Analysis Of Eight Classifications Of Hot-dip Galvanizing Process

The hot-dip galvanizing process can be divided into the following eight categories:

(1) Single steel plate method

This method generally uses hot-rolled laminated sheets as raw materials. First, the annealed steel sheets are sent to the pickling workshop, and the oxygen hot-dip galvanized iron scale on the surface of the steel sheets is removed with sulfuric acid or hydrochloric acid. After pickling, the steel plate is soaked in the water tank immediately and waits for galvanizing, which can prevent the steel plate from re-oxidation. After pickling, water cleaning, squeezing, drying, hot-dip galvanizing in a zinc pot, oiling, and chromizing treatment.

(2) Sendzimir Law

  Combining the annealing process with the hot-dip galvanizing process, the in-line annealing mainly includes an oxidation furnace and a reduction furnace. A square rectangular tube steel strip is directly heated to about 450 degrees by the gas flame in the oxidation furnace to burn off the residual rolling oil on the surface of the strip steel and purify the surface. Then the strip steel is heated to 700-800 degrees to complete recrystallization annealing, the temperature before entering the zinc pot is controlled at about 480 degrees through the cooling section and finally enters the zinc pot for galvanizing without contact with air.

(3) Silas Law

Firstly, the steel strip is degreased by alkali washing, and then the scale on the surface is removed with hydrochloric acid. After washing and drying, it enters a vertical in-line annealing furnace directly heated by a gas flame. By strictly controlling the ratio of gas and air in the furnace, so that it can be incompletely burned under the condition of excess gas and insufficient oxygen, a reducing atmosphere is formed in the furnace. It is heated rapidly to reach the recrystallization temperature and the strip steel is cooled under a low-hydrogen protective atmosphere, and finally, the rectangular tube is immersed in the zinc liquid under a closed condition for hot-dip galvanizing.

(4) Improved Senjimi

It is a superior hot-dip galvanizing process method; it connects the independent oxidation furnace and reduction furnace in the Sendzimir method by a passage with a smaller cross-sectional area, so that the preheating furnace, reduction furnace, and cooling section are in the entire annealing furnace inside constitutes an organic whole.

(5) Wet hot-dip galvanizing

The solvent on the surface of the rectangular tube enters the zinc solution covered with molten solvent for hot-dip galvanizing without drying. The disadvantage of this method is that it can only be galvanized in a lead-free state, and the alloy layer of the coating is very thick and the adhesion is very poor. The zinc dross that generates in addition accumulates in the interface place of zinc liquid and lead liquid and can not be deposited at the bottom of the pan, so the steel plate pollutes the surface because of passing through the zinc layer.

(6) Wheeling Fage

 The continuous galvanizing production line includes a series of pretreatment processes such as lye degreasing, hydrochloric acid pickling, water rinsing, solvent coating, drying, etc., and the original plate needs to be annealed in a bell furnace before entering the galvanizing line for galvanizing. The production process of this method is complicated, and the production cost is high. What is more important is that the products produced by this method often have solvent defects, which affect the corrosion resistance of the square rectangular tube coating.

(7) American Steel Federation Act

It is a variant of the Sendzimir method. It only uses an alkaline electrolytic degreasing tank to replace the degreasing function of the oxidation furnace, and the rest of the process is basically the same as the Sendzimir method. After the original plate enters the production line, it is first electrolytically degreased, then washed with water, dried, and then recrystallized and annealed in a reduction furnace with protective gas, and finally enters a zinc pot for hot-dip galvanizing under sealed conditions. In this method, since the steel strip is not heated by an oxidation furnace, the oxide film on the surface is relatively thin, which can properly reduce the hydrogen content of the protective gas in the reduction furnace. However, since the steel strip enters the reduction furnace without preheating, it undoubtedly increases the thermal load of the reduction furnace and affects the life of the furnace.

(8) Sharon Law

This method is to spray hydrogen chloride gas onto the strip steel in the annealing furnace and make the strip steel reach the recrystallization temperature, so it is also called the gas pickling method. Pickling with hydrogen chloride gas can not only remove the scale on the surface of the rectangular tube but also remove the grease on the surface of the strip steel. Since the surface of the strip steel is corroded by the oxidizing gas, pitting is formed, so the coating obtained by the Sharon method is sticky. The attachment is particularly good. However, due to serious corrosion of equipment, the resulting high cost of equipment maintenance and renewal.