“ Corrosion and Anodes in the Focus ”
Immersed in water, the underwater hull is the perfect target for a natural phenomenon that is well known by boat owners: corrosion
. It occurs when metal elements are in contact with water. This is evidently the case with boats, since even on polyester boats several major components under water contain metal (motor base, propeller shaft, rudder, seacock and through-hull fitting, ...), and for boats made from steel or aluminium there is, apart from these parts, of course the hull itself. Even if the so-called "galvanic" corrosion is inevitable, there is still a solution to limit its impact on your boat and its components: anodes.
This electrochemical process attacks of course the most electrically active metals. An electric field (corrosive) develops around the propeller, the shaft, the rudder, and, as is to be expected, along the metal hull, especially if the boat is moving or if there are currents.
In order to preserve the boat, an element (anode) must be used, which has the potential that it will be attacked first
An anode is made of a strongly electropositive metal, so it is the anode whose metal elements will be dissolved first and who will be eaten up by corrosion. By the way, the notion of "sacrifice" has given it its name of sacrificial anode.
Anodes can be made of three active materials:
(for navigation in freshwater)
(for navigation in salt water)
- An aluminium alloy
Which type of anode should I choose?
Your choice depends on your boat and on your type of navigation (inland waterways / coastal). In general, aluminium anodes assure good protection and offer a longer service life than those of zinc or magnesium. Apart from being suitable for any type of boat or navigation, they also do not endanger the environment (in contrast to zinc which is slightly hazardous).
Since an anode has been conceived to disintegrate over time, it must be replaced before it has totally disappeared. In theory, when navigating on inland waterways, an anode must be replaced every five years, but in reality this depends on the boat and on its environment. In any case, it is important to check their status on a regular basis.
A very strong or a very insufficient use of anodes can actually be a hint that there is a problem somewhere else on your boat. An anode that is not eaten up by corrosion does not do what it is meant to do, and you can be sure that some other part of your boat has taken over the role of a “sacrifice”.
On the other hand, if you note a very strong corrosion process of your anodes, this can indicate that there is a weak current on your boat, either to your neighbours or to electric installations on land. Be careful, since this latter type, called electrolytic corrosion (electrolysis)
, induced by a stray current, has proven to be particularly fast and invasive. Damage can occur within a few days, in some cases even within a few hours only! To limit these risks of electrolytic corrosion, we recommend to install not only anodes, but also a galvanic isolator.