By Alina, Project Engineer, Ziyang Bozong Greenhouse Industry Co., Ltd.
"Hot-dip galvanized frame" looks like a specification. Really it is a claim, and it can describe processes that behave nothing alike after four years in a working greenhouse.
Here is the difference in one sentence. Hot-dip galvanizing puts the finished part into molten zinc at around 450 °C, so the zinc bonds to every surface it reaches, inside the tube and out. Pre-galvanizing, also called mill-galvanizing, coats the steel coil before anyone cuts, punches or welds it.
That word "before" is the whole problem.
A greenhouse frame is mostly thin-wall tube, between 1.5 mm and 3 mm. It gets cut to length, drilled for bolts and joined.
With pre-galvanized steel, each of those operations exposes bare metal. The outside of the tube keeps its coating. The cut face, the drilled hole and the weld do not.
Condensation forms on a greenhouse frame every night and runs downward. It collects at exactly those points. Rust begins inside the joint, where you cannot see it and cannot paint it.
With hot-dip galvanizing this cannot happen, because all the shaping is finished before the zinc goes on. We specify our frames this way for that single reason. The inside wall of a hollow section matters as much as the outside, and only a dipping process reaches it.
ISO 1461:2022 sets minimum average coating thickness by steel thickness:
| Steel thickness | Minimum average coating |
|---|---|
| Under 1.5 mm | 45 μm |
| 1.5 to 3 mm | 55 μm |
| 3 to 6 mm | 70 μm |
| 6 mm and over | 85 μm |
Greenhouse tube sits in the 1.5 to 3 mm band, so 55 μm is the figure that applies to most frames you will be quoted. Pre-galvanized coil typically runs 12 to 30 μm, and the cut ends carry effectively nothing.
A lot of suppliers quote coating weight in grams per square metre rather than microns. You will see figures such as 275 g/m² on a datasheet, and that figure on its own is not enough to compare anything.
The conversion is fixed. One micron of zinc is about 7.1 g/m² on a single side.
So 275 g/m² means roughly 39 μm if it is a single-side measurement, or roughly 19 μm per side if it is a double-sided total. Chinese tube standards such as GB/T 3091 usually express coating weight as a double-sided total. International standards usually express it per side.
Same number, two different frames, and the gap between them is a factor of two. Ask for the coating in microns and ask whether the figure covers one side or both. That single question removes most of the ambiguity.
None of this needs a laboratory.
Ask for the coating specification in microns. Then ask whether the number is single-side or double-sided. A supplier who cannot answer clearly has told you something useful.
Look at a cut end. On a dipped frame the cut face is coated. It may feel slightly rougher than the tube surface, but it is grey zinc, not bare steel. On a pre-galvanized frame it is raw metal, and on a sample that has been outside for a season it is already rusting.
Check the welds. A weld bead is the highest-risk point on any frame, because welding burns off whatever coating was there before.
Ask for a magnetic coating thickness gauge reading on three random tubes. The gauge is inexpensive. On a container order this is a fair request, and a supplier with nothing to hide will not object to it.
This part depends on where the greenhouse stands, and anyone who gives you a single fixed number for every country is simplifying too much.
Zinc is consumed by the atmosphere around it. In dry inland air, corrosion runs slowly. On a coast, airborne salt can push the rate several times higher. Inside a greenhouse the conditions are worse than open air in one specific way: humidity stays above 80% for much of the year, the frame is wet every night, and fertilizer salts and agricultural chemicals are present in the air.
We design our frames for a 15 to 20 year service life in normal agricultural conditions.
That figure sits alongside a 10-year structural warranty and a 5-year warranty against corrosion perforation. The numbers are deliberately different, because they answer different questions.
Design life is a technical estimate. Structural warranty covers the frame's safety. The corrosion warranty covers perforation, meaning the zinc is gone and the steel is going. Mixing them into one claim is how quotations end up contradicting themselves.
Hot-dip galvanizing adds cost to the frame. On a container load that difference is real money, and there is no honest way to pretend it is not.
The comparison that matters is not the two purchase prices. It is what you are holding in year eight.
A pre-galvanized frame that starts rusting at the joints leaves you repainting, replacing sections, or in the worst case replacing the structure. On an installed greenhouse none of those options are cheap, and none of them are possible to undo properly. You cannot re-dip a frame that is already standing.
We have been building greenhouses in Ziyang, Sichuan for more than 35 years. The factory covers over 180,000 m² and employs 378 people, including 18 design engineers and 35 installation engineers. Every main part of our frames is hot-dip galvanized after processing, which means the cut ends and the tube interiors are coated, not just the surfaces you can see.
We publish our coating specification in microns, and we state whether it is single-sided or double-sided. That is the only way the number is worth anything to you.
If a supplier quotes you a coating in g/m² and will not say which convention it uses, you already have your answer about how they work.
Alina, Project Engineer Ziyang Bozong Greenhouse Industry Co., Ltd. — Sichuan, China
