A solar mounting structure is expected to survive 25 years outdoors while carrying a live electrical asset. Get the corrosion specification wrong and it fails early — not spectacularly, but quietly, at the fastener threads and the cut edges, where nobody looks until the array starts moving. This guide summarizes how to specify corrosion protection by environment for the three material families used in PV mounting: aluminium, galvanized steel, and fasteners. It condenses our full 23-page reference — the Corrosion Protection Guide (2026, PDF), which carries the underlying tables and 36 primary sources.
Table of Contents
- Step 1 — classify the environment (ISO 12944 C1–CX)
- Aluminium: anodizing class by category
- Galvanized steel: zinc thickness by category
- When galvanizing alone is not enough (duplex)
- Fasteners: the part most often under-specified
- Ready-to-use specification wording
- Quick FAQ
Step 1 — classify the environment (ISO 12944 C1–CX)
Every corrosion decision starts here, and skipping it is the most common mistake. ISO 12944-2 sorts atmospheres into corrosivity categories by how fast they consume metal:
| Category | Typical environment | Example sites |
|---|---|---|
| C1–C2 | Dry indoor / rural low-pollution | Inland farms, dry highland |
| C3 | Urban, light industry, moderate humidity | Suburban rooftops, inland cities |
| C4 | High-humidity industrial or coastal atmosphere | Coastal towns, chemical zones |
| C5 | Heavy salt spray, heavy industry, high humidity | Surf coast, Gulf industrial |
| CX | Extreme marine, splash zone, strong chlorides | Offshore, jetties, splash zones |
The key discipline: pick the category from the actual site — distance to surf, prevailing wind, industrial pollution — not from a national average. A rooftop 500 m from a surf beach is C5 even in a “mild” country.
Aluminium: anodizing class by category
Structural aluminium (we use AL6005-T5) is inherently corrosion-resistant thanks to its natural oxide, but for a 25-year design life it is anodized to a controlled thickness. Anodizing classes (ISO 7599 / QUALANOD) are named by film thickness in microns — AA10 means a 10 µm anodic film. Thicker film, harsher environment:
| Category | Typical anodizing class | Note |
|---|---|---|
| C1–C2 | AA5–AA10 | Indoor/rural; AA10 is a robust general minimum |
| C3 | AA15 | Standard outdoor architectural |
| C4 | AA20 | Coastal / industrial atmosphere |
| C5–CX | AA25 | Heavy marine; confirm sealing quality |
Our anodized AL6005-T5 profiles achieved a class 10 rating — the highest — in SGS CASS accelerated salt-spray testing (ISO 9227), evidence you can pull from our test reports. For coated (rather than anodized) aluminium, organic powder coating to QUALICOAT / EN 12206-1 is the alternative, specified by film thickness per the coating datasheet.
Galvanized steel: zinc thickness by category
Hot-dip galvanizing (HDG) to ISO 1461:2022 protects steel two ways: the zinc is a barrier, and it sacrifices itself to protect exposed steel at cut edges. Coating life is roughly proportional to zinc thickness, because the zinc is consumed at a rate set by the environment. The Galvanizers Association corrosion map illustrates this with an 85 µm coating: at zinc corrosion rates of 0.5 / 1 / 1.5 / 2 / 2.5 µm per year, average life is about 170 / 85 / 57 / 43 / 34 years respectively.
One nuance that trips up specifications: ISO 1461:2022 grades minimum zinc thickness by steel thickness, not by environment. The 85 µm figure many people quote as “the ISO minimum” is only the standard mean minimum for steel thicker than 6 mm. Thin-wall PV members (often 1.5–3 mm) fall in a lower band:
| Steel thickness | Min. mean zinc (ISO 1461:2022) |
|---|---|
| > 6 mm | 85 µm |
| > 3 mm to ≤ 6 mm | 70 µm |
| ≥ 1.5 mm to ≤ 3 mm | 55 µm |
| < 1.5 mm | 45 µm |
So the correct way to write it is: “HDG to ISO 1461:2022, minimum mean coating thickness per Table 3; project minimum 85 µm where expressly specified.” That preserves a higher project requirement without mislabelling 85 µm as an ISO default for every member — and where a thin member must reach ≥80/85 µm, the galvanizer confirms manufacturability and provides measured certificates.
