The pH of environment in concrete usually is greater than 12.5, which causes a passive film on the surface of embedded steel reinforcement. However, this protective film breaks down when concrete reacts with atmospheric carbon dioxide or chloride ions (present in aggressive environments such as seawater or de-icing salt) penetrate into the hardened concrete and reach the surface of reinforcing steel.
Corrosion products cause expansive stresses in the concrete which leads to cracking and spalling.
Cathodic protection (C.P.) is a well-accepted technology for the corrosion prevention of the reinforcing steel in concrete.
Figure:
-Deteriorated structure
-CP Installation
-Rehabilitated structure after CP Installation
Source: http://www.corrosionservice.com/reinforced_concrete.asp
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