Fe2+ + 2e- Chloride Penetration - Mechanism
The penetration of chloride ions into reinforced concrete structures is the major contributing factor for inducing corrosion of the steel reinforcement. For corrosion to occur, four conditions must be satisfied:
The steel in concrete is usually protected from corrosion due to the high pH (~ 12.5) of the hydration products. At this pH level, the rate of the corrosion reaction will be negligible. Permeation of chlorides at the rebar level can reduce the pH to a level at which the oxidation reaction for corrosion may occur (corrosion may occur at pH values < 9). At this point, the half-cell reaction begins to take place. The ferrous atoms in the steel become oxidized:
Fe Fe2+ + 2e-
At the same time, water and oxygen atoms react to produce excess hydroxyl ions:
2 H2O + O2 + 4e- 4OH-
The ferrous and hydroxyl ions may combine to form insoluble compounds (rust). It is well documented that the presence of chloride ions accelerates corrosion, but the exact mechanism is still under some debate. Most accept that chlorides are more effective in penetrating the oxide film that exists about the rebar. Another theory is that the chlorides bond with the ferrous ions produced by corrosion forming a soluble compound. This compound can diffuse away from the steel, exposing additional ferrous atoms, and allowing corrosion to continue. The soluble ferrous chloride breaks apart some distance away from the electrode and ferric hydroxide precipitates. This theory may explain the staining by rust that is observed in concrete where corrosion is rampant.
The oxidation of iron atoms in steel reinforcement has dual adverse effects. The first and most obvious is the reduction of steel cross section due to the "rusting away" of bars that may lead to inadequate areas of steel necessary for carrying applied loads. When steel corrodes, The rust produced will have significantly higher volume than the original steel. This volume increase induces internal stresses on the surrounding concrete. The corroded steel essentially pushes outward on the concrete, causing it to delaminate, crack and eventually spall.