Hard anodizing in non-sulfuric acid solutions
Hard anodizing in non-sulfuric acid solutions is relative to traditional sulfuric acid solutions. It uses other acid solutions such as oxalic acid, phosphoric acid, chromic acid, etc. as electrolytes. It is a hard anodizing process developed for specific materials and performance requirements. This type of process can overcome the limitations of sulfuric acid solutions in certain applications. For example, hard anodizing with oxalic acid solution can produce a thicker and denser oxide film, which is suitable for occasions with high requirements for corrosion resistance; phosphoric acid solution is suitable for hard oxidation of aluminum alloys. The resulting oxide film has good lubricity and is often used on friction parts such as pistons. The choice of non-sulfuric acid solution needs to be determined according to the substrate material and coating performance requirements. The oxide films generated by different electrolytes have significant differences in composition, structure and performance.
Hard anodizing with oxalic acid is one of the most widely used non-sulfuric acid systems. The electrolyte is primarily oxalic acid, typically at a concentration of 3%-10%. The oxidation temperature is controlled at 0-15°C, lower than the processing temperature of sulfuric acid. The oxide film produced in oxalic acid can reach a thickness of 50-150μm, with a hardness comparable to that of sulfuric acid (300-600 HV), but exhibits superior corrosion resistance. This is because the oxalate ions participate in the formation of the oxide film, making it denser. This process is particularly suitable for hard anodizing aluminum alloys. The resulting oxide film is dark gray and can be used directly or dyed. It is widely used in the aerospace industry to strengthen components.
Hard anodizing with phosphoric acid solutions is primarily used where excellent lubricity is required. The phosphoric acid concentration in the electrolyte is 10%-20%, and the treatment temperature is 20-40°C, which is higher than that of sulfuric and oxalic acid systems. The resulting oxide film has high porosity and contains phosphate ions, making it self-lubricating. While its hardness is slightly lower than that of sulfuric acid systems (200-400 HV), it exhibits excellent wear resistance and seizure resistance, making it a common surface treatment for friction components such as engine pistons and bearings. Another advantage of phosphoric acid oxidation is its adaptability to high-silicon aluminum alloys, reducing the film porosity caused by silicon , thus expanding the range of materials suitable for hard anodizing.
Hard anodizing with chromic acid solution was once used in high-end fields due to the strong bonding between the oxide film and the substrate. However, its electrolyte contains highly toxic hexavalent chromium, which is harmful to the environment and human body. It has now gradually been replaced by environmentally friendly processes. However, it is still used in a small number of applications in certain situations where extremely high bonding strength is required (such as aerospace precision parts). Its oxide film is relatively thin (5-20μm), gray-green in color, and has excellent corrosion resistance and insulation properties. To reduce toxicity, some processes use trivalent chromium instead of hexavalent chromium. Although the performance is slightly different, the environmental performance is significantly improved.
The process parameters for hard anodizing in non-sulfuric acid solutions are more strictly controlled, and the current density, temperature, and treatment time need to be adjusted according to the type of electrolyte. The current density of the oxalic acid system is usually 1-3A/dm², and the treatment time is 30-90 minutes; the current density of the phosphoric acid system is 2-5A/dm², and the treatment time is 20-60 minutes. Due to the large differences in the conductivity and stability of non-sulfuric acid electrolytes, more precise temperature control and stirring systems are required to ensure a uniform and stable oxidation process. In the future, hard anodizing in non-sulfuric acid solutions will develop in the direction of environmental protection and functionality. By optimizing the composite electrolyte formula, it will improve performance while reducing environmental risks and expand its application in new energy, high-end manufacturing and other fields.
