Polarized half-cell potentials

Polarization. The net current flow produced in a cell results in a deviation of each half-cell potential from the equilibrium value listed in Table 3.3. This deviation from equilibrium is termed polarization, the magnitude of which is given the lowercase greek symbol eta, q and is called the overpotential, E-E°. There are two primary types of polarization activation polarization and concentration polarization. 

Exercise 26-19 The biologically important quinone called plastoquinone is similar to CoQ, except that the CH30— groups of CoQ are replaced by CH3— groups. What differences in properties would you expect between plastoquinone and CoQ and their respective reduction products Consider half-cell potentials (see Exercise 26-15), solubility in polar and nonpolar solvents, and relative acidity. 

Electrochemical cells employed to carry out voltammet-ric or amperometric measurements can involve either a two or three electrode configuration. In the two electrode mode, the external voltage is applied between the working and a reference electrode, and the current monitored. Since the current must also pass through the reference electrode, such current flow can potentially alter the surface concentration of electroactive species that poises the actual half-cell potential of the reference electrode, changing its value by a concentration polarization process. For example, if an Ag/AgCl reference electrode were used in a cell in which a reduction reaction for the analyte occurs at the working electrode, then an oxidation reaction would take place at the surface of the reference electrode ... 

Regardless of the cause of the electron flow at the interface, deviations of the half-cell potentials along the interface from their equilibrium values are functions of the current density. These deviations reflect the polarization behavior of the reaction, a phenomenon of... 

In the derivations of Eq 3.14 and 3.19 for the metal oxidation current density, iox M, and the metal-ion reduction current density, ired M, it was not necessary to restrict the half-cell potential to its equilibrium value. Deviation from E M will occur if the potential of either the metal or the solution is changed, resulting in an overpotential defined in general by Eq 3.1. More specifically, small deviations are associated with charge-transfer polarization, and the overpotential is designated as ... 

These equations are frequently called the Tafel equations for the oxidation and reduction components of the half-cell reaction (Ref 3). Thus, the polarized potentials should plot as linear functions of the logarithm of current density as shown in Fig. 3.9(a). Note that the lines cross when iox m = hed,M = i0,M at the equilibrium half-cell potential, E M. 

A representative anodic polarization curve for iron in a buffered environment of pH = 7 is shown in Fig. 5.4. The solid curve is representative of experimental observations the dashed curve is an extrapolation of the Tafel region to the equilibrium half-cell potential of -620 mV (SHE) and aFg2- = 10 6. This extrapolation allows estimation of an exchange current density of 0.03 mA/m2. The essentially steady minimum current density of the passive state is ip = 1 mA/m2. 

Fig. 5.4 Representative polarization curve for iron in buffered solution of pH = 7. Dashed curve extends to the half-cell potential of iron with ape2+ = 10-6. Letters along curves relate to reactions (details can be found in the text) that are dominant in the associated potential range.

All of the curves in Fig. 5.6 start in the active dissolution potential range and hence do not show the complete polarization curve for the iron extending to the equilibrium half-cell potential as was done in Fig. 5. 4. This extension was shown as dashed lines and the equilibrium potential was taken as -620 mV for Fe2+ = 10 6. Qualitatively, the basis for estimating how the active regions of the curves in Fig. 5.6 would be extrapolated to the equilibrium potential can be seen by reference to Fig. 4.16. There, the corrosion potential is represented as the intersection of the anodic Tafel curve and the cathodic polarization curve for hydrogen-ion reduction at several pH values. It is pointed out that careful measurements have shown that the anodic Tafel line shifts with pH (Ref 6), this shift being attributed to an effect of the hydrogen ion on the intermediate steps of the iron dissolution. 

For metals such as titanium and chromium, the active peak in the anodic polarization curve may occur below the half-cell potential for the... 

Common experience reveals that iron easily oxidizes (rusts) but resists reversal back to the iron base metal. In electrochemistry, this process is known as electrode polarization. A result of polarization is higher electrode resistance to current flow in one direction versus the other. With polarization, the electrode half-cell potential value also tends to vary from table values and depends on the direction and magnitude of electrode cunent flow. It is desirable fw electrodes to be electrochemically reversible since this prevents the process Of polarizatimi. Electrode polarization is a problem in biopotential measurements because it is associated with electric instability. It gives rise to offset potentials between electrodes, electrode noise, and high resistance. 

