The mechanism of pitting of high purity iron in NaCl solutions

Pitting of high purity iron was studied in chloride containing solutions, at various pH values, with and without borate buffer. It was found that for low pitting potential values, in alkaline solutions, where Fe(OH)_{2-oxide phase} is the initial passivation species, localized acidification, or OH^? ion depletion, was a necessary and sufficient condition for passivity breakdown. At high pitting potential values, where the presence of an adsorbed layer of Fe(OH)_{3-oxide phase} leads to passivity, OH^? ion depletion was a necessary but not a sufficient condition for passivity breakdown. Specific anion adsorption steps had to be assumed to explain the specificity of the aggressive anions, at those potentials, and the absence of Fe III species in the corrosion products inside the pits.