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Hot-Dip Galvanizing vs Galvanizing + Powder Coating for Highway Guardrails
Release time:
2025-12-09
In 2025, more than 85 % of new and renovated expressways in China specify double-protection guardrails — it has become the industry gold standard.
1. Hot-Dip Galvanizing Only (Single Galvanizing)
Process Flow
- Steel coil → Slitting → Punching & forming wave shape
- Alkaline degreasing → Pickling (HCl) → Water rinsing
- Fluxing (ZnCl₂ + NH₄Cl solution) → Preheating
- Immersion in 450–460 °C molten zinc bath for 6–8 minutes
- Withdrawal + air knife to remove excess zinc
- 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
- Surface grinding & polishing (remove zinc dross and spikes)
- Chemical pretreatment (chromating or phosphating) for maximum adhesion
- Automatic electrostatic powder spraying (outdoor-grade polyester powder)
- Curing at 180–200 °C for 20–30 minutes in continuous oven
- 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
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