Analytical and numerical modeling have been used to extend the understanding of pitting corrosion in a one-dimensional geometry. The approach is based on that of Galvele [1] and later experimental studies that supported many of Galvele’s findings [2], but with more detail and new features. We have examined both ideal solutions, and non-ideal solutions where the diffusivity (D) is a function of concentration (c) for any species, as well as cation complexation by chloride.

The essential result that Galvele obtained was:

E_pit = A – B log [NaCl],

where E_pit is the pitting potential and B should be 0.059 V at 298 K.

Laycock and Newman [2] clarified that the quantity A contained the kinetics of the anodic dissolution reaction, and defined a quantity that they called the transition potential, E_T, which is the potential where the anodic current density i becomes equal to the anodic limiting current density, i_L.

The origin of Galvele’s equation is an analytical solution of the governing reaction-transport equation (Nernst-Planck equation) considering an unreacting ion such as Cl^-. In this solution, as refined by Newman [3], the second term in Galvele’s equation is the IR drop in the pit at the limiting current density i_L. Importantly, i_L depends on [NaCl] because the contribution of migration to the outward flux of dissolved metal cations increases as [NaCl] decreases.

In the present work, we have examined the following refinements, with the following preliminary outcomes:

The variation of B with [NaCl] – B is constant at 0.059 V over a certain range of [NaCl], but decreases at high [NaCl]. This is easily understood physically. At low [NaCl] there appear to be small deviations from 0.059 V that are still under investigation.

The effect of introducing a D(c) – this turned out to be an intriguing area of study, and new results will be presented at the meeting. Briefly, the effect of varying diffusivity is not as great as might be assumed.

Cation complexation – this is the latest aspect to be investigated. Changes in B with complexation are expected. Such changes may explain the different empirical values of B found in different materials including stainless steels.

References:

1. Jose R. Galvele, Transport Processes and the Mechanism of Pitting of Metals. J. Electrochem. Soc., 1976, 123(4), 464-474.
2. N.J. Laycock, R.C. Newman, Localised Dissolution Kinetics, Salt Films and Pitting Potentials. Corrosion Science, 1997, 39(10-11), 1771-1790.
3. R.C. Newman, Pitting Corrosion of Metals, The Electrochemical Society Interface, 2010, 19(1), 33-38.