Formation of Salt Films during Passivation of Iron

Pure iron in the form of a shielded electrode facing upward was anodically polarized in hydrochloric, perchloric, and sulfuric acid solutions. Ferrous salt films formed in all three electrolytes at potentials and current densities above threshold values determined by mass transport. In perchloric and sulfuric acids, oxide passivation occurred underneath the salt film at potentials above the passivation potential. Analysis of kinetic and transport conditions under and within these salt films indicated that salt films are necessary precursors to oxide passivation in perchloric and sulfuric acid solutions. Oscillatory phenomena during passivation of iron can be explained by formation and dissolution of salt films coupled to p H changes under the salt film due to electrolytic migration.