The electrical double layer in a nanopore in a barrier surface film

The equilibrium solution composition of a cylindrical nanopore in a surface oxide film was simulated, using a modified Poisson–Boltzmann equation accounting for water polarization effects on the ionic hydration energy. The composition of such nanopores is of interest from a corrosion perspective, since they play prominent roles in several mechanisms for the breakdown of passive films leading to pitting corrosion. The example system considered was a nanopore in an oxide film on aluminum. The pore was exposed to a NaCl solution of variable pH, with or without borate ions which are representative of ions, which adsorb strongly on the oxide. For pores with radii as small as the Debye length, the potential distribution and composition of the pore are controlled by ionic adsorption on the pore walls, and are weakly influenced by the potential drop through the surface film. The presence of strongly adsorbing borate anions excludes chloride ions from the pore solution. Since chloride ions initiate metallic corrosion, this effect points to a corrosion inhibition mechanism for such adsorbing anions.