Deep-eutectic solution adding threonine additive associating with supercritical carbon dioxide to enhance zinc electroplating characteristics

Abstract

Deep-eutectic solvent (DES) was used widely as an eco-friendly and sustainable process in the zinc (Zn) electroplating in this study. Both the conventional and supercritical carbon dioxide (SC-CO2) assisted electroplating processes were employed in the Zn plating on the copper substrate, respectively. Moreover, the effects of ascorbic acid (AA), nicotinic acid (NA) and threonine (Thr) additives were also investigated systematically. The Zn coating In the XRD result, the Zn texture of (0 0 2) and (1 0 1) as the (1 0 0) gradually vanished as the increase of Thr additive. Whether the use of the additives, the hardness of the prepared Zn coating from SC-CO2 assisted process was higher than conventional counterparts by 30 %. Moreover, the corrosion current density from the potentiodynamic polarization curve tended to be more passive by 55 %, manifesting that the AA and NA additives in the SC-CO2 assisted Zn electroplating was better in anticorrosion capability comparing its counterpart without additives. The current efficiency shew SC-CO2 Zn coating with Thr additive could gain a 15 % enhancement in compared to the conventional Thr-free Zn coating, making the coating more noble. Microstructures investigations using SEM showed the absence of cracking on surface. On the cross-sectional examination using TEM, a dual dense and porous layer structure were observed. EDS analysis revealed the Cu ions dissolving in the DES solution and reacted with Zn ions to for Cu-Zn alloy compounds as well as solid solution of Cu in the Zn coating while the TEM-EDS analysis showed the existence of an extraordinary Zn(Cu) and interfacial Cu-Zn alloy dual-layers. The possible ion-diffusion induced by DES mechanism causing this dual-layer and solid solution structure during the electroplating process in the DES electroplating solution was proposed.