Cathodic protection is a procedure used to protect an object from corrosion by making it a cathode.
Stray ground currents flow through neutral earthing points in substations, earth electrodes in HVDC Earth Return, Traction Substations, etc.
The underground metallic pipelines and cable sheaths are corroded by electrolytic action from unidirectional electric currents in the ground.
The external coating of insulating tape is the basic protection. In addition, Cathodic Protection is the most widely used method to prevent such corrosion.
External DC voltage is applied. The external DC voltage is obtained from a small motor-generator, engine-generator, batteries, solar cell-battery combination, transformer/ rectifier set.
The negative lead is connected to the underground metal pipe/cable/structure to be protected to make the part 0.25 to 0.3 V negative with respect to the surrounding soil or liquid.
The positive lead is connected to the ground at some distance. Or magnesium or zinc rods are sunk in the ground and are connected externally to the pipe/cable/structure.
These rods are anodic to the pipe and get "sacrificed" due to corrosion and the main pipe is protected.
The distance between consecutive cathodic protection installations varies between 0.4 km and 3 km depending upon the coating on the line and magnitude of ground currents.
The positive lead grounding should have low resistance earthing achieved by irrigation, preparation of the soil bed,
Advantages :
(1) The main advantage of cathodic protection over other forms of anti-corrosion treatment is that it is simply by maintaining a dc current and its effectiveness may be monitored continuously.
(2) Cathodic protection is commonly applied to a coated structure to provide corrosion control to areas where the coating may be damaged.
(3) It may be applied to the existing structures to prolong their life.
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