Reaction Mechanism Analysis of Zn Electrodeposition Process in Acidic Sulphate Bath Using Potentiodynamic Polarization Technique
Authors: Paul Twinkle , Srinivasan Ramanathan 
Published: February 2020

Zinc (Zn) metal is commonly known as a sacrificial anode, and is widely used as corrosion protection coating for ferrous materials. Electrodeposition of Zn is a popular method of coating substrates and it can be performed in acidic or alkaline conditions [1]. Zn electrodeposition is a two electron multi step reaction, and it is accompanied by hydrogen evolution reaction in acidic conditions. Hydrogen bubble formation during Zn deposition can affect mass transport of different species in the electrolyte bath [2]. In addition, mass transfer of cations can limit the reaction rate at typical deposition conditions. Zn electrodeposition process has been modeled in the literature [3-6].The models employ either a single step simple electron transfer reaction with mass transfer effects or multi-step reactions which neglect mass transfer limitations.
In this work, we present experimental results of Zn electrodeposition in aqueous acidic sulphate solution, and propose a model to explain the observed results. The model (Figure 1a) involves a multi step mechanism with diffusional mass transfer effects, Hydrogen evolution reaction (HER) occurring in parallel with the Zn electrodeposition process.
All the electrochemical experiments were carried out using a Zn rotating disk electrode (rde) in a conventional three electrode cell and conducted with IVIUMSTAT electrochemical workstation. Pt mesh was used as the counter electrode and Hg/Hg2SO4 (saturated K2SO4) was used as the reference electrode. The kinetics and mass transfer effects of the process were studied by potentiodynamic polarization (PDP) technique. PDP experiments were performed as a function of Zn salt concentration, electrolyte bath pH and electrode rotational speed. The concentration of Zn salt was varied from 0.01 M to 1 M. The electrolyte bath pH was varied from 2 to 4. The electrode rotational speed was varied from 400 to 1600 rpm to characterize mass transfer effects during the process. A supporting electrolyte (Na2SO4) at 1 M concentration was used to reduce the effects of solution resistance. A few experiments were also conducted without Zn salt (blank solution at pH 2) to quantify the Hydrogen evolution on Zn rde.
Figure 1b shows the PDP curves obtained for blank solution at pH 2 and different rotational speeds to quantify the hydrogen evolution rate alone. Figure 1c shows the PDP curves of Zn electrodeposition process at pH 2 and different rotational speeds. As expected, the current magnitude increases with an increase in cathodic bias, and saturates at large cathodic potentials. The saturation current increases with an increase in electrode rotational speed.
Reaction mechanism analysis was employed to describe the deposition mechanism of Zn in acidic sulphate baths. Several multi step mechanisms were evaluated to model the polarization data acquired in kinetic limited regime. A seven step mechanism [4] as mentioned in figure 1a involving three deposition steps and Volmer Heyrovsky (VH) steps for hydrogen evolution reaction was found to be the best mechanism in the kinetic limited regime. Further analysis on mechanistic investigation to include mass transfer effects during the Zn electrodeposition is in progress.

References

[1] A.G. Association, A Comparative analysis of process and performance characteristics - Zinc coatings, www.galvanizeit.org, 2011, pp. 1-13.
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[3] Epelboin I, Ksouri M, W. R, On a model for the electrocrystallization of zinc involving an autocatalytic step, Journal of Electrochemical Society, 122 (1975) 1206-1211.
[4] Ganne F, Cachet C, MaurinG , Wiart R, Chauveau E, P. J, Impedance spectroscopy and modelling of zinc deposition in chloride electrolyte containing a commercial additive, Journal of Applied Electrochemistry 30 (2000) 665-673.
[5] H. Van Parys, G. Telias, V. Nedashkivskyi, B. Mollay, I. Vandendael, S. Van Damme, J. Deconinck, A. Hubin, On the modeling of electrochemical systems with simultaneous gas evolution. Case study: The zinc deposition mechanism, Electrochimica Acta, 55 (2010) 5709-5718.
[6] I. Zouari, F. Lapicque, An electrochemical study of zinc deposition in a sulfate medium, Electrochimica Acta 37 (1992) 439-446.

Figure 1