One of the major applications of anodized aluminum involves the production of finishes resembling bronze, brass, gold, silver and their alloys. In such cases apart from the choice of suitable coloring materials, an important part is played by the conditioning and preparation of the surface of the metal prior to anodizing. A mechanically polished surface become dull after the first four minutes of anodizing in sulfuric acid. The absence of a metallic luster effects the appearance, the dyed film giving the appearance of a plastic surface. To retain the metallic lustrous, it is necessary to produce a passive film capable of withstanding the action of sulfuric acid. This can be achieved by electro polishing. After electro polishing, the surface of the metal retains its luster even after one hour's anodizing. A pretreatment of this nature also helps the production of more intense color. The necessary passivation of the aluminum can also be achieved by chemical polishing.
The surface of aluminum after chemical polishing in the above solutions has a bright, lustrous but slight etched surface, which does not become dull as a result of anodizing. Treatment in the acid solutions produces a somewhat higher luster than treatment in the alkaline solution. The addition of urea reduces the etching of the metal. Compared with chemical polishing, however, electropolishing gives a better smoothing of the surface and a higher luster. The polished aluminum parts are subsequently anodized in a 15 percent solution of sulfuric acid at 18-22 oC with 10-20 A/sq.ft. with current density for 15-30 minutes.
A gold color can be obtained by dyeing with mixtures of different dyes, red and yellow, orange and yellow, or yellow, red and black. All the dyes must light fast and resistant to leaching out by water. Depending on the composition of the dye bath, different shades of gold can be obtained, simulating the color of pure gold and of its alloys with copper and silver. A color corresponding to that of the pure gold is given by a dye bath consisting of 1.6 kg acid orange 2G and 0.4 kg Lacquer yellow 3 dispersed in 1000 liters of water. This dye bath is used at 60-65 oC and the parts are immersed for 1-2 minutes. The dyed films are sealed treating with boiling water for 20 minutes. The purity of the shade and the intensity of the color produced depend on the composition and structure of the metal. For imitating gold, aluminum of more than 99 percent purity and some alloys of aluminum with manganese are suitable.