Anodic oxidation of aluminum and aluminum alloys
Anodizing of aluminum and aluminum alloys is a surface treatment technology developed based on the characteristics of aluminum material. It generates a dense aluminum oxide film on its surface through electrolysis, which significantly improves the corrosion resistance, wear resistance and decorative properties of aluminum and aluminum alloys. Aluminum is an active metal that naturally forms a thin and loose oxide film in the air. However, the protective performance of this natural oxide film is limited and it is easily destroyed. Anodizing technology can produce an artificial oxide film with a thickness of 5-20 microns (up to hundreds of microns with special processes). This oxide film is firmly bonded to the substrate and has extremely high chemical stability. It can effectively protect aluminum and aluminum alloys from corrosion by corrosive media and is an indispensable key process in the processing of aluminum products.
The principle of anodizing aluminum and its alloys is to utilize electrolysis to induce an oxidation reaction on the aluminum surface and form an oxide film. During the electrolysis process, the aluminum workpiece acts as the anode, and a lead or stainless steel plate acts as the cathode. Both are immersed in an electrolyte (usually a sulfuric acid solution). When a direct current is applied, an oxidation reaction occurs at the anode, where the aluminum loses electrons to form aluminum ions, which combine with oxygen in the electrolyte to form aluminum oxide. A reduction reaction occurs at the cathode, where hydrogen ions gain electrons to form hydrogen gas. As the reaction proceeds, the aluminum oxide film gradually thickens, developing a dual-layer structure: a dense inner barrier layer and a porous outer columnar structure. The porous layer provides excellent adsorption, facilitating subsequent dyeing and sealing processes, while the barrier layer ensures a strong bond between the oxide film and the substrate.
The process parameters of anodizing of aluminum and aluminum alloys have a significant impact on the quality of the oxide film, mainly including electrolyte concentration, temperature, current density, voltage and processing time. The concentration of sulfuric acid electrolyte is usually 10%-20%. Too high a concentration will cause the oxide film to dissolve faster and the film layer to become thinner; too low a concentration will cause the film layer to grow slowly. The temperature is generally controlled at 15-25℃. Rising temperature will accelerate the dissolution of the oxide film, so a cooling system is needed to maintain a stable temperature. The current density is 1-2A/dm². Too high a current density can easily lead to surface ablation, while too low a current density can slow down the film growth. The voltage gradually increases with the thickness of the oxide film, usually between 12-20V. The processing time is determined according to the required film thickness, generally 20-60 minutes, and the film thickness is roughly proportional to the processing time.
Post-anodizing post-processing of aluminum and aluminum alloys is crucial for improving performance, primarily involving dyeing and sealing. Dyeing utilizes the porosity of the oxide film by immersing the workpiece in an organic or inorganic dye solution, allowing the dye molecules to adsorb within the pores. This creates a variety of colors to meet decorative needs. Commonly used organic dyes include acid dyes and direct dyes, while inorganic dyes include iron salts and cobalt salts. Sealing involves filling the pores of the oxide film through physical or chemical methods to improve its density and corrosion resistance. Common sealing methods include hot water sealing (hydrating aluminum oxide to form aluminum hydroxide to fill the pores), steam sealing, and nickel salt sealing. Hot water sealing is the most widely used due to its ease of operation.
Anodizing technology for aluminum and aluminum alloys is widely used in various industrial fields, including construction, transportation, electronics, aerospace, and more. In the construction industry, anodized aluminum profiles are aesthetically pleasing, weather-resistant, and easy to clean, making them widely used in doors, windows, curtain walls, railings, and more. In the transportation sector, anodizing aluminum components for automobiles and high-speed rail systems protects them from wind, rain, erosion, and frictional wear. In the electronics industry, the insulating and wear-resistant properties of anodized films make them ideal for electronic component casings. In the aerospace sector, high-thickness anodized films provide reliable protection for aluminum alloy components, meeting the requirements of use in extreme environments. With the advancement of technology, specialized processes such as color anodizing and hard anodizing have emerged, further expanding the application range of aluminum and aluminum alloys and enabling them to play a vital role in more high-end fields.
