Conclusions
The extreme current densities form a WC electrode interface which cannot be described by common models of diffusion based on diluted systems. Close to the interface, an adherent, supersaturated, viscous film of polytungstates is formed which is continuously dissolved and reproduced. The cell potential is mainly determined by the electrolyte resistance between sample and counter electrode (Relectrolyte = 0.4 Ω cm2 in our setup). The dissolution proceeds in active state up to 30 A cm−2 . At current densities > 30 A cm−2 , an additional layer with a pseudo-ohmic resistance of 0.2 Ω cm2 is formed. This is not a current independent layer with a given Bspecific resistance^ but reflects a passive state and high-field oxide films with thicknesses around 10 nm. The complex interaction between current, field strength, and oxide thickness yields a constant resistance of the oxide film.
The formation of an oxide film and, therefore, the active/ passive transition is also indicated by the onset of oxygen evolution which cannot take place on bare metal surfaces but requires oxide films [8]. The current density of active/ passive transition is not a natural constant but varies with experimental parameters such as electrolyte flow rate and cell geometry. Oxygen evolution consumes about 20% of the anodic charge. However, the interaction of ionic and electronic currents is small due to completely different conduction mechanisms.
