Alloys deposition potential

Note that essentially the same behavior as for the Ni-P alloy deposition was observed in electrodeposition of other iron-group alloys, such as Co-W and Ni-W alloys. Namely, the deposition current in the presence of Na2W04 (the W-source of the Co-W and Ni-W alloys) started to flow at a more positive potential than in the absence of Na2W04, indicating that the electrodeposition of the Co-W and Ni-W alloys occurs by essentially the same mechanism as that of the Ni-P alloy, suggesting the presence of a general mechanism for the induced co-deposition of these alloys. 

As mentioned above, the Ni-P (Co-W, Ni-W) alloy deposition current in the presence of the P-source (W-source) starts to flow at a more positive potential than in... 

Thus, co-deposition of silver and copper can take place only when the silver concentration in the electrolyte falls to a very low level. This clearly indicates that the electrolytic process can, instead, be used for separating copper from silver. When both silver and copper ions are present, the initial deposition will mainly be of silver and the deposition of copper will take place only when the concentration of silver becomes very low. Another example worth considering here is the co-deposition of copper and zinc. Under normal conditions, the co-deposition of copper and zinc from an electrolyte containing copper and zinc sulfates is not feasible because of the large difference in the electrode potentials. If, however, an excess of alkali cyanides is added to the solution, both the metals form complex cyanides the cuprocyanide complex is much more stable than the zinc cyanide complex and thus the concentration of the free copper ions available for deposition is considerably reduced. As a result of this, the deposition potentials for copper and zinc become very close and their co-deposition can take place to form alloys. 

As mentioned earlier, the current efficiency also depends on the presence of additives and/or of impurities which may co-deposit or may influence the electrochemical reaction or may affect the overvoltages of the desirable and the undesirable reactions. The impurities which are more noble would be deposited this would not only contaminate the metal but would also consume charge for undesirable reactions. Additives may be deliberately added when depositing alloys, so that the deposition potentials of the different metals involved could be brought closer however, in most other cases these are considered as harmful impurities. The electrolyte, therefore, needs to be purified with respect to such impurities in order to improve the current efficiency. 

The electrodeposition of alloys at potentials positive of the reversible potential of the less noble species has been observed in several binary alloy systems. This shift in the deposition potential of the less noble species has been attributed to the decrease in free energy accompanying the formation of solid solutions and/or intermetallic compounds [61, 62], Co-deposition of this type is often called underpotential alloy deposition to distinguish it from the classical phenomenon of underpotential deposition (UPD) of monolayers onto metal surfaces [63],... 

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]. 

Focusing on the deposition potential of species A, the activity of species A in the alloy, aA, can be expressed as... 

If it is assumed that the charge transfer kinetics for the deposition of A are independent of the substrate composition, i.e., rjA is constant with respect to the alloy composition, then the potential shift in the deposition potential of species A due to the deposition of an A-B alloy is simply... 

Fig. 7. Alloy composition versus (a) the shift in the aluminum deposition potential, AEai and (b) the shift in the copper/zinc deposition potential Ai cu,Zn, for the deposition of fee Cu-Al and hep Zn-Al alloys, calculated by using Eq. (12) and Eq. (13) and the free energy curves from Figure 6. Reproduced from Stafford et al. [104] by permission of The Electrochemical Society.

Fig. 8. Experimentally observed co-deposition potentials versus the calculated A ai values for alloys having a composition of 1 a/o Al. Adapted from Stafford et al. [104] by permission of The Electrochemical Society.

Cr-Al, Mn-Al, and Ti-Al alloys can be obtained from acidic melt solutions containing Cr(II), Mn(II), or Ti(II), respectively, only if the deposition potential is held very close to or slightly negative of the thermodynamic potential for the electrodeposition of aluminum, i.e., 0 V. From these observations it can be concluded that the formal potentials of the Cr(II)/Cr, Mn(II)/Mn, and Ti(II)/Ti couples may be equal to or less than E0 for the A1(III)/A1 couple. Unlike the Ag-Al, Co-Al, Cu-Al, Fe-Al, and Ni-Al alloys discussed above, bulk electrodeposits of Cr-Al, Mn-Al, and Ti-Al that contain substantial amounts of A1 can often be prepared because problems associated with the thermodynamic instability of these alloys in the plating solution are absent. The details of each of the alloy systems are discussed below. 

