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) nF
that contains 1-M Fe2 ions. The other part consists of an analogous copper half-cell. The two metal electrodes are electrically connected 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 pertinent 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 experimental result that copper is more noble than iron. Indeed, a table can be created which ranks all metals (or their electrode reactions) in decreasing order of inertness to corrosion along with their emf’s compared to a reference for which the hydrogen reaction 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 making 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).
9.2.Electrochemical Corrosion 161 TABLE 9.1.Standard emf series for selected elements in 1-M solution at 25C Electrode potential,or emf Electrode reaction Eo(V) Au3++3e-→Au +1.42 Cathodic Pt2++2e-→Pt +1.2 Ag++1e-→Ag +0.80 Cu2++2e-→Cu +0.34 2H++2e-→H2 0.000 Sn2++2e-→Sn -0.14 Ni2++2e-→Ni -0.25 Fe2++2e-→Fe -0.44 Cr3++3e-→Cr -0.74 Zn2++2e-→Zn -0.76 A13++3e-→Al -1.66 Mg2++2e-→Mg -2.36 Anodic (Oxidation) where R and F are gas and Faraday constants,respectively (see Appendix ID),T is the absolute temperature [set equal to 298.2 K or 25C in the right side of Eq.(9.10)].n is again the number of electrons transferred,and Cion is the molar ion concentration.As expected,one obtains E Eo for a 1-M solution.Equation(9.10) shows that a tenfold increase in the ion concentration leads to a rise of the half-cell potential of 59.2 mV for a single electron re- action. Galvanic It is customary to rank metals and alloys in a simpler way,that Series is,in terms of their relative reactivities to each other in a given environment.Table 9.2 presents such a list,called the galvanic series.It includes some commercial metals and alloys that have been exposed to sea water.No emf's are usually included in a galvanic series. Galvanic The galvanic series of electrochemical corrosion explains a num- Corrosion ber of important mechanisms.To start with,when two different metals (such as an iron and a copper pipe)are electrically con- nected and are exposed to an electrolyte (e.g.,ordinary water), the less noble metal (here,the iron)corrodes near the junction (Figure 9.4).This is an example of a mechanism called galvanic corrosion.It can be essentially prevented by inserting an insu- lated coupling(plastic)between the two unlike pipes.(This tech- nique does not prevent galvanic corrosion in the case when,for example,copper is dissolved "upstream"and then plated on the steel pipe "downstream".)Galvanic corrosion also occurs in a
where R and F are gas and Faraday constants, respectively (see Appendix II), T is the absolute temperature [set equal to 298.2 K or 25°C in the right side of Eq. (9.10)], n is again the number of electrons transferred, and Cion is the molar ion concentration. As expected, one obtains E E0 for a 1-M solution. Equation (9.10) shows that a tenfold increase in the ion concentration leads to a rise of the half-cell potential of 59.2 mV for a single electron reaction. It is customary to rank metals and alloys in a simpler way, that is, in terms of their relative reactivities to each other in a given environment. Table 9.2 presents such a list, called the galvanic series. It includes some commercial metals and alloys that have been exposed to sea water. No emf’s are usually included in a galvanic series. The galvanic series of electrochemical corrosion explains a number of important mechanisms. To start with, when two different metals (such as an iron and a copper pipe) are electrically connected and are exposed to an electrolyte (e.g., ordinary water), the less noble metal (here, the iron) corrodes near the junction (Figure 9.4). This is an example of a mechanism called galvanic corrosion. It can be essentially prevented by inserting an insulated coupling (plastic) between the two unlike pipes. (This technique does not prevent galvanic corrosion in the case when, for example, copper is dissolved “upstream” and then plated on the steel pipe “downstream”.) Galvanic corrosion also occurs in a Galvanic Series Galvanic Corrosion 9.2 • Electrochemical Corrosion 161 TABLE 9.1. Standard emf series for selected elements in 1-M solution at 25°C Electrode potential, or emf Electrode reaction E0 (V) Au3 3e Au 1.42 Cathodic Pt2 2e Pt 1.2 Ag 1e Ag 0.80 Cu2 2e Cu 0.34 2H 2e H2 0.000 Sn2 2e Sn 0.14 Ni2 2e Ni 0.25 Fe2 2e Fe 0.44 Cr3 3e Cr 0.74 Zn2 2e Zn 0.76 Al3 3e Al 1.66 Mg2 2e Mg 2.36 Anodic (Oxidation)