Statistical variation of pitting potential and induction time for pit generation has been studied based on a stochastic theory which had been developed for the fracture of solid materials caused by applied stress. In order to obtain a large number of data, a multichannel pitting corrosion testing apparatus was developed, which can measure the pitting potential, or the induction time for 12 specimens in 1 experiment using 1 potentiostat. Analysis of experimental data for Type 304 stainless steel in 3.5% NaCl solution clarifies that the pitting process at a constant potential consists of 3 successive processes, each of which has a different pit generation rate depending on the potential and time. Only the first process is detected by the pitting potential determined by the potential sweep method. Experiment proves the prediction given by the theory that the pitting potential increases with the square root of the potential sweep velocity. Linear dependence of the pit generation rate on the potential suggests that the pitting process is controlled not by an electrochemical reaction, but by an electromechanical breakdown of the passive film.

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