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Zinc Coating Details to BS EN ISO 1461
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Another property of the galvanized coatings on siliconrich
steels is the colour. During the galvanizing process, a
zinc layer builds up on the zinc-iron alloy layers which are
adhering to the surface of the steel. The reaction rate can
be such that this pure zinc layer is transformed completely
to zinc-iron alloy before the article has had time to cool.
This results in a coating which can be much darker in
appearance, varying in colour and thickness across the
surface of the galvanized item. This appearance does not
alter the corrosion resistance of the zinc coating. Due
to the variations in coating thickness associated with
deep galvanizing of silicon-rich materials it is normal to
specify the finish as ‘deep galvanized to twice the coating
thickness specified by BS EN ISO 1461’.
Deep Galvanizing to BS EN ISO 1461
The use of silicon-rich steels allows much heavier
galvanized coatings to be obtained. Average coating
thicknesses of two to three times that for mild steel can
be achieved. It is for this reason that silicon-rich steels are
termed ‘reactive’ steels and the galvanizing process ‘deep
galvanizing’.
The influence of the silicon does not increase consistently
but rather follows a curve as shown in the following
diagram. This curve gives average values and variations
can be expected between different silicon-rich steels with
the same silicon content but from different steel casts.
These variations are attributed to the fact that whilst the
total silicon contents can be equal, the amount of silicon
that is bound to oxygen within the steel can vary. More
or less silicon is then dissolved in the steel, and it is only
this amount that influences the reaction. The silicon can
be unevenly distributed on the surface of the steel and
this will lead to uneven variations in the coating thickness
after galvanizing.
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Wet storage stain
Galvanized steel is protected from corrosion by a layer of
zinc-iron alloys and a layer of pure zinc. After galvanizing, a
protective zinc carbonate film forms over the surface of the
zinc. The formation of this protective layer is only possible
when the galvanized surface is exposed to free flowing
air. Stacking freshly galvanized articles in contact with one
another prevents the free circulation of air, and in wet or
humid conditions, may result in the development of wet
storage stain. Wet storage stain, often referred to as white
rust, appears as a white, powdery covering. The white
rust, comprising of zinc oxide and zinc hydroxide corrosion
products, is voluminous and can appear to be more
detrimental to the galvanized coating that it actually is.
Wet storage stain can be prevented by correct transport
and storage provisions. For transportation over long
distances, galvanized items should be protected by
waterproof cover to prevent moisture ingress. For storage,
galvanized items should be kept off the ground in a dry
environment. If stacked in a potentially wet environment,
the galvanized items should be separated from one another
to provide free circulation of air. If possible, the stacking
should be at an angle to facilitate drainage of water.
In normal use, light wet storage stain is not serious and
does not reduce the life expectancy of the galvanized
coating. The affected area should be dried and exposed
to the atmosphere to allow the zinc to form a protective
carbonate layer. The appearance of the wet storage
stain will gradually fade to that of a normally weathered
galvanized steel. Where more stubborn wet storage stain
deposits are evident, these should be removed using a
stiff bristle (non wire) brush and, if necessary, a cleaning
solution should be used. Typical solutions would be
ammonia A citric acid based clear such as Metsoak C4900
with a 10% dilution v/v, the cleaning solutions should be
thoroughly rinsed off after treatment and the article allowed
to dry.
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Life expectancy of
zinc coatings
The life expectancy of a zinc coating is largely determined by
its thickness. Thicker coatings give longer life (the period to
first maintenance). When exposed to atmosphere the zinc
coating will weather and corrode, leading to a gradual
diminution in the coating thickness. Under conditions of normal
atmospheric exposure the level of corrosion is low and is
typically at a rate which is between 1/10th and 1/40th of that of
the steel base.
When subject to conditions of high humidity orcondensation,
the rate of corrosion of the zinc coating can be increased
significantly.
The level of contamination in the atmosphere can also
adversely affect the corrosion rate of the zinc coating. The
most significant contaminant accelerating the corrosion
rate of zinc is sulphur dioxide (SO2). The resistance of zinc
to atmospheric corrosion is dependent on the protective
zinc carbonate film which forms on the surface of the zinc.
The sulphur dioxide reacts with moisture to destroy the
protective film and this leads to the corrosion of the zinc
coating.
Research undertaken by the Galvanizers Association has
resulted in the publishing of a series of charts depicting
the average atmospheric corrosion rate for zinc for the
United Kingdom and Ireland. These charts indicate that the
average local atmospheric corrosion rates for zinc have
decreased, reflecting the general decrease in the levels of
sulphur dioxide in the atmosphere.
Current atmospheric corrosion rates for zinc within the
United Kingdom and the Republic of Ireland are given in
the Zinc Millennium Map and are in the range of 0.5μm
to 2.5μm per year (corrosion categories C2 – C3 to ISO
14713).
Please see overleaf page for Zinc
Millennium map.
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