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Galvanising

Which Protective Finish do I Need on my Brackets?

There are many ways of protecting metal from the weather. Common methods adopted for protecting steel aerial support brackets include painting,zinc electroplating or passivisation and hot-dip galvanising.

Painting:

Painting can be the lowest-cost method but usually gives relatively weak protection. One industrial method adopted for painting steel brackets is to immerse the component in a bath of 'paint' (actually a specialist coating formulation). This process may be proceeded by a de-greasing stage, followed by draining and drying, the latter being either natural air dried or accelerated by the use of ovens.

Electroplated zinc and passivisation:

Electroplated zinc and passivisation ("zinc and pass.") provides good protection, but with a fairly limited lifetime when used outdoors. Electroplated coatings are thin and will corrode away, typically in under ten years. Chromate passivisation is a chemical post-treatment which is nearly always used in conjunction with electroplated zinc finishes. passivisation imparts the familiar greeny-yellow colour. Thinner passivisation can give a white or shiny finish; this looks cleaner when new but is less effective. Although passivisation improves corrosion performance in salt spray tests, it does not greatly increase the lifetime of a part such as an aerial bracket in typical outdoor conditions.

Hot-dip galvanising:

Hot-dip galvanising is the process of dipping steel components into a bath of molten zinc, following a chemical cleaning pre-treatment. The zinc reacts with the steel to form inter-metallic alloy layers, ensuring intimate metallurgical bonding between the substrate and the zinc outer surface. A galvanised zinc coating is very much thicker possibly an order of magnitude thicker than a passivated electroplated zinc finish and has a correspondingly longer lifetime.

Section 17 of the CAI Code of Practice for the Installation of Terrestrial and Satellite TV Reception Systems states that hot dip galvanised brackets should always be used. This is for good reason; the installer will have peace of mind that a good job has been done if dip galvanised products have been installed. A galvanised bracket or mount will always be considered to have superior protection compared to a painted or plated item. Use of the inferior finishes inevitably leads to rusting once the coating has failed and this in turn can lead to unsightly staining of brick work and masonry, as is all too often seen!

Brackets/mounts that are supporting a substantial amount of weight should, ideally always be galvanised. Weld strength will be maintained and the life span of the component will be greatly extended if the product is galvanised. The installer should recognise that a galvanised part is always the best choice, especially when bearing in mind the wind loading on a bracket/mount when the aerial or dish are installed.

The following is a summary of the Zinc Millennium Map Project, which was an investigation into the background atmospheric corrosion rate of zinc coatings in the UK and Ireland

The Zinc Millennium Map Project

It is widely known that levels of atmospheric sulphur dioxide (SO²) have been falling for several decades due to the changes in prime power source fuels. The result of the Zinc Millennium Map Project† demonstrates how the reduction in SO² levels since 1991 has helped to increase the life expectancy of hot dip galvanised coatings.

Hot dip galvanising, in conjunction with its many other benefits, including lowest life time cost, reliability and coating toughness, is the most effective corrosion protection system available.

Zinc Galvansing Map

The table below shows expected lifetimes for galvanised and electroplated zinc coatings for each of the five corrosion categories shown in the map.

Corrosion category

1

2

3

4

5

Average corrosion rate ( mm/year)

0.5

1

1.5

2

2.5

Life - galvanised 55mm

110 years

55 years

37 years

28 years

22 years

Life - zinc and pass. 8mm

16 years

8 years

5.3 years

4 years

3.2 years

Life - zinc and pass. 5mm

10 years

5 years

3.3 years

2.5 years

2 years

Data for painted finishes are not available.

Figures above are based on a typical 3mm steel product.

For further information please refer to http://www.galvanizing.org.uk/

Acknowledgements

  • CAI (Confederation of Aerial Industries).
  • The Galvanisers Association.

Annual average atmospheric corrosion of zinc, UK and Republic of Ireland, 1998-2000.

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