Cathodic Protection (CP)

Reviewed by Raghvendra GopalCheckmark
Last updated: May 24, 2023

What Does Cathodic Protection (CP) Mean?

Cathodic protection is a technique used to protect metal surfaces from corrosion by making the metal surface the cathode of an electrochemical cell.

Cathodic protection is typically used in environments where metals are exposed to corrosive environments, such as saltwater, soils and concrete. It is commonly used to protect buried pipelines, offshore structures and marine vessels.


Corrosionpedia Explains Cathodic Protection (CP)

Cathodic protection can be achieved using two main techniques:

  1. Galvanic cathodic protection.
  2. Impressed current cathodic protection.

Galvanic cathodic protection involves the use of a sacrificial anode, typically made of a more easily oxidized metal, such as zinc, aluminum or magnesium. The sacrificial anode is connected to the protected metal surface using a conductor, such as a wire or rod. When the metal surface and the sacrificial anode are immersed in an electrolyte, such as seawater, the sacrificial anode corrodes instead of the metal surface, thereby providing protection.

Impressed current cathodic protection involves the use of an external power source, such as a rectifier, to provide a direct current to the metal surface. This current forces the metal surface to become the cathode, thereby protecting it from corrosion. Impressed current cathodic protection is typically used for larger structures or in environments where a galvanic cathodic protection system is not feasible.

Cathodic protection's effectiveness can be quantified using various parameters, including the potential difference between the metal surface and the electrolyte, the current density and the polarization resistance.

The potential difference is a measure of the voltage required to achieve cathodic protection and is typically expressed in volts. The current density is a measure of the current required to achieve cathodic protection and is typically expressed in amperes per square meter (A/m2). The polarization resistance is a measure of the resistance of the metal to the flow of electrons, and is related to the corrosion rate by the Tafel equation.

Cathodic protection does, however, come with its downsides. For one, installing it can be costly — and this cost can increase depending on the complexity of the structures undergoing CP.

Cathodic protection systems also require routine maintenance, including a periodic visual inspection. In the case of impressed current cathodic protection, managers must also factor in the ongoing cost of electricity.

Finally, CP often isn't an ideal protection method for large metal surfaces that do not have a barrier coating — for example, the bottoms of large, welded tanks. That's because, due to the natural voltage drop while current is flowing, it's nearly impossible for even a well-designed cathodic system to maintain the proper voltage across an uninsulated long metal span.

Because of the limitations above, many experts consider CP a secondary protection method to barrier coatings. However, cathodic protection is quite effective on pipelines with an epoxy barrier coating.



Cathodic Protection System

Cathode Protection

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