NONEQUILIBRIUM FLUCTUATION THEORY ON PITTING DISSOLUTION .1. DERIVATION OF DISSOLUTION CURRENT EQUATIONS

Following the foregoing papers, in this paper, mathematical procedures to describe the time development of tile unstable nonequilibrium fluc tuations in pitting dissolution are presented: First, general equation s depicting the time development of the amplitudes of the fluctuations are deduced, which are the same as those describing nucleation phenom ena. Then, with the same definition as nucleation, the average values of the fluctuations are decided, and measurable pitting current is cal culated. Actually, some analytical equations are derived for some char acteristic parts of the current-time transient. As a result, after app lying a constant potential step to the electrode, a slow relaxation cu rrent, which follows a double-layer charging current, is decided as th e diffusion current of the fluctuations. This current gives the values of passive film coverage and average critical concentration fluctuati on on the completely active surface. Moreover, from the minimum curren t attained by the diffusion current, the ratio of the average critical concentration fluctuation to the autocorrelation distance is determin ed. Finally, resultant pitting growth current arising from the unstabl e growth of the fluctuations is deduced as a function of supporting-el ectrolyte concentration and overpotential, which is attributed to the fact that the instability occurs from the electrostatic interaction of the solution particles in the double layer. (C) 1997 American Institu te of Physics.