How long a piece can be galvanized in one immersion, what happens to the steel that is longer than the kettle, and how to design a member so the question never arises.
The longest piece a galvanizer can coat in one immersion is set by the internal dimensions of the kettle — and a piece can be dipped diagonally, so the usable length is the kettle’s diagonal rather than its length. Anything longer than that has to be double dipped: one end is immersed, withdrawn, the piece is turned end for end, and the other end is immersed to complete the coating.
Double dipping is a normal, accepted practice. It is also the single thing most likely to change how a finished member looks and behaves, and it is the reason kettle size decides more galvanizing enquiries than price does.
Indo East’s kettle is 7.0 m long × 1.2 m wide × 1.6 m deep, working at 450–460 °C. Members up to about 7 metres go in on one dip. Longer members are double dipped, and we say so before quoting rather than after the steel is in the zinc.
The most common misreading of a galvanizer’s capacity.
| Kettle dimension | What it limits | Can double dipping get around it? |
|---|---|---|
| Length | The longest member that fits in one immersion | Yes. Progressive dipping roughly doubles the treatable length, provided at least half the piece fits in the bath |
| Width | The widest assembly — a portal frame, a wide bracket, a bolted sub-assembly | No. A piece wider than the kettle cannot be dipped at all, at any number of dips |
| Depth | How much of the piece is submerged in each pass, and therefore the practical dipping angle | No. Depth is a hard ceiling and it is what makes the diagonal work |
This is why “how long is your kettle?” is only the first of three questions. A 12 metre kettle that is 1 metre wide will refuse an assembly a 7 metre kettle 1.2 metres wide accepts.
Four consequences. None of them is a defect, and all four are worth knowing before you specify.
In order of how much they help. The first one solves the problem outright.
| Approach | What it means in practice | Worth it when |
|---|---|---|
| Split the member | Design the long member as two or more sections within kettle length, joined by a bolted splice or a site weld after galvanizing | Almost always, on anything you are still free to redesign. A splice plate costs less than a distorted member |
| Design the vent and drain holes for speed | Generous, corner-located openings let the piece go in and come out faster, which shortens the time the two halves spend at different temperatures | Every time. See vent and drain hole design |
| Brace it temporarily | Temporary stiffeners or bracing across the section, removed after galvanizing | Slender or asymmetric sections that would otherwise warp |
| Agree the lifting orientation first | Which end goes in, at what angle, on what jig — settled between you and the galvanizer, not improvised at the kettle | Any double dip. It is the cheapest risk control available |
| Change the coating | Thermal spray zinc, or hot dip galvanizing the sub-assemblies and metallising the field joints | Genuinely oversized fabrications that cannot be split |
Worth knowing before an inspector is standing at your consignment.
Five questions. The first three eliminate most vendors before price is discussed.
7.0 m × 1.2 m × 1.6 m, 450–460 °C, in our own kettle at Jangalpur. Members up to about 7 metres are galvanized in a single immersion to IS 2629, IS 4759 and ASTM A123 at 75–85 microns, read on a calibrated dry film thickness gauge, batch by batch.
Longer members are double dipped. We raise it when we read the drawing, not when the steel is in the zinc, and we will tell you what the overlap will look like and where it will fall on your member before you place the order.
And the part most suppliers leave out: if your member has to be single dipped and it is longer than our kettle, we are the wrong works for that piece and we will say so. There are kettles in this country at 12 metres and more. Knowing which question you are actually asking is worth more to you than winning the enquiry is to us.
Because the galvanizing is on our own floor rather than sub-contracted, the dipping decision and the coating record come from the same works — see in-house versus sub-contracted galvanizing.
Answered from our own kettle, not from a brochure.
Double dipping, also called progressive dipping, is galvanizing a piece that is longer than the kettle by immersing it in two passes. One end goes in, the piece is withdrawn and turned end for end, and the other end is immersed to complete the coating. It roughly doubles the length a kettle can handle, provided at least half the piece fits in the bath at a time.
About 7 metres. Our kettle is 7.0 metres long by 1.2 metres wide by 1.6 metres deep and works at 450 to 460 degrees Celsius. A piece can be dipped at an angle, so the usable length is the kettle's diagonal rather than its flat length, but 7 metres is the figure to design to. Longer members are double dipped and we raise that before quoting.
No, but it changes two things. The overlap band where the two immersions meet carries heavier coating, and very heavy coating is more brittle and can flake under impact. And the piece is heated and cooled unevenly, which introduces thermal stress and can distort slender or asymmetric sections. The steel itself is not weakened; the risk is dimensional and cosmetic.
Yes, at first. Where the two dips meet there is a darker band of heavier zinc. It is cosmetic and it fades as the whole surface weathers to a uniform matt grey. Where it matters is architecturally exposed steel, and where it falls on a faying surface or a bolted connection it may need buffing back so the joint closes properly.
No. Double dipping extends length only. Width and depth are hard limits set by the kettle, and a piece wider than the bath cannot be galvanized in that kettle at any number of dips. This is why the width of an assembly, not just the length of a member, belongs in the enquiry.
None of them prohibits it. The coating requirement applies to the finished member as a whole, including the overlap, and the number of immersions is a process decision rather than a compliance one. Coating thickness figures in these standards are minimums with no maximum, so the heavier coating in the overlap band does not fail on thickness. Appearance and adherence requirements still apply.
Design the long member as two or more sections within the kettle length and join them with a bolted splice or a site weld after galvanizing. A splice plate is cheaper than a distorted member and it removes the overlap band, the thermal gradient and the extra handling in one decision. Where the member cannot be split, generous corner-located vent and drain holes and temporary bracing reduce the distortion risk.
Whoever agreed to it in writing, which is why it should be settled before the order. Distortion in double dipping is a known and predictable risk on long, slender, thin-walled and asymmetric sections, and it is the most common dispute in job-work galvanizing. We raise it at drawing stage and agree the lifting orientation and jigging before the piece reaches the kettle.
No, and we will tell you so rather than take the order. Our kettle is 7 metres and a 10 metre member would have to be double dipped here. There are kettles in India at 12 metres and beyond, and if a single immersion is a hard requirement for your piece, that is where it should go. If the member can be split, or if a double dip is acceptable once you have seen where the overlap falls, we are glad to quote.
No. Kettle length decides what will fit; it says nothing about pre-treatment, coating control, testing or documentation. A larger bath is the right filter when your members are long, and the wrong one when they are not. For pieces inside our kettle the questions that actually separate galvanizers are whether the galvanizing is in-house or sub-contracted, what coating figure is certified, and on what instrument it is read.
We will tell you whether it goes in on one dip, what a second dip would do to it, and whether splitting it is the better answer — before there is a quotation on the table. A formal offer typically goes back within 48 business hours.
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