- ISO 1461 minimum mean zinc coating: 85 µm for steel over 6 mm, 70 µm for over 3 to 6 mm, 55 µm for 1.5 to 3 mm.
- ISO 9223 classes atmospheres from C1 (very low corrosivity) to CX (extreme).
- Zinc loss per year in ISO 9223: about 0.7 to 2.1 µm in C3, 2.1 to 4.2 µm in C4, 4.2 to 8.4 µm in C5.
- Coating thickness is measured with a magnetic gauge (ISO 2178); uniformity can be checked with the IS 2633 test.
- Powder coat over galvanizing on decorative poles is commonly 60 to 80 µm.
Why the coating decides the pole's life
Steel does not fail on a street light pole because it gets tired. It fails because it rusts, usually at the base where water, soil, dog urine and road splash all meet. The coating is the only thing standing between the steel and that environment. That is why two poles of the same height and thickness can last eight years or thirty.
Hot-dip galvanizing in plain words
The finished pole is cleaned in acid, dipped in flux, and lowered into molten zinc at about 450°C. The zinc reacts with the steel and forms layers of zinc-iron alloy, topped with pure zinc. Because it is a bath, the zinc reaches inside the pole as well as outside, and into corners a paint brush would miss.
The zinc protects the steel in two ways. It is a barrier, and if the coating gets scratched, the zinc around the scratch corrodes first and protects the exposed steel. Paint cannot do the second part.
How thick should galvanizing be?
| Steel thickness | Minimum local coating | Minimum mean coating |
|---|---|---|
| Over 6 mm | 70 µm (505 g/m²) | 85 µm (610 g/m²) |
| Over 3 mm up to 6 mm | 55 µm (395 g/m²) | 70 µm (505 g/m²) |
| 1.5 mm up to 3 mm | 45 µm (325 g/m²) | 55 µm (395 g/m²) |
| Under 1.5 mm | 35 µm (250 g/m²) | 45 µm (325 g/m²) |
Most decorative pole shafts are 3 to 6 mm thick, so a realistic minimum mean is around 70 µm. Base plates, usually thicker than 6 mm, should reach 85 µm. If a supplier promises ‘85 microns everywhere’ on a 3 mm shaft, ask how they measured it.
How long will it last where you are?
The ISO 9223 standard classifies air by how corrosive it is. Zinc wears away slowly and fairly predictably, so you can make a rough life estimate.
| ISO 9223 class | Typical Indian setting | Zinc loss per year | Rough life of 70 µm zinc |
|---|---|---|---|
| C2 (low) | Dry inland towns, rural areas | 0.1 to 0.7 µm | Very long; many decades |
| C3 (medium) | Most inland cities: Nashik, Pune, Nagpur, Aurangabad | 0.7 to 2.1 µm | About 30 to 100 years |
| C4 (high) | Industrial areas, coastal towns a few km inland | 2.1 to 4.2 µm | About 17 to 33 years |
| C5 (very high) | Sea-front roads, Mumbai marine drive-type locations | 4.2 to 8.4 µm | About 8 to 17 years |
| CX (extreme) | Splash zones, jetties | 8.4 to 25 µm | Only a few years |
These are rough guides. Base zones, where soil and water sit against the pole, usually corrode faster than the shaft. That is why a good design adds protection at the base: a collar, a bitumen band below ground, or a raised concrete plinth.
Powder coating on top: the duplex system
Powder coat over galvanizing is called a duplex system. The powder coat shields the zinc from the weather, and the zinc protects the steel if the powder coat is chipped. Together they typically last noticeably longer than the two coatings would add up to on their own, which is why duplex is the standard choice for decorative poles.
Two things make or break it:
- Surface preparation. Fresh zinc is smooth and powder does not stick well to it. The galvanized surface should be lightly sweep-blasted or chemically treated before coating. Ask your supplier how they prepare it.
- Outgassing. Galvanized steel can release trapped gas during curing, which leaves pinholes. Good coaters use degassing steps and suitable powders.
Polyester powders are the usual choice outdoors because they hold colour under Indian sun. Epoxy powders chalk and fade in UV, so keep them for primers or indoor use.
Tests to ask for
| Test | What it checks | When |
|---|---|---|
| Magnetic thickness gauge (ISO 2178) | Zinc and powder thickness at several points | Every batch; you can do spot checks on delivery |
| Mass of coating (stripping test) | Zinc weight per m² on samples | Type test or batch sample |
| Uniformity test (IS 2633) | Even spread of zinc | Batch sample |
| Cross-cut adhesion | How well powder sticks | Batch sample |
| Salt spray (ISO 9227) | Resistance of the coating system | Type test, useful for coastal projects |
A pocket coating thickness gauge costs less than one pole. For large orders, it is worth having one on site and checking a few poles from each truck.
Common shortcuts to watch for
- ‘Galvanized’ meaning cold galvanizing paint, not hot-dip.
- Galvanizing outside only (electro-galvanized sheet or zinc spray), leaving the inside bare.
- Powder coat applied straight on fresh zinc without preparation, peeling in a year.
- Thickness quoted from a certificate, not measured on your batch.
- Welding done after galvanizing, with burnt zinc at the joint only touched up with silver paint.
References
- IS 4759 — Hot-Dip Zinc Coatings on Structural Steel and Other Allied Products — Bureau of Indian Standards
- ISO 1461 — Hot dip galvanized coatings on fabricated iron and steel articles — International Organization for Standardization
- ISO 9223 — Corrosion of metals and alloys: Corrosivity of atmospheres, classification — International Organization for Standardization
- ISO 2178 — Non-magnetic coatings on magnetic substrates: Measurement of coating thickness (magnetic method) — International Organization for Standardization
- IS 2633 — Methods for testing uniformity of coating of zinc coated articles — Bureau of Indian Standards
Frequently Asked Questions
What galvanizing thickness is required for street light poles?
ISO 1461 sets a minimum mean of 85 µm for steel over 6 mm thick, 70 µm for over 3 to 6 mm, and 55 µm for 1.5 to 3 mm. IS 4759 has similar requirements; check the edition named in your tender.
How long does galvanizing last?
It depends on the air. In typical inland Indian cities (ISO 9223 C3), zinc loses about 0.7 to 2.1 µm a year, so a 70 µm coat can last decades. On a sea-front road (C5), the same coat may need attention in about 8 to 17 years.
Why is my powder coat peeling off a galvanized pole?
Usually poor surface preparation. Fresh zinc must be lightly blasted or chemically treated before powder coating. Outgassing during curing can also cause pinholes and weak spots.
How can I check galvanizing on delivery?
Use a magnetic coating thickness gauge (ISO 2178) at several points on a few poles from each lot, and compare with the supplier's batch report.
कोल्ड गैल्वनाइज़िंग और हॉट-डिप में क्या फर्क है?
कोल्ड गैल्वनाइज़िंग ज़िंक वाला पेंट है, जो पतला होता है। हॉट-डिप में पूरा पोल पिघले ज़िंक में डुबोया जाता है, जिससे अंदर-बाहर मोटी और मज़बूत परत बनती है।
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