I. Overview:
Steel parts (such as fasteners) go through a series of chemical and electrochemical reactions in a solution containing phosphate. The process of forming a phosphate protective film that is difficult to dissolve in water on the surface is called phosphating of steel. . The color of the film formed after phosphating changes with the carbon content in the steel. Low carbon steel and medium carbon steel are off-white; high carbon steel and carburized products are black. Since this kind of film has more pores that are filled, oiled and painted, it has better corrosion resistance, lubricity and good insulation performance in the atmosphere. (Under normal conditions, the breakdown voltage is 250~380V. After coating treatment, the breakdown voltage can be 1000V.) The thickness of the film is generally 5-20um, and the combination with the steel is firm.
Second, the principle of chemical film formation:
Dihydrogen phosphate can be hydrolyzed in the phosphating solution to increase the iron, zinc and manganese ions in the solution. When a steel part is placed in a solution, a series of chemical reactions take place. At this time, nitrate plays a catalytic role, intensifies the interaction between phosphate and parts, so that the concentration of dihydrogen phosphate and orthophosphoric acid increases continuously, and when it reaches saturation, iron, zinc, and manganese ions crystallize and precipitate on the surface of parts The reaction continues, the crystal grains grow continuously on the surface of the part, and the phosphating film is formed.
3. Phosphating process
Degreasing and derusting-water washing-phosphating-water washing-phosphating post-treatment
4. Post-phosphating treatment
Purpose: To increase the corrosion resistance and rust resistance of the phosphating film, the usual method is to add sealant and oil immersion.