Cathodic polarization schematic curve

Fig. 5.9 Schematic representation of relative positions of the net polarization curves to the individual curves for anodic metal polarization and cathodic hydrogen and water polarization, pH = 1. Curve M, anodic polarization for metal (e.g., Fe-18% Cr) curve H, cathodic polarization for H+ curve W, cathodic polarization for H20 curve SC, sum of cathodic polarization for H+ and H20 curve N, net curve for anodic and cathodic polarization. Note Curve N coincides with curve M above-100 mV and with curve H below-350 mV.

Fig. 8(b) shows schematically the curves for the process of self-passivation X and Y represent two possible cathodic polarization curves for an electron acceptor present in the... 

FIGURE 14-1 Schematic representation of cathodic polarization curve of i platinum electrode in acidic solution of metal ion M". ... 

FIGURE 22.17 Metallic passivation schematically illustrated by anodic and cathodic polarization curves of corroding metals (a) active corrosion, (b) unstable passivity, and (c) stable passivity i+ = anodic metal dissolution current and i = cathodic oxidant reduction current. 

Fig. 4.1 7 Schematic examples of the effects of changes in the relative positions of anodic and cathodic polarization curves due to inhibitors, with the resultant Ecorr and lcorr values, (a) Effects of cathodic inhibitor. Note that lcorr is decreased and Ecorr is decreased, (b) Effects of anodic inhibitor. Note that lcorr is decreased and Ecorr is increased, (c) Effects of cathodic and anodic inhibitor...

In this section, the relative positions of several schematic anodic and cathodic curves are presented. The sum anodic (X iox) and sum cathodic (X ired) curves are shown relative to the individual curves, and then the net curves are shown as representative of what would be observed experimentally. Figure 5.8 shows an anodic polarization curve (M) repre-... 

Fig. 5.10 Schematic representation of the net anodic and cathodic polarization curves, N, for the anodic metal, M, and for the cathodic hydrogen, H, polarization curves. Note that the net curves deviate from curves M and H only near Ecorr. SC is the sum of cathodic polarization for H+and H20. pH = 1...

The above relationship is equally applicable if either the metal oxidation-rate curve or the reduction-rate curve for the cathodic reactant does not obey Tafel behavior. To illustrate this point, three additional schematic pairs of individual anodic and cathodic polarization curves are examined. In Fig. 6.3, the metal undergoes active-passive oxidation behavior and Ecorr is in the passive region. In Fig. 6.4, where the total re-... 

Fig. 6.3 Schematic experimental polarization curves (solid curves) assuming active-passive behavior for the individual metal-oxidation curve and Tafel behavior for the individual cathodic-reactant reduction curve (dashed curves)...

Figure 17.1. Polarization curves that show effect of passivator concentration on corrosion of iron. An oxidizing substance that reduces sluggishly does not induce passivity (dotted cathodic polarization curve) (schematic).

Essentially, it is a mixed potential phenomenon in which the anodic and cathodic polarization processes are carried out using a reverse scan rate. The scan rate is reversed at a predetermined potential, leading to a cathodic polarization in the passive region until both anodic and cathodic curves intersect The output of tMs technique is schematically shown in Figure 6.11. 

Rates of corrosion can also be measured using an electrochemical technique known as potentiodynamic polarization. The potential of the test metal electrode relative to a reference electrode (commonly the saturated calomel electrode SCE) is varied at a controlled rate using a potentiostat. The resultant current density which flows in the cell via an auxiliary electrode, typically platinum, is recorded as a function of potential. The schematic curve in fig. 2 is typical of data obtained from such a test. These data can provide various parameters in addition to corrosion rate, all of which are suitable for describing corrosion resistance. The corrosion potential F corr is nominally the open circuit or rest potential of the metal in solution. At this potential, the anodic and cathodic processes occurring on the surface are in equilibrium. When the sample is polarized to potentials more positive than Scon the anodic processes, such as metal dissolution, dominate (Anodic Polarization Curve). With polarization to potentials more negative than Scorr the cathodic processes involved in the corrosion reaction such as oxygen reduction and hydrogen evolution dominate (Cathodic Polarization Curve). These separate halves of the total polarization curve may provide information about the rates of anodic and cathodic processes. The current density at any particular potential is a measure of the... 

With increasing x in Laj. Sr MnOj, the overpotential decreases with the onset of a minimum for the composition x 0.5 as shown in Fignre 12.11. As for electrical condnctivity, the best performance is achieved with Co-containing lanthanum manganite. Hanunouche has shown that the cathodic polarization curve, schematically drawn in Figure 12.12, can be divided into two domains separated by a specific transition potential At low cathodic polarization (E > E,), the oxide cathode behaves as a classic metallic electrode, i.e., platinum. In this region, the reactions involved are... 

Fig. 2 Schematic V-I curve for a typical SOFC, which shows OCV, cathodic polarization loss, anodic polarization loss, and ohmic loss...

The electrochemical mechanism of dissolution is illustrated schematically by the simplified polarization diagram shown in Fig. 2. The open circuit potentials of the cathode process, E, and the anodic process. Eg, are the equilibrium potentials of the corresponding partial reactions of Eqs. 28 and 29. The dissolution current corresponds to the steady-state rate of dissolution. The corresponding dissolution potential of the dissolving solid lies between the equilibrium values of the cathodic and anodic reactions. From the figure, it also follows that conditions which shift the point of intersection of the anodic and cathodic polarization curves by decreasing their slopes, lead to an increase in dissolution rate. Conversely, an increase in the slopes of the curves lowers the dissolution rate. 

FIGURE 123 Schematic shape of polarization curves during an anodic and a cathodic potential scan. 

