Equilibrium potential 6.26

The Nemst equation above for the dependence of the equilibrium potential of redox electrodes on the activity of solution species is also valid for uncharged species in the gas phase that take part in electron exchange reactions at the electrode-electrolyte interface. For the specific equilibrium process involved in the reduction of chlorine ... 

For a simple electron transfer reaction containing low concentrations of a redox couple in an excess of electrolyte, the potential established at an inert electrode under equilibrium conditions will be governed by the Nemst equation and the electrode will take up the equilibrium potential for the couple 0/R. In temis of... 

Since any current resulting from tire anodic reaction must be consumed by tire catlrodic reaction, tire catlrodic current,7, must be equal to tire airodic current As a consequence, tire equilibrium potential of a metal (e.g. Fe) tlrat is immersed into air aqueous electrolyte will be adjusted by tire condition tlrat = j This is... 

Figure C2.8.2. (a) Schematic polarization curves for tire anodic and catliodic reaction of an Fe/Fe electrode. The anodic and catliodic branches of tire curve correspond to equations (C2.8.12 ) and (C2.8.13 ) respectively. The equilibrium potential of tliis electrode will adjust so tliat j = j tire corresponding potential is of Fe. (b)...

When no net current is flowing, the equilibrium potential of the fuel cell is given by the Nernst equation ... 

Tafel Extrapolation Corrosion is an elec trochemical reac tion of a metal and its environment. When corrosion occurs, the current that flows between individual small anodes and cathodes on the metal surface causes the electrode potential for the system to change. While this current cannot be measured, it can be evaluated indirectly on a metal specimen with an inert electrode and an external electrical circuit. Pmarization is described as the extent of the change in potential of an electrode from its equilibrium potential caused by a net current flow to or from the electrode, galvanic or impressed (Fig. 28-7). 

Equation (2-38) is valid for every region of the surface. In this case only weight loss corrosion is possible and not localized corrosion. Figure 2-5 shows total and partial current densities of a mixed electrode. In free corrosion 7 = 0. The free corrosion potential lies between the equilibrium potentials of the partial reactions and U Q, and corresponds in this case to the rest potential. Deviations from the rest potential are called polarization voltage or polarization. At the rest potential = ly l, which is the corrosion rate in free corrosion. With anodic polarization resulting from positive total current densities, the potential becomes more positive and the corrosion rate greater. This effect is known as anodic enhancement of corrosion. For a quantitative view, it is unfortunately often overlooked that neither the corrosion rate nor its increase corresponds to anodic total current density unless the cathodic partial current is negligibly small. Quantitative forecasts are possible only if the Jq U) curve is known. 

When cathodic polarization is a result of negative total current densities 7., the potential becomes more negative and the corrosion rate lower. Finally, at the equilibrium potential it becomes zero. In neutral water equilibrium potentials are undefined or not attainable. Instead, protective potentials are quoted at which the corrosion rate is negligibly low. This is the case when = 1 flA cm (w = lOjUm a ) which is described by the following criteria for cathodic protection ... 

Anode Polarization-the difference between the potential of an anode passing current and the steady-state or equilibrium potential of the electrode with the same electrode reaction. 

Table 1.7 shows typical half reactions for the oxidation of a metal M in aqueous solutions with the formation of aquo cations, solid hydroxides or aquo anions. The equilibrium potential for each half reaction can be evaluated from the chemical potentials of the species involved see Appendix 20.2) and it should be noted that there is no difference thermodynamically between equations 2(a) and 2(b) nor between 3(a) and 3(b) when account is taken of the chemical potentials of the different species involved. 

It should be emphasised that potential-pH diagrams can also be constructed from experimental E -I curves, where E is the polarised potential and / the current. These diagrams, which are of more direct practical significance than the equilibrium potential-pH equilibrium diagrams constructed from thermodynamic data, show how a metal in a natural environment (e.g. iron in water of given chloride ion concentration) may give rise... 

It follows from equation 1.14 that for any constant ratio of a /a the E vs. pH relationship will be linear with a slope -0-059m/z, and that when = 1 fhe intercept of the curve on the E axis (i.e. pH= 0) will be E, the standard equilibrium potential, which by definition is the potential when the species involved in the equilibrium are at unit activity. 

