Theoretical calculations and electrochemical investigation of additives in aqueous methanesulfonic acid for lead electrodeposition

The methanesulfonate system is gradually replacing the traditional highly polluting fluoride electroplating lead (Pb) system with its eco-friendly advantage, and searching for suitable additives is crucially important to obtain uniform Pb coatings in the methanesulfonate system. In this work, a potential additive of thiourea suitable for methanesulfonate-based Pb electroplating was screened based on the theoretical calculations and cyclic voltammetry tests, and the effect of thiourea in the Pb electrodeposition was investigated. Specifically, the adsorption-coordination mechanism was proposed to explain the effect of thiourea by calculating the Bader charge density difference of thiourea on Pb surface and its interaction with Pb2+. Meanwhile, the effects of thiourea on the electrochemical behavior of lead electrodeposition were investigated by cyclic voltammetry and chronoamperometry. The nucleation and growth of Pb follow 3D diffusion-controlled progressive nucleation at the low potential and high thiourea concentration but follow 3D diffusion-controlled instantaneous nucleation at the high overpotential. The appropriate concentration of thiourea could improve the surface morphology of lead coating and make the layer flat and dense. The influence of thiourea on the crystalline orientation and grain size of the coating was analyzed, which indicated that the increase of thiourea concentration favored the formation of the Pb (220) crystal plane and the decrease of grain size.