Reversible potentials alloys

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],... 

A simple calculation shows the effects of such a leak rate. Consider a 0.5 1L test solution volume having 4 pl./h of 4 M CE added (the KC1 concentration in a saturated calomel electrode). In a 24 h test the initial CE-free solution will develop a concentration of 7.7 X 10 5 M CE. Although low, this concentration will lead to pitting in many alloys (3), confounding interpretation of the results. A second effect of filling solution leakage into the bulk solution involves the cations that are released. In the case of the SCE, there is a small concentration of Hg2+ that makes its way into the bulk. Once in the bulk solution, the mercurous ions can be deposited electrochemically onto any surface at a potential below the reversible potential for Hg deposition. This deposition of metallic mercury can cause dramatic changes in the surface behavior. 

Simultaneous Discharge of Cations.—If a solution contains two cations, there is a possibility that simultaneous discharge may occur this problem is not only of interest in connection with the electrodeposition of alloys, but it is important in the deposition of single metals, since aqueous solutions always contain hydrogen ions. Were it not for a variety of complicating factors, such as the influence of one metal on the deposition potential of the other, the situation would, in principle, be relatively simple provided the discharge (reduction) potentials of the two ions were the same, simultaneous deposition would occur. For example, the reversible potential of a metal A in a solution of its ions of activity oa+, i.e., of the electrode A, A+, would be given by... 

Depolarization of metal deposition sometimes occurs when two metals which separate simultaneously form compounds or solid solutions. The reversible potential of a solid solution generally lies in between those of the pure constituents hence an alloy containing both metals may be deposited at a potential that is less cathodic than that necessary for the less noble constituent in the pure state. This probably accounts for the fact that zinc and nickel are deposited simultaneously at a potential of about — 0.6 volt, whereas that required for pure zinc is nearly 0.2 volt more cathodic. The simultaneous deposition of the iron-group metals is partly due to the similarity of the discharge potentials, but the formation of solid solutions also plays an important part. Although the deposition potentials of cobalt and nickel are lower than that of iron, the cathodic deposit almost invariably contains relatively more of the latter metal. ... 

Electrocatalysis is manifested when it is found that the electrochemical rate constant, for an electrode process, standardized with respect to some reference potential (often the thermodynamic reversible potential for the same process) depends on the chemical nature of the electrode metal, the physical state of the electrode surface, the crystal orientation of single-crystal surfaces, or, for example, alloying effects. Also, the reaction mechanism and selectivity 4) may be found to be dependent on the above factors in special cases, for a given reactant, even the reaction pathway [4), for instance, in electrochemical reduction of ketones or alkyl halides, or electrochemical oxidation of aliphatic acids (the Kolbe and Hofer-Moest reactions), may depend on those factors. 

The reversible potential of this reaction is 1.229 volts (RHE). In this region of potential most metals dissolve or form ionically conducting oxides. Thus, it is possible to study the reaction only on noble metals and their alloys. However, it should be possible to investigate the mechanism of this reaction on nonmetals (e.g., semiconducting oxides). 

Most of the work on mechanism determinations has been on noble metals and their alloys. Even on these substrates thin oxide films form at potentials cathodic to the reversible potential of the over-all reaction and, in essence, the oxygen evolution reaction occurs always on oxide-covered surfaces. However, the corresponding cathodic reaction can occur both on oxide-covered or bare surfaces. 

Fig. 7.1b the oveipotential for electrodeposition of metal A is slightly lower than that for metal B, i.e., the polarization curves are almost parallel. Hence, the electrodeposition of alloy commences at the potential r(B)> while the alloy contains more metal A than B. If the difference between r(A) and r(B) is high and the overpotential for electrodeposition of the more noble metal A is lower than that for the less noble metal B, the third case presented in Fig. 7.1c applies in such a case, alloy electrodeposition is impossible. The difference between the reversible potentials of two metals could be changed (lowered) by the change of metal ion concentration (activity), and in most cases, this is achieved by the complexation. 

Simultaneous electrodeposition of two metals is possible even if the difference in their reversible potentials is high, if the appUed current density for alloy electrodeposition is higher than the diffusion limiting ciurent density for the electrodeposition of the more noble metal. Such a case is schematically presented in Fig. 7.2. 

If the difference between the reversible potentials of metals A and B is sufficient, and the constituents of the alloy mix in the solid state forming solid solution and a metal B passivates in the electrolyte used (case b), replacement reaction will not take place during the off-time (/ = 0). Such a case is schematically presented in Fig. 7.37. The current density change is presented in (a), while corresponding potential change is presented in (b). During the current density pulse, everything is the same as in a previous case. The absence of replacement reaction is... 

The polarization curves corrected for IR drop for the processes of Fe, Ni, and Fe-Ni alloy powder electrodeposition from ammonium chloride-sodium citrate containing supporting electrolyte in the presence of Fe(ll) and Ni(II) species are shown in Fig. 8.14. In the case of Fe(II) salts, polarization curve for iron electrodeposition (Fe) was placed at more positive potentials than that for nickel (Ni) as it is expected from the values of their reversible potentials. The polarization curves for Fe-Ni alloy powder electrodeposition are placed in between, and all of them were placed at more positive potentials than expected from the Ni/Fe ratio, indicating anomalous codeposition. 

The situation becomes more complex in the case of an alloy in which the apphed potential is above the reversible potential for the most reactive component but below the reversible potential of the others. In that case, selective dissolution of the more reactive component occurs, but problems arise with the dissolution description provided in Figure 4.2a. As seen in Figure 4.2b, once the dissolntion of the more reactive A atoms at kink sites uncovers a noble B atom, that reaction site is shut down. In the same manner, even if reactive A atoms are dissolved from the step edge faces or terraces occurs, eventually, the surface becomes enriched in B atoms and dissolntion stops. As this does not occnr—as seen in Figure 4.1— the dissolution description provided in Figure 4.2 is lacking. 

The metal sample on which the KMC algorithm operates can comprise a single or multiple components with one of the components having a substantially lower reversible potential than the others. Table 4.2 provides a list of reversible potentials for some oxidation-reduction reactions focused but are available in the literature. In general, while dissolution will proceed if an externally applied potential or local galvanic couple drives the potential of the sample above its reversible potential, selective dissolution will in general occur only for potentials that fall within the gap between the reversible potentials for the components of the alloy. 

If only ions of the less noble component are present, the reversible potential of the alloy electrode will be determined basically by the free-energy change of the less noble component in the alloy. 

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