Effect of thiourea during nickel electrodeposition from acidic sulfate solutions
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I. INTRODUCTION
ORGANIC additives have been used in electroplating baths, to modify the properties of the deposits. In the nickeldeposition 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 100 mg dm3, is an extremely good brightener and leveler in Wattstype nickel baths. Nevertheless, the role of thiourea in the nickel-deposition process, with respect to CE, deposit charU.S. MOHANTY, Postdoctoral Fellow, is with the Department of Material Science and Engineering, National Cheng Kung University, Tainan, 701, Taiwan, Republic of China. Contact e-mail: [email protected] B.C. TRIPATHY, Scientist, and S.C. DAS, Head and Deputy Director, are with the Department of Hydrometallurgy, and V.N. MISRA, Director, is with Regional Research Laboratory (CSIR), Bhubaneswar-751013, India. Manuscript submitted November 16, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
acteristics, crystallographic orientations, and the polarization behavior of the cathode from the acidic sulfate solutions, has not been studied in detail. The present article thus investigates the effect of thiourea on the CE, deposit quality, surface morphology, and crystallographic orientations of deposits during nickel electrodeposition from acidic sulfate solutions. In addition, the electrochemical behavior of the electrodeposited solutions, with and without thiourea, was also studied and discussed. II. EXPERIMENTAL The rectangular flow-through cell used in this work was identical to that described in our previous article.[18] Ultrapure water (Millipore Milli Q system) was used for preparing the
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