Nanocrystalline Ni-Cu alloy plating by pulse electrolysis

Pulse electrolysis and the effect of pulse parameters on the composition of Ni-Cu alloys deposited from a citrate bath has been studied. Coherent, smooth and bright coating is obtained by precise control of the pulse time, relaxation time and peak current density. Stirring, high pH and high temperature is shown to increase the copper content of the deposit. X-Ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies revealed that the deposited Ni-Cu alloy is nanocrystalline in nature (crystallite size similar to 2.5-28.5 nm) and it exists in a single FCC-phase. The strain and mean crystallite size has also been estimated from X-ray diffraction line broadening analysis. Knoop microhardness for pulse current (PC) plated samples is higher than the direct current (DC) plated sample and the internal stress is lower for the PC sample. The corrosion resistance of the pulse-plated Ni-35.8 wt.% Cu alloy, as evaluated by potentiodynamic polarisation studies in deaerated 3 wt.% NaCl solution at 50 degrees C, is better than that of the DC-plated alloy and the commercial Monel-400. (C) 2000 Elsevier Science S.A. All rights reserved.