An Electrochemical and X‐Ray Photoelectron Spectroscopy Study of the Passive State of Chromium

The passive state of chromium has been examined in by a variety of electrochemical techniques and x‐ray photoelectron spectroscopy. The potentiodynamic behavior of the passive electrode may be described by high‐field film growth kinetics. The passivating oxide film is 5–25 Å thick and consists of chromic ions bridged together by a network of water, hydroxide, and oxide ligands. The film thickness is a linear function of potential while the electrode capacitance is inversely proportional to the applied potential. Thus, the passive film behaves as a simple dielectric with a dielectric constant of ∼25 and an ionic resistivity of ∼10 13 Ω‐cm. The distribution of potential across the electrode is consistent with the kinetics of film growth and dissolution. The onset of transpassivity is revealed by a distinct transition in the capacitance‐potential characteristic of the electrode. Cyclic voltammetry demonstrates that this transition is associated with the presence of higher valent chromium species in the film.