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 ffective liquid-phase mass transfer boundary-layer thicknesses during active dissolution and cupric sulfate salt film thicknesses during passive tching 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 following Sherwood-Schmidt-Reynolds number correlation was developed from these relaxation experiments Sh = 1.39 Sc ~ Re ~176 The effective mass transfer boundary-layer thicknesses were used to accurately predict the critical current density for pre~passive CuSO4 salt film formation. Experimentally determined salt films ranged in thickness from 9.2 • 10-4 to 3.2 • 10- 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 tothe disk rotation speed raised to the 1/3 power