corrosion prevention of galvanised steel

Architect's memo 2: August 1980

Traditionally the zinc coating of steel has been regarded as a protective coating in itself, being painted only for aesthetic reasons. Increased corrosion rates of zinc, particularly in industrial and marine environments, have lead to the realisation that not protecting the zinc from corrosion is wasteful of a decreasing resource.

Standard primers for zinc however, do have a number of drawbacks such as:

1. They are two-component mixtures; such as zinc dust-zinc etch oxide, two pack etch primers

2. They have very critical recoat times; such as etch primers, calcium plumbate

3. They contain toxic components, such as etch primers, calcium plumbate

Standard acrylic paints adhere well to galvanised steel but do little to reduce the corrosion of the reactive zinc.

It was recognised that there existed a gap in the primer range that could be filled with a particular product conforming to the following parameters:

1. Would adhere in the short and long term to new and weathered galvanised steel

2. Would passivate the reactive zinc surface to retard corrosion

3. Would have a secondary effect as a ferrous metal primer

4. Would be a one-pot material

5. Would have a non-critical recoat time

6. Would be suitable for roofs collecting potable water

7. Would be able to be topcoated with a variety of topcoats (such as alkyd, acrylic, chlorinated rubber)

Although this does appear to be a tall order, these parameters have all been incorporated in Resene Galvo One. This product now has a technical history of nine years and a commercial history of six. It has been widely accepted by specifying authorities, and is also being manufactured overseas. The possibility of negotiating further overseas manufacturing rights is presently being investigated.

Resene Galvo One is suitable as a lap primer as well as on new or weathered iron.

It can be applied by brush, roller, conventional or airless spray, with complete safety to personnel. It stops zinc corrosion and effectively welds itself to galvanised iron while its white colour ensures good topcoat coverage of gutters and downpipes.

Resene Galvo One has a dry time of five hours at 18°C and can be recoated after twelve hours. It has a theoretical coverage of twelve square metres per litre with a dry film thickness of 35 microns. Usually one coat is sufficient, but where severe corrosion is present an extra coat is recommended.

The choice of topcoat should be governed by the environment in which it is being used.

Acrylics are suitable for use in most environments giving adequate protection with excellent aesthetic appeal. The use of micaceous iron oxide pigmentation while reducing the colour range increases the protective qualities. In industrial and severe marine environments, barrier type coatings such as Resene Armourchlor HB-F or Resene Micabond are required to maximise protection.

For further information on this subject refer to the following data sheets in the Resene One-Line Specification manual: Resene Galvo One, Resene Hi-Glo, Resene Micabond, Resene Armourchlor HB-F.

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