Understanding defects, testing and inspection - Concrete Repair - page 10

Figure 1.7 Low cover and carbonation induced corrosion.

cathodic (non-corroding) region develops where oxygen gas and moisture
which have diffused through the concrete to the steel surface react with
electrons produced from the anodic reaction to produce hydroxyl ions.

2Fe → 2Fe 2+ + 4e - anodic region

O 2 + 2H 2 O + 4e - → 4OH - cathodic region

The iron then reacts with the hydroxyl ions to form iron hydroxide and
then, by further oxidation, iron oxide or rust ( Figure 1.8).
Corrosion of the reinforcement will not occur if the pore solution of
the concrete in which the steel is embedded remains sufficiently alkaline
(assuming chlorides are absent and that stray electric currents are not
present). In an alkaline state, a passive oxide layer forms over the steel
reinforcement which prevents corrosion. However, carbon dioxide (and
sulfur dioxide) in the air are acidic gases, and react with the concrete to
cause alkalinity to be lost from the surface inwards: a gradual process
known as carbonation. In good-quality concrete of low permeability, the
rate of movement of the carbonation front is very slow, and provided the
concrete has a reasonably low water to cement ratio, and sufficient concrete
cover has been provided, the integrity of the reinforcement will last for the
lifetime of the structure.