Hard anodizing in sulfuric acid solution
Hard anodizing with sulfuric acid is currently the most widely used hard anodizing process. Using sulfuric acid as the primary electrolyte, and through precise control of process parameters, it produces a thick, hard oxide film on aluminum and aluminum alloy surfaces. This process offers advantages such as low cost, ease of operation, and excellent film properties. It is suitable for hard anodizing of most aluminum and aluminum alloys and plays an important role in aerospace, automotive, and mold processing. Compared to other electrolytes (such as oxalic acid and chromic acid solutions), sulfuric acid offers greater adaptability and can process a wide range of aluminum alloys. The resulting oxide film exhibits high hardness and wear resistance, making it one of the preferred hard anodizing processes in industrial production.
The principle of hard anodizing in sulfuric acid solution is basically the same as that of ordinary sulfuric acid anodizing, but the process parameters are more stringent. In sulfuric acid electrolyte, the aluminum workpiece acts as the anode and undergoes oxidation reaction under the action of direct current to form an aluminum oxide film. Sulfuric acid in the electrolyte mainly plays the role of conductivity and controlling the dissolution rate of the oxide film. Its concentration and temperature have a significant impact on the growth and performance of the oxide film. During the hard anodizing process, the sulfuric acid concentration is usually controlled between 10% and 20%. Too high a concentration will accelerate the dissolution of the oxide film, which is not conducive to the thickening of the film layer; too low a concentration will result in poor electrolyte conductivity and slow oxidation reaction. At the same time, the electrolyte temperature needs to be strictly controlled at 0-5°C. The low temperature can inhibit the dissolution of the oxide film and promote the growth of the film layer. This is the key to achieving hard anodizing in sulfuric acid solution.
The process parameters for hard anodizing with sulfuric acid solutions significantly influence the quality of the oxide film. These include sulfuric acid concentration, electrolyte temperature, current density, voltage, and treatment time. The sulfuric acid concentration is generally 15%-20%, which ensures electrolyte conductivity while effectively controlling the dissolution rate of the oxide film. The electrolyte temperature is a critical parameter and must be maintained between 0-5°C. An ice bath or refrigeration unit is typically used for cooling, and temperature fluctuations should be controlled within ±1°C. Otherwise, the oxide film thickness will be uneven and the hardness will decrease. The current density is 2-5A/dm². The initial current density should be low and gradually increased to the set value as the oxide film forms to avoid surface ablation. The voltage increases gradually with increasing oxide film thickness, typically between 20-60V, with a maximum of no more than 70V to avoid sparking and oxide film breakdown. The treatment time is determined by the desired film thickness and is generally 60-120 minutes, with film thicknesses of 50-150 microns possible.
The hard anodizing process using sulfuric acid solution consists of pretreatment, electrolytic oxidation, and post-treatment. Pretreatment thoroughly removes oil, scale, and machining marks from the workpiece surface. The steps include degreasing, pickling, neutralization, and water washing. Degreasing uses an alkaline solution or organic solvent to remove surface grease. Pickling uses a dilute nitric acid solution to remove scale and rust. Neutralization eliminates residual acid after pickling, typically using a dilute sodium carbonate solution. Water washing ensures a clean surface and prevents impurities from entering the electrolyte. During the electrolytic oxidation stage, the pretreated workpiece is placed in a sulfuric acid electrolyte, electrodes are connected, and the cooling system and power supply are activated. The process is carried out according to the set process parameters, with the electrolyte constantly stirred to ensure a uniform temperature. Post-treatment includes water washing, sealing, and drying. Water washing removes residual sulfuric acid solution from the surface. Sealing uses hot water or steam to fill the pores of the oxide film and improve corrosion resistance. Drying is performed at 60-80°C to prevent cracking of the oxide film.
The hard anodizing technology of sulfuric acid solution has significant advantages, but also has certain limitations. Its advantages are: the electrolyte is low-cost and easy to obtain; the process is mature, easy to operate, and suitable for large-scale production; the oxide film has high hardness and good wear resistance, which can meet the requirements of most working conditions; and it is suitable for a variety of aluminum and aluminum alloy materials. The main limitations include: high requirements for electrolyte temperature control and high energy consumption; poor treatment effect on high-silicon aluminum alloy, which easily leads to loose oxide film; dark oxide film color and poor decorative effect. To overcome these limitations, the industry often uses composite sulfuric acid electrolytes with added organic acids (such as oxalic acid, citric acid) or rare earth elements to improve the performance and process adaptability of the oxide film. In the future, with the development of energy-saving technologies and new additives, hard anodizing technology using sulfuric acid solution will become more efficient and environmentally friendly, and its application in the industrial field will become more extensive.
