Electrochemical studies of the pitting of austenitic stainless steel

Abstract

Electrochemical impedance and noise measurements have been obtained both below and above the pitting potential on 316 stainless steel immersed in 3% NaCl solutions. The data are interpreted using an extended model of film rupture and repassivation which incorporates both stochastic processes and modified kinetics of film formation and metal dissolution. Pit initiation is considered as an electrocrystallization process with the chloride ions playing a major role by complex formation with the adsorbed intermediates produced on the metal surface. Pit growth and arrest is essentially seen as kinetic competition between the formation of non-passivating species (MOMOHCl) ads, (MOMCl) ads and passivating species [MOOH] ads, [MOMOH] ads. The pitting model proposed incorporated aspects from well-established theories. Passive film rupture is considered as a normal occurrence which increases in intensity with increase of potential and/or aggressive ion concentration. The electrostrictive model, involving chloride ion adsorption on the outer film, is therefore appropriate but the crack-heal process is stochastic. Progression from initial film rupture to pit propagation is also controlled by the potential, the chloride ion concentration and diffusion both within the initial crack and the growing pit. With increase of potential the influence of adsorbed intermediates on the metal dissolution kinetics becomes increasingly important.