When galvanizing alone is not enough (duplex)
For C4–C5 sites, or where a 25-year low-maintenance target exceeds what bare zinc thickness can deliver, the answer is a duplex system: HDG plus an organic topcoat. The two protect synergistically — the coating slows zinc consumption, and the zinc prevents underfilm corrosion — so service life is commonly 1.5–2.3× the sum of the individual systems. Duplex film thickness is selected per ISO 12944-5:2019 Table B.3 / Annex D, not by borrowing the bare-steel numbers. For CX (offshore, splash) neither zinc nor duplex should be extrapolated from the C5 table — that needs ISO 12944-9 or a project-specific dedicated system.
Fasteners: the part most often under-specified
Fasteners are the cheapest parts and the first to fail, because they cannot simply inherit the structure’s coating table. ISO 1461:2022 itself says galvanized products with their own product standard (fasteners) follow that product standard. Selection by grade:
| Fastener type | Standard | Best-fit environment |
|---|---|---|
| HDG carbon/alloy bolt | ISO 10684 | C1–C3; C4 only with extra zinc/sealing |
| Thermal-diffusion (TDC) bolt | ISO 17668 / 14713-3 | C3–C5 where high performance needed |
| SS304 / A2 | ISO 3506-1 A2 | C1–C3; keep away from chloride deposition |
| SS316 / A4 | ISO 3506-1 A4 | C4–C5 coastal / chloride exposure |
Two rules keep fastener specs honest. First, HDG bolt life is judged by zinc thickness and environmental corrosion rate (ISO 9223 / 14713), not by salt-spray hours — neutral salt spray (ISO 9227) is a quality check, not a life prediction. Second, for C4/C5, do not just write “standard HDG” without a life calculation; switch to TDC, SS316/A4, or HDG-plus-sealing. Where salt-spray targets are used for TDC quality control, typical acceptance rises with category: ≥240 h (C2) → ≥480 h (C3) → ≥720 h (C4) → ≥1,000 h (C5), no red rust on significant surfaces. Our own coated roof screws pass 1,000 h neutral salt spray with no red rust.
Ready-to-use specification wording
Copy-ready lines you can drop into a drawing or purchase order:
Aluminium: “Structural aluminium AL6005-T5, anodized to ISO 7599, class AA[15/20/25] to suit ISO 12944 category [C3/C4/C5].”
HDG steel: “Hot-dip galvanized to ISO 1461:2022, minimum mean coating thickness per Table 3; project minimum 85 µm where expressly specified and confirmed manufacturable by the supplier.”
Stainless fasteners: “Stainless fasteners to ISO 3506-1 A2-[70]/A4-[70]; for coastal/chloride exposure use A4 in preference to A2; control crevice corrosion, galling and galvanic contact with aluminium/zinc.”
The full guide adds the HDG-bolt zinc-thickness table, the duplex application control points, and hydrogen-embrittlement notes for high-strength fasteners — download it here (PDF, 0.9 MB).
Quick FAQ
Q: Is stainless always better than galvanized for solar mounting?
A: Not automatically. In chloride environments A4 stainless resists red rust better, but it must be detailed against crevice corrosion and galvanic contact. Inland, a correctly specified HDG or anodized-aluminium system is both cheaper and entirely adequate.
Q: Our project just says “hot-dip galvanized.” Is that enough?
A: It is under-specified. Without a thickness basis (ISO 1461:2022 Table 3) and an environment category, “HDG” alone does not define a service life. Add both.
Q: Can I use salt-spray hours to compare products’ lifespans?
A: Only as a quality/consistency check within the same coating type. Salt-spray hours do not convert to years of outdoor life; use environmental corrosion rates and coating thickness for life prediction.