Process for electrolytic coloring of the anodic oxide film on a aluminum or aluminum base alloys
United States Patent 1 Yanagida etal. A  
 1 PROCESS FOR ELECTROLYTIC COLORING OF THE ANODIC OXIDE FILM ON A ALUMINUM OR ALUMINUM BASE ALLOYS [75] Inventors: Kiyomi Yanagida; Tadashi Hirokane; Tadashi Tsukiyasu; Tomoari Sato, all of Nagoya, Japan [73] Assignee: Sumitomo Chemical Co., Ltd.,  
 Osaka, Japan 22 Filed: Aug. 26, 1974 21 Appl. No.: 500,761  
 [30] Foreign Application Priority Data 3,634,208 1/1972 Kuroda 204/35 N [451 Apr. 15, 1975 3,654,100 4/1972 Nagai et a1 204/35 N 3,661,729 5/1972 Miyakawa et a1..... 204/35 N 3,664,932 5/1972 Patric, 204/35 N 3,669,856 6/1972 Gedde 204/58 3,704,209 11/1972 Patrie 3,751,350 8/1973 Ueki 204/58 3,798,137 3/1974 Kampert 204/35 N FOREIGN PATENTS OR APPLICATIONS 2,112,927 9/1971 Germany 204/35 N Primary Examiner--John H. Mack Assistant Examiner-Aaron Weisstuch Attorney, Agent, or FirmSughrue, Rothwell, Mion, Zinn &amp; Macpeak [57] 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.  
 11 Claims, No Drawings 1 PROCESS FOR ELECTROLYTIC COLORING OF THE ANODIC OXIDE FILM ON A ALUMINUM OR ALUMINUM BASE ALLOYS BACKGROUND OF THE INVENTION 1. Field of the Invention 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.  
 2. Description of the Prior Art One of the known processes for producing electrolytically colored coatings on aluminum surfaces involves the anodic oxidation of aluminum in an aqueous solution containing an organic acid (for example, as disclosed in U.S. Pat. Nos. 3,031,387 and 3,486,991).  
  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. However, 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.  
  In the electrolytic coloring process as described in German OLS 2,112,927, a bronze color is obtained when anodized aluminum is electrolyzed in an aqueous solution containing a water-soluble nickel salt, a redbrown color is obtained in an aqueous solution of a copper salt, bronze to black colors are obtained in aqueous solutions of tin salts, a bronze color is obtained in an aqueous solution of a cobalt salt, and a yellow color is obtained in an aqueous solution of an iron salt. However, in practicing this process on an industrial scale, the coloring is not stable and a defect that might be called oxide spalling may occur. This renders it difficult to obtain a uniformly colored oxide film on aluminum in a stable manner.  
  This phenomenon also occurs in the electrolytic coloring process as described in Japanese Patent Publication No. 28,585/1972 wherein anodized aluminum is subjected to alternating current electrolysis prior to the application of a direct current electrolytic coloring.  
  As the result of various investigations on elucidating the cause of the aforesaid difficulty, it has been found that 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.  
 SUMMARY OF THE INVENTION 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.  
  As the result of various investigations, it has been found that the aforesaid objects of this invention can be attained by the following process.  
  That is, 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.  
 DETAILED DESCRIPTION OF THE INVENTION The invention is explained below in detail.  
  First of all, 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. Usually, 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. However, 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. Furthermore, in using an anodic oxidation bath containing an aromatic sulfonic acid such as sulfosalicylic acid or sulfophthalic acid as the main component, 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.  
  According to the present invention, 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  
  Also, as described above, 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.  
  The current density employed in the cathodic electrolysis is usually about 0.05 to 3.0 amp/dmf&#34;, 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 coloring mechanism of the cathodic electrolysis has not yet been completely clarified but as the result of various investigations, the following conclusions can be drawn. These will be explained using a nickel salt, for example, as the main component for the electrolytic bath.  
  1. 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.  
  2. At the bottom of the micropores of the oxide film, hydrogen gas is generated together with the coloring of the oxide film by the reduction of the nickel ions and when the volume of the hydrogen gas generated in the micropores of the oxide film reaches a definite amount, the coloring by the reduction of the nickel ions is completed at this point. If the electrolysis is further continued, only the generation of hydrogen gas occurs ultimately resulting in a breaking and peeling of the oxide film.  
  3. 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.  
  4. When the aluminum thus subjected to the first cathodic electrolysis is further treated with an anodic electrolysis for a short period of time, it becomes possible to continue again the coloring of the oxide film of the aluminum by a cathodic electrolysis and because in the second cathodic electrolysis the concentration of nickel in the micropores of the oxide film is higher than that in the first cathodic electrolysis, the color formed by the second cathodic electrolysis is deeper than the color obtained in the first cathodic electrolysis.  
  In addition, 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. When the electrolysis is carried out at a low current density, 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.  
