Hot-dip against electro-galvanised steel: which coating for which job

Hot-dip and electro-galvanising both put zinc on steel, but they are not interchangeable. Hot-dip immerses steel in molten zinc and builds a metallurgically bonded coating, from about 20 micrometres per side on continuously coated sheet to between 45 and 85 micrometres on batch-galvanised fabrications. Electro-galvanising deposits pure zinc electrolytically, at roughly 2.5 to 10 micrometres per side on cold-rolled flat product to EN 10152 and typically 5 to 25 micrometres on plated components to ISO 2081, giving a smooth, uniform surface with a fraction of the corrosion life. The choice follows the service environment: outdoors and structural means hot-dip, while indoor parts that need a fine finish and paint adhesion are where electro earns its place.

The two processes in short

Hot-dip galvanising immerses cleaned steel in a bath of molten zinc at around 450 degrees Celsius. The zinc reacts with the steel surface and forms a series of iron-zinc alloy layers with a pure zinc layer on top, so the coating is grown into the steel rather than laid on it. Electro-galvanising, also called electrolytic zinc coating or electroplating, passes the steel through an electrolyte and deposits zinc onto the surface using an electric current, at ambient temperature. The result is a layer of pure zinc, deposited to a thickness the line controls precisely. One process is metallurgical, the other electrochemical, and almost every practical difference follows from that.

Coating thickness: the difference that decides everything

Thickness is where the two processes separate, and the gap is an order of magnitude rather than a matter of degree. Batch hot-dip galvanising to EN ISO 1461 is specified by minimum mean coating thickness against the steel section: 45 micrometres for steel under 1.5 mm, 55 for 1.5 to 3 mm, 70 for 3 to 6 mm and 85 micrometres for steel above 6 mm. The standard sets a second and lower requirement alongside it, the minimum local thickness, at 35, 45, 55 and 70 micrometres for the same four bands, and a compliant coating has to satisfy both. Continuously hot-dip coated sheet to EN 10346 carries less: a Z275 coating is 275 g/m2 counted over both surfaces, which works out at roughly 20 micrometres per side, with the standard tabulating a spread of 15 to 27 micrometres around that figure. Electro-galvanised product to EN 10152 sits far below both.

The two electrolytic entries are not a contradiction. EN 10152 governs zinc applied on the strip line to cold-rolled flat product, where the coating is deliberately thin because the material will be pressed and painted afterwards. ISO 2081 governs zinc electroplated onto finished articles such as brackets, clips and small fittings, in four classes from Zn 5 to Zn 25 micrometres, where a thicker deposit is practical because the part is already made. Threaded fasteners are a separate case: ISO 4042 covers electroplated coating systems for fasteners and takes precedence over the general electroplating standards, because bolt and screw coatings raise thread-fit and hydrogen embrittlement questions that a general standard does not address. Both processes are electro-galvanising; the difference is what is being coated and at what stage.

How the coatings are designated

The two families use different notation, which is a common source of confusion on an order. Continuous hot-dip coatings use the Z designation followed by the total coating mass over both surfaces in grams per square metre, so Z275 means 275 g/m2 across the two faces together. Electrolytic coatings to EN 10152 use ZE followed by a figure for each side, from ZE25/25 through ZE50/50 and ZE75/75 to ZE100/100, corresponding to about 2.5, 5, 7.5 and 10 micrometres per side. Electroplating to ISO 2081 uses a third notation again, built on the minimum local thickness in micrometres, as in the class Zn 12. That figure never travels alone in a full designation, which also carries the basis metal, any stress relief, the conversion coating and any sealant, so Fe/Zn12/A is a specification where Zn 12 by itself is only a class. EN 10152 also allows different coatings on each face, such as ZE75/25, or coating on one side only, both by agreement between the two parties rather than as a stock condition. Reading Z275 and ZE100/100 as comparable figures is a mistake: one is a mass over both sides, the other a thickness per side.

