1.What is the raw material preparation process?
Substrate selection
Common materials: carbon steel such as Q235, Q345, 45# steel, or low alloy steel (such as 20Cr).
Specifications: round bar diameter is usually 5~200mm, cut to specified length according to order.
Surface pretreatment
Degreasing: Use alkali solution (such as sodium hydroxide) or organic solvent to remove surface oil stains to avoid affecting the adhesion of the zinc layer.
Rust removal: Remove the oxide scale by pickling (hydrochloric acid or sulfuric acid solution) to expose the metal substrate.
2.What is the core process of hot-dip galvanizing?
Plating treatment
After pickling, immerse in the plating solution (mixed solution of zinc chloride + ammonium chloride) to form a zinc chloride salt film to prevent secondary oxidation and promote zinc liquid infiltration.
Hot dip galvanizing
Zinc pot heating: The zinc liquid temperature is controlled at 430~460℃, and the zinc purity is ≥99.5%. ◦ Zinc dipping process: The round rod is vertically immersed in the zinc liquid, and the residence time is adjusted according to the diameter to ensure uniform adhesion of the zinc layer.
Heat diffusion and cooling
Lifting and temperature control: After the round rod is lifted out of the zinc pot, the excess zinc liquid is blown away by an air knife (high-pressure air) to control the thickness of the zinc layer.
Cooling: Natural air cooling or water cooling, so that the zinc layer solidifies to form an iron-zinc alloy layer (Zn-Fe alloy layer) and a pure zinc layer.
3.What does post-processing include?
Surface treatment
Passivation: Immerse in chromate solution (such as passivation solution) to form a passivation film to enhance corrosion resistance (chromium-free passivation can be used for environmentally friendly processes).
Oiling: Spray anti-rust oil, suitable for storage in humid environments.
Quality inspection
Zinc layer thickness: Detected with a magnetic thickness gauge.
Appearance inspection: Visually remove zinc nodules, missed plating, surface cracks and other defects.
Adhesion test: Bend test or hammer test to ensure that the zinc layer does not fall off.
4.What are the critical control points of the process?
Pickling concentration: hydrochloric acid concentration 15%~20%, temperature 50~80℃, avoid over-pickling and substrate loss.
Zinc liquid impurities: regularly salvage zinc slag, control the iron content <0.03% to prevent the zinc layer from becoming brittle.
Cooling rate: large diameter round bars need to be cooled by circulating water to control temperature, to avoid stress cracking caused by temperature difference between inside and outside.
5.What are the common defects and solutions?
Uneven zinc layer: adjust the rod lifting speed and calibrate the air knife
Missed plating: strengthen pickling and replace the plating solution
Too thick zinc layer: shorten the zinc immersion time and increase the zinc solution temperature