Nonequilibrium fluctuation theory on pitting dissolution. III. Experimental examinations on critical fluctuation and its growth process in nickel dissolution

Following part II, current–time transient in nickel pitting dissolution has been examined. After the passive film is broken, the resultant diffusion current decreases to the minimum value. The minimum current, as shown in part I, gives the ratio of the average critical concentration fluctuation on the completely active surface to its autocorrelation distance. Therefore, using the experimental data of the minimum current mentioned above and the average concentration fluctuation obtained in part II, the critical autocorrelation distance has been calculated. Since, in the stable region, the fluctuation is suppressed to maintain that the electrode surface is flat and stable, the autocorrelation distance tends to approach infinity. Therefore, the distance at the critical state is expected to be extraordinarily large; the value of about 1 mm has been actually obtained. This value is regarded to have some generality independent of the characteristics of the reaction because it is purely determined by the thermal motion of the solution particles. In fact, it is approximately agreeable with the value obtained for silver nucleation. Then, the fluctuations start unstable growth. As shown in part I, reflecting the fact that the instability arises from the electrostatic interaction of the solution particles in the electric double layer, the growth current substantially depends on the concentration of the supporting electrolyte and the externally applied electrode potential.