INSTABILITY OF NONEQUILIBRIUM FLUCTUATIONS ON PITTING DISSOLUTION .1.OCCURRENCE OF INSTABILITY

Pitting dissolution in the presence of aggressive ions such as chlorid e ions has been examined from the viewpoint of unstable growth of none quilibrium fluctuations. In a solution with aggressive ions, the passi ve film on a metal surface is ruptured and repaired by the thermal mot ion of solution species. As a result, the electrostatic equilibrium pr oduced by the electrical double layer is always locally broken This br eakdown, i.e., fluctuation, does not occur at the Nernstian equilibriu m, so that we can call it ''nonequilibrium fluctuation,'' which has a positive value and forms a diffusion layer outside the double layer. O n examining the mechanism of instability for the fluctuations, a new t ype of instability induced by the electrostatic interaction in the ele ctric double layer has been found. From the unstable conditions obtain ed, this kind of, instability takes place by the strong specific adsor ption of anions onto the electrode surface and has two different modes of instability: (i) from the beginning, the surface is specifically a dsorbed and (ii) there is no specific adsorption at the initial stage. The former allows continuous transfer from the stable state to pittin g corrosion, whereas for the latter suddenly after specific adsorption occurs, corrosion is expected to proceed. Moreover, a critical potent ial which decides the start of the the instability corresponds to the flat distribution of the potential in the diffuse layer within the ele ctrical double layer. Finally the fluctuation in the diffusion layer, in this case, is always stabilized.