The four main problem areas associated with the powder coating of hot dipped galvanised products are:

1) Poor adhesion
2) Pin holing
3) Choice of polyester resin & gloss level - for aesthetics
4) White Rust

Poor adhesion

A powder coat will give good initial adhesion to metal surface, provided it is degreased and free from loose dust or other contamination. However, once in service, the paint film will allow permeation of moisture and gases such as oxygen and sulphur dioxide

This can result in a complete breakdown of adhesion and the film can detach from the surface often in a dramatic way, further more such permeation will promote corrosion of the metal accelerating the process.

Pin Holing

The predominant reason for pin holing lies within the substrate itself. During the hot dip galvanising process gasses can get trapped within the metal skin and then during the curing of the powder coating the trapped gasses start to escape through the paint film. It is for this reason that Pin holing occurs. However this should not be a catalyst for rejection. It is recognised within BS6497 that this can be a problem and is regarded as being acceptable if not excessive.

Aesthetics

Different physical and chemical characteristics of the same of paint may have varied reactions with a galvanised surface. Some types of powder coatings containing polyesters may not be suitable for use over galvanised steel. It is this reason that specially formulated powders containing anti gassing agents are made by powder manufacturers for standard fencing colours. These anti gassing agents do not eliminate the problem of pin holing but minimises it. Even a difference in gloss level will give a different finish. A matt finish is considered more appropriate than full gloss finish because, the latter one will reflect all metal imperfections through the finish. When specifying colours please consult us to see if the requested powder is available in appropriate grade.

White rust

A white film (sometimes called white rust) may appear on zinc surfaces during storage or shipment. The film is found on material with newly galvanised, bright surfaces and especially in such areas as crevices between closely packed sheets and angle bars. This white rust can form if the surfaces come into contact with condensate or rainwater and the moisture does not dry quickly. Zinc surfaces that have developed a normal protective layer of Zinc Carbonate are seldom attacked.

When Zinc coatings corrode openly in air, zinc oxide and zinc hydroxide are normally formed. In the presence of atmospheric carbon dioxide, these compounds are transformed to basic zinc carbonate. If the supply of air to the surface of zinc coating is restricted, as in narrow crevice then sufficient carbon dioxide is not supplied for the formation of the normal later of zinc carbonate.

The layer of zinc oxide and zinc hydroxide is voluminous and porous and adheres loosely to the zinc surface. Consequently, it does not protect the zinc surface against oxygen in the water. Corrosion can therefore proceed as long as there is moisture left on the surface. When white rust occurs the objects should be arranged so their surfaces dry rapidly. The attack ceases and with a free supply of air to the surfaces the normal protective layer of zinc carbonate forms.

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