Key takeaways
- ISO 1461 sets a local minimum and a mean minimum for each steel thickness band; a 3 mm part needs 45 µm at every reading and a 55 µm mean, so one low reading fails the lot even when the average is comfortable.
- Coating mass and thickness convert at 7.2 g/cm³, so 505 g/m² is 70 µm and 325 g/m² is 45 µm; a report that gives only grams can be checked with a calculator.
- A 3 mm bracket might carry about 20 g of zinc, which is why coating cost tracks surface area rather than weight, and why doubling the thickness of the steel barely changes the galvanising price.
- Reactive steel with silicon between 0.03% and 0.12% produces a coating of 150 µm or more, which ISO 1461 accepts because it sets no maximum, and which cracks when the part is bent or will not pass a nut.
- Gravimetric testing to ISO 1460 takes precedence over magnetic gauge readings in a dispute, so record both the reading and the method on the receipt report.
A galvanising certificate arrives with a 3 mm bracket, and the buyer's gauge reads 47 µm at the worst point. The certificate says the part complies. The gauge says the number is above 45 µm. Both are right, and the part may still be non-conforming, because ISO 1461 sets two requirements for that thickness band: 45 µm at every reading and a mean of 55 µm across the reference area. A single low point is not the only way to fail.
ISO 1461:2022 specifies the properties of hot dip galvanised coatings on fabricated iron and steel articles dipped in a zinc melt, and the test methods used to check them. It is a coating specification, not a design guide and not a life prediction. Knowing which of those three jobs you are asking it to do decides whether the certificate means anything.
The sections below give the minimum values by thickness, the arithmetic that converts mass to thickness, the receipt check that catches the failures, and the design limits the standard deliberately leaves alone.
What ISO 1461 covers, and what it does not
The scope is narrow and worth reading before the drawing note is written. The standard applies to fabricated articles, including certain castings, that are dipped in a zinc melt containing not more than 2% of other metals. It sets the minimum coating and the methods for measuring it.
It excludes three families explicitly. Continuously galvanised sheet, wire and mesh are out, because the coating is applied at the mill and belongs to the mill's product standard. Tube and pipe galvanised in automatic plants are out. Fasteners and other products that have their own coating standard are out, which is why an M8 bolt is specified to ISO 10684 rather than to ISO 1461. The standard also does not cover after-treatment or additional coating, so a duplex system of zinc plus powder is two specifications, not one.
That last exclusion causes more confusion than any other. A galvanised frame that is then powder coated has a zinc coating governed by ISO 1461 and a paint or powder system governed by something else, usually a corrosion-protection specification. The adhesion between them, the pretreatment before the powder and the total film build are not in ISO 1461 at all. The standards and compliance hub collects the other scope notes in this series.
The minimum coating values by thickness
The values below are the minimums for articles that are not centrifuged, which covers most fabricated frames, brackets and panels. The local column applies to every reading; the mean column applies to the average over the reference area. The mass values are the same requirement expressed in grams per square metre.
| Steel thickness | Local min thickness | Local min mass | Mean min thickness | Mean min mass | Ganesh note |
|---|---|---|---|---|---|
| > 6 mm | 70 µm | 505 g/m² | 85 µm | 610 g/m² | Base plates and heavy brackets; expect 85–100 µm in practice |
| > 3 mm to ≤ 6 mm | 55 µm | 395 g/m² | 70 µm | 505 g/m² | The band most structural sheet-metal parts fall into |
| ≥ 1.5 mm to ≤ 3 mm | 45 µm | 325 g/m² | 55 µm | 395 g/m² | Canopy frames and rack uprights at 2–3 mm |
| < 1.5 mm | 35 µm | 250 g/m² | 45 µm | 325 g/m² | Thin panels warp at bath temperature; design for it or avoid the process |
| Castings ≥ 6 mm | 70 µm | 505 g/m² | 80 µm | 575 g/m² | Castings are not a sheet-metal route; the values are for reference |
| Castings < 6 mm | 60 µm | 430 g/m² | 70 µm | 505 g/m² | As above |
| Threaded parts, centrifuged, > 6 mm | 40 µm | 285 g/m² | 50 µm | 360 g/m² | Centrifuging removes excess zinc so the thread assembles |
| Threaded parts, centrifuged, ≤ 6 mm | 20 µm | 145 g/m² | 25 µm | 180 g/m² | The lower mass is a thread requirement, not a corrosion saving |
The table is a floor, not a target. A galvaniser that delivers 85 µm mean on 6 mm steel is inside the standard and gives a longer life than one that delivers exactly 70 µm, because life scales roughly with coating mass in a given environment. What the standard prevents is the opposite case: a part that looks grey and even but carries half the zinc it should.
