Introduction: Solar PV mounting structures face decades of outdoor exposure, so choosing materials that withstand the environment is crucial. Engineers use ISO corrosion categories (C1 through CX) to gauge how aggressive the atmosphere is toward metals. These categories, defined by ISO 9223/12944, range from C1 (very low corrosivity) – like a dry indoor space – up to CX (extreme corrosivity) – such as offshore marine climates . Understanding these categories helps in selecting appropriate materials (like Zn-Al-Mg coated steel, aluminum, or hot-dip galvanized steel) and protective measures. Below we break down each category’s typical corrosion rates and environments, then discuss how common PV mounting materials perform in each class.
ISO Corrosivity Categories (C1 to CX)
The ISO categories describe environmental severity by the corrosion rate observed on standard metal samples in one year. Here’s a quick overview of categories C1 through CX, their environments, and approximate corrosion rates (for bare steel, as a reference):
- C1 – Very Low:Dry, clean environments with almost no pollution or moisture. Typically indoor spaces like climate-controlled offices or warehouses. Corrosion of steel is negligible (≤1.3 µm thickness loss per year) , essentially no rusting in practice.
- C2 – Low:Slightly more moisture or pollution than C1. Examples include rural areas with clean air or unheated buildings that might get some condensation . Steel corrodes at >1.3 up to ~25 µm/year in this class . In real terms, metal in a C2 atmosphere rusts very slowly – think of a sheltered farm shed or inland desert climate.
- C3 – Medium:Moderate atmospheric aggressiveness. Typical of urban or light-industrial areas with some sulfur dioxide pollution, or coastal areas with low salt exposure . Also humid indoor environments like food processing facilities can fall here . Expect steel corrosion rates >25 to 50 µm/year . Example: a city environment or ~1–3 km from the ocean (low salt spray) would be C3. Metal structures here corrode at a noticeable but manageable rate.
- C4 – High:Significant pollution or salt in the atmosphere. Includes industrial areas and coastal regions with moderate salinity . For instance, a site within 1 km of the sea or an area with chemical plants is often C4. Steel loss is >50 to 80 µm/year , so corrosion prevention needs to be robust. Indoor pools or shipyards are cited as examples – high humidity and chlorides drive faster rusting .
- C5 – Very High:Very aggressive atmospheres – e.g. coastal marine with high salt or heavy industrial zones with constant humidity/chemical fumes . Steel can corrode >80 to 200 µm/year in C5 conditions . Structures near the surf line of the ocean, or in facilities with continuous condensation and corrosive chemicals, fall in C5. Protective coatings often need to be premium here to survive long term.
- CX – Extreme:The most severe class, typically reserved for offshore and extreme industrial Examples are offshore platforms with salty spray, tropical seaside areas with constant salt-laden humidity, or industrial sites with extreme pollutants . Steel may lose >200 up to 700 µm per year in such an environment – meaning unprotected steel would rust disastrously fast. CX covers what used to be beyond C5 (often called C5-Marine offshore). It’s generally understood that special materials or heavy-duty coatings are required for CX scenarios.
(Note: The ISO categories are defined separately for different metals. For instance, the same coastal environment that is “C5” for steel might only be “C3” in terms of aluminum corrosion, since aluminum corrodes much less in that environment . But for simplicity, engineers often refer to the category by the steel standard as a measure of overall severity.)
ISO atmospheric corrosion categories (C1 through C5) with example environments and corrosion rate ranges. Higher categories (C4, C5) correspond to coastal or industrial areas with more aggressive conditions, while C1–C2 are dry or clean environments.
Material Performance in Different Corrosivity Classes
Now, let’s look at three common materials for PV mounting structures – Zn-Al-Mg coated steel, aluminum alloys, and hot-dip galvanized (HDG) steel – and discuss where each is suitable (or not) across various corrosion categories. We’ll use real-world examples (coastal vs. desert vs. industrial settings) to illustrate.
Zinc-Aluminum-Magnesium (Zn-Al-Mg) Coated Steel
Zinc-Aluminum-Magnesium alloy coatings (applied by hot-dip process) are a newer development in corrosion protection. A well-known example is SOZAMC® by Shougang, which contains high aluminum（5~7%） and magnesium（2~4%）in the coating . This composition gives it unique self-protecting qualities – it forms a dense, stable film that even shields cut edges from corrosion better than regular zinc galvanizing . Here’s how Zn-Al-Mg performs:
- Excellent for C3–C4 environments:In moderate urban, industrial, or coastal (low-to-medium salinity) conditions, Zn-Al-Mg coatings significantly outlast normal galvanizing. Field tests around the world show corrosion rates about 3× lower than regular HDG steel in similar exposure . This can translate to roughly three times the service life. For example, a Zn-Al-Mg coated solar mounting in a coastal farm (C4) will corrode much more slowly than a standard galvanized one – extending the time to first maintenance by years . Many large solar farms have adopted Zn-Al-Mg coated steel to reliably hit 25+ year lifetimes even in tough C3/C4 climates .
