Hard anodizing technology
Hard anodizing technology is a special anodizing process that generates a thicker and harder oxide film on the metal surface by optimizing electrolysis parameters and electrolyte formula, thereby significantly improving the metal’s wear resistance, corrosion resistance and high temperature resistance. Compared with ordinary anodizing, the oxide film thickness generated by hard anodizing can reach 20-200 microns, and the microhardness is usually between 300-600HV. Some processes can even exceed 1000HV. It can effectively resist mechanical wear and chemical corrosion and is an ideal choice for surface treatment of high-end mechanical parts, tools and wear-resistant parts. This technology is widely used in aerospace, automobile manufacturing, mold processing, petrochemical and other fields, such as surface strengthening treatment of engine pistons, hydraulic piston rods, mold cavities and other parts.
The core principle of hard anodizing technology is the same as that of ordinary anodizing, which is to use electrolysis to cause an oxidation reaction on the metal surface to form an oxide film. However, there are significant differences between the two in terms of process implementation. Hard anodizing usually uses a lower electrolyte temperature (0-10°C) to slow down the dissolution rate of the oxide film and promote the thickening of the film; at the same time, it increases the current density (2-5A/dm²) to accelerate the oxidation reaction. In addition, the concentration and composition of the electrolyte are specially formulated to inhibit the dissolution of the oxide film and promote the formation of a dense and hard oxide film. During the electrolysis process, the metal workpiece acts as the anode and an oxide film is continuously generated on its surface. The growth and dissolution of the oxide film are in a dynamic equilibrium. By precisely controlling the process parameters, the film growth rate can be made greater than the dissolution rate, thereby obtaining a thicker hard oxide film.
The hard anodizing process primarily consists of three stages: pretreatment, electrolytic oxidation, and post-treatment. Pretreatment significantly impacts the quality of the oxide film and requires thorough removal of oil, rust, scale, and machining marks from the workpiece surface. Degreasing, pickling, and mechanical polishing are typically performed. Degreasing can be performed with alkaline solutions or organic solvents to ensure a grease-free surface. Pickling uses dilute nitric acid or phosphoric acid solutions to remove scale and rust. Mechanical polishing improves surface finish and reduces defects in the oxide film. The electrolytic oxidation stage is the core of the process. The pretreated workpiece is placed in a specific electrolyte, with controlled parameters such as temperature, current density, voltage, and treatment time. The temperature is generally maintained between 0-5°C and maintained by a refrigeration system. The current density is 2-5A/dm², and the voltage increases with film thickness, typically between 20-60V. Treatment time is determined by the desired film thickness and is generally 30-120 minutes.
The oxide film produced by hard anodizing has a unique structure and properties. The oxide film has a multi-layer structure, with an inner layer being a dense barrier layer and an outer layer being a porous columnar structure, but the porosity is lower than that of ordinary anodized films, and the film layer is thicker and denser. This structure gives the oxide film extremely high hardness and wear resistance, while also possessing certain corrosion resistance and insulation properties. In addition, the oxide film is firmly bonded to the substrate, is not easy to fall off, and can withstand large impacts and loads. However, hard anodized films also have some limitations, such as the film layer is relatively brittle and prone to cracking; the color is dark, usually gray-black or dark brown, and the decorative effect is poor; the processing process consumes a lot of energy, and the production cost is relatively high.
With the development of industrial technology, hard anodizing technology continues to innovate and optimize. The development of new electrolyte formulas has improved the performance of oxide films. For example, the addition of organic acids or rare earth elements can further enhance the hardness and toughness of the film. Advances in low-temperature refrigeration technology have enabled precise control of electrolyte temperature, improving the stability of oxide film quality. The application of automated production lines has reduced labor costs and increased production efficiency. Furthermore, composite treatment techniques (such as sealing or applying wear-resistant coatings after hard anodizing) further enhance the overall performance of metal surfaces. In the future, hard anodizing technology will develop towards high efficiency, energy conservation, and environmental protection, playing a vital role in more high-end manufacturing fields and providing a superior solution for the long-term wear protection of metal parts.
