SOME ASPECTS OF THE FLUCTUATIONS OF THE PASSIVE CURRENT ON STAINLESS-STEEL IN PRESENCE OF CHLORIDES - THEIR RELATION TO THE PROBALISTIC APPROACH OF PITTING CORROSION

This paper emphasizes the relation between current fluctuations in the passive range preceding the pitting potential of AISI 304 stainless steel in the presence of chloride ions and the electrochemical behavior of the passive film under potentiodynamic non-steady conditions. It is shown that neither the electrode potential nor the anodic charge stored in the film determines completely the prepitting activity. Both the amplitude and frequency of the occurrence of bursts decrease with the degree of maturity of the film. AC capacitance measurements in correlation with DC passive current suggest a discrimination between charge storage and completion of the full passive state. These data are briefly discussed in relation to the description of pit nucleation by a Poisson process. It is concluded that, in contrast with recent literature data, the influence of the sweep rate cannot be accounted for by a pit generation rate depending only on the potential without considering the degree of non-stationarity of the film. Moreover, no potential is found at which a stable statistical balance is established between breakdown and repassivation, as should be normally observed for a birth and death process.