Key takeaways
- Vent total area must be at least 25% of a hollow section's internal cross-section, with a 12 mm minimum hole; a 40 × 40 × 2 mm tube needs about a 20 mm vent at each end.
- A 2 m steel frame expands about 10.4 mm between 25 °C and 460 °C, and the part that heats unevenly is the part that bows, so thin sheet needs stiffeners or a different finish.
- ISO 1461 sets 85 µm average and 70 µm local minimum for steel 6 mm and thicker, falling to 45/35 µm below 1.5 mm; the coating adds to every hole and thread.
- Leave a 2 mm gap between overlapping surfaces or seal the joint continuously; a tighter gap traps acid that weeps out weeks after the part is installed.
A 2.5 mm canopy frame arrives at the galvaniser with a sealed 40 × 40 mm tube across its top rail. The galvaniser has two choices: drill his own 12 mm holes and coat it, or reject it. If he drills, the holes are in the wrong place and the customer sees them. If he coats it sealed, the air inside expands at 450 °C and finds the weakest weld.
Design for galvanising is the discipline of letting air out, letting zinc in, letting zinc out, and accepting that the part will be at 450 °C while all of that happens. Get the vents right and the coating is uniform. Get the heat right and the part stays flat.
This article covers sheet and light fabrication from 1.5 mm to 10 mm. Ganesh does not hot-dip galvanise in-house: we cut, form and weld the part, and galvanising sits downstream as a separate operation. What we control is the geometry that decides whether the galvaniser can coat it.
What does design for galvanising have to solve?
Four things, in this order: air must escape, zinc must reach every surface, zinc must drain, and the part must survive 445–460 °C without losing its shape.
The first three are hole placement and hole size. The fourth is section design, weld sequence and gauge. A part that passes the first three and fails the fourth comes back coated and bent, and no amount of touch-up fixes a frame that no longer fits.
The corrosivity class sets how much coating the part needs, but it does not change the venting. ISO 12944-2 classifies the environment from C1 indoors to C5 coastal or chemical, and a C4 canopy and a C2 canopy need the same holes. The class changes the coating thickness and whether a duplex system follows.
Vent and drain sizing
Size the total vent area at 25% of the internal cross-section, with a 12 mm minimum hole and at least one hole at each end of a hollow member.
The arithmetic is simple. A 40 × 40 × 2 mm tube has a 36 × 36 mm bore, or 1,296 mm². A quarter of that is 324 mm², which is a 20.3 mm hole. A 25 × 25 × 2 mm tube has a 21 × 21 mm bore, 441 mm², and needs 110 mm², or a 12 mm hole. That is where the 12 mm rule comes from: it is the practical minimum for the smallest common section, not a universal answer.
On a 100 × 100 × 3 mm section the bore is 8,836 mm² and the vent area is 2,209 mm². One 53 mm hole is awkward to cut and weakens the section, so use two 40 mm holes at opposite corners. Total area is what matters; a single large hole is not better than two smaller ones.
Vent at the high point, drain at the low point, and put them diagonally opposite so the zinc has to travel through the whole cavity. Holes on the same end let the zinc short-circuit across the top and leave the far end uncoated.
Why the part distorts
Steel expands about 12 µm per metre per degree Celsius. A 2 m frame heated from 25 °C to 460 °C grows 10.4 mm. If every part of the frame grew together, that would be harmless: the whole part would be 10.4 mm longer and come back to size on cooling. It does not grow together.
A 2.5 mm sheet heats through in seconds; a 6 mm boss or a welded pad takes longer. The sheet expands first, pushes against the colder thick section, and buckles where it is unsupported. The zinc freezes the buckle in place. The same thing happens on cooling, in reverse, and the residual stress stays in the part.
The stress behind that is not small. If a 1 m width is fully restrained, the thermal stress is EαΔT: 210,000 MPa × 12 × 10⁻⁶ × 435 K, or about 1,096 MPa. That is four times the yield strength of mild steel, so the part cannot stay restrained. It buckles, and the buckle is what the galvaniser sees when the part comes out.
The practical rules follow from that:
- keep large unsupported panels above 3 mm, or add a stiffener;
- avoid a flat panel larger than about 0.5 m² with no break;
- balance the weld sequence around the neutral axis;
- stress-relieve heavily restrained weldments before galvanising;
- expect 1–3 mm of bow per metre on 2–3 mm sheet and design slotted holes or shims for it.
A frame that is 2 m long and welded on one side only will bow towards the weld. That is not a galvanising defect; it is a welding sequence that the bath reveals.
