1. Hot-Dip Galvanizing Only (Single Galvanizing)

Process Flow

1. Steel coil → Slitting → Punching & forming wave shape
2. Alkaline degreasing → Pickling (HCl) → Water rinsing
3. Fluxing (ZnCl₂ + NH₄Cl solution) → Preheating
4. Immersion in 450–460 °C molten zinc bath for 6–8 minutes
5. Withdrawal + air knife to remove excess zinc
6. Water quenching or air cooling → Chromate passivation → Inspection & packing

Protection Mechanism

• Sacrificial anode protection: zinc corrodes preferentially to steel
• Self-healing: a dense zinc carbonate layer (zinc patina) forms naturally and seals the surface

Zinc Coating Standards (China JT/T 281-2007 & GB/T 18226)

• Class A (expressway standard): ≥ 600 g/m² both sides ≈ 85 μm
• Class B (provincial roads): ≥ 350 g/m² ≈ 50 μm Leading manufacturers often achieve 1000–1200 g/m² (≈140+ μm) for extended life

Advantages

• Metallurgical bond between zinc and steel — extremely strong adhesion
• 100 % coverage, including internal holes, welds, and corners
• Lowest initial cost, ideal for mass production

Drawbacks

• Silver-white spangle appearance that darkens over time
• Soft zinc layer easily scratched during transport (white marks, cosmetic only)
• Faster zinc consumption in aggressive environments (coastal, heavy pollution)2. Hot-Dip Galvanizing + Powder Coating (Double Protection)

Why Add Powder Coating After Galvanizing?

The zinc layer is already excellent, but it is slowly “consumed” in corrosive environments. The powder coating acts as a tough physical barrier, almost completely stopping zinc loss and extending service life by decades.

Complete Process After Galvanizing

1. Surface grinding & polishing (remove zinc dross and spikes)
2. Chemical pretreatment (chromating or phosphating) for maximum adhesion
3. Automatic electrostatic powder spraying (outdoor-grade polyester powder)
4. Curing at 180–200 °C for 20–30 minutes in continuous oven
5. Cooling → Quality inspection → Packing

Powder Coating Standards

• Typical thickness: 80–120 μm (can reach 200 μm for extreme conditions)
• Most popular colors: RAL 6029 Traffic Green, RAL 9006 White Aluminium
• Finish: matte, semi-gloss or high-gloss available

Double Protection Mechanism

1st barrier: Polyester coating blocks water, oxygen, and chloride ions 2nd barrier: Even if the coating is scratched, the underlying zinc continues sacrificial protection → True “1 + 1 >> 2” effect

4. Real-World Project References

• Shenzhen–Shantou Expressway (Guangdong): 100 % galvanizing + green powder coating → still looks brand-new after 15 years
• Beijing–Shanghai Expressway (early sections): originally single galvanizing → many sections replaced with double-protection after 10–12 years
• Hangzhou Bay Bridge (world’s longest sea-crossing bridge): exclusively double-protection since 2008 → zero corrosion reported in 2024 inspection

5. Final Recommendation – How to Choose in 2025

Choose Hot-Dip Galvanizing Only if:

• Project is inland with low rainfall and low pollution
• Budget is very tight
• Expected service life 15–20 years is acceptable

Choose Galvanizing + Powder Coating (Double Protection) if:

• Project is expressway, Class-1 highway, bridge, or tunnel
• Located in coastal, high-salt, acid-rain, or heavy-industrial areas
• You need 30+ years of maintenance-free performance
• Visual appearance and landscape coordination are important