The method of forming a water-insoluble phosphate film on the metal surface is called phosphating.
The appearance of the phosphating film is dark gray, gray or black gray, without luster. The thickness of the phosphating film is generally 5-20μm. Phosphate film is relatively stable under general atmospheric conditions, and its corrosion resistance is 6 to 24 times that of bluish film.
Phosphating film has a microscopic pore structure and has good adsorption capacity for paints and oils, so phosphating is often used in conjunction with coating methods such as painting. Dichromate filling, oil immersion or painting after phosphating can further improve its corrosion resistance.
Phosphating film is relatively stable in animal oil, vegetable oil, mineral oil, and also relatively stable in some organic solutions (such as benzene and toluene).
Phosphating film can withstand high temperatures of 400~500°C. Therefore, some springs that work at high temperatures, such as the springs of shells, are usually phosphated.
It is best to use sandblasting before phosphating the spring. After sandblasting, it should not be left for too long, and phosphating should be carried out immediately. If there is no sandblasting equipment, chemical degreasing and pickling methods can also be used to remove oil stains.
The spring produces a large amount of hydrogen during the phosphating process, so the spring after phosphating has hydrogen embrittlement. For springs in key parts, dehydrogenation treatment should be carried out after phosphating treatment.