Initiation and Pitting Potential

The presence of chloride ions produces, at more positive potentials than Epi localized breakdown of the passive film and thus allows attack on the underlying metal. For potentials below Ep, the action of chloride is, to a first approximation, negligible. The chloride content being equal, Ep, decreases as the pH of the pore solution in concrete decreases and as temperature and porosity increase. Epj, is difficult to measure with laboratory measurements since during the measurement significant variations of pH and chloride level in the concrete near the surface of the steel can be introduced, altering the result 

For a given potential of the steel, the highest content of chlorides compatible with conditions of passivity is the critical chloride content (or chloride threshold) at that potential. As already discussed in Chapter 6, for structures exposed to the atmosphere (whose reinforcement operates at a potential around 0 V SCE) the critical level is usually considered to be in the range of 0.4% to 1 % of the cement content. For structures immersed in water (whose reinforcement operates instead at a much lower potential, around —400 to —500 mV SCE) or for reinforcement that is cathodically polarized for any reason, the chloride threshold is much higher. 

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