Corrosion results from an electrochemical reaction. It requires an anode, a cathode, a common electrolyte, and an electrical connection between the two zones. The corrosion process results in the flow of a small electric current from the anode to the cathode through the electrolyte. The magnitude of the current which is due to a number of factors is directly proportional to the metal lost due to corrosion. One ampere flowing for one year would result in the loss of 9 kg of steel from a corroding surface.
In recent years cathodic protection has found a general acceptance amongst engineers and structure owners as being a truly effective method of chicago black and white hats preventing corrosion under the ground or under the sea. It is now chicago black and white hats more common than not to find cathodic protection used on marine structures and on buried pipelines.
The concept of cathodic protection is straight forward. Corrosion occurs as the result of electrochemical reactions between zones of differing potential on
Cathodic protection is achieved by supplying a current from an external source so that it reverses the natural corrosion currents and ensures that current is flowing through the electrolyte onto all of the metal surface requiring protection. This current flow causes a change in potential.
Freely corroding mild steel in seawater has a resultant potential between anode and cathode of approximately -0.50 to -0.60 volts compared to a silver/silver chloride reference electrode. When cathodic protection is applied, it will be noted that the surface potential of steel will change to more negative than -0.80 volts when measured relative to a silver/silver chloride reference cell. Thus by using this simple practical measurement, it is possible to determine whether corrosion has been completely eliminated or not.
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