Duplex systems, as we define them, are materials that
have been painted or powder coated after galvanizing.
Each individual project raises unique reasons as
to why a duplex system should be utilized, and the
advantages for choosing to do so are many:
Extended Corrosion Resistance
The most obvious and most important reason for
using a duplex system, is the added corrosion
protection it provides. Other systems
simply can't match the corrosion resistance
provided by painting or powder coating
over hot-dip galvanized steel.
Synergistic Effect
It’s typical for a duplex system to provide
corrosion protection 1.5-2.5 times longer than
the sum of the lifetime of zinc or paint, used
individually. For example, if a galvanized coating
is expected to be maintenance-free for 40 years
and a paint system is expected to last ten years,
galvanizing and paint combined should protect the
steel substrate for at least 75 years, or 1.5 times the
sum of both systems.
Economic Benefit
Because duplex systems greatly extend the
service life of a product, maintenance costs are
significantly decreased. Additionally, a product
lasts longer before it must be replaced, thus
decreasing the life-cycle cost. The cost of a
product that has been protected by galvanizing and
painting or powder coating, is lower over the entire
life of the product than most single system methods
of corrosion protection.
Ease of Repainting
As the paint film weathers, the zinc in the
galvanized coating is present to provide both
cathodic and barrier protection, until the structure
is repainted. The exposed zinc surface then can
be repainted with minimal surface preparation.
Aesthetics
Galvanizing has an attractive metallic-gray
appearance, suitable for a myriad of applications,
but painting or powder coating can offer aesthetic
advantages. One might choose to paint over a
galvanized coating so that a project matches its
specific environment—such as a stadium, theme
park, or natural habitat.
Safety Marking
A duplex system of galvanized
steel and paint or powder coating
may be used to conform to safety
regulations. For example, the Federal
Aviation Administration (FAA)
requires structures over 200 feet
tall to be painted in the alternating
pattern of white and orange.
Color Coding
Painting or powder coating over galvanized steel
also increases safety in many environments, by
color-coding gas, steam, or chemical pipes,
identifying hazardous work areas and walkways,
and marking high-voltage electrical lines and
equipment.
Extending the Life of Previously Galvanized Steel
Paint is a logical choice to extend the life of
galvanized structures, once the zinc coating has
substantially and naturally weathered away.
Instead of being completely replaced, the
galvanized structure can easily be painted,
extending its useful life. Organic zinc-rich paints
are specifically suited to this application.
Repair of Hot-dip Galvanized Steel
Zinc-rich paints can also be used to touch-up and
repair damaged areas on a galvanized coating, in
order to comply with ASTM A 780, Repair of
Damaged and Uncoated Areas of Hot-Dip
Galvanized Coatings. Repairing a galvanized
coating will significantly extend the useful life of
a product.
