Evans polarization curves

Fig. 3. Hypothetical Evans diagram and polarization curve for a metal corroding in an acidic solution, where point A represents the current density, /q, for the hydrogen electrode at equiUbrium point B, the exchange current density at the reversible or equiUbrium potential, for M + 2e and point...

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

The data discussed in Figs. 20 through 24 represent the type of data in a polarization curve combinations of potential and applied current density. Figure 26 shows a complete polarization curve for the iron in acid systems for which the Evans diagram is shown in Fig. 25. The Evans lines are included as dotted lines in the figure. The difference between the Evans diagram and the polarization curve is that the polarization curve data display applied current densities, whereas the Evans diagram displays the reaction rates in terms of current densities. 

Figure 26 Polarization curve that would result for Evans diagram of Fig. 25. The Evans lines are included as well.

Consider the two materials whose polarization curves are shown in Fig. 31. Both the polarization curves and the Evans lines are shown for both materials. Material 1 is the more noble material (i.e., it has a more positive Ec0II) and has a lower circuit corrosion rate when it is uncoupled. If the surface area of the two materials is the same and the materials are coupled, then the two material-solution interfaces must come to the same potential. In a manner identical to that used for the example of iron in acid used to introduce Evans diagrams, the potential and current at which this condition is met can be found by applying the conservation of charge to the sysytem ... 

As discussed in detail in Chapter 2, the corrosion potential is determined by the intersection of the sum of the anodic Evans lines and the sum of the cathodic Evans lines. For active-passive materials, the only new wrinkle is the increased complexity of the anodic line. Since the anodic line is not single-valued with respect to current density, three distinct cases can be considered. In all cases, the condition E /a = X Ic determines the position of the corrosion potential, and the condition im = z a - ic determines the appearance of the polarization curve... 

In the presence of oxidizing species (such as dissolved oxygen), some metals and alloys spontaneously passivate and thus exhibit no active region in the polarization curve, as shown in Fig. 6. The oxidizer adds an additional cathodic reaction to the Evans diagram and causes the intersection of the total anodic and total cathodic lines to occur in the passive region (i.e., Ecmi is above Ew). The polarization curve shows none of the characteristics of an active-passive transition. The open circuit dissolution rate under these conditions is the passive current density, which is often on the order of 0.1 j.A/cm2 or less. The increased costs involved in using CRAs can be justified by their low dissolution rate under such oxidizing conditions. A comparison of dissolution rates for a material with the same anodic Tafel slope, E0, and i0 demonstrates a reduction in corrosion rate... 

Figure 5 Schematic Evans diagram and resulting potential-controlled polarization curve for a material that undergoes an active-passive transition and is in a reducing solution. The heavy line represents the applied currents required to polarize the sample.

Figure 6 Schematic Evans diagram and resulting potential-controlled polarization curve for a material that undergoes an active-passive transition and is in an oxidizing solution. The heavy line represents the applied currents required to polarize the sample. If the sample did not undergo an active-passive transition, it would corrode at a much higher rate in this solution, as is indicated by the intersection of the dotted line and the cathodic curve.

For the case shown in Fig. 8, the anodic and cathodic Evans lines intersect at three points. The polarization curve for this situation appears unusual, although it is fairly commonly observed with CRAs. At low potentials, the curve is identical to that shown in Fig. 5. However, just above the active-passive transition, another Ecmi appears followed by a loop and yet a third ECljU before the passive region is observed. The direction (anodic or cathodic) of the applied current density for each region shown in the polarization curve of Fig. 8 is indicated, showing that the loop consists of cathodic current. The origin of the cathodic loop is the... 

Figure 8 Schematic Evans diagram and potential-controlled polarization curve for a material/environment combination that exhibits a cathodic loop. Note that the direction of the applied current changes three times in traversing the curve.

Figure 24 Schematic Evans diagram and polarization curve illustrating the origin of the negative hysteresis observed upon cyclic polarization for materials that do not pit. Line a represents the (unchanging) cathodic Evans line. Line b represents the anodic Evans line during the anodically directed polarization, while line c represents the anodic Evans line for the material after its passive film has thickened because of the anodic polarization. The higher corrosion potential observed for the return scan (E (back)) is due to the slowing of the anodic dissolution kinetics.

Figure 28 Schematic Evans diagrams and polarization curves for a material in a solution containing a redox couple that acts as a chemical potentiostat. The i used in the Evans diagram for the O/R redox couple is that relevant to the material of interest. In the absence of the redox couple, the material obtains Ec, i. In the presence of the redox couple, the material obtains Econ2. If Econ2 is above the pitting potential, the material will be rapidly attacked.

The discussion so tar should nave indicated the great importance of the polarization curves In aecernuning the corrosion characteristics of metals, LaQue (36) has stressed this fact and reports extensive data on the deviation of corrosion potentials from the standard values. His values should correspond to the mixed potential discussed above. The important roie of polarization functions iias also been created by Evans (34), Brown (37), Wesley (38), and Muller (39). 

The polarization curves shown in 4(d) are typical of systems in which anodes and cathodes are separated by corrosion products and/or solutions of some resistance. The corrosion rate (current density) depends on the average resistance between the anodes and cathodes and the potential which is measured depends on the position of the probe electrode in relation to the resistance separating the anode and cathode. In a corroding system of this type there is a current flowing through the solution which can be calculated by measuring the potential differences between two spaced electrodes in the solution and the resistance of the solution. Evans and his co-workers did this with iron in bicarbonate solutions and showed that all the corrosion could be accounted for by the currents flowing between anodes and cathodes, (2). This confirmed the electrochemical mechanism of corrosion. 

Figure 4. Evans-type polarization curves (a) anodic curve intersecting two possible cathode reactions (b) diffusion control of cathodic reaction (c) diffusion control of anodic reaction and (d) resistance polarization, measure E will depend on where probe is in relation to anode and cathode...

These equations are straight lines in a plot of E versus log i, such as that in Fig. 2. This plot of versus log i is called an Evans diagram. Actually, the Butler-Volmer equation is described better by the curve in Fig. 5. The net current at the reversible potential is zero because the forward and reverse current, each equal to the exchange current density, balance each other. The log of the current density approaches negative infinity at the reversible potential at which the net current density goes to zero, and the polarization curve points down at the reversible potential when plotted on semilogarithmic axes. 

Fig. 8 Relationship of measured polarization curve to the Evans diagram for a corroding active metal in an acid.

The effect of area ratio is handled with experimental polarization curves in a similar fashion to that shown for schematic Evans diagrams in Fig. 11. Figure 13 shows experimental polarization curves for metals... 

HG. 9—Creating polarization curves from Evans diagrams. 

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