Cathodic protection

Cathodic protection is a technique of prevention against electrochemical corrosion applied to metallic structures that are in contact with electric conductivity environments. It consists in circulating continuous current between an electrode, called anode, and the surface of the structure to be protected, called cathode. This current lowers the electrical potential on the metallic surface, up to excluding the corrosive phenomenon. The cathodic protection technique officially appeared in England in 1824, and it was affirmed in America a century later, and throughout the world beginning from 1960 and 1970. Cathodic protection was first applied in Italy to protect lead conduits of telephone cables around 1930, and later on to protect oil pipelines, gas pipelines, aqueducts and structures at risk to corrosion.

Application methods of cathodic protection

For the correct application of a cathodic protection system and to contain costs, the covering and coating of the structure that must be protected is determinant. For this reason, in function of dimension and the surrounding environment, two systems are applied:

Sacrificial anode system – it does not need electric energy, and simply consists of connecting the surface to be protected to a metal whose electric potential is less noble. Considering the slight difference of value between the two metals, motor work, the essential condition is always present in sea water where the sacrificial anodes are utilized with success, less in the ground where often the elevated resistance value makes the system ineffective. The metals normally used to protect steel in sea water are aluminium and zinc. Magnesium is usually utilized in the ground because of major motor work.
Impressed current system – the distribution of the protective current occurs by means of a cathode feeder on which negative pole is connected to the structure to be protected and on the positive pole the anodic ground electrode. This system needs electric energy, but disposes of a considerable motor work that permits its application on the ground and to protect big dimension structures. The advantages are in function of the electricity supplied by the plant. The resistance of the circuit is determinant for its almost total dependence from that of the anodic ground electrode.

A cathodic protection system with impressed currents is essentially composed of:

Dissipator

To maintain the value in Ω of the resistance low, the dissipater is dimensioned during the planning phase according to resistance of the ground in which it will be placed. To reduce contact resistance with the environment, the anodic material is also placed in backfill.

The dissipators can be placed into two groups:

Surface dissipators

Scavo per dispersore Etroubles The cost is limited, but wide spaces are necessary and also areas that are permitted according to regulations. The dissipator and backfill material are laid in an excavation of 1,50 mt. with a distance of 1000 mt. from the structure to be protected. This kind of dissipator once utilized because of the available spaces is now no longer in use.

Depth dissipators

Depth dissipators are carried out by drilling between 80 and 120 mt. in depth and 13 and 20 cm. in diameter. The anodic material and the backfill are lowered into the well, leaving a space of about 40 mt. between the top of the dissipator and the ground surface.

This kind of dissipator offers many advantages, particularly the necessity of limited spaces, better distribution of the current of protection with less electric interferences. With these advantages, the cost of a vertical dissipater is slightly higher than a horizontal one.

Anodic material normally installed:

- In sea water: Anodes in activated titanium

- In the ground: Iron and Iron silicon

During the planning phase the anodic material is chosen according to the current the plant will supply, the area in which the dissipator will be laid and its duration in time.

Cathodic feeder

The choice of the material is fundamental for the function of the plant, above all in the presence of electric fields deriving from railroad traction lines.

The feeders are as follows:

- Simple feeder

Every setting must be carried out by the operator.

- Constant current automatic feeder

Independently alternates out voltage with varying resistance of the anode dissipator and always supplies the same current

- Constant potential automatic feeder

It autonomously varies the supplied current, increasing or decreasing according to the potential of the set up structure

- Constant potential automatic feeder with basic current.

It is the most commonly used in Italy and the applied parameters are two:

- The basic current. The feeder always distributes that current even when the value of the potential of the structure is more negative than the one planned. The equipment increases the value of the current distribution autonomously. The value of the current and the duration of the intervention are proportional to the electric interference, usually due to the railroad traction lines

Corrosion costs

The corrosion cost is estimated (from specialized journals on the subject) at about 4% of the internal gross product of an industrial country. 70% of this extremely high cost is unavoidable because it is a necessary tax to contrast the corrosion phenomenon.
The remaining 30% is instead avoidable and due to the incorrect or misapplication of know how regarding cathodic protection.


