Electrodeposition and Properties of Electroplating Layers Obtained Using Tin-Zinc Alloy Deposited from Alkaline Electrolyte

The results of developing a process for electrodepositing tin-zinc (Sn–Zn) alloy coating layers from an alkaline electrolyte are given. The study aims to replace toxic cadmium and lead coating layers with an environmentally friendly alternative with the same functionality. The optimal electrolyte composition is established: Na2SnO3 (0.14–0.16 mol/L), ZnO (0.05–0.07 mol/L), NaOH (90–110 g/L), sodium citrate (10–15 g/L), and sodium lauryl sulfate (0.8–1.2 g/L). Deposition parameters are as follows: current density 1.0–1.2 A/dm2, temperature 40–45°C, and pH 11.0–11.5. Under these conditions the deposition rate is about 12–15 μm/h. The zinc content in the coating layer is 30% at a current density of 1.0 A/dm2 and temperature of 40°C. This is sufficient to provide for a cathodic protection of the substrate and for making a barrier layer. The maximum current efficiency (78–82%) is achieved in a 10 g/L sodium citrate and 1 g/L sodium lauryl sulfate electrolyte. The coating layers feature corrosion resistance above 1000 hours in a 3.5% NaCl solution and 600–800 hours under salt spray conditions. The measurements give the following specifications for the layer: microhardness 880–1177 MPa, friction coefficient 0.2–0.3, and contact resistance 15–25 μΩ cm. Introducing sodium citrate into the electrolyte results in better coating morphology, whereas adding sodium lauryl sulfate improves surface roughness and suppresses hydrogen release. The study demonstrates that Sn–Zn coating layers surpass regularly used Sn–Pb compositions in corrosion resistance and environmental safety.