Selective leaching of NiAl₃ and Ni₂Al₃ intermetallics to form Raney nickels

Abstract

Annealed single-phase NiAl₃ and Ni₂Al₃ materials were leached with 20wt% aqueous NaOH solution to remove the aluminium. At temperatures of 274 to 323 K, NiAl₃ leached according to linear kinetics, yielding porous nickel which was friable and disintegrated. At these temperatures Ni₂Al₃ was unreactive, but at 343 to 380 K it leached according to parabolic kinetics, producing a strong, tightly adherent rim of residual material. The Ni₂Al₃ reaction proceeded in two steps, firstly to produce a two-phase mixture of Ni₂Al₃ plus nickel, and secondly to produce nickel alone. In both stages the detailed microstructure of the prior alloy was preserved, implying that the mechanism is selective dissolution. The surface adsorption properties of the nickel residues were obscured by reprecipitated aluminA. However, metal crystallite size measurements showed that a large nickel surface area was potentially available.