It is known that metals in general, and therefore metal pipes or pipes equipped with metal armour, in contact with soil or water are transformed by natural tendency into metal oxides or salts. This transformation is caused by an electrochemical phenomenon by which, due to exchanges of electric currents between the pipe and the ground, a passage of metal in ionic form occurs. This removal of elementary metallic particles from the pipe leads over time to the formation of craters or even perforation of the pipe.

There are two main environmental conditions that can cause corrosive phenomena on underground metal pipes:

1 – Aggressiveness of the soil as a consequence of their chemical-physical constitution which is always very variable both locally and over time (aeration, humidity, pH, etc.). These corrosive phenomena can be favored by some bacterial species which enhance the electrochemical reactions already underway.

2 – Stray currents dispersed by electric traction railways or tramways, which use the tracks as current return conductors to the power substations. To protect steel pipes from these corrosive phenomena, their passive protection (coatings, insulating joints, various insulations, etc.) must first be foreseen during the design and installation phase, with the aim of reducing current exchanges. electrical between the pipes and the ground.

Passive protection can be tested after backfilling has taken place and without carrying out excavations along the route of the pipelines by means of a series of simple electrical measures.

In all those cases where the presence, even if only suspected, of stray currents or the particularly aggressive nature of the laying soil suggests a high possibility of corrosion, after backfilling has been carried out and sometimes with the system already in operation, appropriate measurements are carried out to establish the need for the installation of cathodic protection and the characteristics to be assigned to it Cathodic protection has the purpose of bringing the pipes to a potential level with respect to the ground such that corrosive phenomena are not possible. It can be obtained by sending, through a special auxiliary electrical circuit, a current from the ground to the pipe in the opposite direction to that of the corrosion currents, using power units or reactive anodes.

Each power supply group consists of a power supply, an anode disperser and the respective measurement places.

The power supply consists of a transformer, a saturable magnetic reactor to keep the current supply constant as the load varies (e.g. due to variations in soil humidity), a bridge of rectifiers and measuring instruments. The power supply is connected to the single-phase low voltage electricity distribution network. (160, 220, 280 or 380 V) and has the negative pole connected to the pipe and the positive pole to the anode electrode (see fig. 1).