Polarization curve regions

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

In tlie polarization curve of figure C2.8.4 (solid line), tlie two regimes, activation control and diffusion control, are schematically shown. The anodic and catliodic plateau regions at high anodic and catliodic voltages, respectively, indicate diffusion control tlie current is independent of tlie applied voltage and7 is reached. 

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). 

In the polarization curve for anodic dissolution of iron in a phosphoric acid solution without CP ions, as shown in Fig. 3, we can see three different states of metal dissolution. The first is the active state at the potential region of the less noble metal where the metal dissolves actively, and the second is the passive state at the more noble region where metal dissolution barely proceeds. In the passive state, an extremely thin oxide film called a passive film is formed on the metal surface, so that metal dissolution is restricted. In the active state, on the contrary, the absence of the passive film leads to the dissolution from the bare metal surface. The difference of the dissolution current between the active and passive states is quite large for a system of an iron electrode in 1 mol m"3 sulfuric acid, the latter value is about 1/10,000 of the former value.6... 

The polarization relations found in the region of high polarization are usually plotted semilogarithmically as AE vs. log i (Eig. 6.1). These plots are straight lines, called Tafel lines (curve 1 in Eig. 6.1), when relation (6.3) holds. More complicated polarization functions are found at many real electrodes in the region of high polarization. Sometimes several Tafel sections can be distinguished in an actual polarization curve (curve 2 of Eig. 6.1) each of these sections has its own characteristic values of parameters a and b). 

FIGURE 6.1 Polarization curves in the region of high polarization (1) Tafel (2) complex. 

Fig. 6.4a, curve 2), and the polarization curve is of unusual shape in the region of high anodic CD where i 5S> (the oxidizing agent is the anodic reaction product, hence this relation is possible). In this region... 

Thus, in the region of very high anodic or cathodic polarization, the RDS is always the first step in the reaction path. The transfer coefficient of the full reaction which is equal to that of this step is always smaller than unity (for a one-electron RDS), while slope i in the Tafel equation is always larger than 0.06 V. When the potential is outside the region of low polarization, a section will appear in the polarization curve at intermediate values of anodic or cathodic polarization where the transfer coefficient is larger than unity and b is smaller than 0.06 V. This indicates that in this region the step that is second in the reaction path is rate determining. 

FIGURE 14.4 Schematic polarization curve with harrierless (section AB) and activationless (section CD) regions. 

FIGURE 15.9 Anodic polarization curves recorded at a platinum electrode in the region of high anodic potentials in the presence of acetate ions (1) total current (2) partial current of oxygen evolution (3) partial current of oxidation of adsorbed species. 

Often, it will be found that currents for a given reaction cannot be measured at all metals at the same value of potential. At some metals the currents would be too low for a reliable, sufficiently accurate determination at others they might be too high for a satisfactory experimental realization. A comparison will then be possible only after an extrapolation of data obtained in a different region of potentials, to the value of selected for comparison. This extrapolation may not be sufficiently reliable where the Tafel section of the polarization curve is too short or indistinct. 

Scan Rates Sweeping a range of potentials in the anodic (more electropositive) direction of a potentiodynamic polarization curve at a high scan rate of about 60 V/h (high from the perspective of the corrosion engineer, slow from the perspective of a physical chemist) is to indicate regions where intense anodic activity is likely. Second, for otherwise identical conditions, sweeping at a relatively slow rate of... 

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).

Spiro [27] has derived quantitative expressions for the catalytic effect of electron conducting catalysts on oxidation-reduction reactions in solution in which the catalyst assumes the Emp imposed on it by the interacting redox couples. When both partial reaction polarization curves in the region of Emp exhibit Tafel type kinetics, he determined that the catalytic rate of reaction will be proportional to the concentrations of the two reactants raised to fractional powers in many simple cases, the power is one. On the other hand, if the polarization curve of one of the reactants shows diffusion-controlled kinetics, the catalytic rate of reaction will be proportional to the concentration of that reactant alone. Electroless metal deposition systems, at least those that appear to obey the MPT model, may be considered to be a special case of the general class of heterogeneously catalyzed reactions treated by Spiro. 

Figure 3 shows polarization curves for the anodic oxidation of H2CO at various metal electrodes recorded by Ohno et al. [38] in a solution maintained at 25 °C and containing EDTA (a commonly used complexant in electroless Cu solutions) and maintained at a pH = 12.5. After exhibiting exceptional activity at potentials less than -0.8 Y (SCE)2, the activity of Cu decreases at ca. 0.3 Y (SCE) this region of activity is more than adequate for electroless deposition of Cu. Although they... 

Ultimately, the catalyst performance of a real fuel cell is of the greatest importance. The DEFC polarization curves for the two PtSn anode catalysts are tested and shown in Fig. 15.9. The characteristic data are summarized in Table 15.4. The PtSn-1 catalyst shows a strongly enhanced electron-oxidation reaction (EOR) activity and much better performance in both the activation-controlled region (low-current density region) and... 

The performance of a fuel cell is most often reported in the form of a polarization curve. Such a curve is shown in Figure 3. Roughly speaking, the polarization curve can be broken down into three main regions. At low currents, the behavior of a fuel cell is dominated by kinetic losses. These losses... 

The second part of the polarization curve is the ohmic regime. In this region, the potential varies linearly with the current density. This allows for Ohm s law to be used for modeling purposes... 

Consider first the polarization curve (i.e., Tafel plot) for the anodic halfreaction occurring in corrosion of stainless steels (Fig. 16.8). The diagram for the active region is much the same as has been seen for other anodes (Figs. 15.4 to 15.7). As Eh is increased to a certain specific value, however, a sudden and dramatic drop in the anodic current density i occurs, corresponding to formation of an oxide film. At higher Eh, i remains constant at a very low level (the horizontal scale in Fig. 16.8 is logarithmic), and the metal has become passive, that is, effectively immune from corrosion. 

Figures 16.8 and 16.9 show only the anodic polarization curves for corrosion cells. The important question is, where do these curves intersect with the polarization curves for likely cathodic reactions, such as hydrogen evolution or oxygen absorption The intersection point defines the corrosion current density icorr and hence the corrosion rate per unit surface area. As an example, let us consider the corrosion of titanium (which passivates at negative Eh) by aqueous acid. In Fig. 16.10, the polarization curves for H2 evolution on Ti and for the Ti/Ti3+ couple intersect in the active region of the Ti anode. To make the intersection occur in the passive region (as in Fig. 16.11), we must either move the H+/H2 polarization curve bodily...

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