Reinforced Concrete is a composite material. We have steel to take the tension and concrete the compression. Now, reinforcing steel is susceptible from damage due to corrosion and it needs to bond with steel to actually work as a composite. Quite interestingly in an alkaline environment, the surface atoms of the steel get oxidized to form an thin oxide layer (thickness of about 10 nm) of γFeOOH.
This film is stable in the highly alkaline environment of concrete. However, with time and exposure to Carbon di Oxide in environment which gets dissolved in water and enters the concrete through the permeability and cracks the pH of concrete reduces. As a result, this layer decomposes. And, deleterious agents start attacking the reinforcements resulting in corrosion.
The corroded reinforcement expands in size and spalls the cover leading to further exposure and further corrosion till the structure fails