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The effect of thiourea on the cathodic current efficiency (CE), deposit quality, crystallographic orientations, surface morphology, and polarization behavior of the cathode was investigated during nickel electrodeposition from acidic sulfate solutions for 2 hours at 60 °C. A slight decrease of 3 to 4 pct in the CE was observed, when the concentration of thiourea was increased from 2 to 40 mg dm^sup -3^. The nickel deposit quality deteriorated significantly at higher thiourea concentrations; the surface morphology deteriorated and the contamination of the nickel deposits increased. The presence of thiourea affected the peak intensities of the crystal planes. Cyclic voltammetric studies on nickel deposition at 25 °C revealed depolarization behavior of the cathode at lower thiourea concentrations, ≤10 mg dm^sup -3^; however, a mixed behavior is observed at higher thiourea concentrations. These changes were also observed in the exchange current density (i^sub 0^) values.
I. INTRODUCTION
ORGANIC additives have been used in electroplating baths, to modify the properties of the deposits. In the nickel-deposition process, organic additives have been used as brighteners, levelers, wetting agents, and stress reducers. Scholter[1] used benzene and naphthalene trisulfonic acids as commercial brighteners during nickel electrodeposition from a Watts bath. Burkhart et al.[2] found that thiouronium salts act as a brightener in aqueous nickel-electroplating baths. Saccharin, p-toluene sulfonamide, sodium m-benzene disulfonate, sodium 1,3,5 napthalene trisulfonate, and O-sulfobenzaldehyde[3] have been used as stress reducers in Watts-type nickel baths. Nayak and Karunakaran[4] obtained bright and smooth electrodeposits of nickel from a Watts bath, using a combination of sodium naphthalene 2-sulfonate and an organic additive. Mohanty et al.[5] investigated the effect of pyridine, 2-picoline, and 4-picoline during nickel electrodeposition from acidic sulfate solutions. They observed that these particular additives do not have a significant effect on the current efficiency (CE); however, they affected the surface morphology and the crystallographic orientation of the nickel deposits. Also, 2- and 4-picoline produced smoother, more compact, and more leveled nickel deposits, as compared to pyridine.
Thiourea[6-15] has been used commercially in copper electrorefining and electrowinning, in order to produce smooth and bright copper deposits. Rogers et al.[16] have found that thiourea acts as both leveler and brightener in Watts-type nickel baths. Hoekstra and Trivich[17] have also observed that thiourea, in the concentration range of 50 to...