Effect of ultrasound on mass transfer during electrodeposition for electrodes separated by a narrow gap

This work reports an investigation on mass transfer by ultrasound agitation during electrodeposition on electrodes separated by a narrow inter-electrode gap. Polarisation experiments were performed to identify the mass transfer limiting current. The limiting current density was used to calculate mass transfer boundary layer thicknesses which were used to develop mass transfer correlations. Experiments were carried out using a cell with parallel copper discs which were positioned at gaps of 1, 0.5 and 0.15 cm. The distance between the ultrasonic probe and electrodes was varied between 3 and 1.5 cm. The polarisation data showed clear limiting current plateaux when the distance between the electrodes was larger, however significant distortions were observed when the gap was 0.15 cm. It was found that lower ultrasound powers of 9-18 W/cm2 provided more effective agitation at narrower electrode gaps than powers exceeding 18 W/cm2. Sherwood correlations showed that in this system, developing turbulence occurs for larger inter-electrode spacing, whereas for narrow electrode gaps fully turbulent correlations were obtained. A 2-D current distribution model showed that potential distortions that were observed in the polarisation data were caused by the close placement of the metallic US probe to the two parallel electrodes.