Abstract
The formation of 
from a molten 2 
electrolyte containing up to 0.17 mol/liter Ni(II) has been investigated using a variety of electrochemical techniques. The standard reversible potential for Ni/Ni(II) is found to be in the range of 0.86 to 0.93 V (vs. Al). In a nickel‐free electrolyte aluminum deposition on tungsten occurs via instantaneous nucleation upon an upd aluminum layer. In contrast, bulk nickel deposition occurs by progressive formation and diffusion‐limited growth of three‐dimensional nuclei. The number of nickel atoms forming a critical nuclei, 
, is dependent on overpotential. At potentials below 0.750 V, 
with the active sites on the electrode playing the role of critical nuclei. These sites are occupied according to first‐order kinetics. At potentials above 0.7 V compact nickel deposits are obtained. As the potential is decreased below 0.6 V 
formation occurs. Between 0.6 and 0.0 V alloy composition is a function of potential. The rate of the aluminum partial reaction is first order in the Ni(II) concentration which makes alloy composition independent of Ni(II) concentration over the range investigated. Separate experiments demonstrate that aluminum underpotential deposition (upd) on nickel occurs in this potential regime. Thus, alloy formation may be envisioned as aluminum upd proceeding simultaneously with diffusion‐limited nickel deposition. The upd reaction occurs rapidly such that the alloy composition is determined by the free energy of alloy formation. When the potential is decreased below the reversible potential of aluminum, 0.0 V, phase formation is complicated by a competition between alloy formation and overpotential driven kinetics of aluminum deposition.