Report on hot-dip galvanizing surface treatment technology - The industrial cornerstone of long-term steel corrosion protection
I. Functions and Core Values
Core functions
- Super anti-rust: zinc layer thickness 60-150μm, salt spray test up to 600-1500 hours (5-8 times of ordinary paint)
- Self-healing characteristics: Zinc layer scratches through sacrificial anode to protect the substrate (electrochemical protection)
- Mechanical protection: the hardness of zinc-iron alloy layer is HV250, and the scratch resistance is excellent
- Maintenance free life: anti-corrosion life>20 years in outdoor environment (ISO 1461 standard)
Key problem solving
| Industry pain points | Hot galvanizing solutions |
| Bridge/transmission tower corrosion risk | zinc layer + alloy layer double protection |
| Rust prevention in dead corners of complex structural parts | 450℃ full immersion coating with molten zinc |
| Rapid corrosion in high humidity environment. | Zinc layer consumption rate is only 1-2μm/year |
II. Working Principle
Microstructural stratification (from substrate to exterior):
① δ Phase (FeZn₇, 7-12% iron content) → ② Phase ζ (FeZn₁₃, 6% iron content) → ③ η phase (pure zinc layer)
(FIG.: Metallurgical bonding structure and electrochemical protection mechanism of hot-dip galvanized layer)
Liquid zinc metallurgy combined with electrochemical protection mechanism
III. Process operation process
Seven steps to strictly control the process chain
- De-fatting and oil removal (alkaline solution, 70℃×10min) → water washing
- Pickling and rust removal (15-20% hydrochloric acid, room temperature for 30min) → water washing
- Assistive deposition treatment (ZnCl₂+NH₄Cl solution, 60℃×2min)
- Hot galvanizing (zinc bath temperature 445-465℃, immersion time 3-8min)
- Centrifugal zinc throwing (high speed rotation to remove residual zinc and control film thickness)
- Water cooling (sudden temperature drop to prevent zinc layer cracking)
- Quality inspection and repair:
- Thickness detection: magnetic thickness gauge (ISO 1461 requires ≥85μm)
- Adhesion test: Hammer method without peeling (ASTM A123)
IV. Comparison with other processes
| Features | Hot galvanizing | Electro-galvanizing | Spraying plastics |
| Corrosion resistance life | 20-50 years (outdoor) | 5-15 years | 8-15 years |
| Thickness uniformity of the film | Angle> plane (thickened by 20%) | Uniform but easy to thin at the corners | Uniform coverage |
| Resistant to mechanical damage | ★★★★★ (high hardness of alloy layer) | ★★☆☆☆ (soft pure zinc layer) | ★★★☆☆ (toughness of coating) |
| Environmental protection property | Zinc smoke treatment(environmental protection equipment is required) | Chromium-containing wastewater | Zero VOC |
| Cost | ¥80-150/㎡ (including pretreatment) | ¥20-40/㎡ | ¥25-50/㎡ |
Project demonstration:
The tower of the coastal wind farm (hot-dip galvanized) has been in service for 18 years without corrosion, while the painted structural parts have been repaired three times during the same period
V. Application Scenario Guide
● Preferred fields:
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- ✅ Outdoor heavy structures (power grid towers/steel structures for Bridges)
- ✅ Corrosive environment (chemical plant/ocean platform)
- ✅ Maintenance-free requirements for facilities (high speed guardrail/light poles)
● Use scenarios with caution:
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- ❌ Precision fitting parts (plating thickness fluctuation ±15%)
- ❌> 200℃ working condition (zinc-iron alloy layer embrittlement)
- ❌ Food contact equipment (risk of zinc ion migration)
Conclusion: Hot-dip galvanizing has become an irreplaceable process in heavy-duty corrosion protection due to its ultra-long lifespan, self-healing properties, and extreme environmental adaptability.
Mingli Metal's intelligent hot-dip galvanizing line supports processing of super-large components (up to 12m) with real-time zinc composition monitoring (precise control of aluminum content at 0.005-0.02%), providing a full lifecycle protection solution for infrastructure projects.
Appendix: real scene of hot-dip galvanizing production line
Figure: workpiece immersion in zinc → centrifugation → cooling process (zinc liquid temperature monitoring system)
● Technical appendix
- International standard benchmarking:
- Thickness requirement: ISO 1461 (85μm)> ASTM A123 (78μm)
- Process innovation:
- Zinc-nickel alloy coating (0.8-1.3%Ni) can reduce zinc ash by 50%
- Environmental protection upgrade:
- Acid mist recovery system (HCl recovery rate> 90%)
- Waste heat power generation technology (reduce energy consumption by 30%)