Analysis of convective mass transfer by potential relaxation III. Active and passive copper dissolution at a rotating disk

A linear potential relaxation technique was used successfully to determine effective liquid-phase mass transfer boundary-layer thicknesses during active dissolution and cupric sulfate salt film thicknesses during passive etching for a copper rotating disk electrode immersed in a CuSO₄-H₂SO₄ electrolyte. Experimentally measured concentration over-potentials immediately after current interruption were found to decay linearly with the square-root of time. A theoretical model was derived to calculate liquid-phase effective mass transfer boundary-layer thicknesses or salt film thicknesses from the slope and intercept of the linear potential decay regime. For active copper dissolution, effective mass transfer boundary layer thicknesses were ≈ 56% smaller than those for cathodic deposition. The effective mass transfer boundary-layer thickness were used to accurately predict the critical current density for prepassive CuSO₄ salt film formation. Experimentally determined salt films ranged in thickness from 9.2 × 10^-4 to 3.2 × 10^-3 cm, depending on the anode potential, the passivation time, and the disk rotation speed. Steady-state film thicknesses were dependent on the 1/3 power of the applied potential and were inversely proportional to the disk rotation speed raised to the 1/3 power.