PROTECTION FROM CORROSION

A metal is generally considered to be immune to corrosion, or in a state of immunity, when it is thermodynamically stable in relation to the medium in which it is contained. For each metal, its balance potential is defined by comparison with a non-polarizable electrode of reference. In order to achieve the best contact with the external surroundings, this has to be placed on the soil surface, right above the buried structure to be monitored. The state of immunity is maintained until the metal acts like a cathode compared to other metals or to the external environment or, in physical terms, until the metal’s potential retains the value of -1 V, in relation to the soil.
To support their state of immunity, pipelines can be given two different methods of protections: an Active protection and a Passive protection.

PASSIVE PROTECTION
Passive protection is a set of preventive measures to electrically and mechanically insulate pipelines from their surroundings and other external structures.
The aim of Passive protection is:

• to prevent the electrolytic corrosion of the structures; 
• to aid the Active protection.

In order to accomplish its functions, a protective coating should have the following characteristics:

high insulation resistance;

• minimal permeability to water vapours and oxygen;
• almost perfect adherence to the metallic protected surface;
• absence of discontinuities and of through cracks;
• high mechanical resistance (to rips, cuts, hits, etc);
• high chemical and physical inertia in relation to the medium in contact.

Provided that it is almost impossible to produce a coating that can incorporate all the characteristics listed above, the project engineer has the task to choose the most suitable coating according to the project data he/she is aware of. Some of the characteristics mentioned above are so important in the protection of pipelines that it is worth to reiterate their relevance, especially regards to adherence and continuity. In reality, of course, it is almost impossible to obtain a perfectly adherent and continuous coating. .

ACTIVE PROTECTION
Passive protection can reduce and delay in great measure the phenomenon of corrosion, but cannot stop it from happening altogether. This is particularly true in the weak points of the system where the sheath or coating is damaged due to manufactory faults, to accidental damage of the coating during pipe laying, to the natural degradation of the coating or to particularly aggressive environmental conditions. Last but not least, another accelerating factor of corrosion can be a peculiar and localized electric nature of the soil. Therefore, the passive protection has to be complemented by the Active Protection or Cathodic Protection of pipes. The aim of this type of defence is to discipline the flow of electric currents and ultimately obtaining protection by forcing the surface of the entire protected structure to become a cathode. Total protection is achieved when in any point of the structure and in any given instant the potential is equal or inferior to the threshold of corrosion immunity. This is obtained by creating, between the soil and the buried pipelines, a controlled electric circuit that applies direct current on the pipes, straying in the soil and appropriately dispersed into it, and letting the current leave the pipes at predetermined points through one or more of the circuit’s metal conductors.
There are two ways to obtain such controlled flow:

• sacrificial anodes system;
• Impressed Current Cathodic Protection systems.

he decision of using one or the other depends from the unique characteristics of the protected structure, as well as those of the surrounding soil in which they are laid.

INSTALLATION
SACRIFICIAL ANODES SYSTEM
These installations are realized using a metal connection to the structure to be protected with a less noble metal than the one with which the structure is made of. Such protective action is incrementally more effective with the growing difference in nobility between the two metals, i.e. creating a greater potential difference between the two poles of the electric pile consisting in the protected structure, the surrounding soil and the protective metal.
These types of installations are called “sacrificial” because in this case the protection is achieved at the expenses of the anodic material that is subject to corrosion. The protective action is assured until there is anodic material to be “sacrificed”. It goes without saying that the anode has to be periodically substituted to safeguard the process.
The most common anodic materials are: Aluminium, Zinc and Magnesium. The choice of the most suitable metal depends on what the metal the structure is made of and on the resistivity of the environment in which it is laid.
For example, for the protection of buried steel structures, anodes made of Magnesium are generally used, normally laid in a bed of electrolytic backfill to support a constant corrosion, This diminishes their earth resistance (and therefore increase the emitted current) and provides a uniform distribution of the electric field. Generally, the current supplied by each anode is of a modest amount as it depends on the difference in potential between the poles of the electric pile, which is related to the intrinsic characteristics of the system and to the size of the anode itself that, realistically, always turns out to be of limited dimension. This in turn affects the dimension of the total metallic surface, which in the end results to be quite modest, that can be protected with the sacrificial anode type of installation. For extensive structures, it would be necessary to employ an elevated number of anodes, linked one to the other in a parallel connection. In reality, when dealing with very large structures or pipelines running for several kilometres, it is more convenient to realize Impressed Current Cathodic Protection system to provide a complete protection.
Advantages and disadvantages

