The breakdown of passive films potentiostatically formed on iron in borate and phosphate solutions of pH 8.42 and 11.50 is investigated in 0.5 kmol/m3 NaCl solution at a constant potential of −0.195 V(SHE) higher than a critical pitting potential of −0.295 V in the same solution. The investigation revealed that the logarithmic plots for pitting current (ip) or amount of electric charge (Qp) vs time curves were composed of two straight lines with different slopes after a short period in which ip scarcely flows. The time, τ, required for the transition from the first to the second line was found to be a linear function of passivation potential irrespective of anion species and solution pH in which a passive film was formed. However, the slope of their linear relation depended on anion species and solution pH. It is believed that the first straight line corresponds to pit nucleation and the second one to pit growth. The dependence of τ on log (Cl−) gives the critical chloride ion concentration of 0.6 mol/m3 below which the period of pit nucleation becomes infinite. The breakdown of the passive films on iron is discussed in terms of the thickness and the ion selective property of the passive films, and the mechanism of its breakdown is proposed.