The effect of microclimates
Where the surface is shielded from the cleansing influence of rainwater, which would otherwise wash pollutants (particularly coastal and other salts) from the surface, uncoated steel and hot dip galvanizing are more susceptible to corrosion than other corrodible metals.
The corrosion in these shielded locations, for example under verandahs and open roofed areas, can be typically 3 to 5 times greater than that for surfaces exposed to rain. In such cases, the appropriate paint systems detailed under Service Requirements 3 to 5 should be considered and the paint system would need to extend beyond the microclimate by at least 150 mm.
Even in quite benign locations, where maximized service life is required, the application of one coat of a two pack epoxy primer at 75 µm DFT to non-rain-washed surfaces, after cleaning and degreasing the surface, can significantly extend the service life of the structure. This added coat insulates the hot dip galvanizing from the accumulated pollutants. As this coat will not be exposed directly to sunlight, a two-pack epoxy primer (refer to AS 3750.13 or APAS 2971) will be quite durable.
In a similar vein, while a hot dip galvanized structure might be essentially exposed to the atmosphere, at some points it may be in contact with the ground, buried in soil or may be exposed to intermittent or continuous ponding of rainwater. In such situations localised painting or the application of a protective tape or wrap may be needed to avoid premature corrosion in these areas. Again, the protective coating would need to extend well beyond the microclimate. AS/NZS 4680 provides some commentary on this issue and the
GAA can provide further advice.