Why 275 g/m² on a pre-galvanized sheet and 610 g/m² on a fabricated member are not the comparison they look like, and when each route is actually the right one.
Pre-galvanized steel is coated as flat coil, before it is cut, punched, drilled or welded. Steel galvanized after fabrication is coated as the finished part, so the zinc covers the cut edges, the hole walls, the weld and the inside of every hollow. Every operation you perform after the coil leaves the coating line removes coating; every operation you perform before the kettle gets coated.
That is the whole difference, and it decides the answer far more often than price does. If the part is only bent, pre-galvanized material can be the correct and cheaper choice. If the part is cut, drilled or welded and then has to stand outdoors, it is not.
Indo East galvanizes after fabrication — process to IS 2629, coating mass to IS 4759, in a kettle 7.0 m long × 1.2 m wide × 1.6 m deep at 450–460 °C. We do not roll-form pre-galvanized sheet, and where that is genuinely the better route for your part we will say so rather than sell you a dip you do not need.
Two coating masses, two different denominators. Read the units before you compare the figures.
Coating mass on sheet is written total, both sides inclusive. Coating mass on fabricated steel is written per surface. So a sheet specified at 275 g/m² carries roughly 137 g/m² on each face — about a fifth of what IS 4759 asks of a structural member, not a half and certainly not the same.
| Route | Standard | Coating requirement as the standard writes it | Zinc on one exposed face |
|---|---|---|---|
| Pre-galvanized sheet, common class | IS 277 class 275 ASTM A653 G90 / EN 10346 Z275 | 275 g/m² total, both sides | ≈ 19 microns |
| Pre-galvanized sheet, heaviest common class | IS 277 class 600 | 600 g/m² total, both sides | ≈ 42 microns |
| Hot dip galvanized tube | IS 4736 | 360 g/m² over inside plus outside | ≈ 50 microns average |
| Galvanized after fabrication, 3–5 mm | IS 4759 | 460 g/m² per surface | ≈ 64 microns |
| Galvanized after fabrication, over 5 mm | IS 4759 | 610 g/m² per surface | ≈ 85 microns |
Conversion throughout uses 7.14 g/m² of zinc per micron of coating. IS 277 states the mass as “the total mass of zinc, both sides inclusive”; IS 4736 divides the total zinc by the total area inside plus outside; IS 4759 states it per surface. Three standards, three denominators — which is why a purchase specification should always name the standard, not just the number.
Nothing here is an argument against pre-galvanized material. It is an argument for knowing what you have left.
| Operation | What happens to the coating | Does it matter? |
|---|---|---|
| Roll forming or press braking | The coating bends with the steel. A tight radius can craze the coating on the outside of the bend but it stays attached | No. This is what pre-galvanized material is designed for |
| Shearing or sawing | A bare steel edge the full thickness of the section | Depends on thickness. Zinc throws sacrificial protection a short distance across a cut edge — useful on thin sheet, not on a 6 mm plate |
| Punching or drilling | A bare hole wall, and bare steel under the washer face where the coating was dragged | Yes, outdoors. Bolt holes are where water sits |
| Welding | Coating burned off either side of the weld, the weld itself bare, and zinc oxide fume released | Yes, always. Welded pre-galvanized work needs repair to IS 2629 or ASTM A780, and repair paint is not galvanizing |
| Galvanizing after all of it | Coating goes on last, so cut edges, hole walls, welds and hollow interiors are all coated | — |
There is a real case for it, and pretending otherwise would be selling.
Any one of these on its own settles it.
Where this argument is being had most often in India right now.
Module mounting structures are the live case, because the two routes really do compete there. Pre-galvanized sections arrive coated and are bolted, not welded, which removes the strongest objection. Three things still decide it:
The honest answer: pre-galvanized is defensible on an inland, low-humidity, bolted, fully cut-and-capped design where the developer has accepted the edge condition in writing. On a coastal or high-humidity site, or where the O&M contract carries a corrosion warranty, galvanizing after fabrication is the cheaper decision over the life of the plant. See solar module mounting structures.