What the bath does to the surface
Zinc drains before it freezes, and where it drains slowly the coating is thicker. A horizontal face can collect 100–150 µm, a vertical face 60–90 µm, and a drain hole leaves a tear or a spike that has to be ground. Hang the part at 30–45° so the zinc runs to a corner, and put the drain at the lowest point of that corner rather than in the middle of a face.
The same drainage decides where the coating is thin. A surface shielded by another part, or a pocket that fills and empties slowly, comes back at 30–50 µm — under the ISO 1461 local minimum even though the rest of the part passes. Jig the part so every face is exposed, and do not let two parts touch in the bath: the contact area stays bare, and no thickness gauge will find it.
Contact marks are a drawing decision. If a part is hung from a hole, that hole is bare on the inside. Nominate the hanging point on the drawing, or accept a zinc-rich touch-up applied by the galvaniser. Neither is a defect if it is specified before the part is made.
Coating thickness and what it does to holes and threads
ISO 1461 sets the coating mass and thickness by steel thickness, because thick steel holds more zinc in the alloy layer. The minimums are average and local, and both matter.
| Parameter | ISO 1461 requirement | What it does to the part | Ganesh limit or note |
|---|---|---|---|
| Steel ≥ 6 mm | 85 µm average, 70 µm local | Hole shrinks 0.14–0.17 mm on diameter | Fabricated up to 40 mm |
| Steel 3–6 mm | 70 µm average, 55 µm local | Hole shrinks 0.11–0.14 mm | Common bracket and frame gauge |
| Steel 1.5–3 mm | 55 µm average, 45 µm local | Hole shrinks 0.09–0.11 mm; distortion risk rises | Stiffeners or a different finish below 3 mm |
| Steel < 1.5 mm | 45 µm average, 35 µm local | Coating is thin and uneven; warping likely | We would rather not galvanise large thin panels |
| Vent area, hollow section | 25% of internal area | Below this, the interior coats unevenly | 12 mm minimum hole |
| Overlap gap | 2 mm minimum | Below 2 mm, acid traps and weeps | Seal weld if the gap cannot be opened |
| Thread M8 and below | Coating closes the thread | Chase or tap after coating | Specify the class after galvanising |
| Thread M10 and above | Coating adds 0.09–0.17 mm on diameter | Chase with a tap | ISO 965-4 covers galvanised external threads |
| Drain hole | At the lowest point | Zinc must leave before it freezes | Tilt the part in the jig or the bath |
| Bend radius | ≥ 1t, preferably 2t | Tight bends crack the zinc at the outside | Form before galvanising, never after |
| Salt spray, duplex | 480–720 h | Coating life depends on the zinc, not the paint | ISO 9227 neutral salt spray |
The Ganesh column is about fabrication, not the bath. We cut and form from 0.8 mm to 40 mm, weld the frames, and can hold the ±0.5 mm general sheet tolerance before galvanising. We cannot hold that tolerance after the bath on a thin part, because the distortion happens downstream. If the assembly tolerance is tight, the drawing should say which features are machined or drilled after galvanising.
Holes, threads and clearance
Every hole loses 2× the coating thickness on its diameter. A 10 mm fastener hole coated at 70 µm per surface becomes 9.86 mm. That is still a clearance hole for an M8 bolt, but it is no longer a locating hole for a 10 mm pin.
Threads are the harder case. A galvanised M8 bolt will not run into an M8 nut that was tapped before coating, because the zinc on the bolt adds 0.09–0.17 mm to the pitch and major diameters. ISO 965-4 defines tolerances for galvanised external threads intended to mate with internal threads tapped to position H or G after coating. The simpler production route is to tap or chase after galvanising, and to say so on the drawing.
Overlapping surfaces
Two sheets laid back to back with a 0.5 mm gap are a capillary. Acid from the pickling stage enters, the zinc cannot reach the surface, and the acid weeps out of the seam months later as a rust stain. Either open the gap to 2 mm so both surfaces coat, or close it completely with a continuous seal weld so nothing enters.
Spot welds in the middle of an overlap do not seal it. A plug weld leaves an annular gap around the plug unless the weld is continuous. If the design needs an overlap for stiffness, a 2 mm gap plus a structural adhesive after galvanising is cleaner than a trapped void.
What Ganesh runs
The route here is cut, form, weld, then galvanise outside. The 12 kW fibre laser cuts up to 20 mm, the 250 T press punches hole-intensive panels, and the 200 T, 4,450 mm brake forms sections to 10 mm. Frames are welded in three CLOOS robotic cells or at the 30+ manual stations, with fixtures built in-house.
