DX51D and galvanised steel: grades, zinc coatings and corrosion protection
Galvanised steel is steel coated with zinc for corrosion protection, and DX51D is its standard sheet grade. The zinc layer protects the steel sacrificially, even at a scratch, which is why galvanised steel is the economical choice for structural, outdoor and agricultural work. This guide covers DX51D and the EN 10346 grades, the zinc coatings from Z275 to Magnelis, the two galvanising routes, corrosion life by environment, and the galvanised range ULAMEX supplies as sheet, profiles, fences, gratings and wire.
What galvanised steel is
Galvanising coats steel with a layer of zinc that protects it in two ways. It is a physical barrier between the steel and the atmosphere, and it is sacrificial: the zinc corrodes in preference to the steel, so even a scratch or cut edge stays protected as the surrounding zinc gives up itself first. This self-healing, cathodic protection is what gives galvanised steel a service life measured in decades.
The base steel is usually a structural grade such as S235JR or S355J2; the corrosion performance is set by the coating type and weight. Galvanised steel costs far less than stainless steel and is the standard for structural frames, fencing, gratings, agricultural buildings and outdoor steelwork.
Two galvanising routes: continuous and batch
There are two ways to apply zinc, and the right one depends on whether you start with sheet or with a finished fabrication.
| Route | Standard | Coating | Used for |
|---|---|---|---|
| Continuous hot-dip (coil and sheet) | EN 10346 | Z, ZM, AZ classes (for example Z275) | Galvanised sheet and strip, profiles formed from coil |
| Batch hot-dip (after fabrication) | EN ISO 1461 | Thicker coating, 45 to 85 micrometres | Fences, gratings, posts, welded structures |
| Electro-galvanising | – | Thin, 5 to 25 micrometres | Indoor, light-duty, where a thin coat is enough |
Both hot-dip routes immerse the steel in molten zinc at around 450 degrees C, forming a metallurgical zinc-iron bond. Batch galvanising applies a thicker coating and is the structural standard; continuous coating gives the thinner, uniform Z classes used for sheet and formed profiles. Electro-galvanising is a thin electroplated layer for light indoor use only. The two processes are compared in detail, with coating thickness bands and service life by environment, in hot-dip against electro-galvanised steel.
The hot-dip galvanising process
Batch hot-dip galvanising follows a controlled sequence. The steel is first degreased, then acid-pickled to remove rust and mill scale, rinsed, and fluxed in a zinc ammonium chloride solution that keeps the surface active. It is then dipped into molten zinc at around 450 degrees C, where the zinc reacts with the steel to form a series of zinc-iron alloy layers, and finally withdrawn and cooled.
The result is not a coating that sits on the surface but a metallurgical bond: hard zinc-iron alloy layers (gamma, delta and zeta) grow into the steel, with a softer outer layer of pure zinc. This bonded structure is why hot-dip galvanising is tough, abrasion-resistant and self-healing at a scratch, and why it lasts for decades.
DX51D and the EN 10346 grades
For continuously galvanised sheet and strip, the grade follows EN 10346. DX51D is the entry-level, general-purpose grade, suited to bending and shallow forming, and is by far the most common galvanised sheet. Where deeper forming is needed, the higher DX grades have more elongation. For load-bearing sheet, the structural S grades carry a guaranteed yield strength.
| Grade | Type | Use |
|---|---|---|
| DX51D | Forming, entry grade | General bending and shallow drawing, the standard galvanised sheet |
| DX52D, DX53D, DX54D | Forming, higher ductility | Deeper drawing and more complex formed parts |
| S220GD to S550GD | Structural | Load-bearing sheet and sections with a specified yield strength |
A galvanised sheet is specified by grade and coating together, for example DX51D+Z275: the grade sets the forming behaviour and the coating sets the corrosion protection.
The continuous galvanising line
Sheet, strip and coil are galvanised on a continuous line rather than by batch dipping. The steel strip runs through cleaning and annealing and then a bath of molten zinc, and as it leaves the bath an air knife blows off the excess to set the coating weight precisely, which is how the Z classes such as Z275 are controlled. The line coats both sides evenly and continuously, giving the thin, uniform coating used for DX51D sheet and for profiles roll-formed from coil. Batch galvanising, by contrast, coats a finished fabrication all over in a single dip and gives a thicker coating.
