Zinc, overvoltage

Zinc and tin The electrodeposition of Zn [52] has been investigated in acidic chloroaluminate liquids on gold, platinum, tungsten, and glassy carbon. On glassy carbon only three-dimensional bulk deposition was observed, due to the metal s underpotential deposition behavior. At higher overvoltages, codeposition with A1... 

Cathode Nickel may be an alternative for platinum metals in alkaline solutions due to its low hydrogen overvoltage and catalytic activity. The activity is especially high at the very fine dispersed Raney nickel , which is available from a layer of a nickel alloy on the cathode surface by dissolving the alloy metal (aluminum or zinc) in alkaline solution prior to use (e.g. [23, 24]. Raney nickel usually is not stable against oxygen and self-ignition in air may be possible). 

SO that the concentration of [Zn ] under the same conditions will be 10 g-molecule/L. With these ionic concentrations, the deposition potentials of copper and zinc in the absence of any polarization can each be calculated from Eq. (11.1) to be about —1.30 V. It should be mentioned here again that in practice, Eq. (11.1) refers to reversible equilibrium, a condition in which no net reaction takes place. In practice, electrode reactions are irreversible to an extent. This makes the potential of the anode more noble and the cathode potential less noble than their static potentials calculated from (11.1). The overvoltage is a measure of the degree of the irreversibility, and the electrode is said to be polarized or to exhibit overpotential hence, Eq. (11.2). 

The most important factor in electrolytic reduction (electroreduction) is the nature of the metal used as a cathode. Metals of low overvoltage - platinum (0.005-0.09 V), palladium, nickel and iron - give generally similar results of reduction as does catalytic hydrogenation [727]. Cathodes made of metals of high overvoltage such as copper (0.23 V), cadmium (0.48 V), lead (0.64 V), zinc (0.70 V) or mercury (0.78 V) produce similar results to those of dissolving metal reductions. 

The distance between the current curves of two metals may sometimes be controlled by temperature as the value of the overvoltage varies under its effect. The depositi m potentials of nickel and zinc on electrolysis of their salts in aqueous ammonia solution at 20 °C differ only slightly because the high overvoltage for nickel reduces the difference between the respective reversible potentials, thus making simultaneous deposition of both metals quite possible. On the other hand overvoltage for nickel at 90 °C is considerably lower so that the difference between the deposition potentials increases and simultaneous deposition of nickel and zinc is no l nger possible. 

The mode of attack on metals described above is referred to as the hydrogen evolution type of corrosion, since gas is actually evolved at the more noble portion of the system. This type of corrosion requires a combination of two metals, the nobler one of w4iich has a low overvoltage, but it is not necessary that the metals should be in a massive form. For example, the addition of a small quantity of copper sulfate to an acid solution greatly expedites the rate at w hich zinc dissolves copper is deposited, by replacement, on various parts of the zinc and a large number of local short-circuited cells are set up. Another possibility, which frequently arise., is that the base metal should have in-... 

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