Zn-Al-Mg (ZM) coating is a zinc alloy with added aluminium and magnesium, applied to steel strip on a continuous hot-dip line. The European standard (EN 10346) designates it “ZM”. ArcelorMittal's Magnelis® is the best-known example; Zn-Al-Mg coatings sold by other steelmakers under their own trade names belong to the same family.
It has quickly become the standard for solar piles, purlins and rails. This guide explains how ZM differs from galvanizing, how coating classes are read and how to choose one for a solar project.
How Zn-Al-Mg works
Conventional galvanizing is almost pure zinc. Zn-Al-Mg adds a few percent of aluminium and magnesium (Magnelis, for example, has about 3.5% Al and 3% Mg). The magnesium makes the corrosion products that form on the surface far denser and more stable, markedly slowing the rate at which the coating is consumed.
The second difference is at cut edges. On a profile formed from pre-coated coil, sheared and punched edges expose bare steel. With Zn-Al-Mg, corrosion products containing zinc and magnesium spread over those edges and seal them — the so-called self-healing effect. On heavily punched piles and rails this directly affects service life.
Compared with galvanizing
Three coating routes are used for solar steel: continuous galvanizing on coil (Z), batch hot-dip galvanizing of finished parts (EN ISO 1461) and Zn-Al-Mg on coil (ZM).
| Property | Galvanized (Z) — continuous | Batch galvanized — EN ISO 1461 | Zn-Al-Mg (ZM) — continuous |
|---|---|---|---|
| Where it is applied | To coil, before forming | To parts, after fabrication | To coil, before forming |
| Standard | EN 10346 | EN ISO 1461 | EN 10346 |
| Coating | Z275–Z600 (Z275 ≈ 20 µm/side) | Mean min. 45–85 µm by steel thickness | ZM120–ZM430 (ZM310 ≈ 25 µm/side) |
| Cut and punched edges | Exposed, limited protection | Coated, as galvanizing follows fabrication | Self-healing through corrosion products |
| Life at equal coating mass | Baseline | — | Markedly longer than galvanized (producer data) |
| Distortion of thin profiles | None | Possible (≈450 °C bath) | None |
| Typical solar use | Purlins and rails, mild environments | W-beams, welded parts | Piles, purlins, rails; C3–C5, agricultural, coastal |
Reading coating classes
An EN 10346 class name gives the total coating mass on both sides in g/m²: ZM310 means 310 g/m² of Zn-Al-Mg in total, with a typical thickness of about 25 µm per side. Because Zn-Al-Mg is less dense than pure zinc, the same g/m² corresponds to a slightly thicker layer.
| Class | Total (g/m², both sides) | Typical thickness (µm per side) |
|---|---|---|
| ZM120 | 120 | 10 |
| ZM250 | 250 | 20 |
| ZM310 | 310 | 25 |
| ZM430 | 430 | 35 |
| Z275 | 275 | 20 |
| Z450 | 450 | 32 |
| Z600 | 600 | 42 |
Corrosion categories and a simple life estimate
ISO 9223 classifies atmospheric corrosivity from C1 to CX and gives zinc's first-year corrosion rate for each category. Solar sites are mostly C2–C3; sites near the coast, industry or livestock farming can reach C4–C5.
A simple linear estimate gives a feel: a profile with 20 µm per side (Z275) is consumed in roughly 10–30 years in C3 (0.7–2.1 µm/year) and 5–10 years in C4 (2.1–4.2 µm/year). For a plant targeting 25–30 years that means either a heavier coating or a slower-consuming one such as Zn-Al-Mg. The rates are first-year values; long-term rates usually fall (ISO 9224), and real life varies with rainfall, pollutants and orientation — the estimate is only a first idea.
Below ground it is soil corrosivity, not the atmospheric category, that governs: resistivity, pH, moisture, chloride and sulphate content. These are measured in the geotechnical survey; in corrosive soils a heavier ZM class (e.g. ZM310–ZM430) and, where needed, a corrosion allowance are assessed together.
| Category | Environment (example) | Zinc (µm/year) |
|---|---|---|
| C2 — low | Rural, dry inland | 0.1 – 0.7 |
| C3 — medium | Urban, light industry, low-salinity coast | 0.7 – 2.1 |
| C4 — high | Industrial areas, coastal | 2.1 – 4.2 |
| C5 — very high | Humid industry, high-salinity coast | 4.2 – 8.4 |
Why Zn-Al-Mg became the solar standard
Punched profiles: piles and rails carry hundreds of holes and slots, and Zn-Al-Mg protects those edges by itself.
No distortion of thin sections: the roughly 450 °C bath of batch galvanizing can distort long, thin-walled profiles; forming from pre-coated coil avoids that risk.
Same or longer life at lower coating mass: which lowers cost per ton and zinc consumption.
Agricultural and ammonia environments: producer data show Zn-Al-Mg outlasting galvanizing in ammonia-laden air — an advantage near livestock farming and on agrivoltaic sites.
On site
Avoid cutting and drilling on site; where it cannot be avoided, seal the cut surface with zinc-rich paint. Store profiles ventilated and slightly inclined: in closed, damp stacks white rust (white corrosion product) can form on the coating. Protect pile heads with a suitable driving cap and repair any coating damaged during driving.
We roll our C, U and sigma piles, purlins and rails from ZM120–ZM430 or Z275–Z600 coated coil and supply W-beams hot-dip galvanized. To choose the right class for your project, contact us from our solar steel profiles page.