IS 4736: Hot-Dip Galvanized Coatings on Steel Tubes

IS 4736 defines the hot-dip zinc coating on a mild-steel tube: coating mass, uniformity, adhesion, freedom from defects and verification. It is a coating specification, not the underlying tube specification. A Kanpur buyer must therefore name the tube standard, dimensions, class or wall, finish and applicable IS 4736 coating requirement on the same enquiry. The amended requirement used by the BIS steel-tube product manual is a minimum zinc-coating mass of 360 g/m². The older 300, 270 and 220 g/m² class model should not be copied into a current purchase order.

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, 50 NB Medium, hot-dip zinc coated to IS 4736:1986 with applicable amendments, minimum 360 g/m²." For a water-well casing order, use the tube specification approved by the designer and add the same coating reference only where it is required. A purchase order that says only "galvanized" leaves the edition, coating evidence, surface condition and acceptance basis unclear.

How Kanpur buyers specify the amended IS 4736 coating

Start with the approved BOQ or drawing. Copy the tube standard, size, wall or class, end condition and zinc-coating requirement, without turning a trade nickname such as "heavy galvanizing" into a substitute value. Where the amended IS 4736 clause governs, state the 360 g/m² minimum and require the quotation to repeat the same basis. Fire, water-well and public-works buyers should keep any project-specific coating requirement exactly as written by the designer or tender authority.

Do not infer that an indoor line may use an obsolete lower coating value, and do not use IS 4736 to select the tube material. Boiler tubes, sour-service lines and chemical duties follow their governing material and design documents. If a tender names a different coating system, resolve the conflict in writing rather than treating hot-dip zinc as a universal substitute.

Current coating-mass buying basis

How the tube specification pairs with the coating requirement

The coating requirement and the tube specification answer different questions, so the order must name both. The examples below show how a Kanpur buyer can keep those fields separate. They are RFQ patterns, not universal design selections; the approved BOQ or drawing remains controlling.

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.

How to specify galvanized pipe on the RFQ

A galvanized RFQ needs two standards on one line and the commercial details. State the tube specification with its size and wall/class, then the amended IS 4736 coating requirement in g/m². Keep price, tax, inspection, packing and delivery as separately quoted fields so a commercial comparison does not alter the technical acceptance basis.

• Structural spec with edition: IS 1239 Part 1 edition 2004 or IS 4270 for casing.
• Wall designation and grade: copy the exact class/grade language from the governing tube specification and approved BOQ.
• Size: NB with OD in mm, for example 50 NB at 60.3 mm OD.
• Coating: IS 4736:1986 with applicable amendments and minimum 360 g/m² where the amended clause governs.
• Length with ends: 6 m standard, or 4 m and 5 m, in plain end or threaded and coupled.
• Certificate with inspection: MTC 3.1 showing measured coating mass, and TPI where the tender asks.
• Delivery basis: ex-warehouse Kanpur or delivered site, with packing, freight and legally applicable tax shown on the current quotation.

Why coating evidence matters for water-well casing

Water chemistry, coating damage, handling, threading, storage and installation all affect corrosion performance, so coating mass alone cannot promise a fixed service life. The useful procurement control is traceability: name the approved casing specification, state the coating requirement, define the contract test evidence, and reconcile the delivered marking and lot record before installation. A Kanpur buyer comparing a lower coating result with the amended 360 g/m² basis should hold the lot for written technical review rather than estimate years of life from a marketing table.

Worked example: coating mass on an 80-length order

Take 50 NB medium class at OD 60.3 mm with wall 3.6 mm from the site weight table. Theoretical mass is 5.03 kg per m and 30.2 kg per 6 m length, from W equals OD minus wall, times wall, times 0.02466. Eighty lengths give 80 times 30.2 kg, which is 2416 kg of MS base weight.

Do not add a universal percentage to that 2,416 kg substrate figure. Coated mass depends on actual surface geometry and the verified zinc result, while dispatch billing may use an agreed theoretical or actual-weight basis. Ask the quotation to state that basis, use the supplier's certified coated-lot or invoice weight for the commercial total, and apply GST at the legally applicable rate for the invoiced item and date. The MTC sample and weight chart help separate product evidence from the theoretical steel calculation.

Procurement mistakes and site inspection points

In practice most galvanized disputes trace to vague wording on the PO. The coating class was missing, the MTC showed no measured mass, or the threads were cut before dipping with no repair note. Each of these is cheap to prevent and costly to fix after laying.

• Ordering "galvanized" without the IS 4736 edition/amendment and coating-mass basis.
• Accepting an MTC that states coating within spec instead of measured g per sq m values.
• Skipping the copper sulfate uniformity check on restocked lots with mixed heat numbers.
• Receiving threaded ends with damaged zinc and no passivation or touch-up record.
• Storing GI bundles on bare soil where trapped water stains the zinc before installation.

Standards and compliance clauses

IS 4736:1986 with its applicable amendments sets the hot-dip zinc-coating rules for mild-steel tubes. The ordered tube standard separately controls product scope, dimensions, wall/class, grade and tests. Keep those documents separate on the RFQ and resolve any edition or scope conflict with the designer and supplier before the PO is released.

Where a QCO and BIS licence apply to the ordered tube, verify the CM/L and marking against the exact product standard. Define sampling, coating-mass evidence and any third-party test in the contract rather than assuming a generic certificate settles every dispute. The Quality, MTC and TPI hub shows the document chain, and the GI price list provides dated commercial bands that still require a current quotation.

Key points

• IS 4736 describes the zinc layer only; IS 1239 or IS 4270 describes the tube beneath it.
• Use the amended 360 g/m² minimum where the applicable IS 4736 clause governs the order.
• Every galvanized RFQ should state the tube specification, dimensions, coating standard, minimum mass, evidence, length and ends.
• Do not estimate coated invoice mass from a universal percentage; use the agreed and certified lot-weight basis.

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, drill and borewell pipes in Kanpur, fire system pipe selection.

FAQ: IS 4736 galvanized coating

References

• BIS steel-tube product manual (IS 4736 clause 5.1 amended 360 g/m² basis): Bureau of Indian Standards PDF.
• IS 4736:1986 text with amendments, used as a supporting copy for clause and amendment context: standards text mirror.