Galvanization refers to the surface treatment technology of coating a layer of zinc on the surface of a metal, alloy or other material for aesthetic and antirust effect. The main method is hot galvanizing.
Zinc is called amphoteric metal because it dissolves easily both in acids and bases. Zinc changes little in dry air. In humid air, the surface of zinc can form a dense, basic zinc carbonate film. In the environment of sulfur dioxide, hydrogen sulfide and sea, the corrosion resistance of zinc is poor. Especially in the atmosphere of high temperature and high humidity containing organic acids, the zinc coating is easy to be corroded. The standard electrode potential of zinc is -0.76V. For steel matrix, zinc plating is an anode coating, which is mainly used to prevent steel corrosion. The protective performance is closely related to the thickness of the coating. After passivation treatment, dyeing or coating light protecting agent, zinc coating can significantly improve its protective and decorative properties.
Cold galvanizing is also called the electric galvanized. Using the electrolytic equipment, the pipe fitting was deoiled and pickling and put into a solution of zinc salt and connected with the cathode electrolytic equipment. Place zinc plate across the tube and connect it to the anode electrolytic equipment. By turning on the power supply and using the current moving from positive to negative, a layer of zinc can be deposited on the pipe fitting. Cold - plated tube parts are first processed and then galvanized.
In the rotating barrel containing the plating, glass balls, zinc powder, water and accelerant, the glass ball as the impact medium rotates with the barrel. The mechanical and physical energy is generated by friction and hammering with the surface of the plating. Under the action of chemical accelerator, the zinc-coated powder is "cold welded" to the surface of the plating, forming a smooth, uniform and fine coating with a certain thickness.
Hot dip galvanized
The main method of galvanizing the surface of steel sheet is hot galvanizing.
Hot galvanizing is the development of older methods of hot galvanizing that have been used in industry in France for 170 years since 1836. However, the hot 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: original plate preparation, pretreatment, hot dip plating, post-treatment, finished product inspection, etc. According to custom, the hot galvanizing process is divided into two categories, that is, outline annealing and inline annealing. The outline annealing includes: wheeling, continuous hot galvanizing of strip steel. Inline annealing includes Sendzimir, advanced Sendzimir method, Selas and Sharon.
1. The principle
Because zinc does not change easily in dry air. But in moist air, the surface produces a very dense zinc carbonate film that effectively protects the interior from corrosion. In addition, when the plating layer is damaged for some reason and the substrate is exposed, the zinc and steel matrix form a micro battery, which makes the fastener matrix become the cathode and protected. It is widely used in automobile transportation and other industries, but it is necessary to use trivalent chromium passivation layer and zinc nickel alloy plating closed coating to reduce hexavalent chromium passivation harmful and toxic layer.
2. Performance featuresThe zinc plating layer is thick, crystal fine, uniform and without porosity, and has good corrosion resistance. Is pure, the zinc plating layer in acid, alkali corrosion slower, such as fog, can effectively protect the fasteners matrix galvanized layer formed after chromate passivation, color white, army green, beautiful and easy, have certain adornment sex, because the galvanized layer has good ductility, and therefore can be rushed, rolling, cold bending and forming and not damage the coating.
3. Scope of applicationElectric galvanizing involves the domain more and more extensive, the fastener product application already pervades mechanical manufacturing, make galvanization to spend a net, electron, precision instrument, chemical industry, traffic transportation, spaceflight to wait to have the great significance in national economy.
Galvanizing process basically has hot galvanization and cold galvanization two kinds. Cold galvanizing is also called electric galvanizing. Electrogalvanization is mainly discussed here. There are many kinds of galvanizing. But from the PH value of galvanizing bath cent, basically have two kinds: alkaline galvanizing and acid galvanizing.Alkaline zinc platingAlkaline galvanizing process refers to the PH value of the bath is alkaline. However, due to the different complexing agent, it can be divided into cyanide galvanizing and zincate galvanizing. Cyanide galvanizing is a very old kind of plating. There are mainly three components in the plating solution: main salt zinc oxide, complex agent sodium cyanide and conductive salt sodium hydroxide (commonly known as caustic soda).
