Overvoltage values

Cathode material the hydrogen overvoltage value varies from one cathode material to another. It can be quite high at certain electrodes made of lead and tin. 

Should there be no interaction of the deposited metals (i. e. should no alloy be formed), a simultaneous deposition of different metals is possible only when their deposition potentials are equal. As these deposition potentials are determined by the sum of reversible (reduction) potentials and of the corresponding overvoltage value [Pg.151]

The peroxide ion 0 appears only on the electrode surface, as a reaction intermediate, and is transformed either in oxygen or oxide ion. For a given overvoltage value, n, the total current I is caused for one half by reaction (8-1) and for the second one, by reaction (8-2). If and I e... 

Each overvoltage value Avill behave in an Ohmic manner. Yet Figure 2.12, which uses the experimental readings taken by [LEF 11], proves that the value of the equivalent resistances will be able to vary to a greater or lesser extent with the current. 

The polarization curves are S-shaped with a linear region at overvoltage values lower than ca, 120 mV. The linear part is narrower, the higher the chloride ion activity. Concomitantly, the steady-state Tafel slope increases from 30 mV dec" at [Cl"] < lO M to about 60 mV dec" for [Cl"] = 2M, as the product acf an increases(see Fig. 27). At higher overvoltage, the potential of unpolarizability (or the current of unpolarizability) is reached, and the current increases sharply (Fig. 28). 

Overvoltage. Overvoltage (ti. ) arises from kinetic limitations or from the inherent rate (be it slow or fast) of the electrode reaction on a given substrate. The magnitude of this value can be generally expressed in the form of the Tafel equation... 

An interesting field of application is the protection of tantalum against hydrogen embrittlement by electrical connection to platinum metals. The reduction in hydrogen overvoltage and the shift of the free corrosion potential to more positive values apparently leads to a reduced coverage by adsorbed hydrogen and thereby lower absorption [43] (see Sections 2.1 and 2.3.4). 

Other types of polarization are caused by specific features in the various steps of the electrochemical reaction that produce a potential shift relative to the effective equilibrium potential (i.e., that which already accounts for the prevailing values of surface concentrations). These types of polarization, which may differ in character, are jointfy termed activation polarization. The value of activation polarization is sometimes called the overvoltage (this term should be reserved for the complete ceU see Section 2.5.2). 

N.B. The values for zero current density are for lowest c.d at which bubble formation could just be observed, usually about 5 10 5 amp cnT2. At higher current density the overvoltage increases. 

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