Zinc coating classes: Z275 and beyond
The Z number is the total zinc mass in grams per square metre across both sides of the sheet. A higher number means more zinc and longer protection.
| Coating | Zinc mass (both sides) | Approximate per side |
|---|---|---|
| Z140 | 140 g/m2 | About 10 micrometres |
| Z275 | 275 g/m2 | About 20 micrometres |
| Z350 | 350 g/m2 | About 25 micrometres |
| Z450 | 450 g/m2 | About 32 micrometres |
Z275 is the common general-purpose coating for outdoor and industrial sheet; heavier classes such as Z350 and Z450 are used where the environment is more corrosive. For galvanised tube, EN 10240 applies instead.
Coating thickness to EN ISO 1461
For batch hot-dip galvanising, EN ISO 1461 sets the minimum coating thickness by the thickness of the steel: thicker steel takes a thicker coating, and a thicker coating lasts longer. The mean coating thickness values below are the reference on the galvanising certificate.
| Steel thickness | Mean coating (min.) |
|---|---|
| 6 mm and above | 85 micrometres |
| 3 mm to below 6 mm | 70 micrometres |
| 1.5 mm to below 3 mm | 55 micrometres |
| below 1.5 mm | 45 micrometres |
Steel with a higher silicon content can grow a thicker coating still. The galvanising certificate confirms the coating weight and thickness for the delivered batch.
Magnelis and zinc-aluminium-magnesium coatings
Beyond pure zinc, EN 10346 also covers zinc-aluminium-magnesium (ZnAlMg) coatings, designated ZM and marketed as Magnelis. The magnesium and aluminium make the coating far more durable than pure zinc for the same thickness, and it gives genuine self-healing at cut edges, protecting the bare steel across several millimetres of a cut face. ZM coatings outperform an equivalent Z coating in aggressive C4 and C5 environments, which is why they are increasingly specified for solar mounting, agricultural and coastal structures. Where corrosion is severe, a ZM coating is the upgrade from standard Z.
Other coatings: Aluzinc, Galfan and galvannealed
EN 10346 covers several metallic coatings beyond pure zinc and ZM, each suited to a particular need.
| Coating | Composition | Best for |
|---|---|---|
| Z (zinc) | Pure zinc | General galvanised sheet and profiles |
| ZM (Magnelis) | Zinc-aluminium-magnesium | Aggressive C4 and C5, solar mounting, coastal |
| AZ (Aluzinc) | Aluminium-zinc (about 55 percent aluminium) | Heat reflectance and atmospheric durability, roofing |
| ZA (Galfan) | Zinc with about 5 percent aluminium | Improved formability and corrosion over pure zinc |
| ZF (galvannealed) | Zinc-iron alloy | A paintable, weldable surface |
For most work the choice is between a Z class and a ZM coating; the aluminium-rich AZ, ZA and ZF coatings are specified where reflectance, formability or paint adhesion is the priority.
Corrosion life by environment
Coating life depends on the corrosivity of the environment, classified by EN ISO 9223 from C1 (indoor dry) to C5 (severe marine or industrial), and on the coating thickness. The figures below are for thicker batch hot-dip coatings; thinner sheet coatings such as Z275 give proportionally shorter protection, and a ZM coating extends life in the harsher categories.
| Environment (EN ISO 9223) | Hot-dip galvanised life to first maintenance |
|---|---|
| C2 rural or suburban | 50 to 100 years |
| C4 industrial or coastal | 25 to 40 years |
| C5 severe marine | 15 to 25 years |
Galvanised steel applications by sector
Galvanised steel is the default corrosion-protected steel across several sectors.
Construction and structural. Purlins, frames, gratings and secondary steelwork use galvanised profiles and sections for decades of maintenance-free service. Fencing and perimeter. Posts, panels and gates rely on galvanised and ZM coatings for outdoor life. Agriculture. Sheds, pens and equipment in damp, corrosive environments. Solar mounting. Ground-mount and roof structures increasingly use ZM-coated steel for its cut-edge protection. Infrastructure. Crash barriers, walkways, gratings and lightning protection. In each case the grade and coating are matched to the load and the environment.
Welding galvanised steel
Galvanised steel can be welded, with precautions. The zinc must be removed from the weld zone, typically 10 to 25 mm each side of the joint, and the area must be well ventilated because heated zinc gives off fumes. After welding, the bare area is restored with a zinc-rich paint or spray galvanising to close the corrosion gap. ULAMEX supplies galvanised products intended for both bolted and welded structural connections.
