Hot Dip Galvanized Fasteners
Hot dip galvanized fasteners are bolts, nuts, washers, and other hardware that have been coated with a layer of zinc by being immersed in molten zinc (at around 450°C / 842°F). This process forms a durable, corrosion-resistant coating, making these fasteners ideal for outdoor and structural applications.
Hot Dip galvanization Process for fasteners
Surface Preparation: This is crucial for a successful Zinc coating.
- Degreasing/Caustic Cleaning: Removes dirt, oil, grease, and paint.
- Rinsing: Washes off the cleaning solution.
- Pickling: Immersing the fasteners in an acidic solution (e.g., hydrochloric or sulfuric acid) to remove mill scale and rust.
- Rinsing: Washes off the acid.
- Fluxing: Applying a zinc ammonium chloride solution to remove any remaining oxides and prevent further oxidation before dipping in the zinc bath. This also helps the molten zinc wet and adhere to the steel.
Galvanization: The cleaned fasteners (Nut, Bolts, Washer etc.) are dipped in a molten zinc with temperatures around 455°C to 480°C and high temperatures between 530°C and 560°C). During immersion, the zinc reacts with the iron in the steel to form a series of zinc-iron alloy layers, with a final layer of pure zinc on the outermost surface.
Centrifuging or Spinning: Once the metallurgical reaction is complete and the fasteners are completely coated, the zinc is drained from the basket. Fasteners are often spun at high speed to remove excess zinc, especially from threads, ensuring proper nut fit. Larger items may be brushed.
Cooling/Quenching: The fasteners are cooled in water to solidify the zinc, sometimes in a quench tank, to solidify the Zinc coating.
| Key Benefit | Description |
|---|---|
| Corrosion Resistance | Zinc forms a protective layer and sacrificial barrier to protect the steel. |
| Mechanical Durability | Zinc layer is metallurgically bonded, providing wear resistance. |
| Weather Resistance | Ideal for humid, coastal, or industrial environments |
| Long Service Life | Lifespan can exceed 50 years in mild environments, 20–30 years in aggressive zones |
Threading Considerations
The thickness of the coating means that threads on galvanized bolts are made undersize before coating, and nuts are tapped oversize after galvanizing to ensure they fit together. It’s important not to mix galvanized bolts with non-galvanized nuts or vice-versa without considering thread compatibility.
- Galvanizing adds 0.25–0.50 mm to external threads → undersizing required.
- Use “galvanized nut and bolt pairs” for best performance and thread tolerance.
- Threads are often tapped after galvanizing for nuts to fit properly.
Applications of Hot-Dip Galvanized Fasteners:
Due to their excellent corrosion resistance, Hot-dip galvanized fasteners are widely used outdoor or corrosive environments:
- Construction: Steel structures, Bridges, buildings, Guardrails Bolts, Crash barrier Fasteners and handrails.
- Infrastructure: Highway structures, signage, lighting poles Bolts, and utility towers (power transmission Bolt, telecommunications).
- Marine Environments: Docks, piers, and shipbuilding (though stainless steel might be preferred in direct saltwater immersion for critical applications).
- Agriculture: Fencing, animal enclosures, and agricultural equipment.
- Outdoor Equipment: Playground structures, and outdoor recreational equipment.
- Industrial Applications: Manufacturing plants, and processing facilities where corrosion is a concern.
| Issue | Solution |
|---|---|
| Hydrogen Embrittlement | Use mechanical galvanization or avoid HDG on fasteners >10.9 grade steel |
| Thread Fit Issues | Pair with HDG-matched nuts or use oversized tapping |
| Appearance | Finish is matte grey, rougher than zinc electroplating |
| Electrical Conductivity | HDG is non-conductive; not suitable for grounding without prep |
Hot-dip galvanized fasteners offer a robust and reliable solution for long-term corrosion protection in a wide array of demanding applications, making them a popular choice in many industries.