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Hot galvanizing is developed from the older hot plating method, and has a history of one hundred and forty years since the application of hot galvanizing to industry in France in 1836. However, the hot dip galvanizing industry has been developed on a large scale in the past 30 years with the rapid development of cold rolled strip steel.
The production process of hot galvanized sheet mainly includes: raw plate preparation → pre-plating treatment → hot dip plating → post-plating treatment → finished product inspection and so on. According to the custom, the hot-dip galvanizing process is often divided into two categories: off-line annealing and in-line annealing according to different pretreatment methods, namely:
Wet method
Hot dip galvanizing of off-line annealed sheet steel
Wheeling process for hot dip galvanizing
Inline annealing Sendzimir process (protective gas process) improved Sendzimir process
American steel process (same as Kawasaki process)
The Selas method
The Sharon method

2. Off-line annealing: that is, before the hot-rolled or cold-rolled steel plate enters the hot-dip galvanized line, recrystallization annealing is first carried out in the bottom type annealing furnace or cover type annealing furnace, so that there is no annealing process in the galvanized line. The steel plate must maintain a clean pure iron active surface free of oxides and other dirt before hot-dip galvanizing. This method first removes the annealed surface oxide sheet by pickling, and then coats it with a layer of zinc chloride or a solvent composed of ammonium chloride and zinc chloride to protect it, thereby preventing the steel sheet from being oxidized again.
 

(1) Wet hot dip galvanizing: the solvent on the surface of the steel plate enters the zinc liquid covered with molten solvent on the surface without drying (that is, the surface is still wet) for hot dip galvanizing. The disadvantages of this approach are:
a. Can only be galvanized in the lead-free state, the alloy layer of the coating is very thick and the adhesion is very bad.
b. The generated zinc slag is deposited at the interface of zinc liquid and lead liquid and cannot be deposited at the bottom of the pot (because the specific gravity of zinc slag is greater than the zinc liquid and less than the lead liquid), so that the steel plate through the zinc layer polluted the surface. Therefore, this method has been basically eliminated.
 

(2) Sheet steel: This method is generally the use of hot-rolled laminated plates as raw materials, first the annealed steel plate into the pickling workshop, with sulfuric acid or hydrochloric acid to remove the surface of the steel sheet by oxygen hot galvanizing method. After pickling, the steel plate immediately enters the water tank to soak and wait for galvanized, which can prevent the steel plate from being reoxidized. After pickling, water cleaning, squeezing dry, drying, into the zinc pot (temperature has been maintained at 445-465 ℃) hot dip galvanizing, and then oil and chrome treatment. The quality of the hot-galvanized sheet produced by this method is significantly improved than that of the finished product of the wet galvanized method, and it is only valuable for small-scale production.
 

(3) Huilin process heat: the continuous galvanizing production line includes a series of pre-treatment processes such as lye degreasing, hydrochloric acid pickling, water washing, solvent coating, drying, etc., and the original plate needs to be annealed before entering the galvanizing line. The production process of this method is complicated, the production cost is high, and the main thing is that the product produced by this method often has solvent defects, which affects the corrosion resistance of the coating. And the AL in the zinc pan often reacts with the solvent on the surface of the steel plate to produce aluminum trichloride and is consumed, and the adhesion of the coating is deteriorated. Therefore, although this method has been available for nearly 30 years, it has not been developed in the world hot dip galvanizing industry.

3. In-line annealing: It is the raw plate with coil directly provided by the cold rolling or hot rolling workshop as hot dip galvanizing, and the gas protection recrystallization annealing is carried out in the hot dip galvanizing line. Hot dip galvanizing methods belonging to this industry include: Senjimire process, improved Senjimire process, United Steel process (with Japan Kawasaki process); The Law of Silas; The Sharon Law.
Senjimir method: It is a combination of annealing process and hot dip galvanizing process, and its in-line annealing mainly includes oxidation furnace and reduction furnace. The strip steel is directly heated to about 450 degrees by the gas flame in the oxidation furnace, and the residual rolling oil on the surface of the strip steel is burned off and the surface is purified. After the strip steel is heated to 700-800 degrees to complete recrystallization annealing, the cooling section controls the temperature before entering the zinc pan at about 480 degrees, and finally enters the zinc pan galvanized without contact with air, therefore, the Senzimir method has a high yield and good quality of galvanizing, and this method has been widely used.
 

