IS 4736 is the Indian Standard that defines what “galvanized” actually means on a steel tube — the zinc-coating mass, the uniformity test, the adhesion requirements, and the freedom-from-defect criteria. It is a coating spec, not a structural spec: the underlying tube is supplied to IS 1239 Part 1 (water and gas), IS 4270 (borewell casing), or IS 3589 (large-OD water mains), and IS 4736 sits on top describing how the hot-dip zinc layer must look on the finished tube. Specify the coating class explicitly when ordering galvanized pipe — the difference between heavy, medium, and light coating is the difference between a 30-year borewell casing and a 10-year one.
What IS 4736 actually covers
IS 4736 (current edition IS 4736 : 1986, reaffirmed) sets out the requirements for hot-dip zinc coatings on welded or seamless mild-steel tubes. The standard does not address the steel composition, the tube's mechanical properties, or its dimensional tolerances — those come from the underlying structural spec (IS 1239, IS 4270, IS 3589). What IS 4736 covers is the zinc layer itself: how much zinc per unit area, how evenly distributed, how well bonded, and how to verify all of that on the finished pipe.
A complete galvanized-pipe specification therefore cites two standards together — for example, “IS 1239 Part 1 medium class, galvanized to IS 4736 heavy coating” or “IS 4270 borewell casing pipe galvanized to IS 4736 (single-spot 300 g/m² min)”. A purchase order that says only “galvanized” without citing the coating class is incomplete, and the mill is free to ship the lightest economy coating that meets the bare minimum.
Coating mass classes (g/m²)
| Coating class | Single-spot min (g/m²) | Triple-spot avg min (g/m²) | Typical application |
|---|---|---|---|
| Heavy | 300 | 350 | Borewell casing, fire-system mains, long-life water supply |
| Medium | 270 | 350 | IS 1239 medium-class GI for general water lines |
| Light | 220 | 290 | Indoor low-corrosion lines, short-life applications |
Test methods — uniformity and mass
- Mass-of-coating test — the zinc layer is stripped chemically (hydrochloric acid solution) and the mass loss measured. Gives the coating-mass result in g/m².
- Copper sulfate (Preece) test — the pipe is dipped repeatedly in standardised CuSO₄ solution; the number of dips before copper deposits appear measures coating uniformity.
- Adhesion test — the coating must withstand normal handling and bending without flaking.
- Visual examination — freedom from blistering, peeling, bare spots, ash inclusions, or excessive lumpiness.
Why coating mass matters for borewell casing
Borewell casing pipe sits permanently submerged in groundwater that varies in pH and chloride content across India — slightly acidic in many parts of UP and Bihar, saline in coastal Gujarat and Tamil Nadu, hard and mineral-rich across the Deccan plateau. The zinc coating is the only barrier between the underlying steel and the corrosive aqueous environment, and the coating life is roughly proportional to coating mass: a 350 g/m² heavy IS 4736 coating gives 15 to 30 years of corrosion protection in typical groundwater, while a 220 g/m² light coating is exhausted in 8 to 12 years. For a 100-metre borewell where re-drilling and re-casing costs lakhs, paying the modest premium for heavy-coating pipe is almost always the right economic call.
Related standards and pages: IS 1239 — MS pipes for water & gas, IS 4270 — borewell casing pipes, IS 3589 — large-OD water pipes, all steel pipe standards, galvanized iron pipes catalogue, drill & borewell pipes, fire system pipe selection.
FAQ — IS 4736 galvanized coating
What does IS 4736 specify?
What are the coating mass classes — 300, 270, 220 g/m²?
How is the coating tested and verified?
IS 4736 vs IS 4270 — what is the difference?
Why does the coating class matter for borewell pipe?
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