Aluminum is a light and soft non-ferrous metal that oxidizes very easily in the air. So, people came up with a solution: the aluminum anodizing process, which grows a protective layer on its surface to make it durable, and it can be dyed in all sorts of colorful shades, which is really pleasing to the eye.
The principle of aluminum anodizing is creating a decorative and protective layer on items made of aluminum and its alloys. The process of forming an aluminum oxide film on aluminum involves using direct current at a sufficient voltage through a suitable sulfuric acid electrolyte. In this setup, the aluminum item acts as the anode, while another material serves as the cathode. Industrial aluminum profiles, after being electrified for a certain amount of time, gradually develop an oxide layer of the desired thickness.
After the film is produced, except for some aluminum with its natural color that doesn't need treatment, the next step is coloring. Generally speaking, aluminum coloring mainly falls into the following types:
1. Organic coloring
2. Inorganic coloring
3. Electrolytic coloring
4. Electroplating coloring
Next, we'll mainly discuss the principles, pros and cons, and usage scenarios of commonly used organic coloring and electrolytic coloring:
1. Electrolytic coloring film is based on a transparent anodized film produced by primary electrolysis in sulfuric acid. The anodized film is then colored electrolytically in a solution containing metal salts using alternating current (also called secondary electrolysis film). The weather resistance, light resistance, and service life of the electrolytic coloring film are much better than those of organic dye films, and its energy consumption and coloring cost are much lower than overall coloring films. Currently, it is widely used for coloring architectural aluminum profiles, but the color options are limited, usually only including antique copper, black, gold, and maroon. Also, it is not easy to control the process.
2. Organic coloring: Organic coloring is based on the adsorption theory of substances. Adsorption can be divided into physical adsorption and chemical adsorption. In industrial aluminum profiles, physical adsorption occurs when molecules or ions are adsorbed via electrostatic forces, while chemical adsorption occurs through chemical forces. The combination of these two results in organic coloring, which is usually done at a certain temperature. Because anodic oxide films have high porosity and strong adsorption capacity, they easily take up organic dyes. This method allows for quick coloring, bright colors, and simple operation. After coloring, a sealing treatment ensures that the dye firmly adheres within the film's pores, enhancing the film's corrosion resistance and stain resistance, allowing the beautiful color to last. It is suitable for a large number of aluminum products that are not intended for outdoor use, indoor aluminum industrial products, and decorative items. The appearance comes in a wide variety of vibrant colors, satisfying modern people's aesthetic tastes and enhancing the market competitiveness of the products. Organic coloring has some characteristics and requirements for the oxide film:
1. The oxide film layer should have enough porosity.
2. The inner wall of the pores should maintain a certain level of activity.
3. Aluminum anodized in sulfuric acid produces a colorless and porous film, making it the most suitable for dyeing.
4. The oxide film layer must be of a certain thickness; a thinner layer can only take a very light color.
5. Hard anodized films and those produced by conventional chromic acid oxidation are not suitable for organic dyeing.
6. The oxide film layer should be complete, uniform, and free of scratches, sand holes, pitting, or other defects.
7. The film itself should have an appropriate color and no metallographic differences, such as uneven grain sizes or severe segregation. Therefore, aluminum alloys have certain requirements as well; if elements like silicon, magnesium, manganese, iron, copper, or chromium are too high, the oxide film can turn dull, causing color variations when dyed.
