Galvanized steel is a composite of steel and zinc. It combines the strengths of both materials, strength and plasticity of the steel, and a corrosion-resistant coating. It is used in a wide range of industries including construction, household appliances and automobiles. .
In the atmosphere, the corrosion resistance of zinc is much stronger than steel, usually under the conditions of zinc corrosion resistance is 25 times the iron and steel.Zinc plating on the surface of the steel plate, through the physical role and electrochemical role, so that steel corrosion corrosive medium from the role.
The main process of strip galvanizing Hot-dip galvanizing is the use of steel and zinc melting point difference is relatively large. We know that the melting of pure iron is 1534 ℃, the melting point of zinc is only 419 ℃, so that the treated strip can be directly immersed in molten zinc liquid zinc.
We can see this schematic diagram, this is the strip after annealing so far into the zinc pot, in the case of air, zinc plated zinc after the finished zinc strip. We can see an air knife to control the thickness of the zinc layer, and then you can see there are four small red, this is if the production of alloy products, strip to re-heating in this place, 7-15% Fe content In the coating which is alloyed products, follow-up is to be cooled.
Classification of hot – dip galvanizing products
According to the coating to points, you can use this process to produce hot-plated coating can be basically divided into the following six kinds:
GI (pure zinc coating);
GA (zinc – iron alloy coating);
55% Al-Zn (aluminum-zinc coating; GALVALUME);
5% Al-Zn (zinc-aluminum coating, GALFAN);
Aluminizing (aluminum), aluminum can be divided into two categories, one is pure aluminum plating, one is aluminum-plated silicon products;
Zinc-aluminum-magnesium products, in recent years the new coating zinc-aluminum-magnesium products, such as Zn-5Al-3Mg. Zn-Al-Mg is a family of products that can include different Mg and Al contents.
In general these six can be produced by hot-dip, so we can simply put them are classified as a class of hot-dip galvanized products.
At present, our country is mainly pure zinc, zinc and iron and aluminum are three kinds of aluminum, aluminum is also a new Baosteel, aluminum products in the country is also slowly developing.
Conventional coating characteristics
1.Hot-dip pure zinc products, the code is GI.
Hot-dip pure zinc products are characterized by surface appearance, good corrosion resistance, good processing.
It is divided into two kinds, one is the normal zinc flowers, there is a zinc-free flowers. Before the hot-dip galvanizing products because of zinc in the liquid lead can not be extracted very pure, the surface there are always some zinc flowers, so our old concept of galvanized inside is a zinc flower. With the needs of the automotive industry, hot-dip galvanized car plate to do painting requirements, zinc flowers on the painting is affected, and later by zinc and zinc liquid lead content reduced to dozens of ppm later, We can produce products that do not have zinc flowers or very little zinc flowers. For some special purposes like the construction of the above or like a large zinc flower, we can add zinc in the lead or antimony and other elements, access to large and beautiful zinc flower.
2.Alloyed products, its code is GA.
This product has the advantage of coating the surface of the paint adhesion is particularly good, after painting the corrosion resistance is also very good, while its welding is also very good.
But its surface morphology is gray, not suitable for bare use, if not in the case of coating we are not recommended because it contains 7-15% of the coating inside the iron, if not painted, This part of the iron will generate a very pale light red rust. Although the corrosion resistance of red rust will not be further expanded, but the appearance is not very good.
So the main use of zinc-iron alloy or in the use of coating with the occasion, just like the car’s outer panel and refrigerator side panels, you can directly use GA products, like the refrigerator processing, it can not do pretreatment direct dusting.
3.Aluminum zinc products
Al-Zinc is characterized by corrosion resistance is particularly good, the appearance of the surface is also very beautiful. Its zinc flower looks like pretty fish scales, very beautiful, you can directly use.
It is the corrosion resistance of our ordinary hot-dip galvanized 2-6 times, its high temperature performance is also better, can be used in the case of 300 ℃ does not change color, short-term use, at 700 ℃ also has a good Of the color retention, and has an excellent heat reflection effect.
So this kind of product is now a large number is used in the construction of household appliances industry.
Why there are different hot-dip galvanizing products？We may ask, why do we have different hot galvanized products?
Like pure zinc, zinc iron, aluminum zinc and zinc aluminum magnesium, in fact, the development of so much of the coating mainly from the use of the above to consider.Since each coating has its own characteristics, eight properties are listed here, from the following table, from the formability, corrosion resistance, sacrificial protection, corrosion resistance (after molding), paint adhesion, corrosion resistance (After painting), weldability, heat reflectivity of these eight aspects of the pure zinc, aluminum zinc and zinc iron were compared.Â
Â