Anions, migration into occluded

Anion Migration into Occluded Regions. Anions in the external environment, particularly chloride ions, will migrate into the occluded region as a consequence of the potential difference between the solution at the metal/environment interface in the pit and the solution at the external surface or, equivalently, in response to the increase in positive charge resulting from the increased cation concentration in the bottom of the pit. Chloride ions are known to stabilize the hydrolysis reactions and actually further lower the pH (Ref 19). If the increase in metal-ion concentration associated with the anodic current density at the pit inter-... 

Taken together, the processes described above tend to lead to a concentrated, acidic solution of metal salts within the crevice. Metallic materials tend to dissolve rapidly upon polarization in these solutions. Thus, maintenance of the occluded solution composition is critical to stability of crevice corrosion. As mentioned above, diffusion acts to destabilize crevices by dispersing the concentrated solution, whereas migration effects tend to concentrate aggressive anionic species into the crevice. 

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