Corrosion phenomena

Corrosion phenomena and cathodic protection.

General information and methods.

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).



Normally the current supplied does not exceed approximately 70%. than the nominal one of the power supply to compensate for any greater current requests (e.g. to extend the cathodic protection to other pipes).

Anode earth electrodes are generally made of recycled ferrous material and have a low resistance to earth in order to reduce electricity consumption to a minimum; their duration is expected to be 15 years

Using the measurement stations for the power supplies, it is possible to check with mobile instruments the current supplied, the direct voltage at the output terminals and the potential of the pipe with respect to the ground (Cu/ Cu SO4) at the point of installation of the power supply. Using the measuring stations for the earth electrodes, it is possible to follow the functioning of the various elements of the earth electrode over time and detect any inefficiencies.

The power supply units do not require particular checks or maintenance, but only periodic readings of the instruments installed on the power supply panel.

The following table shows, depending on the average degree of insulation, the maximum lengths and surfaces of the pipes, the possibilities of use and the annual electricity consumption of the small, medium and large power units.

Type 1
Type 2
Type 3 (*)
Type 4 (*)
Good insulation
lenght max. (km)
Surfaces. max. (m2)
Sufficient insulation
lenght max. (km)
surfaces max. (m2)
Average annual electricity consumption (KWh)

From this table we can see the considerable importance that the insulation of the pipes has on the type of system to be used and consequently on its installation and operating cost

The reactive anodes used for the cathodic protection of the external surfaces of underground pipelines are normally made of magnesium-based alloys; they are distributed along the route of the pipes and buried at a distance of a few meters and at a depth of 1.5 ÷ 2 m. The tube-anode connection is made using an underground copper cable.

The number and weight of the anodes varies depending on the diameter, length and insulation from the ground of the pipelines to be protected; for underground pipes, a duration of 10 years is generally expected, after which the anode material must be replaced.

Reactive anodes are normally used for pipelines of limited development and only in cases where these are not affected by stray currents and cross soils with a low average electrical resistivity.

In the past, electrical drainage has also been installed for poorly insulated pipes affected by stray currents of railway or tram origin. With this system, the electric currents circulating within the pipes were conveyed to the tracks of the railway or tram lines through a copper cable, rectifiers or relays (so that the current passed only from the pipes to the tracks and not vice versa) and regulation resistors. However, drainage systems are currently out of use because they require continuous checks and frequent adjustments.

For the cathodic protection of the internal surface of pipelines conveying aggressive liquids (sea water, sewage, industrial water, salt solutions in general, etc.), reactive rings can be advantageously used together with an insulating coating (epoxy resins, bitumen, etc.). installed at the ends of the pipes (see pic. 2). These rings complete the protection in areas where the coating is missing (e.g. at the welds of

the tubes).