ELECTRODEPOSITION OF NI1-XALX IN A CHLOROALUMINATE MELT

The formation of Ni1-xAl(x) from a molten 2 AlCl3-NaCl electrolyte con taining up to 0.17 mol/liter Ni(II) has been investigated using a vari ety of electrochemical techniques. The standard reversible potential f or Ni/Ni(II) is found to be in the range of 0.86 to 0.93 V (vs. Al). I n a nickel-free electrolyte aluminum deposition on tungsten occurs via instantaneous nucleation upon an upd aluminum layer. In contrast, bul k nickel deposition occurs by progressive formation and diffusion-limi ted growth of three-dimensional nuclei. The number of nickel atoms for ming a critical nuclei, n(c), is dependent on overpotential. At potent ials below 0.750 V, n(c) = 0 with the active sites on the electrode pl aying the role of critical nuclei. These sites are occupied according to first-order kinetics. At potentials above 0.7 V compact nickel depo sits are obtained. As the potential is decreased below 0.6 V Ni1-xAl(x ) formation occurs. Between 0.6 and 0.0 V alloy composition is a funct ion of potential. The rate of the aluminum partial reaction is first o rder in the Ni(II) concentration which makes alloy composition indepen dent of Ni(II) concentration over the range investigated. Separate exp eriments demonstrate that aluminum underpotential deposition (upd) on nickel occurs in this potential regime. Thus, alloy formation may be e nvisioned as aluminum upd proceeding simultaneously with diffusion-lim ited 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 alu minum, 0.0 V, phase formation is complicated by a competition between alloy formation and overpotential driven kinetics of aluminum depositi on.