The Process — Five Stages
1. Degreasing — alkaline bath removes oil, grease and mill markings.
2. Acid pickling — hydrochloric or sulphuric acid removes mill scale and surface rust.
3. Fluxing — zinc-ammonium chloride flux primes the surface for zinc adhesion.
4. Galvanizing — immersion in molten zinc at 450 °C; iron-zinc alloy layers form a metallurgical bond.
5. Quenching & inspection — water or air cool, then coating thickness, finish and adherence are checked to IS 4759.
IS 4759 — Coating Classes
IS 4759:1996 specifies hot-dip galvanized coatings on structural steel products. Standard classes:
• Class 5 (45 microns / 325 g/m²) — Light duty
• Class 6 (65 microns / 470 g/m²) — Medium duty
• Class 7 (85 microns / 610 g/m²) — Standard for transmission towers
• Class 8 (115 microns / 815 g/m²) — Heavy industrial exposure
Standard for towers, bridges and outdoor structural sections is Class 7 or higher.
Hot-Dip Galvanized vs Pre-Galvanized (GP) vs Electroplated
• Hot-dip galvanized — thick zinc coating (60 to 100 μm), metallurgically bonded, 25 to 50 year life. Best for structural sections and outdoor use.
• Pre-galvanized (GP / GI) — thinner zinc coating (20 to 40 μm) applied to coils before fabrication. Suitable for ducting, light hollow sections.
• Electroplated zinc — very thin coating (5 to 15 μm), cosmetic only, not for structural use.
When to Galvanize
Mandatory or strongly recommended for:
• Transmission and telecom towers
• Coastal structures within 5 km of the sea (Mumbai's entire coastline)
• Outdoor handrails, fencing, gates and gratings
• Buried structural sections
• Solar mounting structures
• Roof trusses in industrial environments
Galvanizing Job Work at SIIPL
SIIPL undertakes hot-dip galvanizing job work for customer-supplied structural sections, fabricated assemblies and components. Coating classes from IS 4759 Class 5 to Class 8 are available, with batch-wise certificates issued for each lot. Contact our team for capacity, rates and turnaround.
FAQ
How long does hot-dip galvanizing last?
In urban atmospheric exposure, an 85 micron Class 7 coating lasts 30 to 50 years before first maintenance. In coastal or industrial environments, expect 20 to 30 years.
What is the difference between GI and hot-dip galvanized?
GI (Galvanized Iron) usually refers to sheet steel pre-galvanized at the mill with a thinner coating. Hot-dip galvanizing is the post-fabrication batch process described here, producing a much thicker, more durable coating.
Can welded fabrications be galvanized?
Yes — and it's the standard sequence: fabricate, weld, drill, then galvanize. Welds and cut edges receive the same zinc coverage as the parent metal.
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