Method of neutralizing surface color caused by anodizing aluminum alloys
United States Patent 1 1 1 1 3,874,902 Andrus Apr. 1, 1975 METHOD OF NEUTRALIZING SURFACE [56] References Cited COLOR CAUSED BY ANODIZING UNITED STATES PATENTS ALUMINUM ALLOYS 2.030.236 2/1936 Andcrau 204/38 A [75] Inventor: Joseph Moore Andrus, Park Ridge, 2.683.113 7/1954 Prancc ct a1 204/38 A L 2.814.576 11/1957 Zickendruht et a1. 148/6.1 2.969.351 1/1961 Grossmann 148/6.1 [73] Assignee: Stauffer Chemical Company. 3,767.474 10/1973 Cohn 148/61 Westport. Conn. [22] Filed: Mav 28, 1974 Pl&#39;llll(ll Exunziner Cameron K. Weiffenbach A surface color modification caused by anodizing cer- 1 Cl 7/71 M, 148/61 204/35 tain aluminum alloys is neutralized by sealing the an- 204/3632O4/38 204/42 odized surface in an aqueous solution containing 21 [51] Int. Cl B44d 1/16, C231 5/04, C23f 1 s ecific dye. The dye neutralizes said color modifica- [58] Field of Search 117/71 M; 148/6.1;  
 tion without leaving a color of its own.  
 6 Claims, No Drawings METHOD OF NEUTRALIZING SURFACE COLOR CAUSED BY ANODIZING ALUMINUM ALLOYS BACKGROUND OF THE INVENTION The present invention relates to the correction of a surface color modification caused by anodizing certain aluminum alloys and more particularly to a method of neutralizing said color modification with a dye containing sealing solution.  
  It is known that anodizing certain aluminum alloys can modify the surface color. In copper-containing alu minum alloys, for example, this color appears as a yellow cast. ln some applications, the appearance of this cast is not detrimental to the final product. When the product is to be used for decorative purposes, however, the cast may be considered undesirable, for example. in the case of aluminum automobile bumpers.  
  A known method to eliminate or reduce this undesired cast is to lower the current density during anodizing to a level whereby the cast is diminished. This method has the disadvantages of slowing production and producing an anodic coating of inferior corrosion resistance.  
  Of course, manufacturers have the option to plate the aluminum surface instead of anodizing it. lf the aluminum is to be used for automobile bumpers, for example, it can be electroplated with nickel and chromium. Electroplating, however, is expensive and the corrosion resistance of the resulting surface is unpredictable.  
  Contrary to the objectives of the present invention, some decorative uses of aluminum require a colored surface. In these cases, a surface color modification caused by anodizing may be desired and can even be enhanced with dyes. Moreover, dyes of any desired color, if used in sufficient quantity, may be used to impart color to the anodized surface. Dyeing is most commonly carried out prior to scaling. However, some methods of dyeing and scaling in a single step are known.  
  In U.S. Pat. No. 2,683.] 13. a method of coloring aluminum ice trays is described wherein the anodized tray is placed in an aqueous solution of an organic dye. The dye is added to the solution in a sufficient quantity to yield the desired color. By maintaining the dye bath at a temperature of between about 175 and 210F. the surface is sealed simultaneously with dyeing.  
  Another method of dyeing and sealing anodized aluminum in a single step is described in US. Pat. No. 3,079,309. In this method, anodized aluminum is colored by applying an ink composition consisting essentially of a water soluble organic hydroxy compound having a boiling point greater than 212F., a coloring material dissolved therein. a viscosity control agent compatible with and chemically inert toward the remaining constituents in the composition and a finely divided filling agent consisting of porous particles of solid. inorganic pigment which also is chemically inert toward the remaining constituents in the composition.  
  In accordance with a method described in US. Pat. No. 3,767,474, surface color modifications caused by anodizing may be enhanced. or in the alternative, a different color may be imparted to the anodized surface by sealing the anodized surface with a hot aqueous solution containing disodium 4-dodecylated oxydibenzene sulfonate. a soluble hydrolyzable metallic salt and a dye compatible with the other constituents of said solution.  
 SUMMARY OF THE INVENTION It has now been discovered that the undesired cast appearing in the anodized surface of certain aluminum alloys can be simply and economically neutralized by contacting said surface with an aqueous scaling solution containing a specific dye. Thus, in accordance with the present invention there is provided a new method for eliminating undesired surface color modifications imparted to aluminum as the result of anodizing.  
  Prior to anodizing, the aluminum workpiece can be chemically brightened if desired. The workpiece then is anodized at a current density sufficient to produce an anodic coating having good corrosion resistance. It is preferred to anodize at a current density between about 12 and 24 amperes per square foot for about 15 to 30 minutes. If the anodizing step causes an undesired cast to be imparted to the anodized surface, a dye is selected to neutralize that cast.  
  it is critical to select the dye on the basis of its ability to neutralize the undesired cast without leaving a color of its own. Dyes having the color characteristics necessary to neutralize the undesired cast are selected on the basis of a color system such as the Munsell color system. When the Munsell system is used, the hue corresponding to the undesired cast is found on the hue Munsell color circle. The color opposite the position, on that circle. of said corresponding hue, i.e. from that position, is normally the neutralizing color. A dye having said neutralizing color then is selected. Suitable dyes include the monoazo acid metallized type, among others.  
