Electrodeposition of zinc-nickel alloys from ammonia-containing baths

This paper describes the use of ammonia-containing baths for Zn-Ni alloy electrodeposition. Buffering properties of the ammonia/ammonium couple limit the local change in pH in the vicinity of the electrode surface caused by simultaneous hydrogen evolution. In addition, it is shown that the divalent zinc and nickel species exist in the form of Zn(NH3)(4)(2+) and Ni(NH3)(6)(2+) complexes over a large pH range. The electrochemistry of the deposition at pH 10 was investigated by galvanostatic experiments and cyclic voltammetry, and compared with deposition from ammonium chloride baths at pH 5. The Ni content in the alloys were found to be 40-60% higher from the ammonia-containing bath than from the acidic baths. Reduction of divalent ions and hydrogen evolution were shown to occur at potentials 250 mV more cathodic than with baths at pH 5; the deposition mechanism may be affected by complexation of the metal cations by ammonia.