Atmospheric corrosion
resistance of hot dip galvanizing.
The life expectancy of a hot dip galvanized coating is roughly proportional
to the thickness of the coating. Therefore, in any given environment it is
possible to predict how long a coating will last before repair is required.
With a galvanized coating life expectancy is based upon the appearance of 5%
red rust on the steel surface.
The graph below highlights the relationship between the protection life of
a galvanized coating and its thickness.
Clearly,
the resistance to atmospheric corrosion of hot dip galvanized
steel depends upon climatic factors such as humidity, rainfall,
proximity to the coast and the presence of pollutants. The
latter can have a particularly detrimental effect upon the
performance of the coating depending which pollutants are
present.
Corrosion rates of hot dip galvanized steel at coastal locations
(generally within 1 km of the high water mark) can be high
but duplex coating in these areas can provide effective corrosion
protection. Industrial gases such as sulphur dioxide and
nitrous oxides attack the zinc coating, as do ammonia gases.
In rural areas galvanized coatings can last up to 100 years.
This performance may be compromised where excessive application
of insecticides and fertilizers occurs which, in combination
with moisture, can attack the protective coating. Interestingly,
galvanized coatings weather to a whiter colour in marine
climates compared to the darker colours experienced in rural
and industrial climates. In some instances the coating colours
to a red, which is often mistaken for rusting. This discoloration
occurs as the iron/zinc alloys react to form a protective
film with the red iron corrosion product. In certain environments
the iron/zinc alloys give better protection to the underlying
steel than pure zinc. Studies have been carried out by the
CSIR to determine the performance of hot dip galvanized coatings
throughout South Africa. The map below provides the potential
user with a guideline as to the protection afforded by hot
dip galvanizing throughout the country.
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