Converting mass to thickness, and why both appear
The two units are the same measurement at a nominal coating density of 7.2 g/cm³. Divide grams per square metre by 7.2 and you get micrometres: 505 ÷ 7.2 = 70.1, 395 ÷ 7.2 = 54.9, 325 ÷ 7.2 = 45.1. Multiply micrometres by 7.2 to go the other way.
That matters because certificates arrive in whichever unit the galvaniser's software prints, and buyers compare the wrong row. A report that says 480 g/m² is a 66.7 µm coating, which passes a 55 µm local requirement on 5 mm steel and fails nothing. A report that says 52 µm mean on the same part fails the 70 µm mean requirement by 18 µm, a shortfall of about 130 g/m².
There is a physical check that makes the number concrete. A 3 mm bracket with 0.05 m² of significant surface at 325 g/m² carries 16.25 g of zinc. At the mean requirement of 395 g/m² it carries 19.75 g. Twenty grams of zinc on a bracket is enough to explain why galvanising is priced on surface area and handled weight, not on the mass of the steel: doubling the plate thickness doubles the steel but barely changes the coating.
Reading a coating report: local, mean and the receipt check
The common failure is not a coating that is uniformly thin. It is a coating that passes at the easy points and fails at the difficult ones: near a weld, on a sheared edge, at the bottom of a drainage path, or on a surface that faced the bath entry. That is exactly why the standard separates the local and mean requirements.
Take five readings across a 3 mm bracket: 60, 58, 54, 49 and 47 µm. Every reading is above the 45 µm local minimum. The mean is 53.6 µm, which is below the 55 µm mean requirement, so the part fails. Reverse the pattern with readings of 70, 68, 62, 44 and 61: the mean is 61 µm and passes, but the 44 µm reading fails the local minimum, so the part fails again. Both criteria have to hold at once, and neither one alone is a pass.
The measurement rules matter as much as the numbers. Thickness readings are not taken on cut surfaces, within 10 mm of an edge or a corner, or on flame-cut faces, because those areas are unrepresentative and the gauge reads the substrate as much as the coating. Magnetic induction gauges need calibration on the same substrate, and a coated reference coupon is not a substitute for checking the gauge on the day. In a dispute, gravimetric testing to ISO 1460 takes precedence over thickness readings, which is why the receipt report should say which method produced the number.
What the standard does not control: reactive steel, threads and distortion
Three problems arrive with galvanising and none of them is a coating thickness failure.
The first is reactive steel. Steels with silicon between about 0.03% and 0.12%, or above 0.25%, and with elevated phosphorus, react faster in the zinc and produce coatings of 150–250 µm. ISO 1461 sets no maximum, so the coating complies while the part does not: it can craze and flake when bent, it changes the fit of a mating surface, and it will not pass a nut. The fix is at the steel order, not at the bath, so a part that will be bent after coating should not be made from a reactive heat.
The second is threads. Seventy micrometres of zinc on each flank adds about 0.14 mm to the pitch diameter of an M8, which exceeds the clearance in a standard 6H nut. Threads are either over-tapped before dipping or centrifuged to the lower mass in the table, and fasteners follow their own standard. A drawing that says "galvanise all over, including threads" without saying which route is a drawing that will produce a joint that cannot be assembled.
The third is distortion. The bath runs at about 450 °C, which stress-relieves the fabrication and can move a thin panel or a long frame out of flatness. Welding before galvanising is normal, but the sequence and the thickness have to allow for the movement, and a 1.5 mm panel with an unsupported span is a design that will come back warped. Vent and drain holes at the high and low points are part of the same design work: molten zinc has to get in and out before it freezes, and a sealed hollow section cannot be galvanised at all.
Where ISO 1461 stops, and what replaces it
The standard does not say how long the coating will last. Life depends on the environment, the coating mass and the maintenance regime, and the environment is classified in ISO 12944-2 by corrosivity category, from C1 interior to CX offshore. A 70 µm coating in C3 urban-industrial inland behaves differently from the same coating on a coastal site, and no clause in ISO 1461 changes that.
It does not define design. Venting, drainage, avoidance of overlapping surfaces, thread allowances and bending radii belong to the design guidance in ISO 14713, which is where a drawing should look before it specifies a coating. It does not cover over-coating, as the scope says, so a duplex system needs a second specification and a compatible pretreatment. It does not cover repair beyond a stated minimum for renovated areas, and a repair patch at 100 µm is not the same coating as the original.