Amisol process: It is a variation of the Senjimir process, which only uses an alkaline electrolytic degreasing tank to replace the degreasing effect of the oxidation furnace, and the rest of the process is basically the same as the Senjimir process. After the original plate enters the operation line, it is first degreased by electrolytic, then washed and dried, then recrystallized and annealed by a reduction furnace with protective gas, and finally entered into a zinc pot under sealed condition. Because the strip steel is not heated by the oxidation furnace, the oxide film on the surface is thin, and the hydrogen content of the protective gas in the reduction furnace can be appropriately reduced. In this way, it is beneficial to furnace safety and reducing production cost. However, because the strip steel is not pre-heated into the reduction furnace, this undoubtedly increases the heat load of the reduction furnace and affects the life of the furnace. Therefore, this method is not widely used.
 

Silas process: also known as direct flame heating method; First of all, the strip steel is degreased by alkali washing, and then the surface of the oxide is removed with hydrochloric acid, and after washing, drying and then into the vertical line annealing furnace directly heated by the gas flame, through strict control of the flame ratio of gas and air in the furnace, so that it is incomplete flame burning in the case of excess gas and insufficient oxygen, so as to create a reducing atmosphere in the furnace. The strip is heated quickly to the recrystallization temperature and cooled in a low hydrogen protective atmosphere, and finally immersed in zinc liquid under closed conditions for hot galvanizing. The equipment is compact, the investment cost is low, and the output is high (up to 50/ hour). However, the production process is complex, especially when the unit stops running, in order to avoid burning the strip steel, it is necessary to use the method of moving the furnace horizontally away from the steel strip, so there are many operational problems, so the hot-dip galvanizing industry uses this method very little.
 

Sharon process: In 1939, the United States Sharon Company put into production a new hot-dip galvanized unit, so it is also called Sharon process. This method is to spray hydrogen chloride gas into the annealing furnace and make the strip steel reach the recrystallization temperature, so it is also called gas pickling method. The use of hydrogen chloride gas pickling, not only can remove the strip surface of the oxide, but also remove the grease on the surface of the strip, because the surface of the strip is corroded by the oxide gas, forming a pocked surface, so the use of the Sharon process obtained by the coating adhesion is particularly good. However, due to the serious corrosion of the equipment, the resulting high equipment maintenance and renewal costs. Therefore, this method is rarely used.
 

Improved Senjimmy: It is a more superior hot-dip galvanizing process method; It connects the independent oxidation furnace and reduction furnace in the Senjimir process by a small cross-sectional aisle, so that the entire annealing furnace, including the preheating furnace, reduction furnace and cooling section, constitutes an organic whole. The practice has proved that this method has many advantages, such as high quality, high yield, low consumption and safety, which have been gradually recognized by people. Its development rate is very fast, since 1965, almost all the new lines of operation have adopted this method, and in recent years, the old Senjimir units have been mostly transformed in accordance with this method.
 

With the discovery of industry, hot-dip galvanized products have been used in many fields, the advantage of hot-dip galvanized is that the anti-corrosion life is long, to adapt to the environment has been a very popular anti-corrosion treatment method. It is widely used in power tower, communication tower, railway, road protection, road light pole, Marine components, building steel structure components, substation ancillary facilities, light industry, etc.
 

Hot-dip galvanizing has a long anti-corrosion life, but the anti-corrosion life is not the same in different environments: 13 years in heavy industrial areas, 50 years in the ocean, 104 years in the suburbs, and 30 years in the city.