1. Why is surface cleaning of steel plates necessary before galvanizing? What are the specific steps involved?
During the rolling process, rolling oil, iron powder, and other contaminants remain on the surface of the steel plate. These impurities hinder the contact between the molten zinc and the steel plate, leading to incomplete galvanizing or poor coating adhesion.
Therefore, pre-galvanizing cleaning is crucial and mainly includes three steps:
Degreasing Cleaning: First, the strip steel is sprayed and brushed with a high-temperature alkaline solution to effectively remove surface grease and dirt.
Electrolytic Cleaning: The strip steel is treated as an electrode in an electrolyte solution. The bubbles generated by electrolysis "strip" away extremely fine residues from the surface, achieving a deep purification effect.
Rinsing and Drying: The surface of the strip steel is rinsed clean with deionized water and then thoroughly dried with hot air to prepare it for subsequent annealing in the furnace.
2. Why is annealing necessary? What happens during the annealing process?
After cleaning, the steel sheet still has a thin oxide film on its surface, which prevents the zinc liquid from wetting it. Therefore, the core objectives of the annealing process are twofold:
Restoring material properties: By heating the strip steel above its recrystallization temperature (usually exceeding 730°C), the internal stress caused by the previous cold rolling process is eliminated, restoring the steel's inherent plasticity and ductility.
Reducing surface activity: In an annealing furnace filled with a reducing atmosphere of hydrogen and nitrogen, the furnace gases reduce the oxide film to a pure, "sponge-like, active pure iron surface." This allows the strip steel to react rapidly with the zinc liquid when it enters the zinc bath, ensuring a perfect metallurgical bond.
3. How to control zinc layer thickness and form the final surface during galvanizing?
After annealing, the strip steel is immediately immersed in a molten zinc bath at approximately 460°C for hot-dip galvanizing. After exiting the zinc bath, the key device determining the final surface quality and coating thickness is the air knife.
Coating Control: The air knife consists of a pair of slit nozzles that spray high-speed compressed gas. By precisely adjusting the gas pressure, flow rate, and distance from the strip steel, excess molten zinc can be blown away, thus precisely controlling the zinc layer thickness. The typical coating thickness ranges from approximately 45-275 g/m² (G30 to G90 grades).
Zinc Flower Formation: After exiting the zinc bath, the strip steel enters a cooling tower for rapid cooling. The size and presence of zinc flowers depend on the composition of the molten zinc and the control of the cooling rate.
Normal Zinc Flowers: Allowing the zinc layer to cool and solidify naturally and slowly forms a distinct zinc crystal pattern.
No Zinc Flowers/Small Zinc Flowers: By adding elements such as antimony and bismuth to the molten zinc and accelerating the cooling rate, the growth of zinc crystals is inhibited, resulting in a more uniform and fine surface.
4. What are the important surface treatments after galvanizing? What are their functions?
After the zinc coils are removed from the zinc bath and cooled, the following post-treatments are typically performed to enhance corrosion resistance or impart additional functions:
Passivation: The most common basic treatment. A chemical conversion film (such as chromate or chromium-free passivation film) is formed on the surface of the galvanized layer using a chemical solution. This effectively prevents white rust from forming on the galvanized coils due to moisture during storage and transportation. This is a temporary surface protection.
Oil Treatment: After passivation, a very thin layer of rust-preventive oil is applied to further enhance rust resistance.
Phosphating Treatment: A phosphate chemical film is formed on the zinc layer surface. This film significantly improves the adhesion of subsequent paint or powder coating, making it particularly suitable for sheets requiring subsequent painting.
Fingerprint Resistant Treatment: An organic composite film is coated on the surface. This not only prevents rust but also prevents fingerprints from being left behind. It is mainly used in fields with high aesthetic requirements, such as household appliances and electronic equipment.
5. How do these surface treatments affect the performance of the final product?
Different surface treatments endow galvanized products with drastically different properties to meet the diverse needs of downstream customers.
Surface Treatment (Code) Core Functions and Applications
Passivation (C) Provides short-term protection against white rust and is the most basic corrosion-resistant treatment.
Passivation + Oiling (CO) Adds an oil film on top of the passivation film, providing stronger rust protection, suitable for sea transport or long-term storage.
Phosphating (P) As a pretreatment layer for subsequent painting, significantly improves paint adhesion.
Lacquer Sealing (L) Coats an extremely thin organic film, providing some corrosion resistance and fingerprint resistance.
Fingerprint Resistant (N) Designed specifically for the home appliance and electronics industries, preventing fingerprints from being left by frequent touches.