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

Pitting dissolution in the presence of aggressive ions such as chloride ions has been examined from the viewpoint of unstable growth of nonequilibrium fluctuations. In a solution with aggressive ions, the passive film on a metal surface is ruptured and repaired by the thermal motion of solution species. As a result, the electrostatic equilibrium produced by the electrical double layer is always locally broken This breakdown, i.e., fluctuation, does not occur at the Nernstian equilibrium, so that we can call it ''nonequilibrium fluctuation,'' which has a positive value and forms a diffusion layer outside the double layer. On examining the mechanism of instability for the fluctuations, a new type of instability induced by the electrostatic interaction in the electric double layer has been found. From the unstable conditions obtained, this kind of, instability takes place by the strong specific adsorption of anions onto the electrode surface and has two different modes of instability: (i) from the beginning, the surface is specifically adsorbed and (ii) there is no specific adsorption at the initial stage. The former allows continuous transfer from the stable state to pitting corrosion, whereas for the latter suddenly after specific adsorption occurs, corrosion is expected to proceed. Moreover, a critical potential which decides the start of the the instability corresponds to the flat distribution of the potential in the diffuse layer within the electrical double layer. Finally the fluctuation in the diffusion layer, in this case, is always stabilized.