Thus the tendency for an electrochemical reaction at a metal/solution interface to proceed in a given direction may be defined in terms of the relative values of the actual electrode potential E (experimentally determined and expressed with reference to the S.H.E.) and the reversible or equilibrium potential E, (calculated from E and the activities of the species involved in the equilibrium). 

Although the zones of corrosion, immunity and passivity are clearly of fundamental importance in corrosion science it must be emphasised again that they have serious limitations in the solution of practical problems, and can lead to unfortunate misconceptions unless they are interpreted with caution. Nevertheless, Pourbaix and his co-workers, and others, have shown that these diagrams used in conjunction with E-i curves for the systems under consideration can provide diagrams that are of direct practical use to the corrosion engineer. It is therefore relevant to consider the advantages and limitations of the equilibrium potential-pH diagrams. 

Consider now the transfer of electrons from electrode II to electrode I by means of an external source of e.m.f. and a variable resistance (Fig.. 20b). Prior to this transfer the electrodes are both at equilibrium, and the equilibrium potentials of the metal/solution interfaces will therefore be the same, i.e. Ey — Ell = E, where E, is the reversible or equilibrium potential. When transfer of electrons at a slow rate is made to take place by means of the external e.m.f., the equilibrium is disturbed and Uie rat of the charge transfer processes become unequal. At electrode I, /ai.i > - ai.i. 3nd there is... 

By definition, electrode II at which oxidation is the predominant reaction is the anode, whereas electrode I at which reduction is the predominant reaction is the cathode. It is apparent that the removal of electrons from Ag will result in the potential of its interface becoming more positive, whilst the concomitant supply of electrons to the interface of Ag, will make its potential become more negative than the equilibrium potential ... 

It is apparent (Fig. 1.21) that at potentials removed from the equilibrium potential see equation 1.30) the rate of charge transfer of (a) silver cations from the metal to the solution (anodic reaction), (b) silver aquo cations from the solution to the metal (cathodic reaction) and (c) electrons through the metallic circuit from anode to cathode, are equal, so that any one may be used to evaluate the rates of the others. The rate is most conveniently determined from the rate of transfer of electrons in the metallic circuit (the current 1) by means of an ammeter, and if / is maintained constant it can eilso be used to eveduate the extent. A more precise method of determining the quantity of charge transferred is the coulometer, in which the extent of a single well-defined reaction is determined accurately, e.g. by the quantity of metal electrodeposited, by the volume of gas evolved, etc. The reaction Ag (aq.) -t- e = Ag is utilised in the silver coulometer, and provides one of the most accurate methods of determining the extent of charge transfer. 

The equilibrium potentials and E, can be calculated from the standard electrode potentials of the H /Hj and M/M " " equilibria taking into account the pH and although the pH may be determined an arbitrary value must be used for the activity of metal ions, and 0 1 = 1 is not unreasonable when the metal is corroding actively, since it is the activity in the diffusion layer rather than that in the bulk solution that is significant. From these data it is possible to construct an Evans diagram for the corrosion of a single metal in an acid solution, and a similar approach may be adopted when dissolved O2 or another oxidant is the cathode reactant. 

Fig. 1.38([Pg.112]

In the case of chromium in 1 N H2SO4 transpassivity occurs at about 1 1 V (below the potential for oxygen evolution, since the equilibrium potential in acid solutions at pH 0 is 1 23 V and oxygen evolution requires an appreciable overpotential) and is associated with oxidation of chromium to dichromate anions ... 

The pitting potential (point D in Figure 1.42) is sometimes treated as a type of equilibrium potential. There are empirical reasons for this. The pitting potential is often observed to decrease linearly with log (Cl ) (the chloride activity or concentration), giving an apparently Nernstian form . 

The values in Table 2.16 show how the potentials obtained under service conditions differ from the standard electrode potentials which are frequently calculated from thermodynamic data. Thus aluminium, which is normally coated with an oxide film, has a more noble value than the equilibrium potential 3 + / = — 1-66V vs. S.H.E. and similar considerations apply to passive stainless steel (see Chapter 21). 

Equilibrium Potential-pH Diagrams with Anions at Various Temperatures... 

Fig. 8.6 Potential and pH ranges for the stress-corrosion cracking of ferritic steels in various environments, together with the pH-dependent equilibrium potentials for reactions involving Fe - Fej04, H - H and Fej04 - Fe203 (after Congleton eial. ...

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