Bonded coating against deposited coating

The metallurgical difference matters as much as the thickness. Because hot-dip coatings grow through iron-zinc alloy layers, they are bonded into the steel and resist knocks, abrasion and handling damage in a way a deposited layer does not. Those alloy layers are harder than the steel beneath them, at roughly 179 to 250 on the Vickers scale against about 159 for mild steel, and they make up much of the 45 to 85 micrometres of a batch coating, which is why hot-dip galvanised sections survive transport, site handling and erection. The bond is metallurgical rather than mechanical, and bond strength is an order of magnitude above that of applied paint, so the coating resists peeling and flaking far better than a deposited layer. That resistance has two recognised limits worth specifying around. The alloy layers are hard but brittle, so severe bending after galvanising can crack them, which is why parts are formed before coating. Steel chemistry matters just as much: silicon in the reactive range drives runaway alloy growth and produces a dull, excessively thick coating that is fragile at the edges, so steel intended for galvanising is selected for its silicon content rather than assumed to be suitable. An electro-galvanised layer is pure zinc laid on the surface at a few micrometres, tough enough for pressing and forming but easy to scratch through to bare steel. On a component that will be handled, drilled or bolted on site, that difference shows up quickly.

Corrosion life in service

Zinc works in two ways, and the larger of the two is often overlooked. Most of the service life comes from barrier protection, the zinc corroding slowly under its own patina in place of the steel, while sacrificial action operates at defects, cut edges and scratches where steel is actually exposed. Both mechanisms consume zinc, so service life within a given environment is broadly proportional to how much zinc is present. A batch hot-dip coating at 85 micrometres carries about 17 times the zinc reservoir of an electro-galvanised coating at 5 micrometres, and lasts accordingly.

Putting numbers to that means working from corrosion categories rather than from adjectives. Zinc loss runs at roughly 0.1 to 0.7 micrometres per year in a rural atmosphere, 0.7 to 2.1 in an urban one, 2.1 to 4.2 in an industrial atmosphere and 4.2 to 8.4 in a marine one. An electro-galvanised coating of 5 micrometres therefore lasts a matter of months in a marine atmosphere, between one and two years in an industrial one, and perhaps two to seven years in an ordinary urban environment. A batch hot-dip coating of 70 to 85 micrometres in a rural or urban atmosphere runs from about 33 years to well beyond a century. In a dry heated interior the picture changes again: at under 0.1 micrometres per year, even a few micrometres of electro-galvanised zinc will outlast the equipment it is part of.

Two cautions apply to any such figure. Zinc loss is not linear in time, because the patina thickens and becomes more protective, so the first year consumes roughly twice the zinc of a later average year and published service lives differ by about a factor of two depending on which rate an author used. The governing question is never which coating is better in the abstract, but how aggressive the environment is and how long the part has to survive in it. Environment classes and expected life are set out in the guide to galvanised steel corrosion protection.

Appearance and surface finish

The surfaces look different because they are made differently. Hot-dip galvanising gives a bright metallic finish that may show spangle, the crystalline pattern formed as the zinc solidifies, along with some variation in texture and the occasional run or drip on batch work. It is a functional finish, not a decorative one. Electro-galvanised sheet comes off the line smooth, matt and uniform, with no spangle and a consistent appearance across the coil, which is why it is the choice where the finish will be seen or painted. EN 10152 recognises this directly by defining 2 surface qualities, type A for a normal surface and type B for the improved surface required where appearance is more demanding, so the finish can be specified rather than assumed. Where a hot-dip coated part has to look uniform, it is normally painted or powder coated over the zinc rather than left bare.

Forming, pressing and paint

Electro-galvanised material is made for cold forming, and EN 10152 covers cold-rolled flat products for exactly that purpose in thicknesses from 0.35 mm to 3 mm. The thin, ductile zinc layer follows the steel through pressing and deep drawing without flaking, and the smooth surface takes paint, primer and adhesive well, which is why it remains a standard choice for enclosures, cabinets and pressed parts where the finish will be seen. One distinction matters here, because the 2 hot-dip routes behave differently. Thick batch coatings applied to finished fabrications are poorly suited to severe forming afterwards, so batch hot-dip parts are formed first and coated second. Continuously coated sheet to EN 10346 is a different product entirely, thin enough at about 20 micrometres per side to be pressed and drawn on the same lines as electro-galvanised material, which is why it has taken over much of the flat product work that electro-galvanised sheet once held.

