Induced codeposition mechanism

Zeng Y, Li Z, Ma M, Zhou S (2000) In situ surface Raman study of the induced codeposition mechanism of Ni-Mo alloys. Electrochem Comm 2 36-38... 

In this section, the process of electrodeposition is reviewed briefly, and its place in the general context of electrode reactions and charge transfer across the metal/solution interface is set (Section 1.1). In Section 1.2, special emphasis is given to deposition of alloys, and particularly to anomalous deposition of alloys (Sections 1.2.3 and 1.2.4). Next, the phenomenon of induced codeposition is defined, and possible mechanisms are discussed briefly (Section 1.2.5). Several electroless (Section 1.2.6) and electrodeposition processes, in which induced codeposition plays a role, are mentioned. A more extensive discussion of electrodeposition of W-, Mo- and Re-based alloys is included in Section 2. Typical... 

With respect to the last type of mechanism, Gileadi and coworkers recently studied the induced codeposition of W-Ni from moderately alkaline baths containing citrate as a complexing agent, with or without ammonia. The range of bath compositions and operating conditions is shown in Appendix A. The working hypotheses in these studies were that ... 

The difficulty in attempting to determine the mechanism of alloy deposition from the current-potential relationship observed in complex solutions, which sometimes contain more than one ligand, was alluded to in the introduction to this chapter (cf., Section 1.2.2). The comments made here are not meant to criticize the experimental work presented in these papers in the field of induced codeposition of Mo with iron-group metals. It is only given to show the limits of validity of mathematical models, particularly when the solution is complex and the number of freely adjustable parameters is large. 

Gomez et al. " electrodeposited Co-Mo magnetic alloys from a sulfate-citrate bath on carbon electrodes. Although the focus of their paper was not on elucidating the mechanism of induced codeposition, it was suggested that hydrogen could not be responsible for the deposition of Mo in the Co-Mo system, because its concentration was fairly low and because another mechanism should explain the need for citrate or polycarboxylate anions in solution. The deposition process was foimd to be favored when molybdate was present in solution, even at very low concentrations. Hence, the authors adopted the model of Podlaha and Landolt, according to which a mixed-metal complex of cobalt(II), citrate and molybdenum dioxide is adsorbed on the surface and promotes Mo reduction. 

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 ... 

Many publications were devoted over the years to explain the mechanism by which induced codeposition proceeds. None have been proven beyond doubt, and one has to look for the explanation that is consistent with the widest range of experimental observations. 

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