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The difference between hot-dip galvanizing and cold galvanizing of malleable steel pipe surface treatment

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The difference between hot-dip galvanizing and cold galvanizing of malleable steel pipe surface treatment
Latest company news about The difference between hot-dip galvanizing and cold galvanizing of malleable steel pipe surface treatment

       Malleable Iron pipe fittings, as an important fitting used in water pipelines, are often exposed to water and air, so their corrosion resistance is very important. Malleable steel pipe fittings are generally galvanized on the surface of the product for anti-rust treatment. Among them, galvanizing is divided into two types: hot-dip galvanizing and cold-dip galvanizing. So, how to distinguish hot and cold galvanizing?

 

        They can be distinguished from the appearance. Because the cold galvanized coating is uniform, its surface is close to the blank, and it has the unique surface unevenness of the blank product, which looks smooth and bright. Due to electrical shielding, cold-plated parts generally have no galvanized layer inside (except for special galvanizing techniques). The hot-dip galvanized pipe fittings have a thicker surface zinc layer due to the late hot-dip galvanizing layer and the poor fluidity of the zinc solution, and the surface of the hot-dip galvanizing layer will oxidize to produce a white protective layer, so the color is generally darker. , And the surface smoothness is far less than that of cold-plated products.

 

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       Now some manufacturers use some ungalvanized black parts to spray a layer of silver paint on the surface and sell them as galvanized pipe fittings. The method of distinguishing is also very simple. When two pipes are knocked on each other, the silver paint on the surface of the pipes will fall off. In addition, from the color distinction, if the silver paint pipe fittings do not fall off, the paint layer is very bright, cold galvanized pipe fittings are dark and bright, and there is generally no galvanized layer inside, while hot-dip galvanizing will become dark after leaving the factory for a period of time. Ash.


        So which performance is better for cold-plated pipe fittings or hot-dipped pipe fittings?


        A specific analysis from the galvanizing process of the product:


        Hot-dip galvanizing, also called hot-dip galvanizing, is a method of immersing pipe fittings in molten zinc to obtain a metal coating.
Principle: Hot-dip galvanizing has good coverage, dense coating and no organic inclusions. As we all know, the anti-atmospheric corrosion mechanism of zinc includes mechanical protection and electrochemical protection. Under atmospheric corrosion conditions, there are protective films of ZnO, Zn(OH)2 and basic zinc carbonate on the surface of the zinc layer, which can slow down the corrosion of zinc to a certain extent. The protective film (also known as white rust) is damaged and a new film is formed. When the zinc layer is seriously damaged and the iron matrix is ​​endangered, zinc will produce electrochemical protection for the matrix. The standard potential of zinc is -0.76V, and the standard potential of iron is -0.44V. When zinc and iron form a microbattery, zinc is dissolved as an anode. It is protected as a cathode. Obviously, hot-dip galvanizing has better atmospheric corrosion resistance to base metal iron than electro-galvanizing.

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        The formation process of the hot-dip galvanizing layer is the process of forming an iron-zinc alloy between the iron matrix and the outermost pure zinc layer. The iron-zinc alloy layer is formed on the surface of the workpiece during hot-dip coating, which makes the iron and pure zinc layer very close. Good combination, the process can be simply described as: when the iron workpiece is immersed in molten zinc, a solid solution of zinc and α iron (body core) is first formed on the interface. This is a crystal formed by dissolving zinc atoms in the base metal iron in a solid state. The two metal atoms are fused, and the attraction between the atoms is relatively small. Therefore, when zinc reaches saturation in the solid solution, the two element atoms of zinc and iron diffuse each other, and the zinc atoms that have diffused (or infiltrated) into the iron matrix migrate in the matrix lattice, and gradually form an alloy with iron, and diffuse The iron and zinc in the molten zinc form an intermetallic compound FeZn13, which sinks into the bottom of the hot-dip galvanizing pot, which is called zinc dross. When the workpiece is removed from the zinc immersion solution, a pure zinc layer is formed on the surface.

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Pub Time : 2021-01-05 13:21:19 >> News list
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