Galvanized Steel Sheet Coatings: Z120 vs Z180 vs Z275 Explained
What zinc coating designations like Z120, Z180 and Z275 mean, how coating mass affects corrosion life, and which to choose for Saudi coastal, industrial and inland conditions.
Saudi Steel Work Editorial Team · · 3 min read
Short answer: The number in Z120, Z180 or Z275 is the minimum total zinc coating mass in grams per square metre on both sides combined. Z275 carries more than twice the zinc of Z120 and so protects for much longer. Heavier coatings are recommended for coastal and industrial sites in Saudi Arabia.
Galvanized (GI) sheet is coated with zinc on a continuous line. The coating designation tells you how much zinc you are buying — and therefore how long the steel will resist corrosion.
Reading the designation
Under EN 10346, Z stands for pure zinc coating and the number is the minimum total coating mass on both sides in g/m² (triple-spot test).
| Designation | Total zinc (g/m²) | Approx. thickness per side |
|---|---|---|
| Z100 | 100 | ~7 µm |
| Z120 | 120 | ~8.5 µm |
| Z180 | 180 | ~12.5 µm |
| Z275 | 275 | ~19.5 µm |
| Z350 | 350 | ~25 µm |
American equivalents under ASTM A653 use G-numbers: G60 ≈ Z180, G90 ≈ Z275.
Why coating mass matters
Zinc protects steel by acting as a barrier and by corroding sacrificially. Zinc corrodes slowly at a rate depending on the environment, so life is roughly proportional to coating thickness. Doubling the zinc roughly doubles the time to first rust in the same environment.
Saudi environments
Corrosivity varies widely across the Kingdom:
- Coastal (Jeddah, Dammam, Jubail, Yanbu, Jazan): high humidity and chlorides — heavier coatings or pre-painted products recommended
- Industrial: pollutants accelerate zinc loss
- Inland desert (Riyadh, Qassim): generally lower corrosivity, but dust and occasional humidity matter
ISO 9223 corrosivity categories help formalise this — see corrosion protection in Saudi Arabia.
Choosing a coating
| Application | Typical choice |
|---|---|
| Indoor ducting, panels | Z120–Z180 |
| Roofing and cladding substrate | Z180–Z275, or Galvalume AZ150 |
| Exposed coastal | Z275+ or pre-painted |
| Structural decking | Z275 |
Sheet galvanizing vs hot-dip after fabrication
Continuous-line sheet coatings are thinner than hot-dip galvanizing applied to fabricated steel (often 85 µm or more). For structural steelwork outdoors, see hot-dip galvanizing explained.
Buying tip
Ask for the coating mass on the mill certificate. "GI sheet" without a coating class is incomplete.
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