  In subjecting the faintly colored aluminum to an anodic electrolysis, when the electrolytic coloring of the anodized aluminum is completed or during the ca thodic electrolysis for coloring the anodized aluminum, the direction of the electric current from a D. C. power source is changed using a change-over switch, whereby a direct current electrolysis is conducted using the aluminum as the anode. There is no particular limitation on the current density used in the anodic electrolysis but a better result is obtained by employing a current density substantially same as that used in the first cathodic electrolysis. Also, in the anodic electrolysis an effective result is obtained in a short period of time but when a low current density is employed, a comparatively longer period of time is required, for example, 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. These conditions are, however, selected depending to the color of the oxide film desired and the composition of the bath and, of course, a period of l to 2 minutes can be employed as the case may be.  
  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. Thus, the anodized oxide film of the aluminum is colored more deeply than the colored coatings obtained in the first cathodic electrolysis.  
  If the color of the oxide film on the aluminum is lighter than the color desired, the aluminum is further subjected to a second anodic electrolysis and a third cathodic electrolysis, whereby the oxide film is colored more deeply. By further repeatedly applying the anodic electrolysis and the cathodic electrolysis to the anodized aluminum, the oxide film of the aluminum can be colored far more deeply.  
  Thus, 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. For example, when an electrolytic coloring bath containing a water-soluble nickel salt is employed, 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. On the other hand, when the 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.  
  In addition, 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 above embodiment of this invention was explained for the use of one electrolytic coloring cell for conducting the cathodic electrolysis and the anodic electrolysis alternatingly but in the process of this invention, the cathodic electrolysis and the anodic electrolysis can be conducted in a different electrolytic cell.  
 In the latter case the employment of the change-over switch is, of course, unnecessary and a great advantage is obtained in being able to treat a large amount of articles continuously. In conducting the cathodic electrolysis and the anodic electrolysis in separate electrolytic cells, it is preferable that 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. However, if the solution in the electrolytic cell for the anodic electrolysis enters the electrolytic coloring cell for the cathodic electrolysis carried by aluminum articles treated in the latter electrolytic cell, 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.  
  The invention is explained more specifically by the following examples but the invention is not tobe construed as being limited to these examples.  
  Unless otherwise indicated, all parts, percents, ratios and the like are by weight.  
 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. After washing the samples with water, 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.  
  Each of the samples thus treated was washed with water and then sealed for 30 minutes in boiling water.  
  The shades of color of the samples thus treated were evalulated with the lightness Y being measured using a color difference meter, and the results obtained being shown in the following table.  
  As is clear from the results, in the first coloring treatment or the first cathodic electrolysis, the coloring was completed under the conditions applied to Sample 2 and even if the period of time of electrolysis was prolonged, no additional coloring occurred (see Sample Also, even Sample 2 for which only a light yellowbrown color was obtained since the coloring had reached the limit, could be colored more deeply by further subjecting the sample to the anodic electrolysis and the cathodic electrolysis as in Sample 3. Furthermore, it can be clearly understood from Sample 4 having a dark-bronze color that by further repeating the anodic treatment and the cathodic treatment, the color of the oxide film of the sample became deeper and deeper.  
 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. In the same manner, 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.  
  Each of the samples thus treated waswashed with water, subjected to a sealing treatment, and then the lightness Y of the surface thereof was measured, the results being shown in the following table.  
  As is clear from the results shown in Table 2, from the electrolytic coloring bath having a low sodium ion 15 content, a quite deep dark bronze color was obtained as in Sample 5 but by applying the process of this invention, a deeper color was obtained and by applying repeatedly the coloring treatments further, a deep black color was obtained for Sample 7.  
  As illustrated in the above-described examples, the  
 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.  
  Thus, according to the process of this invention, 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.  
  While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.  
 What is claimed is:  
  1. 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.  
  2. The process as claimed in claim 1, wherein the anodic oxide coating is obtained by anodically oxidizing the article in an anodic oxidation bath containing sulfuric acid or an aromatic sulfonic acid.  
  3. The process as claimed in claim 1, wherein said first cathodic electrolysis for the article is conducted in an aqueous electrolytic coloring bath containing a watersoluble nickel salt.  
  4. The process as claimed in claim 3, wherein said water-soluble nickel salt is nickel sulfate, nickel chloride, or nickel acetate.  
  5. The process as claimed in claim 1, wherein said first cathodic electrolysis for the article is conducted in an aqueous electrolytic coloring bath containing a copper salt, a tin salt, a cobalt salt, or an iron salt.  
 current electrolysis and a cathodic electrolysis wherein- 6. The process as claimed in claim 1, wherein said electrolytic coloring bath used for the first cathodic electrolysis further contains boric acid or sulfuric acid for adjustment of the pH and electric conductivity of the electrolytic coloring bath.  
  7. The process as claimed in claim 1, wherein said first cathodic electrolysis for the article is conducted at a current density of about 0.05 to 3.0 amp./dm. at a temperature of about to 40C., and for about 2 seconds to 3 minutes.  
  8. The process as claimed in claim 1, wherein said subsequent anodic electrolysis and the cathodic electrolysis are conducted in the same electrolytic coloring .same conditions as in the first cathodic electrolysis.  
  11. The process as claimed in claim 1, wherein said anodic electrolysis is conducted in an electrolytic bath different from the electrolytic coloring bath for the cathodic electrolysis.