Nickel Electrodeposition from CaCl 2 -Based Highly Concentrated Aqueous Baths

Abstract

The present study investigated the electrodeposition of nickel from hydrate melts comprising highly concentrated aqueous solutions of CaCl 2 . The objective of this work was to elucidate the manner in which the extreme chloride activity of such melts affected both electrocrystallization kinetics and hydrogen co-deposition. Visible spectra demonstrated that the high chloride concentrations in these solutions transformed symmetric Ni(II) aquo ions into lower-symmetry chloride complexes. Consequently, the kinetics of the Ni electrodeposition process were enhanced, as evidenced by decreased charge-transfer resistance compared with that associated with a conventional Watts bath. The Ni electrodeposited from these hydrate melts exhibited a random crystallographic orientation whereas a {100} preferred orientation was obtained from the Watts bath. Thermal desorption spectroscopy indicated the substantial co-deposition, or occlusion, of atomic hydrogen in the form of vacancy-hydrogen clusters in the resulting Ni films. This co-deposition occurred despite a current efficiency of nearly 100%. Interestingly, enhanced hydrogen co-deposition together with the suppression of gaseous H 2 evolution were previously observed during the electrodeposition of other metals and appear to be characteristic of electrodeposition from chloride-based hydrate melts.