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Procedures for Design Detailers
Scope
Design engineers, architects and end users are increasingly specifying
galvanizing, not only for its proven corrosion performance, but also for
its natural appearance as an architectural decorative finish.
This specification refers to the procedures required when aesthetic
appearance is sought, additional to that referred to in
AS/NZS4680:1999. For a self finish or post treatment with a finish such
as paint or powder coating the following should apply.
Galvanizing is of course a cast coating with a clean metallic and textured
surface which may be enhanced with further dressing as an attractive
self finish. It must not however be confused with spray painted finishes
over fine metals, planed timbers or plaster.
Consultation with the Galvanizer
To achieve the best possible appearance, steel designers should consult
with the galvanizers to ensure the steel detailing will produce the
desired result.
Important procedures
- Ensure that the galvanizer receives details of the finish required.
A test sample of the steel should be galvanized as the standard for
the job.
- Advise the galvanizer at the time of order that an architectural finish
is specified.
- Optimum results in processing steel to produce an architectural
galvanizing finish will depend on the steel surface preparation.
- Rough areas surrounding the welding or cutting should be buffed to a
uniform appearance.
- Sharp edges will normally be heavily coated with zinc but may
sometimes need post galvanizing inspection and dressing.
- At the design stage, ensure that the article to be galvanized will fit
inside the galvanizing bath in a single dip and discuss with the
galvanizer provisions for adequate filling, venting and draining.
Ensure that the design will allow even zinc runoff. This can have a
significant effect on the ability to achieve a smooth finish. As a
general rule, the drainage should be a minimum of 30% of the cross
sectional area of the article.
Specification Details
Specify steel which when galvanized, will produce a ‘normal’ coating
in appearance. Certain steels containing silicon and / or phosphorous
can produce coatings which can be dull or patchy grey in colour, with
a rough finish and may be brittle. As a general rule the following
criteria should be used
% Si < 0.04 %
% Si + (2.5 x % P) < 0.09 %
- While dull coating denotes thicker and longer lasting protection, appearance is adversly affected.
In extreme cases shot blasting of deformed surfaces may be
necessary depending on the finish required. Examples of a
requirement for blasting would be structural CHS (circular hollow
sections) particularly lacquer coated in the warehouse where part
removal has allowed corrosion and texture to affect the steel surface.
- Specify the steel to be galvanized in accordance with Australian
Standard AS/NZS4680:1999 and highlight the need for the coating to
be free from runs, dags, spikes, uneven surfaces and roughness or
other defects that could affect appearance.
- Welding quality should be free from porosity, all weld slag removed
and grinding marks minimized.
- Inspection of the finished galvanized work must take place at the
galvanizer’s yard prior to dispatch.
- To maintain optimum appearance, it is important that care be taken to
ensure the coating is not dirtied or damaged during transportation,
site storage and or erection.
- If the galvanized coating is to be subsequently painted (duplex
coating) the galvanizer should be advised of the paint system to be
used at the time of order, as certain operating procedures may need
to be taken to ensure the paint coating can be properly applied.
- Differing time periods will temporarily show brightness variations at
the early stages of exposure but are a transitory effect.
- Care is required with site storage to provide open ventilation which
prevents moisture entrapment with resulting wet storage stain. This
is normally a temporary condition which does not affect the life of the
coating.
- As the galvanizing surface texture can vary and is dependent on steel
chemical composition, consult with the galvanizer on procurement.
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