- Adequate in low-corrosivity areas:In desert or rural inland climates (C1–C2), Zn-Al-Mg coatings are almost overkill – they will have extremely low corrosion loss per year. The dry environment means the protective coating might last well beyond the project life with virtually no degradation. (E.g. a Zn-Al-Mg coated tracker in an arid desert sees so little moisture that it stays pristine).
- Borderline for C5–CX extremes:In very high salinity coastal (C5) or offshore (CX) conditions, manufacturers often limit the use of bare Zn-Al-Mg coatings. The corrosion rate, while slower than normal zinc, is still significant in constant salt spray. Standard Zn-Al-Mg coated sheets (of typical thickness) might not meet a 20-30 year requirement in a C5 marine environment unless additional measures are taken . For example, ArcelorMittal’s data shows that thin Zn-Al-Mg coatings (≈10–20 µm per side) are “not appropriate” for long-term use in C5 marine atmospheres without further protection . In such cases, options include using a heavier coating layer, a protective paint overcoat (duplex system), or avoiding carbon steel altogether. So for offshore PV (CX) or extremely corrosive industrial sites, Zn-Al-Mg alone may be insufficient – though it still performs better than plain galvanizing if used.
- Real-world usage:Zn-Al-Mg coated steel has been successfully used in coastal solar installations and even environments like agricultural greenhouses (where fertilizer or ammonia vapors accelerate corrosion). Its ability to protect scratches and cut edges is a big advantage when assembling PV structures on-site. Example: The mounting structures in one of the world’s largest solar farms (in a tropical coastal region) were made with Magnelis® Zn-Al-Mg steel, chosen to ensure long-term durability in a humid, salty atmosphere . After a decade, these structures have minimal corrosion, validating the improved performance of Zn-Al-Mg coatings.
Shougang SOZAMC® can warranty use to C5.
Aluminum Alloys
Aluminum is widely used in solar PV frames and racks thanks to its natural corrosion resistance. Aluminum forms a thin oxide layer that shields the metal from further rust – it doesn’t “rust” like steel does (no red iron oxide flaking). Common alloys (e.g. 6xxx series extrusions) are lightweight, strong, and corrosion-resistant, making them ideal for module frames and railings. Considerations for aluminum:
- Great in most atmospheres (C1–C4):Aluminum holds up very well in low, medium, and even high corrosivity environments. In urban/industrial atmospheres (C3) or moderate coastal climates (C4), aluminum components typically see only slight surface oxidation. In fact, a harsh environment that would be classified as C5 for steel might only equate to about C3 severity for aluminum . For example, an aluminum rack in a coastal city will form a dull oxide or a few pits, but generally won’t experience the deep material loss that steel would in the same air. This is why aluminum frames are preferred in humid and coastal regions – they can easily survive 25+ years with minimal maintenance . Even without coatings, aluminum parts on seaside PV systems usually endure with just cosmetic corrosion.
- Coastal/marine (C5) performance:In marine environments with salt spray, aluminum can corrode via pitting, but it’s relatively slow and localized. The aluminum oxide film actually resists chloride attack to an extent, so long as the surface isn’t continuously wet. Many coastal solar installations report that aluminum frames and rails last for decades in salty air . Any corrosion is often limited to a whitish powdery buildup or tiny pits, which are usually superficial. Regular rain rinsing or occasional cleaning can further prevent salt buildup and keep corrosion to a cosmetic level . For instance, aluminum PV module frames in a beachfront solar farm might show slight pitting after 10+ years, but they remain structurally sound and firmly intact. (Manufacturers also certify modules with salt-mist corrosion tests, ensuring the aluminum frame and stainless fasteners can handle marine air .)
- Dry desert (C1/C2) conditions:Aluminum is extremely well-suited for deserts and arid regions. The lack of moisture means the oxide layer stays intact and corrosion is practically zero. Example: Aluminum mounting structures in Middle Eastern desert solar plants (low pollution, low humidity) have virtually no corrosion over their lifetime – the metal looks almost new after years of operation. Here, aluminum’s longevity far exceeds galvanized steel, which might still develop a patina in the rare event of moisture.
- Special considerations:While aluminum itself resists atmospheric corrosion, engineers must watch for galvanic corrosion when aluminum contacts other metals. If aluminum is in contact with a more noble metal (e.g. copper, or even steel) in the presence of an electrolyte (water, especially salt water), the aluminum can corrode preferentially . For PV systems, this means isolating aluminum parts from copper wires, galvanized steel bolts, etc., in coastal areas. Using plastic/nylon washers, appropriate coatings, or separating dissimilar metals prevents galvanic couples. Also, very alkaline or acidic environments outside normal atmospheric ranges can attack aluminum’s oxide film – for example, fresh concrete (wet concrete is highly alkaline at pH ~13) will eat into aluminum on contact . Thus, aluminum bases should be protected if encased in concrete foundations. Overall, however, aluminum alloys are a safe choice for corrosion resistance: they’re maintenance-free in most environments, needing only occasional cleaning. It’s no surprise that industry guidance often recommends aluminum frames for long-lasting durability in humid/coastal climates .
