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What is hot-dip galvanized finish?

What is hot-dip galvanized finish?
Issue Time:2018-07-18

What is hot-dip galvanized finish? 

Galvanizing Also called hot dip galvanizing and hot dip galvanizing: It is an effective metal anti-corrosion method, which is mainly used in metal structure facilities in various industries. The rust-removed steel pieces are immersed in a molten zinc solution at a temperature of about 500° C., so that a zinc layer is adhered to the surface of the steel member, so that the purpose of corrosion protection is achieved. Hot-dip galvanizing process: finished pickling - washing - plus plating solution - drying - rack plating - cooling - chemical - cleaning - grinding - hot galvanized finish. Hot-dip galvanizing was developed from the older hot-dip coating method. Since France applied hot-dip galvanizing to the industry in 1836, it has been more than 170 years old. In the past 30 years, with the rapid development of cold-rolled strip, the hot-dip galvanizing industry has been developed on a large scale.


Development

Because the standard electrode potential of zinc is negative to iron, the galvanized layer in the water and moist air has the function of sacrificial anode to protect the steel base, which can greatly extend the service life of the steel. The galvanized layers commonly used in industry include hot-dip galvanizing, electrogalvanizing, mechanical galvanizing, and thermal spraying (plating) of zinc. Among them, hot-dip galvanizing accounts for about 95% of the total galvanized amount, and the amount of zinc used for hot-dip galvanizing is It accounts for 40% of zinc production worldwide and about 30% of zinc production in China.

Hot-dip galvanizing is a process technology in which steel, stainless steel, cast iron, and other metals are immersed in a molten liquid metal or alloy to obtain a plating layer. Hot-dip galvanizing is the most widely used and most cost-effective steel surface treatment method in the world today. Hot-dip galvanizing products play an inestimable and irreplaceable role in reducing corrosion, prolonging life, and saving energy and materials. At the same time, the coated steel is also a high-value-added short-term product that is fostered and developed by the state.

With the implementation of the Western Development Strategy, the in-depth development of projects such as West-to-East Power Transmission, West-East Gas Transmission, South-to-North Water Diversion, the Three Gorges Project, rural power grid, and the second-generation network of urban power grids, China's hot-dip galvanizing industry has entered a new round of high-speed development stage.


Protective performance

The thickness of the electrogalvanized layer is usually 5 to 15 μm, and the hot-dip galvanized layer is generally 35 μm or more, and even up to 200 μm. Hot dip galvanizing coverage is good, the coating is dense, no organic inclusions. As we all know, the mechanism of zinc resistance to atmospheric corrosion is mechanical protection and electrochemical protection. In the atmospheric corrosion conditions, the surface of the zinc layer has ZnO, Zn (OH) 2 and basic zinc carbonate protective film, to a certain extent, slow down the corrosion of zinc. The protective film (also called 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 electrochemically protect 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. Protected as a cathode. Obviously hot-dip galvanizing is superior to electro-galvanizing on the atmospheric corrosion resistance of base metal iron.


Galvanizing layer forming process

The process of forming a hot-dip galvanizing layer is a process of forming an iron-zinc alloy between the iron matrix and the outermost pure zinc layer. When the surface of the workpiece forms an iron-zinc alloy layer during the hot dip plating, only the iron and the pure zinc layer are formed. For a good combination, the process can be simply described as follows: When the iron workpiece is immersed in the molten zinc liquid, a solid solution of zinc and alpha iron (body center) is first formed at the interface. This is a crystal formed by the dissolution of zinc atoms in the solid state of the base metal iron. The fusion between the two metal atoms is relatively small. Therefore, when zinc is saturated in the solid solution, the two elements of zinc and iron diffuse into each other, and the zinc atoms diffused into (or infiltrated into) the iron matrix migrate in the matrix of the matrix, gradually forming an alloy with iron, and diffuse. The iron in the molten zinc liquid forms an intermetallic compound with zinc and sinks into the bottom of a hot-dip galvanizing pot, namely, dross. When the workpiece is removed from the zinc bath, the surface forms a pure zinc layer, which is a hexagonal crystal. Its iron content is not more than 0.003%.

 

Hot-dip galvanizing advantages

1. Low processing cost: The cost of hot-dip galvanizing and rust prevention is lower than that of other lacquer coatings;

2. Durable: In the suburban environment, the standard galvanized rust-proof thickness can be maintained for more than 50 years without repair; in the urban or offshore areas, the standard hot-dip galvanized rust-proof layer can be maintained for 20 years without repair ;

3. Good reliability: Metallurgical bonding between the galvanized layer and the steel becomes a part of the steel surface, so the durability of the coating is more reliable;

4, the toughness of the coating: galvanized layer forms a special metallurgical structure, this structure can withstand mechanical damage during transportation and use;

5. Comprehensive protection: Each part of the plated part can be plated with zinc, which can be fully protected even in the depressions, sharp corners and hidden places;

6. Save time and effort: The galvanization process is faster than other coating construction methods, and it can avoid the time required for painting on the construction site after installation.

7. Low initial cost: Under normal circumstances, the cost of hot dip galvanizing is lower than that of applying other protective coatings. The reason is very simple. Other protective coatings such as sandblasting paint are labour-intensive processes, whereas hot dip galvanizing processes are high. Mechanized, tightly controlled in-plant construction.

8, the test is simple and convenient: hot-dip zinc layer can be visually and simply non-destructive coating thickness test for testing

9. Reliability: The specifications of hot-dip zinc are generally performed in accordance with BS EN ISO 1461, which restricts its minimum zinc layer thickness so that its rust-proof period and performance are reliable and predictable.

 

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