The New Aluminum and the Sealing Process of Aluminum Alloys

Aluminum and aluminum alloys have been widely used because of their superior material properties. At the same time, the decoration and protection technology of the material's surface has also been rapidly developed. Among these surface protection technologies, the anodic oxidation, electrolytic coloring, and sealing treatment processes are the most commonly used on the surface of aluminum and aluminum alloys.
 
After aluminum or aluminum alloys are subjected to anodizing treatment, an oxide film (Al2O3) will be formed on the metal's surface. The surface layer of the oxide film has a porous structure with a loose texture and a dense bottom layer. For good appearance or other special requirements, the oxide film is often electrolytically colored to color the surface of the material. Because the surface layer of the oxide film has a loose texture, its wear resistance and corrosion resistance are poor. Therefore, the key process of sealing treatment is required after oxidation and coloring.
 
The principle of sealing treatment is to hydrolyze the porous oxide film through the hot water or chemical solution to form Al(OH)3. A variety of hydrolysates are formed in the chemical sealing process. The micropores on the surface of the oxide film are filled after volume expansion, so that the wear resistance and corrosion resistance of the surface of the workpiece are greatly improved. At present, people have developed a variety of sealing technologies, such as boiling water thermal sealing, high-temperature steam sealing, medium temperature chemical sealing, and cold sealing.
 
Boiling water heat sealing is mainly to boil the dyed workpiece in the boiling water for about 30 minutes. At present, this process requires high requirements for the water's quality. It must be distilled water which is deionized, with pH values from 5 to 7, and temperatures 98℃. High-temperature water vapor sealing is a sealing process proposed in Germany. It is a process that puts the dyed workpiece in the steam with high pressure and temperatures between 100℃ and 110℃. The sealing speed of this method is slightly faster, and the effect is a little better. However, high-pressure steam is required to be in a closed container, and this method has high equipment costs and is not suitable for processing large items.
 
The medium temperature chemical sealing uses strong oxidizing dichromate to interact with the oxide film at a high temperature of 90℃ and form alkali aluminum chromate, alkali aluminum dichromate precipitation and hydrate of aluminum oxide to seal the pores. It is considered to be a sealing technology with better corrosion resistance for the oxide film in various sealing methods.
 
Cold sealing is a sealing process firstly proposed in Italy. It is a process that immerses the workpiece in a chemical solution mainly contained NiF2 at room temperature, and forms a mixture of hydrate deposits with Ni(OH)2 and AI(OH)3 through ion exchange and hydration reactions, thus sealing the pores of the oxide film. This method has low requirements on water, low energy consumption at room temperature, and short sealing time. The disadvantage of the technology is that nickel causes serious environmental pollution and personal damage. This method has not been recognized internationally so far.
 
The existing sealing technology meets the requirements for conventional applications. However, in some areas where the surface cleanliness of the workpiece is demanding, such as inertial confinement fusion, aerospace, etc. Most of the above-mentioned sealing technologies cannot meet the requirements. When the parts which are subjected to the sealing process are wiped with alcohol, fading will occur greatly. The wear resistance and corrosion resistance of the oxide film's surface are poor, which is easy to pollute the clean environment. The cost of individual processes that can meet the requirements is very high, which is dozens of times higher than that of ordinary processes.
 
In order to solve this problem, researchers from the Advanced Optical Technology Laboratory of Optics and Precision Machinery of Xi'an Institute proposed a sealing process of anodic oxidation and electrolytic coloring for aluminum alloys. This process has a long period, and the hydrolysis process changes from gradually being fast to being slow due to the gradual decrease of the hydrolysis in temperature, which provides sufficient time for the hydrolysate to fully and densely adhere to the oxide film's surface. The rapidly dehydrated and cracked hydrolysis film caused by the workpiece being rapidly cooled from high temperature in the conventional boiling water sealing process is avoided through the process of slowly reducing the temperature to room temperature. Cracks on surfaces will greatly reduce the wear resistance and corrosion resistance of the surface. Use a scouring pad dipped in alcohol to repeatedly wipe the workpiece which is subjected to an ordinary sealing treatment of anodic oxidation and electrolytic coloring, and the scouring pad is as clean as ever. The sealing treatment of anodic oxidation and electrolytic coloring has advantages of simple technology, low costs, no pollution, and a good sealing effect. It has been verified in inertial confinement fusion and aerospace loads and has now been patented in China.