Polarization curve, electrochemical

Electrochemical methods, such as anodic polarization curves electrochemical impedance spectroscopy (EIS), and measurement of the corrosion potential (open circuit or rest potential) are primarily laboratory tests. They require experience in interpretation of the results, but have the advantage of very short test times. As such, they are important in mechanistic studies, but certain commercial uses also exist. 

The atmospheric corrosion occurs frequently under thin electrolyte layers (TELs) or even adsorbed layers. The thickness of electrolyte has important role on corrosion phenomena such as the mass transport of dissolved oxygen, the accumulation of corrosion products, and the hydration of dissolved metal ions. The corrosion behavior of pure magnesium was investigated under aerated and de-aerated TELs with various thicknesses by means of cathodic polarization curve, electrochemical impedance spectroscopy (EIS) and EN measurements (Zhang et al, 2008). 

Figure C2.8.3. A tliree-electrode electrochemical set-up used for the measurement of polarization curves. A potentiostat is used to control the potential between the working electrode and a standard reference electrode. The current is measured and adjusted between an inert counter-electrode (typically Pt) and the working electrode.

FIG. 28-9 Typical electrochemical polarization curve for an active/passive alloy (with cathodic trace) showing active, passive, and transpassive regions and other important features. (NOTE Epp = primary passive potential, Ecaa- — freely corroding potential). 

Figure 11. Schematic diagram of anodic polarization curve of passive-metal electrode when sweeping electrode potential in the noble direction. The dotted line indicates the polarization curve in the absence of Cl-ions, whereas the solid line is the polarization curve in the presence of Cl ions.7 Ep, passivation potential Eb, breakdown potential Epit> the critical pitting potential ETP, transpassive potential. (From N. Sato, J, Electrochem. Soc. 129, 255, 1982, Fig. 1. Reproduced by permission of The Electrochemical Society, Inc.)...

Figure 10.9. Stationary polarization curve obtained with 10 mol % V205 - 90 mol % K2S207 catalyst at 440°C. 2 Reproduced by permission of the Electrochemical Society.

Indicator electrodes are used both for analytical purposes (in determining the concentrations of different substances from values of the open-circuit potential or from characteristic features of the polarization curves) and for the detection and quantitative characterization of various phenomena and processes (as electrochemical sensors or signal transducers). One variety of indicator electrode are the reference electrodes, which have stable and reproducible values of potential and thus can be used to measure the potentials of other electrodes. 

The current is recorded as a function of time. Since the potential also varies with time, the results are usually reported as the potential dependence of current, or plots of i vs. E (Fig.12.7), hence the name voltammetry. Curve 1 in Fig. 12.7 shows schematically the polarization curve recorded for an electrochemical reaction under steady-state conditions, and curve 2 shows the corresponding kinetic current 4 (the current in the absence of concentration changes). Unless the potential scan rate v is very low, there is no time for attainment of the steady state, and the reactant surface concentration will be higher than it would be in the steady state. For this reason the... 

Electrochemical methods of protection rest on different precepts (1) electroplating of the corroding metal with a thin protective layer of a more corrosion-resistant metal, (2) electrochemical oxidation of the surface or application of other types of surface layer, (3) control of polarization characteristics of the corroding metal (the position and shape of its polarization curves), and (4) control of potential of the corroding metal. 

Other electrochemical techniques covered include measurements of the corrosion potential, the redox potential, the polarization resistance, the electrochemical impedance, electrochemical noise, and polarization curves, including pitting scans. A critical review of the literature concerned with the application of electrochemical techniques in the study of MIC is available [1164]. 

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. 

Fig. 11. Potentiostatic polarization curve on Co3P, measured in a droplet of DI water. The Tafel region is marked [125]. (Reprinted by permission of The Electrochemical Society).

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

Various carbon-based catalysts for the electrochemical oxygen reduction have been tested in the air gas-diffusion electrodes [7]. The polarization curves of the air electrodes were measured when operating against an inert electrode in 2 N NaCl-solution. The potential of the air electrodes was measured versus saturated calomel electrode (SCE). 

The shapes of the polarization curves shown in Fig. 3, including those generally observed for other reducing agents, are invariably complex, and not representative of simple electrochemical reactions. From the inception of modern electroless deposition practice, a number of mechanisms have been advanced to describe the electroless deposition process, many of which dealt with Ni-P. These classical mechanisms include the following ... 

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... 

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. 

Abstract The flotation mechanism is discussed in the terms of corrosive electrochemistry in this chapter. In corrosion the disolution of minerals is called self-corrosion. And the reaction between reagents and minerals is treated as inhibition of corrosion. The stronger the ability of inhibiting the corrosion of minerals, the stronger the reagents react with minerals. The two major tools implied in the research of electrochemical corrosion are polarization curves and EIS (electrochemistry impedance spectrum). With these tools, pyrite, galena and sphalerite are discussed under different conditions respectively, including interactions between collector with them and the difference of oxidation of minerals in NaOH solution and in lime. And the results obtained from this research are in accordance with those from other conventional research. With this research some new information can be obtained while it is impossible for other methods. 

Oxygen/reduction polarization curves from RDE measurements for Ru,92MO(,Q8Se04, Ru/C, and Pt/C in 0.6 M H2SO4,60°C. (T. J. Schmidt et al.. Journal of the Electrochemical Society, 4 2620 (2000). Reproduced by permission of The Electrochemical Society.)... 

Polarization curves for Hj/Oj fuel cells at 50°C, 1 atm pressure. Curve A Nation impregnated (brush coated) PTFE-bound electrode (0.35 mg/cm Pt loading) curve B PTFE-bound catalyst layer (Pt loading 4 mg/cm ) curve C PTFE-bound electrode (Pt loading 0.35 mg/cm. (Based on Ticianelli, E. A. et al. Journal of the Electrochemical Society 1988 135 2209-2214. By permission of The Electrochemical Society.)... 

The initial drop in the polarization curve is due to the sluggish kinetics of the ORR at the temperatures normally used for current polymer-electrolyte fuelcell operation (<100 °C). A typical electrochemical... 

Figure 4. Model and experiment comparison of polarization curves for air or oxygen at different gas pressures and at 70 °C using eq 20. (Reproduced with permission from ref 12. Copyright 1995 The Electrochemical Society, Inc.)...

Experimental validation of SOFC models has been quite scarce. Khaleel and Selman °° presented a comparison of 1-D electrochemical model calculations with experimental polarization curves for a range of... 

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