Key takeaways
- Hot-dip galvanising to ISO 1461 puts 45–85 µm per face on fabricated steel, so a 10 mm hole closes to 9.83 mm and every mating dimension has to be re-drawn.
- Zinc plating at 8–12 µm adds 10–24 µm on a diameter, which fits most clearance holes, but 8 µm of zinc on C4 duty is gone in about three years at a 3 µm/year loss rate.
- Coating mass is the honest comparison: 85 µm of zinc is 607 g/m², roughly ten times the 57 g/m² in an 8 µm electroplated film.
- Galvanised sheet to a coating-mass designation and hot-dip galvanising of a fabricated part are different products; a drawing that says galvanise has to name which one.
A 10 mm clearance hole is a 10 mm hole until it is galvanised. Then it is 9.83 mm, and the M10 bolt that was specified with 0.3 mm of clearance now has 0.13 mm, or none if the coating runs heavy on one side. Nothing about the drawing changed. The process did.
Zinc plating versus galvanising is usually argued as a corrosion question, and it is really a tolerance and geometry question with a corrosion answer attached. Electroplating adds 5–25 µm per face. Hot-dip galvanising adds 45–85 µm per face and heats the part to about 450 °C on the way. One route fits a thread; the other does not fit a thread without overtapping. One protects a bracket for two years in coastal air; the other protects a structural frame for thirty.
This article gives the coating masses, the dimensional allowance and the corrosion life for both, using a 3 mm CRCA bracket and a 6 mm fabricated frame as the worked cases. It also separates galvanised sheet from a galvanised fabrication, because a drawing that says "galvanise" without saying which is a drawing that will be quoted two different ways.
Zinc plating versus galvanising: how much zinc each adds
Hot-dip galvanising to ISO 1461 deposits a minimum average of 85 µm on steel 6 mm and thicker, 70 µm on 3–6 mm, 55 µm on 1.5–3 mm and 45 µm below 1.5 mm. Zinc plating to ISO 2081 is specified in thickness classes, and 5, 8, 12 and 25 µm cover most sheet-metal work.
The difference in zinc reserve is roughly ten to one. Zinc has a density of 7.14 g/cm³, so 85 µm is 607 g/m² of zinc, and 8 µm is 57 g/m². Corrosion life follows that mass, not the label on the process.
ISO 12944-2 gives the first-year zinc loss by corrosivity category: 0.1–0.7 µm/year in C2, 0.7–2.1 in C3, 2.1–4.2 in C4 and 4.2–8.4 in C5. Divide the coating thickness by the loss rate and the comparison stops being close:
- 85 µm galvanising in C4 at 3 µm/year gives about 28 years;
- 8 µm plating in C4 at 3 µm/year gives about 2.7 years;
- 12 µm plating in C3 at 1.4 µm/year gives about 8.5 years;
- 85 µm galvanising in C3 at 1.4 µm/year gives about 60 years, which is why practical experience caps the claim at 20–50 years rather than the arithmetic.
The arithmetic ignores mechanical damage and porosity in a thin plated film, but it ranks the two routes correctly, and it answers the question a buyer asks: why is the cheaper coating not equivalent?
The numbers to design against
| Parameter | Zinc plating (ISO 2081) | Hot-dip galvanising (ISO 1461) | Ganesh limit or note |
|---|---|---|---|
| Coating thickness per face | 5–25 µm | 45–85 µm by steel thickness | 85 µm minimum average at 6 mm and above |
| Coating mass | 36–180 g/m² | 325–610 g/m² | 8 µm is 57 g/m²; 85 µm is 607 g/m² |
| Dimensional change on a diameter | 10–50 µm | 90–170 µm | Allow in the drawing, not at the galvaniser |
| 10 mm hole after coating | 9.99–9.98 mm | 9.83–9.91 mm | Ream or drill oversize before dipping |
| M8 internal thread | Fits with masking or plugging | Overtap about 0.4 mm, or cut after | ISO 10684 covers galvanised fasteners |
| Process temperature | 20–60 °C | About 450 °C | Heat distortion and temper loss above 200 °C |
| Distortion risk on 1.5 mm panel | None | High | Minimum 2 mm, symmetrical sections |
| Vent and drain holes | Not required | 8–12 mm at high and low points | Closed sections without them fill or trap air |
| Salt spray, neutral | 96–500 h depending on thickness and passivation | 1,000 h and above | ISO 9227, red rust as the criterion |
| Hydrogen embrittlement risk | Yes, above 1,000 MPa tensile | Low, but acid pickling precedes the dip | Bake plated parts within 4 h at 190–220 °C |
| Coating on cut edges | Thin; plating follows the surface with little edge build | Full thickness, because it is dipped | The reason galvanising suits laser-cut outdoor parts |
| Coating inside a box section | Poor throwing power | Good, if vented | Neither coats a sealed cavity |
| Repairability on site | Patch plating not practical | Zinc-rich repair allowed | ISO 1461 permits repair above a size limit |
| Relative cost per m² | Lower | Higher | Driven by zinc mass and handling, not by process time |
The Ganesh column is short because neither process runs in-house. We fabricate, blast, pretreat and coat; zinc plating and hot-dip galvanising sit with specialist processors. That makes the coating thickness a drawing decision that we have to design around rather than a process we can tune, and it is better to settle it before the part is cut.
