What is Hot-Dip Galvanising?
Hot-dip galvanising (HDG) is the process of coating steel with zinc by immersing the fabricated part in a bath of molten zinc at approximately 450°C (840°F). The zinc reacts metallurgically with the steel surface to create a series of zinc-iron alloy layers, topped by a layer of pure zinc.
Unlike paint or electroplating — which sit on the surface — HDG coating is metallurgically bonded to the steel. It cannot be chipped or peeled like paint. It provides both:
- Barrier protection: The zinc layer prevents moisture and oxygen reaching the steel
- Cathodic (sacrificial) protection: Zinc corrodes sacrificially, protecting exposed steel at scratches and cut edges
HDG Coating Thickness on Fasteners
Per IS 1573 and ASTM A153, HDG coating thickness on threaded fasteners is:
- M6–M12: Minimum 45 µm average
- M12–M36: Minimum 50 µm average
- Above M36: Minimum 50 µm average
For comparison, zinc electroplating provides only 5–12 µm. HDG provides 4–10× more zinc than zinc plating, which translates to dramatically longer service life.
Predicted service life of HDG fasteners (ISO 14713):
| Environment | HDG life at 50µm | Zinc Plate life at 8µm | |-------------|----------------|-----------------------| | Rural atmospheric | 50+ years | 8–12 years | | Urban/suburban | 25–35 years | 5–8 years | | Industrial (SO₂) | 15–25 years | 3–6 years | | Coastal (>500m from sea) | 10–20 years | 2–4 years | | Severe marine | 5–8 years | 1–2 years |
The Critical Detail: HDG Bolts Require Tapped Nuts
This is the most common mistake when procuring HDG fasteners: you cannot use standard (non-HDG) hexagon nuts on HDG bolts.
Here's why: The 50µm zinc coating on bolt threads adds approximately 0.1mm to the thread diameter. A standard DIN 934 nut machined to 6H tolerance will not thread onto the coated bolt. Forcing it will damage the zinc coating on the bolt threads — eliminating the corrosion protection at precisely the most vulnerable location.
Solution: HDG nuts must be manufactured with an oversize internal thread — "tapped oversize" or "HDG cleared" (6AZ tolerance per ISO 4759). This extra thread clearance accommodates the zinc coating and allows free assembly.
Practical guidance:
- Always purchase HDG bolts and HDG nuts together as a matched set
- Never mix HDG bolts with non-HDG nuts
- If you receive HDG bolts and nuts won't thread on, the nuts are non-HDG — return them
Applications for HDG Fasteners in India
Construction:
- Structural steel connections in PEB (pre-engineered buildings)
- Column base plate anchor bolts
- Roof truss purlin connections
Infrastructure:
- Bridge structural bolt assemblies
- Highway and flyover steel components
- Power line and telecom tower connections
Solar energy:
- Ground mount structural connections (inland sites)
- Module racking primary structural bolts
Telecom:
- Tower leg connections, bracing bolts, U-bolts
- Equipment shelter anchoring
HDG vs Zinc Plated vs Dacromet — When to Use Which
Zinc plated (electroplated): Indoor applications, mild environments where 5–10 year life is acceptable. Low cost. Cannot be used on Grade 10.9/12.9 without HE management.
HDG: Outdoor structural, construction, infrastructure, telecom towers. 15–50 year life. Cannot be HDG on bolts above M36 without HE management.
Dacromet: High-strength bolts (10.9/12.9) in outdoor environments — provides good corrosion resistance without hydrogen embrittlement risk. RoHS compliant (chromate-free). Thinner coating (5–8 µm) — maintains thread fit without nut tapping.
PGS Fasteners HDG Range
PGS Fasteners stocks a comprehensive range of hot-dip galvanised fasteners:
- Hex bolts DIN 933/931 Grade 8.8 HDG, M6 to M52
- Heavy hex bolts ASTM A325 Type 1 HDG, 1/2" to 1-1/2"
- Foundation anchor bolts HDG, various configurations
- Hex nuts DIN 934 / IS 1367 Grade 8, tapped oversize for HDG assembly
- Heavy hex nuts ASTM A194 2H HDG
- Plain washers DIN 125 HDG
- Spring washers DIN 127 HDG
All supplied with EN 10204 2.2 test documentation. 3.1 MTCs available on request for critical applications.