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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 passivation and hot-dip galvanizing.

 

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 preceded by a degreasing stage and followed by draining and drying, the latter being either natural air drying or accelerated by the use of ovens.

 

Electroplated zinc and passivation ("zinc & 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 passivation is a chemical post-treatment which is nearly always used in conjunction with electroplated zinc finishes. Passivation imparts the familiar greeny-yellow colour. Thinner passivation can give a white or shiny finish; this looks cleaner when new but is less effective. Although passivation 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 galvanizing 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 intermetallic alloy layers, ensuring intimate metallurgical bonding between the substrate and the zinc outer surface. A galvanized 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 galvanized 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 galvanized products have been installed. A galvanized 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 brickwork and masonry, as is all too often seen!

Brackets/mounts that are supporting a substantial amount of weight should, ideally always be galvanized. Weld strength will be maintained and the life span of the component will be greatly extended if the product is galvanized. The installer should recognise that a galvanized 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 galvanized coatings.

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

 

 

The table below shows expected lifetimes for galvanized 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 ( m m/year)

0.5

1

1.5

2

2.5

Life – galvanized 55 m m

110 years

55 years

37 years

28 years

22 years

Life – zinc & pass. 8 m m

16 years

8 years

5.3 years

4 years

3.2 years

Life – zinc & pass. 5 m m

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 3 mm steel product

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



Acknowledgements

  • CAI (Confederation of Aerial Industries)
  • The Galvanizers Association

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