Galvanizing refers to the surface treatment technology of plating a layer of zinc on the surface of metal, alloy or other materials for the purpose of beauty and rust prevention.The main method is hot dip galvanizing.
Zinc is soluble in acids and alkali, so it is called amphoteric metal.Zinc changes little in dry air.In moist air, the zinc surface will form a dense basic zinc carbonate film.Containing sulfur dioxide, hydrogen sulfide and Marine atmosphere, zinc corrosion resistance is poor, especially in high temperature and high humidity containing organic acid atmosphere, zinc coating is easy to be corroded.The standard electrode potential of zinc is -0.76v. For steel matrix, zinc coating belongs to anodic coating, which is mainly used to prevent the corrosion of steel. Its protective performance has a great relationship with the thickness of coating.The protective and decorative properties of zinc coating can be significantly improved by passivation, staining or coating with protective agent.
The principle is to rapidly oxidize the surface of iron and steel products to form a dense oxide film protective layer.There are two commonly used methods of blackening: traditional alkaline heating blackening and late blackening at room temperature.But the effect of room temperature blackening process on low carbon steel is not good.It is better to blacken A3 steel with alkali.Alkalescent blackening is subdivided, have again blackening and two blackening distinction.The main components of black liquor are sodium hydroxide and sodium nitrite.The temperature required for blackening is wide, ranging from about 135 degrees Celsius to 155 degrees Celsius, and you get a nice surface, but it takes some time.In practical operation, attention should be paid to the quality of rust and oil removal before blackening of the workpiece, and the passivation oil immersion after blackening.The quality of blackening often varies with these processes.Metal “bluing” medicinal liquid adopts alkaline oxidation or acid oxidation.The process of forming an oxide film on the metal surface to prevent corrosion is called “bluing”.The oxide film formed on the surface of black metal after “bluing” treatment, the outer layer is mainly ferric oxide and the inner layer is ferrous oxide.
High – strength bolts are generally used in important joints, subject to greater tension and shear.The last step in bolt processing is heat treatment, commonly known as quenching, to increase the strength of bolts.However, hydrogen embrittlement occurs easily in the process of galvanizing bolts.Hydrogen embrittlement is usually characterized by delayed fracture.This reduces the strength of high-strength bolts.Therefore, the surface black produced by the reheat treatment of high-strength bolts is a relatively stable oxidation film.It will not rust when it is not in contact with corrosive substances.
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Post time: Sep-09-2019