains a pH adjustor in an amount sufficient to provide the required pH and is selected from the group consisting of alkali and alkaline earth metal carbonates, hydroxides and tri-basic phsophates.
• ferricyanide solution contains an inhibitor for aluminum that prevents darkening of the oxide coating.
• a process for coloring aluminum and its alloys including the steps of cleaning the aluminum to the extent necessary, oxidizing the aluminum and contacting the aluminum with a colorant, the improvement comprising formation of said oxide coating by chemical means consisting of contacting the surface of said aluminum with an aqueous, alkaline solution of a ferricyanide salt, said solution consisting of at least 0.01 moles per liter of a ferricyanide, a pH adjustor in an amount sufficient to provide required pH and an inhibitor for aluminum to prevent darkening of the oxide coating, said aqueous, alkaline solution of the ferricyanide salt having a pH varying between about 10 and 13.
• ferricyanide salt is an alkali or alkaline earth metal ferricyanide.
• aqueous, alkaline solution of the ferricyanide comprises potassium ferricyanide in an amount from 0.1 to moles moles per liter, potassium carbonate in an amount from 0.1 to 0.5 moles per liter, trisodium phosphate in an amount from 0.04 to 0.50 moles per liter and water to 1 liter.
• An aqueous solution for oxidizing aluminum and its alloys in a process for coloring aluminum comprising a ferricyanide salt in an amount of at least 0.01 moles per liter, a pH adjustor in an amount sufficient to provide an alkaline solution having a pH varying betweenabout 10 and 13, and an inhibitor to prevent darkening of the oxide coating, said inhibitor being present in an amount of at least 0.01 moles per liter of solution.
• the solutioh of claim 21 having the following composition: alkali or alkaline earth metal ferricyanide 0.1 0.5 moles per liter alkali metal carbonate tribasic phosphate water UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENTNO: ,765,952 DATED October 16, 1973 INVE I Bertha S. Tuttle and Jekabs OZolins It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

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Citations (2)

• 1971
  • 1971-08-10 US US00170668A patent/US3765952A/en not_active Expired - Lifetime
• 1972
  • 1972-07-10 FR FR7224870A patent/FR2148423B1/fr not_active Expired
  • 1972-07-17 IT IT27087/72A patent/IT962921B/it active
  • 1972-07-18 CA CA147,423A patent/CA975265A/en not_active Expired
  • 1972-07-25 CH CH1105972A patent/CH564610A5/xx not_active IP Right Cessation
  • 1972-08-04 AU AU45291/72A patent/AU470202B2/en not_active Expired
  • 1972-08-07 GB GB3671772A patent/GB1363041A/en not_active Expired
  • 1972-08-07 SE SE7210265A patent/SE384539B/xx unknown
  • 1972-08-09 DE DE2239255A patent/DE2239255C3/de not_active Expired
  • 1972-08-09 DE DE2264970*A patent/DE2264970A1/de not_active Ceased
  • 1972-08-10 JP JP47079559A patent/JPS526255B2/ja not_active Expired

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