Spongy-deposits

Ionic Displacement. Electromotive Series. When a strip of zinc is placed in a solution of copper sulphate, it is noticed that a spongy deposit of copper metal soon appears on the surface of the zinc, and that the solution loses its blue color. Then if the solution is tested for the presence of copper and zinc ions by adding ammonium sulphide, it is found that this reagent gives a white precipitate. This test shows that copper ions are now absent and that zinc ions are present because we know ammonium sulphide will precipitate black copper sulphide from a solution of copper ions, and white zinc sulphide from a solution of zinc ions. Since ordinary pieces of metal are not charged, it is obvious that the reaction consists in a transfer of the positive charges of the copper ions to the zinc atoms, or, more strictly, of negative electrons from the zinc atoms to the copper ions ... 

There are two conditions under which the particles of active metal placed on the surface of a macroelectrode can represent microelectrodes. The first condition is that the substrate is conducting but inert relative to the process under consideration. The second one is that the grains are sufficiently small to permit activation control of the electrochemical process on them, making thus, mixed overall control, as in the case of the tips of growing dendrites,11,12,31 or during the induction time of the formation of spongy deposits.32-34... 

When the Cd in the electroljde is depleted to such an extent that Hs begins to evolve at the cathode, the solution must be replenished with CdS04 to avoid formation of a spongy deposit (the latter also appears at excessive current densities). 

On the basis of all the above facts, it can be concluded that the formation of a spongy deposit on an inert substrate may be caused by mass-transport limitations when the nucleation rate is low. Hence, suitable conditions for the formation of spongy deposits arise at low overpotentials in systems where /l < io U, 54]. 

As was mentioned before, at a fixed value of the overpotential, the growth of a spongy deposit is possible if condition given by Eq. (2.128) is fulfilled. 

It should be noted that some other possible mechanisms of spongy deposit formation have been considered in a qualitative way, as reviewed in Refs. [114, 115], but the mechanism presented above seems to be the most probable [115]. However, the mechanism of formation of a spongy deposit over an initial coating, which is not seen in the case of cadmium but occurs in zinc deposition, requires clarification [54, 113]. For instance, the mechanism of spongy growth initiation in this case has not been elucidated. 

The nucleus of spongy deposit, i.e., hedgehog-like particle, appears when amplification of surface coarseness on the nucleus in spherical diffusion control growing starts. It was shown earlier [118] that this amplification is very fast so the induction time when growing nucleus enters mixed control can be taken also as induction time of spongy formation. It follows from Eqs. (2.139) and (2.140) that and decrease with increasing overpotential. 

On the other hand, it was also shown [54] that spongy deposit can be formed only if around each grain with radius r y, growing under spherical diffusion control, a diffusion layer of the same thickness is formed, as illustrated earlier (Eq. (2.129)). 

At an overpotential of 300 mV, the conditions of the spherical diffusion control around the growing grains are fulfilled and dendritic-spongy deposit is formed, as can be seen from Fig. 2.44. 

Prevention of the Formation of Spongy Deposits and the Effect on Dendritic Particles... 

EPCR is used in the charging of silver-zinc storage batteries, to prevent, or to delay, the formation of spongy and dendritic deposits of zinc [20, 21]. It is impossible to obtain smooth deposits of zinc from alkaline zincate solutions during prolonged deposition at a constant rate due to formation of spongy deposits at lower and dendritic deposits at higher overpotentials [21, 22]. 

Spongy deposit formation can, however, be completely prevented by the PO deposition [21], as illustrated in Fig. 4.4. 

The increase in the number of nuclei formed with increasing deposition time can be seen in Fig. 1.27 a, b, and a spongy deposit is formed as can be seen in Fig. 1.27b. The spongy growth takes place on a relatively small number of nuclei, as is shown in Fig. 1.27b, c. 

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