IS 4759:1996 is the Indian Standard (BIS) for hot-dip zinc coatings on structural steel and other allied products. IS 4759 covers hot-dip galvanizing (HDG) of structural steel and hardware — zinc coating applied by dipping in molten zinc. HDG provides 45-85µm zinc coating (much thicker than electroplating IS 1573 at 5-25µm). Standard protection for transmission towers, poles, guardrails, and outdoor steel structures.
Specification for hot-dip galvanized coatings on structural steel, hardware, and allied products covering coating mass, uniformity, adhesion, and testing.
Key reference values — verify against the current code edition / project specification.
| Reference | Value | Clause |
|---|---|---|
| Subject | Hot-dip galvanized coating on structural steel/articles | Scope |
| Min avg, steel ≥6 mm | ≈ 610 g/m² (local min ≈ 550) | Acceptance |
| Min avg, 5–6 mm | ≈ 460–610 g/m² band by thickness | Acceptance |
| Mass→thickness | µm ≈ g/m² × 0.142 | Conversion |
| Verify | IS 6745 (mass) + IS 2633 (uniformity) + adhesion | QC |
| Read with | IS 2629 (practice) / IS 6745 / IS 2633 | Cross-ref |
IS 4759 specifies hot-dip zinc coatings on structural steel and other allied products — the standard for galvanising of steel products to provide corrosion protection through a sacrificial zinc layer. Hot-dip galvanising is the most common corrosion protection for outdoor / weather-exposed steel in India.
Use IS 4759 for galvanising: - Steel structures (PEB, transmission towers, lattice masts) - Steel sections + tubes (IS 4923, IS 1161) - Steel reinforcement (rare; usually epoxy-coated instead) - Bolts + nuts (IS 1364) - Wire rope (IS 2266) - Sheet metal (roofing sheets, cladding, ducts) - Pipe + tube (water supply, structural, conduits) - Wire mesh + chain link - Crash barriers + guard rails (IRC:99:2018) - Highway sign posts (IRC:67:2012)
Galvanising provides: - 50-75+ year corrosion life in mild atmosphere - 25-50 years in industrial atmosphere - 15-30 years in marine atmosphere - Self-healing scratches (zinc galvanises adjacent steel) - Maintenance-free (vs paint requiring periodic touch-up)
vs alternatives: - Painted steel: cheaper initial cost; needs 5-10 year touch-up cycle - Stainless steel: best corrosion; expensive (5-10× cost of galvanised mild steel) - Aluminium: lightweight + corrosion-resistant; specific applications (cladding, roof) - Epoxy-coated: alternative for rebar in concrete; less common for structural
IS 4759:1996 coating classes (zinc per unit surface area):
| Class | Total zinc coating (g/m²) | Minimum local thickness (µm) | Use | |---|---|---|---| | Class A1 | 700 | 100 | Heaviest; marine + severe industrial | | Class A2 | 600 | 85 | Severe industrial | | Class B1 | 530 | 75 | Industrial / coastal | | Class B2 | 460 | 65 | Industrial general | | Class C1 | 380 | 55 | Atmospheric general (most common) | | Class C2 | 305 | 45 | Mild atmospheric / indoor |
Coating mass = ~7.14 µm × 1 g/m² (zinc density 7.14 g/cm³).
