160 9.Degradation of Materials (Corrosion) shrinks grows Fe Cu FIGURE 9.3.Schematic representation of Fe2+solution two electrochemical half-cells in which Cu2+solution two metal electrodes are immersed in Anode Cathode 1-M solutions of their ions (Galvanic couple). Membrane that contains 1-M Fe2+ions.The other part consists of an analo- gous copper half-cell.The two metal electrodes are electrically con- nected through a voltmeter for reasons that will become obvious momentarily.After some time one observes that the iron bar has slightly decreased in size,that is,some of the solid Fe atoms have transferred as Fe2+into the solution.On the other hand,some copper ions have been electroplated onto the copper electrode, which,as a consequence,has increased somewhat in size.The per- tinent reaction equations are thus as follows: Fe→Fe2++2e- (9.8) Cu2++2e-→Cu. (9.9) During the entire process,a voltage (called electromotive force, or emf)of 0.78 V is measured.One concludes from this experi- mental result that copper is more noble than iron.Indeed,a table can be created which ranks all metals (or their electrode reac- tions)in decreasing order of inertness to corrosion along with their emfs compared to a reference for which the hydrogen re- action has been arbitrarily chosen;see Table 9.1.(The standard hydrogen reference cell consists of an inert platinum electrode that is "immersed"in 1 atm of flowing hydrogen gas at 25C.) The electromotive forces,Eo,listed in Table 9.1 are valid for 1-M solutions and for room temperature(25C)only.For other conditions,the potential of a system needs to be adjusted by mak- ing use of the Nernst equation: +In C+00392 og CV). (9.10) nFthat contains 1-M Fe2 ions. The other part consists of an analo￾gous copper half-cell. The two metal electrodes are electrically con￾nected through a voltmeter for reasons that will become obvious momentarily. After some time one observes that the iron bar has slightly decreased in size, that is, some of the solid Fe atoms have transferred as Fe2 into the solution. On the other hand, some copper ions have been electroplated onto the copper electrode, which, as a consequence, has increased somewhat in size. The per￾tinent reaction equations are thus as follows: Fe
Fe2 2e (9.8) Cu2 2e
Cu. (9.9) During the entire process, a voltage (called electromotive force, or emf) of 0.78 V is measured. One concludes from this experi￾mental result that copper is more noble than iron. Indeed, a table can be created which ranks all metals (or their electrode reac￾tions) in decreasing order of inertness to corrosion along with their emf’s compared to a reference for which the hydrogen re￾action has been arbitrarily chosen; see Table 9.1. (The standard hydrogen reference cell consists of an inert platinum electrode that is “immersed” in 1 atm of flowing hydrogen gas at 25°C.) The electromotive forces, E0, listed in Table 9.1 are valid for 1-M solutions and for room temperature (25°C) only. For other conditions, the potential of a system needs to be adjusted by mak￾ing use of the Nernst equation: E
E0 R nF T ln Cion
E0 0.0 n 592 log Cion[V], (9.10) 160 9 • Degradation of Materials (Corrosion) Fe Cu shrinks grows Fe2+ solution Cu2+ solution Anode Cathode Membrane e– e– FIGURE 9.3. Schematic representation of two electrochemical half-cells in which two metal electrodes are immersed in 1-M solutions of their ions (Galvanic couple).