Condition of steel surface prior to hot-dip galvanising
Hot-dip galvanising involves immersing steel in a zinc bath containing molten zinc at a temperature of 450 ºC. During the galvanising process, the zinc forms an alloy with the steel and an unbreakable layer is created. However, this layer can only form on an absolutely clean steel surface. If the steel has not been adequately cleaned, non-galvanised areas are left as black spots that stand out on the silver zinc coating.
This means that steel has to be chemically pretreated before it can be hot-dip galvanised, with rust, mill scale, thin greasy layers, and various types of oil being removed.
Conditions for hot-dip galvanising
For a good galvanising result, the incoming materials must satisfy a number of conditions:
- the steel quality must be satisfactory: see NEN-EN-ISO-1461;
- vent and drain holes, and suspension holes, must have been made in the correct manner and in the right places (see also: Drill holes);
- materials must be free of impurities that cannot be removed by the standard chemical pretreatment;
- materials must not consist of a combination of galvanised steel and unprocessed steel.
That means incoming materials must be free of:
- paint and/or varnish (or traces);
- welding sprays that are too thick or contain silicone;
- welding slag;
- thick oil and/or grease layers;
- marks with pastel crayon, paint or inappropriate markers;
- adhesives, glues, stickers etc.;
- old zinc coatings.
Result of hot-dip galvanising by use of paint letters
Ungalvanised spot by grease and/or paint
Material with zinc or traces of zinc on it, for example because it has been galvanised previously, can only be galvanised again if the old zinc coating has been removed entirely. For degalvanising, the Rotocoat galvanising plants use a special degalvanising bath, also known as the strip bath. Clients sending material that requires degalvanising should inform Rotocoat accordingly in advance. Degalvanising is a time-consuming process that needs to be planned so that delivery times can be met.
Surfaces that cannot be galvanised
It sometimes happens that people do not want some parts of an object to be galvanised, examples being screw threads or places where welding needs to be done. These areas can be covered with an appropriate material. Alternatively, the zinc coating can be removed mechanically after galvanising has taken place. Rotocoat wishes to emphasise that there should be consultation in advance with the galvanising plant about the best approach because this depends very much on the object being galvanised and the components that should not be galvanised.
Rolling flaws, splinters or overlaps should be removed before sending the material. If this is not done, these flaws can be seen and felt as sharp protrusions after galvanising.
If materials go through the Duplex Route and are given another coating after hot-dip galvanising, they must meet specific requirements. For a good result and a long coating life, it is important for the substrate to be free of sharp edges and burrs. For more information: technical information about powder coating.