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 of coloring an anodized aluminum or aluminum alloy article by subjecting the anodized article to an electrolytic treatment with a direct current in an aqueous electrolytic coloring bath containing a metal salt, the oxide film on the article can be colored in a stable manner and in a deep color by applying repeatedly, after subjecting the article to a cathodic direct current electrolysis in an electrolytic coloring bath containing the metal salt, an anodic direct current electrolysis and then further a cathodic direct current electrolysis to the article.
• the present invention relates to a process for electrolytically coloring an anodic oxide coating on aluminum or an aluminum base alloy (for brevity the term aluminum in this description includes articles made of aluminum or aluminum alloys and hereinafter the term aluminum will be used throughout) and more particularly, the invention relates to an improvement in the process of coloring anodized aluminum by subjecting it as a cathode to an electrolytic treatment with direct current in an electrolytic coloring bath Containing a water-soluble metal salt.
• Another process comprises electrolyzing aluminum which has been subjected to a preliminary anodic oxidation in an electrolytic bath containing a watersoluble .metal salt.
• Examples of this later process include known processes such as the inorganic coloring process disclosed in U.S. Pat. No. 3,382,160 using an alternating current electrolysis, and a process as disclosed in German OLS 2,1 12,927 using a direct current electrolysis in an electrolytic coloring bath containing a metal salt.
• the process using direct current electrolysis is superior to the aforesaid other conventional processes in such points that (l) the aluminum article can be colored in a short period of time, (2) the cell voltage required for performing the electrolytic coloring is low, (3) the coloring is applicable to various aluminum articles including plates, extrusions, castings, etc., and (4) the coloring can be easily operated and a superior color is obtained.
• the cause of the difficulty lies in the presence of impurities such as sodium ion, potassium ion, etc., dissolved in the electrolytic coloring bath and the variation in the pH value of the electrolytic coloring bath. That is, in practicing the aforesaid process on an industrial scale, the electrolytic coloring bath is contaminated by a build-up of various impurities which may arise from the water for the coloring bath, chemicals added to the coloring bath, and aluminum articles treated in the coloring bath as well as from the surroundings in an anodizing treatment plant. However, it is difficult to avoid completely contamination of the electrolytic coloring bath with such impurities.
• An object of this invention is, therefore, to provide an improved process for electrolytically coloring anodized aluminum which can provide a superior and uniformly deeply-colored oxide coating on the aluminum without the above-described difficulty induced from contamination of the electrolytic coloring bath and variation in the pH value.
• Another object of this invention is to provide an economically improved electrolytic coloring process for anodized aluminum employing an electrolytic coloring bath which does not require repeatedly purifying the bath of contamination and frequently reconstituting the bath.
• Still another object of this invention is to improve the process for coloring an anodized aluminum using a direct current electrolysis in an electrolytic coloring bath containing a water-soluble metal salt so that the anodized aluminum can be uniformly colored in a stable manner unaccompanied by the aforesaid difficulty.
• the present invention provides an electrolytic coloring process for the anodized oxide film formed on the surface of aluminum by subjecting the aluminum having the anodized oxide film in a thickness of at least 6 microns to a direct current electrolysis in an aqueous electrolytic coloring bath containing a water-soluble metal salt, which comprises applying first to the anodized aluminum a cathodic electrolysis for coloring wherein the aluminum as the cathode is subjected to a direct current electrolysis and then applying to the aluminum alternatingly one or more times an anodic electrolysis wherein the aluminum as the anode is subjected to a direct electrolysis and further a cathodic electrolysis for coloring wherein the aluminum as the cathode is subjected to a direct current electrolysis.
• the anodization treatment or anodic oxidation treatment in this invention is conducted for forming an anodized oxide film on the surface of the aluminum and in this case, aluminum having thereon an oxide film of a thickness of at least 6 microns formed by the anodization in an anodic oxidation bath containing sulfuric acid and/or an aromatic sulfonic acid can be uniformly colored in the subsequent electrolytic coloring steps in a stable manner and further the colored oxide film thus formed on aluminum has high weatherability.
