A module is warranted for 25–30 years. The steel under it often rusts in five. In coastal Mumbai, Chennai or Visakhapatnam, and near factories and chemical plants, corrosion is the main structural risk, ahead of wind. The material and coating you choose decide whether the structure outlives the modules.
How zinc protects steel
Steel rusts when water and oxygen reach it. A zinc coating protects it in two ways:
- Barrier: zinc covers the steel.
- Sacrifice: where the coating is scratched, zinc corrodes in preference to the exposed steel nearby, protecting it.
Zinc itself corrodes slowly, at a rate that depends on the environment. The coating's life is roughly thickness ÷ corrosion rate.
Corrosivity categories
ISO 9223 groups sites by how fast zinc corrodes (first-year rates):
| Category | Environment | Zinc loss per year | Indian examples (typical; confirm for the site) |
|---|---|---|---|
| C2, low | Dry inland, low pollution | 0.1–0.7 µm | Jaipur, Indore, Bengaluru suburbs |
| C3, medium | Urban, mild industrial, or several km from the coast | 0.7–2.1 µm | Pune, Hyderabad, Delhi NCR |
| C4, high | Coastal or industrial | 2.1–4.2 µm | Mumbai, Chennai, Kochi, industrial estates |
| C5, very high | Sea-front, heavy industry, high humidity with salt | 4.2–8.4 µm | Beachfront Chennai, Puri, Kutch coast, near chemical plants |
Worked example: coating life
A hot-dip galvanised structure with an 80 µm zinc coating (a common specification for rooftop solar; check the job specification). Using the upper rate of each category:
- C3: 80 ÷ 2.1 = 38 years
- C4: 80 ÷ 4.2 = 19 years
- C5: 80 ÷ 8.4 = about 9.5 years
A pre-galvanised Z275 section has 275 g/m² of zinc on both sides together, about 19 µm per side (zinc density 7.14 g/cm³). In C3: 19 ÷ 2.1 ≈ 9 years. In C4: under 5 years. And its cut ends and drilled holes have no zinc at all.
The lesson is plain: inland, an 80 µm HDG structure comfortably outlasts the modules. On the coast it may not, so specify a thicker coat, a better coating system, aluminium, or a maintenance plan.
The material options
Hot-dip galvanised steel (HDG, "HDGI")
Steel members are cut, drilled and welded first, then dipped in molten zinc at about 450 °C. Zinc covers every face, edge and hole, and bonds with the steel as zinc-iron alloy layers. This is the standard for quality elevated RCC and ground structures.
Check: dull grey, even coat (spangle or matt), no bare spots, no lumpy drips blocking holes, coating thickness measured with a gauge.
Pre-galvanised (pre-gal) steel
Sheet that was galvanised at the mill and then roll-formed into channels and cut on site. Cheap and light, with a thin coat (Z120 to Z275) and bare cut edges and holes. Acceptable for dry inland sites if edges are touched up; a poor choice near the sea or for 25-year life without maintenance.
Zinc-aluminium-magnesium (ZAM or ZM) coated steel
Mill-coated sheet with a zinc alloy containing aluminium and magnesium. It corrodes much more slowly than plain zinc of the same thickness, and its corrosion products tend to cover and partly protect cut edges as it weathers. Increasingly used for rooftop rails and purlins. Ask for the coating class and the maker's corrosion data.
Aluminium
Extruded aluminium alloy profiles (commonly 6063-T6 or 6005-T5/T6), often anodised (15 µm or more).
- Light, corrosion-resistant, no coating to fail, easy to cut on site.
- Costlier per kg and weaker per section than steel, so sections are deeper.
- Standard for mini-rails, module clamps and many coastal rooftop systems.
Fasteners
Fasteners are where structures fail first.
| Fastener | Use | Note |
|---|---|---|
| SS304 (A2) | Inland sites, clamps, rail bolts | Common default |
| SS316 (A4) | Coastal, chemical or swimming-pool sites | More resistant to chlorides |
| HDG bolts | Structural steel joints | Match to HDG members |
| Zinc-plated (electroplated) bolts | Avoid outdoors | Thin coat (a few µm), rusts within a year or two |
Stainless-on-stainless threads can gall (seize) if tightened fast or dry. Use anti-seize compound and tighten steadily.
Protecting cuts and holes on site
Every site cut, drill hole or weld on galvanised steel removes the zinc there.
- Avoid site drilling by ordering pre-drilled, pre-cut members.
- Where unavoidable, clean the spot and apply zinc-rich cold galvanising paint (high zinc dust content in the dry film), in two coats.
- Never weld galvanised members on the roof: zinc fumes are toxic and the joint will rust.
Choosing by site
| Site | Recommended |
|---|---|
| Inland city (C2–C3): Pune, Jaipur, Hyderabad | HDG steel (about 80 µm) with SS304 fasteners; or aluminium |
| Coastal city (C4): Mumbai, Chennai, Kochi | Heavier HDG or ZAM with SS316 fasteners, or aluminium; isolate dissimilar metals; inspect yearly |
| Sea-front or chemical industry (C5) | Aluminium or a duplex system (HDG plus paint), SS316 fasteners, a specific corrosion design and an inspection plan |
Key points
- Coating life ≈ zinc thickness ÷ zinc corrosion rate for the site's category.
- An 80 µm HDG coat lasts decades inland but may last only 10–20 years on the coast.
- Pre-galvanised sections have thin coats and bare cut edges; avoid them where life or corrosion matters.
- Use SS304 fasteners inland and SS316 near the sea; never use zinc-plated bolts outdoors.
- Isolate aluminium from copper and steel, and touch up every site cut with zinc-rich paint.
Check yourself
- A 70 µm HDG coat in a C4 site corroding at 3.5 µm a year lasts roughly how long?
- Why do pre-galvanised channels rust first at their ends?
- Which fastener grade would you specify for a rooftop 500 m from the sea in Chennai?
Answers
- 70 ÷ 3.5 = 20 years.
- They are cut after galvanising, so the cut ends (and any holes) have no zinc coating.
- SS316 (A4), with isolation between dissimilar metals.