At present, in industrial applications, most of them are oxidized or phosphated on the surface of steel workpieces to improve their protective and decorative properties. Generally speaking, compared with phosphating treatment, the film obtained by oxidation treatment has richer color and better decorative performance, but poor corrosion resistance. Compared with the oxidation treatment, the color of the phosphating treatment film is obviously dark gray, which is not as good as the oxide film, but the corrosion resistance is worse than that of the oxide film. Therefore, to make the phosphating film have good corrosion resistance and good decorative effect, the method of black phosphating treatment of steel parts can be used.
The film layer obtained by black phosphating treatment of steel parts is black, with good adhesion, uniform color, continuous film layer and fine crystal. In industrial application environment and marine atmosphere, the corrosion resistance of black phosphating film is higher than that of ordinary phosphating film, and the film has better light absorption, heat absorption, insulation and other properties. The black phosphating film generally does not affect the accuracy of the workpiece, and can reduce the diffuse reflection of the surface. Therefore, the parts of some electromechanical equipment on the market are protected and decorated with black phosphating film, especially the precision cast steel parts are used the earliest. The black phosphating process has been widely used in various industries such as machinery, electronics, instrumentation, aviation, defense industry, ships and automobiles.
The typical formula and process conditions of black phosphating solution are shown in Table 2-27.
Factors affecting the quality of black phosphating
  details as follows:
(1) concentration. The solution plays a major role in film formation, and at the same time can deepen the color of the phosphating film. However, if the concentration is too large, the quality of the phosphating film will be deteriorated.
(2) concentration. The solution can not only promote the film formation, but also deepen the color of the phosphating film. The properties of Ni2+ are similar to those of ions in the solution, such as Ni2+, which participate in the film formation, and can partially replace the phosphate film, refine the crystal grains of the phosphate film, deepen the color of the phosphate film, and enhance the combination of the phosphate film and the substrate. force to improve the wear resistance of the phosphating film.
(3) Surfactant. A small amount of surfactant in the phosphating solution can improve the wettability and permeability of the phosphating solution on the surface of the steel workpiece, improve the uniformity and compactness of the phosphating film crystal, and improve the corrosion resistance of the phosphating film layer. Too much surfactant can easily affect the bonding force and wear resistance between the phosphating film and the substrate, and the dosage should be paid attention to.
(4) Stabilizer. Adding a stabilizer to the phosphating solution can form a stable chelate compound in the phosphating solution to prevent it from being oxidized, reduce the phosphating reaction speed, reduce the amount of sediment, and obtain fine crystals, uniform and consistent color. Deep, corrosion-resistant phosphating layer. Commonly used stabilizers are tartaric acid and the like.
(5) Accelerators. Adding accelerators to the phosphating solution can accelerate the phosphating reaction, reduce the phosphating temperature and shorten the phosphating time. Generally speaking, a strong oxidant can be used as an accelerator, but the oxidizing property of the accelerator is too strong, and it is easy to accelerate the oxidation, increase the amount, and increase the amount of sediment, passivate the surface of the steel, and even fail to form a phosphating film. Commonly used accelerators are nitrate, nitrite, molybdate, chlorate, etc.
(6) Free acidity, total acidity and acid ratio. The free acidity, total acidity and acid ratio of the phosphating solution are important factors affecting the quality of the phosphating film. If the free acidity is too high, the surface of the phosphating film will be rough and porous, the sediment of the solution will increase, and the adhesion of the film to the substrate will be poor; if the free acidity is too low, the phosphating film will be too thin and even unable to form a phosphating film. If the total acidity is too high, the phosphating reaction speed can be accelerated, but the film layer is very thin or no film is formed; if the total acidity is too low, the phosphating speed will be slow and the film layer will be rough. In general, the acid ratio of the phosphating solution should be controlled within a suitable range, so that the free acidity and total acidity should be adjusted within the range of acid ratio.
(7) Processing temperature. In the phosphating process, when the temperature is set to the upper limit, the point with low energy can be activated, so that it can also become the active center of crystallization, so that the number of crystal nuclei is increased, and the speed of crystal film formation is accelerated. If the temperature exceeds the upper limit, the acid ratio will change, the reaction speed will be accelerated, the crystal of the phosphating film will be coarse, the pores will be enlarged, the slag-containing phosphating film will be formed, the corrosion resistance will be reduced, and the stability of the phosphating solution will be affected. Therefore, the phosphating temperature should be controlled within the most appropriate range, which can not only ensure the speed of phosphating and the quality of the film, but also stabilize the phosphating solution and reduce the phosphating sediment.
Black phosphating process
Generally speaking, the black phosphating process is: degreasing and degreasing → cold water washing → pickling and rust removal → cold water washing → surface adjustment → black phosphating → cold water washing → hot water washing → drying.
Application of black phosphating process
The black phosphating process of steel parts plays an important role in the protection of steel materials. Since the reform and opening up, researchers have done a lot of work on the black phosphating process, and developed a variety of black phosphating solutions. Due to the unique advantages of the black phosphating process and the black phosphating film, the black phosphating technology has gradually replaced the original concentrated alkali high temperature blackening treatment.
According to the needs of some special conditions, black phosphating treatment is carried out on steel workpieces. The phosphating film obtained by the black phosphating technology researched and developed has both the uniform black of the oxide film and the fine and precise crystal characteristics, as well as the phosphating film. good wear resistance and lubricity. The specific implementation is as follows:
(1) Phosphating solution formulation and process conditions. The formula and specific process conditions of the phosphating solution with excellent performance are shown in formula 3 of Table 2-27 above.
(2) Black phosphating process: degreasing and degreasing → cold water washing → pickling and rust removal → cold water washing → surface adjustment → black phosphating → cold water washing → hot water washing → drying.