Localised dissolution kinetics, salt films and pitting potentials

Dissolution kinetics within artificial pit electrodes have been studied and related to the transition in real pits from metastability to stability. For 302 and 316 stainless steel, two regimes of growth were identified; mixed activation/ohmic control was found at lower potentials, but at higher potentials growth was under diffusion control with a metal salt film present on the electrode surface. The transition potential, E-T, between these two regimes was found to increase linearly with the log of the limiting current density, i(lim). A model for the anodic dissolution kinetics is proposed which accounts for this observation and predicts that real pits will also grow in either active or salt filmed slates depending on their current density. Fitting potentials, E-pit, and transition potentials, E-T, were measured as a function of chloride concentration. The differences in E-pit, between 302 and 316, or between different chloride concentrations for one alloy, were equal to the corresponding differences in E-T measured at current densities in the range 1-5 A cm(-2), typical of real pits. This is consistent with the idea that E-pit is the potential above which pits are able to progress from metastable to stable growth, and therefore depends on the kinetics of pit dissolution rather than the breakdown resistance of the passive film. Metastable pitting occurs at potentials well below E-pit. The variation of E-pit with chloride concentration and molybdenum alloying is completely explained by the effects of these variables on kinetics within the pit environment as manifested in the measurement of E-T. On an absolute scale, E-T (at 1-5 A cm(-2)) falls within the range of E-pit values measured for surface finishes from 120 to 1200 grit. (C) 1997 Elsevier Science Ltd.