Cathodic protection is an use of an impressed current to prevent or to reduce the rate of corrosion of a metal in an electrolyteby making the metal the cathode for the impressed current.

The structure to soil potential of the structure to which Cathodic Protection is applied is made more negative than its natural potential. Usually –0.85 Volt to –1.5 Volt as measured with a special meter with respect to Cu-CuSO4 half-cell should be sufficient to prevent corrosion of the structure indefinitely.

There is no definite answer to this question. Each structure has its own current requirements, electrolyte resistivity maintenance schedule etc and hence each structure has to be considered individually. This also applies when selecting different metals for Sacrificial Anode system.

The source of electrical energy may be derived from a Transformer Rectifier unit or it may galvanic in origin as when iron and an active metal like zinc are connected to each other and are immersed in a common electrolyte, thereby protecting the iron at the expense of zinc.

Since a structure that has to be protected by Cathodic Protection must receive direct current from the electrolyte surrounding the structure and since air is poor conductor of electricity it is not possible to prevent atmospheric corrosion by Cathodic Protection.

Cathodic Protection is achieved if it is continuously and efficiently applied. It must be ‘ON’ all hours of the day and all of the year round. Occasional breakdown of power system is not dangerous if plate thickness is not just absolute minimum.

This can be applied on all structures when continuously immersed in water or permanently buried in soil. Oil, gas and water pipes, wharves, jetties, tanks, tankers, steel pilings, ships hull, heat exchangers, lead sheathed cables, etc are but a few cased in steel industry where cathodic protection has been used with large measure of success. Water and gas boards, oil industries, ports and sewage plants are large users of this system.

It is possible to protect the internal surfaces if the anodes are bonded inside the structure containing electrolyte.

Power Consumption is not very high when a good electrically insulating coating is applied on the structure to be protected.

The cost depends upon various factors like corrosive conditions of the environment, coating and its quality, and in case of pipelines on the thickness of the pipe shell. However against this direct cost, the cost of increased maintenance and repairs, increased depreciation, larger capital investment due to higher factor of safety, loss of revenue due to breakdowns and wastage, loss of public goodwill, damage suits etc is to be considered. Extra cost due to coating and cathodic protection is more than justified. As a rough measure, this cost comes to 5% to 10% of the cost of the structure.

Losses sustained by industry, defence forces and municipalities amount to many billions of dollars annually. Losses may be divided by direct and indirect losses. By direct losses meant cost of replacing corroded structures. Indirect losses are more difficult to assess. Examples of indirect losses are shut down, loss of product, loss of efficiency, contamination of product etc.