Rehabilitation of concrete structures must incorporate some form of corrosion control; otherwise, the effect would be purely cosmetic and additional costly repair work would be necessary in a few years. Interest has been rapidly increasing in the technique of cathodic protection, which has been shown to actually stop corrosion. Cathodic protection uses electrical means to reverse the direction of the mechanism that causes steel to rust in concrete.
THE MECHANISM OF CORROSION
In the normally alkaline concrete environment, a passivating oxide layer initially forms over the steel reinforcement. However, when salt such as sodium chloride or calcium chloride is present in the concrete, the chloride ions break down this passivating layer and the reinforcement is unprotected against corrosion. Furthermore, because the electrical potential of steel is different in concrete containing different concentrations of salt, there will be differences in electrical potential between one part of the reinforcement cage and another. There is a flow of current from one piece of rebar to another. The steel that is more positive (anodic) corrodes, while the steel that is more negative (cathodic) does not corrode. The rust that forms around the anodic bars occupies a much larger volume than the original steel, and this causes tremendous bursting forces to build up in the concrete around the reinforcement.
THE PRINCIPLE OF CATHODIC PROTECTION
To install cathodic protection, an electrode is buried in the concrete near the reinforcement and connected to the positive terminal of a direct current power source so that this new electrode is forced to act as the anode. The reinforcement is connected to the negative terminal. The external power is adjusted so that a net positive current flows into the entire rebar cage. This overcomes any corrosion current flowing between areas within the cage that previously were anodic and cathodic and it stops further corrosion.