Hot-Dip Galvanized (HDG) Steel
Hot-dip galvanizing – dipping steel components in molten zinc – is a classic and economical way to protect steel from rust. The zinc coating serves as a sacrificial barrier: it corrodes slowly in place of the steel. HDG steel is heavily used in PV mounting (for piles, bolts, purlins, etc.), especially in large-scale ground mounts. Its suitability depends on coating thickness and environment:
- Ideal for low to moderate corrosivity (C1–C3):In mild atmospheres, galvanized steel can last a very long time. For example, in a C2 rural environment, zinc corrodes at only about 0.1–0.7 µm per year . Even a standard ~85 µm thick galvanizing could theoretically protect steel for well over 50 years in such conditions. In a typical C3 environment, the zinc corrosion rate of 0.7–2.1 µm/yr means a 100 µm coating might not need maintenance for 47–100+ years . Real-world example: Many agricultural installations or small-town solar farms (C2/C3) use galvanized steel structures that show little to no rust even after decades – just a dull gray zinc patina. HDG steel is a cost-effective, reliable choice for these moderate climates.
- Higher corrosivity (C4):In a C4 industrial or coastal (modest salt) environment, galvanizing will still protect the steel, but the zinc layer is consumed faster (roughly 2–4 µm per year by ISO definition ). For long life in C4, usually a thicker zinc coating or additional paint is recommended. For instance, a coastal PV mount near the ocean (but not in direct salt spray) might use HDG steel with 100–140 µm of zinc. This could provide on the order of 25–40 years of protection before significant touch-ups are required. Expect to see the zinc turn chalky white (zinc oxide layer) over time; that’s normal. Maintenance tip: Checking galvanizing thickness and planning for possible re-galvanizing or painting at mid-life in C4 areas is wise if 30-yr service is needed.
- Very high to extreme (C5–CX):C5 marine or industrial environments push galvanized steel to its limits. Zinc corrodes at 4 to >8 µm per year in C5 climates , so a 85 µm coat could be largely gone in ~10–20 years. In CX offshore conditions, zinc loss can be as high as 8–25 µm/yr – at those rates a galvanized layer can fail in just a few years if thin. Thus, bare galvanizing is often not sufficient for long-term corrosion resistance in C5/CX. Engineers often resort to duplex systems (galvanizing + a durable paint or powder coat) for such environments, or specify stainless steel for critical hardware. For example, a solar array built on a coastline with constant salt spray may see red rust on HDG steel after only 5-6 years if the coating was too light. To prevent that, one might galvanize to a very heavy grade and then epoxy-coat it, achieving a combined protection that withstands the aggressive atmosphere. In summary, standard HDG steel is not recommended as the sole protection in severe coastal (C5) or offshore (CX) sites if a 25+ year life is expected .
- Arid desert scenarios:On the flip side, desert solar farms (C1/C2) are a perfect fit for galvanized steel. With virtually no rainfall and low humidity, the zinc coating remains largely intact for decades. There are solar plants in Sahara-like climates where galvanized support structures show only slight weathering after 20+ years. The biggest concern in deserts is often mechanical (wind-blown sand erosion) rather than chemical corrosion. HDG steel in these areas is very durable, making it an economical choice compared to more expensive coatings.
- Practical example:Consider a PV mounting structure in an industrial coastal city (C4/C5 boundary) – perhaps near a petrochemical plant by the sea. If it’s made of regular galvanized steel, within a few years you’d notice the shiny zinc dulling to white, then dark gray. By year ~5-10, you might spot brown rust at joints or cut edges as the coating wears off. A switch to Zn-Al-Mg coated steel in this scenario might delay that rusting significantly (thanks to the improved corrosion resistance) . Or, using aluminum components would result in no rust at all, just occasional powdery corrosion byproducts. This illustrates how material choice becomes critical as the corrosion category rises. In benign settings, galvanized steel is perfectly fine (and most economical), but in coastal and industrial zones one should evaluate upgraded coatings or materials to ensure the solar infrastructure remains sound over its lifetime.
Conclusion: Every solar project should account for its site’s corrosion category when selecting mounting materials. To sum up guidelines: use basic galvanizing confidently in dry or mildly corrosive areas (C1–C3), consider Zn-Al-Mg coated steel for extra safety in moderate-to-high corrosion areas (C3–C4, or lower C5) for its extended durability, and lean on aluminum (or enhanced protection systems) for the harshest coastal/industrial conditions. By matching materials to ISO corrosion classes, engineers can ensure solar PV structures stay safe and rust-free throughout decades of clean energy production.
Sources: Understanding of ISO categories and material performance is based on ISO 12944/9223 standards and industry data , as cited above. Always refer to manufacturer guidelines and corrosion test results for final material selection in critical projects.