Electrodeposition concentration polarization

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). 

Kvokova and Lainer electrodeposited pure Re and Re-Cr alloy on Mo substrate. For the deposition of Re itself, two baths were used one containing perhhenic acid, and the other containing potassium perrhenate. In the first bath, the discharge of hydrogen ions was enhanced. The authors attributed the low overpotential of hydrogen on Re to its lattice parameter (a = 2.758 A). However, a justification to this theory has not been proposed. For both deposition of Re and Re-Cr alloy, the concentration polarization was foimd to be insignificant compared to the activation polarization. 

For the electrodeposition of laminar metal coatings, two conditions must be fulfilled (1) The reversible potentials for metals A and B must be sufficiently different so that at a given current density, the less noble one (B) virtually does not electrodeposit during the electrodeposition of the more noble one (A) until complete concentration polarization with respect to ions of metal A takes place (2) within the duration of the current density pulse, Send s equation [15] for diffusional polarization is obeyed with respect to concentration change, resulting in transition from electrodeposition of metal A to electrodeposition of metal B after well-defined transition time. 

The cathodic overpotential equation derived in Chapter 4 for concentration polarization is very useful in electrowinning operation. In analyzing the electrodeposition of metal MJ " cations from aqueous solution, the cathodic overpotential and current density for stationary and rotating-disk electrodes are related by... 

There are several mathematical expressions for polarization overpotential for various cases. In non steady-state electrochemical systems the mathematical manipulation of polarization overpotential is not trivial, so as in systems exhibiting chemical reactions preceding the electrochemical ones. For the relatively simple case of steady-state mass transfer, in which insoluble R is produced in the reaction O -i- ne < R (e.g., O is a metal ion being electrodeposited) the polarization overpotential is expressed as t/c = fp In ( ) where i/c is the concentration overpotential, R is the -> gas constant, n the number of electrons that pass in the electrochemical reaction, F is -> Faraday constant, j is current density, and f is limiting current density. For small potential deviation from equilibrium, namely, for small currents this expression can be simplified by Taylor expansion to t]c =... 

The thickness distribution of electrodeposits depends on the current distribution over the cathode, which determines the local current density on the surface. The current distribution is determined by the geometrical characteristics of the electrodes and the cell, the polarization at the electrode surface, and the mass transfer in the electrolyte. The primary current distribution depends only on the current and resistance of the electrolyte on the path from anode to cathode. The reaction overpotential (activation overpotential) and the concentration overpotential (diffusion overpotential) are neglected. The secondary... 

The effect of ultrasound on the process of tellurium anodic dissolution in alkaline solutions was studied by the method of plotting polarization and galvanostatic curves [148]. Tests were made in NaOH solutions (concentrations of 0—20 g/L), subjected to the action of ultrasound at a frequency 17.5 kHz and using Te electrodeposited under ultrasound. The anodic polarization curves plotted without ultrasound and in its presence shifted with increased NaOH concentration towards negative values as a result of the increasing rate of Te anodic dissolution. The presence of ultrasound inhibited the process of Te anodic dissolution, probably due to the desorption of OFT anions from the anode surface. This sonoelectrodeposited Te thus showed greater corrosion resistance in alkaline solution than that deposited... 

A precipitation of insoluble salts on an electrode surface can be initiated and controlled by varying the concentration of metal ions. The most effective method is electrodeposition of mercuric and mercurous salts induced by an anodic polarization... 

The preconcentration of trace metals by electrodeposition is an integral part of anodic-stripping voltammetry. The method consists of the preelectrolysis of the stirred solution with a small mercury drop or solid electrode as the cathode (112-114). The metals, which are deposited and dissolve in the mercury, are then stripped from the amalgam after a suitable rest period by a reversal of the electrode potential. The resulting current-polarization curve is characteristic of the metal and its concentration. Concentrations as low as 10 M of metal ions require a preelectrolysis of about 60 min or longer. Other electrodes such as mercury films, platinum, gold, silver, and various forms of carbon have been used (77 ). 

The polarization curves for nickel, copper, and cadmium electrodeposition (in all solutions, concentrations of depositing ions were 0.070 M) are shown in Fig. 1.13, while corresponding Tafel plots and the results of linear polarization experiments... 

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. 

Figure 9A shows the potential dependence of the TEC transfer function measured with AgNO concentration of C = 0.01 mol/dml In addition to the mass transport loop and the space charge branch a middle frequency loop is visible in the positive part of the plan. Its amplitude is as much higher as polarization increased and completely disappears at the limiting current value where the process is under pure mass transport control. The middle frequency loop points out the previous results obtained during nickel electrodeposition... 

The polarization curves corrected for IR drop for the processes of pure cobalt (Co), pure nickel (Ni), and Co-Ni aUoy (powders) electrodeposition from ammonium chloride-ammonium hydroxide containing supporting electrolyte are presented in Fig. 5.3 (in this case the total concentration of cations in aU investigated solutions was 0.1 M). As can be seen, cobalt electrodeposition (Co) commences at about —1.1 V, while sharp increase of current density (massive Co... 

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