The difference between hot dip galvanizing and cold galvanizing on the surface of Malleable Iron pipe fittings
- 14 Sep, 2022
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Malleable Iron pipe fittings, as important pipe fittings in water pipelines, are often in contact with water and air, so their corrosion resistance is very important. Malleable Iron pipe fittings are generally galvanized and rust-proof on the surface of the product. Galvanizing is divided into hot-dip galvanizing and cold galvanizing. So, how to distinguish between hot-dip galvanizing and cold galvanizing?
From the appearance, due to the uniform cold galvanized layer, its surface is close to the billet, with a unique surface unevenness, and the billet has a smooth and bright appearance. Due to the phenomenon of electrical shielding, cold galvanized parts usually do not have a galvanized layer inside (except for special galvanizing processes). The zinc layer on the surface of the hot-dip galvanized pipe is thicker because the hot-dip galvanized layer is late and the fluidity of the zinc solution is poor. In addition, the surface of the hot-dip galvanized layer will oxidize to produce a white protective layer, so the color is generally gray, and the surface smoothness is much lower than that of cold-dip galvanized products.
At present, some manufacturers use some non-galvanized black parts to spray a layer of silver paint on the surface, and then sell them as galvanized pipe fittings. The discrimination method is also very simple. If two pipes collide with each other, the silver paint on the surface of the pipe will peel off. In addition, according to the color, if the silver powder paint pipe fittings do not fall off, the paint layer is very bright, the cold galvanized pipe fittings are dark and bright, and there is generally no galvanized layer inside, and the hot-dip galvanized pipe fittings will turn dark gray after a period of time.
So, for cold galvanized pipe fittings and wholesale hot galvanized pipe fittings, which performance is better?
Hot dip galvanizing, also known as hot dip galvanizing, is a method of dipping pipe fittings in molten zinc to obtain a metallic coating.
Principle: Hot-dip galvanizing has good coverage, dense coating, and no organic inclusions. It is well known that the mechanisms of zinc's resistance to atmospheric corrosion include mechanical protection and electrochemical protection. Under atmospheric corrosion conditions, there are ZnO, Zn(OH)2 and alkaline zinc carbonate protective films on the surface of the zinc layer, which slow down the corrosion of zinc to a certain extent. If this protective film (also known as white rust) is damaged, a new protective film will form. When the zinc layer is severely damaged and endangers the iron substrate, zinc produces electrochemical protection of the substrate. The standard potential of zinc is -0.76v and the standard potential of iron is -0.44v. When zinc and iron form a miniature battery, the zinc dissolves as the anode and the iron is protected as the cathode. Obviously, the atmospheric corrosion resistance of hot-dip galvanizing to base metal iron is better than that of electro-galvanizing.
The formation process of the hot-dip galvanized layer is the process of forming an iron-zinc alloy between the iron matrix and the outer pure zinc layer. During hot-dip galvanizing, an iron-zinc alloy layer is formed on the surface of the workpiece, so that the iron and the pure zinc layer are well combined. The process can be simply described as: when the iron piece is immersed in molten zinc, zinc and zinc are first formed on the interfacial alpha iron (bulk core) solid melt. This is a crystal formed by dissolving zinc atoms in the solid state of the base metal iron. Two metal atoms are fused together, and the attraction between the atoms is relatively small. Therefore, when zinc reaches saturation in a solid melt, the two element atoms of zinc and iron diffuse into each other. The zinc atoms diffused (or penetrated) into the iron matrix migrate in the matrix lattice and gradually form alloys with iron. The iron and zinc diffused into the molten zinc solution form an intermetallic compound fezn13 and sink into the bottom of the hot-dip galvanizing pot, that is, zinc slag. When the workpiece is taken out of the zinc immersion solution, a pure zinc layer is formed on the surface.
The electrolytic galvanizing industry, also known as cold galvanizing, uses electrolytic equipment to degrease and pickle the workpiece and put it into a solution containing zinc salts to connect the negative electrode of the electrolytic equipment; place a zinc plate on the other side of the workpiece to connect it to the The positive electrode of the electrolysis equipment is turned on, and the directional movement of the current from the positive electrode to the negative electrode is used to deposit a dense, smooth and uniform zinc layer on the workpiece. The cost of electrogalvanizing is generally about 300 yuan per ton of cross pipe fitting, which is much lower than that of hot-dip galvanizing. The galvanized layer on the surface of the electro-galvanized product is completely separated from the iron base (a typical manifestation is that the surface of the chrome-plated faucet commonly used in my house can be directly torn off), and the combination of the zinc layer and the iron base is unreliable and easy to fall off. In fact, the anti-rust effect of cold galvanizing is far less than that of hot-dip galvanizing. Its effect is at best as good as, or even worse than, anti-rust paint, especially in critical areas such as structural stress. Since the paint layer is easy to peel off, the rust prevention effect is even worse. The hot-dip galvanized layer is not easy to fall off, and although it will rust, its service life is much longer than that of cold-galvanized products.
In short, the difference between cold galvanizing and hot galvanizing can be summarized as follows:
1. The hot-dip galvanized layer is several times thicker than the cold-dip galvanized layer.
2. The cold galvanized layer has poor adhesion and is easy to fall off. The hot-dip galvanized layer is firmly bonded and not easy to fall off.
3. The cost of cold galvanizing is very low, basically one-tenth of that of hot-dip galvanizing, and the price advantage is great.
4. Due to the use of cyanide, hexavalent chromium and trivalent chromium in the process of cold galvanizing, the waste water will pollute the environment greatly. The state stipulates that galvanized steel pipes for construction shall not be cold galvanized. Only a few small factories or workshops produce steel pipes that are still cold galvanized. Malleable Iron pipe fittings exported abroad are all hot-dip galvanized pipe fittings or black fittings (ie, non-galvanized pipe fittings).
5. The surface of cold galvanized is smooth, and the surface appearance is better than that of hot-dip galvanized Malleable Iron pipe fittings. Cold plated parts in pipe fittings are often difficult to plate unless a better cold plating process is employed.