Surface Studies on Passive Iron

Surfaces of zone‐refined iron in sulfate solutions of p H between 1 and 15 have been studied by measurement of electrode capacity and by chronopotentiometry. Capacities were measured as a function of potentiostatically controlled average electrode potential by a recently developed pulse technique. The capacity curves thus obtained were approximately plateaus over potential regions in which oxygen or hydrogen evolution occurred at >10 µa cm −2 . In intermediate potential regions the curves had shallow minima as low as 4 µfarad cm −2 near the potential region of oxygen evolution and peaks with maxima as great as 165 µfarad cm −2 near the potential region of hydrogen evolution. The capacity values depended on the history of the potential variations applied to the electrodes. Chronopotentiometry indicated the presence of at least two sorts of reducible surface material under most of the experimental conditions. Results are interpreted to indicate the presence of a surface material with properties which depend on the electrode potential and solution environment during its formation and which are significantly different from those of any known crystalline, stoichiometric iron oxide. Such properties of the material of a passive iron surface may be reasonably explained by a previously presented hypothesis of a surface material formation sequence initiated by the adsorption of solution species.