Underpotential Deposition of Zn in Presence of Iron-group Metals.

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Underpotential deposition of Zn in which Zn was codeposited at the potentials more positive than its equilibrium potential was found in the electrodeposition of Zn-Ni alloys from aqueous solutions. The underpotential deposition of Zn discussed in this study proceeded continuously with appreciable high rate, which was different from the so-called "underpotential deposition" reported in the previous papers. This phenomenon was observed only in the anomalous codeposition of Zniron-group metal systems such as Zn-Ni, Zn-Co and Zn-Fe but did not appear in the normal codeposition of Zn-Cu and Zn-Cd systems. The chemical states of the alloy constituents were investigated by XPS and Zn as well as Ni in the electrodeposits obtained in the underpotential region was found to be electroreduced to metallicstate.The underpotential deposition of Zn in the presence of iron-group metals was enhanced with an increase in temperature, a decrease in metal concentration ratio of in in the bath and the addition of chloride ion to the baths. Since it was known that these changes in the operating conditions diminished the inherent overpotential of iron-group metals, the underpotential deposition of Zn was related to the polarization behavior of iron-group metals during alloy deposition.Further, the microstructure of the alloys deposited at the underpotential region was investigated by X-ray diffraction technique. It was difficult that the mechanism of the underpotential deposition of Zn was explained in terms of the decrease in the activity of metallic Zn in the alloys.Consequently, since the underpotential deposition of Zn is observed in the anomalous codeposition of Zn-iron-group metal alloys, the mechanism of the underpotential deposition should be discussed in connection with that of the anomalous codeposition.