• an aqueous sulfuric acid solution having a concentration of from about 5 to 30% by weight, preferably 10 to 20% by weight is used as the anodic oxidation bath and the anodic oxidation bath can further contain a small amount of an organic acid such as oxalic acid, etc., or a salt of such an organic acid. It is preferred, in this case, that the anodizing treatment be conducted with direct current at room temperature (about 20-30C) and a current density of about 1 amp./dm. or, occasionally at a high current density about 3.0 to 5.0 amp./dm.
• the abovedescribed values of the sulfuric acid concentration, the current density, and the bath temperature can be changed to some extent with effective coloring, as the case may be, if the thickness of the oxide film formed is at least 6 microns.
• the anodization is preferably conducted in an aqueous solution of the aromatic sulfonic acid having a concentration of about 10% by superimposing an alternating current on a direct current.
• the thus anodized aluminum is subjected to the first cathodic electrolysis for coloring without applying a sealing treatment of the micropores of the oxide film, in which the anodized aluminum as the cathode is subjected to a direct current electrolysis in an aqueous solution, as an electrolytic coloring bath, containing a metal salt.
• the electrolytic coloring bath used in the first cathodic electrolysis contains generally, as the main component, a water-soluble metal salt such as a nickel salt, a copper salt, a tin salt, a cobalt salt, or an iron salt and further the bath can contain, if desired, a suitable amount of an acid such as boric acid, sulfuric acid, etc., for adjusting the pH and the electrolytic conductivity of the electrolyte. Moreover, in order to obtain a desired color in the anodized oxide film on the aluminum, the bath can also contain an ammonium salt and/or one or more other metal salts.
• the first cathodic electrolysis will be explained more specifically by referring to an example using a watersoluble nickel salt as the main component for the electrolytic coloring bath. That is, in this case nickel sulfate, nickel chloride, nickel acetate, etc., can be used as the water-soluble nickel salt and the concentration of the nickel ions as the main component can vary over a wide range. For example, when nickel sulfate is employed as the main component, a more desirable colored oxide film is obtained at a nickel sulfate concentration of about to 100 g./liter. Sufficient coloring is, of course, obtained using nickel sulfate concentrations outside this general range. I
• the electrolytic bath used in this invention can contain further an acid such as, for example, boric acid, for adjusting the electric conductivity of the bath and in this case the amount of the boric acid is suitably about 10 to 50 g./liter for obtaining a stable and uniform color.
• an acid such as, for example, boric acid
• the current density employed in the cathodic electrolysis is usually about 0.05 to 3.0 amp/dmf", more preferably about 0.1 to 2.0 amp./dm.
• the temperature of the bath is usually at substantially room temperature (about to 30C) but can be selected appropriately in the range of from about 10C. to 40C.
• the period of time required for the first cathodic electrolysis can be appropriately selected depending on the color of the anodized oxide film desired and the current density employed. That is, in general, as the period of time of electrolysis increases, the color obtained tends to become deeper. However, when a high current density is employed in the electrolysis, sufficient coloring is obtained in a few seconds. Generally, a suitable period of time for performing the cathodic electrolysis is from about 2 seconds to 3 minutes.
• the anodized oxide film of the aluminum iscolored by the reduction of the nickel ions at the bottom of the micropores of the oxide film, the reduced nickel is present as metallic nickel and nickel compound, and as the amount of the reduced nickel increases, the color of the oxide film becomes deeper.
• the elapsed period of time until the coloring is completed depends upon the electrolyte composition and the electrolytic conditions but when the electrolytic coloring bath contains impurities which hinder the occurrence of coloring, such as, in particular, sodium ions, potassium ions, and aluminum ions, the elapsed period of time until the coloring is completed decreases depending upon the concentration of the impurities. Therefore, in such case, the amount of the reduced nickel ions is less and hence faintly colored oxide coatings on the aluminum are obtained.
• the end of the electrolytic coloring can be readily confirmed be detecting the condition when the cell voltage begans to change greatly in employing a constant-current electrolysis or the condition whenthe electric current begans to change greatly in employing a constant voltage electrolysis.
