Pitting corrosion stabilizing factors

The discussion of various experimental results has shown that different effects may serve as stabilizing factors for localized corrosion. It depends on the stage of development of a corrosion pit and the environmental conditions which one is the most effective. Therefore one has to refer to the special situation under discussion. Often the controversial points of view of authors depend on the different situations that are examined and discussed. It was in the aim of this contribution to sort out the different factors and to describe the conditions in which they are effective. In this sense, localized corrosion is a very complicated process involving different effects. 

Due to the stability of intermediate complexes between the metal substrate and the aggressive anions, pitting corrosion does not occur for chromium metal. Stability constants of CrX complexes are smaller than 1, for instance it is 1 when X is C1 and lO" when it is 1. In addition, exchange of CT and HjO ligands between the inner and outer sphere of chromium halide complexes is extremely slow. Together these factors causes insolubility of CrClj in cold water due to very low dissolution rate of Cr. Therefore the presence of a Cr-Cl complex at the surface will not increase the dissolution rate because it will dissolve very slowly by itself. In the case of this exchange is very rapid. Similarly Fe-Cr alloys are more resistant to pitting in Cl" solution than is pure Fe. 

The stabilization of the growth of corrosion pits may be influenced by several complicating factors depending on the state of their development. However, the presence of halides within the electrolyte is a necessary condition. Pit growth will stop when halide-containing electrolytes are replaced by halide-free solutions [70]. One needs a bulk concentration of at least 0.0003 M to cause stable pit growth [6,... 

Beyond this formal presentation of the pitting potential, it should be noted that all factors increasing the corrosion products production (K) or decreasing their dilution in the electrolyte (D) should favor pit stabilization ... 

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