At a glance
| Topic | What the source says | Source | What to write or ask |
|---|---|---|---|
| The standard | BS EN ISO 1461:2022, hot dip galvanized coatings on fabricated iron and steel articles. BSI lists it as Current, published 30 September 2022. | BSI Knowledge listing | Name the edition: hot dip galvanized to BS EN ISO 1461:2022. |
| Minimum coating by steel thickness | Over 6 mm: 70 µm local, 85 µm mean. Over 3 to 6 mm: 55 and 70. 1.5 to 3 mm: 45 and 55. Under 1.5 mm: 35 and 45. Castings: 70 and 80 (6 mm and over), 60 and 70 (under 6 mm). | GA, and HDG Datasheet 4a (Oct 2024) | The thickness grows with the steel; you do not choose it per piece. Thin parts distort most: ask the galvaniser. |
| Thicker than the table | Grit blasting before dipping, or a reactive (higher-silicon) steel. Agree it with the galvaniser first; the figure is a nominal target, not a guarantee. | GA; Datasheet 4a | Rarely needed. A thick coating can be dull grey and easier to damage. |
| Sealed hollow sections | Must be vented, for safety, with holes diagonally opposite and close to the ends. | GA, venting and drainage page | Draw the holes on the shop drawing. Ask the galvaniser to approve their size. |
| Overlapping welded plates | Over 100 cm² sealed by a continuous weld: vent them (overlap table in the full guide). | GA page, citing EN ISO 14713-2:2020 | Air trapped between plates can turn to steam in the bath and burst the part. |
| How long it lasts | The Galvanizers Association says most environments allow a maintenance-free life of 60 years, and sometimes over 100. | GA, EN ISO 1461 page | A trade body's claim, not a warranty, and it depends on the environment. |
Thickness is in micrometres (µm; 1,000 µm is 1 mm). The bands are for articles that are not centrifuged. Local minimum means the lowest reading at any point; mean minimum means the average over the article.
What the standard sets, and what it leaves open
BS EN ISO 1461 is the standard for hot dip galvanized coatings on fabricated iron and steel articles. It sets the coating’s properties and the test methods. The Galvanizers Association (GA) says the thickness depends mainly on the thickness of the steel, with the surface condition and the steel’s chemistry also having an effect, and that the coating often exceeds the minimum.
| Steel thickness (articles not centrifuged) | Local minimum, µm | Mean minimum, µm |
|---|---|---|
| Over 6 mm | 70 | 85 |
| Over 3 mm, up to 6 mm | 55 | 70 |
| 1.5 mm and over, up to 3 mm | 45 | 55 |
| Under 1.5 mm | 35 | 45 |
| Castings, 6 mm and over | 70 | 80 |
| Castings, under 6 mm | 60 | 70 |
Four points follow for a specifier.
- Name the standard and the edition. GA’s datasheet 4a (October 2024) says that for the vast majority of applications galvanizing can be specified simply as hot dip galvanized to EN ISO 1461:2022. In the UK the standard is published by BSI, which lists the 2022 edition as current.
- Check what the standard does not cover. BSI’s listing excludes continuously galvanized sheet, wire and mesh, tube and pipe galvanized in automated plants, and products such as fasteners that have their own standards. Those are a separate request to the supplier.
- Thicker is possible but is a decision. GA says grit blasting before dipping, or a more reactive steel, can give a thicker coating, but only for certain jobs, and the requirement should be agreed with the galvaniser before it is written. Datasheet 4a calls the thicker figures nominal: a target, not a guarantee. A coating grown entirely as zinc-iron alloy can be dull grey and more easily damaged in handling.
- Checking is simple. GA says thickness can be checked on site with a magnetic gauge without damaging the coating. Ask for readings on the approved sample.
The galvanizer normally works for the steel fabricator, so the galvanising clause must be in the specification issued to the fabricator, who passes it on (GA, EN ISO 1461 page).
Design for galvanising: vents, drains and size
The steel is lowered into zinc at 450°C and lifted out again (GA, design detailing). Molten zinc has to reach every surface and drain away again, and air has to escape. GA states the rule for hollow sections plainly: where they are sealed they must be vented, for reasons of safety.
- Hollow sections: a vent hole at each sealed end, in the end plate or the section wall, diagonally opposite one another and as near to the end (or to the wall the end plate joins) as is practical. Holes should be as large as possible. Cut corners (a V or U notch, or ground-off corners on square and rectangular tube) can replace holes if they have the same area.
- Every sealed section of a fabrication needs its own holes, including tubes inside a larger frame.
- Stiffeners, gussets and diaphragms should have their corners cropped so zinc flows past; open-top tanks need stays to limit distortion.
- Hole size: GA publishes a table of hole sizes by section, which this sheet does not reproduce. Ask the galvaniser for the size, and put it on the drawing.
- Size: the bath sets the largest piece. GA lists the plants and says the bath size is only a guide, because the real limit depends on how the piece is dipped. Over-length work can sometimes be double-dipped, only after advice from the galvaniser.