Cut edges and repair
Where a galvanised part is cut, drilled or welded after coating, the bare steel is exposed and must be restored to close the corrosion gap. The repair is made with a zinc-rich paint or by zinc spray (metallising), applied to the cleaned area to the required thickness. On continuously galvanised sheet a cut edge is protected by the sacrificial action of the surrounding zinc across a few millimetres, and a ZM coating extends that self-healing further. Where possible, cut and drill before galvanising so the coating covers the finished part.
White rust and storage
White rust is a white, powdery zinc hydroxide deposit that can form when moisture is trapped between stacked, newly galvanised components. It looks alarming but is superficial and does not affect long-term protection. It is prevented by storing galvanised material with adequate ventilation and spacing, so moisture does not sit between surfaces.
Choosing the right coating
Match the coating to the environment and the job. The table is a starting point; for a borderline case, send the location and exposure with your enquiry.
| Situation | Coating |
|---|---|
| General outdoor and industrial sheet | Z275 |
| More corrosive industrial exposure | Z350 or Z450 |
| Coastal, severe (C4 to C5), solar mounting | ZM (Magnelis) |
| Batch-galvanised fabrications and structures | Hot-dip to EN ISO 1461 |
| Roofing needing heat reflectance | AZ (Aluzinc) |
Where galvanised steel is not the right choice
Galvanised steel covers most structural and outdoor work, but there are limits. In strongly acidic or alkaline environments, and in the most aggressive marine service, the zinc is consumed quickly and stainless steel or a ZM coating is the better answer. Galvanising is not for high-temperature service: above roughly 200 degrees C the coating degrades, and zinc melts at about 419 degrees C. For food-contact and hygienic surfaces, stainless steel is specified rather than galvanised. Matching the material to the environment is what a durable installation depends on.
Galvanised steel versus stainless steel
Galvanised and stainless steel solve corrosion differently. Galvanised steel gives excellent corrosion resistance at a much lower cost and is ideal for structural, outdoor and agricultural work. Stainless steel resists chemicals, high temperatures and chlorides better, and is required for food, pharmaceutical, architectural and marine surfaces; see our guide to 304 and 316 stainless steel. For most construction and infrastructure, hot-dip galvanised steel is the cost-effective choice; where the environment is chemically aggressive or the surface must be hygienic, stainless is specified instead.
Galvanised steel in construction
Galvanised structural steel used in load-bearing construction is designed to the Eurocodes and fabricated to EN 1090. ULAMEX operates EN 1090-1 EXC2 and ISO 3834-2, and supplies galvanised structural products with CE marking and the Declaration of Performance where applicable, plus a galvanising certificate confirming the zinc coating and a 3.1 mill certificate on request. A structural base grade (S235JR or S355J2), a controlled coating and full documentation are what a CE-marked or audited project requires.
Duplex systems: galvanising plus paint
A duplex coating combines hot-dip galvanising with a paint or powder-coat top layer. The two together last longer than the sum of each alone, because the paint protects the zinc and the zinc protects the steel if the paint is damaged. A duplex system also adds colour, which is why galvanised-then-powder-coated steel is used for architectural steelwork, fencing and solar structures that need both a long life and a finish. The galvanised surface is prepared before coating so the paint adheres.
Galvanised product range in detail
The ULAMEX galvanised range covers four product groups. Galvanised profiles include C, U and Z purlins for roofing and cladding frames, plus angles and flat bars for general fabrication. Fences and systems cover 3D welded panels, posts and gates for perimeter and security fencing. Gratings and platform steps, in press-locked and welded forms, provide walkways, flooring and access platforms. Wire and band irons cover construction wire, fencing wire and lightning-protection conductors. All are galvanised on a structural base of S235JR and S355J2 to the relevant European standards.
Sustainability and lifecycle
Both steel and zinc are fully recyclable, and a hot-dip galvanised structure that lasts 50 years or more with no maintenance has a low lifecycle cost and footprint compared with a painted structure that needs repeated recoating. The zinc coating is applied once and protects for decades, and at end of life the galvanised steel is recycled without separating the zinc. For projects assessed on whole-life cost or environmental criteria, this durability is a large part of why galvanised steel is specified.
