INSTABILITY OF NONEQUILIBRIUM FLUCTUATIONS ON PITTING DISSOLUTION .2. DETERMINATION OF THE CRITICAL CONDITION

The mechanism of the fluctuations which are necessarily induced by the breakdown of the electrostatic equilibrium has been examined in part I. An intense specific adsorption is essential to the progress of pitting. At the same time, the critical condition for unstable growth corresponds to a flat potential distribution in the diffuse layer within the electric double layer. Therefore, in the stable region of the fluctuations, the electrode is kept in electrostatic equilibrium and apparently behaves as an ideal polarized electrode. A method to determine the critical potential by the extrapolation from the stable region is developed theoretically. As a result, critical potential equations corresponding to the flat distribution of the potential in the diffuse layer are obtained for various experimental conditions. By applying these equations to actual systems, the adsorbed states of ions at the interface can be examined. As an experimental demonstration of the theory, critical potentials in nickel pitting dissolution in NaCl solution have been measured. Consequently, as expected, intense specific adsorption of Cl- ion is observed. At the same time, the adsorbed states of the other ions coexisting with Cl- ion are made clear.