NONEQUILIBRIUM FLUCTUATION THEORY ON PITTING DISSOLUTION .2. DETERMINATION OF SURFACE COVERAGE OF NICKEL PASSIVE FILM

Following part I, in this paper, the experiments for nickel pitting di ssolution 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 cu rrent, a slow relaxation current, i.e., the diffusion current of the f luctuations was expected. The current must have been a function of pas sive film coverage. Therefore, if the passive him breaks so slowly, su ch diffusion current cannot be observed. However, it was actually foun d that the film is rapidly broken at the final stage of the double-lay er charging, and the coverage is determined by the slope of the Cottre l plot of the diffusion current. Then, it was also shown that in the a bsence of Cl- ions, the film have already been broken at least about 1 5%, which may be attributed to the passivity breakdown effect of H+ io ns. Finally, by the use of extrapolation of the data to the infinity o f Cl- ionic concentration, the average critical concentration fluctuat ion without any passive film was obtained. (C) 1997 American Institute of Physics.