| ALWC
incorporates an electrolytic corrosion cell. The anodic
attack site can, in the same way as with normal marine
corrosion (and corrosion of ships), be protected against
by the use of Cathodic Protection (CP) making the steel
cathodic in respect of an applied anode.
CP can be ‘galvanic’ (using a sacrificial
anode) See image 1
or ‘impressed current’ (IC) (driving a ‘contra-corrosion’
cell) See image 2.
Both have been shown to be effective against ALWC,
and could be installed at construction stage or retrofitted
to older quays. As with all installations there will
be features which are best taken into account at design
stage to optimise cost and efficacy.
Both types of CP require careful design, and possibly
sample testing, to give sufficient, but not excessive,
current density at all points of the structure. Having
created the passive environment, adjustments will be
necessary in the case of the IC system. Monitoring and
adjustments will be required, but at increasing time
intervals over the life of structure.
It is often recommended for new structures that a combination
of CP and a coating be applied at installation. Allowing
minimal use of the CP over a start-up period and full
use only if necessary after breakdown of the coating
layer.
This paint and CP combination should give the best
protection with life cost economy. This is, however,
less easy to achieve for retrofitted applications, but
the same economies can be achieved by maximising the
work at one time. Operators of quays will be particularly
aware of this in relation to disruptive down-time See
image 3.
The use of CP is a skilled application, requiring experienced
design, monitoring and maintenance. Please contact us
for more information on this. These days monitoring
of applied current systems can be carried out remotely
via phone link.
CP may be less than 2% of the cost of constructing
a new quay and if installed prudently could save repair
costs of 10 times the amount and more See
image 4.
Other life costs are electricity for impressed current
systems and replacements anodes for galvanic systems.
The latter being a design trade-off between initial
anode size/cost and the replacement period; this could
be typically 10 or 20 year intervals. Both systems should
include for monitoring.
Also see:
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