Corrosion occur in concrete reinforcement due to the increase in the volume of oxidized compounds as compared to the base metal dissolved in steel reinforcement. Due to that increase in the volume the tensile forces in steel start cracking the concrete around it.
The alkalinity of concrete prevents the steel from corrosion. The reserve of calcium hydroxide is very high, so there is no need to expect steel to corrode, even when water penetrates into the concrete.
Every small crack develops in concrete lead to concrete damage. However the environment influence and carbon dioxide are particular and thus reduce the PH (12.6 to 8.0) of concrete which lead to the removal of alkalies. The reduction of PH value leads to the corrosion of reinforcement.
There are two mechanisms by which the corrosion of steel can occur.
1: Direction oxidation
2: Electrochemical reaction
There is no corrosion below 30% relative humidity and at 70- 80% relative humidity, the rate of corrosion is very high. The pitting corrosion can take place when the chloride ions are present in a certain amount called pitting potential.