The process of forming a metallic oxide film on the surface of metal products by electrolytic treatment is called anodic oxidation treatment. Anodic oxidation treatment can be applied to aluminum alloy, magnesium alloys, and titanium alloy products. Among them, anodic oxidation treatment to aluminum alloy products is most popular. After anodic oxidation treatment, several microns - hundreds of microns of the oxide film can be formed on the surface of the aluminum part. Compared with the natural oxide film of aluminum alloy, its corrosion resistance, wear-resistance, and decorative properties have been significantly improved. A thin layer of alumina is formed on the surface of anodic aluminum or its alloys. Its thickness is 5-30 microns, and the hard anodic oxide film can reach 25-150 microns.
Direct current sulfuric acid anodization is the most widely used method because it is suitable for the anodic oxidation treatment of aluminum and most aluminum alloys. The film is thicker, hard and wear-resistant, and can obtain better corrosion resistance after sealing. Besides, the film is colorless and transparent, with strong adsorption capacity and easy to be colored. Moreover, the treatment voltage is low, the power consumption is low, and the treatment process is good. Voltage cycle must be changed, which is conducive to continuous production and automation of practical operation. Last but not least, sulfuric acid is less harmful to people than chromic acid, and has advantages of wide supply and low price.
The oxide film itself is a porous structure, so after the formation of the oxide film, it needs to be sealed. The common sealing methods include High-temperature hydration reaction sealing, steam sealing, inorganic salt sealing. To anodize aluminum parts with color, a dyeing process is introduced after the formation of the oxide film and before the sealing. The dyeing process filled the pores in the oxide file with colored dyes.
Hard anodizing is basically the same process as anodizing but with higher voltage, higher current, and lower temperature agent to form thicker and harder oxide film.
The dyeing process can also be introduced in hard anodizing.
1000, 3000, 4000, 5000 and 6000 Series aluminum alloy works quite well in anodizing. They form different colors easily and oxidize layer provides good protection.
2000 series aluminum alloys can be anodized, but the process does not work as well as for alloys in terms of corrosion resistance. Because of the high copper content in this series of alloys, the dyed color tends to be darker than other series of alloys and sometimes pitting.
7000 series aluminum alloys can form a fairly good oxide layer in terms of anti-corrosion. A clear anodizing on 7000 series looks a bit ‘brown'. But this group of alloys still can be color anodized with fairly good result.
The oxide layer's thickness formed by anodizing is 5~30um for normal anodizing and 25~150um for hard anodizing. The pre-treatment for anodizing involves acid and alkaline cleaning, which can ‘eat-up' size of the treated parts. For tight tolerance aluminum parts, the anodizing process has to be carefully executed. All parameters such as acid cleaning time, current, voltage, etc have to be carefully regulated.
For prototype parts or small volume parts, it is still easier to do a clear anodizing or a black anodizing as those 2 colors are most popular and easy to find. In an average for any color, there is a setup cost, so if a rather rare color is required, the setup cost would fall on to the customer who requires that color, while if the clear or the black color is the choice, setup cost would fall on large numbers of customers who requires the colors.
Anodizing for Cast Aluminum Alloy
In the alloy, copper will make the oxide film reddish, destroy the quality of electrolyte and increase the oxidation defects; silicon will make the oxide film grey, especially when the content is more than 4.5%, the effect is more obvious; iron will exist in the form of black spots after anodizing due to its characteristics.
Cast aluminum alloys and die castings generally contain high silicon content. The anodic oxide film is dark, and it is impossible to obtain colorless and transparent oxide film. With the increase of silicon content, the color of anodic oxide film ranges from light gray to dark gray to black-gray. Therefore, anodizing oxidation does not work well for cast aluminum alloy.
Commonly used die-casting aluminum alloys can be divided into 4 main categories:
Aluminium-silicon alloy, mainly including ADC1, A413.0, ADC3, A360.
Al-Si-Cu alloy mainly includes A380, ADC10, A383, ADC12, B390, ADC14.
Al-Mg alloy, mainly contains 5180, ADC5, ADC6.
Aluminum manganese cobalt alloy DM32, aluminum manganese magnesium alloy DM6
For Al-Si and Al-Si-Cu alloys, as its name implies, the main components are silicon and copper and aluminum. Normally, the content of silicon is between 6-12%, the content of copper is next, and the content of iron is usually between 0.7-1.2%. This kind of alloy is very hard to oxidize and dye. Even with desilication oxidation, it is still difficult to achieve the desired effect. For Al-Si alloy or aluminum alloy with high copper content, it is difficult to form an oxide film, and even it is formed, the film tends to be dark, gray and glossy.
For Al-Mg alloy, the oxide film is easier to be formed and the quality of the film is better than the other first 2 kinds of cast alloy. It can be oxidized and colored. But the protection against corrosion and the color effect are far from satisfaction.
Only the last groups of cast alloy - aluminum manganese alloys work well with anodizing process regards to anti-corrosion and coloring.
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