SELECTIVE LEACHING OF NIAL3 AND NI2AL3 INTERMETALLICS TO FORM RANEY NICKELS

Annealed single-phase NiAl3 and Ni2Al3 materials were leached with 20 wt% aqueous NaOH solution to remove the aluminum. 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.