Repair is a real option. It is not an equivalent one, and the standards say so.
Five lines that prevent the argument at inspection.
Stated plainly, including the part we do not do.
We galvanize after fabrication. Process to IS 2629, coating mass to IS 4759, standard coating range 75–85 microns, and where a specification calls for 610 g/m² we galvanize to the equivalent thickness at the top of that range or above and certify the mass rather than only the gauge reading. Our kettle is 7.0 × 1.2 × 1.6 m — see double dipping and kettle size if your member is longer than that.
We do not roll-form pre-galvanized coil and we do not sell GI sheet. That means we have no commercial reason to talk you out of it — so when a part is genuinely a bend-only, indoor, thin-gauge job, the honest answer is that it does not need us.
Ten questions, answered the way we would answer them on the phone.
Both are hot dip processes, but they are applied at different points. Pre-galvanized steel passes through molten zinc as flat coil on a continuous line, before it is made into anything. Batch hot dip galvanizing dips the finished fabricated part. The coating chemistry is similar; what differs is how much zinc there is and, far more importantly, which surfaces have any.
On a typical IS 277 class 275 sheet, roughly 19 microns on each face, because the 275 g/m² figure counts both sides together. IS 4759 asks for 610 g/m² per surface on steel over 5 mm thick, which is about 85 microns. So the batch coating is around four times heavier on the face you can see, and it exists on faces the coil process never reached at all.
Yes, but only over a short distance and only for a while. Zinc is anodic to steel, so it corrodes preferentially and throws protection across a small gap. The distance it can bridge depends on coating thickness and how conductive the surface moisture is. It works well on thin sheet in a mild atmosphere. It does not indefinitely protect the sawn end of a thick section standing outdoors.
You can, and IS 2629 and ASTM A780 both describe how — clean the area and apply zinc-rich paint, zinc solder or sprayed zinc to at least the thickness of the surrounding coating. Understand what you are buying: a repair coat has no iron-zinc alloy layer and no metallurgical bond, so it will not last as long as the coating around it. Both standards frame repair as a remedy for incidental damage, not as a coating method for every weld on a structure.
No. The heaviest common class in IS 277 is 600 g/m², and that is the total across both faces — about 300 g/m² per face. A 610 g/m² per-surface requirement is a batch galvanizing requirement and can only be met by galvanizing the finished article.
It depends which tube. Tube made from pre-galvanized coil is formed from already-coated strip and has a bare internal weld seam. Tube galvanized to IS 4736 is dipped after it is made, so it is coated inside and out — that standard asks for a minimum 360 g/m² taken over the inside plus outside area, and four one-minute dips in the IS 2633 copper sulphate test. If the internal surface matters, ask which one you are being offered.
Per kilogram of steel, usually yes, and on a bend-only indoor part that saving is real. On a cut, drilled or welded outdoor part the comparison is not like for like, because you are buying a coating that will not be present where the part will corrode. The cost to compare is the coating you still have at the cut edge and the bolt hole, not the price of the coil.
Yes. The existing zinc is removed in our acid pickle along with mill scale, and the part is then galvanized normally. There is no benefit to buying pre-galvanized material if it is going to be batch galvanized afterwards — you would be paying for a coating that is dissolved before the dip.
It depends on the site and on how the design handles cut ends. Bolted, inland, low-humidity designs with capped or concealed ends can be defensible in pre-galvanized material. Coastal or high-humidity sites, and any contract carrying a corrosion warranty over a 25-year plant life, point to galvanizing after fabrication — because the coating at the cut end and the bolt hole is what determines when repair starts, and that is exactly where pre-galvanized material has none.
No. We galvanize after fabrication and we do not roll-form pre-galvanized coil, so we have no stake in the answer. If your part is thin-gauge, bend-only and lives under cover, pre-galvanized material is probably the right call and we will tell you so.