What each process does to the part
Electroplating is an immersion process at near room temperature. The part is cleaned, pickled, plated, passivated and dried, and it comes out with a film that follows the surface, including the inside of a tube if the geometry allows the current to throw. Thickness is controlled by current density and time, and the distribution is uneven: a sharp edge plates two to three times the face thickness, and a recess plates less.
That edge build is a mixed blessing. It gives a plated bracket a thick, sacrificial edge, which is where corrosion would otherwise start, but it also means a plated thread can bind on the crest. Barrel plating, used for small parts, gives 5–8 µm and a more even distribution than rack plating at 8–12 µm.
Hot-dip galvanising is a metallurgical process. The part is degreased, pickled, fluxed, dipped in molten zinc at about 450 °C and withdrawn, and the coating forms as a zinc–iron alloy layer under a pure zinc surface. The alloy layer is hard and tightly bonded; the free zinc on top provides the sacrificial reserve. Coating thickness depends on steel chemistry and thickness, withdrawal speed and the silicon content of the steel, which is why ISO 1461 specifies a minimum rather than an exact figure.
The consequences follow the mechanism. Plating changes a dimension by tens of microns; galvanising changes it by tenths of a millimetre. Plating leaves a thin film at a cut edge; galvanising coats the edge at full thickness because the whole part is immersed. Plating can be done on a finished, assembled part at low temperature; galvanising cannot, because the heat distorts thin sections and destroys anything non-metallic.
Why the hole and the thread decide the route
Take the 10 mm clearance hole. Hot-dip galvanising at 70 µm per face reduces it to 9.86 mm, and a heavy run at 85 µm gives 9.83 mm. For an M10 bolt in a 10.2 mm hole, the clearance falls from 0.2 mm to 0.14 mm at best and 0.03 mm at worst. On a frame that is assembled outdoors with a wrench, that is a bolt that will not go in after the coating has aged and picked up grit.
The fix is not to drill the hole 0.3 mm oversize by guesswork. Drill it to the finished size plus twice the maximum coating thickness, and state the post-coating minimum on the drawing. A 10.34 mm hole before dipping gives 10.0 mm after 85 µm per face, with the tolerance band inside the coating allowance.
Threads are worse. An M8 6H nut has a pitch diameter tolerance of about 0.2 mm, and a 6g bolt leaves roughly 32–150 µm of clearance depending on the grade. Galvanising at 70 µm per face adds 140 µm on the diameter, which consumes the entire clearance and then some. The standard answer for hot-dip galvanised fasteners is an oversize thread, covered by ISO 10684, or tapping after galvanising. For a fabricated part with welded nuts or tapped holes, the practical routes are:
- tap oversize by 0.4 mm on the pitch diameter before dipping;
- plug the hole and tap after coating, which leaves bare steel in the thread that needs a zinc-rich touch-up;
- use a through-bolt and nut instead of a tapped hole, so the clearance can be drilled to suit.
Zinc plating does not escape the problem, it shrinks it. At 12 µm per face, an M8 thread loses 24 µm on the diameter, which is inside most 6g/6H allowances but not all of them at the tight end. Masking threads before plating is common on precision parts and costs a plug per hole; chasing after plating risks a chip.
