Chemical oxidation of aluminum and aluminum alloys
Chemical oxidation of aluminum and aluminum alloys involves immersing them in a chemical solution, where a chemical reaction creates an oxide film on the surface. Compared to anodizing, this technique requires no external power source, is simple to operate, and is relatively low-cost. The resulting oxide film is typically 0.5-4 microns thick, with colors ranging from colorless and transparent to light gray or golden, depending on the oxidizing solution used. This oxide film, primarily composed of aluminum oxide and aluminum hydroxide, is thin but possesses excellent adsorption and corrosion resistance, enhancing the bond between aluminum and aluminum alloy surfaces and coatings and adhesives. It is often used as a pretreatment for processes such as painting, printing, and bonding. For example, chemical oxidation followed by spray coating of aluminum alloy doors and windows strengthens the coating’s bond to the substrate, preventing it from peeling off and improving both decorative properties and weather resistance.
The principle of chemical oxidation of aluminum and aluminum alloys is to utilize the chemical activity of aluminum to produce an oxidation reaction in a specific chemical solution, forming a dense oxide film. Aluminum is an amphoteric metal that reacts with both acids and bases. Therefore, chemical oxidation solutions can be divided into two categories: acidic and alkaline. Acidic oxidizing solutions usually contain ingredients such as chromic acid, phosphoric acid, and sulfuric acid. For example, chromate oxidizing solutions oxidize the aluminum surface under acidic conditions, forming an oxide film containing chromate, which has good corrosion resistance and adsorption properties. Alkaline oxidizing solutions are composed of sodium hydroxide, sodium carbonate, sodium silicate, etc. In an alkaline environment, an aluminum oxide film is formed on the aluminum surface, which is suitable for applications where corrosion resistance is not a high requirement. The oxide film formation process includes the dissolution and oxidation of aluminum and the deposition of the film. The oxide film generated by the reaction gradually covers the aluminum surface, preventing further reaction.
The chemical oxidation process for aluminum and aluminum alloys includes pretreatment, chemical oxidation, and post-treatment. Pretreatment is critical to ensuring the quality of the oxide film and requires the removal of surface oil, natural oxide film, and impurities. Pretreatment steps include degreasing, alkaline cleaning, pickling, and water washing. Degreasing can be performed with organic solvents or alkaline solutions to remove surface grease. Alkaline washing uses a dilute sodium hydroxide solution to remove the natural oxide film and surface impurities, but the duration must be controlled to avoid excessive corrosion. Pickling, typically with nitric acid, neutralizes residual alkaline substances and further removes the oxide film. Water washing removes residual chemical reagents and ensures a uniform oxidation reaction. The chemical oxidation stage involves immersing the pretreated workpiece in the oxidizing solution at a specific temperature (usually 20-60°C) for 1-10 minutes, with the specific time determined by the required oxide film thickness. Post-treatment includes water washing, sealing, and drying. Water washing removes residual oxidizing solution from the surface. Sealing can be performed with hot water rinsing or dip-coating with a sealant to fill the micropores of the oxide film and improve corrosion resistance. Drying is performed at room temperature or low temperature to prevent cracking of the oxide film.
Chemical oxidation technology of aluminum and aluminum alloys has many advantages, which makes it widely used in industrial production. First, the process is simple and requires little equipment investment, making it suitable for small and medium-sized enterprises. Second, the processing time is short, the production efficiency is high, and mass production can be achieved. Third, the oxide film has good adsorption properties and can significantly improve the adhesion of the coating, making it an ideal process for pre-treatment before painting. Finally, it is suitable for workpieces of various shapes, especially complex structural parts, and the oxide film has good uniformity. However, this technology also has some limitations: the oxide film is thin and has limited corrosion resistance, which is not as good as the anodized film; the oxide film has low hardness and poor wear resistance; some acidic oxidizing liquids contain toxic substances such as chromate, which pollutes the environment and requires environmentally friendly treatment.
With increasing environmental protection requirements and technological advancements, chemical oxidation technology for aluminum and aluminum alloys has made significant progress in environmental friendliness and functionalization. The development of new environmentally friendly oxidation solutions, such as chromium-free oxidation solutions (based on zirconium, titanium, silicon, and other ingredients), has replaced traditional chromate oxidation solutions, reducing the use of toxic substances and mitigating environmental risks. Furthermore, the development of functional oxide films has expanded their application areas. For example, by adding nanoparticles or rare earth elements, the hardness, wear resistance, and corrosion resistance of oxide films can be improved. By adjusting oxidation process parameters, oxide films can be endowed with special functions such as conductivity and thermal insulation. Furthermore, the combined application of chemical oxidation with other surface treatment technologies (such as chemical oxidation followed by anodizing or nano-coating) can leverage their respective strengths to further enhance the surface properties of aluminum and aluminum alloys. In the future, chemical oxidation technology for aluminum and aluminum alloys will continue to develop in an environmentally friendly, efficient, and multifunctional direction, playing a more important role in the automotive, aerospace, construction, electronics, and other fields.
