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Electrically Induced Corrosion
There are three types of electrically induced corrosion (sometimes called electrolytic corrosion) that can occur when a vessel is immersed in salt or polluted fresh water (the electrolyt).
i) Whenever two dissimilar metals are connected together a voltage is generated. If the circuit is completed through conductive water a current will flow from one metal to the other) and one of the metals will begin to corrode. This is cathodic corrosion. The fitting of a sacrifical anode ensures that the corroded metal is the anode and not the hull of the boat.
ii) If the hull of the vessel is connected to the negative terminal of the battery usually via the engine it is important to ensure that there are no other points on the hull that are connected to battery negative. This is because if there are two negative points on the hull and one is at a slightly differnt potential to the other current can again flow from one point to another resulting in corrosion. This is stray current corrosion. It can also be avoided by isolating all negative connections from the hull but in this case double pole circuit breakers are required in the DC distribution panel, isolating both positve and negative lines in a case of circuit overload. There should also be an indictor that an insulation fault has occurred if either postive or negative had come in contact with the hull.
iii) When a vessel is connected to a mains (230V AC) shore supply, the earth conductor should be taken to the same single earth point on the hull as the DC negative. The other end of the earth conductor is taken via the generating companies distribution grid to a sub station where it is phisically bonder down to planet earth. If a second vessel is then connect to the shore mains supply adjacent to the first vessel a circuit exists between the two vessels via the common earth connection to the sub station and through the conductive water. Alternatively if the vessel connected to the shore mains supply is adjacent to metal object (ie a jetty) that is driven into planet earth again a circuit has been completed. If the potential between the two vessels or one vessel and the jetty is slightly different, a current will flow and corrosion can occur. This is galvanic corrosion. This type of corrosion can be provented by fitting an isolation transformer in the mains supply immediately after the input plug on the vessel. Alternatively a galvanic isolator can be fitted in series with the AC earth conductor to block the low voltage DC galvanic current flow whilst permitting the passage of alternating current associated with the earth conductor.
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