Cathodic reaction potential polarization

The stationary cathodic current-potential polarization curve of the OERR on pyrolytic graphite exhibits the Tafel slope —0.120 V per decade at 25°C, and the stoichiometric number is 2 for the O2 to OH2 electroreduction reaction. For glassy carbon, the Tafel slope is —0.060 V per decade , and the corresponding stoichiometric number is 1, as expected for reaction (19.26) [28]. The OERR is first order in O2 and zero order in OH for both carbons. 

A reference electrode scanned along the metal surface will measure the series of (E"x)n and (E"M)n interface potentials. From these values, solution potentials (t))s) at the metal/solution interface may be calculated (< )s = -E") and presented as in Fig. 4.6. When the anodic and cathodic sites are microscopic relative to the size and position of the reference electrode, identity of the anodic and cathodic sites on a macroscale is lost, and a single mixed or corrosion potential, Ecorr, is measured as discussed previously. There is essentially a uniform flux of metal ions from the surface, and cathodic reactants to the surface, which constitute anodic and cathodic currents. Since the relative areas to which these currents apply usually are not known, the total area is taken as the effective area for each reaction. It is these currents, however, that mutually polarize the anodic reaction potential from E M up to Ecorr and the cathodic reaction potential from E x down to Ecorr. 

Unlike the cathodic reaction, anodic oxidation (ionization) of molecular hydrogen can be studied for only a few electrode materials, which include the platinum group metals, tungsten carbide, and in alkaline solutions nickel. Other metals either are not sufficiently stable in the appropriate range of potentials or prove to be inactive toward this reaction. For the materials mentioned, it can be realized only over a relatively narrow range of potentials. Adsorbed or phase oxide layers interfering with the reaction form on the surface at positive potentials. Hence, as the polarization is raised, the anodic current will first increase, then decrease (i.e., the electrode becomes passive see Fig. 16.3 in Chapter 16). In the case of nickel and tungsten... 

This equation describes the cathodic current-potential curve (polarization curve or voltammogram) at steady state when the rate of the process is simultaneously controlled by the rate of the transport and of the electrode reaction. This equation leads to the following conclusions ... 

Polarization in the cathodic direction accelerates the cathodic reaction and is called cathodic polarization polarization in the anodic direction accelerates the anodic reaction and is called anodic polarization. In Fig. 7-4 the polarization curve is cathodic at potentials more negative and is anodic at potentials more positive than the equilibrium potential E. In electrode reaction kinetics the magnitude of polarization (the potential change in polarization) is called the overvoltage or overpotential and conventionally expressed by symbol ii, which is negative in cathodic polarization and positive in anodic polarization. 

Figure 10-24 shows schematically the electron levels and the polarization curves for a cathodic hole iivjection in an n-type and a p-type electrode of the same semiconductor. The range of potential where the cathodic reaction occurs on the n-type electrode is more cathodic (more negative) than the range of potential for the cathodic reaction on the p-type electrode. The difference between the polarization potential aEd) (point N in the figure) of the n-type electrode and the polarization potential p (i) (point P in the figure) of the p-type electrode at a constant cathodic current i is equivalent to the difference between the Fermi level n r of interior electrons and the quasi-Fermi level of interfacial holes in... 

Fig. 11-6. Polarization curves that can be observed with a corroding metallic electrode (solid curve) compared with anodic and cathodic reaction currents (dashed curve) as Amctions of electrode potential ip (ip ) = anodic (cathodic) polarization current i (i ) = anodic (cathodic) reaction current.

A bare surface of silicon can only exist in fluoride containing solutions. In reality, in these media, the electrode is considered to be passive due to the coverage by Si— terminal bonds. Nevertheless, the interface Si/HF electrolyte constitutes a basic example for the study of electrochemical processes at the Si electrode. In this system, the silicon must be considered both as a charge carrier reservoir in cathodic reactions, and as an electrochemical reactant under anodic polarization. Moreover, one must keep in mind that, according to the standard potential of the element, both anodic and cathodic charge transfers are involved simultaneously (corrosion process) in a wide range of potentials. 