That matters for galvanising because the weld sequence and fixture strategy set the residual stress that the bath releases. The welded assemblies page covers the cell mix, and welded genset base frames shows the kind of heavy frame where distortion control is decided before the weld starts.
Genset base frames, canopy frames and elevator components for Mahindra Powerol, TKE and Indian Railways pass through this fabrication route. Where the finish is galvanising plus powder, the paint shop page covers the powder side of the duplex system, and degreasing galvanised steel before coating covers the preparation that has to happen between the two.
What to put on the drawing
- Galvanising standard and coating thickness: ISO 1461, with the average and local minimum for the steel gauge.
- Vent and drain locations, diameters and total area, with a note that they stay open after coating.
- Overlap condition: 2 mm gap or continuous seal weld, marked on the section.
- Thread callout with the class after galvanising, and which threads are chased.
- Features machined or drilled after galvanising, with the datum they are located from.
- Bend radius of at least 1t, and a note that all forming happens before coating.
- Distortion allowance and the flatness limit if the part is a panel, not a frame.
- Duplex system, if powder follows, including the preparation between zinc and powder.
The design and CAD hub indexes the rest of the drawing notes. For the failure that follows a missed preparation step, why powder coating fails on outdoor enclosures starts at the corrosion site and works back to the surface.
When this route is wrong
Galvanising is wrong for a thin cosmetic panel. Under 1.5 mm, the coating is thin, the distortion is high and the finish is rough enough that it needs filler or a different process. A 1.2 mm cover is a powder-coated part, not a galvanised one.
It is also a functional finish, not a cosmetic one. The zinc is dull grey, the surface has runs and tears at the drain points, and two parts from the same batch will not match exactly. If the visible face has to be smooth and uniform, the sequence is galvanise, prepare, then powder — and the powder has to be specified for a zinc substrate.
It is wrong for a precision assembly with bores and mating faces, unless those features are machined after coating. It is wrong for a sealed box with electronics inside, because the part cannot be vented and the bath is 450 °C.
It is also wrong where the part will be painted and the paint cannot tolerate the zinc surface. A duplex system needs a preparation step between the zinc and the powder, and a powder that adheres to zinc. If the specification calls for a direct powder coat over fresh galvanising without that step, the adhesion test will fail at the first article.
Send the section, the weld map and the corrosion class to request a quote. The vent and drain review happens before fabrication, because a 20 mm hole added after welding is a repair, and a hole added after galvanising is a bare patch.
Frequently asked questions
How big should a galvanising vent hole be?
Size the total vent area at 25% of the hollow section's internal cross-section, with no single hole below 12 mm. A 40 × 40 × 2 mm tube has 1,296 mm² internally and needs about 324 mm² of vent; a 20 mm hole at each end gives 628 mm² and covers the flow requirement with margin. Bigger sections usually need two holes rather than one very large one.
Why does hot dip galvanising distort thin sheet?
The bath runs at 445–460 °C, so a 2 m part grows about 10 mm before it is coated. Thin sheet heats faster than thick sections, expands against them and buckles; the zinc then freezes the shape. Under 3 mm, large unsupported panels are the highest risk, and stiffeners or a different coating are the usual answers.
Do galvanised threads need to be tapped after coating?
Usually yes for anything below M10. The zinc adds 45–85 µm per surface, so an M8 nut will not run onto a coated bolt without chasing. ISO 965-4 exists for galvanised external threads designed to mate with H or G internal threads after coating; the simpler route is to tap or chase after galvanising and specify the class.
Can I galvanise a sealed hollow section?
No. A sealed section traps air, and the air expands violently at 450 °C. The part either blows zinc out of the vent it does not have or comes back with an uncoated interior. Vent at the high point and drain at the low point, with both holes left open in the finished part.
What is the minimum thickness for hot dip galvanising?
ISO 1461 covers fabricated steel from thin sheet upward, but the distortion risk rises sharply below 3 mm. We fabricate from 0.8 mm, and for a large flat panel under 3 mm we would rather discuss a different finish than promise a flat part after the bath.
Sources and further reading
- ISO 1461:2022 — Hot dip galvanized coatings on fabricated iron and steel articles: specifications and test methods · reviewed 3 Nov 2026
- ISO 12944-2:2017 — Corrosion protection of steel structures: classification of environments · reviewed 3 Nov 2026
- ISO 9227:2022 — Corrosion tests in artificial atmospheres: salt spray tests · reviewed 3 Nov 2026
Have a frame that has to survive the galvanising bath?
Send the section, the weld map and the corrosion class. We will review vents, drains, overlaps and distortion risk before the part is fabricated.
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