What to do when the steel will never fit in a kettle — how thermal sprayed zinc compares on bond, thickness, coverage and life, and where each one genuinely wins.
Hot dip galvanizing is an immersion process, so the bath is a hard size limit. Metallizing sprays molten zinc or aluminium onto a blast-cleaned surface, so it has no size limit at all — it can be done on a fabrication too big for any kettle, or on a structure already standing in the field.
That single difference explains most of the decision. Where a part fits in a kettle, galvanizing gives a metallurgically bonded coating that reaches inside hollows, into threads and around blind corners that no spray gun can see. Where it does not fit, metallizing is usually the only route left that is still a zinc coating rather than a paint.
Indo East does not metallize. We hot dip galvanize, in a kettle 7.0 m long × 1.2 m wide × 1.6 m deep. This page exists because the honest answer to some enquiries is that the piece belongs somewhere else, and it is better said before the steel is made than after.
Nine lines. Read the bond row and the coverage row first — they decide more cases than thickness does.
| Hot dip galvanizing | Metallizing (thermal sprayed zinc) | |
|---|---|---|
| How the coating attaches | Metallurgical — iron-zinc alloy layers grown into the steel itself | Mechanical — molten droplets keyed into a blast profile |
| Size limit | The kettle. Ours is 7.0 × 1.2 × 1.6 m | None. Shop or site, any size, already-erected steel included |
| Typical thickness | 85 microns on sections over 5 mm (IS 4759, 610 g/m²) | 100–250 microns depending on environment and design life (IS 5905) |
| Coverage | Everywhere the zinc flows — inside hollows, hole walls, threads, overlaps, blind corners | Line of sight only. Nothing the gun cannot see is coated |
| Surface preparation | Degrease, acid pickle, flux — chemical | Abrasive blast to a specified cleanliness and profile (IS 9954 abrasives) — mechanical, and the whole coating depends on it |
| Coating structure | Dense and continuous | Lamellar and porous by nature; normally sealed to close the porosity |
| Heat into the steel | The whole part reaches 450–460 °C — distortion is a real design consideration | Essentially cold. No thermal distortion |
| Repair in the field | Zinc-rich paint, zinc solder or sprayed zinc, to IS 2629 or ASTM A780 | Spray again. Repair uses the same process as the original coating |
| Cost driver | Weight and handling — priced per tonne | Surface area, access and labour — priced per square metre |
Thickness conversion uses 7.14 g/m² of zinc per micron. The IS 5905 figures are nominal thicknesses for a stated design life and exposure, not minimum coating masses, so the two standards are not comparable line for line — which is exactly why a specification should name one of them and stop.
The Indian standard for sprayed coatings, and the numbers people quote from it.
Internationally the same ground is covered by EN ISO 2063 for thermal spraying of zinc, aluminium and their alloys, and by the joint NACE No. 12 / AWS C2.23M / SSPC-CS 23.00 specification for application. If you are writing an export specification, name one of those rather than describing the process in your own words.
Four cases. In the first one it is not a preference, it is the only option.
Five cases, and the first two are the ones a spray gun can never answer.
It is rarely one or the other across a whole structure.
The usual arrangement on a big fabrication is to galvanize everything that fits and spray only what does not: sub-assemblies, cleats, brackets, hardware and hollow members go through the kettle, and the oversized welded members and the field joints made after erection are metallized. A sealer or a compatible paint system over both gives a consistent appearance and a duplex life.
Two things make this work in practice. Decide the split at drawing stage, not at the paint shop — it changes how members are broken down and where the splices go. And write one coating schedule that says which member gets which treatment, so the inspector is not deciding it for you.
If a member is only slightly over kettle length, splitting it into two bolted sections is almost always cheaper than changing coating system for the whole structure. See double dipping and kettle size.
Six questions. Three of them are about the steel, not the coating.
Said plainly so nobody discovers it late.
We hot dip galvanize, to IS 2629 for process and IS 4759 for coating mass, standard range 75–85 microns, kettle 7.0 × 1.2 × 1.6 m at 450–460 °C. We fabricate, we galvanize what we fabricate, and we take galvanizing job work from other fabricators.
We do not offer metallizing, and we are not going to pretend that a second dip solves a member that is three metres too long. If your fabrication is beyond our bath, the useful things we can still do are tell you so early, help break the design down into pieces that do fit, and galvanize those — and if the whole job belongs with a sprayer, we will say that too.
Ten questions, answered the way we would answer them on the phone.
Metallizing, or thermal spraying, feeds zinc or aluminium wire into a flame or electric arc gun, melts it, and blows the molten droplets onto a blast-cleaned steel surface with compressed air. The droplets flatten and key into the roughened profile, building up a coating layer by layer. In India it is covered by IS 5905; internationally by EN ISO 2063 and by the joint NACE No. 12 / AWS C2.23M / SSPC-CS 23.00 application specification.
Neither is better in general; they fail in different places. Metallizing can be thicker, has no size limit and puts no heat into the steel. Galvanizing bonds metallurgically and coats every surface the zinc reaches, including the inside of hollow sections and the walls of bolt holes, which a spray gun cannot see. Choose by geometry and access first, then by thickness.
IS 5905 gives nominal sprayed thicknesses of roughly 100 to 250 microns depending on exposure and whether you are specifying its twenty-year or ten-year grade. IS 4759 asks for 610 g/m² on structural steel over 5 mm thick, which is about 85 microns. Sprayed coatings are usually thicker — partly because they are porous and partly because the process allows it.
No, beyond the first short distance from the opening. Thermal spray is a line-of-sight process. If internal protection matters, the member has to be galvanized, or designed so the interior is sealed and does not need coating at all.
Essentially no. The steel stays close to ambient temperature, which is why metallizing is chosen for long slender members, thin sections and anything with tolerances that would be lost at 450 degrees. Distortion is a hot dip galvanizing consideration, not a thermal spray one.
It usually needs sealing. A sprayed coating is lamellar and porous by nature, and IS 5905 covers sealing and finishing coats applied for additional protection, requiring that they go on without delay after spraying. A sealer or a compatible paint over the spray is normal practice, not an upgrade.
They are priced on different things, which is why quotations are hard to compare. Galvanizing is priced on weight and handling; metallizing is priced on surface area, access and labour. For repeating parts and anything of modest size, galvanizing is normally the cheaper route by a clear margin. For a single very large fabrication, the comparison can go either way — and for an erected structure there is no comparison to make.
In order: split it into bolted or site-welded sections that fit, which is almost always cheapest; double dip it, if the geometry tolerates the overlap band and the distortion risk; take it to a works with a longer bath; or change coating system to metallizing. We will tell you which of those applies to your drawing before you fabricate, not after.
No. Those standards describe a coating produced by immersion in molten zinc, and a sprayed coating does not meet them however thick it is. Specify IS 5905, EN ISO 2063 or the NACE No. 12 joint standard for sprayed work, and keep the two coating schedules separate on the drawing.
No. We hot dip galvanize, in a kettle 7.0 m long by 1.2 m wide by 1.6 m deep. If your fabrication does not fit, we would rather say so at enquiry stage and help you break the design into pieces that do, than take the order and discover it at the bath.