Bearing bores and dowel fits
A H7 bore at 20 mm has a 21 µm tolerance band. A 12 µm plated film per side consumes 24 µm on the diameter, which is the whole band. Neither zinc process belongs on a bearing bore unless the bore is masked or machined after coating. This is the point where a designer who has been thinking about corrosion has to switch to thinking about fits, and the drawing has to show both the pre-coat and post-coat dimension.
Distortion comes from the 450 °C bath
A hot-dip galvanised part is heated to about 450 °C, held there long enough to reach temperature, and cooled in air or quenched. A 1.5 mm panel with a stiffener welded to one side expands and contracts at different rates through the section, and it comes out bowed. The coating is not the cause; the thermal cycle and the asymmetry are.
Four design moves reduce it:
- Keep the section symmetrical. A stiffener on one face and nothing on the other will bow. Two stiffeners, or a closed box, resist the cycle.
- Stay above 2 mm where the part is a flat panel. Below that, the panel has too little stiffness to hold its shape through the dip.
- Avoid long unsupported spans. A 1,200 mm panel with no return flange will move; a 20 mm return along each edge holds it.
- Let the galvaniser hang it correctly. Parts hung from one point sag; parts hung from two points with the long axis vertical drain better and distort less.
Put the allowance on the drawing as a flatness callout. On a visible canopy panel, galvanising is usually the wrong route.
Hydrogen embrittlement is the plating exception
Acid pickling and electroplating generate hydrogen at the steel surface, and high-strength steel absorbs it. Above about 1,000 MPa tensile strength — spring steel, hardened washers, high-strength fasteners — the absorbed hydrogen can cause delayed cracking hours or days after plating. The control is a bake at 190–220 °C for four hours or more, started within four hours of plating, and it is not optional on those grades.
Hot-dip galvanising carries less hydrogen risk than electroplating, but the acid pickle before the dip still generates it. The same grades need the same conversation with the processor.
For sheet-metal parts in CRCA at 270–350 MPa tensile, neither route carries a realistic embrittlement risk. The warning matters for the brackets, springs and fasteners that get specified alongside the sheet-metal parts.
The third option: galvanised sheet
A part can start life already coated. Zinc-coated sheet to a coating-mass designation, typically 275 g/m² total or about 20 µm per side, is cut, formed and welded without a coating operation. It is the right material for a genset canopy panel or an electrical enclosure that will be painted, because the zinc under the paint provides sacrificial protection at scratches.
Its weakness is the cut edge. Laser cutting, punching and shearing all expose bare steel, and the zinc around the cut protects only a short distance. A 1.5 mm cut edge on a C4 site shows red rust within a year unless the edge is radiused, coated and, where the duty is severe, touched up with a zinc-rich primer before the topcoat.
Galvanised sheet also behaves differently under a laser and a welder. The zinc boils at 907 °C, below the steel melting point, so laser cutting produces a rougher edge and more dross than CRCA, and welding burns the coating back 5–10 mm on each side, leaving a zone that must be cleaned and recoated. Neither is a reason to avoid the material; both are reasons to design the joint and the edge before the part is cut.
What Ganesh actually runs
Parts here are cut on the 12 kW fibre laser, formed on the 200 T brake, welded on three CLOOS robotic cells or the manual stations, blasted in the 16 ft × 16 ft chamber and coated on the seven-tank pretreatment line with powder or wet paint. That route handles CRCA and galvanised sheet, including the edge preparation that a galvanised part needs before painting.
What it does not include is a plating line or a galvanising kettle. Both are specified processes that change the drawing before we can quote, and the useful thing we can do is flag the conflict early: a 10 mm hole called out on a part that will be galvanised, a thread that will not accept a bolt after coating, a 1.5 mm panel that will bow in the dip. Those are the enquiries where a manufacturing review earns its fee before any metal is cut.
The shot blasting page covers the preparation step, and the surface treatment hub indexes the coating articles. Two that pair with this one: design for galvanising on vents, drains and distortion, and galvanised steel corrosion and finish compatibility on what happens when paint meets zinc.
What to put on the drawing
- The standard and the coating mass or thickness, not the word "galvanised". ISO 1461 with a minimum average, or ISO 2081 with a class.
- Which process, if the part could take either, and the reason: cavity coverage, edge protection, tolerance or appearance.
- Pre-coat and post-coat dimensions for every hole, bore and thread that has a fit.
