Brief introduction of galvanizing knowledge

I. Overview

Zinc dissolves easily in acids and also in alkalis, so it is called an amphoteric metal. Zinc changes little in dry air. In moist air, a basic zinc carbonate film forms on the surface of zinc. In sulfur dioxide, hydrogen sulfide and Marine atmosphere, zinc corrosion resistance is poor, especially in high temperature and high humidity atmosphere containing organic acid, zinc coating is easy to be corroded.

The standard electrode potential of zinc is -0.76V, for the steel substrate, zinc coating belongs to the anodic coating, it is mainly used to prevent the corrosion of steel, its protective performance and the thickness of the coating is very important.

The protective and decorative properties of zinc coating can be improved significantly after passivation treatment, dyeing or coating with light protection agent. In recent years, with the development of galvanizing process and the use of high performance galvanizing brightener, galvanizing has entered the protective and decorative application from the simple purpose of protection.

There are two types of galvanizing solution: cyanide plating solution and cyanide - free plating solution. Cyanide plating solution is divided into mild cyanide, low cyanide, medium cyanide, and high cyanide. Cyanide-free plating solution includes alkaline zincate plating solution, ammonium plating solution, sulfate plating solution and ammonia-free chloride plating solution. Cyanide galvanizing solution has good plating ability, and the coating obtained is smooth and fine, which has been used for a long time in production. However, because cyanide is highly toxic and causes serious pollution to the environment, low cyanide, little cyanide and no cyanide galvanizing solutions have been used in recent years.

Galvanizing process

Ii. Principle

In a bath filled with galvanizing liquid, the parts to be plated after cleaning and special pretreatment are used as the cathode, and the anode is made of coated metal. The two poles are respectively connected with the positive and negative terminals of the DC power supply. Galvanizing solution consists of an aqueous solution containing metal-coated compounds, conductive salts, buffers, pH regulators and additives. After electrification, metal ions in the galvanizing solution move to the cathode to form a coating under the action of potential difference. The anode metals form metal ions into the galvanizing solution to maintain the concentration of coated metal ions [1]. In some cases, such as chrome plating, is made of lead, lead antimony alloy made of insoluble anode, it only plays the role of electron transfer, conduction of current. The concentration of chromium ions in the electrolyte is maintained by periodically adding chromium compounds to the bath. In the process of galvanizing, the quality of anode material, the composition of galvanizing liquid, temperature, current density, on-current time, stirring intensity, impurities precipitated and power waveform will affect the quality of the coating, which needs timely control.

Iii. Types

1. Zinc plating with alkaline cyanide

2. basic zincate galvanized

3. ammonium salt galvanized

4. potassium salt galvanized

5. ammonium potassium mixed bath galvanized

6. sulfate galvanized

Three, galvanized layer of post-treatment

1. Dehydrogenation

2. Passivation

3. Coloring

Now the main method of galvanized steel plate surface is hot dip galvanizing.

Hot dip galvanizing is developed from the older method of hot plating and has a history of one hundred and forty years since it was applied to industry in France in 1836. However, hot dip galvanizing industry has been developed on a large scale with the rapid development of cold rolled strip in the past 30 years.

The production process of hot-galvanized sheet mainly includes: preparation of the original plate → pre-plating treatment → hot-dip plating → post-plating treatment → finished product inspection, etc. According to the custom often according to the plating pretreatment method

Different hot dip galvanizing process is divided into two categories: out-of-line annealing and in-line annealing. Namely wet method (single sheet hot dip galvanizing method), off-line annealing (single sheet hot dip galvanizing method), Wheeling method (strip continuous hot dip galvanizing method), in-line annealing Sendzimir method (protective gas method), Improved Senkimire process, Meisel process (as Kawasaki process), Selas process and Sharon process.

1. Out-of-line annealing: Before hot or cold rolled steel plates enter the hot galvanizing line, recrystallization annealing is first carried out in the bottom annealing furnace or cover annealing furnace, so that there is no annealing process in the galvanizing line. The steel plate must maintain a clean active surface of pure iron free of oxides and other dirt before hot dip galvanizing. In this method, the annealed surface oxide sheet is first removed by pickling method, and then coated with a layer of zinc chloride or a mixture of ammonium chloride and zinc chloride solvent for protection, so as to prevent the plate from being oxidized again.

(1) wet hot dip galvanizing: the solvent on the surface of the steel plate is hot dip galvanizing without drying (that is, the surface is wet) into the zinc liquid covered with molten solvent on the surface. The disadvantages of this approach are:

a. Galvanizing can only be done in 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 and lead liquid and cannot be deposited at the bottom of the pot (because the specific proportion of zinc slag is greater than that of zinc liquid and less than that of lead liquid), so the steel plate passes through the zinc layer and contaminates the surface. Therefore, this method has been basically eliminated.

