A MULTISTEP BIRTH AND DEATH MODEL FOR LOCAL BREAKDOWN OF PASSIVITY

We show in this paper how the random processes theory can be used to describe the stochastic aspects of pits nucleation. We also show that the point defect model is consistent with this probabilistic behaviour. We suggest the existence of two pitting potentials: the "absolute" pitting potential E∞, below which no pitting can occur, and the "irreversible" pitting potential Eo, above which pits grow irreversibly. Between E∞ and Eo, the classical nucleation theory allows to calculate the induction time for pitting T; In some cases Log T is found to vary linearly with 1/(E-E∞). Above Eo the pitting rate is nearly proportionnal to the point defects concentration in the passive film. © 1990.