Electrochemical techniques Tafel polarization

Figure 31.1 shows a classic electrochemically measured Tafel polarization diagram [33. The Tafel analysis is performed by extrapolating the linear portions of both cathodic and anodic curves on a log (current) versus potential plot to their point of intersection. This intersection point provides both the corrosion potential con and the corrosion current density for the system unperturbed. This is a very simple yet powerful technique for quantitatively characterizing a corrosion process. The Tafel equation can be simplified to provide Eq. (7) by approximation using a power series expansion. 

Many different electrochemical and non-electrochemical techniques exist for the study of corrosion and many factors should be considered when selecting a technique. Corrosion rate can be determined by Tafel extrapolation from a potentiodynamic polarization curve. Corrosion rate can also be determined using the Stem-Geary equation from the polarization resistance derived from a linear polarization or an electrochemical impedance spectroscopy (EIS) experiment. Techniques have recently been developed to use electrochemical noise for the determination ofcorrosion rate. Suscephbility to localized corrosion is often assessed by the determination of a breakdown potenhal. Other techniques exist for the determinahon of localized corrosion propagahon rates. The various electrochemical techniques will be addressed in the next section, followed by a discussion of some nonelectrochemical techniques. 

Electrochemical corrosion techniques are essential to predict service life in chemical and construction industries. The following direct current (dc) electrochemical methods are used in corrosion engineering practice linear polarization technique, Tafel extrapolation, and open circuit potential vs. time measurements. The alternating current (ac) technique is electrochemical impedance spectroscopy (EIS). This technique uses alternating current to measure frequency-dependent processes in corrosion and estimates the change of polarization resistance as a function of time. 

With the advent of advanced electronics and computerization, electrochemical techniques have evolved rapidly. The most common technologies today are the polarization resistance technique, electrochemical impedance, and Tafel extrapolation. Regardless of the technique used, each relies on the same basic principles in each test, a metallic coupon in an electrolyte is subject to an electrical perturbation. This perturbation is the appUcation of a current from an external source (power supply). This current stimulates the surface corrosion reactions. The voltage (potential) response of the coupon is measured and correlated with the current appUed—a galvanodynamic test. Conversely, the coupon potential is controlled and correlated with the requisite current—a potentiodynamic test. In either case, the resultant current is representative of the rate determining mass transfer or charge transfer rate. This may be related to the corrosion rate. 

Aqueous corrosion is electrochemical in nature. It is therefore possible to measure corrosion rate by employing electrochemical techniques. Two methods based on electrochemical polarization are available The Tafel extrapolation and linear polarization. Electrochemical methods permit rapid and precise corrosion-rate measurement and may be used to measure corrosion rate in systems that cannot be visually inspected or subject to weight-loss tests. Measurement of the corrosion current while the corrosion potential is varied is possible with the apparatus shown in Figure 1.4. 

The study of uniform corrosion and studies assuming corrosion uniformity are probably the most widespread application of electrochemical measurements both in the laboratory and in the field. The widespread use of these electrochemical techniques does not mean that they are without complications. Both linear polarization and Tafel extrapolation need special precautions for their results to be valid. The main complications or obstacles in performing polarization measurements can be summarized in the following categories ... 

The importance of knowing the exact value of the ohmic drop or uncompensated resistance in an electrochemical system has been pointed out by many workers. In studies of the kinetics of electrode processes by potentiostatic techniques, the ohmic potential drop produces a distortion of the steady state polarization curve which, if uncorrected, will yield erroneous values of the characteristic parameters (Tafel slope, reaction orders) of the electrode reactions (Fig. 6.2). 

Nondestructive Electrochemical Methods—Many of the disadvantages discussed above for the Tafel extrapolation method can be eliminated by using the polarization resistance technique. This nondestructive method can be used on systems that are under either activation or diffusion... 

Ehiring corrosion (oxidation) process, both anodic and cathodic reaction rates are coupled together on the electrode surface at a specific current density known ds icorv This is an electrochemical phenomenon which dictates that both reactions must occur on different sites on the metal/electrolyte interface. For a uniform process under steady state conditions, the current densities at equilibrium are related as o = — c = ieorr Ecorr- Assume that corrosion is uniform and there is no oxide film deposited on the metal electrode surface otherwise, complications would arise making matters very complex. The objective at this point is to determine both Ecorr and icorr either using the Tafel Extrapolation or Linear Polarization techniques. It is important to point out that icorr cannot be measured at Ecorr since ia = —ic and current wfll not flow through an external current-measuring device [3]. 

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