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

(3) Huilin process heating: the continuous galvanizing production line includes a series of pre-treatment procedures, such as alkali degreasing, hydrochloric acid pickling, water washing, solvent coating, drying, etc., and the original plate needs to be annealed in cover furnace before entering the galvanizing line. The production process of this method is complicated and the production cost is high. More importantly, the products produced by this method often have solvent defects, which affect the corrosion resistance of the coating. And the AL in the zinc pot often reacts with the solvent on the surface of the steel plate to generate aluminum trichloride and is consumed, and the adhesion of the coating becomes bad. Therefore, although this method has been developed for nearly 30 years, it has not been developed in the world hot dip galvanizing industry.

2. In-line annealing: It is directly provided by cold rolling or hot rolling workshop with coil as the raw plate of hot galvanizing, and gas protected recrystallization annealing is carried out in the hot galvanizing operation line. Hot dip galvanizing methods belonging to this industry include: Senjimire process, improved Senjimire process, United steel process (with Japan Kawasaki process); Silas method; Sharon Method.

Senkimire method: It is to combine the annealing process and hot dip galvanizing process. The in-line annealing mainly includes oxidation furnace and reduction furnace. The 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 strip surface and purify the surface. After that, the strip steel is heated to 700-800 degrees to complete recrystallization annealing, the cooling section controls the temperature before entering the zinc pot at about 480 degrees, and finally enters the zinc pot galvanized without touching the air. Therefore, the Senzimir method has high yield and good quality of galvanized. This method has been widely used.

Meisel process: It is a variation of Senkimire process, it is only using an alkaline electrolytic degreasing tank instead of the oxidation furnace degreasing, the other processes are basically the same as Senkimire process. After the original plate enters the working line, electrolytic degreasing is carried out first, then washing and drying, then recrystallization annealing is carried out in the reduction furnace with protective gas, and finally, hot dip galvanizing is carried out in the zinc pot under sealed condition. In this method, the strip steel is not heated by the oxidation furnace, so the surface of the oxide film is thin, which can properly reduce the hydrogen content of the protective gas in the reduction furnace. In this way, it is beneficial to furnace safety and reduce production cost. However, because the strip steel is not pre-heated into the reduction furnace, it undoubtedly improves 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 process; First, the strip is degreased by alkali washing, then the surface of the oxide skin is cleared with hydrochloric acid, and after washing and drying, it enters the vertical inline annealing furnace directly heated by the gas flame. Through strict control of the flame ratio of gas and air in the furnace, the incomplete flame firing is carried out under the condition of excess gas and insufficient oxygen, so as to create a reducing atmosphere in the furnace. The strip is heated to recrystallization temperature quickly and cooled in low hydrogen protective atmosphere. Finally, the strip is immersed in zinc solution under closed condition for hot-dip galvanizing. This method has compact equipment, low investment cost and high yield (up to 50/ h). But the production process is complex, especially when the unit stops running, in order to avoid burning the strip steel, the furnace needs to move away from the strip. There are many problems in this operation, so this method is rarely used in hot dip galvanizing industry.

Sharon Process: In 1939, the American Sharon Company put into production a new hot-dip galvanizing unit, so it was also called Sharon process. The process 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 process. The use of hydrogen chloride gas pickling can not only remove the oxide skin on the strip surface, but also remove the grease on the strip surface. Because the strip surface is corroded by the oxide gas, the coating obtained by the Sharon process has a particularly good adhesion. However, due to the serious corrosion of the equipment, it causes high equipment maintenance and replacement costs. So this method is rarely used.

Improved Senjimmy: It is a more superior hot-dip galvanizing process; It connects the independent oxidation furnace and reduction furnace in the Senkimire process by a small cross-sectional aisle, so that the whole annealing furnace, including 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. Its development rate is very fast. Since 1965, almost all the new operating lines have adopted this method, and in recent years, most of the old Senkimire units have been modified in accordance with this method.

In the rotating barrel equipped with plating parts, glass ball, zinc powder, water and accelerator, as the impact medium of the glass ball rotates with the barrel, and plating parts surface friction and hammering produce mechanical and physical energy, under the action of chemical accelerator, coated zinc powder "cold welding" to the surface of the plating parts, forming smooth, uniform and meticulous coating with a certain thickness.

The technical standard is ASTM B695-2000 and military C-81562 mechanical galvanized technical standard.

Quality characteristics:

1, smooth appearance, no zinc tumor, burr, silver white;

2, the thickness is controllable, within 5-107μm arbitrary choice;

3, no hydrogen embrittlement, no temperature damage, can ensure the mechanical properties of materials unchanged;

4, can replace part of the need for hot dip galvanizing process;

5, good corrosion resistance, neutral salt spray test up to 240 hours.

Application range of steel nails, nails, fasteners, water pipe joints, scaffolding fasteners, steel wire rope clamp and so on.