A THEORETICAL-ANALYSIS OF THE ELECTROCHEMICAL NOISE DURING THE INDUCTION PERIOD OF PITTING CORROSION IN PASSIVE METALS .3. THE CURRENT NOISE ASSOCIATED WITH HALIDE ISLAND FORMATION ON THE PASSIVE FILM

The halide island model for the initiation of pitting corrosion was tested for the occurrence of the electrochemical noise during the induction period before pits start. The halide island was modeled by a disk of metal halide of radius r and thickness d, formed on the surface of a passive film. A critical condition concerning the size of such islands for the stable formation and growth was determined with the use of irreversible thermodynamic stability criterion. From the stochastic analysis concerning the growth/diminishing processes of halide islands inside the metal oxide film, a distribution function was obtained for the number of halide islands which have radius r and thickness d. The survival probability of pit initiation and the noise level of the current were determined using this distribution function. Good correspondences with the reported experimental results were obtained, indicative of the applicability of the model to passive metals of thin oxide films.