Zinc coating weight is the biggest quality-vs-price decision when importing GI coil. Here is how Z120, Z180 and Z275 actually differ in service life and cost.
Coating Comparison
| Coating | Zinc (g/m2, both sides) | Typical Service Environment | Relative Price |
|---|---|---|---|
| Z120 | 120 | Indoor framing, ducting, dry climates | base |
| Z180 | 180 | General roofing, mild coastal | +2-3% |
| Z275 | 275 | Structural, humid or coastal, unpainted exposure | +5-7% |
Rule of Thumb
Zinc corrodes at a roughly constant rate for a given environment, so coating life scales with weight: Z275 lasts about 2.3x longer than Z120 in the same exposure. For unpainted roofing in coastal areas, Z275 is usually the correct spec — the 5-7% premium buys years of extra life. For PPGI substrate (paint on top), Z80-Z120 is standard because the paint carries most of the protection.
What Importers Get Wrong
- Buying Z275 for PPGI substrate — usually wasted money; the paint system matters more.
- Buying Z60-Z80 for unpainted corrugated roofing — sheets show white rust within a couple of years in humid markets.
- Not checking the MTC — always verify the coating test result, not just the order spec.
FAQ
Is Z275 required by standard anywhere?
Many structural specifications (e.g. for purlins and framing) call for Z275 under EN 10346 or ASTM A653 G90, which is the closest US equivalent.
Can one container mix coatings?
Related Products from ChinaCNU Steel
Yes — we regularly load mixed Z120/Z180/Z275 coils in one 20GP or 40GP container, MOQ 25 tons total.