03 Mar

From the appearance to distinguish, because of the uniform cold galvanized coating, so its surface close to the rough, rough products have a unique surface concavo-convex feeling, looks smooth and bright. Due to the electrical shielding phenomenon, the cold plating parts generally have no galvanized layer (except special galvanizing techniques). The hot-dip galvanizing coating of the pipe fittings is relatively late, and the zinc liquid has poor fluidity, So its surface zinc layer is thicker, and because the surface of hot dip galvanizing layer will be oxidized to produce a white protective layer, so the color is generally gray, and the surface smoothness is far less than cold plating products.

Now there are some manufacturers with some ungalvanized black malleable iron pipe fittings in the surface with spray a layer of silver paint after pretending to be galvanized pipe fittings sold. The distinction method is also very simple, with two pipes knock each other, pipe surface of the silver paint will fall off.

Cold plating pipe fittings and hot plating pipe fitting which performance is better?

From the product galvanizing process to specific analysis:

Galvanization, also known as hot-dip galvanizing, is a method of China black malleable iron pipe fittings in molten zinc to obtain a metal coating.

Principle: Hot dip galvanized coating has good covering ability, compact coating, no organic inclusions. It is well known that the mechanism of zinc against atmospheric corrosion includes mechanical protection and electrochemical protection. ZnO, Zn (OH) _ 2 and basic zinc carbonate protective films are formed on the surface of the zinc layer under atmospheric corrosion. To a certain extent to slow down the corrosion of zinc, this layer of protective film (also known as white rust) is damaged and will form a new film. When the zinc layer is damaged seriously and endangers the iron matrix, the zinc produces electrochemical protection on the matrix. The standard potential of zinc is -0.76V and that of iron -0.44V. Zinc is dissolved as an anode and iron is protected as a cathode when zinc and iron form a micro battery. Obviously, the atmospheric corrosion resistance of hot dip galvanizing is better than electroplating zinc.

The formation process of hot dip galvanizing layer is the process of forming Fe-Zn alloy between the iron substrate and the outermost pure zinc layer. The process can be described simply as follows: when the iron workpiece is immersed in molten zinc liquid, the solid melt of zinc and α iron (body core) is first formed on the interface. This is the matrix metal iron in the solid state dissolved zinc atoms formed a crystal, two kinds of metal atoms is fusion, the gravity between atoms is relatively small. Thus, when zinc is saturated in the solid melt, the atoms of zinc and iron diffuse with each other, and the zinc atoms diffused into (or infiltrated) the iron matrix migrate in the matrix lattice, Gradually forming alloy with iron, and the iron diffused into molten zinc liquid will form intermetallic compound FeZn13 with zinc, and sink into the bottom of hot galvanizing pot, namely zinc slag. A pure zinc layer is formed on the surface of the workpiece when it is removed from the zinc dip solution.

Galvanizing: also called cold galvanizing in the industry, it is to use electrolysis equipment to put the workpiece into the solution of zinc salt after deoiling and pickling, and connect the anode of electrolysis equipment. A zinc plate is placed on the opposite side of the workpiece and connected to the cathode of the electrolytic equipment, the power supply is connected, and the current is moved from the cathode to the anode, and a dense, smooth and uniform zinc layer is deposited on the workpiece. Electroplating zinc cost is generally 300 yuan per ton pipe, far lower than hot galvanizing. Galvanized products surface galvanized layer and iron matrix is completely separate (typical embodiment is our home commonly used chrome faucet surface can directly tear down), so the zinc layer and iron matrix combination is not reliable, easy to fall off. In fact, cold galvanizing rust, far less than hot galvanizing, its effect is at most equivalent to rust paint, or even worse, especially in key positions such as structural stress, because the paint layer is easy to fall off, rust corrosion effect is worse. The hot galvanizing layer is not easy to fall off, although it will also rust, but its life is much higher than cold galvanizing products.

In short, the difference between cold and hot galvanizing can be summarized as follows:

1. Hot galvanizing is several times thicker than cold galvanizing.

2. The adhesion property of cold galvanized layer is very poor and easy to fall off. And hot-dip galvanized layer is solid and hard to fall off.

3. Cold galvanizing cost is very low, basic for hot galvanizing one-tenth, the price advantage is great.

4. Due to the use of cyanide, hexavalent chromium and trivalent chromium in cold galvanizing process, the waste water causes great environmental pollution. The pipe fittings of malleable steel exported abroad are hot-dip galvanized pipe fittings or China black malleable iron pipe fittings (non-galvanizing pipe fittings).

5. The surface layer of cold galvanizing is smooth, and the surface sense is better than that of hot galvanizing pipe fittings. Cold plating of pipe fittings is difficult to plate unless a better cold plating process is adopted. 

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