A cathodic protection system prevents corrosion by making a metal surface the cathode of an electrochemical cell. Corrosion, a natural electrochemical process, occurs when a metal in a corrosive environment (an electrolyte, such as soil or water) loses electrons and oxidizes. By supplying a steady flow of electrons, cathodic protection halts the oxidation reaction on the protected structure, transferring the corrosion to an external, more expendable component.
This technology is critical for safeguarding metallic infrastructure that is either buried underground or submerged in water, such as:
- Pipelines for oil, gas, and water
- Storage tanks
- Ship and boat hulls
- Offshore oil platforms and wind farms
- Rebar in concrete structures
Two types of cathodic protection
1. Sacrificial (galvanic) anode cathodic protection
This method involves connecting the structure to be protected with a more electrochemically active metal, known as a sacrificial anode.
- Mechanism: Based on the galvanic series of metals, the more active anode (made of a metal like zinc, aluminum, or magnesium) corrodes preferentially in the presence of an electrolyte. This forces the protected structure to become the cathode, where no corrosion occurs.
- Usage: It is ideal for smaller structures or applications where there is no external power source, such as water heaters and small-diameter pipelines.
- Replacement: The anodes are gradually consumed over time and must be replaced periodically. Galvanized steel is a common example, where a zinc coating acts as a sacrificial anode to protect the underlying steel from rust.
2. Impressed current cathodic protection (ICCP)
ICCP uses an external DC power source, like a rectifier, to supply a protective electrical current to the structure.
- Mechanism: The rectifier connects the protected structure to the negative terminal and an array of durable, inert anodes to the positive terminal. The power source “impresses” a current through the anodes, forcing electrons toward the structure and preventing corrosion.
- Usage: This system is more suitable for large-scale projects, such as long pipelines and offshore platforms, where a higher current is needed.
- Adjustability: ICCP systems can be adjusted to maintain optimal current output and consistently monitor the level of protection.
Benefits and considerations
Key benefits
- Extends asset life: Cathodic protection systems can significantly prolong the operational life of metallic infrastructure.
- Environmental protection: By preventing leaks from pipelines and storage tanks, CP helps avoid environmental contamination.
- Cost-effective: When combined with protective coatings, CP can be an economical method for corrosion control.
Important considerations
Regular monitoring: To ensure the system remains effective, especially for ICCP, regular monitoring and maintenance are essential.
Requires an electrolyte: The system is only effective when the metal structure is in contact with an electrolyte, such as soil or water, and does not work in atmospheric conditions.
Potential issues: Improperly designed or maintained systems can lead to problems, including coating damage from excessive current (cathodic disbondment) or hydrogen embrittlement in high-strength steels.