The corrosion of steel reinforcement is the depassivation of steel with the reduction in the alkalinity through carbonation.

What is Corrosion in Concrete?

Corrosion is defined as the destruction or the deterioration of the materials due to the chemical or electrochemical reaction with the environment and also the loss of steel due to the formation of rust. The corrosion of steel reinforcement is the depassivation of steel with the reduction in the alkalinity through carbonation. Corrosion deteriorates concrete because the product of corrosion – ferric oxide, brown in color occupies a greater volume (More than 2 to 10 time) than steel and exerts substantial bursting stress on the surrounding concrete.

Corrosion of Steel Reinforcement in Concrete

The corrosion of steel reinforcement causes an increase in the volume of oxidized compounds when compared with the volume of the base metal that is dissolved. This increase in the volume results in the tensile forces, that leads to cracks in the concrete around the reinforcement steel. The corrosion mechanism can be explained through the figure given below. 
Fig.1. Corrosion of Steel Reinforcement in R.C.C 

The corrosion of the steel in concrete is an electrochemical process. When there is a difference in the electrical potential along the steel reinforcement in concrete, an electrochemical is set up. 

In the steel, on part becomes the anode and the other part will form as a cathode. This system is connected by means of an electrolyte. The pore water in the hardened concrete will act as this electrolyte. The positively charged ferrous ions Fe2+at the anode pass into the solution, while the negatively charged free electrons e- will pass through the steel into the cathode, where they are absorbed by the constituents of the electrolyte and combine with water and oxygen to form hydroxyl ions (OH-). These travel through the electrolyte and combine with the ferrous ions to form ferric hydroxide which is converted by further oxidation to rust. The reaction involved are explained below: 

Anodic Reaction

Fe → Fe++ + 2e‑

Fe++ + 2(OH)- → Fe(OH)2 (Ferrous Hydroxide)

4Fe(OH)2 + 2H2O + O2 → 4Fe(OH)3 (Ferric Oxide)

Cathodic Reaction

4e- +O2 +H2O → 4(OH)- 

Fig.3.Corrosion Chemical Reactions

Note: No corrosion will take place if the concrete is dry or probably below the relative humidity of 60% because enough water is not there in order to promote the corrosion. Complete immersing of concrete under the water will also not cause corrosion as this condition won’t promote the diffusion of oxygen into the concrete. This is why the porosity and permeability is found as a great study for understanding the concrete properties. 

The products of corrosion occupy a volume as many as six times the original volume of steel reinforcement, depending on the oxidation state. The increased volume of rust will exert thrust on cover of concrete resulting in cracks, spalling or delamination of concrete. Due to these, concrete will lose its integrity. The cross-section of reinforcement progressively reduces and hence will result the structure to collapse.