Cathodic polarization curves

FIG. 28-14 Logic sequence Diagram 3 used to qualitatively evaluate the degree of corrosion for systems that cannot be evaluated by extrapolation of the cathodic polarization Curve 2 from a CBD. 

Based on cathodic polarization curves, Dexter and Gao concluded that the increase of E for 316 stainless steel exposed to natural seawater was due to an increased rate of the cathodic reduction of oxygen at a given potential. It is not possible from Ecorr or polarization curves to decide whetlier the increase in Ecorr is due to thermodynamic effects, kinetic... 

The straight lines for the partial CD t andT in Fig. 63b intersect at the equilibrium potential AE = 0. The value of CD corresponding to the point of intersection is that of the exchange CD f, according to Eq. (6.11). It follows that the exchange CD can be determined when the linear sections of the anodic or cathodic polarization curve, which have been measured experimentally and plotted as log i vs. AE, are extrapolated to the equilibrium potential. Moreover, according to Eq. (6.19) the exchange CD can be determined from the slope of the polarization curve near the equilibrium potential when the curve is plotted as i vs. AE. 

Figure 1T2 shows anodic d cathodic polarization curves for the partial CD of dissolution 4 and deposition 4 of the metal and for the partial CD of ionization 4 and evolution 4 of hydrogen, as well as curves for the overall reaction current densities involving the metal (4) and the hydrogen (4). The spontaneous dissolution current density 4 evidently is determined by the point of intersection. A, of these combined curves. 

The surface of the base metal is anodically polarized under the effect of local cells. For a graphical analysis of the phenomena, one must construct the polarization curves for the partial currents at the base metal as well as the overall anodic 4 vs. E curve reflecting the effective rate of dissolution of this metal under anodic polarization. The rate of the cathodic process, 4, at the inclusions is described by the corresponding cathodic polarization curve (since the surface areas of anodic and cathodic segments differ substantially, currents rather than current densities must be employed here). At open circuit the two rates are identical. 

Electrochemical corrosion processes also include a number of processes in organic chemistry, involving the reduction of various compounds by metals or metal amalgams. A typical example is the electrochemical carbonization of fluoropolymers mentioned on p. 316. These processes, that are often described as purely chemical reductions, can be explained relatively easily on the basis of diagrams of the anodic and cathodic polarization curves of the type shown in Fig. 5.54. 

Horita et al. [97] studied the electrochemical polarization performance of Laj x SrxCo03 (x = 0.2, 0.3, 0.4) cathodes on (La, Sr) (Gd, Mg)03, LSGM electrolyte. With an increase of Sr content in LSC, the conductivity increases above 1400 Scm-1 (for x = 0.3, 0.4). The temperature dependence of the conductivity shows metallic behavior, especially above x = 0.3. The polarization activity for the 02 reduction increases with the Sr content in LSC. The cathodic polarization curves at the porous... 

Fig. 7-4. Anodic and cathodic polarization curves of an electrode reaction E= electrode potential ...

Figure 8-7 shows the anodic and cathodic polarization curves observed for a redox couple of hydrated titanium ions Ti /Ti on an electrode of mercury in a sulfuric add solution the Tafel relationship is evident in both anodic and cathodic reactions. FYom the slope of the Tafel plot, we obtain the symmetry factor P nearly equal to 0.5 (p 0.5). 

The polarization curves shown in Fig. 8-27 correspond to Eqns. 8-62 to 8-65 while the electrode is in the state of band edge level pinning. As examples, Fig. 8-28 shows the cathodic polarization curves of several redox reactions at an... 

Fig. 8-28. Cathodic polarization curves for several redox reactions of hydrated redox particles at an n-type semiconductor electrode of zinc oxide in aqueous solutions (1) = 1x10- MCe at pH 1.5 (2) = 1x10 M Ag(NH3) atpH12 (3) = 1x10- M Fe(CN)6 at pH 3.8 (4)= 1x10- M Mn04- at pH 4.5 IE = thermal emission of electrons as a function of the potential barrier E-Et, of the space charge layer. [From Memming, 1987.]...

Figure 8-42 illustrates the anodic and cathodic polarization curves observed for an outer-sphere electron transfer reaction with a typical thick film on a metallic niobium electrode. The thick film is anodically formed n-type Nb206 with a band gap of 5.3 eV and the redox particles are hydrated ferric/ferrous cyano-complexes. The Tafel constant obtained from the observed polarization curve is a- 0 for the anodic reaction and a" = 1 for the cathodic reaction these values agree with the Tafel constants for redox electron transfers via the conduction band of n-lype semiconductor electrodes already described in Sec. 8.3.2 and shown in Fig. 8-27. 

Fig. 8-42. Anodic and cathodic polarization curves observed for electron transfer of hydrated redox particles at an electrode of metallic niobium covered with a thick niobium oxide NbsOs film (12 nm thick) in acidic solution reaction is an electron transfer of hydrated redox particles, 0.25MFe(CN)6 /0.25M Fe(CN)g, in 0.1 M acetic add buffer solution of pH 4.6 at 25 C. =...

