The critical current density at which massive Zn deposition began was measured in Zn and Zn-Ni alloy plating baths containing sodium citrate.
Results were as follows:
(1) The Zn deposition potential from citrate-containing baths was 0.1V less positive than from citrate-free baths, iudicating that citrate behaves as a complexing agent with Zn ion.
(2) Ni partial polarization curves also shifted to a less positive region with the addition of citrate in Ni and Zn-Ni alloy plating baths.
(3) The addition of sodium citrate and a decrease in pH in baths increased the Zn critical current density.
(4) pH in the cathode layer attained the value for Zn hydroxide formation at the critical current density in both citrate-containing and citrate-free baths.
Since the pH for Zn hydroxide formation is higher in citrate-containing than citrate-free baths, the critical current density at which adsorbed Zn hydroxide suppresses the hydrogen evolution rate to initiate massive Zn depositions is increased by the addition of citrate to baths.