Overpotential alloy electrodeposition

Fig. 7.1 Schematic presentation of the characteristic cases for alloy electrodeposition, (a) The overpotential for electrodeposition of the more noble metal A is higher than that for the less noble metal B (b) The overpotential for electrodeposition of metal A is slightly lower than that for metal B (c) Alloy electrodeposition is impossible (Reprinted from Ref. [5] with kind permission from Springer)...

Electrodeposition deposition of a metal or alloy onto a substrate by electrochemical reduction of its ions from an electrolyte under the application of a cathodic overpotential. 

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. 

Kvokova and Lainer electrodeposited pure Re and Re-Cr alloy on Mo substrate. For the deposition of Re itself, two baths were used one containing perhhenic acid, and the other containing potassium perrhenate. In the first bath, the discharge of hydrogen ions was enhanced. The authors attributed the low overpotential of hydrogen on Re to its lattice parameter (a = 2.758 A). However, a justification to this theory has not been proposed. For both deposition of Re and Re-Cr alloy, the concentration polarization was foimd to be insignificant compared to the activation polarization. 

Without the colloid present (i.e., electrodeposition from pure aqueous media), a Pt-rich catalyst was formed, typically only on the outer surface of the three-dimensional support, without significant penetration into the matrix. For codeposition throughout the thickness of the support of binary and ternary catalyst formulations, with atomic compositions relevant to fuel cell application, the presence of the colloidal system was essential. The mechanism of action for the surfactant or water-in-oil microemulsion is believed to be related to selective blocking of the surface, creating a high-Pt electrocrystallization overpotential, thereby lowering the Pt electrodeposition rate relative to the alloying elements (e.g., Ru, Mo, or Sn). 

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. 

The schematic indicating the potential region where OPCD occurs is shown in Fig. 2 using the example of CoFe alloy. The schematic considers electrodeposition process from the solution which is at standard conditions (P°, Cqq2+ = Cpg2+ = 1 mol). In order to obtain desired composition of CoFe (50 50) alloy, the concentrations of Co and Fe ions in the solution have to be appropriately adjusted together with the potential (overpotential) or current at which the alloy deposition occurs. Typical approach towards the solution and deposition potential (current) design involves experiments where the concentration of more noble metal, Co, is such that C(-q2+ < Cp 2+, so that Co deposition occurs under mixed control for a... 

The results of electrochemical impedance spectroscopy show that Al coating leads to an increase in the polarization resistance of a bare Mg alloy by one order of magnitude in 3.5 wt% NaCl solution. Furthermore, the potentiodynamic polarization results show that Al-coated Mg alloy can be passivated, and a wider passive region with a lower passive current density can be obtained if the Al is electrodeposited at a lower applied current or a low cathodic overpotential. The passivity of the co-deposited Al/Zn film is slightly inferior to that of the pure Al coating. 

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