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Corrosion is by far the greatest course of failure
of cooling water equipment. It takes place to some degree whenever
water, metal and a depolarizing agent such as oxygen are brought
together. Also, water inherently contains aggressive species such as
chloride and sulfate. Corrosion is a chemical process, therefore,
its rate of reaction will increase with temperature. In general, a
low pH will increase the rate of corrosion, while inhibiting scale.
A high pH will decrease corrosion and promote scale formation. Low
pH can result from gases such as sulfur dioxide being scrubbed from
the air by the cooling tower; acidic process fluids being leaked in
at the heat exchangers; or an overfeed of acid, if being used to
control the pH in the cooling tower.
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Non-chemical factors such as water velocity, or deposit
formation, also play a considerable role in localized corrosion,
which can accelerate equipment failures. Because corrosion is an electrochemical process, any condition
that causes one point on a metal surface to differ from another will
allow a localized corrosion cell to establish.
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Localized corrosion is produced by crevices, deposits of any type and local stresses.
Low flow areas can allow suspended solids to settle out, and
localized corrosion can then take place under the resulting deposit.
This is one of the most common causes of localized corrosion in
cooling water systems.
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Another form of localized corrosion is galvanic coupling
corrosion caused by the joining of two different metals in the
system. Galvanic corrosion is well known and understood,
particularly in the case of copper and steel. The areas of these
different metals which form the galvanic cell will also play a part
in the damage caused by corrosion- A large anodic area (more
corrodible) connected to a small area of cathodic (more corrosion
resistant) metal is commonly acceptable because the total corrosion
is spread over a large area. When the reverse situation is present,
a small anodic area connected to a large cathodic area, corrosion
will be accelerated because the corrosion is concentrated in a small
area which can quickly lead to the failure of that area.
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When two metals from this galvanic series are in contact in
solution, the corrosion rate of the more anodic (corrodible) metal
increases and the corrosion rate of the more cathodic (corrosion
resistant) metal decreases.
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