Electrochemical tests Tafel extrapolation

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. 

As with all electrochemical studies, the environment must be electrically conductive. The corrosion rate is directly 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 locm corrosion. 

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. 

DC polarisation tests took place in a three-electrode electrochemical cell where the area of the working electrode (modified Al-2024 T3) exposed to the corrosive solution was 1 cm. For these measurements, a platinum foil and a SCE were used as the counter and the reference electrodes, respectively. Potentiodynamic tests were carried out by scanning the potential between -1 V and 0 V vs SCE at a rate of 1 mV s. The resulting graphics were fitted to the Tafel plots allowing the corrosion current and potential, and the polarisation resistance to be extrapolated. Corrware software was used for fitting. 

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