You probably don’t need a degree in chemistry to understand that one of the most likely things to happen to metal in a wet environment, especially a saltwater environment, is corrosion. However, in addition to good old oxidization, there is another type of corrosion we need to protect against; electrolysis.
Electrolysis will corrode our yacht’s metal propellor, through hull fittings, bow thruster or even engine if we do not take measure to protect it. Stopping such corrosion is pretty much impossible in the marine environment and so, instead, a ‘weaker’ metal is attached to the metal we want to protect. That metal effectively allows itself to be eaten up by the surrounding electrolytic action and acts protects adjacent metals from damage. It is therefore referred to as a sacrificial anode and its sole purpose is to be the first line of defence, and the first metal to corrode.
On most saltwater boats the sacrificial anode is usually made of zinc although magnesium and aluminium can also be used. In fact, magnesium may sometimes be better, but will usually not last as long as zinc. To check corrosion and the effectiveness of anodes take advice from a specialist.
The anodes should be checked at least annually and when any of them is over 50% depleted it should be replaced. Most anodes have a backing plate / gasket and this should also be replaced at the same time as should the washers. Bolts used should have their threads greased and then they should be painted to protect them.
If you have a racing yacht where faired underwater hull shape is important, a portable anode hung off the guardrail may sometimes suffice, but make sure you always rig it properly when alongside - and don’t forget it when leaving the marina!
Most permanent anodes are to be seen fitted to the hull / drive shaft and also internally by way of a pencil anode on the engine / bow thruster.