Pit Formation in the Cathodic Polarization of Passive Iron III. Stimulation of Pit Initiation and Uniform Dissolution by Chelating Agents

Pit formation on iron was stimulated by chelating agents at lower concentrations: the effect was largest in EDTA followed by NTA, IDA, and G in this order. This is caused by the cathodic reduction rate of the passive film being stimulated in the presence of the chelating agent, and therefore the potential determined by a coupling of the cathodic reduction of the passive film and the anodic dissolution of bare iron shifts to a more positive value, which leads to the stimulation of pit formation. In the presence of higher concentrations of chelating agents, however, iron did not cause pitting but caused iron to dissolve uniformly, which was due to the adsorption of the chelating agent onto the local anodic area or to the inhibition effectiveness of the ferrous chelating agent complex.