Reaction Model for Iron Dissolution Studied by Electrode Impedance: I . Experimental Results and Reaction Model

Steady‐state polarization curves and electrode impedances were measured during the dissolution of iron in solution acidified by the addition of . These experiments were performed within very wide p H (0–5), current density (up to 0.1 A · cm −2 ), and frequency (10 −3 –10 5 Hz) ranges. Three time constants, in addition to the high‐frequency capacitive loop attributed to the double layer capacity and the charge transfer resistance, were observed before the onset of the passivation process. The experimental results were quantitatively interpreted by computer simulation on the basis of a reaction model including three dissolution paths. At low current densities, the dissolution path, which can be related to the consecutive mechanism, controls the overall rate. At higher current densities, a self‐catalytic path, implying a ferrous intermediate, determines the overall current. Another self‐catalytic path, with monovalent iron, plays an important role in the electrode impedance and the prepassivation process although its contribution to the current is not prevalent at any p H.