Cathodic protection of underwater constructions can be carried out using galvanic anodes (sacrificial anodes) or an external DC source and auxiliary impressed current anodes.
Galvanic anodes are made of metals with a potential more negative than the protected structure. The structure and the anode electrically connected to it (eg. welded) form a galvanic cell and the corroding anode provides cathodic protection current. This current flows from the anode to the electrolytic environment (water, earth) and then from the electrolyte it leaks through the protective coating defects to the structure, changing its potential to a more negative one (cathodic polarization). Full protection against corrosion is achieved when the construction potential is reduced to the protection potential, which for steel is equal to -0.8 V vs. silver/silver chloride reference electrode (Ag/AgCl).
In order to polarize the structure to the protection potential, it is necessary to provide appropriate cathodic protection current density. The required current density is closely linked to the state of the protective coating, i.e. the better the barrier properties of the coating, the lower the protective current density. The average design values of protective current density for bare steel in seawater are 80-100 mA/m2. The quantity of anodes should be selected so as to ensure that the entire surface of the protected structure is polarized to the potential of full cathodic protection. If the amount of current supplied is too small (too few anodes), the structure will not be polarized to the protective potential. At potentials more positive than -0.8 V vs. Ag/AgCl electrode there occurs partial cathodic protection, where corrosion processes are not completely inhibited.
Protection by external power source consists in forcing the current flow between the auxiliary anode and the protected structure. Similarly to the first case, the structure is in a state of cathodic polarization, i.e. its potential is reduced. The anodes, on the other hand, are not subject to dissolution but oxygen is released. The size of the anode must be selected adequately to the surface of the protected structure. The most frequently used anodes are made from titanium and coated with mixed metal oxides.