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canopy coating

Architect's memo 57: April 1999

Our long, narrow country, caressed on all sides with salt-laden sea breezes, bathed with lambent moist air, should be conducive to the rapid corrosion of metals.

The fact that New Zealand's typical rate of corrosion is less than what would be expected is due to the beneficial effects of our relatively high rainfall. Our clean rain washes off the corrosion-promoting salt and consequently reduces corrosion.

Take away the rain, and you bring on major potential for corrosion such as that which is present in the microclimate created by the typical canopy.

Normally a canopy is created by building a metal frame (often from steel tubular section), hanging it from the side of a building, and covering it with clear protective sheeting (normally glass).

Experience has shown that the best protection for steel is achieved by a tripartite approach. Firstly the steel is coated with zinc which will, 'in extremis', nobly sacrifice itself (although paradoxically it is less noble) to protect the steel. To avoid such self-sacrifice, the zinc is coated with a thick barrier of epoxy, using the tightly cross-linked network of the epoxy to keep moisture and dissolved salts at bay. The third, most glamorous, member of the team is then applied in a urethane coating to supply the colour and the desirable long term surface characteristics such as gloss, cleanability, abrasion resistance, etc.

The application of the zinc is a specialised art, invariably done off-site. The steel needs special preparation before it is hot-dipped, metal-sprayed, or coated with a zinc-rich coating. Regardless of which process, the frame will have to be assembled on site which will involve welding, or joining by bolting together or fitting with threaded sleeves. Either way, problems exist whether it is the reinstatement of the welded areas, or the avoidance of crevice corrosion between bolted flanges or threaded sleeves.

The profile of the steel sections, coupled with the general impracticality of spraying in such situations, makes uniform application difficult. The high performance coatings needed require to be easily applied by brush or mit.

Finally, when the coating system has at last been assembled, it is often savagely penetrated by drills and taps in order to affix the covering. This process exposes bare metal and risks the possibility of dissimilar metal corrosion should the fasteners not be carefully chosen. Protection of such penetrations requires the use of sealants or deformable inert washers. Swarf from such operations must also be carefully removed in order to avoid disfiguring surface rust.

Such are the difficulties encountered under canopies that successful longevity needs a maintenance program. The simplest, and most efficient, is to turn on the rain at regular intervals by washing with clean water.

Resene products useful in this area are:

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