Conditions for Electrodeposition of Alloys

The metals immersed in the solution of their simple salts establish the reversible potential. The values of the reversible potentials for different metals could differ for about 3 V. Electrodeposition of metals could take place only at potentials more negative than the reversible ones. Accordingly, in the solutimi of ions of two metals (cf. Cu and Zn ) with one being on the positive side of the potential scale (vs. SHE) (Cu) and another one being on the negative side of the potential scale (Zn), intensive electrodeposition of Cu could take place at potentials at which Zn would not electrodeposit at aU. Taking into account that the reversible potentials of metals could change with the presence of different anions in the solution (com-plexation of metal ions) and that the rates of electrodeposition of different metals are usually different, it is possible to achieve conditions for simultaneous electrodeposition of these two metals [1,5]. 

For simultaneous electrodeposition of two metals, A and B, their electrodeposition potentials E) must be identical, (A) = (B), i.e.. 

The condition defined by Eq. (7.1) could be accepted only as a first approximation, since the potential of the metal electrodeposition is undefined quantity if the value of corresponding current density is not known. It appears that a better definition of the conditions for simultaneous electrodeposition of two metals would be current density (/) at which both metals electrodeposit with approximately the same current density. More precisely, for two-component alloy to be electrodeposited with the molar ratio of the more noble metal x and the less noble metal (l x), assuming that Faraday s law is obeyed, the following relations should be fulfilled  

The condition defined by Eq. (7.6) could be achieved by proper adjustment of three essential variables the concentration of the electrodepositing ions at the electrode/solution interface (where the discharge occurs), the electrode potential, and the temperature [1,5]. 

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. 

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