The principle involved in this method is to force the metal to be protected to behave like a cathode, thereby corrosion does not occur.
There are two types of cathodic protections:
- Sacrificial anode method
- Impressed current method
Impressed current method
- During corrosion, electrons flow from anode to cathode and corrosion current flows fromcathode to anode.
- In this method, an impressed current is applied in opposite direction to that of corrosioncurrent; thereby nullify the effect of corrosion current, and converting the corrodingmetal from anode to cathode.
- In this method, the insoluble material (Like stainless steel, platinum, scrap iron, graphiteor high silica iron) is usually kept in backfill made up of gypsum or any such material,which can help in increasing electric contact with soil.
- This insoluble material is buried underground. Usually D.C. current by using battery ordry cell is applied to it and it is then connected to the structure to be protected.
- As a result of impressed current, the burried material starts acting as anode anddeteriorates and the structure to be protected acts as cathode.
- The burried anode has to be replaced periodically.
- This anode may be single (as in simple applications) or many anodes (as in pipe lines).
Applications: This type of cathodic protection by impressed current is applied to open water boxcoolers, water tanks, buried water or pipelines, condensers, transmission line towers, marine piers, laid-up ships etc.
