Short answer: for ground-mount posts and heavy members, use steel hot-dip galvanized after fabrication. For roll-formed rails and purlins, zinc-aluminium-magnesium (ZAM) coated strip is a strong choice: an independent review found ZM310 at least as durable as much heavier zinc coatings. Use aluminium where weight matters most, mainly on rooftops, and isolate it from steel. Specify fasteners for the corrosivity category.
Most PV structures combine materials. The useful question is not which material is best, but which one each component needs at your site’s corrosivity.
Key numbers
- An independent Steel Construction Institute review (2024) found ZAM coating ZM310 at least equivalent to Z600 zinc in typical conditions, for surfaces and cut edges, and its design life greater than a 55 µm batch-galvanized coating; ZM430 outlasts 70 µm.
- Field data in that review put zinc’s thickness loss at about 1.5× that of ZAM at inland sites and 2.4× at a coastal site. Salt-spray claims of 5–10× overstate real outdoor performance.
- Aluminium alloy 6005A-T5 has a density of 2.70 g/cm³, about a third of steel’s 7.85, but a lower yield strength (about 260 vs 355 MPa for Q355 steel) and twice the thermal expansion.
1. What each material is
| Hot-dip galvanized after fabrication | ZAM (Zn-Al-Mg) coated strip | Aluminium (6005-T5 / 6063) | |
|---|---|---|---|
| Process | Finished parts dipped in molten zinc (ISO 1461 / ASTM A123) | Steel strip continuously coated, then formed (EN 10346, “ZM”) | Extruded profiles, often anodised |
| Typical coating | 55–85 µm minimum, by steel thickness | ZM275–ZM430: about 20–35 µm per side | Natural oxide or anodised layer |
| Cut edges and holes | Coated, if cut before galvanizing | Self-healing within limits | Not an issue |
| Weight | Steel | Steel | About one third |
| Best for | Posts, piles, heavy beams, welded parts | Rails, purlins, braces | Rooftop rails, clamps, coastal rooftops |
2. Hot-dip galvanized steel: the default for heavy members
Galvanizing after fabrication covers every face, edge and hole, and coating thickness rises with steel thickness: ISO 1461 requires at least 85 µm mean on steel over 6 mm. That makes it the natural choice for posts, piles and welded parts. Its weak point is thin members: rails of 2–3 mm need only 55 µm, which can set the structure’s life in C4 and C5 environments. Our galvanizing guide has the life table.
3. ZAM: thinner coating, longer life on thin members
ZAM coatings add aluminium and magnesium to zinc. EN 10346 calls them ZM; a common composition is about 93.5 % zinc, 3.5 % aluminium and 3 % magnesium. The coating is thinner than batch galvanizing, but it corrodes more slowly and, where it is cut, zinc and magnesium form a protective layer over the exposed edge.
- Choose the coating mass by steel thickness. Manufacturer guidance: ZM250 or more on steel up to 3 mm, ZM310 or more above 3 mm. Edge self-healing has limits (coating over about 20 µm, steel not too thick relative to the coating).
- Holes punched after coating rely on that self-healing; drilled holes in thick sections may need touch-up.
- Ask for evidence in your category: field exposure data and the supplier’s coating certificate, not only salt-spray hours.
4. Aluminium: light, but mind strength and contact
Aluminium rails weigh about a third of equivalent steel sections, which matters on roofs and for hand carrying. Because aluminium is less stiff and less strong, spans are shorter and sections deeper. It expands about twice as much as steel with temperature, so long rails need expansion joints.
The main risk is galvanic corrosion where aluminium touches steel in damp or salty air: the aluminium corrodes. Separate the two with EPDM pads or insulating washers, use stainless fasteners (A4 / 316 near the sea), and use anti-seize on stainless threads to avoid galling.
5. Our choice by component and environment
| Component | C2–C3 (inland, mild) | C4 (coastal, industrial) | C5 (severe coastal) |
|---|---|---|---|
| Posts, piles, main beams | HDG, ISO 1461 minimum | HDG, thicker coating or ZAM-equivalent life check | HDG plus paint (duplex), or heavier coating |
| Rails, purlins, braces | HDG or ZM275–ZM310 | ZM310–ZM430 | ZM430 or aluminium; check edges |
| Rooftop rails and clamps | Aluminium or ZAM | Aluminium, isolated from steel | Anodised aluminium, isolated |
| Fasteners | HDG or A2 stainless | A2 / A4 stainless | A4 (316) stainless |
This matrix is our engineering guidance for a 25-year design life. Confirm the corrosivity category for the site and the design life required by the owner.
FAQ
Is ZAM better than hot-dip galvanizing?
Per micrometre of coating, yes: it corrodes more slowly. But batch galvanizing applies a much thicker coating. The independent review found ZM310 outlasts 55 µm batch galvanizing and ZM430 outlasts 70 µm; for thick posts with 85 µm or more, hot-dip galvanizing remains the standard.
How many times longer does ZAM last?
Field data suggest about 1.5–2.4 times less thickness loss than zinc, depending on the site. Treat salt-spray multipliers of 5–10 as marketing, not design values.
Can aluminium and galvanized steel be bolted together?
Yes, with isolation: EPDM pads or insulating washers between them, and suitable stainless fasteners, especially in humid or coastal sites.
Which material is cheapest?
Per kilogram, steel is far cheaper than aluminium; aluminium can still win on rooftops through lower weight and faster installation. Compare installed cost and life, not material price alone.
How we help
Our structures plant near Kunming roll-forms steel profiles and runs its own hot-dip galvanizing line, so posts and members can be galvanized to the coating your site needs. If your specification calls for ZAM strip or aluminium components, send it to us and we will tell you honestly what we can supply and how. See our ground-mount and rooftop structures.
Sources: Steel Construction Institute review of Magnelis® ZM310 versus Z600 and batch galvanizing, as published by ArcelorMittal (2024–2025); EN 10346:2015 coating designations; ArcelorMittal Magnelis® technical guidance on coating mass by thickness; ISO 1461 minimum coating thickness; aluminium 6005A-T5 and Q355 steel properties from published material data. The component matrix is our own engineering guidance. Related: hillside design, all mounting systems.