Cathodic curves

Corrosion Rate by CBD Somewhat similarly to the Tafel extrapolation method, the corrosion rate is found by intersecting the extrapolation of the linear poi tion of the second cathodic curve with the equihbrium stable corrosion potential. The intersection corrosion current is converted to a corrosion rate (mils penetration per year [mpy], 0.001 in/y) by use of a conversion factor (based upon Faraday s law, the electrochemical equivalent of the metal, its valence and gram atomic weight). For 13 alloys, this conversion factor ranges from 0.42 for nickel to 0.67 for Hastelloy B or C. For a qmck determination, 0.5 is used for most Fe, Cr, Ni, Mo, and Co alloy studies. Generally, the accuracy of the corrosion rate calculation is dependent upon the degree of linearity of the second cathodic curve when it is less than... 

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... 

The general form of the anodic polarisation curve of the stainless steels in acid solutions as determined potentiostaticaiiy or potentiodynamically is shown in Fig. 3.14, curve ABCDE. If the cathodic curve of the system PQ intersects this curve at P between B and C only, the steel is passive and the film should heal even if damaged. This, then, represents a condition in which the steel can be used with safety. If, however, the cathodic curve P Q also intersects ED the passivity is unstable and any break in the film would lead to rapid metal solution, since the potential is now in the active region and the intersection at Q gives the stable corrosion potential and corrosion current. 

Since in most cases of corrosion in which transport of the cathodic reactant (HjO , dissolved O2, Fe , HNO3 acid, etc.) is rate determined, the anodic curve intersects the cathodic curve at /V, then... 

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. 

Three general reaction types compare the activation-control reduction processes. In Fig. 25-12, in Case I, the single reversible corrosion potential (anode/cathode intersection) is in the active region. A wide range of corrosion rates is possible. In Case 2, the cathodic curve intersects the anodic curve at three potentials, one active and two passive. If the middle active/passive intersection is not stable, the lower and upper... 

Let us introduce into the titrant one Pt indicator electrode vs. an SCE and maintain in the electric circuit a low constant current + /, as indicated by the broken horizontal line in Fig. 3.71. For this line we shall consider the successive points of its intersection with the voltammetric curves during titration and observe the following phenomena as expressed in the corresponding electrode potentials. Immediately from the beginning of the titration E remains high (nearly 1.44 V), but falls sharply just before the equivalence point (E = 1.107 V), and soon approaches a low E value (below 0.77 V) (see Fig. 3.72, cathodic curve +1). 

Figure S-4S shows the polarization curves observed, as a function of the film thickness, for the anodic and cathodic transfer reactions of redox electrons of hydrated ferric/ferrous cyano-complex particles on metallic tin electrodes that are covered with an anodic tin oxide film of various thicknesses. The anodic oxide film of Sn02 is an n-type semiconductor with a band gap of 3.7 eV this film usually contains a donor concentration of 1x10" ° to lxl0 °cm °. For the film thicknesses less than 2.5 nm, the redox electron transfer occurs directly between the redox particles and the electrode metal the Tafel constant, a, is close to 0.5 both in the anodic and in the cathodic curves, indicating that the film-covered tin electrode behaves as a metallic tin electrode with the electron transfer current decreasing with increasing film thickness.

In cathodic area, the Tafel slope in the presence of DDTC is bigger than that in the absence of DDTC, and the cathodic curves imder the conditions of different DDTC concentration are almost parallel and their Tafel slopes only change a little. These demonstrate that the chemisorption of DDTC on the surface of jamesonite electrode also inhibits the cathodic reaction, but the chemisorption amoimt of DDTC is a little and almost not affected by the DDTC concentration due to their negatively electric properties of DDTC anion and the electrode surface. This reveals that there is a little DDTC chemisorption on the mineral even if the potential is lower (i.e., negative potential). 

At 60 minutes only, dc potentiodynamic curves were determined from which the corrosion current was obtained by extrapolation of the anodic Tafel slope to the corrosion potential. The anodic Tafel slope b was generally between 70 to 80 mV whereas the cathodic curve continuously increased to a limiting diffusion current. The curves supported impedance data in indicating the presence of charge transfer and mass transfer control processes. The measurements at 60 minutes indicated a linear relationship between and 0 of slope 21mV. This confirmed that charge transfer impedance could be used to provide a measure of the corrosion rate at intermediate exposure times and these values are summarised in Table 1. 

Figure 15.9 Voltage-current relationship for a typical hydrogen-oxygen fuel cell. The vertical separation between the anode and cathode curves represents the cell output voltage.

In practice, one often has cases where the course of the cathodic and anodic current densities across the electrode with change of overpotential is symmetrical (except for the signs for the same numerical value of T)). It will be seen in the next section that this is so if the symmetry factor, P, is exactly 0.50. More often, the anodic and cathodic curves are nearly symmetrical. However, sometimes they are importantly and even dramatically different. For example, the anodic current is oxidizing and could provoke... 

Fig. 9.21. Tafel plots of the (a) anodic and (b) cathodic current densities for film thickness varying from 4 to 40 A in steps of 6 A. The cathodic curves have been normalized forr = 0. Barrier height= 1.5 eV. VL=VR= 1.5 harmonic. (Reprinted with permission from W. Schmickler and J. Ulstrup, J. Chem. Phys. 19 217, Fig. 3, copyright 1989, American Chemical Society.)...

Fig. 6.26 Peak parameters of the CV curves (peak potentials (a) peak heights (b) half-peak widths (c)) calculated by generating voltammograms from Eqs. (6.160) and (6.166) for different values of the dimensionless rate constant Q. The values of a appear on the curves. Letters a and c in figure c correspond to anodic and cathodic curves, respectively...

The shape of the anodic curve is always the same, independent of EXy but the value of Ek alters the position of the anodic curve in relation to the current axis. For this reason 7p,a should be measured from a baseline that is a continuation of the cathodic curve, as shown in Fig. 9.3. 

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.

Expected Order of Magnitude of Corrosion by CBD The third cathodic curve will generally fall between the first two, or the first and third will coincide and fall positive to the second curve. These two curve configurations are only valid relationships for the projection... 

If no Epit can be determined, but cathodic Curves 1 and 2 cross at E forming a hysteresis loop, crevice-type attack during the rapid scan is indicated. 

The main results obtained are given in Fig. 14. One sees that the relative rate constants calculated at the formal potentials separately from cathodic and anodic current-potential dependences are different. While the rate constants calculated from cathodic curves are virtually independent of alcohol concentration, the constants calculated from anodic dependences exhibit a deep minimum when the concentration of alcohols in the solution is increased. 

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