Chloride ions migration into occluded

Figure 22.32a. The accumulation of chloride ions in the occluded solution reaches a stationary level, at which the rate of inward ion migration equals the rate of outward diffusion of the ions. Furthermore, the anodic chloride ion migration is accompanied by an electroosmotic flow of water molecules into the occluded solution. The final chloride concentration thus established in the steady state is determined by the ratio of the chloride ion migration rate to the electroosmotic water flow rate. It is therefore the transference numbers, Ter an(J tH2o, of chloride ions and water molecules that determine the chloride ion concentration in the occluded solution.

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-... 

The fact that the pH values of the pure metals were lower than the theoretical values was attributed to the formation of hydroxy-chloro complexes of the metal and to the high chloride ion concentration in the pit, and the results highlight the very pronounced decrease in pH that can occur in an occluded cell, particularly when the alloy contains high concentrations of chromium and molybdenum. They also showed that migration of chloride ions into the solution in the pit can result in a 7-12-fold increase in concentration, and that the potential in the pit is in the active region. 

With a cation-selective rust layer, the anodic ion transport across the mst layer carries mainly hydrated protons migrating outward from an occluded solution into the solution bulk as shown in Figure 22.32b. No accumulation of chloride ions and hydrogen ions is thus expected to occur in the occluded solution. Instead, the outward migration of hydrogen ions leads to the basification of the occluded solution, where the precipitation of corrosion products will then be accelerated. 

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