Since in cathodic reactions is always smaller than c°, the concentration polarization has a negative sign, which adds to the activation overpotential in causing the electrode to depart from the equilibrium potential in the negative direction for an electronation reaction. 

The cathodic reactions are normally slower than the anodic reactions and are therefore the rate-determining steps thus the driving force of the corrosion cell reaction (and the overall rate of corrosion) can be slowed down by reducing the difference in potential at the cathode (cathodic polarization). 

Another specialized form of potentiometric endpoint detection is the use of dual-polarized electrodes, which consists of two metal pieces of electrode material, usually platinum, through which is imposed a small constant current, usually 2-10 /xA. The scheme of the electric circuit for this kind of titration is presented in Figure 4.1b. The differential potential created by the imposition of the ament is a function of the redox couples present in the titration solution. Examples of the resultant titration curve for three different systems are illustrated in Figure 4.3. In the case of two reversible couples, such as the titration of iron(II) with cerium(IV), curve a results in which there is little potential difference after initiation of the titration up to the equivalence point. Hie titration of arsenic(III) with iodine is representative of an irreversible couple that is titrated with a reversible system. Hence, prior to the equivalence point a large potential difference exists because the passage of current requires decomposition of the solvent for the cathode reaction (Figure 4.3b). Past the equivalence point the potential difference drops to zero because of the presence of both iodine and iodide ion. In contrast, when a reversible couple is titrated with an irreversible couple, the initial potential difference is equal to zero and the large potential difference appears after the equivalence point is reached. 

Imposing an anodic current density on the iron with an external device results in the generation of the anodic branch of the polarization curve. Increasing the applied anodic current decreases the reduction reaction rate as the surface is polarized in the positive direction. At small anodic current densities, the HER current density is still an appreciable fraction of the anodic current density. Under these conditions the applied current density is less than anodic current density. For example, at a potential of -0.225 V(NHE), 4 is 2 X 10 3 A/cm2, 4pP is 6 X 10 3 A/cm2, and 4 is 8 X 10 3 A/cm2. At sufficiently large anodic current densities (e.g., 10 2 A/cm2 in Fig. 26), the cathodic reaction is insignificant rela-... 

Figure 8 Current-potential relationship for a corrosion process, showing the separation of the anodic and cathodic half-reactions by polarization to positive and negative potentials, respectively.

Implicit in these relationships is that both the anodic and the cathodic halfreactions are polarized far from their equilibrium potentials (i.e., E Ee is large) and hence are irreversible. For the 02 reduction reaction this is inevitably so, but for reactive metals like Cu (and Fe, Zn) this may not be so. 

Corrosion of A1 alloys often occurs under conditions in which the primary cathodic reaction is oxygen reduction. In cathodic polarization experiments, this reaction appears to be under mass-transfer control at potentials near the open... 

In analyzing the polarization data, it can be seen that the cathodic reaction on the copper (oxygen reduction) quickly becomes diffusion controlled. However, at potentials below -0.4 V, hydrogen evolution begins to become the dominant reaction, as seen by the Tafel behavior at those potentials. At the higher anodic potentials applied to the steel specimen, the effect of uncompensated ohmic resistance (IRohmk) can be seen as a curving up of the anodic portion of the curve. 

Formation of a Galvanic Cell. When a metal or alloy is electrically coupled to another metal or conducting nonmetal in the same electrolyte, a galvanic cell is created. The electromotive force and current of the galvanic cell depend on the properties of the electrolyte and polarization characteristics of anodic and cathodic reactions. The term galvanic corrosion has been employed to identify the corrosion caused by the contact between two metals or conductors with different potentials. It is also called dissimilar metallic corrosion or bimetallic corrosion where metal is the conductor material. 

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