Equilibrium alloy electrodeposition

The phase distribution observed in the alloys deposited from AlCb-NaCl is very similar to that of Mn-Al alloys electrodeposited from the same chloroaluminate melt [126 129], Such similarity may also be found between the phase structure of Cr-Al and Mn-Al alloys produced by rapid solidification from the liquid [7, 124], These observations are coincident with the resemblance of the phase diagrams for Cr-Al and Mn-Al, which contain several intermetallic compounds with narrow compositional ranges [20], inhibition of the nucleation and growth of ordered, often low symmetry, intermetallic structures is commonly observed in non-equilibrium processing. Phase evolution is the result of a balance between the interface velocity and... 

Classification of different types of alloy electrodeposition was made by Brenner [3] in 1962, by defining five groups equilibrium, irregular, regular, anomalous, and induced codeposition. More detailed explanations including samples for each type were given in Ref. [5]. 

AU the remarks mentioned above are applicable as well to the electrodeposition of alloys. Using Breimer s classification of alloy electrodeposition, e.g., equilibrium, regular, anomalous, etc., all existing combinations of deposition parameters and their influence on the alloy morphology are analyzed. Interestingly, certain features, which are not recognized in the electrodeposition of pure metals, are observed in the alloy depositiOTi processes. An example includes the spatiotemporal structures, which is discussed in this book. 

In electroplating industrial iron metals, zinc metal electrodeposition is accompanied by the formation of Zn-Ni, Zn-Co, and Zn-Fe alloys, where zinc electrodeposition is known to be anomalous in some cases. The ratio of zinc metal to iron metal in those alloys is sometimes higher than that of the electroplating bath solution, and zinc ions occasionally deposit at potentials positive to the equilibrium potential of zinc ions on zinc metal although is very negative to the equilibrium potentials of iron metals. It can be seen from the study of underpotential deposition of zinc ions " that this is not anomalous, but could be explained as an underpotential deposition phenomenon, to be clarified in further work. 

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. 

A qualitatively different type of precipitation patterns concerns structures formed during the electrodeposition of alloys. In 1938, Raub and Schall observed the formation of propagating wave patterns during the electrodeposition of silver-indium alloy [9, 10]. However, their observation was widely ignored, because no systematic theory was available that could classify these patterns as typical features of systems far from thermodynamic equilibrium. In 1986, the phenomenon was studied by Krastev and Nikolova in the possibly related electrodeposition of a silver-antimony alloy (Figure 11.1) [11]. Furthermore, very similar patterns were observed by Saltykova et al. during the electrodeposition of iridium-ruthenium alloy from molten salts [12]. 

Though alloy deposition is subject to the same scientific principles as individual metal plating, thermodynamics and kinetics of codeposition processes are more complicated than that of the deposition of a single metal. Electrochemical deposition of more than one metal often results in formation of different microstructures and phases. Moreover, electrodeposited binary alloys may or may not be the same in phase structure as those formed by simple melting. Besides, electrodeposited phases are not always in the thermodynamic equilibrium. 

The activity of the metal in the alloy is always less than 1 which yields different values of the equilibrium potential for the component in the alloy as compared to the corresponding elemental bulk metal electrode. According to Eq. (6), in order to form an AB alloy, the applied potential to the electrode surface has to be such that both components of the alloy are at overpotential conditions with respect to the equilibrium potentials of constituents in the alloy (Eq. (7)). In practice, the term OPCD is related to electrodeposition of alloys in the potential range where the applied potential is such that both components of the alloy are at overpotential conditions with respect to the equilibrium potentials of their elemental phase, defined by Eq. (1). In this case, the resulting composition of the electrodeposited alloys is controlled by the combination of several effects (1) kinetics of the electrodeposition of each component itself, (2) transport limitations, (3) conditions at the electrochemical interface, and (4) mutual interaction of adsorbed intermediates. 

---