How long does hot-dip galvanizing last?

In rural/suburban India: 50-100 years. In moderate industrial: 25-50 years. In severe coastal: 15-25 years. Depends on zinc thickness and atmospheric corrosion rate. IS 4759 heavy coating (610 g/m²) lasts longer than light coating (395 g/m²).

Galvanizing vs painting — which is better for outdoor steel?

HDG galvanizing (IS 4759) provides longer maintenance-free life (50+ years vs 10-15 years for paint). Paint is cheaper initially but needs repainting every 10-15 years. For critical structures (towers, bridges), galvanizing is preferred.

IS 4759:1996 — Hot-Dip Zinc Coatings on Structural Steel and Other Allied Products

IS 4759 : 1996

Hot-Dip Zinc Coatings on Structural Steel and Other Allied Products

ASTM A123/A123M · ISO 1461

CurrentFrequently UsedSpecificationMaterials Science · Painting, Coatings and Surface Finishing

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Summary

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 Values

Min coating (steel >5mm)610 g/m² (≈86µm)

Min coating (steel 3-5mm)460 g/m² (≈65µm)

Min coating (steel 1.5-3mm)395 g/m² (≈55µm)

Practical Notes

! HDG provides 50-100 year corrosion life in rural/suburban environments — the most cost-effective protection for outdoor steel.

! Preece test (copper sulphate dip) is the standard uniformity check — bright copper deposit indicates thin/bare spots.

! Steel composition affects galvanizing quality — silicon >0.04% causes dull/rough coating (reactive steel).

! Galvanized surfaces need special paint (zinc chromate primer) if painting over is required.

! All structural bolts, nuts, washers for outdoor steel should also be HDG per IS 1367/IS 6649.

Cross-Referenced Codes

IS 277:2018Galvanized Steel Sheets (Plain and Corrugated...

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IS 1573:2018Electroplated Coatings of Zinc on Iron and St...

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IS 2629:1985Recommended Practice for Hot-Dip Galvanizing ...

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IS 6745:1994Method for Determination of Density of Harden...

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hot dip galvanizingHDGzinc coatingstructural galvanizingcorrosion protection