1.What is the core purpose of hot-dip galvanizing post-treatment?
Remove surface defects to ensure appearance quality.
During the hot-dip galvanizing process, zinc nodules (excess zinc accumulation), zinc slag (impurities in the zinc bath), and residual flux residue may remain on the workpiece surface. These defects not only affect the appearance but can also cause localized corrosion (e.g., water accumulation under the nodules). Post-treatment cleaning is required to remove these impurities, ensuring a smooth and uniform surface finish.
Optimize galvanized coating performance and enhance protective effectiveness.
Some post-treatment processes can improve the structure or condition of the galvanized coating. For example, passivation forms a dense passive film on the surface of the galvanized coating, preventing direct contact between the zinc and air and water, significantly improving its corrosion resistance (especially in humid, acidic, and other harsh environments). Cooling control prevents cracks or stress in the galvanized coating caused by excessive cooling, ensuring strong adhesion. Meeting Subsequent Processing or Use Requirements
If a workpiece requires subsequent processing (such as painting, welding, or assembly), post-processing must ensure these conditions. For example, cleaning the surface of oil or impurities ensures paint adhesion during painting. Stripping (removing the zinc coating from the threads) of threaded parts prevents assembly difficulties caused by thickening of the zinc coating. Furthermore, some industries (such as construction and automotive) have specific requirements for workpiece appearance (e.g., absence of visible scratches and uniform color), requiring post-processing such as polishing and finishing to meet these standards.
2.How to remove surface defects and impurities?
Suitable for small batches of complex workpieces, this method primarily removes defects manually using tools:
Use a wire brush, sandpaper, or an angle grinder to remove surface zinc abrasives, zinc slag, and scale, especially around corners and welds where impurities are prone to buildup.
Use a chisel or hammer to gently remove larger zinc abrasives (pay careful attention to force to avoid damaging the underlying zinc coating).
For threaded parts (such as bolts and nuts), use a tap or die to re-tap/re-thread the threads to remove excess zinc plating and ensure thread fit accuracy. Mechanical cleaning
Suitable for large quantities of regularly shaped workpieces (such as steel pipes, plates, and standard parts), it offers greater efficiency:
Drum cleaning: The workpiece is placed in a drum filled with abrasives (such as quartz sand or steel shot). The rotation of the drum causes the workpiece to rub against the abrasive, removing surface lumps and scum (similar to "polishing").
High-pressure water washing: High-pressure water (typically 10-20 MPa) is used to wash the workpiece surface to remove residual flux, fine zinc slag, and oil. This method is particularly suitable for complex structures that are difficult to reach manually (such as the interior of pipes).
Sandblasting: For workpieces with higher requirements, sandblasting (using compressed air to spray quartz sand or aluminum oxide onto the workpiece surface) can be used. This not only removes defects but also creates a certain surface roughness for subsequent painting.
3.How to improve the protection and stability of the galvanized layer?
Passivation treatment is the most commonly used performance optimization method. It forms a passivation film (primarily composed of chromates, phosphates, etc.) on the surface of the zinc coating through a chemical reaction. It can be categorized by the type of passivating agent:
Chromate passivation: The traditional method involves immersing or spraying the workpiece with a chromate solution (such as potassium dichromate) to form a yellow-green or iridescent passivation film (iridescent passivation films offer enhanced corrosion resistance). The principle is that chromate undergoes an oxidation-reduction reaction with zinc, forming a dense composite oxide film that inhibits zinc dissolution. However, due to the toxicity of hexavalent chromium, its use is currently restricted in some industries, and chromium-free passivation is gradually being replaced.
Chromium-free passivation: An environmentally friendly alternative, commonly used passivating agents include silanes, titanates, and zirconates. These passivating films provide protection by forming an organic or inorganic composite film. For example, silane passivation films offer excellent adhesion and water resistance, making them suitable for applications requiring high environmental standards (such as food packaging and medical devices).
Note: The passivation treatment requires controlled time and temperature (usually room temperature to 60°C, immersion for 5-30 seconds) to avoid excessive thickness of the passivation film, which can lead to cracking. Cooling Control
After the workpiece is removed from the zinc bath, the cooling rate affects the structure of the galvanized layer. If the cooling is too rapid (e.g., directly rinsing a hot workpiece with cold water), the galvanized layer may crack due to thermal stress. If the cooling is too slow, the zinc layer is susceptible to oxidation and discoloration. Therefore, a "stepped cooling" method is necessary:
First, naturally cool in air to 200-300°C (avoid sudden cooling). Then, depending on the needs, cool in cold water (to accelerate setting) or continue air cooling to ensure uniform crystallization and defect-free galvanized layer.
4.How to do zinc removal?
Chemical stripping: Immerse the workpiece where the zinc plating is to be stripped in a dilute hydrochloric acid or nitric acid solution. The reaction between the acid and zinc (Zn + 2HCl = ZnCl₂ + H₂↑) dissolves the zinc coating. Once the zinc plating reaches the desired location, remove the workpiece and rinse with clean water to neutralize the zinc.
Mechanical stripping: For simple structures such as threads, the excess zinc plating can be directly removed through mechanical means such as turning and grinding, avoiding damage to the substrate caused by chemical treatment.
5.How to apply oil or paint?
Oiling: Apply anti-rust oil (such as machine oil or petroleum jelly) to create a physical barrier. This is suitable for short-term storage or non-appearance parts.
Painting: Apply a special zinc primer or polyurethane paint to the cleaned galvanized surface. This double protection of "galvanized layer + paint film" enhances corrosion resistance. This is suitable for applications such as automotive bodies and building steel structures where both appearance and protection are important. (Careful to select a paint that is compatible with the galvanized layer to prevent paint film peeling.)