Phosphating And Phosphating Film
Phosphating is a surface treatment technology that forms a non-metallic, non-conductive phosphate conversion film on metal surfaces through a chemical reaction. Its core principle is to utilize phosphoric acid or a phosphate solution to react with the metal substrate to produce a strongly adherent phosphate film. This film is primarily composed of metallic phosphate crystals and typically ranges in thickness from 0.5 to 50 μm, depending on parameters such as treatment time, temperature, and solution composition. Phosphating is widely used in the surface treatment of metal materials such as steel, zinc, and aluminum, particularly in the automotive manufacturing, machining, and hardware industries, where it is an essential pretreatment step before painting. Compared to other surface treatment technologies, phosphating offers advantages such as low cost, ease of operation, and stable treatment results, significantly improving the corrosion resistance of metal surfaces and coating adhesion.
The formation of a phosphate film is a complex electrochemical and chemical process, primarily involving the dissolution of the metal surface, the diffusion of phosphate ions, and the nucleation and growth of phosphate crystals. When a metal workpiece is immersed in a phosphating solution, the metal surface dissolves first, releasing metal ions (such as Fe²⁺ and Zn²⁺ ), which raises the pH of the solution near the workpiece surface . As the pH rises, phosphate ions ( PO₄³⁻ ) combine with the metal ions to form insoluble phosphate crystals. These crystals first nucleate at active sites on the metal surface (such as grain boundaries and defects), then gradually grow and interconnect, ultimately forming a continuous, uniform phosphate film. Different phosphating systems (such as zinc, manganese, and iron) vary in the crystal structure and properties of their films. For example, zinc-based phosphate films, composed of zinc phosphate crystals, have a dense structure and strong corrosion resistance, making them suitable for paint bases. Manganese-based phosphate films, on the other hand, offer high hardness and wear resistance and are often used to treat friction surfaces.
The properties of phosphate coatings primarily include corrosion resistance, adhesion, hardness, and porosity, which directly impact their application effectiveness. Corrosion resistance is one of the most important properties of phosphate coatings and is typically evaluated through salt spray testing. Zinc-based phosphate coatings can withstand neutral salt spray testing for up to 48-96 hours, effectively preventing rust on metal substrates before coating. Adhesion between the phosphate coating and subsequent coatings is crucial for ensuring coating quality. Its porous structure creates a mechanical bond with the coating, significantly improving adhesion. Test data from an automobile manufacturer showed that the adhesion of the electrophoretic coating on phosphate-treated steel sheets (cross-cut test) reached level 0, while the adhesion of unphosphated steel sheets was only level 3. Furthermore, the hardness of phosphate coatings, generally between 200 and 500 HV, provides some wear resistance, but their high porosity (typically 10%-30%) also lends them to good adsorption capacity, facilitating subsequent painting or lubrication.
Phosphating process parameters significantly impact the quality of the phosphate film, primarily including the temperature of the phosphating solution, pH, treatment time, and the type and concentration of accelerators. The phosphating solution temperature typically ranges from 30-90°C, with different phosphating systems requiring different temperatures. Zinc-based phosphating is generally performed at temperatures between 50-70°C. Excessively high temperatures can result in a rough film, while too low temperatures can lead to slow and uneven film formation. The pH value is a key parameter in controlling the phosphating reaction, typically ranging from 1.5 to 3.0. A low pH value results in rapid metal dissolution, making film formation difficult; a high pH value leads to rapid phosphate crystal precipitation, resulting in a loose film. Treatment time typically ranges from 5-20 minutes. Too short a treatment time results in insufficient film thickness, while too long a treatment time can lead to cracking or detachment. Accelerators (such as nitrates, nitrites, and chlorates) can accelerate the phosphating reaction, shorten treatment time, and improve film quality. For example, adding an appropriate amount of sodium nitrate to the zinc-based phosphating solution can shorten the phosphating time from 15 minutes to 8 minutes, and significantly improve the density of the film layer.
Phosphating technology is trending toward environmental friendliness, high efficiency, and multifunctionality. Traditional phosphating processes use phosphating solutions containing heavy metals such as nickel and manganese, which pollute the environment. With increasingly stringent environmental regulations, nickel-free, manganese-free, and low-slag phosphating technologies have rapidly developed. By adjusting the formulation of nickel-free zinc-based phosphating solutions, high-performance phosphating films can be achieved without the addition of nickel, reducing wastewater treatment costs by over 30%. Regarding efficiency, the development of new accelerators and optimized process parameters have enabled rapid, low-temperature phosphating. For example, at 30-40°C, a qualified phosphating film can be formed within 5 minutes, significantly reducing energy consumption. Regarding multifunctionality, combining phosphating with other surface treatment technologies, such as phosphating-silane treatment and phosphating-passivation treatment, can further enhance the corrosion resistance and coating adhesion of metal surfaces. For example, silane treatment after phosphating can increase salt spray resistance of metal surfaces by 2-3 times. With the continuous advancement of these technologies, the application of phosphating in metal surface engineering will become more extensive, making important contributions to the quality improvement of industrial products and environmental protection development.