• the coloring of the oxide film is completed after a comparatively long period of time while when a high current density is employed, the coloring is completed in a short period of time. If the electrolysis is continued further beyond this point, only the generation of hydrogen gas occurs and ultimately the oxide film will be peeled and spalled.
• the period of time for electrolysis is about 5 to 30 seconds at a current density of 0.2 to 0.5 amp./dm. while when a high current density is employed, a shorter pe riod of time is required, for example, the period of time for electrolysis is about 5 seconds at a current density of 1.0 to 1.5 amp./dm.
• the faintly colored aluminum treated with the abovedescribed anodic electrolysis is subjected to a second cathodic electrolysis in, preferably, the same electrolytic coloring bath as used in the first cathodic electrolysis under electrolytic conditions which can be the same as or different from those conditions used in the first cathodic electrolysis.
• electrolytic conditions which can be the same as or different from those conditions used in the first cathodic electrolysis.
• the aluminum is further subjected to a second anodic electrolysis and a third cathodic electrolysis, whereby the oxide film is colored more deeply.
• the oxide film of the aluminum can be colored far more deeply.
• the process of this invention is quite effective in obtaining a deeply colored anodized oxide coating on aluminum using an electrolytic coloring bath which results in only a light color using a conventional technique in the apparatus employed industrially caused by impurities present in the electrolytic bath.
• the electrolytic bath contains usually a large amount, e.g., about to 30 ppm. of sodium ions and hence only a light color is obtained when the anodized aluminum is colored in the electrolytic coloring bath in a conventional manner.
• anodized aluminum is colored according to the process of this invention using the electrolytic coloring bath containing the abovedescribed impurities, a deep bronze color is obtained and further the anodized aluminum can be readily colored black, which has not previously been possible using an electrolytic coloring bath containing a watersoluble nickel salt in a conventional manner.
• the change from the cathodic electrolysis to the anodic electrolysis in an electrolytic coloring bath can be easily carried out by operating a changeover switch connected to a D. C. power source.
• the composition of the electrolytic bath for the anodic electrolysis be the same as the composition of the electrolytic coloring bath for the cathodic electrolysis but the electrolytic bath for the anodic electrolysis can have a different concentration and different composition than those of the electrolytic coloring bath for the cathodic electrolysis.
• the electrolytic coloring bath for the cathodic electrolysis is contaminated adversely affecting the coloring and hence adverse influences on the electrolytic coloring bath must be avoided by strictly controlling and managing the pH, the electric conductivity, and the composition of the electrolytic bath for the anodic electrolysis.
• EXAMPLE 1 An extruded article of an aluminum base alloy 6063 (AA. designation) was immersed in a 10% aqueous sodium hydroxide solution at C. for 2 minutes and then subjected to a neutralization treatment for 3 minutes at room temperature using a 20% aqueous nitric acid solution. After washing the aluminum sample with water, the aluminum was anodized with direct current using a 15% aqueous sulfuric acid solution as an anodic oxidation bath for 15 minutes at a current density of 2.0 amp./dm. and at a bath temperature of 20C. 1 1C., whereby an anodic oxide coating on the aluminum having a thickness of about 9 microns was formed. In the same manner, four samples, Samples 1, 2, 3, and 4, were prepared.
• Sample 1 was placed as a cathode in an aqueous electrolytic coloring bath containing 50 g./liter of nickel sulfate and 30 g./liter of boric acid as well as sodium chloride (22 ppm. of sodium ions) and was electrolyzed by passing a direct current using a nickel plate as the anode for 30 seconds at a current density of 0.5 amp./dm. and a bath temperature of 20C.
• Sample 2 was also colored using the same direct electrolysis in an electrolytic coloring bath having the same composition as above for 20 seconds at a current density of 0.5 amp./dm. and a bath temperature of 20C.
