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Summary of anodizing methods for aluminum and aluminum alloys

In the past ten years, China's aluminum oxidation coloring technology has developed rapidly. Many factories have adopted new technology and accumulated rich experience in actual production. There are many anodizing processes for aluminum and its alloys that have matured and are being developed, and suitable processes can be selected according to actual production needs.

Before selecting the oxidation process, the aluminum or aluminum alloy material should be known, because the quality of the material and the composition of the material will directly affect the quality of the anodized aluminum product. In this regard, Hong Jiude and Comrade Fan Ji have devoted themselves to the discussion (see Electroplating and Finishing, No. 2, 1982, P.27). For example, if there are defects such as bubbles, scratches, peeling and roughness on the surface of the aluminum, all the rickets will still be revealed after anodizing. The alloy composition also has a direct effect on the surface appearance after anodization. For example, an aluminum alloy containing 1 to 2% manganese is brownish blue after oxidation. As the amount of manganese in the aluminum increases, the surface color after oxidation changes from brownish blue to dark brown. The aluminum alloy containing 0.6 to 1.5% of silicon is gray when oxidized, and white-gray when it contains 3 to 6% of silicon. Zinc-containing is an opalescent color, chrome is a golden to gray uneven color, and nickel is yellowish. In general, only aluminum containing gold and containing more than 5% of titanium contains gold, which is oxidized to give a colorless, transparent, bright, smooth appearance.

After selecting aluminum and aluminum alloy materials, it is natural to consider the selection of an appropriate anodizing process. At present, the sulfuric acid oxidation method, the oxalic acid oxidation method and the chromic acid oxidation method widely used in China have been introduced in detail in the manuals and books, and need not be described. This article is a brief introduction to some of the new processes currently under development in China, as well as some foreign methods.


1. New technologies developed in China

(1) oxalic acid-formic acid mixture exchange rapid oxidation

The oxalic acid-formic acid mixture is used because it is considered that formic acid is a strong oxidizing agent. In such a bath, formic acid acts to accelerate the dissolution of the inner layer (barrier layer and barrier layer) of the oxide film, thereby making it a porous layer (ie, The role of the outer layer of the oxide film. The conductivity of the bath can be increased (that is, the current density can be increased), so that the oxide film can be formed quickly. Compared with pure oxalate oxidation, this solution can increase productivity by 37.5% and reduce electricity consumption (the oxalic acid oxidation method consumes 3.32 degrees per square meter, which is 2 degrees per square meter), saving 40% of electricity.

The process formula is: oxalic acid 4 ~ 5%, formic acid 0.55%, three-phase exchange 44 ± 2 volts, current density 2 ~ 2.5A / dm2, temperature 30 ± 2 ° C.

(2) Mixed acid oxidation

This law was officially incorporated into the Japanese National Standard in 1976 and was adopted by Japan North Star Nikko Household Products Co., Ltd. It is characterized by rapid film formation, high hardness, wear resistance and corrosion resistance of the film. It is higher than the ordinary sulfuric acid oxidation method, and the film is silver-white, which is suitable for printing and coloring products. After the inspection of China's aluminum products industry in Japan, it was recommended for use in 1979. The recommended process recipe is: H2SO4 10-20%, COOHCOOH•2H2O 1-2%, voltage 10-20V, current density 1~3A/dm2, temperature 15-30 °C, time 30 minutes.

(3) Porcelain oxidation

Porcelain oxidation mainly uses chromic acid, boric acid, titanium potassium oxalate as an electrolyte, and is treated with high voltage and high temperature for electrolysis. The film layer looks like a glaze on porcelain, has a high degree of corrosion resistance, good wear resistance, and the film layer can be dyed with organic or inorganic dyes to give the appearance a special luster and color. At present, it is widely used in aluminum cookware, lighters, gold pens and other products, and is very popular among the masses.

(4) National Defense Color Oxidation

National Defense color oxidation is mainly used in the decoration of military aluminum products, so it requires special protection. The oxide film is military green, dull, wear-resistant and durable, and has good protection performance. The process is as follows: first, the oxalic acid is oxidized to form a golden yellow film layer, and then anodized by using a solution of potassium permanganate 20 g/l and H 2 SO 41 g/l. Shenyang Aluminum Products Factory used this process to produce military water bottles and cookware supplies.

(5) Multicolor oxidation

The anodized layer which has been dyed but not sealed is wetted with chromic acid or oxalic acid to spread CrO3, and part of the surface of the dyed product is faded after being wetted by CrO3, and oxalic acid or chromium is used in any part as needed. After pickling, it is generally possible to stop reacting with the image. Then dye the second color or repeat the procedures of CrO3 wiping, flushing, dyeing, etc., and flowers, clouds and other patterns can appear as needed. At present, it is widely used in gold cups, water cups, tea boxes, lighters and other products.

(6) Marble dyeing process

After the oxidized product is first dyed with the first base color, dried, and then immersed in water with grease on the surface, and then lifted or immersed, the oil and water hang naturally, so that the film layer is irregularly striped. Contaminated by grease. When the second color is re-dyed, the oxide film is not dyed by the grease stain, and the second color is dyed without the grease, forming an irregular pattern such as a marble pattern. This method can be seen in the article of Comrade Zhou Shouqi, the state-owned Yangjiang Knife Factory in Guangdong ("Electroplating and Finishing", No. 2, 1982).

