What Is Galvanized Steel Sheet? Coating Weight, Grades and Uses
How hot-dip galvanizing works, what Z100–Z275 coating codes mean, the difference between DX51D and S350GD, and three questions that decide your order.
Galvanized sheet appears in everything from roof panels to white goods bodies, from ventilation ducts to racking systems. The reason is simple: it combines the low cost and strength of steel with the corrosion resistance of zinc. To choose the right product you only need to read three things: coating weight, grade code and thickness.
How hot-dip galvanizing works
After cleaning and annealing, the cold rolled coil is immersed in a molten zinc bath at roughly 450–460 °C. On exit, air knives set the coating thickness precisely. Zinc does not simply stick to the surface; it forms a metallurgical interlayer with the steel. This structure delivers two kinds of protection:
- Barrier protection: the zinc layer physically cuts contact between steel, moisture and oxygen.
- Cathodic protection: even where the coating is scratched or cut, zinc oxidises before the steel and keeps protecting the exposed area. That is why cut edges remain protected over a limited width.
Coating weight: what the Z codes tell you
Codes such as Z100, Z140, Z200 and Z275 state the total amount of zinc on both surfaces in grams per square metre. Z275 means 275 grams of zinc per square metre in total, which corresponds to roughly 20 microns per side.
| Code | Total zinc (g/m²) | Approx. thickness per side | Typical use |
|---|---|---|---|
| Z100 | 100 | 7 µm | Indoor parts, components to be painted |
| Z140 | 140 | 10 µm | Enclosures, racking, ducting |
| Z200 | 200 | 14 µm | Semi-exposed areas, under-roof |
| Z275 | 275 | 20 µm | Facades, agricultural buildings, humid environments |
Heavier coatings extend corrosion life but also raise the risk of cracking during forming, and the cost. For deep-drawn parts, the Z100–Z140 band is usually safer.
Grade codes: from DX51D to S350GD
EN 10346 defines two families. The DX series covers forming grades: DX51D is the basic quality for general bending and roll forming, while DX52D–DX54D offer increasing deep-drawing capability. The S series covers structural grades; S220GD, S250GD, S280GD and S350GD guarantee increasing minimum yield strength and are used in load-bearing applications.
The "+Z" suffix indicates a standard zinc coating, while "+ZM" indicates a zinc-magnesium-aluminium coating. Manufacturers of trapezoidal sheet and purlins usually work with S280GD and above.
Where is it used?
- Roof and facade panels, trapezoidal sheet and sandwich panel skins
- Heating, ventilation and air conditioning ducting
- White goods inner bodies and chassis
- Automotive sub-industry parts and cable trays
- Racking, enclosures, storage and prefabricated building elements
Three questions before you order
- Where will the product work? Indoors or on a facade; is there contact with chemicals or salty air? The answer sets the coating weight.
- How severe is the forming? Simple bending or deep drawing? The answer sets the DX or S series and the coating ceiling.
- Is there a load-bearing role? If the part carries load, structural grade and thickness follow the structural calculation.
Once these three answers are clear, only one task remains: choosing the right form. Galvanized sheet is available as coil, slit coil and cut sheet; slitting widths and cut-to-length tolerances are agreed at order stage.
Cut edges, welding and painting
On galvanized sheet, a cut edge is left bare. The cathodic protection of zinc covers that edge over a limited width; on thin gauges it is acceptable without further treatment, while on heavier products and outdoor exposure an edge repair with a zinc-rich primer is recommended.
Zinc vaporises during welding. This creates a risk of porosity in the weld and calls for effective ventilation for occupational safety. In spot welding, electrode life is shorter than on bare steel and parameters must be adjusted accordingly. If the part will be painted, the passivation layer and oil must be removed and a suitable primer used; otherwise the paint will not adhere and will flake within a short time.
Three common mistakes
- Confusing coating with thickness: Z275 does not mean the sheet is 275 microns thick; it is the total zinc weight across both surfaces.
- Choosing a heavy coating for deep drawing: a thick zinc layer can flake in the die, generate powder on the surface and shorten die maintenance intervals.
- Ignoring the risk of white rust: in damp, unventilated stacks a white oxide forms on the zinc surface. Strength is unaffected but appearance suffers; dry, ventilated stacking with interleaving paper prevents it.
One last note: on galvanized sheet the delivery form also affects the price. Coil is the most economical form; slit coil differs slightly because of slitting labour and edge trim; cut sheet includes shearing and packing costs. If you need a single size in high volume, producing from coil on your own line usually wins; for many sizes in low volume, ready-cut sheet is normally the better choice.
See our galvanized steel sheet page for details, review our trapezoidal sheet options for roofing, and calculate tonnage with our weight calculator.