Any corrosion protection system is an integral part of maintaining process safety. One of them is cathodic protection. For a plant operator, these systems are seldom seen and operators are often not trained on the importance of these systems. A good interview by the Australian Pipeliner with Mr Mark Drager stresses the following points:
"Typically there are two types of CP systems:
Impressed – This CP system works by applying a small current (typically milliamps per kilometre) to the pipeline via units known as transformer-rectifiers. These units convert AC electricity into DC and use this electricity to lower the ‘energy’ of the pipeline. This system enables an asset owner to protect several kilometres of pipeline, provided the AC power remains connected.
Sacrificial – This CP system essentially performs a similar function via the electrical connection made between the pipeline and the buried anodes, namely zinc or magnesium. This system differs in that the DC electricity generated is due to the galvanic difference between the pipeline and the anodes. This system is also limited in protection range but is relatively maintenance free, however the anodes have a finite life and will need to be replaced.
There are few limitations to the usage of CP, but all can be overcome through careful design, construction and operation:
"Typically there are two types of CP systems:
Impressed – This CP system works by applying a small current (typically milliamps per kilometre) to the pipeline via units known as transformer-rectifiers. These units convert AC electricity into DC and use this electricity to lower the ‘energy’ of the pipeline. This system enables an asset owner to protect several kilometres of pipeline, provided the AC power remains connected.
Sacrificial – This CP system essentially performs a similar function via the electrical connection made between the pipeline and the buried anodes, namely zinc or magnesium. This system differs in that the DC electricity generated is due to the galvanic difference between the pipeline and the anodes. This system is also limited in protection range but is relatively maintenance free, however the anodes have a finite life and will need to be replaced.
There are few limitations to the usage of CP, but all can be overcome through careful design, construction and operation:
- Excessive negative potentials;
- May cause pipeline coating disbondment due to the excess hydrogen evolution;
- In some instances hydrogen cracks have also developed within pipelines due to the excess hydrogen evolution;
- Combined with the incorrect coating selected for the operating conditions, it may contribute to the development of stress corrosion cracks;
- Interference (stray currents) from the anode bed can affect neighbouring foreign utilities if the location of the anode bed is too close them; and,
- Disbonded coating or other foreign objects located next to the pipeline can cause a shielding effect. This would mean that the pipeline could not be protected locally at that point of shielding."
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