  Several dyes may appear to be satisfactory. However, each dye must be tested to determine which dyes of those selected will not leave a color of their own&#39;in the anodized surface. Testing may be carried out on a laboratory scale by contacting samples of anodized aluminum having the undesired cast with aqueous sealing solutions containing the individual dyes. (The composition of said solutions is defined below. The sealed samples are visually inspected and the specific dye or dyes that yield best results are thereby selected.  
  The specific dye is added in a concentration of between about 3 and 30 milligrams per liter to water, thus making-up the aqueous sealing solution. Scaling accelerators such as metallic salts, for example. Co or Ni salts, can be added to the solution. The use of accelerators, however, makes color control difficult. lt is critical that the concentration of the dye is below the value that would cause coloring of the anodized surface and is at or above the value necessary to effect neutralization of the undesired cast. Minor variations will easily establish the optimum concentration range for the specific dye. In a continuous process, the optimum concentration range is maintained with the aid of a colorimeter or spectrophotometer.  
  Also critical to the method of the present invention is the maintenance of the pH of the solution within a range that is optimum for sealing and dye absorption. The pH must be between about 5.5 and 6.5. Selection of a pH value is based on the desired rate of sealing. Sealing rate decreases with increased acidity. Once a pH value is selected, it must be controlled to i(). l. Sodium hydroxide or acetic acid is used to control the pH.  
  The temperature of the solution must be maintained at a value sufficient to effect sealing of the anodized surface. A temperature between about l 80 and 2l0F. is selected and maintained at 5F.  
  Sealing according to the present invention is effected by contacting the anodized aluminum with the dyecontaining sealing solution for about to 30 minutes.  
  The present invention will be more fully illustrated in the example which follows:  
 EXAMPLE A high strength aluminum alloy utilized in the manufacture of bumpers for automobiles was chemically brightened by a conventional process. The alloy had the following composition: 1.0 percent magnesium, 0.8 percent copper, 4.5 percent Zinc, less than 0.1 percent iron, less than 0.1 percent silicon, 0.05 percent titanium and the balance; high purity aluminum ingot. Anodizing was effected at a standard current denisty of 14 amperes per square foot until the anodic coating was between about 0.0004 and 0.0005 inch thick. The anodized product had an undesired yellow cast. On the basis of the Munsell Color System, five dyes of neutralizing color were selected, namely, aluminum black-c-&#39; cone, aluminum grey NL paste, aluminumblack 2 LW, aluminum fast black GL paste, and aluminum black V. All dyes were from Sandoz Colors and Chemicals.  
  A sealing solution having a dye concentration of about 15 milligrams per liter was maintained at a pH between 5.6 and 5.7, with sodium hydroxide and acetic acid. A sealing solution temperature of about 205 i 5F. was used. The anodized alloy was immersed in the solution for IS minutes. As the result of experimentation with the above-dyes under these conditions, it was found that aluminum black V (also known as acid black 52, a monoazo metallic dye having the chemical formula: N O SC H -xCr-Na and a color index of l5,7l I) successfully neutralized the undesired yellow cast without leaving a color of its own.  
 Having set forth the general nature and an exemplary embodiment of the present invention, the scope is now.  
 particularly pointed out in the appended claims.  
 What is claimed is:  
  l. A method of neutralizing an undesired cast appearing in an anodized surface of an aluminum alloy, by sealing said anodized surface with an aqueous solution containing a monoazo metallic dye of a color and in a concentration sufficient to neutralize said unde-, sired cast without imparting the dye color to said anodized surface wherein said solution is maintained at a pH necessary for optimum sealing and dye absorption and at a temperature between about and 2 l0 i 0.5F. 2. A method as in claim 1 wherein said aluminum alloy is chemically brightened &#39;prior to anodizing.  
  3. A method of neutralizing an undesired cast appearing in an anodized surface of an aluminum alloy,  
 by sealing said anodized surface for about l0 to 30 minutes with an aqueous solution containing a monoazo metallic dye of a color and in a concentration sufficient to neutralize said undesired cast without imparting the dye color to said anodized surface wherein said solution is maintained at a pH between about 5.5 and 6.5 i- 0] and at a temperature between about 180 and 210 i 5F.  
 4. A method as in claim 3 wherein said aluminum,  
 alloy is chemically brightened prior to anodization.  
 5. A method of neutralizing a yellow cast appearing in an anodized surface of a copper-containing aluminum alloy, by sealing said anodized surface for about 15 minutes in an aqueous solution containing about 15 milligrams per liter of a monoazo metallic dye to neutralize said yellow east, wherein said solution is maintained at a pH between about 5.6 and 5.7 and at a ,tem  
 perature of about 205 i 5F 6. A method as in claim 5 wherein said aluminum alloy is chemically brightened prior to anodizing. 7  
  k =t= 9 =l l