Corrosion -testing methods tafel extrapolation

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. 

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. 

The impossibility of a direct measurement of corrosion rate using electrochemical testing would seem to be discouraging. Application of mixed potential theory allows determination of the corrosion rate using a method known as Tafel extrapolation. 

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 main benefit of the LPR method is that the corrosion rate is determined without significant disturbance of the natural corrosion conditions (the electrode is polarized only slightly from the natural corrosion potential). Because a condition is that the Tafel constants are known, it is often preferable to combine the LPR method with the first described method (extrapolation of linear parts of the polarization curves). To avoid disturbanee of the natural conditions during the test, the complete polarization curves should be reeorded at the end of the exposure period. 

Draw the polarization curves. Determine the eorrosion current by extrapolation of linear parts of the eurves recorded at the end of the test, and by using the LPR measurements obtained at two and four hours, respectively. Assume that the Tafel gradients do not change during the exposure period. Compare and discuss the results. Explain the differences between the corrosion currents determined by die two methods. 

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