Pit initiation at passive iron in borate and phthalate buffer solutions was investigated by separately measuring dissolution rates of Fe(III) and Fe(II) at rotating ring‐dis electrodes. The rate of Fe(III) dissolution grow linearly with the chloride concentration. Fe(II) appears simultaneously with the first pits. At a constant electrode potential positive to the critical pitting potential a stage of pit initiation is observed ending at the time ti of incubation after addition of chloride to the solution. The time ti is independent of the time of prior passivation. The potential dependence of ti described by a relation known from the theory of two‐dimensional nucleation. The physical meaning of this formal analogy is discussed. According to the dependence of ti on pH and chloride concentration the observed mechanism of pit initiation is possible only above a critical chloride concentration c=0.3 mM and below a critical pH 10.4. The absolute values of critical pitting potentials and the dependence of the critical pitting potentials on chloride concentration were found to be a function of the nature and the concentration of the supporting electrolyte. The influence of the supporting electrolyte on the time of incubation was also investigated.
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Heusler et al. (1976) studied this question.