Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 393

TABLE 6.3 Comparison of Impressed Current CP and Sacrificial Anode

CP Systems

Impressed Current CP Systems Sacrificial Anode CP Systems

Power source
and
connections

External continuous power source
required. Can be inadvertently
misconnected, resulting in reversed
direct current (DC) polarity, which
will accelerate corrosion. Isolation
of anodes from the structure is
essential. Fewer connections
required, but more complex.

Independent of any power source.
Straightforward welded or bolted
connections to structure. Cannot be
wrongly connected and connec-
tions are cathodically protected.

Control Simple controls: manual or auto-

matic.
Automatic will maintain potentials
within preset limits but requires
additional fixed reference elec-
trodes. Monitoring required at
regular intervals.

A tendency for current to be self-
adjusting. Dependent on initial
design: if not adequate, will require
additional anodes.

Anodes Usually lighter and fewer numbers.

May affect other structures in close
proximity to anodes and should be
assessed for any interaction.

Relatively heavy and large num-
bers. Bulk of material may restrict
water flow and induce turbulence
and drag.
Less likely to cause interaction with
neighboring structures because
output is low. Lifespan varies with
conditions, so replacement may be
required at different times.

Damage Anodes are lighter in construction

and therefore less resistant to
damage. Loss of an anode can be
more critical to the effectiveness of
the system due to high current
output for each anode.

Anodes are robust and not suscep-
tible to mechanical damage.
Where a system has a large number
of anodes, the loss of a few anodes
has little overall effect on the
system.

Maintenance Equipment designed for long life

but regular checks on electrical
equipment in service are required.

Generally maintenance-free.
Renewals usually required at
regular intervals unless designed
for life of structure.

Environment May cause overprotection, result-
ing in coating disbondment or
hydrogen-induced cracking of
high-tensile steels. Less restricted
by high-resistivity conditions.

Not practical for use in high-
resistivity conditions. Limited
current availability.

(Continued)