The linear polarization technique requires us to polarize the steel whth an electric current and monitor its effect on the half cell potential. It is carried out with a sophisticated development of the half cell incorporating an auxiliary electrode and a variable low voltage DC power supply. The half cell potential is measured and then a small current is passed from the auxiliary electrode to the reinforcement. The change in the half cell potential is simply related to the corrosion current by the equation ... 

Criteria relating linear polarization measurements to deterioration rates, similar to the ASTM C876 (1991) criteria for half cell potentials, have been published (Broomfield et aL, 1993). These show some comparability between different devices and will be discussed below under interpretation (Section 4.11.4). A set of conversion equations is provided in the final report of the Strategic Highway Research Program (SHRP) contract on corrosion rate measurement (Fliz et al., 1992). 

Laboratory tests ivith the guard ring device have shown that the corrosion rate in pits can be up to ten times higher than generalized corrosion. This means that the device is very sensitive to pits. However, linear polarization devices cannot differentiate beUveen pitting and generalized corrosion. That must be done by direct observation of the steel or by careful study of the half cell potentials (Vassie, 1991) and chloride contents. 

Most work on linear polarization probes has been done in chloride corrosion condition. Ho vever, the only methods of assessing carbonated concrete are destructive drilling or coring for carbonation depth measurement and trying to interpret half cell potentials which is difficult (Section 4.7.2). Linear polarization is therefore very useful in asse.ssing carbonated structures, particularly as half cell potentials are so difficult to interpret for carbonation induced corrosion. 

Epoxy coated rebars present particular problems to determining the corrosion condition of the steel. In the first place the bars are electrically isolated from the concrete except at areas of damage. The sizes and locations of the areas of damage are obviously unknown. Attempts to carry out half cell potential surveys and linear polarization measurements have therefore been unable to come up with definitive criteria for corroding and non-corroding areas. The other problem is that the bars are isolated from each other so a connection must be made to each bar measured to be sure that contact is being made. 

One definition of effective cathodic protection is to depress the potential of the cathodes to the level of the anodes, thus stopping current from flowing between anodic and cathodic areas (Mears and Brown, 1938). This works because cathodes are more easily polarized (potential shifted) than anodes. We saw this phenomenon in Section 4.11 where the effect of an external current on the half cell potential allows us to calculate the corrosion rate. 

Corrosion tests that are performed on lollipops are corrosion potential readings as described in ASTM C 876 (Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete), polarization resistance as described in ASTM G 59, using IR correction [3,9], and EIS [9]. Specimens are broken open to visually examine the bars for confirmation of the electrochemical results, and chloride analyses are performed. The chloride analysis correlates the chloride content to the corrosion activity. 

Cathodic protection is an electrochemical polarization process that is widely and effectively used to limit corrosion. Simply stated, it is an electrical system whose energy operates in opposition to the natural electrochemical decomposition process of corrosion. All cathodic protection systems require the artificial development of an alternative corrosion cell with (-) electrons flowing finm the artificially installed anode to the structure in the metallic path. It also requires the flow of (+) ions (atoms or molecules carrying electrical charge) from the anode to the structure by the electrolyte path and/or (-) ions in the opposite direction. For a constant current, the level of protection depends on the polarization slope of the cathodic reaction on the structure. Current can be supplied by a galvanic or impressed current system. In a galvanic system, the electrons flow because of the difference in half-cell potential between the metal of the structure and the cathodic protection anode metal, given that the anode metal is more reactive than the metal of concern. In an impressed current system, an... 

Half-Cell Potential Criterion For steel stmctures, cathodic protection is achieved when polarized at the iron (Fe) equilibrium half-cell potential [3]. In neutral environments (soil and seawater), the half-cell potential is based on the following reactions and it is determined by the Nemst equation... 

When using the half-cell potential criterion as developed through the E-log I method, there is a risk that there will be times when the cathodic system will not completely control the corrosion of the steel. For example, if the concrete is near saturation, the steel can usually be polarized with relatively small current densities. Then, if the rectifier is regulated by a halfcell potential and the concrete dries so that oxygen becomes abundant and the polarized potential drifts significantly less negative, it is likely there will be insufficient current capacity to raise the potential to the protective potential value. 

---