METAR SUPPORT SYSTEMS
The corrosion rate for zinc is generally linear for a given
local environment. This allows predictions of the life
expectancy of a galvanized product, to first maintenance,
based on the zinc coating thickness and the zinc corrosion
rates given in the Zinc Millennium Map. For example,
a hot dip galvanized product with a coating thickness
of 55μm will last approximately 110 years in a location
where the atmospheric corrosion rate of zinc is 0.5μm per
year, and approximately 22 years in a location where the
atmospheric corrosion rate is 2.5μm per year.
METALOK SUPPORT CHANNEL
Life Expectancy for
Zinc Coated Products
Based on Classification
to BS EN ISO 14713
Finishes
The following are available for METAR Cable
Management Systems, components, and accessories:
Galvanizing
Hot dip galvanized to BS EN ISO 1461
(post-galvanized) GY, GA, & GK
Deep Galvanized to BS EN ISO 1461 (post-galvanized) GX
Further information regarding hot dip galvanizing and the
Zinc Millennium Map can be obtained from the Galvanizers
Association.
The Zinc Millennium Map provides specific information
for the United Kingdom and Ireland. For other locations,
reference can be made to BS EN ISO 14713 (Protection
against corrosion of iron and steel in structures – Zinc and
aluminium coatings – Guidelines).
METAR SUPPORT SYSTEMS
Coatings
Epoxy coated over mild steel EY & EA
Epoxy coated over hot dip galvanizing FY & FA
Coatings
A number of coatings have been used for the coating
of cable management products. By far the most cost
effective, versitile, and advantageous is epoxy. Epoxy
coatings are based on thermosetting epoxy resins which
are applied electrostatically as a powder spray which
is cured and hardened in an oven. The powder spray
application ensures complete and even coverage of the
surface. Epoxy coatings give a thin, hard and durable
finish which provides good chemical resistance, excellent
adhesion, and coating flexibility. Epoxy coatings are
available in a variety of colours. Black is supplied as
standard unless otherwise requested.
Galvanizing
Epoxy coatings can be applied directly to mild steel to
give a corrosion resistant finish. The steel products are
subject to a degreasing treatment to remove all surface
contaminants and then epoxy powder coated to a dry film
thickness of 75 microns.
Epoxy over Hot Dip Galvanized Mild Steel
BS EN ISO 14713 provides general guidelines on corrosion
rates for zinc in differing environmental conditions, details
of which are given in the table below.
The coating of steel using zinc, either before manufacture
(pre-galvanized) or after manufacture (post-galvanized)
is a cost effective and practical means of protecting the
steel from corrosion. The zinc coating protects the steel
in three ways. Firstly, the zinc coating weathers at a very
slow rate giving a long and predictable life. Secondly, the
zinc coating corrodes preferentially to provide sacrificial
protection of any small areas of steel exposed through
cutting, drilling, or accidental damage; scratches and
small areas of damage are sealed by weathering products
from the zinc. Thirdly, if the damaged area is larger,
the sacrificial protection provided by the surrounding
zinc prevents ‘creepage’ typically associated with other
protective finishes such as paint coatings.
The thickness of the zinc coating is dependant on the
method of application. The following table shows the
typical zinc coating thicknesses for a number of galvanizing
and related processes, and includes zinc based paints for
comparison purposes.
Using these broad corrosion rates, the following
table shows the life expectancy of galvanized cable
management products for corrosion categories C2 to C5.
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Hot Dip Galvanized
Finishes to BS EN ISO 1461
The hot dip galvanizing process provides a continuous
layer of zinc-iron alloys and zinc on the surface of the
products manufactured in steel. The hot dip zinc coating
provides a continuous barrier to moisture and other
contaminants, thereby protecting the steel substrate.
During the galvanizing process, a layer of zinc-iron alloy
develops on the surface of the steel product. When the
steel product is withdrawn from the zinc bath, a layer of
pure zinc is left on the zinc-iron alloy. The layer of pure
zinc gives a newly galvanized item a bright finish. This
bright finish will gradually fade as the surface layer of the
zinc oxidises, leaving a uniform dull grey appearance.
Zinc coatings compared in terms of coating thickness
Epoxy over Mild Steel
The corrosion rates should be considered as an indication
only and provide a broad means of estimating the life
expectancy of a zinc coating. This information should be
treated as a general guide and further information should
be sought relating to the specific zinc corrosion rates at
the installation site.
METALOK SUPPORT CHANNEL
The average amount of zinc which can be deposited
on a product is expressed in terms of thickness and is
measured in μm. The actual zinc coating thicknesses will
vary depending on the thickness of the steel, the chemical
composition of the steel, and the period of immersion
within the zinc bath. BS EN ISO 1461 specifies a number
of thickness ranges for products to be galvanized, each of
which has a specified minimum average local reading and
minimum mean average reading. Details are given in the
following table.
Whilst hot dip galvanizing provides a long lasting and cost
effective means of protecting steel from corrosion, the
performance of the zinc coating can be enhanced by the
addition of an epoxy coating. This type of finish is
referred to as a duplex coating. The duplex coating can be
used to add colour for aesthetic or safety purposes and
provide additional protection for the steel in aggressive
environments. The epoxy provides resistance to chemical
degradation, and the underlying layer of zinc prevents
creepage under the epoxy coating. The hot dip galvanized
steel products are treated by an acid etch, a chromate
pre-treatment and then epoxy powder coated to a dry
film thickness of 75 microns.
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