Pre-Galvanised (GI) versus Hot-Dip Galvanised (HDG) Steel for Solar Structures
Coating thickness in microns and g/m², IS 277 against IS 4759 and ISO 1461, cut edges, and expected life by environment for GI and HDG solar structures.
Vasistron engineering team · Published 11 September 2026 · Last reviewed
Two processes, two standards
Both pre-galvanised and hot-dip galvanised steel carry zinc applied from a molten bath at about 450 degrees Celsius; the difference is when. Pre-galvanised steel (GI, continuously or Sendzimir galvanised) is coated as a coil before slitting, roll-forming and punching. Hot-dip galvanised steel (HDG, batch galvanised) is dipped after the member is cut, punched, welded or bent, so edges and holes are coated too.
In India, pre-galvanised sheet and coil are specified to IS 277, which defines zinc coating classes by total mass on both surfaces. Hot-dip galvanising of fabricated steel is specified to IS 4759, or increasingly to ISO 1461, which defines minimum coating thickness by the thickness of the steel being dipped. A purchase specification should name one or the other for each member.
Coating thickness: microns and g/m²
Zinc has a density of 7.14 g/cm³, so 1 micron of zinc weighs 7.14 g/m² on one surface. IS 277 coating classes are quoted as the total on both surfaces, so a class must be halved before converting to microns per side.
| Specification | Coating mass or thickness | Per side in microns | Typical use |
|---|---|---|---|
| IS 277 class 120 | 120 g/m² total, both sides | About 8 | Light-duty sheet, not suitable for structure |
| IS 277 class 275 (Z275) | 275 g/m² total | About 19 to 20 | Standard for cold-formed solar purlins, rafters and rails |
| IS 277 class 350 (Z350) | 350 g/m² total | About 24 to 25 | Heavier-duty pre-galvanised members |
| ISO 1461, steel under 1.5 mm | 45 microns mean minimum | 45 | Thin dipped brackets |
| ISO 1461, steel 1.5 to 3 mm | 55 microns mean minimum | 55 | Dipped cold-formed sections |
| ISO 1461, steel 3 to 6 mm | 70 microns mean minimum | 70 | Posts, base plates, rafters |
| ISO 1461, steel 6 mm and above | 85 microns mean minimum | 85 | Carport columns, heavy sections |
IS 4759 follows the same principle as ISO 1461, setting minimum coating mass by steel thickness, with the heaviest class for sections 5 mm and above at [610 g/m², about 85 microns] and lighter classes of the order of [340 to 460 g/m²] for thinner material. ISO 1461 also limits uncoated areas to 0.5 per cent of the surface and 10 cm² per patch, and allows repair with zinc-rich paint or zinc thermal spray.
The practical result is that a hot-dip galvanised member carries three to four times the zinc of a Z275 pre-galvanised member of the same thickness. Because zinc corrodes at a rate that depends on the environment rather than on the coating, service life scales almost linearly with coating thickness.
Cut edges, holes and bends
The known weakness of pre-galvanised steel is the cut edge, which shows bare steel after slitting, shearing or punching. Zinc is anodic to steel, so the zinc on either side of the edge corrodes sacrificially and protects the exposed steel over a short distance, generally accepted as up to [1 to 2 mm]. For sheet up to about 2 mm thick the edge is effectively protected; for 2.5 to 3 mm sections the middle of the edge will show a rust stain within a few years at humid sites, though structural loss is small. Above 3 mm, edge protection is not reliable and the member should be hot-dipped or the edge sealed.
Roll-forming and press-braking to the radii used for C, Z and U sections do not crack a Z275 coating, because the alloy layer on continuously galvanised sheet is thinner and more ductile than on batch-dipped steel. Punched holes behave as cut edges. The thicker, harder HDG alloy layer resists abrasion in transport better but cannot be bent after coating.
Hot-dip galvanising after fabrication has its own constraints. Members need vent and drain holes; closed sections without them can burst in the bath. Cold-formed sections of 1.6 to 2.5 mm can distort from thermal stress, long asymmetric Z sections most of all. Welding must precede dipping, and threads need over-tapping to clear the zinc. This is why cold-formed solar structures are almost always pre-galvanised and only posts, base plates and brackets are hot-dipped.
Expected life by environment
ISO 9223 classifies atmospheric corrosivity into categories C1 to C5 and CX, and gives first-year zinc corrosion rates for each. Categories C2 (low, rural inland), C3 (medium, urban and light industrial), C4 (high, industrial and mild coastal) and C5 (very high, marine and heavy industrial) cover most Indian sites. Time to first rust is approximately the coating thickness divided by the corrosion rate, although the rate falls after the first few years as a stable zinc carbonate patina forms.
