Hot-dip galvanising
How it works
Section titled “How it works”After preparation the finished, assembled steel member is dipped in molten zinc. The reaction between zinc and steel forms alloy layers bonded tightly to the parent metal.
The protection is twofold:
- a barrier layer: it physically seals the steel off,
- cathodic protection: at small points of damage the zinc “sacrifices itself”, so a scratch does not start corrosion.
Its advantages
Section titled “Its advantages”- durable: decades of protection in a suitable environment, without maintenance,
- covers the whole surface: internal faces and cavities are protected too,
- mechanically robust: it withstands erection better than paint,
- self-healing at small points of damage.
Its geometric limits
Section titled “Its geometric limits”These have to be handled at design stage, not in fabrication:
The member has to fit in the galvanising bath. A larger member can only be galvanised in stages or in split form.
Vent and drain holes
Section titled “Vent and drain holes”In hollow sections and cavities the zinc has to flow in and out and the air has to escape. So holes have to be designed — drilling them afterwards is much worse.
Distortion
Section titled “Distortion”The heat of the dip releases and induces stresses. In long, asymmetrical or thin members that can cause deformation.
Bolt holes
Section titled “Bolt holes”Because of the thickness of the zinc layer, hole sizes and the treatment of threads have to be planned in advance.
What it means for the connections
Section titled “What it means for the connections”Weld first, then galvanise
Section titled “Weld first, then galvanise”That is the right order: welding first, galvanising after.
Welding after galvanising
Section titled “Welding after galvanising”To be avoided: the coating locally vaporises, it has to be repaired, and the welding conditions are worse too.
Bolted connections
Section titled “Bolted connections”The natural solution for galvanised members, and what site erection calls for anyway. Welded and bolted connections.
The link with the thin-walled side
Section titled “The link with the thin-walled side”Thin-walled profiles are made from continuously galvanised coil — there the coating is already on the material rather than applied afterwards. Zinc coating.
Where the two kinds of member meet, material pairing and the compatibility of the coatings is a separate consideration.
The point
Section titled “The point”Hot-dip galvanising is the most durable corrosion protection — but it has geometric consequences. Handle them at the drawing board, not in the galvanising plant.