Painting over hot-dip zinc is entirely possible as a duplex system, and the combination outperforms simple addition: industry guidance puts the total service life at 1.5 to 2.3 times the sum of the 2 systems taken separately, because the paint shields the zinc while the zinc prevents corrosion creeping under defects in the paint. Preparation is what makes it work, and the requirement depends on the state of the zinc rather than on painting being difficult in principle. Freshly galvanised zinc is clean but too smooth and needs a profile. Partially weathered zinc, from roughly 2 days to a year old, carries oxides that have to be removed. Fully weathered zinc carbonate is a sound surface and is left in place.

Cut edges, holes and welds

Both coatings share one useful property: zinc protects a small area of exposed steel sacrificially, so a narrow cut edge or a drilled hole does not immediately start rusting outward. The width that can be protected is set by the electrolyte rather than by the coating, because the current has to travel through the moisture film on the surface. Complete protection extends to roughly 2 millimetres of bare steel in an ordinary atmosphere and roughly 5 millimetres in an industrial or marine one, where the film is more conductive. Coating thickness governs how long that protection lasts, not how far it reaches, which is a distinction worth holding on to when a supplier claims a thicker coating covers a wider edge.

The practical consequence is that edge protection depends on the material thickness being cut. An exposed edge on 1 mm sheet is within the range zinc can cover, while an edge on 2.5 mm or thicker material presents too much bare steel and begins to rust within weeks in a severe atmosphere. On electro-galvanised material, cut edges and welds in an outdoor application need to be sealed or painted as a matter of course. The genuine edge advantage of batch hot-dip is simpler than any argument about throwing distance: because the article is galvanised after it has been cut, drilled and welded, the edges and holes are coated rather than bare. Welding either coating burns the zinc off locally, which raises fume control and repair questions covered in the guide to welding galvanised steel.

Which process for which job

The decision is usually settled by environment and handling, not by preference. Anything structural, outdoor, buried, in a wet or coastal atmosphere or expected to last decades without maintenance belongs on hot-dip, where the coating runs from about 20 micrometres per side on coil to 85 on heavy batch work. Anything indoor, dry, formed in a press and finished for appearance is where electro-galvanised material at 2.5 to 10 micrometres fits. The test to apply is simple: if the part will be rained on, the thin coating is the wrong specification whatever it saves.

Cost, and why the cheaper coating is often the expensive one

Electro-galvanised material costs less than hot-dip for the obvious reason that it carries a fraction of the zinc, and for indoor work that saving is real. The mistake is treating the two as substitutes on an outdoor job. A coating of a few micrometres put into an environment that consumes zinc will fail years before a hot-dip coating of 70 or 85 micrometres, and the replacement cost, including access, removal and downtime, dwarfs the difference in material. Specifying the coating to the environment rather than to the purchase price is the whole discipline, and it is the same logic that governs coating class within hot-dip itself.

The standards to specify against

Three standards cover the ground and naming the right one removes ambiguity from an order. EN ISO 1461 covers hot-dip galvanised coatings applied to fabricated iron and steel articles after manufacture, with minimum thicknesses tied to the steel section. EN 10346 covers continuously hot-dip coated steel flat products, including the Z zinc coatings and the zinc-aluminium-magnesium types, where the nominal thickness ordered already includes the coating. EN 10152 covers electrolytically zinc coated cold-rolled flat products for cold forming, with the ZE designations. Stating the standard and the coating designation together, and the certificate type required, is what makes a specification unambiguous. Certificate types are explained in the guide to EN 10204 material certificates.

Galvanised steel from ULAMEX

ULAMEX supplies hot-dip coated steel flat products and sections to EN 10346, including DX51D in Z coating classes and zinc-aluminium-magnesium coatings, and brings 38 years of experience in European steel and aluminium trading, having been in the metals wholesale industry since 1988. Structures manufactured in-house are made under EN 1090-1 EXC2 and ISO 3834-2. Coating application itself, whether batch galvanising after fabrication or electrolytic coating, is carried out by specialist coating plants rather than by a stockholder, so the material is supplied already coated to the standard specified.

Send the coating standard, designation, grade, dimensions and quantity your project calls for, and ULAMEX will return a non-binding quotation with lead time and availability, with a 3.1 inspection certificate available on request. See the guides to galvanised steel DX51D and galvanised steel sheet, our steel and aluminium supply capabilities, or contact the export desk at [email protected] or +48 504 424 761.

Frequently asked questions