New observations on the anodic oxidation of copper in hot acidified copper sulfate solutions
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INTRODUCTION
ALTHOUGH the electrochemical behavior of copper in acidic copper sulfate solutions has been extensively studied, 0-71 no general agreement has been reached pertaining to the phase(s) formed during the anodic polarization of copper. One of the major challenges to the copperrefining process is to fully understand the mechanism of anode passivation, t3'6'8] Many hypotheses have been reported in the literature t2-Hj in this regard, but disagreement persists concerning the mechanism of formation of the major passivating phase(s) and the phenomenon is not yet fully understood. Most of the previous studies I2,3,6,81 have focused attention on the anodic behavior of impure copper anodes in solutions containing contaminants such as As, Sb, Bi, and Ni and how the presence of such impurities in both the electrolyte and the copper anode may affect the passivation process. However, little work has been reported on the mechanism and kinetics of the passivation of pure copper in an impuritiesfree electrolyte. This communication presents results which establish the mechanism on a firmer basis and also provides a different interpretation to that suggested previously. Besides electrochemical studies, X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray analysis (EDS) were performed to determine the composition of the layers. II.
EXPERIMENTAL
Measurements were made on a working copper electrode made of "Gold label" Aldrich Chemicals, Milwaukee, WI, copper (99.99 pct) of a surface area of
0.32 cm 2. The surface area of the auxiliary copper electrode was 15.8 cm 2. The anode and cathode were polished with a series of emery papers, finishing off with the 600 grit grade. Before immersion in the cell, the electrodes were copiously washed with doubly distilled water. All potentials were measured with reference to Hg, Hg2SO4/K2SO 4 sat. (E = 0.640 V/NHE). The reference electrode was connected to tile cell by a salt bridge and terminated with a Luggin capillary. Solutions were made of the following Fisher reagents: CuSO4" 5H20 [0.66 M Cu2+], H 2 S O 4 [1.63 M] and NiSO4"6H20 [0.29 M Ni2+]. The temperature of the 400-ml doublewall cell was fixed at 65 ~ --- 0.5 using a thermostated water-circulating bath. The above conditions indicated in brackets were chosen because they make up a solution which simulates an industrial electrorefining electrolyte, hereafter called the base solution. Cyclic sweep perturbations and potentiostatic measurements were performed with an EG & G potentiostat Model PAR 273 and with a HEWLETT-PACKARD* *HEWLETT-PACKARD is a trademark of Hewlett-Packard Company, Colorado Springs, CO.
x-y-t recorder, Model 7090. The surface morphology of the samples was examined by a scanning electron microscope, model JEOL JSM25S, equipped with a TN 5700 energy-dispersive X-ray analyzer. Surface analyses were conducted by XPS on an ESCALAB-MK II (VG Scientific Ltd., East Grinstead, U.K.,) multitechniques apparatus. Operating conditions were generally 100 W (10 mA emission current at 10 kV) using an A1 or Mg
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