Selection by environment:
| Environment | Recommended class | |---|---| | Indoor (factory storage, not exposed) | C2 | | Mild outdoor (rural, suburban) | C1 | | Industrial atmosphere (urban, vehicle pollution) | B2 | | Coastal (within 5 km of sea) | B1 to A2 | | Marine (near sea splash / spray) | A2 to A1 | | Submerged in fresh water | B1 | | Submerged in seawater | A1 + cathodic protection |
Process: 1. Surface preparation: degreasing → acid pickling → flux dipping 2. Hot-dip: immersion in molten zinc bath at ~450 °C for 1-5 minutes 3. Withdrawal: controlled removal speed (drips back to bath) 4. Cooling: air or water quench 5. Inspection: visual + thickness measurement (digital gauge)
Coating thickness measurement: - Magnetic / eddy-current gauge (non-destructive) - Stripping test (destructive; reference for calibration) - Minimum at any single point per Class table; mean of multiple readings ≥ Class minimum
Repair / touch-up: - Damaged areas re-galvanised (preferred) OR painted with zinc-rich paint (acceptable for small areas) - Maximum touch-up area: 10 % of total surface - Touch-up paint thickness: ≥ 100 µm dry
Galvanising defects + acceptance:
| Defect | Acceptance limit | |---|---| | Bare spots (uncoated) | < 1 % of total area; touch-up required | | Lumps + drips | Not unsightly; functional only | | Surface roughness | Per visual standard | | Adhesion | Hammer / cross-cut test; minor flaking acceptable | | Coating thickness | ≥ class minimum at all points | | Brittleness (heavy galvanised) | No spontaneous cracking |
Inspection cadence: - Sampling per delivery (typically 10 % of items) - Visual + thickness measurement at each - Per-batch test certificate from galvaniser
Cost (typical 2026): - Galvanising: ₹15-25 per kg of steel (for typical thickness) - Total cost (steel + galvanise): ₹85-110 per kg vs ₹60-70 for ungalvanised mild steel - Compared to painting (₹5-15 per kg), galvanising premium is 2-5× — but lifetime value is higher (no maintenance)
Galvaniser shop: - Bath capacity: 8-15 m × 1-2 m × 2-3 m (depth) - Capacity: 5-50 t per day - Indian galvanisers: many regional players; ISO 1461-compliant
Component compatibility: - Threaded fasteners: galvanised separately; thread overlap with mating part - Welded structures: galvanise after welding (not before) - Mixed assemblies (zinc + dissimilar metal): isolate to prevent galvanic corrosion
Service life prediction: - Coating thickness × corrosion rate (per environment) - Mild atmosphere: 1-2 µm/year; 100 µm coating → 50-100 years - Industrial: 3-5 µm/year; 50-25 years - Marine: 5-15 µm/year; 15-25 years
Re-galvanising existing structure: - After 50-75 % of coating consumed - Process: clean + grit-blast existing coating off → re-pickle → re-galvanise - Cost: 60-80 % of new galvanising
1. Wrong class for environment. Class C1 in marine zone = corrosion within 5-10 years. Match class to environment. 2. Surface preparation inadequate. Mill scale / paint not removed; coating poor adhesion. Acid pickling mandatory. 3. Painting over galvanising without surface treatment. Paint adhesion poor; flakes off. Mordant wash or T-wash before painting. 4. Galvanising welded structures pre-welding. Welding burns off zinc; HAZ unprotected. Galvanise after welding. 5. Touch-up area excessive. > 10 % touch-up indicates poor original galvanising; quality issue. Reject + re-galvanise. 6. Mixed Zn + Al / Zn + Cu fasteners. Galvanic corrosion at junction. Use compatible fasteners. 7. Galvanised structure embedded in fresh concrete. Zn corrodes in alkaline (pH 12+) environment; releases hydrogen; potential delamination. Isolate Zn from fresh concrete. 8. Galvanised structure painted with cement-based mortar. Same alkaline corrosion issue. 9. No coating thickness verification at delivery. Galvaniser may under-coat for cost savings. Spot-test on delivery. 10. Repair with non-zinc-rich paint. Doesn't provide cathodic protection. Use zinc-rich primer for touch-up. 11. Confusing electroplating with hot-dip galvanising. Electroplated zinc is much thinner (5-15 µm) than hot-dip (45-100+ µm); much shorter life. Specify hot-dip per IS 4759. 12. Heavy galvanising on thin / heat-treated steel. Hydrogen embrittlement risk; springs / high-strength bolts may crack. Use cadmium plating or other coating instead.
Steel project cascade with galvanising:
1. Design — material + corrosion protection per environment. 2. Steel procurement — base steel per IS 2062:2011 etc. 3. Fabrication — cutting, drilling, welding (do NOT galvanise pre-welding). 4. Galvanising specification — class per environment. 5. Galvanising at approved shop — per IS 4759. 6. Inspection at galvaniser: - Visual: no bare spots, lumps, defects - Thickness: per class minimum - Adhesion test 7. Site delivery + verification: - Spot-test thickness on random samples - Visual inspection for transport damage - Touch-up if needed (zinc-rich paint) 8. Erection — handle to prevent coating damage. 9. Joining — mechanical fasteners (bolts) preferred; if welding required at site, weld + then touch-up. 10. Service — periodic inspection (annual visual; 5-year detailed thickness check). 11. End-of-life or rust signs — re-galvanise OR replace.
Project economics: - Galvanising adds 15-25 % to steel structure cost - Saves 10-30 % over 20-30 year life vs painting (no maintenance cost) - Zero-maintenance design preferable for difficult-access structures (transmission towers, bridges)
IS 4759 + companion codes provide robust corrosion protection framework. Indian galvanising industry is mature; quality + capacity available across major industrial hubs.
| Parameter | IS Value | International | Source |
|---|---|---|---|
| Coating mass (>5mm steel) | 610 g/m² | 610 g/m² (1.8 oz/ft²) | ASTM A123 |