The chloride threshold

The chloride threshold for the initiation of pitting corrosion for a given structure depends on numerous factors. Major factors have been identified in the pH of concrete, i. e. the concentration of hydroxyl ions in the pore solution, the potential of the steel and the presence of voids at the steel/concrete interface.


The hydroxyl ion concentration in the pore solution mainly depends on the type of cement and additions . It was suggested that pitting corrosion could take place only above a critical ratio of chloride and hydroxyl ions . For instance, the Figure shows that the risk of significant corrosion increases as the [Cl–]/[OH–] ratio in the pore solution rises above a certain value. Although several authors confirmed the dependence of corrosion initiation on the [Cl–]/[OH–] ratio, a great variability was found for the threshold value. The great variability of the [Cl–]/[OH–] threshold is, first of all, a consequence of the stochastic nature of the initiation of pitting corrosion; it appears that the chloride threshold can only be defined on a statistical basis.

Source: Bertolini, L. et. al, Corrosion of Steel in Concrete, Wiley-VCH, 2004