1. Metallurgical Structure
In hot-dip galvanizing, the zinc doesn’t just sit on the surface. It metallurgically bonds with the steel, forming a series of iron-zinc alloy layers beneath the outer pure zinc layer. From the steel outward, the typical layer sequence is:
- Gamma (Γ) – Fe₃Zn₁₀
- Delta (Δ) – FeZn₁₀
- Zeta (ζ) – FeZn₁₃
- Eta (η) – Pure zinc
These alloy layers are harder than the base steel, providing mechanical durability in addition to corrosion resistance.
2. Coating Thickness
Coating thickness is typically measured in microns (µm) or by mass (g/m²). Standards like ASTM A123 or ISO 1461 define acceptable thickness levels. For instance:
- A typical hot-dip galvanized structural member might have 85–100 µm (600–700 g/m²).
- Electro-galvanized coatings are much thinner—5–25 µm.
Coating weight affects corrosion performance, and coating uniformity is critical in structural applications.
3. Corrosion Mechanisms
Zinc protects steel through:
- Barrier protection: Prevents contact with oxygen and water.
- Cathodic protection: Zinc has a more negative electrochemical potential than steel. It corrodes preferentially, protecting exposed steel areas even if scratched.
However, in marine environments or acidic/alkaline industrial settings, the zinc layer may degrade faster due to the formation of soluble zinc salts.
4. Performance in Different Environments
- Rural environments: Low corrosion rate (~1 µm/year)
- Urban/industrial: Moderate rate (~3–8 µm/year)
- Coastal/marine: High corrosion rate (~10–20 µm/year)
Engineers often refer to ISO 9223 classifications when estimating service life.
5. Welding & Fabrication Considerations
- Welding galvanized steel releases toxic zinc fumes (zinc oxide). Proper ventilation and PPE are mandatory.
- Post-weld touch-up using zinc-rich paint, zinc metalizing, or soldering may be required to restore corrosion resistance.
- Forming galvanized steel can cause micro-cracks or flaking in the coating, especially if alloy layers are thick and brittle.
6. Paintability and Adhesion
Bare galvanized steel can be painted but requires surface preparation:
- Degreasing
- Surface roughening (brush blast or sweep blast)
- Application of an etch primer or zinc-compatible primer
Galvannealed steel (post-annealed after galvanizing) offers better paint adhesion due to its matte finish and fully alloyed surface.
7. Standards and Specifications
Key standards include:
- ASTM A123: Hot-dip galvanizing of iron and steel products
- ASTM A153: Hardware-specific galvanizing
- ASTM A653: Galvanized sheet
- ISO 1461: Global standard for hot-dip galvanized coatings
8. Limitations
- Not ideal for high-temperature applications (>200°C), where zinc may volatilize or lose adhesion.
- In contact with certain woods (like oak or cedar), tannins can accelerate corrosion.
- Galvanic corrosion can occur if connected to copper or brass in moist environments.
