Effect of Aggressive Anions on Corrosion

Many mechanisms have been proposed for the suppression or acceleration of metallic dissolution by the action of aggressive anions in general. The simple most common theory on the accelerated corrosion due to aggressive anions is the 

In the case of aluminum adsorbed aggressive anions such as chloride can undergo a chemical reaction with the passive film and produce soluble transient compounds such as AKOHI Cl, AlOHClj, and AlOCl, which are easily dissolved into the solution once they are formed. Similarly, soluble FeSO complex forms in presence of another aggressive anion, that is Thus as a result of these adsorption-dissolution processes, the protective oxide film is thinned locally, small pits are made and the corrosion rate of aluminum is greatly enhanced 

When aggressive anions have to be compared with one another, the stability of the intermediate complexes of substrate metal and aggressive anions must be considered. In the specific case of steel corrosion, if an anion, X , is first adsorbed on the steel surface, a surface complex forms in the anodic process, and then the complex is desorbed from the surface.  

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. 

---