4.1.1 Cathodic protection by means of sacrificial
anodes may be used in combination with the coating to prevent or reduce
pitting corrosion starting from local defects in the coating.
4.1.2 The anodes should be designed in terms of
size, weight and distribution to give an adequate life commensurate
with the service period. The anode distribution, type, weight and
dimensions should be shown in relevant documents and be available
for maintenance purposes.
4.1.3 Having determined the number and size of
anodes, the anodes are to be distributed evenly over all the structure
with some emphasis on horizontal surfaces likely to retain water and,
in particular, they should be installed close to the bottom plates
of tanks which are seldom completely dry.
4.1.4 Cathodic protection is without effect when
the tank is empty, and it requires some time (a day or more) to become
effective after the tank has been filled.
4.1.5 The following aspects should be considered:
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.1 tanks' size, shape and areas to be protected;
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.2 extent and location of coated and uncoated
surfaces;
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.3 frequency of ballasting/deballasting operations,
including the percentage of time the tank is filled, the level of
filling; and
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.4 the resistivity of water, its temperature,
etc.
4.1.6 The anode renewal should be carried out
well in time before the old anodes are fully consumed. The renewal
periods should be based on in-service experience.