One of the most prevalent materials in the reinforcement of concrete is steel; the main reason is because of its high tensile. A second almost as valid reason is that steel and concrete cement have almost the same thermal expansion coefficient. This means that when concrete/steel composite expands upon heating all the components of the composite. To insure and strengthen the bonding in a concrete steel reinforced composite the surface of the steel members are processed with the incorporation of contours (or ridged) that is called rebar.
As a result of the hydration reactions of cement, the pore solution of concrete tends to be alkaline, with pH values typically in the range 12.5-13.6. Under such alkaline conditions, reinforcing steel tends to passivate and display negligible corrosion rates.

However, due to the porous nature of concrete, corrosive species and chemical species supporting corrosion reactions can enter the concrete and lead to corrosion problems. Furthermore, corrosive species can enter the mix if "contaminated" mix ingredients are used (water, aggregates, additives).

Corrosion damage to the reinforcing steel results in the build-up of voluminous corrosion products, generating internal stresses and subsequent cracking and spalling of the concrete as shown schematically in the diagram below (Left). Clearly the reinforcing steel is more vulnerable to further corrosion damage after the protective concrete cover is compromised in this manner.

Two important rebar corrosion mechanisms are

• chloride induced rebar corrosion, and
• carbonation.

Reference- http://www.corrosion-club.com/