Architects memo no. 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
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,
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:
Zincilate 10 and Resene
Zincilate 11 - inorganic zinc rich coatings
120 - epoxy zinc for weld touch-ups
510 - epoxy barrier coat
402 and Resene
Uracryl 403 - acrylic urethane topcoats