1. Basic Definition
Hot-dip galvanized bolts are fasteners subjected to pre-treatments (degreasing, pickling, fluxing) and immersed in molten zinc (≈450°C), forming a zinc-iron alloy layer on the surface. The coating thickness (typically 50–150 μm) significantly exceeds that of electro-galvanizing, making it suitable for harsh environments.
2. Key Features**
Superior Corrosion Resistance**: The zinc layer forms a dense zinc carbonate barrier in air, isolating corrosive agents. Damaged areas are protected via "sacrificial anode" corrosion protection.
Stable Mechanical Properties**: The coating bonds firmly to the substrate without compromising strength grades (e.g., 4.8, 8.8, 10.9).
Extended Service Life**: 20–30 years in general atmospheres; 10–15 years in marine/industrial environments.
Appearance**: Gray surface with possible spangle patterns (crystallization marks), slightly coarse texture.
3. Process Flow**
Pre-treatment**: Degreasing (oil removal) → Pickling (oxide removal) → Fluxing (prevents re-oxidation).
Hot-Dip Galvanizing**: Immersion in molten zinc to form coating.
Post-treatment**: Centrifugal removal of excess zinc → Cooling → Passivation (optional; enhances corrosion resistance).
4. Classification & Standards**
By Strength Grade**: Commercial grade (4.8, 5.8); High-strength grade (8.8, 10.9).
By Head Type**: Hex head, round head, square head, etc.
Standards:
- GB/T (China): E.g., GB/T 5782 (hex bolts), GB/T 41 (hex nuts).
- ISO: ISO 4014, ISO 4032.
- ASTM** (U.S.): ASTM A325 (high-strength structural bolts).
5. Applications
- Outdoor Infrastructure: Power transmission towers, bridges, highway guardrails.
- Building Structures: Steel-framed buildings, curtain wall connections.
- Marine Engineering: Docks, vessels (requires supplementary corrosion protection).
- Municipal Engineering: Streetlights, traffic sign supports.