Galvanised steel from ULAMEX
ULAMEX has supplied galvanised steel since 1988, more than 38 years in the trade. The range covers four product groups, galvanised to the relevant European standards on a structural base of S235JR and S355J2.
| Group | Products |
|---|---|
| Galvanised profiles | C, U and Z purlins, angles and flat bars |
| Fences and systems | 3D panels, posts and gates |
| Gratings and platform steps | Press-locked and welded gratings, steps |
| Wire and band irons | For construction, fencing and lightning protection |
| What you get | Detail |
|---|---|
| Coatings | Hot-dip to EN ISO 1461; continuous Z classes (Z275 and others) and ZM to EN 10346 |
| Certification | EN 1090-1 EXC2 and ISO 3834-2; 3.1 mill certificate and galvanising certificate on request |
| Lead time | Typically 2 to 7 working days on stocked lines |
| Single source | Galvanised with structural steel and stainless on one order |
The corrosion resistance of galvanised and ZM-coated steel also makes it the structural choice for PV mounting and outdoor frames. Need galvanised steel for a project? Send the product, grade and coating with your dimensions, and ULAMEX will return a non-binding quotation. Browse the full galvanised steel range, or contact the export desk at [email protected] or +48 504 424 761. For our full range, certification and quote process, see steel and aluminium supply capabilities.
Frequently asked questions
What is DX51D steel?
DX51D is the entry-level, general-purpose grade of continuously hot-dip galvanised steel sheet to EN 10346. It suits general bending and shallow forming and is the most common galvanised sheet grade, usually specified with a zinc coating such as DX51D+Z275.
What does Z275 mean?
Z275 is a zinc coating class under EN 10346: 275 grams per square metre of zinc across both sides of the sheet, roughly 20 micrometres per side. It is the common general-purpose coating for outdoor and industrial galvanised sheet; Z350 and Z450 give more zinc for harsher environments.
What is the difference between hot-dip and electro-galvanising?
Hot-dip galvanising immerses steel in molten zinc at around 450 degrees C, giving a thick, durable coating (45 to 85 micrometres on batch-galvanised structures) with a metallurgical bond. Electro-galvanising applies a thin layer (5 to 25 micrometres) and suits only light indoor use. For structural and outdoor work, hot-dip is the standard.
What is Magnelis or ZM coating?
ZM is a zinc-aluminium-magnesium (ZnAlMg) coating under EN 10346, marketed as Magnelis. The aluminium and magnesium make it far more durable than pure zinc for the same thickness, with strong self-healing at cut edges, so it outperforms a Z coating in aggressive C4 and C5 environments such as coastal and solar mounting.
How long does galvanised steel last?
It depends on the environment (EN ISO 9223) and coating thickness. Thicker batch hot-dip coatings last 50 to 100 years in rural or suburban (C2), 25 to 40 years in industrial or coastal (C4) and 15 to 25 years in severe marine (C5). Thinner sheet coatings last proportionally less, and ZM coatings extend life in the harsher categories.
Can galvanised steel be welded?
Yes, with precautions. Remove the zinc from the weld zone, typically 10 to 25 mm each side, ventilate well because heated zinc gives off fumes, and after welding restore the bare area with zinc-rich paint or spray galvanising.
What is white rust on galvanised steel?
White rust is a white, powdery zinc hydroxide deposit that forms when moisture is trapped between stacked newly galvanised parts. It is superficial and does not affect long-term protection. Store galvanised material with ventilation and spacing to prevent it.
Galvanised or stainless steel: which should I choose?
Galvanised steel gives excellent corrosion resistance at lower cost for structural, outdoor and agricultural work. Stainless steel is used where chemicals, high temperature, chlorides or a hygienic surface are involved, such as food, pharmaceutical and marine applications. ULAMEX supplies both.
What galvanised products does ULAMEX supply?
Galvanised profiles (C, U and Z purlins, angles, flat bars), fences and systems (3D panels, posts, gates), gratings and platform steps, and wire and band irons, on a structural base of S235JR and S355J2, with hot-dip and continuous Z and ZM coatings, with 3.1 and galvanising certificates on request.
What is Aluzinc (AZ) coating?
Aluzinc (AZ) is an aluminium-zinc coating, around 55 percent aluminium, under EN 10346. It offers good atmospheric durability and heat reflectance and is common on roofing sheet. For aggressive or coastal service, a ZM (Magnelis) coating is usually the stronger choice.
What coating thickness does hot-dip galvanising give?
Under EN ISO 1461 the minimum mean coating thickness rises with the steel thickness: about 45 micrometres below 1.5 mm, 55 micrometres for 1.5 to 3 mm, 70 micrometres for 3 to 6 mm and 85 micrometres at 6 mm and above. The galvanising certificate confirms the thickness for the batch.
What is a duplex coating system?
A duplex coating is hot-dip galvanising plus a paint or powder-coat top layer. The two together last longer than the sum of each alone and add colour, which is why galvanised-then-coated steel is used for architectural, fencing and solar structures.