Aluminum and aluminum alloys have been widely used due to their superior material properties. At the same time, the surface decoration and protection technology has also developed rapidly. Among these surface protection technologies, the most commonly used is the anodizing, electrolytic coloring, and sealing processing of the material surface.
After anodized aluminum or aluminum alloy, an oxide film (Al?O?) is formed on the surface of the metal. The surface of the oxide film is a porous structure, the texture is loose, and the underlying layer is dense. For aesthetic or other special needs, the oxide film is often subjected to electrolytic coloring to color the surface of the material. Due to the loose texture of the surface layer of the oxide film, its wear resistance and corrosion resistance are poor. Therefore, following the oxidation and coloring, it is also necessary to perform a key process of sealing treatment.
The principle of closed treatment is that the porous oxide film is hydrolyzed by hot water or chemical solution to form Al(OH)3 (a variety of hydrolysates are formed in chemical sealing), and the pores of the oxide film are filled with pores after volume expansion. Therefore, the wear resistance and corrosion resistance of the workpiece surface are greatly improved. At present, people have developed a variety of closed technologies, such as heat sealing with boiling water, high temperature water vapor closure, medium temperature chemical sealing and cold sealing.
Boiling water heat sealing is mainly used to boil the dyed workpiece in boiling water for about 30 minutes. At present, this kind of process requires high quality water, must be distilled water, and must be deionized, PH value between 5-7, temperature 98 Â°C. High-temperature steam sealing is a closed process proposed by Germany. It is to seal the dyed workpieces in high-pressure, 100Â°C-110Â°C steam. This method is slightly faster and the effect is slightly better. However, high-pressure steam is required in a closed container. This method has high equipment costs and is not suitable for large-scale processing. Medium temperature chemical blocking is the use of dichromate with strong oxidizing properties, and the formation of basic aluminum chromate and basic aluminum dichromate precipitation at high temperature (90Â°C) with oxide film and hydration of aluminum oxide will close the pores It is considered to be a kind of sealing technology with good corrosion resistance of oxide film in various sealing methods. Cold sealing is firstly a closed process proposed by Italy. It is a NiF2-based chemical solution in which the workpiece is immersed at room temperature. Ni(OH)2 and Al(OH)3 are mixed by ion exchange and hydration reactions. Hydrates are deposited so that the pores of the oxide film are closed. This method requires low water quality, low energy consumption at normal temperature and short closing time. The disadvantage of the technology is that nickel has serious environmental pollution and personal injury. This method has not obtained international recognition so far.
The existing closed process basically meets the application requirements of the conventional applications, but in some areas where the cleanliness of the workpiece surface is demanding (such as inertial confinement fusion, aerospace, etc.), most of the above closed processes cannot meet the requirements. After the closed parts are wiped with alcohol, there will be a lot of fading phenomenon, the wear resistance and corrosion resistance of the surface of the oxide film is poor, and it is easy to cause pollution to the clean environment. The cost of individual processes that can meet the requirements is very high, which is several dozen times that of ordinary processes.
In order to solve this problem, Yan Yadong, Wang Feng, He Junhua and other researchers of the Institute of Advanced Optical Technology of the Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences proposed an aluminum oxide anodized electrolytic coloring closure process. Because of the gradual decrease of the hydrolysis temperature and the long cycle of the whole process, the hydrolysis process gradually becomes slower, which provides sufficient time for the hydrolysate to fully and densely adhere to the surface of the oxide film. By slowing the temperature down to room temperature, the phenomenon of rapid dehydration and cracking of the hydrolyzed film caused by the rapid cooling of the workpiece by the conventional boiling water sealing process is avoided, and the surface cracking greatly reduces the wear resistance and corrosion resistance of the surface. In the invention, after the common anodized black dyed workpiece is closed, it is repeatedly wiped with a scouring pad dipped in alcohol, and the scouring pad is clean as before. This technology is simple, low cost, no pollution, good sealing effect, has been verified in inertial confinement fusion and space load, and has now been authorized by the national invention patent.
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