Phosphating is the process of forming a phosphate chemical conversion film through chemical and electrochemical reactions, and the formed phosphate conversion film is called a phosphating film. The main purpose of phosphating is to protect the base metal and prevent the metal from being corroded to a certain extent; Used for priming before painting to improve the adhesion and corrosion resistance of the paint film; Used to reduce oil film lubrication during metal cold working process.
Phosphating is a commonly used pre-treatment technique. In principle, it should belong to chemical conversion membrane treatment. As long as it is applied to the phosphating of steel surfaces, non-ferrous metals such as aluminum and zinc can also be applied to phosphating. The process of immersing a workpiece (steel, aluminum, or zinc) in a phosphating solution (some acidic phosphate based solutions) and depositing a layer of insoluble crystalline phosphate conversion film on the surface is called phosphating.
Blackening is a common method of metal heat treatment. The principle is to create a layer of oxide film on the metal surface to isolate air and achieve the purpose of rust prevention. When the appearance requirements are not high, blackening treatment can be used. The surface of steel parts turns black, some of which are called blue. Bluing treatment is a chemical surface treatment. Its main function is to form a dense oxide film on the surface of the workpiece, prevent corrosion and rust, and improve the wear resistance of the workpiece. Only surface treatment will not affect the internal structure. It is not heat treatment, it is fundamentally different from quenching.
High strength bolts use phosphating, which can also avoid hydrogen embrittlement issues. Therefore, bolts above grade 10.9 in the industrial field generally use phosphating surface treatment. Blackening+oiling is a popular coating for industrial fasteners because it is the cheapest and looks good before fuel consumption. Due to its blackening, it has almost no rust prevention ability, so it will rust quickly without oil.
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