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Aluminum Anodizing

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February 10, 2008

Decorative Finishing

The Last Step of Anodizing is Sealing and Coloring

Anodizing followed by dyeing provide an effective method of finishing aluminum and its alloys for decorative purposes. In conjunction with a special mechanical pretreatment of the surface, anodizing and dyeing produce attractive surface finishes and can be used to imitate the appearance of certain types of wood or stone. The possibility of drying the film in different colors depends on organic dyes or inorganic pigments, the pores in the oxide film adsorb the coloring matter or the later enters into chemical combination with oxide of the film. Given a suitable coloring matter, the colored finished will have a high degree of durability.

The intensity of the color depends on the thickness and structure of the oxide film, on the composition of the dye bath and on the operating condition. For one and the same coloring matter, the intensity of the color increases with increasing thickness and porosity of the film. Film obtained in oxalic acid have their own intrinsic color which modifies that produced by dyeing. Because of their limited thickness, films produced in chromic acid solution are not used for dyeing. The most suitable film for dyeing are obtained by anodizing with direct current in 20 percent sulfuric acid at 15-23 oC, 10-25 A/sq.ft. current density, and 30-45 minutes anodizing time. A color of sufficient intensity is obtained with film having thickness of 0.01 to 0.015 mm. The film obtained on pure aluminum are colorless and transparent. Alloys containing silicon give dark color films containing inclusion of graphitic silicon. Such film are incapable of yielding pure light tones and can only be dyed black, brown or some other dark colors.

The dyeing is preferably done immediately after anodizing. The anodized component should be thoroughly washed free of all residues of the anodizing solution and stored in cold water. Coloring with inorganic coloring matter is effected by precipitating a colored compound in the pores of the film. In this process the anodized parts are immersed first in a solution in one compound, washed in water then immersed in a solution of another compound. In both solution the parts are kept for 5-10 minutes. The solution are used a room temperatures. The compositions of some suitable solutions of this type are given in a table.

A brass color can be imparted to anodic oxide films by treating them in a single solution of ferrous ammonium oxalate and ammonium sulfate. The solution is prepared from 22 kg oxalic acid, 28 kg. ferrous ammonium sulfate, 25-30 liters of liquid ammonia (25 percent) and 1,000 liter water. The freshly prepared solution is used at 45-55 oC, the time of immersion being 3-5 minutes, pH 5-6. After they have been colored with an inorganic pigment, the anodized product are washed with cold and hot water, dried at 90-100 oC and coated with a colorless varnished.

Here are some pigment can use for certain color:

Color : Blue
Solution 1: Potassium Ferro Cyanide (10-15)
Solution 2: Ferric Chloride (10-100)
Pigment Formed: Prussian Blue

Color :Brown
Solution 1 : -do-
Solution 2 :Copper Sulfate (10-100)
Pigment Formed: Copper Ferrocyanide

Color : Black
Solution 1: Cobalt Acetate (50-100)
Solution 2: Potassium Permanganate (15-25)
Pigment Formed:Cobalt Oxide

Color :Yellow
Solution 1: Potassium bichromate (50-100)
Soluiton 2: Lead Acetate (100-200)
Pigment Formed: Lead Chromate

Color :Golden Yellow
Solution 1 :Sodium hyposulfite (10-50)
Solution 2 :Potassium permanganate (10-50)
Pigment Formed: Manganese sesquioxide

Color :White
Solution 1 :Barium Acetate (10-50)
Solution 2 :Sodium Sulfate (10-50)
Pigment Formed: Barium Sulfate

Color :Orange
Solution 1 :Potassium Chromate (5-10)
Solution 2 :Silver nitrate (50-100)
Pigment Formed:Silver Chromate