Home / Services / Hot-dip galvanising / Zinc Coating Thickness under DSTU EN ISO 1461:2024
Hot-dip galvanising
Zinc Coating Thickness under DSTU EN ISO 1461:2024
DSTU EN ISO 1461:2024 sets the minimum zinc coating thickness according to the thickness of the steel. The table below shows what a hot-dip galvanised article is checked against.
Contents
- Why coating thickness depends on steel thickness
- Minimum coating thickness by steel thickness
- Why the coating differs on steel of the same thickness
- How coating thickness is checked
- How many measurements are taken, and where
- What coating acceptance includes
- If the thickness does not meet the requirement
- How uncoated areas are repaired
- Coating thickness and service life
- Fasteners and threaded parts
- How many articles from a batch are measured
Why coating thickness depends on steel thickness
A thick article stays hot longer while it is immersed in the molten zinc, so the layer of iron-zinc alloys on its surface has time to grow thicker. That is why the standard gives several thresholds rather than a single figure. The thicker the blank, the higher the minimum coating thickness it has to reach.
Minimum coating thickness by steel thickness
The main categories under DSTU EN ISO 1461:2024 (harmonised with ISO 1461), by the thickness of the steel article:
| Article steel thickness, mm | Min. local coating thickness, µm | Min. mean coating thickness, µm |
|---|---|---|
| > 6 | 70 | 85 |
| > 3 – ≤ 6 | 55 | 70 |
| ≥ 1.5 – ≤ 3 | 45 | 55 |
| < 1.5 | 35 | 45 |
Values as in Table 3 of DSTU EN ISO 1461:2024 (ISO 1461:2022). “Local” thickness is the average of several readings (at least five) on a reference area of at least 10 cm², not the lowest single reading; “mean” is the average of the local values over all the areas (the mean requirement is higher). For small fasteners and threaded parts galvanised by centrifuging, the standard sets separate, lower thresholds — check for the specific article.
Why the coating differs on steel of the same thickness
Wall thickness is not the only factor. Steel composition matters too, above all silicon and phosphorus: with silicon below 0.03% and phosphorus below 0.02% the coating is thin and bright, while at 0.03–0.14% silicon it can grow excessively thick and dark. The ranges are given in Designing for Hot-dip Galvanising. Low-alloy steel 09G2S contains more silicon than St3, so its coating is usually thicker.
Steel with no more than 0.01% silicon and more than 0.035% aluminium reacts weakly with zinc, and the coating on it can come out thinner than expected. If that keeps articles of steel over 3 mm thick from reaching the table minimum, DSTU EN ISO 1461:2024 (ISO 1461:2022) requires the next lower category to apply. A declaration of compliance, if one is issued, says so.
How coating thickness is checked
Thickness is read with a magnetic gauge, which does not damage the coating. On large articles (poles, masts, gantries) the readings are spread over typical places: a joint, the middle of a section, the zone near the flange. How the areas are chosen is covered in the next section.
Every batch is checked before shipment, and by agreement the results can go into the accompanying documentation.
How many measurements are taken, and where
Factory control is mainly magnetic (ISO 2178), a non-destructive method. In a dispute, thickness is determined from the mass of zinc (ISO 1460). Readings are taken on reference areas of at least 10 cm², at least five on each. On a long article that means roughly the middle, plus points about 100 mm from the edges and from each end. Nothing is measured on cut surfaces or within 10 mm of edges and corners. Small ancillary elements (stiffeners, end plates, brackets) are left out of the check unless otherwise agreed.
If the significant surface of an article (the part that matters for service or appearance) exceeds 2 m², it is assessed on at least three areas. On each, the average must not be below the minimum local thickness, and the average over that article’s areas must not be below the minimum mean thickness. Smaller articles need at least one area. Each must meet the minimum local thickness, and the average over all areas the minimum mean thickness. A sample is drawn from the batch at the purchaser’s request.
What coating acceptance includes
Acceptance means inspecting the surface and measuring thickness before the articles leave the plant. Under the standard it may be carried out by the purchaser or on the purchaser’s behalf. Adhesion is not normally tested, only by separate agreement.
The significant surface is viewed from at least 1 m and must show no blisters or uncoated areas, and no roughness or sharp points that could cause injury. Darker or lighter patches and “orange peel” are not grounds for rejection. Neither is wet storage stain, provided the thickness is not below the minimum. If a control sample falls short on thickness, twice as many articles from the batch are tested.
If the thickness does not meet the requirement
If the doubled control sample also falls short on thickness, the non-conforming articles are set aside or, with the purchaser’s authorisation, re-galvanised where the shape of the article allows it. Uncoated areas follow different rules, covered in the next section.
How uncoated areas are repaired
Uncoated areas may be repaired if each is no larger than 10 cm² and together they make up no more than 0.5% of the surface. Otherwise the article is re-galvanised, unless agreed otherwise. Repair is done with zinc-rich paint, zinc paste, a zinc alloy stick or thermal zinc spraying, and the repair coating must average at least 100 µm unless another thickness is agreed. Typical coating defects and what causes them are covered in the articles Hot-dip galvanising defects and Zinc Coating Defects.
