Alloy induced codeposition

Induced Codeposition of Alloys of Tungsten, Molybdenum and Rhenium with Transition Metals... 

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... 

Although this chapter is about induced codeposition, and is not meant to deal specifically with anomalous alloy deposition, a few general comments would seem to be appropriate. 

Electroless deposition, or autocatalytic plating, may be defined as deposition of a metal coating by a controlled chemical reduction, catalyzed by the metal or alloy being deposited. Electroless deposition has been known for a long time. One of its early uses was the deposition of a mirror-like layer of silver on the internal surfaces of Dewar flasks for improved thermal isolation, and as the back coating of mirrors. Later, it was used for deposition of different metals and alloys, and even for induced codeposition of alloys. 

A phenomenon of induced codeposition, similar to that discussed above for W, is observed when Mo is codeposited with iron-group metals. Similarly to tungsten, molybdenum cannot be deposited alone from aqueous solutions. Electrodeposition of Mo alloys exhibits similar dependencies on experimental variables as that of W. It should be noted that, although the two systems are very similar, some differences are found in the literature, as described bellow. 

NiCit] to the cathode surface. Because the deposition rate of the two metals is coupled, the alloy composition does not vary with rotation rate. In contrast, if the concentration of Ni in solution is comparable to or higher than that of MoO4 , the rate of formation of the Ni-Mo intermediate is limited by the transport of molybdate, while Ni can be deposited in parallel—its rate of deposition being independent of the rate of mass transport. Increasing either the rotation rate or the molybdate concentration—the partial current density of Mo deposition will also increase, while that of Ni deposition will not be affected. Thus, it was proven beyond any doubt that the induced codeposition of Mo with Ni and other iron-group metals was dependent on the existence of the iron-group metal ions in solution. 

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 ... 

This brings us to one of the main point made in this chapter, which is relevant both for anomalous alloy deposition and for induced codeposition in order to understand the process, one should understand the chemistry of the solution, and particularly the distribution of species in plating baths that contain complexes. This type of analysis is shown in Figs. 1, 2, 8, 13 and 14, and has been used in our own work to explain the induced codeposition of tungsten. 

For induced codeposition of Ni-W alloys, we concluded that the precursor for deposition of the alloy is a mixed-metal complex of the type [(Ni)(HWO4)(Cit)] ". This complex is formed from a nickel citrate complex (cf., Eq. 50) and a tungstate citrate complex [(WO4)(Cit)(H)] . It may be somewhat surprising that the nega-... 

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]. 

Although it has been shown that Mo, W, Ti, and Ge could not be separately electrodeposited from aqueous solutions, it was discovered that they could be codeposited with the iron-group metals (Fe,Ni,Co) in the presence of appropriate complexing agents. This type of alloy electrodeposition was defined by Brenner [3] as induced codeposition. 

Podlaha EJ, Landolt D (1996) Induced codeposition I. An experimental investigation of Ni-Mo alloys. J Electrochem Soc 143 885-892... 

Podlaha EJ, Landolt D (1997) Induced codeposition HI. Molybdenum alloys with nickel, cobalt, and iron. J Electrochem Soc 144 1672-1680... 

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... 

The polarization curves are presented in Fig. 5.43. As can be seen, the polarization curves characterized by two inflection points (Fig. 5.43a), as in all previous cases, were obtained. It is important to note that the potential of the beginning of alloy deposition (A) becomes more negative with the increase of molybdate ions concentration (with the decrease of Ni/Mo ratio), as it could be expected, since the potential of the Mo deposition is much more negative than that of Ni [116]. At the same time, a deposition of Mo can only take place in the presence of Ni (induced codeposition [116]). Taking into account that the concentration of Ni ions was constant, it is quite reasonable that the value of current density of the inflection point B does not change with changing Ni/Mo ions concentration ratio (being about —1.2 A cm ). 

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