Installation Manual
N38143 Chapter 6
Page 39
Electrolytic corrosion
Caution: The engine may be damaged by electrolytic corrosion (stray current corrosion) if the correct bonding
procedure is not adopted.
Denition of galvanic and electrolytic corrosion.
Galvanic corrosion is caused when two different metals are immersed in a conductive uid such as seawater
(called electrolyte), with a connection between them, an electric current is generated in the same way as a
battery.
Electrolytic corrosion (stray current corrosion) is caused by a current from an external source such as the boats
battery or shore supply.
Avoiding electrolytic corrosion
The current that causes electrolytic action is called ‘stray current’ which can emanate from two sources.
The rst is the batteries on board the vessel where the negative terminal is earthed to the hull at a central earth
terminal. If other negative connections are made elsewhere on the vessel then the resulting small differences
in voltage between the earth terminals can cause the same chemical action as in galvanic corrosion, but it
must be stressed that this is not GALVANIC CORROSION but stray current known as electrolysis caused by
an external electrical current.
The way to prevent electrolytic corrosion is to ensure a good electrical installation and to bond the genset to
the bonding system in the boat which is providing a low resistance connection between all the metals in contact
with the sea water. The bonding system should be connected to a zinc sacricial anode that is xed to the
outside of the hull below sea level. A typical layout is shown in (A).
The bonding should consist of heavy stranded wire (not braiding or wire with ne strands). It is an advantage
if the wire is tinned. Insulation is also an advantage and should preferably be green in colour. Although the
current carried by the bonding system will not normally exceed 1 amp, the cable sizes should be generous as
shown in the table below:
Length of run to zinc anode Suggested cable size
Up to 30 feet 7 strand / 0.185mm (4mm2)
30 - 40 feet 7 strand / 1.04mm (6mm2)
Electrical system