Tafel extrapolation technique

As with all elec trochemical studies, the environment must be electrically conduc tive. The corrosion rate is direc tly dependent on the Tafel slope. The Tafel slope varies quite widely with the particular corroding system and generally with the metal under test. As with the Tafel extrapolation technique, the Tafel slope generally used is an assumed, more or less average value. Again, as with the Tafel technique, the method is not sensitive to local corrosion. 

Tafel extrapolation technique, the Tafel slope generally used is an assumed, more or less average value. Again, as with the Tafel technique, the method is not sensitive to local corrosion. 

H2SO4 in which the potential was scanned from —20 to -1-20 mV relative to the OCP. A relatively linear response is observed in this potential range. From Fig. 4, the polarization resistance can be found to be 80 cm. Using the Tafel slopes for this system determined from the potentiodynamic polarization curve given in Fig. 3 and the polarization resistance taken from Fig. 4, the corrosion rate is found to he 1.4 x 10 A cm , which is close to the value determined hy the Tafel extrapolation technique. 

Tafel Extrapolation technique (TE) (Figure 32) takes into account the linear parts of the anodic and cathodic curves for determining Rp. 

The value of icon obtained for iron corroding in an aqueous solution (electrolyte) by the Tafel extrapolation technique is 3.74 x 10 " A/m. Find the rate of corrosion in (a) mm/year and (b) mdd. 

The primary use of this laboratoiy technique today is as a quick check to determine the order of magnitude of a corrosion reaction. Sometimes the calculated rate from an immersion test does not Took correct when compared to the visual appearance of the metal coupon. While the specific corrosion rate number determined by Tafel extrapolation is seldom accurate, the method remains a good confirmation tool. 

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. 

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... 

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