Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/04—Anodisation of aluminium or alloys based thereon
  • C25D11/18—After-treatment, e.g. pore-sealing
  • C25D11/20—Electrolytic after-treatment
  • C25D11/22—Electrolytic after-treatment for colouring layers

Definitions

• ABSTRACT In the process for electrolytically coloring an anodically oxidized coating on an aluminum or aluminum base alloy article by subjecting the anodized aluminum or aluminum base alloy article as a cathode to an electrolytic coloring treatment with a direct current in an electrolytic-coloring bath containing a nickel salt, an improvement which comprises conducting the electrolytic coloring treatment at a pH in the range of from 2.0 to 5.5 to obtain a uniformly colored film.
• the present invention relates to a process for coloring an anodically oxidized coating on an aluminum or aluminum base alloy article (hereinafter referred to as aluminum for brevity).
• the invention relates to a process for electrolytically coloring an aluminum article which has previously been anodically oxidized, which comprises subjecting the anodically oxidized aluminum as a cathode to an electrolytic coloring treatment with a direct current in an electrolytic coloring bath containing a nickel salt to obtain a uniformly colored film.
• a primary object of this invention is to provide an improved process for electrolytically coloring an anodically oxidized coating on an aluminum by subjecting the aluminum having the anodized oxide film in a thickness of at least 6 microns thickness to a direct current electrolysis as a cathode in an electrolytic coloring bath containing a nickel salt, the improvement comprising conducting the electrolytic coloring treatment in an electrolytic coloring bath at a pH in the range of 2.0 to 5.5.
• the purpose of the first anodic oxidation treatment of this invention is the formation of a practically useful anodically oxidized coating on the surface of the aluminum and, in particular, the anodically oxidized coating I having a thickness of at least 6 microns which has been formed on the aluminum in an anodic oxidation bath containing sulfuric acid and/or an aromatic sulfonic acid as a main component can be uniformly colored with the electrolytic coloring treatment in a stable manner and further the colored film has high weatherability.
• an aqueous sulfuric acid solution having a concentration of from about 10 to 30% by weight, perferably 10 to 20% by weight is usually used as the anodic oxidation bath and an aluminum article is subjected to the anodizing treatment with a direct currect at room temperature (about 2030C) and a current density of about 1 ampere/dm or, occasionally, at a higher current density of 3.0 to 5.0 amperes/dm
• stable coloring treatment can be attained when the above anodizing conditions, i.e., sulfuric acid concentration, current density and bath temperature are changed to some extent, so long as the thickness of the anodically oxidized coating formed on the aluminum article is above 6 microns.
• the anodization is preferably conducted in an aqueous solution of the aromatic sulfonic acid having a concentration of about 10% by weight by superimposing an alternating current on a direct current.
• the coloring treatment of the anodized article is then carried out without applying a sealing treatment by a direct current electrolysis using the aluminum article as a cathode in an aqueous solution containing a nickel salt as an electrolytic coloring bath.
• the electrolytic coloring bath used in this invention contains a water-soluble nickel salt as the main component and the electric conductivity of the bath can usually be controlled by adding thereto an appropriate amount of boric acid, sulfuric acid, etc.
• the water-soluble nickel salt are nickel sulfate, nickel chloride and nickel acetate, and the concentration of the nickel ion as the main component of the electrolytic coloring bath can vary over a wide range. For example, when nickel sulfate is used as the nickel salt, a desired colored film is obtained at a nickel sulfate concentration of 15 to I00 g/liter.
• the electrolytic coloring bath used in this invention can contain further boric acid, preferably at a concentration of 10 to 50 g/liter, for adjusting the electric conductivity of the bath whereby a more uniformly colored film can be obtained in a stable manner.
• the pH of the electrolytic coloring bath is maintained at 2.0 to 5.5 since when the coloring treatment is 'conducted at a pH value outside the above range, the resulting color of the colored film becomes faint and the colored film tends to be peeled and spalled. That is, the coloring treatment cannot be conducted in a stable manner at a pH value outside the above range. Also, in a practical industrial operation, since a wide variety of bronze-like colors can be obtained uniformly and with better reproducibility by maintaining the pH of the electrolytic coloring bath in a range of 3.0 to 4.5, it is the electrolytic coloring bath in the range of from 3.0
• the procedure for the electrolytic coloring treatment is advantageously carried out using a current density in a range of 0.05 to 3.0 amperesldm preferably 0.1 to 2.0 amperesldm
• the bath temperature can be room temperature with a satisfactory result, but the coloring operation can be conductedat a temperature in the range of from about 10 to about 40C.
• Thetime required for the electrolytic coloring treatment can be appropriately selected depending upon the color tone desired. Generally, as the period of time of electrolysis increases, the color of the film obtained becomes deeper. However, when a high current density such as about 2.0 amperes/dm is employed in the electrolysis, a sufficiently deep color is obtained in a short period of time, e.g., 2 to 5 seconds and when a low current density such as 0.1 to 0.3 ampere/dm is employed, a sufficiently deep color can be obtained at a comparatively longer period of time, e.g., l to 3 minutes.