| Environment | ISO 9223 category | Zinc loss, first year | Z275 GI, 20 microns | HDG, 70 microns |
|---|---|---|---|---|
| Inland dry, Telangana interior, rural Rajasthan, Karnataka plateau | C2 | 0.1 to 0.7 microns/year | [30 to 60+ years] | [100+ years] |
| Urban or light industrial, Hyderabad periphery, Pune, Nagpur | C3 | 0.7 to 2.1 microns/year | [12 to 30 years] | [35 to 100 years] |
| Heavy industrial or 5 to 20 km from coast, Vizag hinterland, Chennai outskirts | C4 | 2.1 to 4.2 microns/year | [6 to 12 years] | [18 to 35 years] |
| Coastal within 1 to 5 km, ports, refineries, Kutch salt flats | C5 | 4.2 to 8.4 microns/year | [3 to 6 years] | [9 to 18 years] |
At inland Telangana sites, Z275 outlasts a 25-year plant, and upgrading to Z350 or HDG buys margin against local wet spots rather than the general climate. From C4 upwards, HDG or a zinc-aluminium-magnesium coating is needed for a 25-year life, and at C5 HDG benefits from a duplex system with an organic topcoat.
When to specify which
- Pre-galvanised Z275 to IS 277: purlins, rails, rafters and bracing up to 2.5 mm in C2 and C3 environments.
- Pre-galvanised Z350 or higher: the same members at C3 sites with high humidity, near effluent or fertiliser plants, or where the owner asks for a longer coating warranty.
- Hot-dip galvanised to IS 4759 or ISO 1461: driven posts and piles, base plates, brackets over 3 mm, carport columns and any welded assembly. Needed for a 25-year life in C4 and C5.
- Zinc-aluminium-magnesium coated coil to EN 10346, or 55 per cent aluminium-zinc to IS 15961: thin pre-galvanised members at C4 sites where hot-dipping is impractical. Aluminium-zinc should not be used in contact with fresh concrete or mortar.
SS304 fasteners on a zinc-coated structure form a galvanic couple, but the small cathode area of the bolt against the large anodic structure keeps zinc loss negligible; a zinc-plated bolt in a stainless assembly is the harmful reverse. Aluminium module frames on zinc-coated rails are acceptable, with EPDM or nylon isolation at C4 sites or worse.
Comparison table
| Item | Pre-galvanised (GI) | Hot-dip galvanised (HDG) |
|---|---|---|
| Governing standard | IS 277 | IS 4759, ISO 1461 |
| When coated | Coil, before forming and punching | After fabrication |
| Typical coating | Z275, about 20 microns per side | 55 to 85 microns by steel thickness |
| Cut edges and holes | Bare; sacrificially protected to about [1 to 2 mm] | Fully coated |
| Practical steel thickness | 0.8 to 3 mm | 1.5 mm upwards; distortion risk below 2.5 mm |
| Welding | Not after coating; bolted assembly | Weld before dipping |
| Surface | Smooth, spangled or matt | Rougher, grey, may show drips |
| Added cost over plain steel | [Rs 8 to 14 per kg] | [Rs 14 to 22 per kg] plus transport to plant |
| Lead time effect | None; coil in stock | [3 to 10 days] per batch |
| Suits | Cold-formed purlins, rails, rafters; C2 and C3 sites | Posts, base plates, welded frames; C4 and C5 sites |
About Vasistron
Greentek Vasistron Private Limited, Dhulapally, Hyderabad, manufactures the SolBase Terra ground mount, SolBase Aero carport and canopy, and SolBase Tecta rooftop and shed structures from GI cold-rolled C, Z and U sections with SS304 fasteners and no site welding. Coating class and the use of hot-dip galvanised posts are specified per project against the ISO 9223 category of the site.
Questions on this topic
- What is the difference between GI and HDG steel?
- GI or pre-galvanised steel is zinc-coated as a coil to IS 277 before it is cut and formed, typically at 20 microns per side for the Z275 class. HDG or hot-dip galvanised steel is dipped after fabrication to IS 4759 or ISO 1461 at 55 to 85 microns, so cut edges, holes and welds are also coated.
- Is Z275 pre-galvanised steel adequate for a 25-year solar plant in Telangana?
- At inland Telangana sites in ISO 9223 category C2, zinc loss is 0.1 to 0.7 microns per year, so a 20 micron Z275 coating gives an expected life of [30 years or more] before first rust. Z275 is adequate for cold-formed purlins and rails there; posts and base plates are usually hot-dipped.
- Do cut edges on pre-galvanised solar sections rust?
- The bare edge is protected by sacrificial corrosion of the zinc on either side over roughly [1 to 2 mm]. Sections up to about 2 mm thick are effectively protected. Thicker edges may show rust staining at humid sites, and members over 3 mm should be hot-dip galvanised.
- Can cold-formed C and Z sections be hot-dip galvanised after forming?
- Yes, but sections under about 2.5 mm risk distortion in the 450 degree Celsius bath, long asymmetric Z sections most of all, and the members need vent and drain holes. This is why solar purlins and rails are normally pre-galvanised and only posts, base plates and brackets are batch-dipped.
- Which coating should be used for a coastal solar plant?
- Within 5 km of the coast, ISO 9223 category C4 or C5 applies and zinc loss can exceed 4 microns per year. Structural members should be hot-dip galvanised to ISO 1461 at 70 to 85 microns, thin members should use a zinc-aluminium-magnesium coating, and at C5 a duplex system with an organic topcoat over HDG is advisable.