Overlapping surfaces are a second hazard. GA warns that air trapped between plates can be turned to superheated steam in the bath and cause an explosion. For overlaps larger than 100 cm² that are sealed by a continuous weld, it shows the following (from its page, which is based on EN ISO 14713-2:2020 and says further guidance is given there):
| Overlap area, a | Action the Galvanizers Association shows |
|---|---|
| a up to 100 cm² | A tight weld all round. The steel must be dry when welded, and the overlapping parts smooth and assembled without gaps. |
| Over 100, up to 1,000 cm² | In diagonally opposite corners: 2 holes of at least 12 mm, or 2 gaps of at least 25 mm in the weld. |
| Over 1,000, up to 2,500 cm² | Either 4 holes of at least 12 mm at the corners, or 4 weld gaps of at least 25 mm. |
| Over 2,500 cm² | Holes of at least 12 mm (or weld gaps of at least 25 mm) at the corners and all round at least every 300 mm from the corners. |
Large overlaps can also be vented by drilling holes at least 12 mm across at 300 mm spacing, or by welding intermittently with gaps (GA). A 100 cm² overlap is, for example, 10 cm by 10 cm, or 20 cm by 5 cm.
Distortion, dirty steel and joining
These are the points that decide whether a galvanised piece looks right when it comes out.
- Distortion. GA says it usually comes from stresses built in to the steel being released as it heats, whether inherent or put there by welding, cold forming and hole punching. It advises symmetrical welds, small welds and avoiding big changes of section. Lifting holes and vent holes affect distortion too, so agree them with the galvaniser.
- Metals you cannot galvanise. Plain carbon steel, some low-alloy steels and iron and steel castings can be galvanised. Soldered or brazed parts should not be. A fabrication of mixed steels and surfaces can galvanise unevenly, and GA suggests grit blasting the whole assembly if different materials cannot be avoided.
- Heavy cold work. Steel bent through a tight radius may suffer strain age embrittlement and should be stress relieved first (GA). A rare combination of high-strength steel, a crack starter and high residual stress can crack in the bath; GA points to a British Constructional Steelwork Association publication and says to ask it for advice. That is for the structural engineer, not the designer.
- Contamination. Grease, tar, paint and weld slag cannot be removed by the galvaniser’s chemical cleaning and leave black bare spots. Burnt-on anti-spatter spray and baked-on cutting fluid cause the same. The fabricator, not the galvaniser, is responsible for delivering clean steel, and re-galvanising costs extra (GA).
- Joining. Galvanised parts can be bolted, welded, riveted or bonded. Bolted joints are best made after galvanising; welds damage the coating locally and must be renovated (GA). Galvanised fixings against stainless, brass or copper are a corrosion risk: see galvanic corrosion and dissimilar metals.
Appearance: what to approve on a sample
BS EN ISO 1461 is a performance standard for the coating, expressed in thickness and tests. The GA pages read for this sheet do not offer an appearance grade, and the standard’s own text on appearance was not read. For steel that will be seen, agree the look on a sample, in the light of the room, and keep it with the order.
- Colour and finish vary. GA describes a coating of hard zinc-iron alloy layers under a soft outer layer of zinc, and notes that a very thick, fully alloyed coating is dull grey. How bright or dull a surface looks depends on the steel and the dip, so approve a sample of the actual steel.
- White powder after storage. If galvanised steel stays damp and cannot dry, a white powdery layer called wet storage stain forms (GA datasheet 11). Specify dry, ventilated storage and handling with spacers, not wrapped or stacked wet.
- Staining and marks. GA datasheet 16 lists dirt, timber spacers, seepage from welds, swarf and rust at damaged spots as causes of discolouration that can be mistaken for failure. Plan for repair of damaged areas before the steel is handed over.
- Visible welds. Weld dressing and grinding are done before dipping. See weld finish and visible welds for what to specify.
Painting or powder coating over galvanising
A galvanised coating and an organic coating over it together make a duplex system. GA gives three reasons to add the second coat: colour (for appearance, camouflage or safety), a longer economic life, and extra protection in very aggressive environments. If the paint is damaged, the zinc beneath still stops red rust from undercutting it, so repair is less urgent (GA datasheet 15, summary page).
GA’s advice is that every paint system must be made for galvanised steel and applied to the maker’s instructions, and that the surface is prepared by one of four recognised methods:
| Preparation before painting | What the Galvanizers Association says |
|---|---|
| T-wash (a modified zinc phosphate solution) | Generally considered the best method on new galvanising. Not for weathered galvanising. Do not let it pool on horizontal faces. Painted within 30 days, with white salts brushed off. |
| Etch primer | Used successfully, but there is no colour change to show it has reacted everywhere. Suits older, weathered galvanising. |
| Sweep blasting | Fine copper slag or similar, at no more than 40 psi (2.7 bar), to remove the minimum of oxide. Angular iron grit must not be used. |
| Weathering | Effective only after at least six months outdoors, then abrasive pad or brush, a hot detergent wash and a rinse. Not for marine environments with high chloride. |
GA adds that a few products are applied directly with no pre-treatment. It names two-pack polyurethane and acrylic urethane as common top coats, and high-build epoxy and vinyl systems as increasingly used. For powder coating over galvanising, schemes and standards exist: see the powder coating sheet. For which corrosion category a piece needs, see corrosion categories and coating durability.
