Galvanizing specifications arrive written to one of two standards, and the two express the same requirement in different units. That causes more confusion than it should.
IS 4759 specifies mass
| Product class | Minimum coating mass (g/m²) | Indicative thickness (µm) |
|---|---|---|
| Grey iron and malleable iron castings | 610 | ~86 |
| Fabricated steel 5 mm thick and above | 610 | ~86 |
| Fabricated steel 2 to 5 mm thick | 460 | ~65 |
| Fabricated steel 1.2 to 2 mm thick | 340 | ~48 |
| Threaded work 10 mm diameter and above | 300 | ~42 |
| Threaded work under 10 mm diameter | 270 | ~38 |
The thickness column is a derived conversion at 7.14 g/m² per micron. IS 4759 itself specifies only mass.
ISO 1461 specifies thickness, local and mean
| Article and thickness | Local thickness min (µm) | Local mass min (g/m²) | Mean thickness min (µm) | Mean mass min (g/m²) |
|---|---|---|---|---|
| Steel above 6 mm | 70 | 505 | 85 | 610 |
| Steel above 3 up to 6 mm | 55 | 395 | 70 | 505 |
| Steel 1.5 to 3 mm | 45 | 325 | 55 | 395 |
| Steel under 1.5 mm | 35 | 250 | 45 | 325 |
| Castings 6 mm and above | 70 | 505 | 80 | 575 |
| Castings under 6 mm | 60 | 430 | 70 | 505 |
Where they meet
For the heaviest class the two standards agree exactly. IS 4759's 610 g/m² for steel 5 mm and above is the same requirement as ISO 1461's 610 g/m² mean for steel above 6 mm. If your specification is written to ISO and your galvanizer works to IS, that equivalence is usually all you need to establish.
The structural differences are worth knowing. ISO 1461 distinguishes local from mean values — a single measurement may fall to the local minimum provided the average holds. IS 4759 makes no such distinction. ISO also bands by section thickness in four steps where IS uses three plus separate classes for threaded work.
One more: IS 4759 assumes normal or rural atmospheric exposure. Marine and industrial exposure requires an increased coating “by mutual agreement” — IS 4759 has no heavy-duty class of its own. If the structure is going up near the coast, say so in the enquiry.
The three acceptance tests
| Test | Standard | Requirement |
|---|---|---|
| Mass of zinc coating | IS 6745:1972 | Stripping / gravimetric method against IS 4759 Table 1 |
| Uniformity of coating | IS 2633:1986 | Four one-minute dips in copper sulphate without adherent copper deposit |
| Adhesion | IS 2629:1985 | Pivoted hammer and knife test — no flaking or peeling ahead of the tool |
| Freedom from defects | IS 4759 cl. 6.2 | Uniform, adherent, free from flux, ash, bare patches, blisters and runs |
There is a quiet advantage to the Indian standard here. ISO 1461 states that no suitable international standard exists for adhesion testing of hot dip galvanizing on fabricated articles, so adhesion is tested only by agreement between the parties. IS 2629 defines a pivoted hammer test and a knife test. The Indian route therefore gives you a more defined adhesion criterion than the international one, not less.
Before you send work to the kettle
The single most common reason job work is turned away is missing venting and drainage holes. A sealed hollow section in a 450 °C zinc bath is a pressure vessel with moisture inside it. IS 2629 makes venting mandatory; ASTM A385 gives the practical guidance on hole placement and size.
Also worth flagging at enquiry: the steel grade, because high tensile steel needs embrittlement precautions per IS 6158; any article that must not be quenched; and any previous paint or coating, which has to be removed before pickling.
