Geometric Evolution of Flux from a Corroding One-Dimensional Pit and Its Implications on the Evaluation of Kinetic Parameters for Pit Stability

The flux from a one-dimensional (1-D) artificial pit electrode corroding under a salt film was examined using experimental and modeling techniques. Finite element simulations showed that the flux at shallow depths was consistently lower than analytically determined calculations for 1-D Fickian diffusion. This deviation was due to a substantial contribution of the external hemispherical boundary layer to the overall diffusion length. Increasing the pit diameter resulted in a larger boundary layer, which in turn affected the flux characteristics to greater depths. Data from experiments and simulation converged with the theoretical 1-D calculations only when pit depths approached nearly ten times the pit diameter. The experimental data from this artificial pit study as well as data from related published work were observed to span the range bounded by the numerically simulated and the analytically determined flux predictions. Comparison with published pit stability phenomenology showed that only deep pits provided kinetic data based on the cation concentration gradient unadulterated by bulk chloride effects. Finally, this work also provided insight into the origin of the dependence of the measured repassivation potential with pit depth, contributing towards a quantitative framework relating the various critical factors governing pitting.