Galvanising

How does Galvanising work, step by step?

1. 1

   Step 1: Design the fabrication for the bath

   Galvanising is designed in. The size and shape of the fabrication have to suit the bath available, and where a member is longer than the bath the galvaniser is consulted about double dipping, which puts a visible line where the two immersions meet, or the member is split and jointed on site. Hollow sections, box members, sealed pockets and back-to-back angles need vent and drain holes sized and positioned so that zinc and gases can move freely and so that nothing is trapped. Assemblies of very different thicknesses, long slender members and heavily welded frames are reviewed for distortion risk. Lifting points, jigging points and the way the item will hang in the bath all affect the finish, because zinc drains downwards and the orientation decides where runs and pooling end up.

2. 2

   Step 2: Fabricate, then clean off everything that would stop the reaction

   All cutting, drilling, welding and fabrication is completed before galvanising, because zinc cannot be cut or welded through afterwards without destroying it locally. Welds are made continuous where possible, and welding slag, flux residue and spatter are removed, since none of them will react with zinc and each will leave a bare patch. Paint, markings, oil, grease and heavy scale are removed. Identification is applied in a form that survives the process rather than in paint that will not. Anything applied to the surface that the pre-treatment cannot remove will show up as a defect in the finished coating, and by then the item is at the works and the fix is a re-clean and a re-dip.

3. 3

   Step 3: Degrease, pickle and flux

   The pre-treatment is what makes the reaction possible. The steel is degreased to remove oils and greases, rinsed, then pickled in acid to strip the mill scale and rust and leave chemically clean steel, and rinsed again. It is then fluxed, which cleans the surface at a microscopic level and leaves a thin protective film that stops it oxidising again in the moments between the flux tank and the bath. The item is dried before immersion, because moisture entering molten zinc flashes to steam violently. Each stage has a purpose and none is optional. Most coating defects that appear at the bath are traced back to something that went wrong or was skipped in pre-treatment.

4. 4

   Step 4: Immerse in the zinc and let the alloy layers form

   The dried, fluxed fabrication is lowered into the molten zinc at a controlled rate, held until it reaches bath temperature and the alloy layers have developed, then withdrawn slowly so the excess zinc drains cleanly rather than freezing as runs and icicles. Immersion times are a matter of minutes for light items and longer for heavy sections, and the galvaniser judges them from the mass and thickness of the work. The thickness of the coating that forms is largely a function of the steel itself, particularly its chemistry and its surface condition, and of the thickness of the section - which is why the coating on a heavy section behaves differently from the coating on thin sheet, and why the specification is agreed with the galvaniser rather than assumed.

5. 5

   Step 5: Cool, dress and inspect

   The work is cooled in air or quenched, then dressed - runs, icicles, drips and rough edges are removed, and threads and machined faces that have to fit are cleaned out. Bolted assemblies are trial-fitted where tolerances matter, because the coating takes up clearance in holes and on faying surfaces and a hole that was a fit before galvanising may not be one after. The finish is inspected for bare patches, inclusions, ash and lumps, and the coating is checked against the specification. Appearance varies - some items come out bright and some matt grey, and both can be entirely sound, since the grey is a function of the steel chemistry rather than a fault.

6. 6

   Step 6: Handle, erect and make good on site

   Galvanised steel is handled with fabric or soft slings rather than chains, stacked with spacers and stored so that water cannot sit between closely packed surfaces - newly galvanised steel left nested in the wet develops white storage staining, which is unsightly and consumes coating. On site, any damage, any site-welded connection and any hole cut or drilled after galvanising leaves bare steel that has to be made good, normally with a zinc-rich paint or a zinc spray applied to a prepared surface. The zinc around a small area of damage will protect it sacrificially for a while, but repair is still carried out and recorded. Where the steelwork is being painted or powder coated as a duplex system, the zinc surface is prepared as the paint system requires and a compatible primer is used.