MS and GI pipe can share the same steel-tube substrate; GI adds a specified zinc coating that changes corrosion protection, fabrication precautions and quoted cost. It does not create a stronger pressure class. A Kanpur buyer should compare the same tube standard and wall, then let the service environment, joining method, coating requirement and approved project specification decide between black and galvanised supply.
What MS and GI pipe actually are
MS pipe is welded mild-steel tube made by cold-forming hot-rolled coil into a cylinder and closing the seam by electric resistance welding (ERW). It ships black — as-welded, sometimes lightly oiled — with no corrosion protection beyond the mill scale. GI pipe is that same MS tube after a separate hot-dip galvanising step: the pipe is cleaned, fluxed and dipped in a bath of molten zinc at roughly 450°C, which reacts with the steel surface to form a metallurgically bonded zinc-iron alloy layer topped with pure zinc. Neither designation is a standard on its own — the governing document for the steel tube is IS 1239 Part 1, and for the zinc coating it is IS 4736; a pipe is only correctly called “GI to spec” when both are cited together on the PO.
How Kanpur PWD and JJM bidders choose MS or GI
Kanpur fabricators and water-project bidders often receive drawings that say only “MS” or “GI”. That is not enough to buy from. A painted welded frame, an exposed railing, a water line and an irrigation assembly can require different substrate standards, coating systems, joints and inspections. Preserve the BOQ and approved drawing, record the exposure and water chemistry, and ask the responsible engineer to resolve any missing coating or material requirement. Boiler, marine and aggressive chemical duties require their own code-led material review rather than an automatic jump to one product named on this page.
MS vs GI side-by-side — buyer table
| Attribute | MS pipe (black) | GI pipe (galvanised) |
|---|---|---|
| Base material | Mild-steel ERW substrate | Mild-steel ERW substrate + zinc coating |
| Manufacturing route | Hot-rolled coil → forming → ERW seam | Same as MS, then hot-dip galvanising bath |
| Zinc coating mass | None | 360 g/m² min (IS 4736:1986 as amended) |
| Outdoor durability | Depends on the specified paint/coating and exposure | Depends on zinc mass, detailing, damage and exposure |
| Pressure rating | Same as base wall class | Same as base wall class (zinc adds no strength) |
| Commercial comparison | Compare on a dated like-for-like quote | Include coating, finishing, inspection and repair scope |
| Welding | Use the approved material-specific WPS | WPS must also control zinc fumes and coating repair |
| Indian standard | IS 1239 Pt 1 (NB sizes); IS 3589 (water mains) | IS 1239 Pt 1 + IS 4736 (zinc coating spec) |
| End finish typical | Plain, bevelled or screwed | Threaded and coupled per IS 1239 |
| Primary applications | Frames, scaffolding, painted structures, fabrication | Plumbing, potable water, outdoor rails, agriculture |
When to choose MS pipe
Black MS may suit a fabricated assembly when the approved design uses welded joints and a separate paint, lining or external coating system. It avoids damaging a zinc layer during fabrication, but the base steel still needs the specified surface preparation and corrosion protection. The engineer must account for internal exposure, drainage, crevices, buried conditions and maintenance access. Compare the complete protective system and its inspection scope rather than assuming black pipe is cheaper over the life of every installation.
When to choose GI pipe
GI may be appropriate where the project specification calls for hot-dip zinc and the service chemistry, temperature and joining method are compatible with that coating. Potable-water acceptance depends on the applicable project and public-health requirements, not merely the letters GI. Outdoor exposure also varies with moisture, contaminants, cut edges, trapped water and contact with dissimilar metals. Threaded, coupled or other joints must be specified explicitly, and any damaged coating needs the approved repair system.
Welding GI — what changes
Arc welding heats the zinc well beyond the point at which hazardous zinc-containing fumes can form, and it damages the coating around the joint. Work must follow the approved welding procedure, safety data, ventilation or extraction plan and site controls. Zinc removal distance, weld preparation and coating repair are project-specific; the coating specification should identify the accepted repair material, dry-film requirements and inspection. For a fabricated assembly, the designer may instead specify galvanising after fabrication if geometry, venting, distortion risk and the governing coating procedure permit it.
Standards to put on the RFQ
For MS, specify the substrate standard, size, class, manufacturing route, finish and length. For GI, add the applicable coating standard and the coating requirement from the approved project specification. Do not treat the word “galvanised” as proof of a particular process or coating mass; require marking, inspection and documentary evidence appropriate to the PO. See our IS 1239 reference and ERW pipes product page for the related technical fields.
