The effects of lead on the electrochemical and adhesion behavior of zinc electrodeposits
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INTRODUCTION
L E A D can be detrimental to the properties of zinc, and controlling its concentration is important in various deposition processes. In the electrolytic extraction of zinc, it is desirable to minimize the lead to insure acceptable purity for high-grade applications. One specific example of the adverse influence of lead in zinc is the poor adhesion it gives to electrogalvanized steel When the lead content is too high, the zinc coating can be easily removed from the steel substrate after heating in the temperature range of 215 ~ to 280 ~ Insoluble lead-silver anodes and the source of zinc for the electrolyte, whether in the form of metal oi~ a compound, are possible sources of lead contamination. For instance, good quality zinc oxide or special high-grade zinc contains enough lead to supply 2 mg/1 lead in an electrolyte containing 100 g/1 zinc. tL2] The concentration of lead is likely to increase with time, since the processes are closed loop in nature, and it is very probable that the saturation limit of lead in the electrolyte will be reached with continued operation. Unfortunately, there has not been any extensive research conducted on the fundamental behavior of lead during zinc electrolysis. One objective of the present study was to provide some basic electrochemical data which might be of assistance in controlling its influence during zinc electrolysis. Since the theoretical reduction potential of the lead ions present in the electrolyte is - 0 . 2 2 3 V(SHE) at 10 mg/1, codeposition with the zinc should occur. The deposition rate of lead may be quite low, since it will be depositing at its limiting current density at such low concentrations. The diffusion coefficient of lead in the VIJAY SR1NIVASAN, Research Metallurgist, is with National Steel Corporation, 1745 Fritz Drive, Trenton, MI 48183. J. SAMI CUZMAR, Postdoctoral Fellow, and THOMAS J. O'KEEFE, Curators' Professor of Metallurgical Engineering, are with the Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO 65401. Manuscript submitted April 6, 1988. METALLURGICAL TRANSACTIONS B
electrolyte can be evaluated by using the rotating disc electrode method based on the Levich equation t31 for mass transport under diffusion-controlled conditions and laminar flow, iL = 0.62nFAO2/a u-1/6 tol/2Cb
[1]
where iL is the limiting current, n the number of electrons involved in the reaction, F the Faraday's constant, A the electrode area, D the diffusion coefficient, v the kinematic viscosity, to the electrode angular velocity, and Cb the bulk concentration. Once the diffusion coefficient has been determined for a particular electrolyte, the mass transport coefficient for the cell design and operating conditions (current density, relative flow rate, etc.) must be evaluated. The determination of the mass transfer coefficient by the use of tracer ions has been described in the literature. E4,51The rate at which the lead will deposit at its limiting current density in an actual process is determined by iL = nFkCb = n F D C
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