February 10, 2008

Mechanism of Coloring

The mechanism of coloring the anodic oxide film with an inorganic pigment differ from that of dyeing with organic dyestuffs, as in the latter case, there occurs not only adsorption of the dye in the film but also chemical reaction between them. The oxide film can be dyed directly in solutions of acid dyes as used for wool and silk and of substantive dyes as used for cotton. For dyeing with a basis bye, the film must first be treated with a solution of tannin. Typical dyes capable of directly dyeing the oxide film include aniline violet N, aniline catechol G, aniline yellow G, aniline brown M, aniline green B, aniline blue 2B, aniline orange G, aniline bronze G and other dyestuffs containing HSO3 and COOH groups in their molecules. The basic dyes, which will dye the oxide film after the latter has been treated with a solution of tannin, include methanic acid, toluidine blue, fuchine, chrysoidine, safranine and other dyestuffs containing the basic groups NH2 and N(CH3)2. Alizarin dyes and mordant and acid mordant dyes of the alizarin type can dyed the film directly without a preliminary treatment with tannin and show a greater fastness to light than substantive or basic dyes.


The intensity of coloration increase with the thickness of the film and with the concentration of the dye in the dye bath. In anodizing with a view to subsequent dyeing, the amount of electricity passed through the work should be minimum 400-600 A/sq.ft. To prepare the dye bath, the necessary amount of the dye (this may range from 1 to 10 g/liter) is dissolved in hot water, the solution is boiled for 10-15 minutes, allowed to stand and then filtered through  linen filter. Dyeing is done by immersing the parts for 10-15 minutes, the temperature of the dye bath being 60-80 oC. While in use the dye is stirred. The dyed parts are washed and then immersed in hot water (90-100 oC) for 20-30 minutes. This treatment contributes to the sealing of the film.


The usual chromate sealing process is not suitable for dyed films because it leads to some oxidization of the dye. The following solution are recommended for sealing of the dyed film.

-          A solution prepared by dissolving 4.2 kg nickel sulfate, 0.7 kg cobalt sulfate, sodium acetate and 5.3 kg boric acid in 1000 liters of water.

-          A solution prepared by dissolving 5.5 kg sodium acetate, 0.1 kg cobalt acetate and 3.5 kg boric acid in 1000 liters of water.


These solutions are used at 80-85oC, their pH being 4.5-5.5. The treatment is carried out for 15-20 minutes. After sealing the dyed parts are washed in water and dried at 70-80oC. To increase their resistance to corrosion, it recommended to fill the remaining pores of the film by coating with a colorless varnish or by dipping in a molten wax (paraffin or ceresin). The excess of these materials is removed by wiping the surface. To improve the appearance of anodized parts, after dyeing, they may be buffed on soft wheels with a polished paste. The polishing paste forced into the pores of the films has a certain sealing effect.

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