Inspectioneering

Cathodic Protection

Cathodic Protection is a process that is used to protect materials from galvanic corrosion using either sacrificial anodes or an impressed current. It can, in principle, be applied to any metallic structure in contact with a bulk electrolyte. In practice though, its main use is to protect steel structures buried in soil or immersed in water.

The reason cathodic protection is needed is because of the corrosion which occurs in aqueous solutions due to an electrochemical process where anodic and cathodic reactions take place eventually causing the material to corrode. Cathodic protection controls this corrosion by making the material work as a cathode of an electrochemical cell. This is achieved by placing in contact with the metal to be protected another more easily corroded metal to act as the anode of the electrochemical cell.

There are two main types of systems of cathodic protection: sacrificial anodes and impressed current. While they involve different methods, they both rely on the same underlying scientific process.

Sometimes also known as galvanic anodes, sacrificial anodes are made using using alloys of zinc, magnesium, and aluminium. They are designed to have less negative electrochemical potential than the cathode, the material they are protecting. This leads to an electron flow from the anode to the cathode caused by difference in electrochemical potential between the anode and the cathode. This prevents the reactions that cause the corrosion in the cathode, while the anode continues to corrode in it’s place. Sacrificial anodes do occasionally require inspection and replacement.

For larger structures, sacrificial anodes are not easily able to deliver enough current to provide adequate protection for the entire structure. In those cases, impressed current is available. Impressed current tends to be more complex than sacrificial anodes and involves using anodes connected to a DC power source, which when attached to the component supply of high energy electrons to stifle anodic reactions on a metal surface preventing galvanic corrosion. Like with sacrificial anodes, this prevents corrosion in the material.

Impressed current anodes tend to be tubular solid rod shapes or continuous ribbons of various specialized materials, such as high silicon cast iron, graphite, mixed metal oxide or platinum. They tend to get consumed a slower rate than sacrificial anodes do and don’t need to be replaced as frequently.

This process has many applications within the industry most often protecting equipment that is either underground or submerged. Specifically the process is often used to protect pipeline and storage systems, marine transport and equipment, metal-reinforced structures, and underground storage tanks. It is also often used to protect steel, water or fuel pipelines and storage tanks, steel pier piles, ships, offshore oil platforms, and onshore oil well casings.

 

Related Topics

Coatings Condition Monitoring Locations (CMLs) Corrosion Control Documents (CCDs) Damage Mechanisms Flue Gas Dew Point Corrosion Green Rot Pitting Corrosion Positive Material Identification (PMI) Unit Corrosion Assessments

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