Aluminum Coil Anodizing Process

Processing steps of anodized aluminum coils

★ Mechanical polishing – degreasing – water washing – chemical polishing – water washing – anodizing – water washing – closing – mechanical brightening.

★ Chemical polishing products: aluminum alkaline polishing liquid

★ Anodizing products: aluminum anodizing liquid

★ Closing products: aluminum coil coloring closing liquid

Aluminum coil anodizing and dyeing process

Aluminum products treated by dyeing method have beautiful and bright colors, higher corrosion resistance, wear resistance and insulation than general aluminum products. Suspend the aluminum workpiece in a suitable electrolyte solution and use it as an anode for electrolysis. During the electrolysis process, the hydroxide ions in the water release electrons at the anode to become water and new ecological oxygen, which oxidizes the aluminum into a thicker aluminum oxide film. Because this process is oxidized by the metal product as the anode, it is called anodizing. After anodizing, the aluminum product is colored, closed and processed to become a dyed product.

Dyeing process of anodized aluminum coils

• Pretreatment

During repeated mechanical processing, aluminum parts are stained with more grease, a small amount of abrasives, dust and imperfect oxide films. These substances have poor conductivity and cannot be anodized, so they need to be pre-treated. The method is to use carbon tetrachloride, trichloroethylene, gasoline or toluene as a cleaning agent, immerse the aluminum parts, rinse with a brush, then air dry, and then immerse in water for repeated cleaning. After the oil is gone, rinse with hot water immediately. If a black film is formed on the surface, it should be soaked in a 32% nitric acid solution for 20 seconds to remove the black film, and finally rinse with cold water. Immerse in distilled water for the preparation of oxide film.

• Anodizing

1. Manufacture of sulfuric acid electrolyte: It is mixed with 18-20 kg of sulfuric acid and 80-82 kg of deionized water. At this time, the specific gravity of the solution is about 1.125-1.140. Sometimes, in order to obtain an oxygen electrode oxide film with good protective performance, a small amount of oxalic acid is generally added to the sulfuric acid electrolyte.

2. Oxidation process: Install the circuit surface, use the aluminum parts to be dyed as anodes and immerse them all in the electrolyte, then turn on the power and control according to the following process conditions.

The electrolyte temperature is controlled at 12-25℃, the anode current density is 1-2A/dm², and the voltage in the tank is between 13-23V. The time is about 30-40 minutes.

After the above process operation is completed, take the aluminum parts out of the electrolyte at any time, rinse the acid with clean water, and pay more attention to the concave parts, otherwise there will be white spots. After the acid is cleaned, immerse it in clean water for use.

• Dyeing

After the aluminum parts are anodized, the surface forms a colored complex that can be adsorbed by covalent bonds or hydrogen bonds, and the color is presented.
Dye selection: Dyes are divided into inorganic dyes and organic dyes. Inorganic dyes are mostly composed of inorganic salts. When dyeing, the aluminum parts are immersed in compound A and B solutions respectively to form colored compounds to achieve the dyeing purpose.

Dyeing operation of anodized aluminum coils

1. Single-color dyeing method: Immerse the aluminum products that have been anodized and washed with clean water in a coloring solution at 40-60℃. Soaking time: 30 seconds to 3 minutes for light colors; 3-10 minutes for dark colors and black. Take out after dyeing and wash with clean water.

2. Multi-color dyeing method: If two or more different colors are dyed on the same aluminum part, or landscapes, flowers and birds, figures, and texts are printed, the procedures are very complicated, including paint masking method, direct printing and dyeing method, foam plastic dyeing method, etc. The above methods are different in operation, but the principles are the same. The paint masking method is introduced as follows: This method mainly applies quick-drying and easy-to-clean varnish thinly and evenly on the yellow that is really needed to mask it. After the paint film is dry, immerse all the aluminum parts in a dilute chromic acid solution to remove the yellow color of the unvarnished part, take it out, wash off the acid with clean water, dry it at low temperature, and then dye it red. If you want to dye the third or fourth color, you can follow the above method.

• Closing

After washing the dyed aluminum parts with water, immediately put them into 90-100℃ distilled water and boil them for 30 minutes. After this treatment, the surface becomes uniform and non-porous, forming a dense oxide film. The dye applied for coloring is precipitated in the oxide film and can no longer be wiped off. The oxide film after closing is no longer adsorbent, and its wear resistance, temperature resistance and insulation are enhanced.

Dry the surface of the aluminum parts that have been closed, and then polish it with a soft cloth to get beautiful and bright aluminum products. If you dye multiple colors, after closing treatment, the protective agent applied on the aluminum parts should be removed. Wipe off a small area with cotton dipped in acetone, and a large area can be immersed in acetone to wash off the paint.

Precautions for anodizing of aluminum coils

◆ 1. After the aluminum coils are washed with oil, they should be oxidized immediately and should not be left for too long. When making the oxide film of aluminum parts, they should be completely immersed in the electrolyte. The tank voltage should be stable and consistent from beginning to end. The same batch of products must be completely consistent, which should also be followed during dyeing.

◆ 2. During the anodizing process, aluminum, copper, iron, etc. in the electrolyte solution are continuously added, affecting the gloss of aluminum. When the aluminum content is greater than 24 g/L, the copper content is greater than 0.02 g/L, and the iron content is greater than 2.5 g/L, the electrolyte should be considered for replacement.

◆ 3. When purchasing raw materials and dyes, high-purity products should be selected, because generally when there are slightly more impurities or when mixed with glauber powder or dextrin, the dyeing effect is poor.

◆ 4. When dyeing a lot, the dye liquid is thick at first and then light, and the dyed color will show different shades, so attention should be paid to timely mixing with slightly thick dye liquid to keep the consistency of dye liquid concentration as much as possible.

◆ 5. When dyeing multiple colors, dye the light color first and then the dark color, dyeing in the order of yellow, red, blue, brown and black. Before dyeing the second color, the spray paint should be dry so that the paint is close to the aluminum surface, otherwise the dye will penetrate and the boundary of the rough edge will be unclear.

Original Source:Anodized Aluminum Sheet Coil