Phosphating process for steel
The phosphating process for steel is a systematic process encompassing three main stages: pretreatment, phosphating, and post-treatment. Each stage has stringent process requirements that directly impact the quality and performance of the phosphate film. The purpose of pretreatment is to remove impurities such as oil, rust, and scale from the steel surface, ensuring uniform and continuous formation of the phosphate film. Pretreatment typically includes degreasing, pickling, and water washing. Degreasing can be performed with an alkaline degreaser or organic solvent to remove surface grease; pickling uses hydrochloric acid or sulfuric acid solutions to remove rust and scale; and water washing is used to remove residual degreaser and acid to avoid contamination of the phosphating solution. For workpieces with relatively clean surfaces, the pretreatment steps can be simplified, but the surface must be free of oil and rust.
The phosphating treatment stage is the core link in forming the phosphating film, and parameters such as the composition, temperature, treatment time and pH value of the phosphating solution need to be strictly controlled. The composition of phosphating solution varies depending on the type of phosphating. Zinc-based phosphating solution mainly contains zinc phosphate, zinc nitrate, zinc oxide and other ingredients, while manganese-based phosphating solution is mainly manganese phosphate and manganese nitrate. The concentration of the phosphating solution needs to be controlled within a certain range. Too high a concentration will result in a rough phosphating film, while too low a concentration will make it difficult to form a complete film layer. Temperature has a significant effect on the rate of phosphating reaction. According to the treatment temperature, it can be divided into high-temperature phosphating (80-90℃), medium-temperature phosphating (50-70℃) and room-temperature phosphating (15-35℃). High-temperature phosphating has a fast reaction speed and a dense phosphating film, but it also has high energy consumption. Room-temperature phosphating has low energy consumption and is easy to operate, but it has a slow reaction speed and requires the addition of a catalyst to improve reaction efficiency.
Treatment time is another important parameter in the steel phosphating process, typically ranging from 5 to 20 minutes. The specific time depends on the type of phosphating, temperature, and desired film thickness. If the treatment time is too short, the phosphate film will be too thin and incomplete; if the treatment time is too long, the film will become rough, loose, or even fall off. The pH value also has a significant impact on the formation of the phosphate film. It is generally controlled between 2 and 5. Different types of phosphating solutions have different optimal pH ranges. The pH value of zinc-based phosphating solutions is generally 2.5-3.5, and that of manganese-based solutions is 3.5-4.5. A high pH value accelerates the phosphating reaction but also makes the film loose. A low pH value can lead to excessive corrosion of the steel surface and affect the adhesion of the phosphate film.
During the phosphating process, the solution requires regular maintenance and adjustment to ensure stable performance. Parameters such as concentration, pH, total acidity, and free acidity should be regularly tested. If any parameters deviate from the optimal range, appropriate chemicals should be added promptly for adjustment. Sediment should also be regularly removed from the solution to prevent it from adhering to the workpiece surface and affecting the quality of the phosphate film. For continuous production phosphating lines, an overflow system should be used to continuously replenish the solution to maintain a stable composition.
Post-treatment is the final stage of the steel phosphating process and primarily includes washing, passivation, and drying. Washing removes residual phosphating solution from the workpiece surface to prevent the formation of powdery frosting after drying. Passivation treatment can utilize a chromate solution or a chromium-free passivating agent to further enhance the corrosion resistance of the phosphate film. Drying removes moisture from the workpiece surface to prevent rust. Drying temperatures typically range from 80-120°C, and drying time varies depending on the size and shape of the workpiece. Improper post-treatment can lead to defects such as rust and powdering of the phosphate film, so strict adherence to process requirements is essential.
The phosphating process for steel must be rationally selected and optimized based on the workpiece material, application requirements, and subsequent processing. For example, zinc-based phosphating, used as a pre-painting treatment, typically utilizes a medium- or room-temperature process, achieving a phosphate film thickness of 1-5 microns. Manganese-based phosphating, used for wear resistance treatment, typically utilizes a high-temperature process, achieving a film thickness of 10-30 microns. By optimizing process parameters, a high-performance phosphate film can be obtained, providing reliable protection for steel workpieces.
