Zinc anodes
Cathodic protection is a technique to protect from corrosion metal structures that are exposed to an electrolytic environment such as sea water, fresh water, soils, chemicals, which can be aggressive towards the structures.
Sea water specifically has electrolytic behaviour, what does that mean?
In simple terms it means that seawater is capable of transporting or conducting electricity in the form of positive and negative ions that are dissolved in the water, which is why the electrolyte is also referred to as an ionic conductor.
Now, what happens is that on the surfaces of metals immersed in seawater, corrosive electrochemical reactions take place, aided by the presence of oxygen and hydrogen, so that in the typical case of boats when we have different metals immersed (stainless steel axles, bronze propellers, etc.), some will act as an anode and some as a cathode; well, but in practice?
It means that in the case of seawater we have a transport of material in the form of ions from the positive anode to the negative cathode, in short, the anode is slowly consumed by losing material until it is completely disintegrated, which is why the more electronegative metals are protected.
By using zinc anodes, the other metals in the water are automatically protected, as zinc comes first on the electronegativity scale, which is why they are also called sacrificial anodes; it is better (especially for our wallet) to consume a zinc than a propeller or an axle!
There are two main forms of cathodic protection (at the cathode we have already mentioned that there are propellers, axles etc. etc.).
One is passive protection: sacrificial anodes are used, which are fixed near the cathode to be protected and are connected to it.
The other system is active cathodic protection: in practice, current is supplied to the system and the electronegativity of the metal is thus altered, bringing it into the 'safe' range.
In practice, there are a few things to take into account if we want our propellers and axles not to be brutally sacrificed!
1) The sacrificial anode must not be painted under any circumstances! Paint prevents the exchange of current, so that it is the propellers and axles that are corroded!
2) When mounting, ensure that there is good contact between the surface of the anode and the surface on which it is to be installed.
3) If 50% of the anode is corroded, it must be replaced in order to ensure its function, which is vital for the equipment, and during the year the anodes must be changed at least once.