Fig. 8-43. Anodic and cathodic polarization curves observed for a redox electron transfer at metallic tin electrodes covered with an anodic oxide Sn02 film of various thicknesses d in a basic solution reaction is a redox electron transfer of 0.25 M Fe(CN)6 A).25 M Fe(CN)6 in 0.2 M borate buffer solution of pH 9.1 at 25°C. d = film thickness dj = 2 nm ...

Fig. 9-4. Anodic and cathodic polarization curves measured for transfer of divalent cadmium ions (dissolution-deposition) at a metallic cadmium electrode in a sulfate solution (0.005MCd + 0.4MS04 ) i (i )= anodic (cathodic) reaction current a = Tafel constant (transfer coefficient). [From Lorenz, 1954.]...

Fig. 9-5. Anodic and cathodic polarization curves observed for transfer of divalent iron ions (dissolution-deposition) at a metallic iron electrode in a sulfuric add solution at pH 4 (0.5MFesS04-)-0.5MKaS04) = anodic iron dissolution (cathodic iron...

Fig. 10-28. Polarization curves for cell reactions of photoelectrolytic decomposition of water at a photoezcited n-type anode and at a metal cathode solid curve M = cathodic polarization curve of hydrogen evolution at metal cathode solid curve n-SC = anodic polarization curve of oxygen evolution at photoezcited n-type anode (Fermi level versus current curve) dashed curve p-SC = quasi-Fermi level of interfadal holes as a ftmction of anodic reaction current at photoezcited n-type anode (anodic polarization curve r re-sented by interfacial hole level) = electrode potential of two operating electrodes in a photoelectrolytic cell p. sc = inverse overvoltage of generation and transport ofphotoezcited holes in an n-type anode.

It then follows that the anodic polarization curve intersects the cathodic polarization curve (i = i ) at the corrosion potential as illustrated in Fig. 11-5. 

As described in Sec. 11.3, the spontaneous corrosion potential of a corroding metal is represented by the intersection of the anodic polarization curve of metal dissolution with the cathodic polarization curve of oxidant reduction (Figs. 11—5 and 11-6). Then, whether a metal electrode is in the active or in the passive state is determined by the intersection of the anodic and cathodic polarization curves. 

The cathodic ctirrent of this reaction increases with increasing concentration of nitric add as shown by cathodic polarization curves (dashed curve) in Fig. 1 l-14(b). 

A mixed polarization diagram (where the polarization behavior of the two different electrodes is represented) for the sphalerite-hypersteel combination is given in Fig. 1.10 (Vathsala and Natarajan, 1989), in which the cathodic polarization curves for the sphalerite and the anodic polarization curves for the hypersteel ball material are seen to overlap. The active nature of the ball material is evident. The current values were observed to be lower in the absence of oxygen which indicated a lower anodic dissolution of the hypersteel grinding medium in the absence of oxygen. 

Data presented in Figure 6.20 can be used to evaluate the Tafel slope b [Eq. (6.62a)] for the deposition and dissolution of copper. From the cathodic polarization curve we obtain the Tafel slope b as... 

Figure 10, Cathodic polarization curve of p-type GaP and anodic polarization curve of n-type TiOg in 0.5M HgSO with illumination by an ultrahigh-pressure...

Figure 11 Cathodic polarization curves on 2024-T3 in actively oxygen-sparged 1 M NaCl solution with (a) dichromate additions and (b) 0.3 vol% peroxide and dichromate additions. (From A. Sehgal, G. S. Frankel, B. Zoofan, S. Rokhlin. J. Electrochem. Soc. 147, 140 (2000).)... 

The objective of the mass transport lab is to explore the effect of controlled hydrodynamics on the rate at which a mass transport controlled electrochemical reaction occurs on a steel electrode in aqueous sodium chloride solution. The experimental results will be compared to those predicted from the Levich equation. The system chosen for this experiment is the cathodic reduction of oxygen at a steel electrode in neutral 0.6 M NaCl solution. The diffusion-limited cathodic current density will be calculated at various rotating disk electrode rotation rates and compared to the cathodic polarization curve generated at the same rotation rate. 

Cathodic polarization curves for mild steel exposed to air-saturated tap water and tap water containing 200 ppm of CeCl3 [7] showed that oxygen reduction is the primary cathodic reaction in aerated solutions devoid of CeCl3 and the current density is considerably reduced by the addition of CeCb. A pale yellow film was also observed on the sample. 

At intermediate potentials (+ 0.8 to - 0. 2 volts) stainless steel becomes passive owing to the formation of stoichiometric M 203 oxides. For lower potentials (lower than -0.2 volts) corrosion takes place again since the oxides are partly reduced before dissolution. At fee lowest potentials (from --0.3 to -0,5 volts) fee corrosion decrease represents the classical cathodic polarization curve. 

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... 

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