Corrosion Will Occur in All Types of Water with Dramatic Effect

Metals of differing electrochemical potential when in contact with one another form Galvanic cells. The metal with a lower potential in the galvanic cell will be anodic and will corrode. The same effect can occur in areas of different electrochemical potential in a single piece of metal such as a steel plate. Any craft moored and operating in fresh, salt or estuarine water is at risk from corrosion and the effects can be costly.
Corrosion on Steel & Aluminium vessels can be identified as either areas of localised pitting to the hull plate, rudders, bilge keels etc. or less obviously in the form of general wastage of the hull plating often occurring below the paint coating. Pitting can lead to the complete penetration of the hull below the waterline. General wastage of the steel can be just as critical, weakening the hull and necessitating expensive re-plating.

Corrosion on Aluminium vessels is also generally in the form of localised pitting to the hull plate, rudders, bilge keels and particularly in way of weld seams. Pitting can lead to the complete penetration of the hull below the waterline necessitating expensive re-plating.
On Wood and GRP vessels the areas of concern are principally the stern gear i.e. The propellers, shafts, shaft brackets, stern tubes and rudders which are expensive to replace and vital to the vessel, the failure of a propeller or rudder could have disastrous consequences. The effects of corrosion can vary from pitting of propellers and shafts to the decomposition of the alloy of propeller. The failure of something as small and inexpensive as a split pin can result in the loss of the propeller.
Stray current leakage is quite often cited as the cause of corrosion on all types of vessel however more often than not the problem can be traced to a galvanic action. Stray current leakage is the action of electrical current from an external power source such as a battery or shore power supply which because of some electrical system fault on board the vessel passes out through the hull or a fitting in the hull and flows through the water causing “Electrolytic” corrosion. Stray current leakage is usually a result of damage or wear to the wiring system or poor installation of wiring or electrical equipment.