FIGURE 22.2 Schematic polarization curves for spontaneous dissolution (a) of active metals (h) of passivated metals. (1,2) Anodic curves for active metals (3) cathodic curve for hydrogen evolution (4) cathodic curve for air-oxygen reduction (5) anodic curve of the passivated metal. 

In electrochemical kinetics, the plot of reaction current (reaction rate) as a fimction of electrode potential is conventionally called the polarization curve. Figure 7—4 shows schematic polarization curves of cathodic and anodic electrode reactions. The term of polarization means shifting the electrode potential from a certain specified potential, e.g. the equilibrium potential of an electrode reaction, to more negative (cathodic) or more positive (anodic) potentials. The term of polarization also occasionally applies to the magnitude of potential shift from the specified potential. 

Figure 10-24 shows schematically the electron levels and the polarization curves for a cathodic hole iivjection in an n-type and a p-type electrode of the same semiconductor. The range of potential where the cathodic reaction occurs on the n-type electrode is more cathodic (more negative) than the range of potential for the cathodic reaction on the p-type electrode. The difference between the polarization potential aEd) (point N in the figure) of the n-type electrode and the polarization potential p (i) (point P in the figure) of the p-type electrode at a constant cathodic current i is equivalent to the difference between the Fermi level n r of interior electrons and the quasi-Fermi level of interfacial holes in... 

Fig. 11-14. (a) Corrosion rate of metallic iron in nitric acid solution as a function of concentration of nitric add and (b) schematic polarization curves for mixed electrode reaction of a corroding iron in nitric add W p, = iron corrosion rate CHNO3 = concentration of nitric add t" (t ) = current of anodic iron dissolution (cathodic nitric add reduction) dashed curve 1= cathodic current of reduction of nitric add in dilute solution dashed ciuve 2 s cathodic current of reduction of nitric add in concentrated solution. [From Tomashov, 1966 for (a).]... 

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.

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 3.16. Schematic representation of the correlation between polarization resistances (anode, cathode, and cell) and polarization curves [23], (With kind permission from Springer Science+Business Media Journal of Applied Electrochemistry, Characterization of membrane electrode assembhes in polymer electrolyte fuel cells using a.c. impedance spectroscopy, 32(8), 2002, 859-63, Wagner N. Figure 6.)...

Using the electrode resistances (R anode and Rcath) gained from simulation with an equivalent circuit, the individual cathode and anode polarization curves can be determined, as shown schematically in Figure 3.16 [23],... 

Since holes are consumed at the surface during the anodic dissolution, the n-type samples show increasing differences between the measured and the calculated capacities with increasing rate of dissolution, i. e., with increasing anodic polarization. In this case d-c potential curves also show deviations from the initial exponential slope. At higher anodic potentials a saturation current occurs. Illumination compensates for or decreases the influence of the anodic current on the concentration cf holes. Fig. 11 shows schematically the influence of anodic dissolution and illumination. For p-type Ge the same effects occur, when electrons are consumed by the electrode reaction, i. e., in the cathodic region. 

Typical curves schematically showing the current-voltage characteristics of a redox system are shown in Fig. 2. Curves A and C are the polarization curves for the anodic and cathodic reaction, respectively. Eeq is die reversible potential for the various concentrations of the cathodic reactant - tne metal ion in the case of a metal - and Cp C2 and the polarization curves for decreasing concentration. 

Figure 8.27. Polarization curves for various concentrations of Fe and Fe. Solid lines are polarization curves (electrode area = 1 cm ) I o, exchange current. Dashed lines are hypothetical cathodic (-i) and anodic ( + /) currents. Curves are schematic but based on experimental data at relevant points.

Figure 8.28. Electnxie polarization curves for oxygen-containing solutions (a) in otherwise pure water and (b) in the presence (nonequilibrium) of some Fe. Curves are schematic but in accord with available data at significant points. Because the net current (a) is virtually zero over a considerable span of the electrode potentials, the exact location of the redox potential becomes difficult to determine or is determined by insidious redox impurities. A mixed potential (b) may be observed at the point where the anodic and cathodic currents are balanced but because the various redox partners are not in equilibrium with each other, it is not amenable to quantitative interpretation.

The change of the corrosion potential in either the anodic or the cathodic direction may correspond to a decrease or increase in the corrosion current. The variation of the corrosion potential and corrosion currents under various conditions can be generalized using schematic polarization curves in Fig. 1.26. The corrosion potential of an active electrode in a solution is Ecorr- -Ecorr. Escort, and Ecorr are the corrosion potentials under changed conditions. 

FIGURE 22.10 Schematic polarization curves for anodic hole-emitting dissolution of an n-type electrode and a p-type electrode of the same semiconductor in the dark and photoexcited conditions [14,15] solid curve = photoexcited, dashed curve = dark, ia = anodic dissolution current, ic — cathodic current, iph — photoexcited dissolution current, and Eft, — flat band potential for n-type and p-type electrodes. 

FIGURE 22.20 Schematic polarization curves for the corrosion of p-type germanium in acid solution the corrosion potential is less positive with the cathodic hydrogen reaction but more positive with the cathodic oxygen reaction. 

We see, as a consequence, that the anodic oxygen production on the part of w-type oxide may be coupled with the cathodic oxygen reduction and/or with the cathodic hydrogen production on the part of the metal surface. The polarization curves of these reactions are schematically shown for a passive metal in Figure 22.36 and for an active metal in Figure 22.37. 

Figure 22.39 shows, for a corroding metal in contact with a p-type oxide, schematic polarization curves of the anodic metal dissolution and the cathodic hydrogen reduction that occurs at the photoexcited oxide part. We see, as a consequence, that metals such as copper, which suffer no corrosion in the absence of oxygen, if in contact with a p-type oxide, may become subject to anodic corrosion with the cathodic hydrogen production on the oxide under the photoexcited or radiation-excited conditions. 

---