Corrosion is one of the greatest enemies
of a ship and its machinery. It is also
the toughest enemy to fight against
especially for people working on board
ship. Iron is one substance which is used
in abundance in the ship from the main
body of the ship to the smallest equipment used, Iron makes its presence
felt in almost every type of parts
and system used on board. Ships are often
exposed to very harsh marine environments.
Due to the environment in
which they operate, the ship's hull is
often vulnerable to environmental
corrosion. The corrosivity of seawater
as regards to general corrosion on
steel increases with increasing
temperature, oxygen content, water
velocity, the content of corrosive
contaminants, eroding particles, and
conductivity. The seawater containing
salt forms a perfect electrolyte with
the ship's hull which is fully made up
of iron or mild steel in order to form a
galvanic cell. Excessive corrosion can
lead to total failure of a part of a
ship or its machinery. When two
dissimilar metals are in contact with
each other in the presence of corrosive
medium or electrolyte, the more active
metal in the galvanic series acts as an
anode and undergoes corrosion. This means
in a galvanic series of metals, the more
active metal acts as anode and undergoes
corrosion and the less active metal acts
as cathode and stays protected. This
electrical current is referred to as
corrosion current and is nothing but a
metal ion and electron transfer process
from the anode which dissolves and
passes into the solution. This simple
cell where the corrosion process takes
place is called a galvanic cell. In order
to fight against this corrosion and
protect the parent material - sacrificial
anodes are used on ships. Metal strips of
top order metals in the reactivity series
serves as anodes and are installed for
cathode protection. These are called
sacrificial anodes. The anode is divided
into two parts namely anode insert and
the anode body. Out of these two the one
which serves as an anode in the process
of electrolysis is termed as anode body,
whereas the other is called anode insert
which is used to securely fix the anode
to the parent surface.
Anodes are categorized under a variety
of parameters. As per the classification
of anodes on the basis of their shape,
there are six types - flat or block-shaped,
teardrop anodes, cylindrical or semi
cylindrical anodes, disc-shaped anodes,
bracelet anodes and tubular anodes.
Also according to the size of anodes they can
be of two types - small-sized and large-sized anodes.
Whereas on the basis of material zinc anodes
and aluminium anodes are preferred in
the maritime industry. The anode can be
secured or fitted to the surface of the
ship to be protected by three common
methods which are - by use of brackets
bolts and welds. Among these three
methods, welding is the most efficient
method which can maintain maximum
electrical continuity and remain in the
closest contact with the parent surface.
Sacrificial anodes work on the principle
similar to electrolysis according to
which if an anode and a metallic strip
are dipped in an electrolytic solution
anode electron will dissolve and deposit
over the metallic strip and make it a
cathode
anodes have to be changed only after
they are completely decayed. The
frequency for changing of anodes
depends on the application where the
anodes have been used. In case the
anodes are attached to the ship's hull, then
they are to be checked during dry dock
which takes place after 2 to 3 years if
the anodes are found completely
corroded then anodes of bigger size
should be fitted. For a fully corroded
anode means that the material used
was of poor quality or a large amount of
material is required to protect the hull
We now know from the galvanic cell and
the sacrificial anodes that a potential
difference between two electrodes is
required for corrosion current to occur
This corrosion current dissolves the
anode in the electrolyte but in the
absence of a potential difference within
the ship's hull, the corrosion current
will be at minimum. This is what the ICCP
or the impressed current cathodic
protection does - It makes the hull to
remain always cathode by keeping the
potential difference to a minimum and
introducing a current opposite to the
natural corrosion current thereby
protecting the anode
Impressed current cathodic protection
systems are the technologically advanced
and long-term solution to corrosion
problems and are regarded as a superior
alternative to sacrificial anode systems.
In the ICCP, the metal to be protected is
connected to an insoluble anode and
current is passed using a DC source
opposite to the corrosion current so
that the corroding material gets
converted from anode to cathode and is
protected from corrosion. This insoluble
anode can be either platinum, Platinised
titanium or any other inert elements. The
same principle is applied to ships. Here
a contact point is taken on the hull of
the ship and is connected to a reference
electrode. This reference electrode is
completely passive insoluble material.
The reference electrode measures the
natural corrosion current which is
nothing but the potential difference
between the hull and this reference cell.
We need to measure this corrosion
current in the hull so that we supply a
DC current which is either equal to or
slightly greater than the natural
corrosion current and is in the opposite
direction to the impressed current anode.
This, in turn, supplies a protecting
current to the hull of the ship making
the hull a cathode, protecting it from
corrosion.