OTDS: Cathodic Protection

Overhead Transmission Distribution Systems

Cathodic protection enables to preserve the outer surface of buried or immersed steel structures by guarding them against electrochemical attacks of the metal by the ambient surroundings, in other words by corrosion. The steel pipes are the main fields of use of this protection. Even old and damaged steel networks can benefit from this technique in the most economic conditions possible.

Definition

A metal in contact with a conductible environment like water or the ground is the center of electrochemical reactions called the battery effect. This phenomenon has a dissolving effect on the metal from the anode to the cathode, in other words from the pipe towards the surrounding area: this is corrosion or oxidation. There exist two complementary means to control this corrosion:

Passive protection:

It is made up of the outer covering which has the effect of creating an electrical barrier between the metal pipes and the surrounding environment.

Active protection:

It consists in bringing in through artificial and outside means the whole of the outer metal surface to a potentially and sufficiently negative level to make the metal completely cathodic and thereby eliminate all risks of external corrosion. The cathodic protection level is the potential value below which the steel cannot corrode. For buried or immersed steel, the level of protection is measured with a Cu/CuSO4 reference electrode which is in close and immediate contact with the metal of -850 mV. It is only within a certain current flow level that the potential is reached.

To achieve the cathodic protection of the buried steel pipes there will be a need to:

Either make a battery with a more electronegative metal than steel (magnesium or zinc): it is sacrificial anode protection.
Principle: by creating a galvanized coupling whose metallic pipes will be the cathode of the battery and the anode a metal chosen for its more electronegative potential (magnesium, aluminium, and zinc).
or link them to an electrical outlet conveniently connected so that the steel becomes the cathode of the system and make sure that the potential of this cathode reaches all the required protection levels: this is decanting current.
Principle: The lowering of the potential of these pipes to the desired value is done by connecting the network, at one or several points, to the negative pole of a direct current electrical outlet. The electrical field distributes itself in the ground, through the ground connection or the spillway. The electrons reach the pipes and penetrate through their lateral surfaces, travel longitudinally in the conductors right to the connection with the negative pole of the rectifier. The result is a lowering of the potential in the network. This lowering of the potential increases from the farthest points of the network connection right to the negative pole of power supply to then reach its maximum above it. It must be sufficient for the protection criterion to be reached and maintained everywhere.
This type of protection is ideally suited for big networks and is very adaptable and adjustable.

An active and economical system.

Cathodic protection supplements well the passive covering protection and this no matter the modifications and the environment of the mains on the long run:

cuts of the outer covering of the pipe due to work done by a third party, near the pipe,
Modification of the soil resistance due to accidental pollution , and variations of the water tables, etc.
A new electrical environment of the pipe such as a newly cathodically protected network (gas, fuel ...),
High-tension power-lines, tramway, high-speed trains,
Draining of wandering currents.

For a minimum investment, the user of a pipe protected cathodically can regularly control the level of electrical potential of the pipe in regards to the surrounding environment and therefore be advised of any accidental degradation or modification of the surroundings of the pipe: this is cathodic surveillance. He will thereby ensure the permanence of his network and his investment: this is the life insurance of your network.

The electrodes used are: