AN ELECTROCHEMICAL AND X-RAY PHOTOELECTRON-SPECTROSCOPY STUDY OF THE PASSIVE STATE OF CHROMIUM

The passive state of chromium has been examined in 1M H2SO4 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 angstrom 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 approximately 25 and an ionic resistivity of approximately 10(13) OMEGA-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.