Pitting Corrosion of Iron in Weakly Alkaline Chloride Solutions

Chloride (Cl^-)-induced pitting corrosion of iron at pH 10.5 and 25°C was examined by conducting quasi-steady-state (potentiostatic) polarization experiments in borate-buffered 0.1 M sodium chloride (NaCl) solutions with buffer concentrations from 0 M to 0.075 M. Values of the film breakdown potential (E_b) were scattered at each buffer concentration, and the scatter band moved to higher potentials with increasing concentrations, indicating increased resistance to pitting. Consistent with this, pitting did not always occur at the higher buffer concentrations. E_b measurements, optical and electron microscopy, x-ray microanalysis, and supplementary polarization experiments in lower-pH borate solutions suggested pitting in the iron-Cl^– system initiated within occluded regions, such as matrix-inclusion interfaces and exposed voids, where pH control was lost because of an inadequate local supply of buffer species. Pitting behavior was consistent with a mechanism dominated by mass transport, in which the presence of Cl^– prevented buffering of occluded regions by the borate specie H₂BO₃^–, allowing the pH to be driven into an acidic domain where the solubilities of ferrous hydroxide (Fe[OH]₂) and ferric hydroxide (Fe[OH]₃) are high.