Magnesium alloy is the lightest metal engineering material in industrial application. Its low density is only 2/3 of aluminum and 1/4 of iron. It has many advantages, such as excellent damping and electromagnetic shielding performance, good heat dissipation and electrical conductivity, convenient processing and forming, easy recycling and meeting the requirements of environmental protection. It has important application value and broad application prospect in automobile, electronics, aerospace, national defense and other fields, and is known as "green engineering materials in the 21st century". An inorganic film composed of oxides or metal salts can be formed on the surface of magnesium alloy matrix by chemical transformation. This film has a good bonding force with the matrix and can prevent the corrosion of corrosive medium on the matrix. Such conversion film itself is generally not too dense, corrosion resistance is not too strong, can only be used for short-term atmospheric corrosion protection. However, it is important that it can serve as the base for the subsequent coating to enhance the binding force between the magnesium alloy substrate and the subsequent coating. According to the type of solution, the chemical conversion films of magnesium alloy can be divided into chromate and chromium-free chemical conversion systems, among which phosphating films are the most widely used ones.
Phosphating is a process in which metals react with dilute phosphoric acid or acid phosphate solution to form a phosphate protective film. It is one of the methods to replace toxic chromate treatment. It produces a layer of non-metallic, non-conductive, porous phosphate film on the metal surface through chemical reactions. In addition, phosphating film can also make the metal surface from a good conductor to a bad conductor, thus inhibiting the formation of metal surface microcells, effectively hinder the corrosion of the coating, can be multiplied to improve the corrosion resistance and water resistance of the coating, so phosphating film has been recognized as a good coating substrate.