Coating thickness and service life
In a given environment, a thicker coating protects the metal for longer. How service life depends on the corrosivity category of the environment (under ISO 9223) is covered in Hot-dip Galvanising Service Life. For how hot-dip galvanising compares with electro- and cold galvanising, see Hot-dip, Electro- or Cold Galvanising.
Fasteners and threaded parts
Small articles (bolts, nuts, washers, fasteners) are galvanised in baskets and spun in a centrifuge straight afterwards to shed excess zinc. For these the standard gives a separate table with lower thresholds than for structures.
| Centrifuged articles | Min. local coating thickness, µm | Min. mean coating thickness, µm |
|---|---|---|
| Threaded, diameter over 6 mm | 40 | 50 |
| Threaded, diameter 6 mm and under | 20 | 25 |
| Other, thickness 3 mm and over | 45 | 55 |
| Other, thickness under 3 mm | 35 | 45 |
As in Table 4 of DSTU EN ISO 1461:2024 (ISO 1461:2022). Local and mean thickness are worked out in the same way as for structures. For a specific article, check the values against the text of the standard.
How many articles from a batch are measured
Not the whole batch is measured but a control sample, and its size depends on the number of articles in the batch (table below).
Example. A batch of 120 poles, each with a significant surface of more than 2 m². By the table (4–500 articles) the sample is at least 3 poles. On each, at least three reference areas are taken, with at least five readings on each: 3 × 3 × 5 = 45 readings at the very least.
| Articles in the batch | In the control sample, at least |
|---|---|
| 1–3 | all |
| 4–500 | 3 |
| 501–1,200 | 5 |
| 1,201–3,200 | 8 |
| 3,201–10,000 | 13 |
| over 10,000 | 20 |
As in Table 1 of the standard. What happens when a sample falls short of the required thickness is described above, in the section on acceptance.
Updated:
Reviewed by: V.V. Padalka, Chief Project Engineer
Zinc coating thickness: frequently asked questions
What coating thickness will my poles or structures have?
It depends on the wall thickness of the article itself: find its category in the table above. On lighting poles the mean coating thickness per ISO 1461 is at least 55 µm for a 3 mm wall and at least 70 µm for a 4 mm wall. After galvanising we measure the actual thickness with a magnetic gauge.
Why is the required mean thickness higher than the local one?
Because they check different things. The lower local threshold allows for the natural spread of thickness between areas; the higher mean threshold keeps the coating from being thin overall. Local thickness is the average of several readings on one reference area, mean thickness the average of the local values over all areas. Both conditions apply: each area not below the minimum local thickness, and the average over all areas not below the minimum mean thickness.
Can articles thinner than 1.5 mm be galvanised?
Yes. For steel thinner than 1.5 mm the standard sets a minimum local thickness of 35 µm and a mean of 45 µm (see the table). Small fasteners and threaded parts galvanised by centrifuging have separate, lower thresholds, so check the figures for the specific article.
Do you check the coating thickness yourselves?
Yes. Every batch is measured with a magnetic gauge before it ships.
Is an article rejected if one reading comes out below the minimum?
No. The standard disregards individual readings below the minimum; what decides is the average on the reference area and the average over all areas.
Why does the coating differ on articles of the same thickness?
It comes down to the steel composition. Silicon and phosphorus speed up the reaction with zinc, so the coating grows thicker and darker. It is best to tell the plant the steel composition before galvanising.
Do the Rules for the Arrangement of Electrical Installations (PUE) set a zinc thickness for poles?
No. PUE (2026 edition), cl. 2.5.19: metal poles of overhead lines above 1 kV up to 750 kV need to be protected against corrosion, as a rule by hot-dip galvanising to DSTU EN ISO 1461. The first paragraph of the same clause also refers to DSTU-N B V.2.6-186 for corrosion protection. For lines up to 1 kV, cl. 2.4.8 refers to that clause and to cl. 2.5.21. How many micrometres of zinc a pole must carry is not stated in the PUE: it comes from the standard (the table above) and from the design. The only zinc coating figures in the PUE are for earth electrodes in the ground (cl. 1.7.117, Table 1.7.5), and they are minimum sizes: for a vertical round earth electrode of Ø16 mm the coating is at least 70 µm, for a horizontal round one of Ø10 mm at least 50 µm. For a specific site, the designer takes the final values from the current text of the PUE.
What is the zinc thickness on bolts and nuts?
For threaded articles galvanised by centrifuging, the standard sets a minimum local thickness of 40 µm and a mean of 50 µm for a diameter over 6 mm; for a diameter of 6 mm and under it is 20 and 25 µm. The thresholds are lower than for structures.
Related products: Hot-dip galvanising · Street lighting columns · Overhead power line masts · Substation gantries
More on this topic: Hot-dip Galvanising Process: Steps and Requirements · Hot-dip Galvanising Defects: Causes and Prevention · Zinc Coating Defects: Runs, Grey Coating, Detachment and Black Spots · Hot-dip, Electro- or Cold Galvanising: Which to Choose · How to Prepare a Steel Structure for Hot-dip Galvanising: Checklist · Designing Steel Structures for Hot-dip Galvanising: Holes, Welds, Stress
Get a quote for your requirements
Send us your requirements — we will calculate the price and lead time.