Hot dip galvanising explained
Galvanising is the default corrosion protection for outdoor steel in Ireland. It is worth understanding what happens in the bath, because it affects how the steel has to be designed.
The process
Finished steel is degreased, pickled in acid to remove mill scale and rust, fluxed, then dipped into molten zinc at around 450 degrees. The zinc reacts with the steel surface to form a series of zinc iron alloy layers with pure zinc on top.
Because it is a reaction rather than a coating laid on the surface, the protection is bonded to the steel, covers every surface the zinc reaches including inside hollow sections, and self heals small scratches sacrificially.
Vent and drain holes
Any sealed hollow section will explode in the bath as trapped air and moisture expand. Every hollow member therefore needs vent and drain holes, sized and positioned so zinc flows in and out freely.
These are designed in during fabrication. They also serve the item for the rest of its life by letting rainwater escape rather than sitting inside.
Distortion and appearance
The heat of the bath can distort thin or asymmetric fabrications, so items are designed with balanced sections where possible. Thin plate and long unsupported spans are the ones to watch.
The finish is a mottled grey and it is not uniform. If appearance matters, powder coating over the galvanising gives an even colour and the longest overall life.
Lifespan
In ordinary Irish inland conditions a galvanised coating lasts decades with no maintenance. Marine and industrial atmospheres are more aggressive and shorten it, though still far beyond any paint system.
We design for galvanising from the outset and arrange the process as part of the job.
Want this priced for your own job? Send us your drawings or a photo and we will come back with a fixed price, usually within one working day.
Common questions
Need this done properly? Talk to the workshop.
Send us drawings, a sketch or a site photo. We come back with a fixed price and a firm lead time, usually within one working day.