Nonequilibrium fluctuation theory on pitting dissolution. II. Determination of surface coverage of nickel passive film

Following part I, in this paper, the experiments for nickel pitting dissolution in NaCl solution were carried out to compare the theoretical equations with experimental data. From the theoretical examination on nonequilibrium fluctuations in part I, after double-layer charging current, a slow relaxation current, i.e., the diffusion current of the fluctuations was expected. The current must have been a function of passive film coverage. Therefore, if the passive film breaks so slowly, such diffusion current cannot be observed. However, it was actually found that the film is rapidly broken at the final stage of the double-layer charging, and the coverage is determined by the slope of the Cottrel plot of the diffusion current. Then, it was also shown that in the absence of Cl− ions, the film have already been broken at least about 15%, which may be attributed to the passivity breakdown effect of H+ ions. Finally, by the use of extrapolation of the data to the infinity of Cl− ionic concentration, the average critical concentration fluctuation without any passive film was obtained.