An electrochemical study of mass transfer in high-rate anodic dissolution of copper

High-rate electrochemical dissolution of copper was studied. Electrochemical methods were used to distinguish between mechanisms that have been proposed to account for mass-transfer limitations in the rate of reaction. Dissolution of copper into phosphoric acid, where the resulting surface finish was smooth and bright, and into cupric-sulfate solutions, where the surface is smooth but dull, was studied. The electrochemical behavior of the systems was characterized with steady-state-polarization and AC-impedance-spectroscopy measurements and supplemented with flow-modulation spectroscopy, also known as EHD-impedance spectroscopy. In all cases, a rotating disk electrode was used to maintain well-defined hydrodynamic conditions.; In flow-modulation spectroscopy, the disk rotation speed is modulated sinusoidally over a wide range of frequencies. This technique has not been used extensively because it is not available commercially and a quantitative analysis of the data requires treatment of the unsteady Navier-Stokes equation. An experimental setup capable of performing flow modulation was built and tested as part of this study; software for quantitative data analysis was also developed. The contribution of ionic migration to the EHD-impedance spectrum was investigated and found to be nearly negligible in high-rate metal dissolution.; AC-impedance measurements show that, contrary to previous claims, there was no solid precipitate present during the electropolishing of copper in concentrated phosphoric acid; instead, the rate-limiting step was diffusion of a complexing species to the surface of the electrode. EHD-impedance measurements were used to determine the diffusion coefficient of the rate-limiting species without any assumptions about its identity. This value of diffusion coefficient was combined with the steady-state results to show conclusively that the complexing species are water molecules. The experimental and theoretical study presented here is the most convincing evidence for the applicability of a water-acceptor mechanism.; The electrochemical etching of copper in concentrated cupric sulfate was also studied. Steady-state measurements and AC-impedance spectroscopy suggested the presence of a salt-film precipitate on the surface of the electrode. In this process, the mass-transfer limitation was diffusion of ion products away from the film surface. The electrochemical measurements permit the characterization of the transport properties of the film. Flow-modulation spectroscopy measurements were consistent with the present model and provided further physical constraints for a more detailed formulation.