COPPER ELECTROPOLISHING IN CONCENTRATED PHOSPHORIC-ACID .1. EXPERIMENTAL FINDINGS

The electropolishing of rotating copper-disk electrodes in concentrate d phosphoric acid is studied by steady-state measurements, ac-impedanc e spectroscopy, and now-modulation spectroscopy The effects of applied potential, rotation speed, and temperature on the response of the sys tem are investigated at the mass-transfer-limited plateau. Both the Ar rhenius behavior of the limiting current and the ac-impedance data sug gest that the rate of reaction is controlled by mass transfer of an ac ceptor to the copper surface and not by precipitation of a salt film. It is not possible to determine from steady-state measurements the dif fusion coefficient of the acceptor because its identity and bulk conce ntration are not known with certainty. Therefore, the steady-state mea surements are complemented with flow-modulation spectra which allow an estimation of the diffusion coefficient of the acceptor without knowl edge of its bulk concentration.