"Galvanized pole" is a vague phrase. Hot-dip galvanizing and cold (electro) galvanizing differ sharply in price — and in lifespan by a factor of five. A lot of cheap street lights are cheap exactly because of this one process. Here is the difference, and how to inspect it on delivery.
The difference at a glance
| Item | Hot-dip galvanizing | Cold / electro galvanizing |
|---|---|---|
| Process | Pickled, then the whole pole is dipped into ~450 °C molten zinc | Electrochemical deposition at room temperature |
| Bonding | Zinc-iron alloy layer — metallurgical bond | Physical adhesion only, weak bond |
| Coating thickness | 65–100 µm, can be thickened on request | Typically only 5–20 µm |
| Outdoor life | 15+ years without rust-through | Rust appears in 2–3 years |
| Cost | Higher; zinc consumption per tonne is significant | About one third of hot-dip cost |
Why cold galvanizing fails outdoors
The cold-galvanized layer is thin and only physically bonded. Scratches and welding heat break its continuity, and rust grows inward from every breach. Poles stand in rain and wide temperature swings all year, so cold-galvanized poles typically start weeping rust around the flange and access door within three to five years, and need replacement in five to eight. The money saved on galvanizing is spent again — doubled — on maintenance and re-purchase.
Acceptance: three ways to catch a thin coating
1. Measure thickness. With a coating gauge, test several points at the top, middle and bottom of the pole and on the flange. Hot-dip galvanizing should average ≥ 80 µm (minimum point ≥ 65 µm); anything below 40 µm is almost certainly cold galvanizing or a spec-cut coating.
2. Look at the surface. Hot-dip galvanizing shows an even zinc spangle pattern in a matte grey; the inner wall is also coated, because the whole pole is dipped. Cold-galvanized poles are bright and smooth outside with essentially no coating inside — the fastest one-glance test.
3. Check cut edges. Around cuts and weld breaches, hot-dip zinc "self-heals" — the surrounding zinc sacrifices itself to protect the steel. A cold-galvanized breach exposes bare steel. On delivery, open one pole and inspect the back of the flange and the inside of the door frame; nothing tells the truth faster.
Procurement note: for street light, CCTV and landscape poles, write "hot-dip galvanized, average coating ≥ 80 µm" into the contract. For the full production sequence, see how a light pole is made.