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... 

The book is divided into 18 chapters, presented in a logical and practical order as follows. After a brief introduction (Chapter 1) comes the discussion of ionic solutions (Chapter 2), followed by the subjects of metal surfaces (Chapter 3) and metal solution interphases (Chapter 4). Electrode potential, deposition kinetics, and thin-fihn nucleation are the themes of the next three chapters (5-7). Next come electroless and displacement-type depositions (Chapter 8 and 9), followed by the chapters dealing with the effects of additives and the science and technology of alloy deposition... 

In ZnCl2-EMIC (1 1) melt containing Cu(I), the electrodeposition of Cu-Zn alloys on tungsten and nickel electrodes was carried out [180]. The composition of the Cu-Zn deposit was changed by deposition potential, temperature, and Cu(I) concentration in a plating bath. 

The Zn-Cd alloys were obtained from ZnCl2-EMIC melt containing Cd(II), during reduction on Pt, Ni, or W electrodes at the potentials sufficient to reduce Zn(II) to metal [183]. The composition of Zn-Cd alloys was dependent on the deposition potential, temperature, and Cd(II) concentration. 

Also, the Pt-Zn electrodeposition on tungsten electrode was studied in ZnCh -EMIC melts containing Pt(II) [185]. The morphology of Pt-Zn alloys is dependent on the deposition potential and Pt(II) concentration in the plating solution. Pt-Zn alloys were also obtained on the electrode surface when Zn( 11) was reduced on Pt electrode. 

Zn- Fe alloys were deposited and studied on the nickel electrode in ZnCl2-EMIC melts [188]. The composition of Zn-Fe alloy was also dependent on the deposition potential and Fe(II) concentration in the plating solution. 

The nse of complexation to allow codeposition of alloys is well known in electroplating. The best-known example is that of brass (Cu/Zn) plating, where cyanide, which is a stronger complex for Cu than it is for Zn, brings the deposition potentials of the two metals, originally far apart, to almost the same value. There is a direct connection between this effect and the equivalent one for CD. This arises from the fact that, for both CD and electrodeposition of alloys (we in-clnde mixed metal compounds in the term alloy), the effect of the complexant is to lower the concentration of free cations. For CD this affects the deposition throngh the solnbility product, while for electrodeposition it affects the deposition potential through the Nemst equation ... 

A photoelectric cell consists of a photosensitive surface (the metal or alloy) deposited inside an evacuated glass bulb fitted with a second metal electrode kept at a positive potential (Figure 2.3). Electrons emitted by the photosensitive surface called the photocathode will then be captured by the positive anode and a current will flow in an external circuit which connects the two electrodes. 

There are four types of fundamental subjects involved in the process represented by Eq. (1.1) (1) metal-solution interface as the locus of the deposition process, (2) kinetics and mechanism of the deposition process, (3) nucleation and growth processes of the metal lattice (M a[tice), and (4) structure and properties of the deposits. The material in this book is arranged according to these four fundamental issues. We start by considering the basic components of an electrochemical cell for deposition in the first three chapters. Chapter 2 treats water and ionic solutions Chapter 3, metal and metal surfaces and Chapter 4, the metal-solution interface. In Chapter 5 we discuss the potential difference across an interface. Chapter 6 contains presentation of the kinetics and mechanisms of electrodeposition. Nucleation and growth of thin films and formation of the bulk phase are treated in Chapter 7. Electroless deposition and deposition by displacement are the subject of Chapters 8 and 9, respectively. Chapter 10 contains discussion on the effects of additives in the deposition and nucleation and growth processes. Simultaneous deposition of two or more metals, alloy deposition, is discussed in Chapter 11. The manner in which... 

---