Cathodic protection is an electrochemical process which halts the natural reaction (corrosion) of metals in a particular environment by superimposing an electrochemical cell more powerful than the corrosion cell. Sacrificial anodes are fitted or bonded to the metal to be protected which in turn, as it has a greater electrical potential than the anode material, becomes cathodic and causes the anode to waste instead of itself.
When sacrificial anodes are correctly installed in a cathodic protection system the corrosion occurs only to the anode, which is replaceable. The number and size of the anodes required to install a cathodic protection system is determined by the type of material and the surface area being protected from corrosion.
When referring to a cathodic protection system the term bonding refers to the connection of the sacrificial anode to a remote metal component such as the propeller shaft or rudder stock and it should be remembered that the entirety of the bonding is critical to the effectiveness of the cathodic protection system.
Several factors determine the type of cathodic protection which should be fitted. Firstly the environment in which the boat / vessel is operating, secondly the size and type of construction and finally the length of time that the boat / vessel is likely to be afloat before the next maintenance slipping.
Important note:
All schemes must be inspected annually and the sacrificial anodes renewed if more than 50% are wasted. You should check the working surface of the sacrificial anode to ensure that it is free of paint and chalky corrosion deposits. When fitting or replacing a sacrificial anode you should also replace the serrated fan disc washers under the nuts and change the anode backing sheet on wood and GRP hulls. Exposed fixing studs, nuts and washers should be well greased or painted after assembly.
Aluminium Alloy shaft anodes are suitable for use in salt and brackish / estuarine waters.
Magnesium shaft anodes are suitable for use in fresh waters.
Zinc shaft anodes are suitable for use in salt waters.
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