Corrosion is the spontaneous chemical reaction of a metal against its surroundings. Particularly, in sea water this chemical reaction proceeds by an electrochemical mechanism involving oxidation of the metal, which results in metal loss. That happens when we have a plate with pitting appearance, at the spots where the coating is not good enough to protect.
The part of the metal surface in the electrochemical mechanism (electron release) is called anodic. The other part of the metal surface where the electron consumption takes place is called cathodic.
If we can supply electrons from an external source, then the electron consumption (cathodic) will speed up and the electron release (anodic) reaction will slow down. As a consequence, the rate of iron dissolution will slow down and the electrode potential will decrease. From the procedure above, the principle of cathodic protection can be derived: by supplying electrons into the metal from an external source, we can slow down its dissolution.
The delivery of the electrons to the metal surface to be protected can be achieved in two ways. Firstly, by using an impressed current technique and secondly by using sacrificial anodes.
By this method the electrical current is delivered to the metal surface from a D.C. power source through an auxiliary anode i.e. the current is forced in or impressed.
The metal surface will act as a cathode
in the shell that formed.
Typical galvanic anode for cathodic protection of a ship's hull (anodes alongside the hull are normally mounted alternately on both sides of dilge keels)
By this method a galvanic cell is formed between the structure to be protected and the sacrificial anode. The electrons pass spontaneously from the anode to the metal surface (cathode).
Thus, the source of the electrons (the sacrificial anodes) must have a more negative electrode potential than the metal surface. Metals and conducting materials commonly used are listed below in such order that each normally act as anode with respect to all the materials which follow it.
MAGNESIUM, ZINC, ALUMINUM IRON AND STEEL, LEAD, BRASS, COPPER, GRAPHITE, COKE E.T.A.
This explains why magnesium, aluminum and zinc alloys are used to protect steel today