Cathodic polarization curve reactions

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. 

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. 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.

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). 

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. 

Yamashita and co-workers studied the effects of ultrasound on the electrodeposition of Zn from ZnBr2 solution [92], They examined the effects of ultrasound on the cathodic polarization curves and the impedance characteristics of the system. From their results, it was shown that ultrasound increased the reaction rates of deposition and dissolution of Zn. However, ultrasound had no effect on the overpotential of the reaction. Electron microscopy studies also showed that uniform and fine crystals of Zn were obtained in the presence of ultrasound, thus increasing hardness of the coating. 

From the following data for the polarization of the hydrogen evolution reaction on iron at a pH = 4, plot the cathodic polarization curve from i on Fe to iIXred H2 ... 

Plot the cathodic polarization curve for the hydrogen reaction on copper using the data in problem 4 but with a change in the value of the exchange current density to i0 on Cu = 1 mA/m2. Why should the polarization curves for hydrogen evolution on copper and iron terminate at the same iu red value ... 

Assume the homogeneous corrosion of iron in a deaerated acid solution at pH = 4 (i.e., the anodic and cathodic reactions are occurring uniformly over any unit area). Plot the anodic polarization curve for iron and the cathodic polarization curve for the hydrogen reaction. Estimate Ecorr, icorr, and the corrosion penetration rate in pm/year. Given ... 

In Chapter 4, analysis of the kinetics of coupled half-cell reactions shows how the corrosion potential and corrosion current density depend on the positions of the anodic and cathodic polarization curves. The anodic polarization curves are generally represented as showing linear or Tafel behavior, and the cathodic curves are shown with both Tafel and... 

In deaerated 1 N H2SO4 (pH = 0.56), hydrogen-ion reduction is the cathodic reaction with the cathodic polarization curve intersecting the iron, nickel, and chromium curves in the active potential region. Hence, active corrosion occurs with hydrogen evolution, and the corrosion rates would be estimated by the intersections of the curves. The curves predict that the titanium will be passivated. However, the position ofthe cathodic hydrogen curve relative to the anodic curves for titanium and chromium indicates that if the exchange current density for the hydro-... 

The sequence of reactions involved in the overall reduction of nitric acid is complex, but direct measurements confirm that the acid has a high oxidation/reduction potential, -940 mV (SHE), a high exchange current density, and a high limiting diffusion current density (Ref 38). The cathodic polarization curves for dilute and concentrated nitric acid in Fig. 5.42 show these thermodynamic and kinetic properties. Their position relative to the anodic curves indicate that all four metals should be passivated by concentrated nitric acid, and this is observed. In fact, iron appears almost inert in concentrated nitric acid with a corrosion rate of about 25 pm/year (1 mpy) (Ref 8). Slight dilution causes a violent iron reaction with corrosion rates >25 x 1()6 pm/year (106 mpy). Nickel also corrodes rapidly in the dilute acid. In contrast, both chromium and titanium are easily passivated in dilute nitric acid and corrode with low corrosion rates. 

The potentiostat can be set to polarize the WE either anodically, in which case the net reaction at the WE surface is oxidation (electrons removed from the WE), or cathodically, in which case the net reaction at the WE surface is reduction (electrons consumed at the WE). With reference to the potentiostatic circuit in Fig. 6.1, determination of a polarization curve is usually initiated by measuring the open-circuit corrosion potential, Ecorr, until a steady-state value is achieved (e g., less than 1.0 mV change over a five-minute period). Next, the potentiostat is set to control at Ecorr and connected to the polarization cell. Then, the set-point potential is reset continuously or stepwise to control the potential-time history of the WE while Iex is measured. If the set-point potential is continuously increased (above Ecorr), an anodic polarization curve is generated conversely, if the potential is continuously decreased (below Ecorr), a cathodic polarization curve is produced. 

The net (or experimental) anodic and cathodic polarization curves in Fig. 6.2 also can be expressed with Ecorr and Icorr as parameters. This form is used in establishing expressions that provide the basis of one of the experimental techniques for determination of Icorr. At the specific condition that E = Ecorr and Iex = 0, Iox M = Ired x = Icorr therefore, the Tafel expressions for the currents of the individual anodic and cathodic reactions can be equated, or... 

Figure 1. Cathodic polarization curves for a rapid electrode reaction calculated from Eq. (7), where j/]j = y, E 0.34 V. (a) z 1 (b) z = 2.

Upon polarization of either electrode, the cell potential moves along the oxidation and reduction curves as shown in Fig. 1.1. When the current through the cell is f, the potential of the copper and zinc electrodes is Cj cu and e zn > and each of the electrodes have been polarized by (Ceq.cu i.Cu) and (Ceq.zn i,z )- Upon further polarization, the anodic and cathodic curves intersect at a point where the external current is maximized. The measured output potential in a corroding system, often termed the mixed potential or the corrosion potential (Tcorr)> h the potential at the intersection of the anodic and the cathodic polarization curves. The value of the current at the corrosion potential is termed the corrosion current (Icon) and can be used to calculate corrosion rate. The corrosion current and the corrosion potential can be estimated from the kinetics of the individual redox reactions such as standard electrode potentials and exchange current densities for a specific system. Electrochemical kinetics of corrosion and solved case studies are discussed in Chapter 3. 

The cathodic polarization curve is constructed using the oxygen electrode equilibrium potential and the cathodic slope, 6<- = —0.05V/decade. The equilibrium cathode potential, geq,c is calculated by applying the Nemst equation to the oxygen reduction reaction, Eq. (4.13), for an oxygen concentration of 1.0x10 mol/1 at pH= 11. 

If the cathodic polarization curves of Fig. 6.5 intersect the anodic curve at a still more noble potential, within the transpassive region, the corrosion rate of, for example, stainless steel, is greatly increased over the corrosion rate at less noble potentials within the passive region, and the corrosion products become Cr207 and Fe ". Transpassivity occurs not only with stainless steels, but also with chromium, for which the potential for the reaction... 

In aerated solutions, the cathodic reaction is the O2 reduction reaction (Eq. 22). In such media, the anodic polarization curve will be the same as in acidic solutions (Fig. 14.6) in the absence of O2. However, the cathodic polarization curve (Fig. 14.7) has a different shape, because the reduction of O2 is a diffusion-controlled process. 

Equations (4.91) to (4.93) can be applied to any cathodic partial reaction for which the charge-transfer step obeys the Butler-Volmer equation. In corrosion, oxygen reduction is often under mixed control. Figure 4.25 shows the cathodic polarization curve for oxygen reduction, measured on a platinum electrode [6]. The shape of the curve suggests a relatively low value for the ratio I o/ltil-... 

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