Polarization alloy electrodeposition

The individual polarization curves for the metals are often modified as a result of interactions resulting from codeposition. If the alloy deposition occurs at low polarization, the nobler metal will be deposited preferentially (Cu in Example 11.1). All factors, however, that increase polarization during electrodeposition, such as high current density, low temperature, and quiescent solution—factors that increase concentration polarization—will favor the deposition of the less noble metal (Zn in Example 11.1). 

Fig. 7.1b the oveipotential for electrodeposition of metal A is slightly lower than that for metal B, i.e., the polarization curves are almost parallel. Hence, the electrodeposition of alloy commences at the potential r(B)> while the alloy contains more metal A than B. If the difference between r(A) and r(B) is high and the overpotential for electrodeposition of the more noble metal A is lower than that for the less noble metal B, the third case presented in Fig. 7.1c applies in such a case, alloy electrodeposition is impossible. The difference between the reversible potentials of two metals could be changed (lowered) by the change of metal ion concentration (activity), and in most cases, this is achieved by the complexation. 

Two types of solutions were examined for Ag-Sn alloy electrodeposition [18] sulfate solution containing thiourea as a complexing agent for Ag ions and pyrophosphate and iodide solution which form a stable complex with both Ag and Sn ions. In sulfate solution, Sn was a normal metal, while Ag was intermediate one due to formation of complexes with thiourea and iodide ions. In pyrophosphate solution, both metals belonged to intermediate ones due to formation of complexes with pyrophosphate and iodide ions. The polarization curves for alloy electrodeposition measured by the potential sweep method (v = 0.5 mV s ) in sulfate and pyrophosphate-iodide solutions are shown in Fig. 7.15. A current density rapidly increased at about —0.07 V versus NHE with the current density plateau up to about —0.27 V versus NHE corresponding to the pure Ag electrodeposition in the sulfate solution. Additional current density increase and plateau at more negative potentials correspond to the alloy electrodeposition ( ). Similar behavior is detected for pyrophosphate-iodide solution ((D). In both electrolytes, electrodeposition of both metals was suppressed due to complexes formation. The content of Ag in both cases abruptly decreased with the increase of electrodeposition current density. 

Fig. 7.15 Polarization curves for Ag-Sn alloy electrodeposition from sulfate ( ) and pyrophosphate-iodide ((2)) solutions (Reprinted from Ref. [18] with the permission of the Japan Institute of Metals and Materials)...

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. 

The electrodeposition of an alloy requires, by definition, the codeposition of two or more metals. In other words, their ions must be present in an electrolyte that provides a cathode film, where the individual deposition potentials can be made to be close or even the same. Figure ll.l depicts typical polarization curves, that is, deposition... 

Electrodeposition of Zn-Ni alloys attracted considerable attention. The influence of electrolyte composition [422-428] hydrodynamic conditions [424] and the electrochemical polarization mode [424,... 

A/dm2, Ni2+ ions discharge into Ni-B° occurs with enhance polarization in comparison with discharge into Ni°. The breaking of the Ni2+ ions discharge rate, when Ni-B° is electrodeposited (at the E< - 0,75), is connected with this alloy surface coverage by the adsorbed dope boron containing anions. So, for example, cathodic polarization at the ik = 2 AJ dm2 maintained the value 30 mV, and at the ik = 4 A/ dm2 - 45 mV. 

Another mechanism for induced codeposition of Mo was suggested by Chassaing et al for electrodeposition of Mo-Ni alloys from citrate-ammonia electrolytes. Electrochemical impedance spectroscopy (EIS) measurements were carried out in order to better understand the different reactions occurring on the electrode surface during deposition. The proposed mechanism is based on a multi-step reduction of molybdate species. A M0O2 layer is formed via reduction of molybdate ion as in Eq. (42). Then, if free Ni is present in solution, this oxide can first combine at low polarization with Ni, following the reduction reaction ... 

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. 

Fig. 7.2 Polarization curves for the electrodeposition of more noble metal (A) and less noble metal (B) /l(A) diffusion limiting current density for the electrodeposition of metal (A), M(B) current density for the electrodeposition of metal (B), /d(all) current density for the electro-deposition of alloy (Reprinted from Ref. [5] with kind permission from Springer)...

Polarization Curves for Co-Ni Alloy Powder Electrodeposition from the Sulfate-Containing Supporting Eiectroiyte... 

In Fig. 8.1a are shown polarization curves corrected for IR drop for the processes of Co, Ni, and Co-Ni alloy powder electrodeposition from 1 M (NH4)2S04 + 0.7 M NH4OH containing supporting electrolyte. Their shape of aU polarization curves is identical, characterized with two inflection points, A and B. For Co electrodeposition, sharp increase of current occurs at about —1.19 V versus Ag/AgCl, while for Ni electrodeposition, this phenomenon is moved to more negative potentials... 

Fig. 8.1 (a) Polarization curves for the electrodeposition of cobalt (Co), nickel (Ni), and Co-Ni alloy powders after IR drop correction recorded for different Ni /Co ions ratios 4.00, 1.50, 0.67, and 0.25 (marked in the figure), (b) The same polarization curves after hydrogen evolution current density subtraction, (c) Corresponding rji vs. E curves for (b) (Reprinted from Ref. [1] with kind permission from Springer)... 

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