Crevice corrosion positive metal ions

Crevice corrosion initiation is based on a pH decrease at crevice bottom due to Eq. (7.5). Positive metal ions formed in the absence of oxygen reduction attract chloride into the crevice, initiating acidification (water hydrolysis) ... 

The mechanism for pitting corrosion is similar to that invoked for crevice corrosion. Rapid metal dissolution in the pit area (with O2 reduction taking place in the adjacent areas) produces an excess positive charge, which induces Cl ion migration, followed by H+ ion formation due to hydrolysis, which results in further dissolution of the metal. Since the O2 concentration in the pit is virtually zero, the pit protects the rest of the metal cathodically by sustaining O2 reduction in the adjacent areas. 

Crevice corrosion occurs as a result of a non-uniform concentration of eletrolyte solution. The local electrode potential varies with the concentration of electrolyte corrosion currents (electron and ion migration) that occur in the metal and solution. For instance a piece of copper immersed in copper sulphate solution of varying composition, tends to be more positive in the region of high copper sulphate concentration due to the enhanced rate of deposition of Cu ions. A flow of electrons through the metal towards this region occurs and the ion deposition is a continuous process. As a result corrosion occurs in the metal where the solution concentration is at its lowest, and Cu ions pass into solution. 

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