What is corrosion
All metal structures immersed in an electrolyte, whether soil, fresh water, sea water or concrete, are naturally subject to degradation through the phenomenon we all know as corrosion, that is the electrochemical process which leads to the deterioration of metal elements. This means that all transport systems using buried or submerged pipelines, such as gas, water and oil transport networks, are subject to corrosion, which leads to enormous maintenance costs due to pipeline leaks, downtime and replacement of components, in addition to problems related to the safety of people and the environment.
How to prevent corrosion
For corrosion to occur, four elements must be present:
- Metallic bonding
To stop or slow down the deterioration process, it will therefore be necessary to eliminate at least one of the four components mentioned above.
Cathodic protection for the prevention of damages due to corrosion
There are various methods for preventing metal corrosion. In this article, we will deal specifically with cathodic protection, an electrochemical technique used to slow down the corrosion of metal structures in contact with a conductive environment, that is, an environment containing enough ions to conduct electricity (such as soil or water, and particularly sea water).
A metallic material placed in a conductive environment is the focus of a process known as battery effect, which means that a current can flow from the tube into the environment, causing the dissolution of the metal.
A metal will corrode at the point where the current separates from the anode.
To develop corrosion, the elements described above give rise to the following effects:
- A current flow through the electrolyte from anode to cathode is produced. It returns to the anode via the metal connection.
- Corrosion occurs whenever the current separates from the metal (pipe, structure, etc.) and comes into contact with the ground (electrolyte). The area where the current separates is called anodic. Therefore, corrosion takes place in the anodic zone.
- The current then flows back to the cathode and, indeed, there is no corrosion in this area. Around the cathode we find the polarisation effect (formation of a passivation film), which reduces the flow of corrosion current.
Corrosion in buried metal pipelines
With regard to buried metal pipelines, the galvanic corrosion process can be generated as a result of various factors; the main ones are:
- Metal contacts between different materials or between different alloys of the same metal (e.g. connection joints) having different electrochemical potentials;
- Presence of stray currents (for example near railways or other cathodic protection systems);
- Presence of electrochemical potentials due to different types of soil (such as clay-sand, the former is an anodic zone and the latter is a cathodic zone).
The current flow is caused by a potential difference between the anode and the cathode. The cathode protection level is the potential value below which the steel cannot corrode.
For buried carbon steel, the level of protection is measured against a Cu/CuSO4 reference electrode in direct contact with the ground and is conventionally set at -850 mV CSE under resistivity conditions of less than 100 Ohm*m.
Any current in the sense of flow separating from the pipeline causes corrosion. The basic idea is to supply electrons to the metal structure (underground pipeline or other) to prevent the dissolution of the metal.
Generally, corrosion control is achieved as follows:
- Galvanic (or sacrificial) anode system
In galvanic anode systems, the (underground) metal structure to be protected is connected to a less noble metal, such as zinc or magnesium.
Once connected, they form a galvanic pair. The sacrificial anode will corrode at a faster speed than it would on its own, while the metal structure will act as a cathode and corrode at a slower speed than it would on its own.
Anodes are sized according to the resistivity of the environment (ground). Anodes are made of materials such as magnesium (Mg), zinc (Zn) or alluminium (Al) and they are usually installed in the immediate proximity of the area to be protected and are connected to it with an insulated conductor.
- Impressed current system
Alternatively to the sacrificial anode system, a metal structure can also be protected by means of a system using an external power supply.
The negative terminal of the transformer rectifier is connected to the structure to be protected, while the positive terminal is connected to an inert material (anode). In this way, a current flow is generated between the anode and the area to be protected in order to slow down the speed of corrosion.
The anodes are connected to a DC current source, such as a transformer rectifier or a converter. The principle is the same as above, except that the anodes are made of corrosion-resistant materials, such as graphite, silicon iron, lead-silver alloys or platinum.
The current to be applied to the system to ensure corrosion protection depends on the surface to be protected, the type of coating used, the type of soil and other factors, ranging from a few µA up to several A.