• the pH of the electrolytic coloring bath is inadequately controlled, it is necessary to immediately adjust the pH of the electrolytic coloring bath to the pH within the above range. That is, when the pH of the electrolytic coloring bath becomes too high, the pH can be easily reduced by adding a small amount of sulfuric acid to the electrolytic coloring bath and, on the other hand, when the pH of the electrolyticcoloring bath becomes too low, the ph can be increased by dissolving a small amount of nickelhydroxide in the electrolytic coloring bath or passing the electrolytic coloring bath through an anion exchange resin.
• the pH of the electrolytic coloring bath can be maintained in the range of 2.0 to 5.5, preferably 3.0 to 4.5.
• the pH of the electrolytic coloring bath can be increased by dissolving nickel hydroxide as a neutralizing agent in the bath.
• Nickel hydroxide used in this procedure can be in the form of a powder or a paste. Generally, a small amount of nickel hydroxide will be sufficient.
• the neutralizing agent used in this procedure is not limited to nickel hydroxide, but on considering the influences of various metal ions on the color of the colored film, the use of a nickel compound is. preferred. Also, this procedure usually exhibits satisfactory results using nickel hydroxide at a 'concentration,ofQfrom-lO to 1000 ppm. and, therefore, the pH control can be effected conveniently at low cost.
• alkalis which-are generally used in the conventional neutralization procedure such as sodium hydroxide, potassium hydroxide, aqueous ammonia, etc.
• alkali metal ions such as sodium ion, potassium ion or the like
• ammonium ions are present in the electrolytic coloring bath, a deep color cannot easily be obtained whereby the color is undesirably limited to specific colors.
• the pH of the electrolytic coloring bath which has decreased below 2.0 can be increased by treating the coloring bath with an anion exchange resin.
• an anion exchange resin In this case, the use of a weak basic anion exchange resin is most preferable since free sulfuric acid is mainly removed by the anionexchange resin and thus the pH of the electrolytic coloring bath can be readily adjusted to a value in the desired pH range.
• EXAMPLE I An aluminum plate (99.2% Al) was immersed in an aqueous 10% sodium hydroxide solution at 60C for 2 minutes, and then subjected to a neutralization treatment with a 20% aqueous nitric acid solution at room temperature. After rinsing the aluminum plate with water, it was subjected to an anodic oxidation treatment in a aqueous sulfuric acid solution for 15 minutes at a current density of 2 amperes/dm and at a bath temperature of 1C.
• the anodized aluminum plate thus obtained was then subjected to electrolytic coloring treatment as a cathode with a direct current in an electrolytic coloring bath containing 50 g/liter of nickel sulfate and g/liter ofboric acid for 1 minute at a current density of 0.3 ampere/dm and at a bath temperature of 20 lC using a nickel plate as an anode.
• the pH of the above electrolytic coloring bath was reduced to 1.5 using sulfuric acid and then the pH of the electrolytic coloring bath was gradually increased to 5.7 by dissolving nickel hydroxide therein, whereby the electrolytic coloring treatment was conducted at various pH values of the bath.
• EXAMPLE 2 alloy article comprising subjecting said article to anodic oxidation in an aqueous anodic oxidation bath consisting essentially of sulfuric acid or an aromatic sulfonic acid to form an oxidized film of a thickness of at least 6 microns and electrolyzing said anodic oxi-
• An aluminum plate 99.2% Al was subjected to the dized article as a cathode immersed in an aqueous same pre-treatment and anodic oxidation treatment as electrolytic coloring bath containing a water-soluble in Example 1, and then subjected to an electrolytic nickel salt selected from the group consisting of nickel coloring treatment with a direct current in an electrosulfate, nickel chloride and nickel acetate using a direct lytic coloring bath containing 35 g/liter of nickel sulfate current, the improvement of obtaining uniformly coland 35 g/liter of boric acid for 2 minutes at a current I ored coating onthe aluminum article by maintaining density of 0.3 ampere/dm and at
• the ment. electrolytic coloring was effected using the electrolytic 2.
• the process according to claim 1, wherein the pH various space velocities to increase the pH of the bath of the electrolytic coloring bath is maintained at 3.0 to whereby the electrolytic coloring treatment was per- 4.5 during said electrolytic coloring treatment. formed at various pH values of the electrolytic coloring 5.
• the color lightness of the resulting colored alumiof the electrolytic coloring bath is maintained at 2.0 to num plates was evaluated as described in Example 1, 5.5 by adding a small amount of sulfuric acid to the and the results obtained are shown in Table 2 below. bath.

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• Chemical Kinetics & Catalysis (AREA)
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• Electrochemical Coating By Surface Reaction (AREA)
• Coloring (AREA)
• Electroplating Methods And Accessories (AREA)

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• 1973
  • 1973-10-20 JP JP11804773A patent/JPS578196B2/ja not_active Expired
• 1974
  • 1974-10-17 NO NO743748A patent/NO743748L/no unknown
  • 1974-10-17 GB GB4512474A patent/GB1439120A/en not_active Expired
  • 1974-10-18 IT IT53623/74A patent/IT1023079B/it active
  • 1974-10-18 FR FR7435170A patent/FR2248340B1/fr not_active Expired
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  • 1974-10-21 DE DE19742449926 patent/DE2449926A1/de active Pending
  • 1974-10-21 US US516523A patent/US3929612A/en not_active Expired - Lifetime

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