What Does Cathodic Shielding Mean?
Cathodic shielding is an electrochemical means of corrosion control in which the oxidation reaction in a galvanic cell is concentrated at the anode and suppresses corrosion of the cathode in the same cell. This is achieved by placing a more easily corroded metal to act as the anode of the electrochemical cell in contact with the metal to be protected.
Cathodic shielding is also known as cathodic protection.
Corrosionpedia Explains Cathodic Shielding
Cathodic shielding is a procedure used to protect an object from corrosion by making the object a cathode. For example, to make a tank a cathode, an anode is attached to it. Both have to be in an electrolyte such as soil or water.
Cathodic shielding is a widely used method for controlling the corrosion of metallic structures in contact with many forms of electrolytically conducting environments such as soil, seawater and natural waters. Cathodic shielding essentially reduces the corrosion rate of a metallic structure by reducing its corrosion potential, bringing the metal closer to an immune state.
Cathodic shielding can be achieved in two ways:
- Galvanic (sacrificial) anodes - Galvanic anode systems employ reactive metals as auxiliary anodes that are electrically connected to the steel to be protected.
- Impressed current - Impressed current systems employ inert anodes and use an external source of DC power to impress a current from an external anode onto the cathode surface.
Cathodic shielding enables the cost-effective and safe operation of grounded and submerged metal structures. It is relatively simple, has proven efficiency and its effectiveness can be monitored continuously. Cathodic shielding is the key to protecting and extending the life of metal equipment.
Cathodic shielding is one of the few corrosion control methods that effectively control corrosion of existing buried or submerged metal surfaces. Cathodic shielding systems are most commonly used to protect:
- Water or fuel pipelines
- Storage tanks
- Steel pier piles
- Offshore oil platforms
- Onshore oil well casings
Cathodic shielding can sometimes be an effective method to prevent stress corrosion cracking (SCC).
The disadvantages of cathodic shielding are:
- Excessive negative potentials can cause accelerated corrosion of lead and aluminum structures due to the alkaline environments created near the cathode.
- Hydrogen evolution at the cathode surface may, on high-strength steels, result in hydrogen embrittlement of the steel, with subsequent loss of strength. This may lead to catastrophic failure.
- May cause disbondment of coatings; the coating would then act as an insulating shield to the cathodic protection currents.
- Cannot be used to prevent atmospheric corrosion of metals.