Zinc electrodeposition simulation in a rotating system

BENGOA, L. N.; PARY, P.; GOÑI, S.M.; SALVADORI, V.O.; SERÉ, P.R.; EGLI, W. A.

Congreso; 11th World Congress of Chemical Engineering; 2023

Zinc electrodeposition is a widespread practice used to prevent metal corrosion. The process conditions in the production of electrogalvanized steel are set to optimize the quality of the coating on the flat portion of the strip, leaving the edges susceptible to current density distribution problems. Generation of dendrite crystals is an undesirable consequence since it causes dents during stamping in the automotive industry. In previous works [1], an experimental device, which reproduces fluid dynamic and electrochemical conditions of an industrial electroplating line, was designed. It consisted of a SAE 1010 steel disk cathode (0.7 mm thickness, 8 and 40 mm of internal and external diameter respectively), mounted on a rotating steel rod isolated with Teflon; a ring of pure Zn was used as anode (72 mm internal diameter). The electrolyte solution contained ZnSO4 and H2SO4. The effect of electrolyte temperature, electrodeposition time, and rotation speed on the thickness of deposited Zn and dendrites growth was experimentally tested. The aim of this work was to develop and validate a mathematical model of zinc electrodeposition.The mathematical model considers the main transport phenomena involved in the process: ternary current distribution, with dissolution of Zn in the anode, Zn ions transport due to convection, diffusion and electric field migration in the solution, and electrodeposition of Zn in the cathode. Deformation in both electrodes and turbulent fluid flow associated to the rotating disc are also included in the model. The described model was implemented in the finite element software COMSOL Multiphysics considering axisymmetric bidimensional geometry. The Multiphysics problem coupled 3 physic interfaces: turbulent fluid flow, Nerst-Plank ternary current distribution and deformed geometry. In preliminary studies, the fluid flow was solved considering different turbulence models, in steady and transient modes, and mesh analysis were performed. The final strategy involved 3 stages: (i) simulation of the steady fluid flow; (ii) simulation of the primary current distribution as initialization of electrochemical study; (iii) simulation of the transient ternary current distribution including deformed geometry. SST turbulence model was employed since its better prediction of the velocity profile over the rotating disk. According to the experimental set-up, a Zn deposit with an average thickness of 14 m should be obtained. The developed model predicts an average thickness between 14.527 and 14.561 m. From the simulated results, the minimum (7.431-7.805 m) and maximum (57.815-67.476 m) local deposit thicknesses are obtained. As expected, the maximum thickness is located at the edges of the disk, where the electric field is more intense. The shapes of predicted deposit are in good agreement with the experimental ones.The model provides a valuable tool to simulate different working conditions and improve the electroplating system.