When you buy S220GD galvanized steel coil, the steel grade is only half of the specification - the other half is the zinc coating weight. Write "Z275" or "Z180" and you change the corrosion life, the price and even the way the material behaves in roll forming. Yet coating designations are one of the most misunderstood parts of an order. This guide explains what Z180, Z275 and Z350 really mean in grams and microns, and how to pick the right class for your project.
What Does "Coating Weight" Actually Mean?
In the European system (EN 10346), the coating class is expressed as the total mass of zinc on both sides of the strip, in grams per square meter (g/m²). A "Z275" coating therefore means a nominal 275 g of zinc over every square meter of strip - about 137 g on each side.
- Triple-spot test - the value written in the designation is the average of three specimens, which is the acceptance basis of the class.
- Single-spot test - the minimum allowed on any one specimen, roughly 10–15% lower than the triple-spot figure.
- Coating thickness - zinc density is about 7.1 g per cubic centimeter, so 1 µm of zinc on one side weighs approximately 7.1 g/m². A quick rule: total g/m² ÷ 2 ÷ 7.1 ≈ microns per side. Z275 ≈ 19 – 20 µm per side.
EN 10346 Zinc Coating Classes At A Glance
| Coating Class | Total Mass, Both Sides (g/m²) | Approx. Per Side (µm) | Typical Application |
|---|---|---|---|
| Z100 | 100 | ≈ 7 | Light indoor parts, back panels, cable trays |
| Z140 | 140 | ≈ 10 | General indoor sections, ducting, housings |
| Z180 | 180 | ≈ 13 | Roll-formed sections for mild indoor / sheltered use |
| Z200 | 200 | ≈ 14 | General structural framing, semi-exposed components |
| Z225 | 225 | ≈ 16 | Light outdoor cladding, agricultural equipment |
| Z275 | 275 | ≈ 20 | Standard for outdoor buildings - purlins, roofing, cladding, solar brackets |
| Z350 | 350 | ≈ 25 | Coastal, industrial and agricultural environments |
| Z450 | 450 | ≈ 32 | Severe corrosion exposure, long design life |
| Z600 | 600 | ≈ 42 | Extreme environments, buried or water-contact structures |
Values per EN 10346 triple-spot test. Micron figures are guidance values; density 7.1 g/cm³. Single-spot minimums: Z275≥235, Z350≥300, Z450≥385 g/m² (both sides).
Z180 vs Z275 vs Z350 - The Practical Comparison
Here is how they compare side by side:
| Aspect | Z180 | Z275 | Z350 |
|---|---|---|---|
| Total zinc mass (g/m²) | 180 | 275 | 350 |
| Thickness per side (µm) | ≈ 13 | ≈ 20 | ≈ 25 |
| Relative corrosion life* | Low – medium | Medium – high | High |
| Relative cost | Lowest | Market standard | Higher (more zinc) |
| Best suited to | Sheltered / indoor sections, cost-sensitive parts, components that are painted | Outdoor metal buildings: purlins, roof & wall cladding, light steel frames | Coastal areas, industrial air, livestock and agricultural buildings |
| Forming behavior | Thinner coating - fewer concerns about zinc flaking at bends | Good; keep bend radii reasonable | Heavier coating - allow slightly larger bend radii to avoid edge flaking |
*Service life depends on actual environment corrosivity (ISO 9223 category), design life and whether cut edges are exposed; the coating class alone does not guarantee a number of years.
How To Choose: A Five-Point Checklist
- Environment. Where will the product live? Dry indoor use rarely needs more than Z180–Z200. General outdoor use starts at Z275. Within a few kilometers of the sea, in industrial air, or in humid agricultural sheds, step up to Z350.
- Design life. A temporary structure or a painted component can accept a lighter coating. A 25+ year building specification will normally call for Z275 as a minimum.
- Cut edges. Roll-formed sections are sheared, punched and drilled - exposed edges rely on sacrificial zinc protection. A heavier coating protects cut edges longer.
- Forming. Very heavy coatings (Z450+) can flake on tight radii. For S220GD bent into C/Z profiles, Z275 and Z350 are well proven; simply give the tooling sensible corner radii.
- Budget. Upgrading from Z275 to Z350 adds a noticeable but small premium relative to the extra decades of edge protection in aggressive environments. Never downgrade below what the environment demands to save a few dollars per ton.
How To Write It In Your Specification
A clear coating requirement prevents disputes at delivery. Follow this format:
Steel: S220GD+Z275 to EN 10346 - thickness 2.0 mm, width 1000 mm, minimized spangle, chromated, lightly oiled. Zinc coating Z275 (triple-spot ≥ 275 g/m² total both sides).
If you need cut-edge protection beyond what pure zinc offers - typical for solar mounting and coastal projects - ask for the +ZM (zinc-aluminium-magnesium) coating instead. GNEE can supply S220GD+ZM on the same base steel for those applications.
FAQ
1. What does Z275 mean on S220GD?
It means 275 g/m² of zinc on the total surface of both sides (triple-spot test, EN 10346), approximately 20 µm per side. Z275 is the most common outdoor structural coating in the world.
2. Is Z180 enough for outdoor use?
For sheltered or semi-exposed parts, sometimes yes. For roof and wall members of a permanent outdoor building, Z275 is the safer minimum - Z180 (about 13 µm per side) corrodes noticeably faster once the first pinholes or edge damage appear.
3. How many years does Z275 last?
There is no single answer - it depends on the environment corrosivity, exposure and maintenance. In a normal rural or urban atmosphere, Z275 cladding typically serves for decades before the first red rust appears; in a marine or industrial atmosphere the same coating degrades much faster, which is why Z350 (or +ZM) is recommended there.
4. Is a heavier coating harder to form?
Slightly. Zinc is softer than steel and can smear or flake on very tight radii. In practice Z275 and Z350 roll-form cleanly into C/Z purlin and cladding profiles when corner radii are designed sensibly; problems are rare.
5. Do you test the coating weight before shipment?
Yes. Every coil is produced to the ordered coating class, and the mill test certificate states the triple-spot and single-spot results. SGS, BV or TUV inspection, including independent coating-weight testing, can be arranged before loading. Contact our team with your environment and design life and we will help you fix the right coating class for your S220GD order.