Anodic oxide growth on tungsten studied by EQCM, EIS and AES

Anodic oxide growth on tungsten in 0.1 M H2SO4+0.4 M Na2SO4 has been studied on stationary and rotating electrodes using the electrochemical quartz crystal microbalance (EQCM), Auger electron spectroscopy (AES) depth profiling, electrochemical impedance spectroscopy (EIS) and the rotating ring disc electrode (RRDE). The sputter rate of tungsten oxide was calibrated with respect to a tantalum oxide reference film using Rutherford back-scattering spectroscopy (RBS). Long time polarisation experiments at constant potential showed that at a stationary electrode the oxide thickness determined by AES increased with time, whereas a steady state was reached after a few seconds under rotating conditions. On the other hand, EIS indicated a time independent oxide thickness on both stationary and rotating electrodes. Both methods yielded a higher absolute thickness for stationary electrodes. The oxide growth behaviour at short times was studied with the EQCM by monitoring the current and the mass change resulting from a potential step. From these data the film growth fraction and the oxide thickness change were calculated. After an initial period of a few seconds dominated by film growth, the rotating electrodes exhibited a significant mass loss contrary to stationary electrodes. RRDE experiments confirmed dissolution during anodic polarisation of a rotating electrode. The results of this study demonstrate the necessity to combine several experimental methods when studying anodic film growth and they illustrate the usefulness of the EQCM which can monitor directly and with sub-second time resolution the mass change of the electrode together with the current density. They show the decisive effect of convection conditions on anodic oxide growth on tungsten. (C) 2004 Elsevier Ltd. All rights reserved.