Stochastic and deterministic behaviours of 316 stainless steel pitting corrosion in phosphate-borate buffer containing sodium chloride

Abstract

The pitting corrosion of 316 stainless steel (SS) in phosphate-borate buffer (pH 8.00) containing 0.5 M NaCl at 25°C is studied through current transients run under a constant potential step, potentiodynamic runs and scanning electron microscopy. According to the potential and time windows different laws for pitting kinetics are observed. For small number of pits, the kinetics of pit growth is governed by a stochastic law whereas for a large number of pits a deterministic law which corresponds to an instantaneous nucleation and growth is obeyed. At the initial stages two different rate constants for pit nucleation are obtained. This fact correlates with two well distinguishable types of pits observed through SEM observations. Results are discussed through a model involving the penetration of chloride ions through the passive film and the existence of at least two types of sites at the metal surface which in turn become centers of pit nucleation. Pit stability is directly related to the Cr(III) content in the passive layer which can be shifted by changing the applied potential with respect to that of the Cr₂O₃/CrO²⁻₄ redox couple.