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Introduction to hot dip galvanizing products
Galvanized steel sheet is a composite material of steel and zinc. Galvanized steel sheet combines the advantages of the two materials. The composite material has steel strength and plasticity and corrosion-resistant coating. It is widely used in industrial fields such as construction, household appliances and automobiles. In this issue, we will first make a brief introduction to hot-dip galvanized products. In the next issue, we will talk about the performance requirements and main brands of hot-dip galvanized products.
Protective effect and galvanizing method of strip galvanizing
Mechanism of zinc coating protecting steel plate
In the atmosphere, the corrosion resistance of zinc is much stronger than that of steel. Under normal conditions, the corrosion resistance of zinc is several times that of steel.
1) Physical function:
The zinc coating on the surface of the steel plate blocks the contact between the steel plate and the external corrosive medium, so that the steel plate is free from the corrosive effect of the corrosive medium.
2) Electrochemical action:
When the galvanized steel plate substrate and the zinc coating are exposed to the corrosive medium environment at the same time (such as the scratch and notch of the zinc coating), because the electrode potential of zinc (- 0.726v) is lower than that of iron (- 0.44v), zinc is anode and steel is cathode, so zinc gives priority to corrosion and protects the substrate, which is called sacrificial anode protection of zinc.
Main process of strip galvanizing
Hot plating and electroplating are two main processes of steel plate galvanizing. Today we mainly talk about hot galvanizing.
Hot dip galvanizing: the melting points of steel and zinc differ greatly. The melting point of pure iron is 1534 ℃, while the melting point of zinc is only 419 ℃. The method of galvanizing the treated strip steel directly into molten zinc is called hot dip galvanizing.
Continuous hot-dip galvanizing of strip steel: the production process of connecting the steel coils and galvanizing the strip steel directly through the zinc pot with molten zinc solution after necessary pretreatment and annealing is called continuous hot-dip galvanizing of strip steel.
Product classification and characteristics
Classification: according to the type of coating, it is divided into 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 coating), zn-5al-3mg (zinc aluminum magnesium coating).
Hot plated pure zinc products (GI)
Features: beautiful surface, good corrosion resistance and good processability.
Normal zinc flower
Large and beautiful zinc flowers can be obtained by adding elements such as lead or antimony to the liquid zinc.
No (small) zinc flower
Hot dip galvanized alloyed products (GA)
Features: zinc iron alloy coated steel plate has good painting adhesion, good corrosion resistance and weldability after painting.
Surface macro morphology
Surface micro morphology
Hot dip aluminized zinc products (galvalume)
Features: good corrosion resistance, 2-6 times that of hot-dip galvanizing. This is the conclusion obtained by exposing the sample to different environments for more than 15 years and comparing it with galvanized steel plate; Good temperature resistance. It can be used at 300 ℃ without discoloration. If discoloration is allowed, the service temperature can reach 700 ℃. It has excellent heat reflecting surface.