Electrolytic corrosion
When immersed in water, an electric current forms between two metals. This phenomenon is called electrolysis (more precisely electrolytic corrosion).
This current moves ions (matter carriers) from the softer metal (the anode) to the harder metal (the cathode). This is the principle of the electric battery. To channel and control this phenomenon, we place anodes in very soft metal which are there to destroy themselves instead of the metal parts of the edge. We speak then of sacrificial anodes.
Zinc for the anode
The softest, most negatively charged metal is zinc. It therefore protects metals such as :
- aluminum
- mild steel
- tin
- copper
- the cast iron
- lead
- stainless steel
The metals are listed here in order of negative potential, with stainless steel being the least sensitive to electrolysis.
Zinc is therefore used as an anode to protect other metals.
Aluminum is very prone to electrolysis. We also understand the danger of putting two different metals in contact without insulating them, for example a bronze propeller on a stainless steel shaft.
There are many sizes and models of anodes. Replace an anode with the same or a larger model.
Avoiding electrical leaks
Electrolysis is a natural phenomenon that can be managed with well calibrated and well placed anodes. But this phenomenon is accelerated when an electric current from a poorly insulated network intervenes.
euros each refit, we renew all its anodes. Here, a nut on the shaft line and an anode at the end of the propeller.
What is the life span of the anodes?
It is considered that anodes should be changed at each fairing, or more often in case of particular wear, so at least once a year. An anode should be changed as soon as it has lost 75% of its mass. It is replaced by an identical model, or if not, a larger one. The anode must offer a good electrical contact with the metal it protects, which is why you should not install an anode on a painted surface. Similarly, anodes should not be painted!
Do not paint the metal part in contact with the anode.
An anode must wear out
If you find your anode as new when you refit it, ask yourself questions. This is not normal. The electrolysis must be done elsewhere. Check that the anode is in electrical contact with the keel, the rudder, the moteuros
And in fresh water?
Fresh water does not conduct electricity like seawater. Salt water is ten times more conductive than river water, itself a thousand times more conductive than rain water.
Therefore, different anodes are used:
- in salt water: zinc anode
- in fresh water: magnesium anode
- in brackish water: aluminum anode