There was no brightener in the early cyanidation solution. With the improvement of people's aesthetic requirements, a brightening agent was added to the zinc plating solution of cyanide solution. The cyanide galvanizing process is stable and the coating is fine. The plating solution has good dispersing ability. According to the number of sodium cyanide content, also divided into high cyanide, cyanogen and low cyanide galvanized three. The biggest disadvantage of cyanide galvanizing is that it is too toxic and harmful to environment.Zincate galvanizing is a rapid process in recent 30 years. Its main component is main salt zinc oxide, complex agent and conductive salt sodium hydroxide (commonly known as caustic soda). In order to obtain the fine dispersing ability of the bright coating, must add the brightener. The main development period of zincate galvanization in China is the great cyan-free galvanization period in 1970s. The famous DPE galvanization process and DE galvanization process have been used till now. Although this process is not as stable and delicate as cyanide galvanizing process. But its greatest advantage is that it is free of cyanide. The damage to the environment is much less. At present, zincate galvanizing has a new development, the defects such as foaming and brittleness have been overcome, and the dispersing ability has been greatly improved, which can be compared with cyanide galvanizing.Acidic galvanizedThe PH of the bath is 4-6.
In the 1970s, the second major achievement of cyanide - free galvanizing was cyanide - free ammonium chloride galvanizing. It keeps pace with zincic acid - base galvanizing. Zinc chloride is the main salt and ammonium chloride is the complexing agent and conductive salt. Citric acid and aminotriacetic acid were the auxiliary complexants. Polyethylene glycol and thiourea were used as coating thinning agents. The coating obtained from this solution is fine and dispersible. The plating solution has little toxicity. However, the main drawback is that the plating solution is unstable, and the leakage of ammonium chloride gas is very corrosive to the electroplating equipment. Current efficiency is also low. Sensitive to impurities. The operating temperature range is narrow. It has been basically eliminated by the potassium chloride galvanizing process. Zinc plating process with potassium chloride (or sodium chloride)Potassium chloride (or sodium chloride) galvanizing process is a new non-cyanide galvanizing process developed in 1980s. Its main composition is: zinc chloride main salt, the general use content is 70-90g/L. Potassium chloride is used as conductive salt, the usage content is different from 140-280g/L, and can be adjusted according to different demands. Boric acid is used as a PH buffer to stabilize the PH between 4.6 and 5.6.
As this plating solution works at room temperature, the solubility of boric acid is not high, and the general content is controlled at 25-30g/L.These three ingredients alone are not suitable for zinc plating. Some additives are needed to create a shiny, fine coating. The advantages of KCL galvanizing are stable plating solution, bright coating and low cost, high current efficiency and non - toxic. The disadvantage is that the dispersion capacity of the plating solution is slightly worse than that of alkaline galvanizing, and the coating is brittle. Nevertheless, as soon as KCL galvanization came into being, it was immediately popular. Rapid development, has occupied half of the galvanization. From the market, the current galvanizing process is mainly two kinds of plating: alkaline zincate and acid KCL galvanizing.
All other galvanizing processes have been dwarfed by the slow retreat from history.There is also a sulfate galvanizing process in acid galvanizing process. Its main component is zinc sulfate. The content is between 250 and 500g/L. Alum or aluminum sulfate as conductive salt. The content is between 30 and 50g/L. Conductive salts are also added with sodium sulfate or sodium chloride. In addition to the above several, but also add some additives, the role is to make fine coating. It was first made of dextrin or gum, and later some special brighteners were invented to make the coating brighter and more delicate. The greatest advantage of sulphate galvanization is that it can be applied with a high current density (1-5a /dm2) and plating speed is fast. But the drawback is that the dispersion of plating solution is very poor, should not be used to plating more complex workpiece.