IS 1239 wall-thickness classes — same table for MS and GI
Class (Light / Medium / Heavy) sets wall thickness for a given NB and is identical whether the pipe ships black (MS) or galvanised (GI) — galvanising adds a coating, not steel. The table below gives IS 1239 Part 1 wall thickness in mm for the sizes most requested in Indian plumbing and water-supply work.
| NB (mm) | OD (mm) | Class A / Light (mm) | Class B / Medium (mm) | Class C / Heavy (mm) |
|---|---|---|---|---|
| 15 | 21.3 | 2 | 2.6 | 3.2 |
| 25 | 33.7 | 2.6 | 3.2 | 4 |
| 40 | 48.3 | 2.9 | 3.2 | 4 |
| 50 | 60.3 | 2.9 | 3.6 | 4.5 |
| 80 | 88.9 | 3.2 | 4 | 4.8 |
| 100 | 114.3 | 3.6 | 4.5 | 5.4 |
| 150 | 165.1 | 4.5 | 4.8 | 5.4 |
See the full IS 1239 class thickness chart for the published nominal dimensions. Class selection must follow the drawing, service calculation, tender and applicable code. Buried, gas, fire-water or potable-water duty cannot be assigned a class from nominal bore alone; corrosion allowance, pressure, jointing, loads and protective systems also matter.
RFQ checklist
- NB size and class: state both, e.g. “50 NB Medium” — class alone without NB is incomplete
- Coating: black (MS) or hot-dip galvanised to IS 4736 minimum 360 g/m² (GI) — name the standard, not just “galvanised”
- End finish: plain, bevelled for weld, or threaded & coupled (T&C) for GI plumbing runs
- Length: 6 m random unless the project needs cut-to-length
- Inspection document: state the type and traceability required by the PO. If IBR governs a steam pipe, Form III-A is the component-specific form; the approved design and competent authority control applicability
- Inspection: TPI only where the tender (PWD/JJM) specifically mandates third-party inspection before dispatch
- Packing and delivery basis: bundled/loose, ex-works Kanpur or freight-paid to site
Worked example — weight and cost check for a 50 NB run
Weight (kg/m) = (OD − wall) × wall × 0.02466 for the site's nominal carbon-steel geometry check. For 50 NB Medium class (OD 60.3 mm, wall 3.6 mm), the result is 5.03 kg/m, or 30.2 kg for a nominal 6 m length. Galvanising adds coating mass, but the added percentage is not universal: it changes with inside and outside surface area, actual zinc mass and tube geometry. Use the ordered standard's tolerance rules and the supplier's stated basis for commercial weight. At receipt, match dimensions, markings and traceability documents rather than treating one theoretical number as proof of coating compliance.
Common procurement mistakes
- Treating GI as a stronger or higher-pressure pipe than MS of the same class — the zinc adds no mechanical strength
- Ordering “galvanised pipe” without the IS 4736 minimum-mass callout, receiving a thin electroplated or spray-zinc coating instead of true hot-dip
- Welding a GI line without grinding back the zinc first, then wondering why the weld porosity is high and the fumes are heavy
- Mixing MS and GI fittings on the same run without matching the coating — an MS fitting on a GI line becomes the corrosion-initiation point
- Assuming JJM or PWD tenders accept either coating interchangeably — most water-supply tenders mandate GI or a specific lined pipe; check the BOQ before quoting MS
Key points
- MS and GI share the same IS 1239 steel substrate and class-based wall thickness
- GI adds hot-dip zinc; durability depends on the specified coating, detailing, damage and exposure
- Pressure rating depends on class (Light/Medium/Heavy), not on coating — GI is not a stronger pipe
- Welding GI requires a procedure that controls zinc fumes, preparation and coating repair
- Name the applicable substrate and coating requirements on the PO rather than relying on MS/GI shorthand
FAQ — MS vs GI pipe procurement
What is the difference between MS and GI pipe?
Is GI pipe stronger than MS pipe?
How much more expensive is GI than MS pipe?
Can GI pipe be welded?
Which Indian standard governs MS and GI pipe?
Related pages
- ERW Pipes & Tubes — product detail
- GI Pipes & Fittings — product detail
- ERW process hub
- IS 1239 standard reference
- IS 4736 galvanising standard reference
- IS 1239 class thickness chart
- GI pipe price list
- MS pipe price list
- MS ERW Pipe supplier in Kanpur
- GI pipes in Kanpur
- How steel pipe pricing works
- ERW vs Seamless comparison
- Request a quote
Sources
- BIS product manual for IS 1239 (Part 1), which cites a minimum zinc-coating mass of 360 g/m² under IS 4736 clause 5.1 for tubes up to 6 mm thick: bis.gov.in — official product manual PDF
- IS 1239 Part 1:2004, Steel Tubes for Water and Sewage — Specification, Part 1 (substrate and Light/Medium/Heavy class walls)
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