- Thread treatment: oversize tap, plug and chase, or cut after coating, with the touch-up specified.
- Vent and drain holes, 8–12 mm, at the highest and lowest point of every closed section.
- Flatness and distortion allowance, as a measurable callout.
- Repair acceptance, including the maximum area that may be repaired with a zinc-rich product and the method.
- Any subsequent coating, with the pretreatment route, because powder over fresh galvanising needs a conversion layer and a vented part.
A drawing that carries those eight items can be quoted without a clarification loop. Send it to request a quote and the fabrication route can be priced with the coating allowance built in rather than added after the first part fails to fit.
When neither zinc route is right
Stainless steel. 304 or 316 needs no zinc. Adding a coating to stainless is a cost with no corrosion benefit, and the pretreatment that suits steel can contaminate the stainless surface.
Aluminium. Zinc plating on aluminium is not a protective system; anodising or a conversion coating plus powder is.
Immersion or C5 duty. A splash zone, a marine deck or a chemical plant needs a duplex system, a higher-build paint system or a different alloy. Galvanising alone at 85 µm is a starting point, not a finish, and the topcoat has to be compatible with zinc.
Precision assemblies with bores and threads. If half the features need masking, the coating cost is in the masking, and a painted or stainless route is often cheaper than a plated one.
High-strength steel without a bake capability. If the processor cannot bake within four hours, do not plate the part. A delayed crack in a loaded bracket is a failure that inspection will not catch.
A 1.2 mm visible panel. It will distort in the dip and the coating will be visible on the face. Use galvanised sheet and coat it, or change the material.
Frequently asked questions
How much does hot-dip galvanising change a dimension?
Each face gains 45–85 µm depending on steel thickness, so a dimension across two coated faces grows by 90–170 µm. A 10 mm hole becomes 9.83–9.91 mm and an M8 thread needs to be cut oversize before dipping. Allow the coating in the drawing, not at the galvaniser.
Can zinc plating replace galvanising outdoors?
Only for light duty. An 8 µm electroplated film with a trivalent passivation gives 96–200 hours of neutral salt spray, while 85 µm of hot-dip zinc gives over 1,000 hours. In C4 coastal air, zinc is lost at 2.1–4.2 µm per year, so the plated film is consumed in a few years.
Why do thin panels distort when galvanised?
The part is dipped in molten zinc at about 450 °C, and a 1.5 mm panel with an asymmetric stiffener heats and cools unevenly. Distortion follows the section, not the coating. Symmetrical sections, balanced stiffeners and a minimum 2 mm thickness reduce it; below that, zinc plating or a coated route is the better answer.
Do galvanised parts need vent and drain holes?
Yes. Air trapped in a closed section prevents zinc from reaching the inside, and molten zinc trapped inside can blow out as the part cools. An 8–12 mm hole at the highest and lowest point of every closed section, plus a 3 mm minimum gap at overlaps, is the usual rule.
Is galvanised sheet the same as a galvanised fabrication?
No. Galvanised sheet carries a factory-applied coating of a specified mass per square metre, typically 20 µm per side, and is cut and formed afterwards, leaving bare steel at every cut edge. A hot-dip galvanised fabrication to ISO 1461 has 45–85 µm and a coating on every surface including cut edges, at the cost of dimensional change and heat distortion.
Does powder coating over galvanising work?
It can, as a duplex system, and it extends life by roughly 1.5–2.5 times the sum of the two. It needs the zinc surface to be properly pretreated and the part to be vented, because trapped air and zinc salts at 190 °C blister the powder. We coat galvanised sheet in-house and would rather discuss the edge condition first.
Sources and further reading
- ISO 1461:2022 — Hot dip galvanized coatings on fabricated iron and steel articles: minimum coating thickness and mass by steel thickness · reviewed 3 Nov 2026
- ISO 2081:2018 — Electroplated coatings of zinc with supplementary treatments on iron or steel, including coating thickness classes · reviewed 3 Nov 2026
- ISO 12944-2:2017 — Corrosion protection of steel structures: corrosivity categories and first-year zinc loss rates · reviewed 3 Nov 2026
Not sure which zinc route the part can take?
Send the drawing with the environment and the mating dimensions. We will flag where coating thickness changes a fit, and quote the fabrication route around it.
Request a quote