Buying Galvanized Steel Coil from China: A Coating-Weight Guide

Galvanized steel coil — GI coil — is among the most widely imported Chinese steel products and, not coincidentally, among the most defrauded. The reason is simple and physical: the zinc coating that determines corrosion resistance and price is invisibly thin, and a buyer cannot tell Z275 from Z120 by looking. That gap between what the eye can see and what the invoice charges for is precisely where coating-weight fraud lives.

What coating weight means and why it sets the price

Coating weight is the mass of zinc per square metre of surface, counting both sides, expressed in classes like Z275 (275 g/m²), Z180, or Z120. More zinc means longer corrosion life and higher cost. When a supplier quotes a price that undercuts the market for a given coating class, the most common explanation is not efficiency — it is that the coil will arrive under-coated, with a certificate that says otherwise.

How coating-weight fraud is committed

• Shipping a lower coating class than ordered — Z120 invoiced as Z275 — because the difference is undetectable without testing.
• Supplying a mill test certificate with the ordered figure while the physical coil does not match it.
• Uneven coating that meets spec at the test point an inspector is shown but falls short across the coil.

All three exploit the same blind spot. The defense is to make coating weight a measured, contractual quantity rather than a number you take on trust. See the documented coating-weight fraud case in the scam library for how this plays out end to end.

How to specify it so there is no ambiguity

Write the coating class, the test standard, and the sampling method into the purchase order. State the coating weight in g/m² both sides, name the applicable standard, and require a mill test certificate tied to the heat number. Ambiguity in the specification is the supplier's friend; precision is yours.

If coating weight is not a measured, contractual number with a rejection clause attached, you are not buying Z275 — you are buying a promise of Z275.

How to verify it on arrival

Coating weight can be checked independently, and for a first order it should be. Pre-shipment inspection can include coating-weight testing by magnetic gauge or laboratory stripping test, sampled across the coil rather than at a single convenient point. Pair that with positive material identification on the base metal and verification of the heat number against the mill. For the full arrival routine, follow the inspection guide on this site, and use the supplier verification checklist before you order so coating-weight testing is built into the deal from the start.