Diagram Evans

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 two dashed lines in the upper left hand corner of the Evans diagram represent the electrochemical potential vs electrochemical reaction rate (expressed as current density) for the oxidation and the reduction form of the hydrogen reaction. At point A the two are equal, ie, at equiUbrium, and the potential is therefore the equiUbrium potential, for the specific conditions involved. Note that the reaction kinetics are linear on these axes. The change in potential for each decade of log current density is referred to as the Tafel slope (12). Electrochemical reactions often exhibit this behavior and a common Tafel slope for the analysis of corrosion problems is 100 millivolts per decade of log current (1). A more detailed treatment of Tafel slopes can be found elsewhere (4,13,14). 

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). 

A typical Evans diagrams for the corrosion of a single metal is illustrated in Fig. 1.26a (compare with Fig. 1.23 for two separable electrodes), and it can be seen that the E -I and E -I curves are drawn as straight lines that intersect at a point that defines and (it is assumed that the resistance for the solution is negligible). E can of course be determined by means of a reference electrode, but since the anodic and cathodic sites are inseparable direct determination of /co by means of an ammeter is not... 

Fig. 1.27 Evans diagrams illustrating (a) cathodic control, (b) anodic control, (c) mixed control, (d) resistance control, (e) how a reaction with a higher thermodynamic tendency ( r, ii) may result in a smaller corrosion rate than one with a lower thermodynamic tendency and (/) how gives no indication of the corrosion rate...

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. 

Figures 1.27a to d show how the Evans diagram can be used to illustrate how the rate may be controlled by either the polarisation of one or both of the partial reactions (cathodic, anodic or mixed control) constituting corrosion reaction, or by the resistivity of the solution or films on the metal surface (resistance control). Figures 1. lie and/illustrate how kinetic factors may be more significant than the thermodynamic tendency ( , u) and how provides no information on the corrosion rate.

Over the years the original Evans diagrams have been modified by various workers who have replaced the linear E-I curves by curves that provide a more fundamental representation of the electrode kinetics of the anodic and cathodic processes constituting a corrosion reaction (see Fig. 1.26). This has been possible partly by the application of electrochemical theory and partly by the development of newer experimental techniques. Thus the cathodic curve is plotted so that it shows whether activation-controlled charge transfer (equation 1.70) or mass transfer (equation 1.74) is rate determining. In addition, the potentiostat (see Section 20.2) has provided... 

Fig. 1.28 Evans diagram illustrating a corrosion process (e.g. a bimetallic couple) in which the area of the cathode is not equal to that of the anode, (o) so that and (b) > S(,...

Evans Diagram diagram in which the E vs. I relationships for the cathodic and anodic reactions of a corrosion reaction are drawn as straight lines intersecting at the corrosion potential, thus indicating the corrosion current associated with the reaction. 

Any fundamental classification of corrosion control must be based on the electrochemical mechanism of corrosion, and Evans diagrams may be constructed (Fig. 1.27, Section 1.4) illustrating... 

In an Evans diagram 89> the mixed potential can easily be found and also be verified by measuring the open circuit potential of a zinc-amalgam electrode in a Cu2+-ion solution. Even the complication by the simultaneous presence of another reducible species, e.g., Pbz+ can be graphically demonstrated for different limiting conditions... 

Fig. 3. Evans-diagram for the cementation of Cu2+ and Pb2 with zinc amalgam of different zinc content. If the zinc concentration in the mercury employed for this special extraction technique is low, the anodic zinc-dissolution current density may be diffusion controlled and below the limiting cathodic current density for the copper reduction. The resulting mixed potential will lie near the halfwave potential for the reaction Cu2+ + 2e j Cu°(Hg) and only Cu2 ions are cemented into the mercury.

Of fundamental importance in understanding the electrochemistry of ion-selective membranes and also of biomembranes is the research in the field of voltammetry at ITIES mainly pioneered by Koryta and coworkers 99 101 . Koryta also demonstrated convincingly that a treatment like corroding metal electrodes is possible 102). For the latter, the description in the form of an Evans-diagram is most appropriate Fig. 4 shows schematically some mixed potentials, which are likely to arise at cation-selective membranes if interfering ions disturb an ideal Nernstian behavior82. Here, the vertical axis describes the galvani potential differences (absolute po-... 

Fig. 5. Tentative mixed potential model for the sodium-potassium pump in biological membranes the vertical lines symbolyze the surface of the ATP-ase and at the same time the ordinate of the virtual current-voltage curves on either side resulting in different Evans-diagrams. The scale of the absolute potential difference between the ATP-ase and the solution phase is indicated in the upper left comer of the figure. On each side of the enzyme a mixed potential (= circle) between Na+, K+ and also other ions (i.e. Ca2+ ) is established, resulting in a transmembrane potential of around — 60 mV. This number is not essential it is also possible that this value is established by a passive diffusion of mainly K+-ions out of the cell at a different location. This would mean that the electric field across the cell-membranes is not uniformly distributed.

Figure 8. Wagner-Traud (Evans) diagram for aluminum in aqueous solution, in the absence of dissolved oxygen.

Fig. 2. Current-potential curves in Evans diagram [29] format for reduction of Cu2+ ions and oxidation of H2CO. and are the equilibrium, or open circuit, potentials for the Cu2+ reduction and H2CO oxidation reactions, respectively. Assuming negligible interfering reactions, the vertical dashed lines indicate the exchange current densities for the two half reactions, and the deposition current for the complete electroless solution. Adapted from ref. 23.

Evans diagram, 7 802-804 Evaporated salt, in water softening,... 

Figure 8.3. Evans diagram of current-potential curves for a system with two different simultaneous electrochemical reactions. Kinetic scheme Eqs. (8.4) and (8.5).

Figure 8.4. Current-potential curves for the reduction of Cu ions and the oxidation of reducing agent Red, formaldehyde, combined into one graph (an Evans diagram). Solution for the Tafel line for the reduction of Cu ions O.IM CUSO4, 0.175M EDTA, pH 12.50, Egq (Cu/Cu ) = -0.47 V versus SCE for the oxidation of formaldehyde 0.05 M HCHO and 0.075 M EDTA, pH 12.50, (HCHO) = -1.0 V versus SCE temperature 25 0.5°C. (From Ref. 10, with permission from the American Electroplaters and Surface Finishers Society.)...

Figure 3.9 Evans diagram showing effect of activation polarization on overpotential for a hydrogen electrode. Reprinted, by permission, from W. Callister, Materials Science and Engineering An Introduction, p. 574, 5th ed. Copyright 2000 by John Wiley Sons, Inc.

Evans diagrams Evansite [12244-16-5] Evaporated milk Evaporation... 

The rate of deposition and the mixed potential are determined on the basis of the mixed-potential theory using the Evans diagram. First, the current-potential curve... 

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