What replaces the missing parts is a two-line specification: the coating standard and the steel thickness band, plus the environment category and the expected life. For a sheet-metal enclosure at 1–2 mm, the more usual route is continuously galvanised sheet with a powder finish, because batch galvanising a thin panel costs distortion and gains little. For a structural frame that will be outside for 20 years, ISO 1461 plus a duplex system is the right answer and the drawing should say so.
What Ganesh runs, and where the line is
Ganesh does not operate a hot dip galvanising bath. The in-house coating route is seven-tank pretreatment, then powder from an 18 ft × 14 ft booth or wet paint, with a 16 ft × 16 ft shot blast chamber ahead of it and a curing oven behind it. Dry film thickness is checked with a gauge and adhesion with a cross-hatch test, which are the two checks that matter for a film that will be handled and bolted.
When a drawing specifies ISO 1461, the finish is outside that route, and the honest answer at quotation is to say so rather than to quote a process we do not run. Where the part needs zinc plus colour, the two specifications have to be written together: the galvanising to ISO 1461, the powder system to its own standard, and the pretreatment between them named. The paint shop capability sets out the booth, oven and test equipment we do run.
Certification is ISO 9001:2015 and ISO 14001:2015, with PPAP and first article inspection available. Customer work includes enclosures and structures for Godrej & Boyce, TKE and Indian Railways, where the coating callout usually comes from the customer's own specification rather than from a general standard. The shot blasting capability covers the 16 ft × 16 ft chamber used ahead of coating.
What to put on the drawing
- The coating standard and edition, written as ISO 1461:2022, not "galvanised".
- The steel thickness band the part falls into, so the local and mean values are unambiguous.
- The significant surface, marked, and any area that may be left uncoated for an earth or a sliding fit.
- Threaded features, with the route: over-tapped, centrifuged, or masked and coated after assembly.
- Vent and drain hole positions, or a note that the fabricator is to add them and report the positions.
- The environment category and expected life, so the coating mass can be judged against a purpose and not only a minimum.
- Whether a duplex system follows, with the second specification named and the pretreatment between the two.
- The receipt check: number of readings per reference area, method, and who witnesses it.
The corrosion protection by coating guide covers what happens when a paint system is specified instead of zinc, and why powder coating fails on outdoor enclosures covers the failure that a coating thickness check will not catch. Send the drawing and the service environment through the quotation route and the reply will say whether the specified finish is the one the part needs.
Frequently asked questions
What coating thickness does ISO 1461 require on 3 mm steel?
45 µm local minimum and 55 µm mean minimum, equivalent to 325 g/m² and 395 g/m². The local value applies to every reading in the reference area, so a single reading of 43 µm makes the part non-conforming even if the average is 60 µm.
How do I check a galvanised coating on receipt?
Take magnetic induction readings at least 10 mm from edges, cut faces and corners, record the minimum and the mean against the band for the steel thickness, and ask for the galvaniser's certificate with its own readings. If the numbers disagree, gravimetric testing to ISO 1460 settles it.
Does salt spray testing prove a galvanised coating is correct?
No. ISO 1461 qualifies the coating by thickness and mass, not by salt spray hours. A salt spray result can show a discontinuity but it does not replace the coating mass check, and the hours do not translate into years of service.
Why is my galvanised M8 bolt too tight?
Seventy micrometres of zinc on each thread flank adds about 0.14 mm to the pitch diameter, which is more than the clearance in a standard 6H nut. Threads must be over-tapped or centrifuged to a lower coating mass, which is why fasteners have their own standard in ISO 10684.
Can I paint over a galvanised surface?
Yes, as a duplex system, but the two specifications must both be named. ISO 1461 does not cover over-coating, and powder or paint adhesion on zinc needs its own pretreatment and its own acceptance criteria.
Does ISO 1461 apply to continuously galvanised sheet?
No. Its scope excludes sheet, wire and mesh that are continuously galvanised, and tube galvanised in automatic plants. Those products are covered by their own mill standards, so an ISO 1461 certificate on a coil is the wrong document.
Sources and further reading
- ISO 1461:2022 — Hot dip galvanized coatings on fabricated iron and steel articles: specifications and test methods, including the minimum local and mean coating tables · reviewed 3 Nov 2026
- ISO 12944-2:2017 — Classification of environments, which converts a coating into a durability expectation by corrosivity category C1 to CX · reviewed 3 Nov 2026
- ISO 9227:2022 — Corrosion tests in artificial atmospheres: salt spray tests, which defines the test conditions but not the acceptance criteria for a galvanised part · reviewed 3 Nov 2026
Is your drawing asking for the right finish?
Send the part, the environment and the expected service life. We will say whether the specified coating system is the one the part needs, and quote the route we can actually run.
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