Page 326 - Ship Construction.DJ Eyres 6Ed

Ch27-H8070.fm Page 315 Wednesday, October 18, 2006 7:01 AM
Corrosion Control and Anti-fouling Systems 315
anodic corrosion reactions are suppressed by the application of an oppos-
ing current. This superimposed direct electric current enters the metal at
every point lowering the potential of the anode metal of the local corro-
sion cells so that they become cathodes.
There are two main types of cathodic protection installation, sacrificial
anode systems and impressed current systems.

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(1) Sacrificial Anode Systems – Sacrificial anodes are metals or alloys
attached to the hull which have a more anodic, i.e. less noble, potential than
steel when immersed in sea water. These anodes supply the cathodic pro-
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
tection current, but will be consumed in doing so and therefore require
replacement for the protection to be maintained.
This system has been used for many years, the fitting of zinc plates in way
of bronze propellers and other immersed fittings being common practice.
Initially results with zinc anodes were not always very effective owing to the
use of unsuitable zinc alloys. Modern anodes are based on alloys of zinc,
aluminium, or magnesium which have undergone many tests to examine
their suitability; high purity zinc anodes are also used. The cost, with vari-
ous other practical considerations, may decide which type is to be fitted.
Sacrificial anodes may be fitted within the hull, and are often fitted in
ึ
ballast tanks. However, magnesium anodes are not used in the cargo-ballast
tanks of oil carriers owing to the ‘spark hazard’. Should any part of the
anode fall and strike the tank structure when gaseous conditions exist an
explosion could result. Aluminium anode systems may be employed in
tankers provided they are only fitted in locations where the potential
energy is less than 28 kg.m.
(2) Impressed Current Systems These systems are applicable to the pro-
tection of the immersed external hull only. The principle of the systems is
that a voltage difference is maintained between the hull and fitted anodes,
which will protect the hull against corrosion, but not overprotect it thus
wasting current. For normal operating conditions the potential difference is
maintained by means of an externally mounted silver/silver chloride refer-
ence cell detecting the voltage difference between itself and the hull. An
amplifier controller is used to amplify the micro-range reference cell current,
and it compares this with the preset protective potential value which is to be
maintained. Using the amplified DC signal from the controller a saturable
reactor controls a larger current from the ship’s electrical system which is
supplied to the hull anodes. An AC current from the electrical system would
be rectified before distribution to the anodes. Figure 27.3 shows such a system.

Originally consumable anodes were employed but in recent systems
non-consumable relatively noble metals are used; these include lead/silver
and platinum/palladium alloys, and platinized titanium anodes are also
used.