Corrosion protection is chosen for the structure and the environment, not for the final colour alone.
Galvanising and paint systems are different approaches to protection. The choice depends on the exposure environment, the geometry of the structure, the requirements of appearance, future repairs and whether proper preparation is possible.
On demanding projects we look at the protection system before fabrication, so that the detailing does not create water traps or points that cannot be reached for maintenance.
The choice follows from the exposure environment and the geometry, not from the colour. Hot-dip galvanising to EN ISO 1461 is not a coating on the steel but an alloy with it, and the thickness follows the thickness of the material: a mean of about 70 µm on steel of 3–6 mm, about 85 µm above 6 mm — which is why thin sheet is not protected as well as a heavy section. EN ISO 9223 grades the environment from C1 to C5, and that grading is done per project, not per region. A duplex system — galvanising and paint together — lasts longest.
How much zinc galvanising puts on
Hot-dip galvanising to EN ISO 1461 is not a layer on top of the steel but an alloy with it. The thickness depends on the material thickness: for steel from 3 to 6 mm a mean of about 70 µm is required, above 6 mm about 85 µm. That is why thin sheet is not protected to the same degree as a heavy section.
The corrosivity of the environment
EN ISO 9223 classifies the environment from C1 to C5. Distance from the sea, prevailing wind and humidity count for more than the region does: two houses three hundred metres apart can fall into different categories. The classification is made per project, not per area.
The design decides before the material does
- closed sections need vent holes to be galvanised properly,
- any point where water stands is the point that will corrode first,
- a duplex system — galvanising plus coating — lasts longest, but the galvanised surface has to be prepared correctly first.
