Electrical Double Layer Capacities of Iron During Forced Cathodic Decay of Passivity

A study of the behavior of the differential capacity of the electrical double layer of high‐purity, zone‐refined iron in , p H 2.3–6.5 has been made during forced cathodic decay of passivity using a fast single pulse technique. The capacity‐time curve shows a maximum >22 µf/cm 2 (but <35 µf/cm 2 ) at the end of the Flade plateau. This is explained on the basis of a desorption step of an as yet undefined charged particle. The capacity at O 2 evolution potentials is 6–8 µf/cm 2 , compatible with an oxide film; at H 2 evolution potentials it is 19 µf/cm 2 , independent of p H in both cases. The charge involved in the Flade plateau is 550–850 µcoul/cm 2 with an average 680 µcoul/cm 2 .