Short answer: specify hot-dip galvanizing to ISO 1461 (or ASTM A123) and size the coating to the site’s ISO 9223 corrosivity category. For steel over 6 mm, ISO 1461 requires at least 85 µm mean. At C3 rates that lasts several decades; on a salty C5 coast it may last under 20 years, so add thickness, a paint system or a zinc-aluminium-magnesium coating.
A PV plant is designed for 25 years or more, and the zinc on its steel is what gets it there. Two numbers decide the specification: the minimum thickness the standard requires, and how fast zinc corrodes at your site.
Key numbers
- ISO 1461 minimum mean coating: 85 µm on steel over 6 mm, 70 µm on 3–6 mm, 55 µm on 1.5–3 mm.
- First-year zinc corrosion under ISO 9223: 0.7–2.1 µm/year in C3, 2.1–4.2 in C4, 4.2–8.4 in C5.
- Rough life of an 85 µm coating at those rates: about 40–120 years in C3, 20–40 in C4, 10–20 in C5 (our arithmetic; see the table below).
1. Minimum coating thickness under ISO 1461
ISO 1461 gives two minimums for articles that are not centrifuged. The local thickness is the average of at least five magnetic readings in one reference area; the mean is the average of all reference areas on the sample. A batch must meet both.
| Steel thickness | Local minimum | Mean minimum |
|---|---|---|
| > 6 mm | 70 µm | 85 µm |
| > 3 mm to ≤ 6 mm | 55 µm | 70 µm |
| ≥ 1.5 mm to ≤ 3 mm | 45 µm | 55 µm |
| < 1.5 mm | 35 µm | 45 µm |
| Centrifuged threads > 6 mm diameter | 40 µm | 50 µm |
Two practical consequences for PV structures. First, purlins and rails roll-formed from 2–3 mm strip only need 55 µm mean, so they set the life of the structure, not the thick posts. Second, bolts and nuts carry the thinnest coating of all; in C4 and C5 sites specify them separately.
2. How fast zinc corrodes at your site
ISO 9223 classifies atmospheres from C1 (very low) to CX (extreme) and gives first-year corrosion rates for zinc in each:
| Category | Typical environment | Zinc loss, µm per year |
|---|---|---|
| C2, low | Dry rural, inland | 0.1–0.7 |
| C3, medium | Urban, mild coastal, humid inland | 0.7–2.1 |
| C4, high | Industrial, coastal | 2.1–4.2 |
| C5, very high | Severe coastal or industrial | 4.2–8.4 |
| CX, extreme | Offshore, tropical salt spray | 8.4–25 |
The category can be measured with exposure coupons over a year, or estimated from the environment: distance from the sea, prevailing wind, humidity, industrial pollution. For a large plant near a coast, measure it or ask the local galvanizing association.
3. Life estimate: our worked table
Divide the coating thickness by the corrosion rate. First-year rates are conservative, because zinc usually corrodes more slowly once its protective layer forms; ISO 9224 gives longer-term averages for detailed design.
| Coating | C3 (0.7–2.1) | C4 (2.1–4.2) | C5 (4.2–8.4) |
|---|---|---|---|
| 55 µm (2–3 mm rails) | 26–79 years | 13–26 years | 7–13 years |
| 70 µm (3–6 mm) | 33–100 years | 17–33 years | 8–17 years |
| 85 µm (> 6 mm) | 40–121 years | 20–40 years | 10–20 years |
The table shows why a single “85 µm” line in a specification is not enough. In C3 the standard minimum covers a 25-year life comfortably. In C4 the thin rails become the weak point. In C5 even the posts may not reach 25 years without extra protection.
4. Options when the standard minimum is not enough
- Thicker hot-dip coating. Steel chemistry (silicon and phosphorus content) and immersion time affect thickness; agree a higher minimum with the galvanizer and the steel supplier together.
- Duplex system. A paint system over hot-dip galvanizing; the two protect each other and the combined life is longer than the sum of the parts. Common for coastal plants.
- Zinc-aluminium-magnesium (ZAM) coated strip. Used for roll-formed rails and purlins. Ask the supplier for test data in your corrosivity category and for how cut edges are treated.
- Stainless or hot-dip galvanized fasteners rated for the category, instead of electro-zinc plated bolts.
5. How to write it into the specification
A clear clause saves arguments at delivery:
“All steel members hot-dip galvanized after fabrication to ISO 1461. Minimum mean coating: 85 µm on members over 6 mm, 70 µm on 3–6 mm, [X] µm on roll-formed members. Site corrosivity: C[n] per ISO 9223. Design life: 25 years without structural repair. Fasteners: [specification]. Galvanizing records with measured thickness per batch to be supplied.”
6. How to check on delivery
- Ask for the galvanizing record with measured coating thickness for each batch.
- Spot-check with a magnetic coating thickness gauge: at least five readings per reference area, on a sample of pieces of each thickness.
- Inspect for bare spots, flux inclusions, blisters and heavy drips at holes and slots.
- Check that any cut edges and holes made after galvanizing were repaired with zinc-rich paint, and that repaired areas are small. ISO 1461 limits repairs to a small share of the surface.
FAQ
Is 85 µm enough for a 25-year solar plant?
In C2 and C3 environments, yes, with a wide margin. In C4 it is borderline for thin members, and in C5 first-year rates suggest 10–20 years, so add thickness or a duplex system.
What is the difference between ISO 1461 and ASTM A123?
Both specify hot-dip galvanized coatings on fabricated steel. ASTM A123 sets minimum average thicknesses by material category and thickness; ISO 1461 uses the local and mean minimums above. Specify the one your project’s code and inspector use.
Should rails and purlins be hot-dip galvanized or ZAM?
Both are used. Galvanizing after fabrication covers cut edges and holes; pre-coated ZAM strip is efficient for roll-forming. Choose based on the corrosivity category and the supplier’s test data.
Can a coating be too thick?
Very thick coatings on reactive steels can be brittle and flake under impact. That is why it is better to agree the target thickness with the galvanizer than to simply ask for “as thick as possible”.
How we help
Our structures plant near Kunming runs its own hot-dip galvanizing line with a 400-tonne zinc kettle, next to its roll-forming lines, so the coating can be matched to each project. Send us the site location and corrosivity class, and we will propose a coating specification for posts, rails and fasteners.
Send us your site and specification
Sources: ISO 1461 Table 3 as summarised by the American Galvanizers Association and Galvanizers Association (UK); ISO 9223:2012 Table 2 corrosion rates, see AGA, HDG corrosion rates for ISO categories C1–C5/X; ISO 9224 and ISO 14713-1 for long-term design. Life estimates are our own arithmetic from these rates. Related: fixed ground-mount structures, hillside design checklist, fixed vs tracker, our production base.