IMPRESSED CURRENT CATHODIC PROTECTION SYSTEM
This type of systems utilizes a DC power source, which operates automatically, allowing to keep constant at any time the value of the voltage between pipe and soil. The power supply unit has the function to supply direct current to guarantee the continuous flow necessary for protection. The values of the current must be such, however,  not to lead to condition of over-protection, as this can damage the coating (cathodic disboding) as well as being potentially dangerous by interfering with third parties’ structures present on the territory. The negative pole of the power source is electrically connected to the structure that has to be protected, which becomes polarized like a cathode. The positive pole is instead connected to the ground anode (groundbed). The appropriate dimension of the groundbed, which can be considered the “heart” of the plant, is assessed on the basis of the amount of current needed, the length of the operation (usually 10/15 years), the consumption related to the amount of supplied current and the resistivity of the surrounding soil. The type of ground electrode appropriate for each system is assessed on the base of the following parameters:

• the nature of the soil at different depths;
• the costs of the installation;
• the necessity to minimize the frequency of the periodic checks needed to guarantee its efficiency and to keep constant its earth resistance and uniform its consumption.

Normally, the groundbeds (especially the horizontal types) are laid in an appropriate bedding of coke dust which ensures:

• an increase of the dispersing surface so that its earth resistance is minimized;
• a reduction of  anodic material consumption, using the backfill itself as an anode;
• a balance of the electrical behaviour of the surrounding soil with a more homogeneous behaviour of corrosion in the anode consumption, thus avoiding localized corrosion of the anode that may lead to its fracture.

Advantages and disadvantages


GROUNDBEDS
In relation to their spatial orientation, groundbeds con be classified as:

• horizontal;
• shallow vertical;
• deep vertical.
The first two types of ground electrodes for the protection of pipelines are usually laid at a distance of 100/150 meters from the protected structure so that the electric field is uniformly distributed. The same distance is normally kept from third parties’ buried structures, to be sure not to cause interferences with them.
In case of deep vertical groundbeds, the distance between the protected structure (or third parties’) and the top end of the anodic array decreases, but it is never less than 40 meters.

For the protection of distribution and supply networks it is common to use deep vertical ground electrodes for the following reasons:

• they take up the minimum space in congested areas;
• the possibilities of interference with other structures are minimized;
• some soils that have  a high resistivity on the surface, often have a good conductivity at depth; therefore, in those cases, the total resistance of a deep vertical groundbed results to be less than an horizontal one;
• there is a better distribution of the current;
• the anodic potential gradient is lower at higher depths;
• they are unaffected by the seasonal variations of the soil;
• they are not subject to third parties excavations and maintenance work form external agents.

Nonetheless, deep vertical groundbeds have also some disadvantages:

• high costs in relation to the current emitted;
• impossibility of repairing connection cables;
• in high-density soil, the increase of the resistance of the ground anode, caused by the stagnation of gasses on its surface, is more pronounced. In these cases, it is necessary to provide them with appropriate vent valves (actually this effect takes place in horizontal groundbeds, too).

The most common materials are: Iron, Graphite, Siliceous, activated Titanium and Magnetite.
                                            

INTERFERENCES

Interferences are variations of the electric state of a structure as a result of the electric field alteration in the environment due to the presence of another structure or to direct currents dispersed by the latter.

Misura del potenziale tubo/terra = Measure of  the pipe/earth potential
Elettrodo di riferimento = Reference electrode
Tester = Tester