Kinetics of Tin–Zinc Alloy Electrodeposition from Alkaline Electrolytes: Diffusion–Kinetic Control and Optimization of Process Parameters

Abstract

We present our kinetic study of tin–zinc (Sn–Zn) alloy electrodeposition from an alkaline electrolyte using potentiodynamic polarization curves and a rotating disk electrode (RDE). The joint tin and zinc discharge is controlled by a mixed diffusion–kinetic mechanism, confirmed by the linear dependence of the inverse current density on the inverse square root of angular RDE rotation velocity. Diffusion coefficients of tin ions D(Sn) ≈ 1.77 × 10^–8 cm²/s and zinc ions D(Zn) ≈ 1.97 × 10^–7 сm²/s were determined, significantly lower than those for simple ions in aqueous solutions due to complexation to form stannates and zincates in alkaline solutions and the influence of additives (sodium citrate and sodium lauryl sulfate). Polarization curves featured zinc depolarization (deposition at –1.10 V instead of –1.20 V), while tin reaches its limiting current at –1.40 V due to diffusion limitations. The alloy composition depends on the deposition potential and RDE rotation velocity: the velocity increasing from 100 to 1600 rpm raises the zinc content from 55 to 65% at –1.45 V. The hydrogen evolution side reaction is significant below –1.8 V, reducing the current efficiency. Thermal studies showed the activation energy to decrease from 18.73 kJ/mol at –1.1 V to 14.42 kJ/mol at –1.4 V, indicating enhanced diffusion control with a negative shift of the deposition potential. Potentiodynamic measurements at sweep rates of 1–100 mV/s confirmed nonstationary factors’ influence on curve shifts. The obtained results are crucial for developing environmentally friendly technologies for protective coatings with controlled compositions and enhanced properties. Future directions include exploring new additives and optimizing the electrolyte composition for providing improved efficiency.