Electrodeposition of alloys

Alloy deposition is almost as old an art and/or science as is the electrodeposition of individual metals. (Brass deposition, for instance, was invented circa 1840 ) In the last analysis, as can well be expected, alloy deposition is subject to the same scientific principles as individual metal plating. Indeed, progress in either of the two has almost always depended on similar advances in electrodeposition science and/or technology. 

The subject of alloy electroplating is being dealt with by an ever-increasing number of scientific publications (close to 200 in the last five years in the Journal of the Electrochemical Society alone ). The reason for this seems to be the vastness of the number of possible alloy combinations and the concomitant possible practical applications. 

Electrodeposited binary alloys may or may not be the same in phase structure as those formed metallurgically. By way of illustration, we note that in the case of brass (Cu-Zn alloy), x-ray examination reveals that apart from the superstructure of 

Fundamentals of Electrochemical Deposition. Second Edition. By Milan Paunovic and Mordechay Schlesinger Copyright 2006 John Wiley Sons, Inc. 

Using specific metal combinations, electrodeposited alloys can be made to exhibit hardening as a result of heat treatment subsequent to deposition. This, it should be noted, causes solid precipitation. When alloys such as Cu-Ag, Cu-Pb, and Cu-Ni are coelectrodeposited within the limits of diffusion currents, equilibrium solutions or supersaturated solid solutions are in evidence, as observed by x-rays. The actual type of deposit can, for instance, be determined by the work value of nucleus formation under the overpotential conditions of the more electronegative metal. When the metals are codeposited at low polarization values, formation of solid solutions or of supersaturated solid solutions results. This is so even when the metals are not mutually soluble in the solid state according to the phase diagram. Codeposition at high polarization values, on the other hand, results, as a rule, in two-phase alloys even with systems capable of forming a continuous series of solid solutions. 

Electrodeposited binary alloys may or may not be the same in phase structure as those formed metallurgically. By way of illustration, we note that in the case of brass (Cu-Zn alloy), X-ray examination reveals that, apart from the superstructure of /3-brass, virtually, the same phases occur in the alloys deposited electrolytically as formed in the melt. Phase limits closely agree with those in the bulk. Debye-Scherrer interference rings indicate the presence of a strong distortion of the lattice, particularly in the a-phase brass. Electrodeposited a-brass, for instance, is 

One recognizes three main steps in the cathodic deposition of alloys or single metals  

Over the past two decades, ionic liquids (ILs) have attracted considerable interest as media for a wide range of applications. For electrochemical applications they exhibit several advantages over the conventional molecular solvents and high temperature molten salts they show good electrical conductivity, wide electrochemical windows of up to 6 V, low vapor pressure, non-flammability in most cases, and thermal windows of 300-400 °C (see Chapter 4). Moreover, ionic liquids are, in most cases, aprotic so that the complications associated with hydrogen evolution that occur in aqueous baths are eliminated. Thus ILs are suitable for the electrodeposition of metals and alloys, especially those that are difficult to prepare in an aqueous bath. Several reviews on the electrodeposition of metals and alloys in ILs have already been published [1-4], A selection of published examples of the electrodeposition of alloys from ionic liquids is listed in Table 5.1 [5-40]. Ionic liquids can be classified into water/air sensitive and water/air stable ones (see Chapter 3). Historically, the water-sensitive chloroaluminate first generation ILs are the most intensively studied. However, in future the focus will rather be on air- and water-stable ionic liquids due to their variety and the less strict conditions under which 

Electrodepositionjrom Ionic Liquids. Edited by F. Endres, D. MacFarlane, A. Abbott Copyright 2008 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31565-9 

Electrodeposition of Al-containing Alloys from Chloroaluminate Ionic Liquids 

The Lewis acidic chloroaluminate ILs are suitable for the electrodeposition of aluminum-containing alloys. Many examples have been published but those that have been reviewed in detail by Stafford and Hussey [1] will not be included in this section. 

Lowering the liquid acidity from 66.7-33.3% to 60.0-40.0% mole fraction results in the disproportionation of [Ti(AlCl4)3] producing TiCl3 and Ti precipitates. 

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Brenner, A., The Electrodeposition of Alloys, Academic Press, New York (1963)... 

The electrodeposition of alloys at potentials positive of the reversible potential of the less noble species has been observed in several binary alloy systems. This shift in the deposition potential of the less noble species has been attributed to the decrease in free energy accompanying the formation of solid solutions and/or intermetallic compounds [61, 62], Co-deposition of this type is often called underpotential alloy deposition to distinguish it from the classical phenomenon of underpotential deposition (UPD) of monolayers onto metal surfaces [63],... 

Brenner, A., Electrodeposition of Alloys. Vol. 1. 1963, New York Academic Press. 

Electrodeposition of alloys Electrolysis has also been used in order to obtain several metal compounds and alloys via the simultaneous co-deposition, from aqueous solutions or fused salts of the metal components. 

Electrodeposition of alloys. Ag-Bi alloys as an example of electro co-deposition... 

The nse of complexation to allow codeposition of alloys is well known in electroplating. The best-known example is that of brass (Cu/Zn) plating, where cyanide, which is a stronger complex for Cu than it is for Zn, brings the deposition potentials of the two metals, originally far apart, to almost the same value. There is a direct connection between this effect and the equivalent one for CD. This arises from the fact that, for both CD and electrodeposition of alloys (we in-clnde mixed metal compounds in the term alloy), the effect of the complexant is to lower the concentration of free cations. For CD this affects the deposition throngh the solnbility product, while for electrodeposition it affects the deposition potential through the Nemst equation ... 

In this chapter some results on the electrodeposition of alloys from ionic liquids are summarized. Many fundamental studies have been performed in chloroaluminate first generation ionic liquids but the number of studies employing air- and water-stable ionic liquids rather than the chloroaluminates is increasing. Currently, new ionic liquids with better electrochemical properties are being developed. For example, Abbott et al. [47] have prepared a series of ionic liquids by mixing commercially available low-cost choline chloride and MCI2 (M = Zn, Sn) or urea and demonstrated that these ILs are good media for electrodeposition for pure metals (see Chapter 4.3). It can be expected that in the near future, the electrodeposition of alloys from ILs may become available for industrial applications. Furthermore, due to their variety, their wide electrochemical and thermal windows air- and water-stable ionic liquids have unprecedented prospects for electrodeposition. 

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