Codeposition, alloys

The development of new alloys as a means of modifying existing electrodeposits for example the production of hard gold by alloy codeposition of copper, cadmium etc. to yield 23 or 18 carat alloys, or the use of zinc alloys for improved electrogalvanised coatings. 

As is evident from Eq. (11.4), copper and zinc are very far apart in the standard EMF series, so alloy codeposition seems next to impossible. Fortunately, the difference can be eliminated (even reversed) by changing the values of the activities. This can be achieved by inducing a considerable change in ionic concentrations via complex ion formation, as discussed in detail below. 

Crousier et al. examined the role of hydrogen evolution in the process of deposition of Mo-Ni alloys on different substrates (glassy carbon, Ni and Pd). It was found that on carbon and Ni substrates, bright and smooth deposits were formed, while on Pd no alloy was formed. This observation was related to easy absorption and diffusion of atomic hydrogen into Pd, which prevented its availability for the alloy codeposition process. Hence, it was concluded that hydrogen plays an important role in the codeposition of the alloy. This conclusion of the authors is, however, not convincing. Firstly, it is known that hydrogen atoms can also permeate into Ni to some extent. Secondly, unsuccessful attempt to deposit Mo-Ni alloys on Pd may also be attributed, for example, to kinetic limitations. 

Multiple Simultaneous Electrode Reactions, Including Alloy Codeposition and Gas Coevolution... 

Composites. Another type of electro deposit in commercial use is the composite form, in which insoluble materials are codeposited along with the electro-deposited metal or alloy to produce particular desirable properties. Polytetrafluoroethylene (PTFE) particles are codeposited with nickel to improve lubricity (see Lubrication and lubricants). SiHcon carbide and other hard particles including diamond are co-deposited with nickel to improve wear properties or to make cutting and grinding tools (see Carbides Tool materials). 

A1 is more noble than Ti, and so at room temperature only codeposits and alloys can be obtained. Furthermore, kinetic factors also play a role in the electrodeposition of the element. 

High strength structural parts when alloyed with small amounts of boron (0.05-0.2 wt.%) produced by codeposition fromNi(CO)4 andB2Hg.l l... 

In addition to these different types of alloys, some studies were also devoted to alternatives to platinum as electrocatalysts. Unfortunately, it is clear that even if some catalytic activities were observed, they are far from those obtained with platinum. Nickel tungsten carbides were investigated, but the electrocatalytic activity recorded for methanol oxidation was very low. Tungsten carbide was also considered as a possible alternative owing to its ability to catalyze the electrooxidation of hydrogen. However, it had no activity for the oxidation of methanol and recently some groups showed that a codeposit of Pt and WO3 led to an enhancement of the activity of platinum. ... 

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. 

The composition of the codeposition bath is defined not only by the concentration and type of electrolyte used for depositing the matrix metal, but also by the particle loading in suspension, the pH, the temperature, and the additives used. A variety of electrolytes have been used for the electrocodeposition process including simple metal sulfate or acidic metal sulfate baths to form a metal matrix of copper, iron, nickel, cobalt, or chromium, or their alloys. Deposition of a nickel matrix has also been conducted using a Watts bath which consists of nickel sulfate, nickel chloride and boric acid, and electrolyte baths based on nickel fluoborate or nickel sulfamate. Although many of the bath chemistries used provide high current efficiency, the effect of hydrogen evolution on electrocodeposition is not discussed in the literature. 

For practical as well as fundamental reasons, there has been considerable interest in the deposition of alloys containing metals such as W, Mo, and Sn. In their pure forms, these metals do not catalyze the oxidation of the usual electroless reducing agents. Therefore, their mechanism of codeposition is intriguing, and developing an understanding of it should help to better understand the mechanism of electroless deposition as whole. Obvious questions in ternary and quaternary alloy deposition include the effect of the third or fourth element containing ions in solution on the rate of electroless deposition, as well as on the P and B contents in the case of alloys such as Ni-P and Ni-B. 

A great number of measurements have been reported for articles electroplated with zinc. The various aims have been evaluation of the corrosion rate of zinc that had been plated in a number of commercial cyanide-free zinc baths," comparison of the corrosion rate of a composite material (zinc with codeposits of various oxides) and of pure zinc deposits," corrosion testing of various alloyed zinc platings (Zn-Ni, Zn-Co, Zn-Fe), with or without subsequent post-treatment. Most of the work in the last category was only recorded in internal reports. The published work consists of an examination of the corrosion behavior of a ctoomated Zn-Fe... 

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... 

It must be understood that in a case such as that illustrated in Figure 11.1, the plating bath is being depleted of metal B ions more quickly than of metal A ions. To keep matters under control (i.e., maintain uniform deposition conditions), metal ions must be replenished in direct proportion to their rates of dep>osition dictated by the specific alloy. It is clear, therefore, that ideally, the polarization curves of the competent metals being codeposited should be identical. It is next to impossible to reahze this condition in practice. 

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). 

An increase in current density tends to increase the proportion of the less noble metal in the alloy deposit. The extent of such change may be expected to be greater in the case of simple primary salt solutions than in complex primary salt solutions, and still more so when the codepositing metals are present in complex ions with a common anion than when the anions of the complex ions are different. In cases where the metals are associated with different complexing ions, a significant change in current density can be accommodated with relatively little change in plate composition. 

The same researchers [184] studied electrodeposition of Zn-Sn alloys on tungsten and GC electrodes from ZnCh-EMIC ionic liquid containing Sn(II). The Zn-Sn codeposits consist of two-phase mixtures of Zn and Sn. 

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 ... 

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