Polarization and Overvoltage

Dissolution and Deposition Potentials.—If a metal is placed in a solution of its ions a reversible electrode represented by M, is set up suppose its potential is E, Imagine now that an external source of potential is applied to this electrode so as to make it an anode of an electrolytic cell (p. 8) this will have the effect of increasing the potential, and since the electrode is reversible it will immediately commence to dissolve (cf. p. 184). It follows, therefore, that when a metallic electrode is made an anode, it wnll begin to dissolve as soon as its potential exceeds the reversible value E by an infinitesimal amount. In other words, the electrolytic dissolution potential of a metal when made an anode should be equal to its reversible (oxidation) potential (cf. p. 243) in the given electrolyte. The actual value depends, of course, on the concentration, or activity, in the solution of the ions with respect to which the metal is reversible. On the other hand, if the particular electrode under consideration is made a cathode, so that its potential is reduced below the reversible value, the reverse process, viz., deposition 

Determination of Anode and Cathode Potentials.—Direct measurement of the potentials at which the various processes occur at a cathode or an anode at various current strengths can be made by means of the apparatus depicted in Fig. 109. The electrode A under examination is 

As the strength of the current is increased the potential is observed to diverge somewhat from the reversible value in the given solution typical results arc shown in Fig. 110 in which the potential of a cathode and an anode of the same metal are plotted as a function of the current density (c.d.). It is the general practice in electrolytic work to use the current density instead of the current strength the former is defined as 

1 For a general treatment, see Glasstone, School Science Review, 328 (1935). 

Variation of current density with cell voltage 

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Construct the cathodic and anodic polarization curves from the overvoltage curves in Exercise 5. How are polarization and overvoltage curves usually determined ... 

Passivation and Passivity Described by Anodic Polarization and Overvoltage Curves... 

Figure 5.1 Anodic polarization and overvoltage curve for iron in 1 N H2SO4, mainly based upon data from Franck, Franck and Weil, and Hersleb and EngeU, collected by Kaesche [5.1]. Overvoltage curve for hydrogen reduction estimated from other data [5.2, 5.3].

Deseription of uniform corrosion by means of polarization and overvoltage curves is relatively simple, since we can consider both the anodic and the cathodie area equal to the total area. 

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