Anomalous alloy deposition

The term anomalous codeposition (ACD) was first introduced by Abner Brenner/ to describe an electrochemical deposition process m which the less noble metal is deposited preferentially under most plating conditions. This behavior is typically observed in codeposition of iron-group metals (i.e. Fe, Co and Ni) or in codeposition of an iron-group metal with Zn or Cd, with either inhibition or acceleration of the rate of deposition of one of the alloying elements by the other.  

As a first approximation, one might expect that the composition of an electroplated alloy would be related to the current observed for each of the elements, when measured alone in the same solution at the same potential. Assume, for simplicity, that both metals are deposited at high negative overpotentials, within the linear Tafel region (where rj/h 1). Then, one could write the 

Typical values of are in the range 30 to 300 mV decade, corresponding to ac values of 2 and 0.2, respectively, but values close to 0.1 V decade are most commonly observed in metal deposition. When the exact value for a specific system is unknown, the approximation = Z c = 0.12 V decade has often been used, although there is no theoretical basis to support this choice, and it would be more accurate to obtain the value of b from a, employing the simple relationship 

The exchange current densities and the Tafel slopes for two metals are in general different, although they may happen to be close to each other for a particular case. The overpotential is not the same for the two metals, of course, although deposition takes place at the same potential, measured with respect to a given reference electrode. In other words, at the deposition potential, Fjep, one has 

Assuming, for simplicity, that the two Tafel slopes are equal, the atom ratio of the two elements in the alloy should be given by 

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Hessami and Tobias [70] extended the mechanisms of Bockris et al. and Matulis etal. of the deposition of single metals (Ni, Fe) to the mathematical modeling of codeposition of Ni—Fe alloys. This mathematical model for the anomalous alloy deposition describes the electrode processes using the calculated interfacial concentrations. The inhibition (reduction) of nickel partial current density during alloy deposition and the anomalous deposition are explained on the basis of the relative concentrations of metal-hydroxide ions, [MOH]+. Calculations show that the [FeOH]+ concentrations are higher than [NiOH]+ because it has a much smaller dissociation constant ([MOH]+ = (M2+)(OH )/A surface sites, and the result of this competition is inhibition (decrease) in Ni deposition in the presence of [FeOH]+ ions. Figure 30... 

A well-known example of anomalous alloy deposition is the plating of Permalloy . Considering that the values of TI for... 

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. 

Anomalous alloy deposition is common in electroplating. Actually, it is so common that it is the rule rather than the exception. What could be the cause of this phenomenon In general, it is postulated that one of the alloying elements forms a hydroxide on the surface, which inhibits the deposition of the other. This could explain the inhibiting effect, but it is much more difficult to explain the enhancement. Another explanation is that the two ions form a mixed-metal complex that, for some reason, is more readily reduced at the surface than each of the metal ions separately. Naturally, such complexes can only be expected to exist in solution... 

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. 

Several binary alloys of technological importance are known to form by way of an underpotential co-deposition mechanism. The abnormal composition-potential relationship observed in Cu-Zn alloys deposited from cyanide-based electrolytes, one of the most widely used commercial alloy plating processes, is attributed to the underpotential co-deposition of Zn [64]. The UPD of Zn is also known to occur on Co and Fe and has been included in treatments focusing on the anomalous co-deposition of Co-Zn [65] and Ni-Zn alloys [66-68]. Alloys of Cu-Cd have been shown to incorporate Cd at underpotentials when deposited from ethylene diamine solution [69-71]. 

An empirical treatment developed by Kolb et al. [81, 82] relating UPD behavior to the difference in work function between the substrate and depositing species has been used to explain anomalous co-deposition behavior observed in Ni-Fe and Ni-Zn alloys [83]. Although the relationship appears to hold for pure underpotential deposition limited to a monolayer, it does not satisfactorily predict bulk alloy behavior. For example, based on work function data alone, one would expect Zn-Al and Sb-Al alloys to be formed by underpotential alloy deposition. Recent reports in the literature, however, indicate that alloying in these systems does not occur [46, 84]. 

