Corrosion
Most metals tend to react with their environment and produce compounds, e.g. iron oxide or 'rust'. The
process involves the movement of electrons and it is this electrochemical reaction which causes the
corrosion on board ship. If two metals, each of a different potential, are immersed in a common
electrolyte, electrons will flow from one metal to the other. The two metals are electrodes. The one
which loses electrons is known as the anode and the one which receives them, the cathode. (The electric
potential is defined as the amount of work energy needed to move a unit of electric charge from a reference point to
the specific point . Potential difference is the difference in the amount of energy that charge carriers have between
two points in a circuit. Voltage is an electromotive force or potential difference expressed in volts.)
The loss of electrons converts a metal into its compounds. Thus the anode corrodes while the cathode
remains unchanged. Corrosion on board ship means the loss of electron from metallic structures to the
environment, i.e. water and air, with the result that oxides (usually 'rust') form on the metal surfaces.
Corrosion will not occur unless oxygen and water are both present. It may be of some use to consider
the process as a 'corrosion triangle' similar to the fire triangle:
Metals which corrode rapidly are known as anodic or ignoble metals and those which resist corrosion
are termed cathodic or noble
The corrosion rates of metals are shown in tables which are known as 'Galvanic Series'
cathodic protection
The most efficient system for combating underwater corrosion is 'cathodic protection'. The basic
principle of this method is that the ship's structure is made cathodic, i.e. the anodic (corrosion) reactions
are suppressed by the application of an opposing current. Cathodic protection, which is only possible
when metals are immersed in an electrolyte, is provided by two systems (1) sacrificial anodes, and (2)
impressed current.
Impressed current systems
Impressed current systems can only be used to protect the immersed external hull. Anodes are fitted to
the hull by maintaining a voltage difference between the anodes and the hull. An AC current from the
ship's electrical system is fed into a rectifier and DC power is supplied to the anodes. A silver/silver
chloride reference cell on the hull measures the current density in the sea water, i.e. the voltage
difference between itself and the hull. The reference cell indirectly regulates the power to the anodes by
means of a controller which amplifies the micro-range reference cell current and compares it with a
predetermined fixed potential difference. The 'difference' between the two readings is fed back to the
rectifier which then alters the current being supplied to the anodes until the predetermined and
reference cell potentials are equal. A potential difference within the range of 180-250 mV would be
suitable for most ships.
Three factors determine the position of the anodes:
1 They must be located where there is the least risk of contact damage.
2 They must interfere as little as possible with the water flow around the ship and to the propeller.
3 The correct currrent density must be maintained.
The impressed current system is more expensive to install than sacrificial anodes but as the latter must
be renewed, the eventual cost of both systems would appear to be similar. The impressed current
method requires trained personnel to supervise the power units. Log sheets containing the daily
readings of volts, millivolts, and the current in amps must be maintained and the reference reading of
millivolts should be compared with the predetermined value. The anodes and reference cells must be
inspected frequently for damage and also to ensure that they have not been painted over.
