Phosphating of steel
Phosphating steel is a surface treatment technique that creates an insoluble phosphate film on the steel surface through a chemical reaction. This film, commonly referred to as a phosphate film, is primarily composed of phosphate compounds such as iron phosphate, zinc phosphate, or manganese phosphate. Phosphate films typically range in thickness from 1 to 50 microns and appear gray, black, or colored. They have a porous structure and effectively improve the corrosion resistance of the steel surface, enhance adhesion to subsequent coatings (such as paint and powder coating), and also act as a lubricant, reducing friction and wear on workpieces during processing. This technology is widely used in the automotive, machining, and hardware industries, and is a key pre-treatment step in coating processes.
The principle of steel phosphating is to use phosphoric acid or a phosphate solution to chemically react with the steel surface, forming a phosphate film. When the steel workpiece is immersed in the phosphating solution, the iron dissolves first. The iron atoms on the steel surface react with the hydrogen ions in the phosphating solution to produce hydrogen gas and ferrous ions, raising the pH of the solution near the workpiece surface. As the pH rises, the phosphate ions in the phosphating solution combine with ferrous and zinc ions (or manganese and calcium ions, etc.) to form insoluble phosphate crystals. These crystals gradually deposit and grow on the steel surface, ultimately forming a continuous, uniform phosphate film. The phosphate film formation process consists of three stages: dissolution, film formation, and crystal growth. The entire process is influenced by factors such as the phosphating solution composition, temperature, and treatment time.
Phosphating treatments for steel can be categorized by the primary components of the phosphating solution: zinc-based, manganese-based, iron-based, and composite phosphating. Zinc-based phosphating is the most widely used, producing a phosphate film with excellent corrosion resistance and paint adhesion, making it suitable for pre-coating applications in the automotive and home appliance industries. Manganese-based phosphating films offer high hardness and excellent wear resistance, primarily used for lubrication and wear-resistant treatment of mechanical parts such as gears and bearings. Iron-based phosphating films are thin, uniform, and low-cost, making them suitable for applications requiring less demanding corrosion resistance. Composite phosphating (such as zinc-calcium and zinc-manganese) combines the advantages of different systems to meet more complex performance requirements.
Phosphating of steel has many advantages. First, the process is simple, easy to operate, does not require complex equipment, and is suitable for large-scale production. Second, the phosphate film is firmly bonded to the substrate and is not easy to fall off. Third, it can significantly improve the coating adhesion on the steel surface and reduce the risk of coating peeling. Finally, the cost is low, the phosphating liquid raw materials are easily available, and the treatment process has low energy consumption. In addition, the phosphating film also has a certain self-repair ability. When the local film layer is damaged, the surrounding phosphating liquid can continue to react to form a new film layer, delaying the occurrence of corrosion.
With increasing environmental protection requirements, steel phosphating technology is also evolving. Traditional phosphating solutions contain heavy metals such as nickel and chromium, which pollute the environment. These solutions are being gradually replaced by nickel- and chromium-free, environmentally friendly solutions. Furthermore, the development of low-temperature and room-temperature phosphating technologies has reduced energy consumption and lowered production costs. In the future, steel phosphating will develop towards environmentally friendly, efficient, and functionalized approaches. Combining these with other surface treatment technologies (such as silane treatment and passivation) will create a more environmentally friendly and efficient surface treatment system.