(7) Chemical etching oxidation

After the aluminum product is mechanically polished and degreased, it is coated with a masking agent or photosensitive, dried and then chemically etched (fluoride or iron salt etchant) to form a concave-convex pattern. After electrochemical polishing and anodizing, it exhibits a strong surface pattern and is comparable to the surface appearance of stainless steel. Now used in gold pens, tea boxes and screens.

(8) Rapid anodizing at room temperature

Usually, H2SO4 oxidation requires cooling equipment, which consumes a lot of electricity. When α-hydroxypropionic acid or glycerin is added, the dissolution of the oxide film can be suppressed, and oxidation can be performed at normal temperature. The film thickness can be increased by a factor of 2 compared to the conventional sulfuric acid oxidation process. The recommended process recipe is:

H2SO4 150~160g/l

CH3CH(OH)COOH 18ml/l

CH2OHCHOHCH2OH 12ml/l

Current density 0.8~12A/dm2

Voltage 12 to 18 volts

Temperature 18~22°C

(9) Chemical oxidation method (also known as conductive oxide film)

The corrosion resistance of the film layer is close to that of the sulfuric acid anodized film. The conductive oxide film layer has a small contact resistance and can conduct electricity, and the H2SO4 anodized film cannot be electrically conductive due to the large contact resistance. The corrosion resistance of the conductive oxide film is much stronger than that of copper plating, silver plating or tin plating on aluminum. The disadvantage is that it cannot be soldered on the film layer and can only be used for spot welding. The recommended process formula is: CrO3 4g / l, K4Fe (CN) 6 • 3H2O 0.5g / l, NaF 1g / l, temperature 20 ~ 40 ° C, time 20 ~ 60 seconds.


Second, the introduction of new foreign technology

In recent years, foreign countries have developed rapidly in the surface treatment of aluminum. The original old processes that cost manpower, electricity and resources have been reformed. Some new technologies and technologies have been widely used in industrial production.

(1) High-speed anodizing

The high-speed anodizing process mainly reduces the impedance of the electrolyte by changing the composition of the electrolytic solution, thereby enabling high-speed anodization with a higher current density. The original process solution uses a current density of 1 A/dm2 to form a film at a rate of 0.2 to 0.25 μ/min. After using this new process solution, the film formation speed can be achieved even with a current density of 1 A/dm 2 . Increasing to 0.4 to 0.5 μ/min greatly shortens the processing time and improves production efficiency. The process and general treatment methods are shown in Table 1.

Table 1

(2) Futian type (high speed oxidation) method

The Futian method has a much shorter processing time than the old process, and the production efficiency can be increased by more than 33%. This method is applicable not only to ordinary anodized films, but also to hard film oxidation. The process is shown in Table 2.

Table 2

If a hard film is to be produced, it is achieved by lowering the temperature of the solution, and the film formation rate is substantially the same as listed above. The relationship between film hardness and solution temperature is as follows:

10 ° C - hardness 500H, 20 ° C - 400H, 30 ° C - 30H

(three) ruby film

The process of forming a ruby film on the surface of aluminum is a novel process, and the color of the film can be comparable to that of the artificial ruby, so that the decorative effect is excellent, and the corrosion resistance and wear resistance are also good. It is also possible to prepare a color having a different appearance by different kinds of metal oxides contained in the solution. The process is: first anodizing with 15% sulfuric acid, using a current density of 1 A / dm2, time 80 minutes. After taking out, the workpiece can be immersed in different concentrations of (NH4)2CrO4 solution according to the degree of color depth, the temperature is 40 ° C, the time is 30 minutes, mainly to let metal ions enter the porous anodized film pore source. Then, sodium hydrogen sulfate (1 gram molecular weight), ammonium hydrogen sulphate (1.5 gram molecular weight), a temperature of 170 ° C, and a current density of 1 A/dm 2 are added. After the above treatment, a purple-red and glittering ruby film can be obtained. . If the immersion is Fe2(CrO4)3, Na2CrO4, the resulting film is blue with deep purple fluorescence.

(4) Asahi method electrolysis coloring

The shallow field method electrolytic coloring is carried out by current electrolysis, and metal cations (nickel salts, copper salts, cobalt salts, etc.) are penetrated into the bottom of the pinhole of the oxide film to form a color. This process has developed rapidly in recent years, mainly because it can obtain bronze and black, which is welcomed by the construction industry. The coloring has a very stable light resistance and is also resistant to harsh weather conditions. This process saves energy compared to natural coloring. Almost all of Japan's architectural aluminum profiles have been colored by this method. This technology and a full set of equipment have also been introduced in Tianjin, Yingkou and Guangdong. Some units in Guangdong have also been tested successfully and applied to production.

(5) Natural coloring method

The natural coloring method is one coloring to complete the coloring. There are also several types of solutions, such as sulfosalicylic acid and sulfuric acid, useful sulfotitanic acid and sulfuric acid, and also sulfosalicylic acid and maleic acid. Since most of the natural coloring methods use organic acids, the oxide film is relatively dense, and the film layer has excellent light resistance, abrasion resistance and corrosion resistance. However, the disadvantage of this method is that in order to obtain excellent color, the composition of the aluminum alloy material must be strictly controlled.


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