Usage:
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The engine may be damaged by electrolytic corrosion (stray current corrosion) if the correct bonding procedure is not adopted. |
Definition of galvanic and electrolytic corrosion.
Galvanic corrosion is caused when two different metals are immersed in a conductive fluid 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 first 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 set 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 sacrificial anode that is fixed to the outside of the hull below sea level.
The bonding should consist of heavy stranded wire (not braiding or wire with fine strands). It is an advantage if the wire is tinned. Insulation is also an advantage and should preferably be green in color. 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:
As many of the connections may be splashed with sea water they should be soldered wherever possible and clamped elsewhere, with the joint protected from corrosion by neoprene paint, or a similar material, to exclude water.
Bonding of aluminium boats is a special case as the various appliances on board should be earth free and therefore to avoid stray currents all appliances must be earthed to a single terminal.
Grounding is required for safety if voltages are high, i.e. when there is a 240 volt generator on board or when a shore line is connected. Grounding (or earthing) must not be confused with the term 'earth return'. Earth return carries current, whereas grounding (earthing) does not.
Another source of unplanned current giving raise to a form of stray current corrosion is an earth connection from a shore line. When a shore line is in use the boat system should be protected from earth leakage by an earth leakage switch on shore but as additional safety there should be a switch on board the boat.
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This section on bonding covers a typical system and has been included for guidance purposes only. It may not be appropriate for your boat. As installations vary it is advised that specific recommendations from a specialist in the subject of electrolytic corrosion are obtained. |