TEXTURAL MODIFICATIONS IN NICKEL ELECTRODEPOSITION UNDER PULSE REVERSED CURRENT

Nickel electrodeposits consist of crystallites whose orientation, microstructure and size depend on the electrolytic conditions. Study of the mechanism of nickel electrocrystallization has indicated that the oriented growth observed is determined by the specific inhibition exerted by the chemical species which exist or are formed on the metal-electrolyte interface (catholyte) and which are selectively absorbed on the continually renewed metal surface. The pulse reversed current (PRC) technique was used to perturb the electrocrystallization process and hence to change the texture and the structural characteristics of nickel electrodeposits. The structural changes that occur in deposit orientation under PRC conditions were studied. Nickel electrodeposition was realized from a Watts-type bath on a rotated cathode, and the electrolysis parameters (current density, rotation velocity, electrolyte pH and temperature) were chosen in such a way that using direct current they led to the preparation of perfectly [100], [211], [210], [110]A and [110]B textured deposits. The deposit texture was determined by X-ray diffractometry and the results are summarized in texture diagrams given as a function of PRC parameters: pulse frequency and duty cycle. It was proved that the texture is directly related to PRC parameters, which provoke strong perturbation in the crystal growth process. This phenomenon is reinforced when both the pulse frequency and the duty cycle are small. The PRC technique affects the nickel electrocrystallization process according to a mechanism analogous to the action of organic additives under direct current conditions by intervening in the catholyte composition and hence in the adsorption-desorption phenomena occurring at the metal-solution interface during the dissolution and the deposition time.