Metal deposition overvoltage

The low current efficiency of this process results from the evolution of hydrogen at the cathode. This occurs because the hydrogen deposition overvoltage on chromium is significantly more positive than that at which chromous ion deposition would be expected to commence. Hydrogen evolution at the cathode surface also increases the pH of the catholyte beyond 4, which may result in the precipitation of Cr(OH)2 and Cr(OH)2, causing a partial passivation of the cathode and a reduction in current efficiency. The latter is also inherently low, as six electrons are required to reduce hexavalent ions to chromium metal. 

The discussion of concentration polarization so far has centred on the depletion of electroactive material on the electrolyte side of the interface. If the metal deposition and dissolution processes involve metastable active surface atoms, then the rate of formation or disappearance of these may be the critical factor in the overall electrode kinetics. Equation (2.69) can be rewritten for crystallization overvoltage as... 

Reaction overvoltage is due to secondary reactions of, e.g., the electrolysis products (for example, the association of gaseous atoms into molecules, the escape of gas bubbles, the formation of a crystalline lattice in the case of metal deposition, also called crystallization overvoltage). 

The combination of the reversible potential with the overvoltage in each case results in the deposition of hydrogen together with iron, cobalt or nickel. Since it is desired to suppress the hydrogen evolution as much as possible, in order to increase the efficiency of metal deposition, the pH of the solution is raised. There is, however, a limit to this increase because of the danger of the precipitation of basic salts. 

Separation of Metals by Electrolysis.—The complete separation of one metal from another is important in quantitative electro-analysis the circumstances in which such separation is possible can be readily understood from the preceding discussion of simultaneous deposition of two metals. The conditions must be adjusted so that the discharge potentials of the various cations in the solution are appreciably different. If the standard potentials differ sufficiently and there are no considerable deposition overvoltages, complete separation within the limits of analytical accuracy is possible this is, of course, contingent upon the metals not forming compounds or solid solutions under the conditions of deposition. Since the concentration of the ions of a deposited metal decreases during electrolysis, the deposition potential becomes steadily more cathodic, and may eventually approach that for the deposition of another metal. For example, if the ionic concentration is reduced to 0.1 per cent of its original value, the potential becomes 3 X 0.0295 volt more cathodic for a bivalent metal and 3 X 0.059 volt for a univalent metal, at ordinary... 

This is a general form of the well-known Gibbs-Thomson (Lord Kelvin) equation applied to the case of electrochemical metal deposition. It gives the size of the critical nucleus and its equilibrium form in terms of the normal distances of the equilibrium form faces from the Wulff point, hi, as a function of the overvoltage. When this form is a regular polyhedron (cr,- = const.), the size of the nucleus can be given by the radius of the inscribed sphere, perit = h, so that... 

These benefits, evident from the introductory remarks, include wide limits of electrochemical stability, high ionic conductivity, good solvent properties for inorganic feedstocks, especially oxide ores, and relatively small cathodic deposition overvoltages. Two disadvantages occurring from these properties include the solubility of many refractory container materials and the frequent mass-transfer-controlled rate to the electrodeposition reaction. The former problem may be overcome in most cases, reverting to metallic vessels if necessary the latter problem will be discussed below. 

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