Hot Dip Galvanising
Hot dip galvanising is the process of dipping a clean steel fabrication in liquid Zinc (Zn).
Galvanising is a commonly used form of corrosion protection for steel. It produces a versatile and tough, metalurgical coating. It is a single coating operation that produces complete coverage and is generally maintenance free.
The galvanising process
Galvanising requires a clean steel surface to form the coating. The fabrication is first immersed in an acid bath to pickle the surface clean. (Note - Acid will not clean any oils, grease, paint or organic coating – these require cleaning with an appropriate solvent or caustic cleaner).
After pickling the acid is rinsed from the surface.
The fabrication is then dipped in a liquid zinc bath at 450-470 oC.
As the fabrication is steadily withdrawn from the bath, the galvaniser attends to any runs or drips to produce a clean coating.
The galvaniser then removes any lifting or hanging wires and fettles any spikes, runs or lumps of zinc that have formed. This level of cleaning may be sufficient for your application.
However, fabrications for timber engineering, solar shading or façade engineering will require further cleaning and fettling.
Likewise fabrications that require subsequent painting or powder coating will also require further cleaning and fettling. Talk to your sales contact about our experience for your application.
Most steel fabrications are suitable for galvanising. However the fabrication needs a pattern of drilled holes or corner cutouts to ensure that the dipped piece can drain the molten zinc as it is withdrawn from the galvanising bath. See more at https://www.galvanizing.org.uk/design-for-galvanizing/
Some fabrications, eg. thin sections or complex shapes incorporating differing section thicknesses, may distort due to the temperature of the bath. Talk to your sales contact about our experience with galvanising.
Corrosion protection mechanism & coating thickness
Galvanising works by providing a protective barrier to the steel surface. The Zn corrodes in preference to the steel and is a sacrificial coating. The rate at which the Zn corrodes determines the life of the coating. Thicker coatings can give longer life protection. There are tables of indicative lifetime protection for the geographical and environmental exposure of galvanised fabrications. See more at https://www.galvanizing.org.uk/corrosion-map/
When dipped the steel forms a layer of complex Fe-Zn compounds and a surface coating of pure Zn. Coating thicknesses are determined by EN ISO 1461. The usual galvanising process produces a coating of ~85µm thick.
The visual appearance of the final surface finish of a galvanised fabrication can vary. Usually it is a bright and shiny - sometimes with a characteristic galvanised ‘spangle’.
However heavier steel sections and thicker coatings result in a surface which can be a dark grey matt finish. The change in appearance does not alter the corrosion resistance of the coating. The coating thickness can also be determined by the base steel. Steels with higher silicon (Si) content also pick up a thicker coating.
Where a heavier galvanised coating for is required for service life, thicknesses of 130µm can be achieved by shotblasting the surface prior to dipping.
Photo of 200µm as measured by Elcometer.
Spin galvanising is a process where the components are centrifuged after dipping in molten zinc. This is mostly used for fasteners and small components to maintain the definition and detail of the galvanised part. This generates a thinner coating thickness. The details are defined by EN ISO 1461.
Painting or Powder Coating after galvanising
Fabrications can be painted after galvanising.
This requires a T-wash or mordant coating prior to application of paint. The galvanised surface is T-washed, allowed to react and then to dry completely.
The paint system should be specifically formulated for use on galvanised steel. The choice ranges from single coat paint systems through to multi coat systems of either two pack polyeurethanes, high build epoxies or MIO. Selecting the correct system depends upon the environment and service requirements. Specialist paint manufacturers will be able to advise appropriately.
Piece size dimensions for galvanising
Daver Steels Quality Management System maintains a number of approved galvanising subcontractors for quality and turnaround time.
Fabrications up to 7m long in single dip are easily managed.
For larger pieces, fabrications up to 17m long in a single dip can be processed.
Smaller components and brackets can be processed by our approved spin galvaniser.
Talk to your sales contact about your options.