Electrodeposition of Zn Alloys with Cu and Sn from Citrate Electrolytes
Authors: Zahmi Salem , Podlaha Elizabeth J 
Published: February 2020

Zinc alloys with copper and tin are of interest for a variety of applications,1–3 and due to their disparate equilibrium potentials are often electrodeposited with complexing agents, such as cyanide, to shift their potentials more closely together. Other more environmentally friendly alternative complexants have been examined, such as citrate, and conditions demonstrated to achieve electrodeposited Cu-Zn4 and Sn-Zn-Cu5 alloys.  Several models of the Zn reduction mechanism have been proposed and involve of an adsorbed intermediate in Zn reduction.6-8In this study, the influence of codepositing Cu (or Sn) on the Zn partial current density and reaction order is reported.

Experimental

Due to the inherent low current efficiency expected in zinc and zinc alloy deposition an upward facing rotating disk electrode was used. Polarization data was obtained for Cu, Sn, Zn, CuZn, and SnZn using a Pt counter electrode and a silver/sliver chloride reference. The aqueous electrolytes contained 10 mM CuSO4, ZnSO4 or SnSO4 , 60 mM sodium citrate, 500 mM Na2SO4 and 1 mM hydroquinone. Three different ZnSO4 concentrations were examined to assess the reaction order of Zn when deposited as an elemental deposit, and when Zn is codeposited with CuZn, and SnZn, where the metal:citrate ratio was maintained at value of 0.5.  The pH was adjusted to 2 using sulfuric acid and electrolytes were at room temperature. Deposits were fabricated at constant potential of -1.25 V vs Ag/AgCl for 30 min, and the composition characterized using X-ray fluorescence spectroscopy. Ohmic drop was measured with electrochemical impedance spectroscopy (EIS). EIS was also used to characterize the deposition of CuZn and SnZn with a frequency range from 0.1 to 100K Hz.

Results

Table 1 provides a sample of the alloy composition at 900 rpm at a constant potential deposition. There is non-linear increase in the Zn deposit content with the addition of zinc ions in the electrolyte. From the composition data, the Zn partial current density and reaction order were determined for the three electrolytes Zn, CuZn, and SnZn. There was a decrease in the Zn partial current density when codeposited with Cu (or Sn) at low Zn concentration and an increase in the Zn apparent reaction order was observed when Zn was codeposited with Cu (or Sn).  This increase in the Zn partial current density and the change in the Zn apparent reaction order when codeposited with Cu or (Sn) is attributed to the presence of an anodic contribution by the less noble Zn. The influence of the codepositing noble elements on the Zn partial current density can be accounted for in the Zn rate expression by including the mole fraction of the deposit Zn concentration to estimate the activity of the solid state.

Conclusions

The current-potential behavior of Zn deposited alone or codeposited with Cu (or Sn) in an agitated environment showed that the total current density was lowered with increasing Zn concentration, due to a decrease in the Zn partial current density itself. The Zn partial current density could be described by a rate expression that includes the Zn mole fraction of the solid state.  

Acknowledgements

The authors wish to acknowledge support by the Embassy of United Arab Emirates in Washington DC, USA

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Figure 1