IS 2629 : 1985Recommended Practice for Hot-Dip Galvanizing of Iron and Steel

IS 2629:1985 is the recommended practice for hot-dip galvanizing of iron and steel — the *how* of galvanizing (surface preparation, design for galvanizing, the dipping process, handling). It is the companion to the *quantum* standards: IS 4759 for coatings on structural steel/sections, IS 1367 Part 13 for fasteners, and IS 4736 for tubes.

Reach for it whenever fabricated steel is to be hot-dip galvanized — gratings, handrails, transmission/communication towers, gantries, cable trays, fasteners, lintels and embedments in aggressive/coastal environments. It is read with:

• IS 4759 — zinc coating requirements on structural steel
• IS 6745 — determination of mass of zinc coating
• IS 2633 — test for uniformity of zinc coating
• IS 209 — zinc ingot (bath metal)

The hot-dip sequence is: degrease → acid pickle → flux → dip in molten zinc (~450 °C) → quench/passivate. The metallurgical zinc-iron alloy layers formed are what give the coating its abrasion and corrosion life — galvanizing is a *reaction*, not a paint film.

The design rules in IS 2629 matter as much as the process:

• Provide vent and drain holes at high/low points of hollow sections — trapped air prevents coating; trapped zinc adds weight and is dangerous (steam explosion)
• Avoid large differences in section thickness in one assembly — they warp from differential cooling
• Avoid overlapping surfaces/skip-welded laps — acid and moisture get trapped and bleed/stain
• Steel chemistry matters: high-silicon/phosphorus ('reactive') steel gives thick, dull, brittle coatings (Sandelin effect)
• Galvanize *after* fabrication wherever possible; mark assemblies for handling distortion

Problem: a 8 mm thick fabricated steel bracket for a coastal walkway — specify and check the galvanizing.

Step 1 — quantum from IS 4759: for steel ≥ 6 mm the typical minimum *average* coating mass is 610 g/m² (local minimum ~ 550 g/m²).

Step 2 — convert to thickness: coating thickness (µm) ≈ mass (g/m²) × 0.142 → 610 × 0.142 ≈ 86 µm average.

Step 3 — process per IS 2629: confirm vent/drain holes were provided in any hollow members; bracket galvanized after welding and drilling.

Step 4 — verification: measure coating by magnetic gauge at several points (average ≥ 610, no point below local minimum); confirm by the IS 6745 strip test on a sample (dissolve coating in inhibited acid, weigh loss → g/m²); check adhesion (no flaking on a stout-knife test) and uniformity per IS 2633.

Accept only if the average *and* local minima are met — a high average can hide a thin, failing spot.

1. No vent/drain holes in hollow sections. This is the single biggest galvanizing defect — uncoated internals, trapped zinc, and a real explosion risk when sealed members are dipped.

2. Painting fresh galvanizing directly. New zinc is too smooth/reactive for paint to key onto — it needs weathering, a mordant 'T-wash', or a tie-coat. Skipping this peels the duplex system.

3. Welding or drilling after galvanizing without repair. The heat-affected/cut zone loses its zinc — it must be restored with zinc-rich paint or metal spray per IS 13871, or it becomes the corrosion initiation point.

4. Specifying 'galvanized' with no grade. Without an IS 4759 coating class the galvanizer is free to give the thinnest commercial coating.

5. Wet-storage staining. Tightly-stacked freshly-galvanized items in the rain form bulky white zinc-hydroxide ('white rust') — store with spacers and ventilation.

• IS 4759 — hot-dip zinc coatings on structural steel and other articles (the quantum)
• IS 1367 Part 13 — hot-dip galvanized coatings on threaded fasteners
• IS 6745 — determination of mass of zinc coating (g/m²)
• IS 2633 — test for uniformity of zinc coating (Preece test)
• IS 4736 — hot-dip zinc coatings on mild steel tubes
• IS 209 — zinc ingot · IS 13871 — repair of damaged galvanizing
• IS 800 — general steel construction (durability/corrosion clause)

IS 2629 is dated (1985) but the metallurgy has not changed, so it is still the working practice document; it has been reaffirmed and is widely aligned in intent with ISO 1461, which many EPC contracts now cite alongside it. For coastal, industrial and transmission-line work, hot-dip galvanizing remains the most cost-effective long-life protection and is frequently combined with a paint topcoat as a duplex system for a multiplicative life increase — but only if the fresh zinc is correctly prepared.

The most common dispute on site is *coating thickness shortfall* and *bare spots at welds/cut ends*. Resolve it by writing the IS 4759 coating class into the spec, doing magnetic-gauge checks at receipt, and insisting on IS 13871 repair of all post-galvanizing modifications — not by arguing after erection.