THE INHIBITIVE EFFECT OF ETHANOLAMINE ON CORROSION ...

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$elements of refractory corrosion in secondary aluminium melting$

246 Metall. Mater. Eng. Vol 18 (4) 2012 p. 241-257 Fig.3. Electric equivalent circuit. CPE is used in this model to compensate non-homogeneity in the system and is defined by two parameters, Y 0 and n. The impedance of CPE is represented by the following equation [21-23]: −1 CPE 0 ( j ) Z = Y ⋅ ω −n (1) where j = (−1) 1/2 , ω = 2πf is frequency in rad s −1 , and f is the frequency in Hz. If n values range from 0.8–1, the impedance of CPE can be considered to be the one of the pure capacitor: Z CPE ( jωC ) −n = (2) and in this case Y 0 gives a pure capacitance (C). The values of charge-transfer resistance obtained from fitting procedure, constant phase element parameter n and double-layer capacitance. The inhibition efficiency, IE, was then calculated using equation (3), where R 0ct and R ct are the charge-transfer resistance values without and with inhibitor, respectively. The values of inhibition efficiency are listed in Table 1. IE R − R = ⋅ (3) ct 0ct % 100 Rct Table 1. Corrosion parameters obtained from EIS measurements for aluminium in 3 wt. % NaCl without and with inhibitor MEA. C inh , mM R ct , Ωcm 2 IE, % n C dl·10 6 , Fcm -2 blank 3195 - 0.914 60.0 1 3436 7.01 0.891 58.2 3 4452 28.23 0.888 54.8 4 5355 40.36 0.887 66.6 5 11560 72.85 0.852 50.5 7 7938 59.97 0.798 102.1 8 4591 30.40 0.858 100.1
Jevremović et al. - The Inhibitive Effect of Ethanolamine on Corrosion... 247 As it can be seen in Table 1 an increase in MEA concentration increases the charge-transfer resistance, R ct values up to 5 mM of ethanolamine due to inhibitor adsorption and then decreases at higher concentrations. The R ct decreases, indicating an increase in the corrosion rate, probably due to desorption of inhibitor. The decrease in double-layer capacitance with increase in MEA concentration up to 5 mM can result from a decrease in local dielectric constant due to the formation of a protective layer on the electrode surface. The thickness of this protective layer increases with increase in inhibitor concentration up to 5 mM, resulting in a decrease in double layer-capacitance. While further increase in inhibitor concentration is followed by double layercapacitance decrease, probably due to the desorption of inhibitor. These results suggest that MEA molecules function by adsorption at the metal/solution interface. The values of the inhibition efficiency calculated from impedance measurements for aluminium at different concentrations of ethanolamine in 3 wt. % NaCl saturated with CO 2 are shown in Fig. 5. It has been observed that inhibition efficiency increased with increase in inhibitor concentration and reached a maximum inhibition efficiency of 73 % at a concentration of 5 mM of ethanolamine. The reduction in the dissolution of aluminuim in the presence of tested inhibitor was attributed to the amino group in the inhibitor molecule. This group is electroactive and interacts with the metal surface to a greater extend. Inhibition efficiency then decreases with further increase in inhibitor concentration. Fig. 5. Dependence of inhibition efficiency, IE, obtained from EIS measurements, on ethanolamine concentrations for aluminium in 3 wt. % NaCl saturated with CO 2 .
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