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Cathodic Protection of Reinforced Concrete StructuresIn reinforced concrete, a passive oxide film is created around the reinforcing steel by the alkaline Portland cement, which protects the steel from corrosion. This oxide film is weakened or destroyed when aggressive ions, like chlorides, attack the concrete around the reinforcing steel causing corrosion of the reinforcing steel. Corrosion is an electrochemical process having anodic and cathodic areas formed on the steel. When the circuit of the corrosion cell is completed by the presence of continuous steel bars connecting anodic and cathodic areas and by immersion in the same electrolyte, corrosion occurs at the anodic regions. The corrosion continues until failure occurs at the anodic area. Cathodic protection is a method to control the corrosion of steel in contaminated concrete that works by making the embedded reinforcing steel cathodic. When the steel becomes cathodic, hydroxyl ions are accumulated around it making it passive for longer time. The reinforcing steel is electrically connected to another metal that becomes the anode with or without the application of an external power supply. The cathodic protection systems that work in the absence of an external power supply and in the presence of a less noble metal (like zinc) to act as anode are referred to as sacrificial passive systems. In case of an impressed current cathodic protection, an external power supply is used to force a small amount of electric current through the reinforcing steel to counteract the flow of current caused by the corrosion process. A metal, like platinum, serves as anode which corrodes at a very slow rate. Cathodic protection is used to protect almost any type of reinforced concrete structure, including horizontal slabs, walls, towers, beams, columns and foundations. However, this method has the following limitations:
- Cathodic protection does not replace corroded steel
- Impressed current cathodic protection systems are not recommended for general usage on prestressed concrete structures because hydrogen produced can make the high-strength steels brittle in nature
- Passive sacrificial systems can be used for post- tensioned structures after detailed corrosion analysis
- Electrical continuity of the reinforcing steel and ionic conductivity of concrete must be confirmed during system installation.