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Cathodic protection Cathodic protection is a technique of prevention against electrochemical corrosion applied to metallic structures that are in contact with electric conductivity environments. It consists in circulating continuous current between an electrode, called anode, and the surface of the structure to be protected, called cathode. This current lowers the electrical potential on the metallic surface, up to excluding the corrosive phenomenon. The cathodic protection technique officially appeared in England in 1824, and it was affirmed in America a century later, and throughout the world beginning from 1960 and 1970. Cathodic protection was first applied in Italy to protect lead conduits of telephone cables around 1930, and later on to protect oil pipelines, gas pipelines, aqueducts and structures at risk to corrosion. Application methods of cathodic protection For the correct application of a cathodic protection system and to contain costs, the covering and coating of the structure that must be protected is determinant. For this reason, in function of dimension and the surrounding environment, two systems are applied: Sacrificial anode system – it does not need electric energy, and simply consists of connecting the surface to be protected to a metal whose electric potential is less noble. Considering the slight difference of value between the two metals, motor work, the essential condition is always present in sea water where the sacrificial anodes are utilized with success, less in the ground where often the elevated resistance value makes the system ineffective. The metals normally used to protect steel in sea water are aluminium and zinc. Magnesium is usually utilized in the ground because of major motor work. Impressed current system – the distribution of the protective current occurs by means of a cathode feeder on which negative pole is connected to the structure to be protected and on the positive pole the anodic ground electrode. This system needs electric energy, but disposes of a considerable motor work that permits its application on the ground and to protect big dimension structures. The advantages are in function of the electricity supplied by the plant. The resistance of the circuit is determinant for its almost total dependence from that of the anodic ground electrode. A cathodic protection system with impressed currents is essentially composed of: Dissipator To maintain the value in Ω of the resistance low, the dissipater is dimensioned during the planning phase according to resistance of the ground in which it will be placed. To reduce contact resistance with the environment, the anodic material is also placed in backfill. The dissipators can be placed into two groups: Surface dissipators The cost is limited, but wide spaces are necessary and also areas that are permitted according to regulations. The dissipator and backfill material are laid in an excavation of 1,50 mt. with a distance of 1000 mt. from the structure to be protected. This kind of dissipator once utilized because of the available spaces is now no longer in use. Depth dissipators Depth dissipators are carried out by drilling between 80 and 120 mt. in depth and 13 and 20 cm. in diameter. The anodic material and the backfill are lowered into the well, leaving a space of about 40 mt. between the top of the dissipator and the ground surface. This kind of dissipator offers many advantages, particularly the necessity of limited spaces, better distribution of the current of protection with less electric interferences. With these advantages, the cost of a vertical dissipater is slightly higher than a horizontal one. Anodic material normally installed: - In sea water: Anodes in activated titanium - In the ground: Iron and Iron silicon During the planning phase the anodic material is chosen according to the current the plant will supply, the area in which the dissipator will be laid and its duration in time. Cathodic feeder The choice of the material is fundamental for the function of the plant, above all in the presence of electric fields deriving from railroad traction lines. The feeders are as follows: - Simple feeder Every setting must be carried out by the operator. - Constant current automatic feeder Independently alternates out voltage with varying resistance of the anode dissipator and always supplies the same current - Constant potential automatic feeder It autonomously varies the supplied current, increasing or decreasing according to the potential of the set up structure - Constant potential automatic feeder with basic current. It is the most commonly used in Italy and the applied parameters are two: - The basic current. The feeder always distributes that current even when the value of the potential of the structure is more negative than the one planned. The equipment increases the value of the current distribution autonomously. The value of the current and the duration of the intervention are proportional to the electric interference, usually due to the railroad traction lines Corrosion costs The corrosion cost is estimated (from specialized journals on the subject) at about 4% of the internal gross product of an industrial country. 70% of this extremely high cost is unavoidable because it is a necessary tax to contrast the corrosion phenomenon. The remaining 30% is instead avoidable and due to the incorrect or misapplication of know how regarding cathodic protection.