• Sample 3 was subjected to a cathodic electrolysis under the same conditions as in the case of electrolyzing Sample 2, then subjected to an anodic electrolysis using a direct current in the same electrolytic coloring bath for 10 seconds at a current density of 0.5 amp./dm. using a change-over switch, and finally subjected again to a direct electrolysis for 20 seconds at a current density of 0.5 amp./dm. using the samples as the cathode.
• Sample 4 was subjected to a series of electrolysis under the same conditions as in the case of treating Sample 3, then subjected to an anodic electrolysis in the same electrolytic coloring bath for 10 seconds at a current density of 0.5 amp./dm. and finally subjected again to a cathodic electrolysis using a direct current in the same electrolytic coloring bath for 20 seconds at a current density of 0.5 amp./dm.
• the electrolytic coloring bath was always maintained at 20C. for Samples 3 and 4.
• EXAMPLE 2 An aluminum plate (99.2% Al) was subjected to the pretreatment as described in Example 1 and subjected to an anodic oxidation treatment in a 15% aqueous sulfuric acid solution for 50 minutes at a current density of 1.0 amp./dm. and a bath temperature of 20C. i 1C., whereby an oxide film having a thickness of about 15 microns was formed.
• three samples, Samples 5, 6, and 7, were prepared. After washing these samples with water, Sample 5 was placed as a cathode in an aqueous electrolytic coloring bath containing 35 g./liter of nickel sulfate and 35 g./liter of boric acid as well as about 1 ppm. of sodium ions and was electrolyzed by passing a direct current using a nickel plate as the anode for 1.5 minutes at a current density of 0.3 amp./dm. and a bath temperature of 25C.
• Sample 6 was subjected to the same cathodic treatment as described above under the same conditions as in the case of treating Sample 5, then subjected to an anodic electrolysis in the same electrolytic bath for seconds at a current density of 0.3 amp./dm. and finally subjected to a cathodic electrolysis for 1.5 minutes at a current density of 0.3 amp./dm.
• Sample 7 was subjected to a series of treatments under the same conditions as in treating Sample 6 and further subjected to an anodic electrolysis for 15 sec- 6 onds at a current density of 0.3 amp./dm. and further to a cathodic electrolysis for 1.5 minutes at a current density of 0.3 amp./dm.
• process of this invention can be quite effectively applied to the use of an electrolytic coloring bath containing, in particular, a water-soluble nickel salt and further sodium ions as impurities to obtain a deep color whereas only a light color is obtained using a conventional process.
• a uniformly and deeply colored oxide film can be formed in a stable manner on aluminum without causing a spalling of the oxide film which is quite important industrially.
• a process for electrolytically coloring an anodic oxide coating on aluminum or an aluminum alloy article by subjecting the article having an anodized oxide film of a thickness of at least about 6 microns to a direct current electrolysis in an aqueous electrolytic coloring bath containing a water-soluble metal salt which comprises applying first to the anodized article a cathodic electrolysis wherein the aluminum article is subjected to a direct current electrolysis as the cathode for at least a few seconds and then applying alternatingly at least once to the article an anodic electrolysis wherein the article as the anode is subjected to a direct the article as the cathode is subjected to a direct current electrolysis.
• anodic oxide coating is obtained by anodically oxidizing the article in an anodic oxidation bath containing sulfuric acid or an aromatic sulfonic acid.

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Citations (11)

• 1973
  • 1973-08-24 JP JP9550073A patent/JPS5339865B2/ja not_active Expired
• 1974
  • 1974-08-22 NO NO743024A patent/NO139693C/no unknown
  • 1974-08-22 IT IT52707/74A patent/IT1019085B/it active
  • 1974-08-23 CH CH1151574A patent/CH588568A5/xx not_active IP Right Cessation
  • 1974-08-23 GB GB3725174A patent/GB1440733A/en not_active Expired
  • 1974-08-23 DE DE2440540A patent/DE2440540C3/de not_active Expired
  • 1974-08-23 FR FR7429040A patent/FR2241634B1/fr not_active Expired
  • 1974-08-23 CA CA207,673A patent/CA1034900A/en not_active Expired
  • 1974-08-26 US US500761A patent/US3878056A/en not_active Expired - Lifetime

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