It should not be surprising that an ion such as WO4 cannot be reduced readily all the way to metallic tungsten. Indeed, it is surprising that there are certain conditions under which it can be reduced. Moreover, alloy deposition is often a complex, and quite unpredictable, process. In what is called anomalous deposition we classify processes that behave imexpectedly - the composition of the alloy cannot be predicted from the current-potential relation of the alloying elements studied each by itself. When forming a Ni-Fe alloy it seems that Fe + ions in solution inhibit the rate of deposition of nickel, while NP ions accelerate the rate of deposition of iron. In the deposition of a Ni-Zn alloy the situation is somewhat different. Here, one finds a complete synergistic effect adding either ion to the solution enhances the rate of deposition of the other metal. 

This mechanism is based on the known importance of hydroxides in other deposition reactions, such as the anomalous codeposition of ferrous metal alloys [38-39], Salvago and Cavallotti claim an analogy with the mechanism of Ni2 + reduction from colloids in support of their proposed mechanism. There is no direct evidence for the hydrolyzed species, however. Furthermore, the mechanism does not explain two experimentally observed facts Ni deposition will proceed if the Ni2 + and the reducing agent are in separate compartments of a cell [36, 37] and P is not deposited in the absence of Ni2 +. The chemical mechanism does not take adequate account of the role of the surface state in catalysis of the reaction. It has no doubt been the extreme oversimplification, by some, of the electrochemical mechanism that has led other investigators to reject it. 

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. 

A hydroxide suppression model first proposed by Dahms and Croll (2) explains anomalous codeposition behavior of zinc-iron group alloys. This explanation was later supported by a number of workers (3) who measured a rise in pH near the cathode surface during the deposition of Zn-Co alloy. In this model it was assumed that the Zn(OH)2 was formed during deposition as a consequence of hydrogen evolution, thus raising pH in the vicinity of the cathode. Zinc would deposit via the Zn(OH)2 layer, while cobalt deposition took place by discharge of Co2+ ions... 

The electrodeposition of Zn-Co and Zn-Fe alloys in an aqueous bath is classified as an anomalous codeposition [44] because the less noble Zn is preferentially deposited with respect to the more noble metal. This anomaly was attributed to the formation of Zn(OH)+ which adsorbs preferentially on the electrode surface and inhibits the effective deposition of the more noble metal. This anomaly was circumvented by using zinc chloride-n-butylpyridinium chloride ([BP]+C1 / ZnCf ) [27] or [EMIMJ+Ch/ZnCh [28] ionic liquids containing Co(II). The Zn-Co deposits can be varied from Co-rich to Zn-rich by decreasing the deposition potential or increasing the deposition current. XRD measurement reveals the presence of CosZ i in the deposited Zn-Co alloys and that the Co-rich alloys are amorphous and the crystalline nature of the electrodeposits increases as the Zn content of the alloys increases. Addition of propylene carbonate cosolvent to the ionic liquid decreases the melting temperature of the solution and allows the electrodeposition to be performed at a lower temperature. The presence of CoZn alloy is evidenced by the XRD patterns shown in Figure 5.2. 

In anomalous deposition of alloys the opposite equation is valid... 

A very interesting and technologically important case of anomalous deposition is the electrodeposition of Permalloy, Fe—Ni alloy (see Sect. 3.8). Glasstone and Symes [65] found that in the deposition of Fe—Ni alloys from solutions containing more than about 20% of the total metal as Fe, the initial deposit contains relatively more Ni than the electrolyte. 

In the electrodeposition of alloys from the iron group of metals, anomalous deposition is observed, i.e., the more electronegative metal tends to be deposited preferentially. In the autocatalytic deposition this behavior is not observed. 

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... 

Figure 8.2 Anomalous deposition of zinc in zinc-nickel alloys. (Reproduced with permission from Ref. [6], 1994, Elsevier.)...

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