Specification wording
Paste into your schedule or specification and replace anything in [square brackets] with your own figures. Wording is a starting point: your specification governs.
Steel fabrications to be hot dip galvanized to BS EN ISO 1461:2022 [by a galvaniser approved by the project], after fabrication and weld dressing. Coating thickness as the standard for the steel thickness; thicker coating [not required / to be agreed in writing with the galvaniser before ordering, nominal [ ] µm]. Fabricator to deliver steel free of paint, grease, weld slag and anti-spatter. Fabricator to pass this clause to the galvaniser.
Every sealed hollow section and every overlapping area of more than 100 cm² sealed by continuous welding to be vented as EN ISO 14713-2:2020 and the galvaniser’s written advice, with holes or cut corners diagonally opposite and close to the ends, sizes [as the galvaniser confirms]. Vent and drain details to be shown on the shop drawings and approved by the galvaniser before fabrication. No sealed section is to be sent for galvanising.
Galvanised steel to be [painted / powder coated] with a system made for galvanised steel by [maker], surface prepared by [T-wash / etch primer / sweep blasting / weathering] as the coating maker instructs, colour [RAL [ ]], gloss [ ] GU. Damage to the coating before handover to be repaired to the same system. [Pre-treatment and first coat applied within [ ] days of galvanising.]
A sample [300 mm] piece of the actual steel, galvanised and [coated], to be approved before production, with thickness readings [by magnetic gauge]. Galvanised steel to be stored dry and ventilated on spacers, not wrapped wet. Bolted joints to be made after galvanising; welded joints to be renovated to [the galvaniser’s repair method].
Terms used on this page
- Hot-dip galvanising
- Dipping a finished steel fabrication in molten zinc so that a zinc coating forms on every surface the zinc reaches, inside and out. The coating is hard zinc-iron alloy layers under a soft outer layer of zinc (Galvanizers Association).
- Local and mean coating thickness
- In BS EN ISO 1461, the local figure is the lowest acceptable reading at any one place, and the mean figure is the average over the article. Both are minimums.
- Vent and drain hole
- A hole or cut corner that lets air escape from a hollow or sealed part and lets molten zinc flow in and out again during dipping.
- Duplex system
- A metallic coating, here galvanising, overcoated with an organic coating such as paint or powder (Galvanizers Association).
- Wet storage stain
- A white powdery layer of zinc corrosion products that forms when galvanised steel stays damp and cannot dry.
Standards and sources
- BS EN ISO 1461:2022: BSI listing: Current, published 30 Sep 2022. Text not read.
- BS EN ISO 14713-1:2017 and BS EN ISO 14713-2:2020: listings only; confirm the edition in force.
- Galvanizers Association (UK): EN ISO 1461 and design for galvanizing pages; HDG Datasheet 4a (October 2024): read 7 Nov 2026.
Questions we are asked
What does BS EN ISO 1461 require?
It sets minimum local and mean coating thicknesses by the thickness of the steel, and the test methods. For steel over 6 mm the minimums are 70 µm local and 85 µm mean; for steel under 1.5 mm they are 35 and 45 µm (Galvanizers Association table for the 2022 edition).
Why do hollow steel sections need holes before galvanising?
Sealed hollow sections must be vented for safety, so air can escape and zinc can flow in and drain, and so the inside is coated too. Holes go diagonally opposite and close to the ends. Ask the galvaniser to confirm the hole size on the shop drawing.
Can I get a thicker galvanised coating?
Sometimes. Grit blasting before dipping, or a more reactive steel, can give a thicker coating, but the Galvanizers Association says to agree it with the galvaniser before specifying. The figure is nominal, not guaranteed, and a thick all-alloy coating can be dull grey and easier to damage.
Can galvanised steel be painted or powder coated?
Yes. This is called a duplex system. Use a system made for galvanised steel, on a surface prepared by T-wash, etch primer, sweep blasting or weathering, and follow the coating maker’s instructions.
Is galvanising right for an interior?
Often it is more than an interior needs, and the steel may be dipped only because it also stands outside or in a damp place. It can also be chosen as a visible raw finish. Approve a sample either way, because colour and finish vary with the steel.
Who is responsible for clean steel and venting?
The Galvanizers Association says the fabricator is responsible for delivering steel free of contamination, and that the galvaniser normally works for the fabricator. Put the clause in the specification you issue to the fabricator, and have the galvaniser approve the vent details.
Download the factsheet
A two-page A4 PDF with the thickness bands, the venting rules, the vent drawing and paste-in specification wording, to keep with your project file.