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Additional Product Design Considerations for Galvanizing |
When
welded items are galvanized, both the cleanliness of the weld area after
welding and the metallic composition of the weld itself affect galvanizing
quality and appearance. Some welding techniques that lead to good results
include:
- Use of an uncoated electrode where possible,
Thorough removal of all welding flux residues if a coated electrode
is used,
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A welding process that produces little or no
slag,
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A submerged arc method for heavy weldments,
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Selection of welding rods providing a deposited weld that is
composed
of the same materials as the parent metal, and
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Avoiding welding rods that are high in silicon.
Materials
that have been galvanized may be welded easily and satisfactorily by all
common welding techniques. The American Welding Society (AWS) has produced
a book that details all aspects of welding galvanized items. In general,
anything that can be welded before galvanizing can be welded with relative
ease after galvanizing. A copy of
the AWS recommended welding practices is available from Art Galvanizing or
the AGA.
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Hot
dip galvanizing is a well established and widely used process for
coating mechanical fasteners and welded joints. The prime mechanical
fasteners used for making field connections in steel are bolts that
conform to ASTM A 307, A 325, and
A 394 and are galvanized in accordance with ASTM A 153/A
153M and CSA standards. Bolted galvanized structural joints can
be designed for both bearing- and friction-type connections.
Galvanized joints of this type have an outstanding performance
history. A wide variety of structures have been built using hot dip
galvanized steel and connected using galvanized steel bolts.
Art Galvanizing has over 75
years of experience in galvanizing various bolts, screws, and other
fasteners including A490 and Grade 8 Bolts.
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PAINTING
GALVANIZED STEEL
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Galvanized
articles are easily and successfully painted. The two factors
critical to success are proper post-galvanizing surface
preparation and proper paint system selection. Galvanized
steel is painted for a number of reasons: aesthetics, safety
marking, and the desire for even longer lasting protection.
Galvanizers must be notified if galvanized
steel is to be subsequently painted so they can avoid applying
a post galvanizing treatment that would affect paint
adhesion.
A corrosion protection system consisting simply of galvanizing
provides long-term, cost-effective corrosion protection. Often
the expected service life of the galvanized coating actually
exceeds the design life of the structure it protects. However,
a duplex system, one consisting of properly selected and
applied paint over galvanizing, greatly extends the period
that effective corrosion protection is provided. A useful
rule of thumb is that a duplex system will provide effective
corrosion protection for one-and-one-half-times the service
life of the sum of the service lives of the individual
systems. For example, if the life of the galvanized coating in
a particular environment is 35
years and the expected service life of the paint system is
10 years, the expected service life of the duplex system will
be 67.5 years [i.e.,
1.5 (35+10)]. While
such duplex systems often will have a premium first cost, they
are usually the lowest life-cycle cost solution. They may be
the only feasible solution for. structures in aggressive
environments where later in-service painting is extremely
difficult or impossible.
The synergy of duplex systems in corrosion protection stems
from three known factors:
- The paint film extends the life of the galvanized
coating by providing additional barrier protection to the
zinc layers,
-
An underlying layer of zinc tends to prolong the life
of a paint coating by preventing an underlying layer of
corrosion from developing, and
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The corrosion products of zinc and its alloys further
retard paint coating damage by sealing cracks and pores in
paint films.
Two
publications, Duplex
Systems: Painting Over Hot Dip Galvanized Steel, and Practical
Guide .for Preparing Hot Dip Galvanized Steel for Painting are
available from Art
Galvanizing or the AGA.
They provide more complete and detailed information about
duplex systems. In addition to providing a wealth of technical
details and a discussion of the suitability of a number of
different types of paints for application after galvanizing,
these publications also present a number of specific
applications and color photographs of duplex systems.
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