THE E MF AND GALVANIC SERIES

3.8 THE E MF AND GALVANIC SERIES

The Emf Series is an orderly arrangement of the standard potentials for all metals. The more negative values correspond to the more reactive metals (Table

3.2 ). Position in the Emf Series is determined by the equilibrium potential of a metal in contact with its ions at a concentration equal to unit activity. Of two metals composing a cell, the anode is the more active metal in the Emf Series, provided that the ion activities in equilibrium are both unity. Since unit activity corresponds in some cases to impossible concentrations of metal ions because of restricted solubility of metal salts, the Emf Series has only limited use for predict- ing which metal is anodic to another.

Tin, for example, is noble to iron according to the Emf Series. This is also the normal galvanic relationship of tin to iron in tinplate exposed to aerated aqueous media. But on the inside of tin - plated iron containers ( “ tin cans ” ) in contact with food, certain constituents of foods combine chemically with Sn 2+ ions to form soluble tin complexes. Reactions of this kind greatly lower the activity of Sn 2+ ions with which the tin is in equilibrium, causing the potential of tin to become much more active, perhaps even more active than iron. The polarity of the iron –

tin couple under these conditions reverses sign. The ratio of Sn 2+ to Fe 2+ within the can must be very small for the reversal of polarity to occur, as can be calcu- lated from φ ° values for iron and tin in accord with the following reaction:

THE EMF AND GALVANIC SERIES

T A B L E 3.2. Electromotive Force (Emf) Series

Electrode Reaction Standard Potential, φ °, in volts at 25 ° C

0.800 Hg 2 2 + + 2 e −

− + e = Ag

= 2 Hg Cu 0.789 − + e = Cu 0.521 Cu 2+

0.342 2H +

+ 2e − = Cu

+ 2e − = H 2 0.000 Pb 2+

Sn 2 + + Sn → + Fe (3.29) and

Fe 2 +

E ( Sn + ) = . 0 136 0 440 − . − log

2 ( Fe 2 +

32 THERMODYNAMICS: CORROSION TENDENCY AND ELEC TRODE POTENTIALS

The cell reverses polarity when E = 0. Hence,

or, the ratio (Sn 2+ )/(Fe 2+ ) must be less than 5 × 10 − 11 for tin to become active to iron. This small ratio can occur only through formation of tin complexes resulting from certain organic substances in foods. Complexing agents in general, such as EDTA, cyanides, and strong alkalies, tend to increase the corrosion rates of many metals by reducing the metal - ion activity, thereby shifting metal potentials mark- edly in the active direction.

Another factor that alters the galvanic position of some metals is the ten- dency, especially in oxidizing environments, to form specifi c surface fi lms. These fi lms shift the measured potential in the noble direction. In this state, the metal is said to be passive (see Chapter 6 ). Hence, chromium, although normally near zinc in the EMF Series, behaves galvanically more like silver in many air - satu- rated aqueous solutions because of a passive fi lm that forms over its surface. The metal acts like an oxygen electrode instead of like chromium; hence, when coupled with iron, chromium becomes the cathode and current fl ow accelerates the corrosion of iron. In the active state (e.g., in hydrochloric acid), the reverse polarity occurs; that is, chromium becomes anodic to iron. Many metals, especially the transition metals of the periodic table, commonly exhibit passivity in aerated aqueous solutions.

Because of the limitations of the Emf Series for predicting galvanic relations, and also because alloys are not included, the Galvanic Series has been developed. The Galvanic Series is an arrangement of metals and alloys in accord with their actual measured potentials in a given environment. The potentials that determine the position of a metal in the Galvanic Series may include steady - state values in addition to truly reversible values; hence, alloys and passive metals are included. The Galvanic Series for metals in seawater is given in Fig. 3.3 . Some metals occupy two positions in the Galvanic Series, depending on whether they are active or passive, whereas in the Emf Series only the active positions are possible, since only in this state is true equilibrium attained. The passive state, on the con- trary, represents a nonequilibrium state in which the metal, because of surface fi lms, is no longer in normal equilibrium with its ions. Although only one Emf Series exists, there can be several Galvanic Series because of differing complexing tendencies of various environments and differences in tendency to form surface fi lms. In general, therefore, a specifi c Galvanic Series exists for each environment, and the relative positions of metals in such Series may vary from one environ- ment to another.

The damage incurred by coupling two metals depends not only on how far apart they are in the Galvanic Series (potential difference), but also on their rela- tive areas and the extent to which they are polarized (see Chapter 5 ). The poten- tial difference of the polarized electrodes and the conductivity of the corrosive environment determine how much current fl ows between them.

LIQUID JUNC TION POTENTIALS

Figure 3.3. Galvanic series in seawater [5] . (Reprinted with permission of ASM International ® . All rights reserved. www.asminternational.org .)