Corrosion potential experimental polarization

The sohd line in Figure 3 represents the potential vs the measured (or the appHed) current density. Measured or appHed current is the current actually measured in an external circuit ie, the amount of external current that must be appHed to the electrode in order to move the potential to each desired point. The corrosion potential and corrosion current density can also be deterrnined from the potential vs measured current behavior, which is referred to as polarization curve rather than an Evans diagram, by extrapolation of either or both the anodic or cathodic portion of the curve. This latter procedure does not require specific knowledge of the equiHbrium potentials, exchange current densities, and Tafel slope values of the specific reactions involved. Thus Evans diagrams, constmcted from information contained in the Hterature, and polarization curves, generated by experimentation, can be used to predict and analyze uniform and other forms of corrosion. Further treatment of these subjects can be found elsewhere (1—3,6,18). 

Although the CMT method was originally developed to measure the corrosion rate at the corrosion potential, it has been demonstrated that it can also be used, with some restrictions, to measure the dissolution rate of a polarized electrode. The device for polarization can be a galvanostat or a potentiostat, the operation of which must not interfere with the pH measurements. Most important, the counter electrode must be in the same cell compartment as the experimental electrode and its content well mixed. 

The experimental arrangement for potentiodynamic polarization experiment is shown in Figure 1.26. The experiment is done using the software, and polarization curves (both anodic and cathodic branches of polarization) are recorded at a suitable scan rate. The software performs the calculations and gives the data for corrosion potential and corrosion current density for the system on hand. 

Experimental studies usually yield good agreement between the rates of corrosion obtained from polarization resistance measurements and those derived from weight-loss data, particularly if we recall that the Tafel slopes for the anodic and the cathodic processes may not be known very accurately. It cannot be overemphasized, however, that both methods yield the average rate of corrosion of the sample, which may not be the most critical aspect when localized corrosion occurs. In particular it should be noted that at the open-circuit corrosion potential, the total anodic and cathodic currents must be equal, while the local current densities on the surface can be quite different. This could be a serious problem when most of the surface acts as the cathode and small spots (e.g., pits or crevices) act as the anodic regions. The rate of anodic dissolution inside a pit can, under these circumstances, be hundreds or even thousands of times faster than the average corrosion rate obtained from micro polarization or weight-loss measurements. 

The both numerical and experimental potential distributions are shown in Figure 7 and they show good agreement. It is noted that the good agreement can be achieved even if the open circuit corrosion potential is unknown, polarization curve is non-linear and unknown and there are the offsets of reference electrodes. 

The earlier sections of this chapter discuss the mixed electrode as the interaction of anodic and cathodic reactions at respective anodic and cathodic sites on a metal surface. The mixed electrode is described in terms of the effects of the sizes and distributions of the anodic and cathodic sites on the potential measured as a function of the position of a reference electrode in the adjacent electrolyte and on the distribution of corrosion rates over the surface. For a metal with fine dispersions of anodic and cathodic reactions occurring under Tafel polarization behavior, it is shown (Fig. 4.8) that a single mixed electrode potential, Ecorr, would be measured by a reference electrode at any position in the electrolyte. The counterpart of this mixed electrode potential is the equilibrium potential, E M (or E x), associated with a single half-cell reaction such as Cu in contact with Cu2+ ions under deaerated conditions. The forms of the anodic and cathodic branches of the experimental polarization curves for a single half-cell reaction under charge-transfer control are shown in Fig. 3.11. It is emphasized that the observed experimental curves are curved near i0 and become asymptotic to E M at very low values of the external current. In this section, the experimental polarization of mixed electrodes is interpreted in terms of the polarization parameters of the individual anodic and cathodic reactions establishing the mixed electrode. The interpretation then leads to determination of the corrosion potential, Ecorr, and to determination of the corrosion current density, icorr, from which the corrosion rate can be calculated. 

Tafel Curve Modeling (Ref 4, 5). Equation 6.5 provides the form of the experimental polarization curve when the anodic and cathodic reactions follow Tafel behavior. The equation accounts for the curvature near Ecorr and Icorr, which is observed experimentally. Physically, the curvature is a consequence of both the anodic and cathodic reactions having measurable effects on Iex at potentials near Ecorr. Tafel-curve modeling uses experimental data taken within approximately 25 mV of Ecorr where the corrosion process is less disturbed by induced corro-... 

The numerical methods [32, 33, 34, 35, 36, 37] developed for the analysis of experimental polarization curves described by the current-voltage characteristic (2) with the aim of determining the electrochemical parameters a, /3 and h, are of considerable importance in the field of basic research as well as in corrosion rate monitoring. They permit, in fact, a more objective evaluation of these quantities with reference to a given potential difference interval AE, removing the degree of subjectivity that is inherent in the graphic determination of the Tafel slopes. 

A numerical study of the influence of the ohmic drop on the evaluation of electrochemical quantities has been conducted, for example, over the AE interval [-20, 20] mV by means of the IRCOM program, which makes use of a polynomial of the sixth degree, considering some experimental polarization curves and taking the values of the electrochemical parameters obtained by the NOLI method. The examples examined have shown that the representation of experimental data by a polynomial of the sixth degree is very good and that the evaluation of the correct order of magnitude of the corrosion current density, in the presence of an ohmic contribution to the electrode potential, requires that the actual values of the Tafel slopes be known. 

It was shown above that the corrosion rate can be determined experimentally from the extrapolation of the linear portions of the polarization curves plotted in semilogarithmic space back to the corrosion potential. In order to perform Tafel extrapolation, it is necessary to polarize the electrode to large potentials on either side of the corrosion potential. It is also possible to determine corrosion rate experimentally using much smaller polarization from the corrosion potential, as is shown in this section. 

This expression was derived in Chapter 1.3. An experimental polarization curve, such as that shown in Fig. 3(b), can be fitted by nonlinear least squares fitting to this expression. Such a fit will yield values for the corrosion rate, corrosion potential, and anodic and cathodic Tafel slopes. Most modern software packages for analysis of corrosion data have this capability. 

Measurements are made by applying a current and monitoring the potential after a relaxation time and repeating the procedure for different anodic and cathodic currents. The experimental procedure for obtaining the polarization diagram of a corroding system requires the initial measurement of the open circuit potential of the system. The open circuit potential value falls between the equilibrium potentials of the anodic and cathodic reactions. When there is no current in the external circuit, an open circuit potential value is equal to the corrosion potential. 

At significantly large overpotentials (>100 mV) the anodic and the cathodic polarization curves become linear. Linear extrapolation of the curves will yield a point of intersection at the corrosion potential with the corresponding current being the corrosion current. The experimental procedure above can also be performed potentio-statically using modem potentiostats that are capable of automatically handling the process. 

Once Rp is known, the corrosion rate can be evaluated using the Stern-Geary equation. Polarization resistance and corrosion current are determined from the current measured close to the corrosion potential. Polarization resistance can be determined with minimum system perturbation with linear polarization resistance or by using EIS. Experimentally determined potential ranges that indicate expected iron corrosion intensity for different-measured corrosion potentials are shown in Fig. 12.3. 

In experimentally establishing a polarization diagram, the first measurement is usually that of the corrosion potential, ( )corr, when the applied current, I, is zero. The working electrode is then polarized either anodically or cathodically to establish one of the dashed lines in Fig. 5.6. The polarization procedure is then repeated, but with /appi reversed, to obtain the second dashed line. Using the... 

To experimentally determine the polarization resistance at the corrosion potential one applies a polarization of a few millivolts in the cathodic and the anodic direction and determines the slope as schematically shown in Figure 4.18. Under these... 

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