Hot-dip vs electro-galvanised steel -
differences, durability and applications
Hot-dip and electro-galvanising protect steel with the same metal, but they differ in coating thickness and therefore in durability. Hot-dip galvanised sheet with a Z275 coating carries around 20 µm of zinc on each side, while electro-galvanised sheet in the common ZE50/50 variant carries around 5 µm. Since protection time increases in proportion to zinc thickness, the difference in durability under the same conditions is roughly fourfold. Electro-galvanising is the choice where a smooth surface for painting and dimensional precision matter, and hot-dip galvanising wherever corrosion protection comes first.
Three galvanising technologies and their standards
The everyday term galvanising covers three different processes. In the sheet market the most important is continuous hot-dip galvanising of coils, the second is electro-galvanising of sheet, and the third is batch galvanising of finished structures.
| Technology | Standard | Typical zinc thickness | Products |
|---|---|---|---|
| continuous hot-dip galvanising of sheet | EN 10346 | approx. 7-42 µm per side (Z100-Z600) | coils, strip, sheets |
| electro-galvanising of sheet | EN 10152 | 2.5-10 µm per side (ZE25/25-ZE100/100) | coils for automotive and household appliances |
| batch hot-dip galvanising | EN ISO 1461 | 45-85 µm on average, depending on steel thickness | finished structures and parts |
| electroplating of small parts | EN ISO 2081 | usually 5-25 µm | bolts, fittings, fasteners |
How the coating is formed
In continuous hot-dip galvanising, steel strip passes after annealing through a bath of molten zinc at around 450°C, and gas knives blow off the excess metal, precisely setting the coating mass. A small addition of aluminium to the bath limits the growth of brittle iron-zinc phases, so the sheet copes well with bending and roll forming. The coating is metallurgically bonded to the substrate.
In electro-galvanising, zinc is deposited on the strip from a salt solution by electric current, at close to ambient temperature. The result is a thin layer of pure zinc, very even and smooth, without the characteristic spangle and without an alloy layer. The material retains the properties of the substrate steel, and the surface is ideal for painting.
Durability in different environments
Zinc corrodes slowly and predictably. EN ISO 9223 gives zinc loss rates for environmental corrosivity categories, which makes it possible to estimate the time until the coating is consumed. The table below is a HEN-STOL calculation based on these values, for surfaces without a paint coating.
| Corrosivity category | Zinc loss per year | Z275 (approx. 20 µm) | ZE50/50 (approx. 5 µm) |
|---|---|---|---|
| C2, low (rural areas, unheated halls) | 0.1-0.7 µm | over 25 years | 7 years and more |
| C3, medium (urban areas, light industry) | 0.7-2.1 µm | approx. 10-29 years | approx. 2-7 years |
| C4, high (industry, coastal zone) | 2.1-4.2 µm | approx. 5-10 years | approx. 1-2 years |
| C5, very high | 4.2-8.4 µm | approx. 2-5 years | approx. 1 year |
In indoor applications both coatings usually last the whole service life of the product. Outdoors the difference becomes fundamental, and electro-galvanised sheet almost always needs an additional paint coating. A thicker zinc layer also protects cut edges better, because zinc protects exposed steel cathodically. Alternative coatings, zinc-magnesium and aluminium-zinc, are described in more detail in the article on types of metallic coatings.
Where electro-galvanising has the advantage
Surface quality. A smooth, uniform surface without zinc spangle is the standard for visible painted parts, for example in automotive and household appliances.
Precision and formability. The thin coating does not change thickness tolerances and copes well with deep drawing.
Resistance spot welding. Less zinc means lower electrode wear in spot welding.
Where hot-dip galvanising has the advantage
In structures, enclosures, profiles, ventilation ducts, roofing and all products exposed to moisture, hot-dip galvanising provides several times longer protection at a comparable material cost. Hot-dip galvanised sheet is also the basic feedstock for roll forming and bending, and where required it takes paint well in a duplex system, as described in the article on painting galvanised steel sheet.
Selection criteria
Service environment of the product. From category C3 upwards, without painting, practically only hot-dip galvanising gives protection for many years.
Finish. Painted parts with high visual requirements point to electro-galvanising or to hot-dip galvanising with minimised spangle and improved surface quality.
Joining technology. In high-volume spot welding a thin coating makes the process easier, while in arc welding both require extraction of zinc fumes.
Total cost. What counts is the cost of protection over the entire service life, not just the price per tonne of sheet.
Available steel grades, coating types and coil, strip and sheet formats are presented in the HEN-STOL steel sheet offer, with separate pages for galvanized steel sheet, cold-rolled steel sheet and hot-rolled steel sheet.
See how we can help
HEN-STOL supplies hot-dip galvanised steel in coils, strip and sheets, in forming and structural grades, from 5 tonnes. Send us the specification and a description of the application, and we will check availability and advise on the coating suited to the service environment of the product.
Frequently asked questions
What is the difference between hot-dip and electro-galvanising?
Coating thickness and the way it is applied. Hot-dip galvanising involves immersing steel in molten zinc at around 450°C and gives around 20 µm per side on sheet with Z275. Electro-galvanising deposits zinc electrolytically, the coating is thinner (on sheet usually 2.5-10 µm per side), smoother and more even.
Which galvanising lasts longer?
Hot-dip galvanising, because the durability of a zinc coating increases in proportion to its thickness. In a C3 urban environment, according to a calculation based on EN ISO 9223, a Z275 coating lasts around 10-29 years, and an electro-galvanised ZE50/50 coating around 2-7 years without additional painting.
How can hot-dip and electro-galvanised steel be told apart?
Hot-dip galvanised sheet often shows visible zinc spangle or a matt silver surface, while electro-galvanised sheet is uniformly smooth and bright. Small electroplated parts often have a coloured passivation, yellowish or bluish. The most reliable method is measuring coating thickness with a magnetic gauge.
Is electro-galvanised steel suitable for outdoor use?
Without additional protection only in mild conditions and for a short time, because the thin zinc layer is consumed quickly. Outdoors, electro-galvanising is usually used as a base for paint, for example in car bodies. For products working outdoors without painting, hot-dip galvanising is chosen.
What do Z275 and ZE50/50 mean?
Z275 is a hot-dip zinc coating to EN 10346 with a mass of at least 275 g/m² in total on both sides, around 20 µm per side. ZE50/50 is an electrolytic coating to EN 10152 with a nominal thickness of 5 µm on each side.
Can hot-dip galvanised sheet be painted?
Yes. Hot-dip galvanising is a good substrate for liquid paints and powder coating, and combining both coatings in a duplex system extends protection 1.5 to 2.3 times compared with the sum of both layers. The conditions are proper surface preparation and a paint designed for zinc.
Sources
- EN 10346, Continuously hot-dip coated steel flat products, 2015
- EN 10152, Electrolytically zinc coated cold rolled steel flat products, 2017
- EN ISO 1461, Hot dip galvanized coatings on fabricated iron and steel articles, 2022
- EN ISO 9223, Corrosion of metals and alloys - corrosivity of atmospheres - classification, 2012
- EN ISO 14713-1, Zinc coatings - guidelines and recommendations for the protection against corrosion, 2017