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The inhibition of 4-(2′-amino-5′-methylphenylazo) antipyrine on corrosion of mild steel in HCl solution
Materials Chemistry and Physics 70 (2001) 268–273
The inhibition of 4-(20-amino-50-methylphenylazo) antipyrine on corrosion of mild steel in HCl solution S.S. Abd El Rehim a , Magdy A.M. Ibrahim a,∗ , K.F. Khalid b a b
Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt Chemistry Department, Faculty of Education, Ain Shams University, Cairo, Egypt
Received 25 February 2000; received in revised form 10 May 2000; accepted 3 August 2000
Abstract The effect of 4-(20 -amino-50 -methylphenylazo) antipyrine (AMPA) on the corrosion of mild steel in a 2 M HCl solution was studied using weight loss and potentiodynamic polarization techniques. All of the data reveal that AMPA acts as an inhibitor in the acid environment; furthermore, polarization data show that the compound behaves as a mixed-type inhibitor. It was found that the inhibition efficiency increases with an increase in AMPA concentration but decreases with an increase in temperature. Flory–Huggins adsorption isotherm and El Awady thermodynamic–kinetic model fit the experimental data of the studied compound. Thermodynamic parameters for corrosion and adsorption processes were obtained from experimental data of the temperature studies. © 2001 Published by Elsevier Science B.V. Keywords: 4-(20 -amino-50 -methylphenylazo) antipyrine; Corrosion inhibition; Mild steel; Adsorption isotherms; Hydrochloric acid solution
1. Introduction
2. Experimental
Since aggressive acid solutions are widely used for industrial purposes, inhibitors are commonly used to control the metal dissolution as well as acid consumption. Most of the well known acid inhibitors are organic compounds containing nitrogen, sulphur and/or oxygen atoms. Moreover, many N-heterocyclic compounds have been proved to be effective inhibitors for the corrosion of metals and alloys in aqueous media [1–3]. The inhibition of steel corrosion by acids was previously studied by some nitrogen containing organic compounds [3–9]. These compounds can adsorb on the metal surface and block the active sites on the surface and thereby reduce the corrosion rate. AMPA has not been investigated as corrosion inhibitor. Accordingly, this work deals with the study of the corrosion inhibition properties of this compound. The aim of this study was to determine the inhibition efficiency of AMPA as an inhibitor for the corrosion of mild steel in 2 M HCl. Two techniques were employed to carry out the measurements: (a) chemical (weight loss) and (b) electrochemical (potentiodynamic polarization).
The chemical composition of mild steel rods used in the present work is: C = 0.12%; Mn = 0.85%; S = 0.055%; P = 0.05%; Si = 0.09% and the remainder iron. The electrodes were polished with a series of emery papers, starting from a coarse one 500 and proceeding in steps to fine grade 1500. The electrodes were washed thoroughly with distilled water and degreased with acetone. AR grade HCl (MERK) was used for preparing solutions. Double distilled water was used to prepare solutions of 2 M HCl for measurements. The inhibitor 4-(20 -amino-50 -methylphenylazo) antipyrine was synthesized in our laboratory [10]. The structure of the compound is given below:
∗ Corresponding author. Fax: +20-2483-1836. E-mail address: [email protected] (M.A.M. Ibrahim).
0254-0584/01/$ – see front matter © 2001 Published by Elsevier Science B.V. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 4 6 2 - 4
Weight loss measurements were carried out using the mild steel rods, each of size 2 cm length and 0.5 cm diameter (surface area = 6.5 cm2 ). The time of immersion was 120 min. For potentiodynamic polarization measurements, the cell used is a conventional three electrode Pyrex glass cell with a platinum wire counter electrode and a saturated
S.S. Abd El Rehim et al. / Materials Chemistry and Physics 70 (2001) 268–273
269
calomel electrode (SCE) as a reference. The working mild steel rod electrode was embedded in Araldite so that its cross sectional area (0.126 cm2 ) was in contact with the solution. The potentiodynamic current–potential curves were recorded by changing the electrode potential automatically from −800 to 1000 mV (SCE) with scan rate of 100 mV s−1 . A potentiostat/Galvanostat (EG&G model 273) connected with PC, was used to perform these experiments. In all experiments, the temperature of solutions was controlled by using a water thermostat.
3. Results and discussion The loss in weight of the mild steel samples in 2 M HCl in the absence and presence of different concentrations (0.001–0.01 M) of AMPA were determined. The weight loss (in gm cm−2 h−1 ) as a function of AMPA concentration and at different temperatures (20–60◦ C) is plotted in Fig. 1. The weight loss decreases with increasing the concentration of AMPA at any given temperature. This indicates that the presence of AMPA in the solution inhibits the corrosion of mild steel by HCl and that the extent of corrosion inhibition depends on the amount of AMPA present. From the determined weight loss values, the inhibition efficiencies, P%, were calculated using the following equation [11]: w (1) P % = w 0 − 0 × 100 w where w 0 and w are the weight loss in the absence and presence of inhibitor, respectively.
Fig. 2. Variation of the inhibition efficiency with the logarithmic concentration of AMPA for the mild steel in 2 M HCl at different temperatures (a) 20, (b) 30, (c) 40, (d) 50 and (e) 60◦ C.
Fig. 2 illustrates the variation of the inhibition efficiency, P%, versus the logarithmic concentration of AMPA at different temperatures. Inspection of these data reveal that the inhibition efficiency increases with increasing the concentration of the inhibitor. The corrosion inhibition can be attributed to the adsorption of AMPA molecules at the steel acid solution interface. However, the inhibition efficiency decreases with an increase in temperature. Such behaviour can be interpreted on the basis that an increase in temperature resulted in the desorption of some adsorbed AMPA molecules from the metal surface. The apparent effective energy, E0 , for the corrosion reaction of mild steel in 2 M HCl in the presence and absence of AMPA was calculated from Arrhenius-type equation log(Rate) = −
E0 +A 2.303RT
(2)
where E0 is the apparent activation energy, R is the universal gas constant and A is Arrhenius factor. A plot of log corrosion rate (gm cm−2 h−1 ) of the mild steel obtained from weight loss measurements versus l/T gives straight lines as shown in Fig. 3. The values of the activation energy obtained from the lines are given in Table 1. An alternative formulation of Arrhenius equation is the transition state equation: 1S 0 1H 0 RT exp exp (3) Rate = Nh R RT Fig. 1. Variation of the corrosion rate with the concentration of AMPA for the mild steel in 2 M HCl at different temperatures: (a) 20, (b) 30, (c) 40, (d) 50 and (e) 60◦ C.
where N is Avogadro’s number, h is Planck’s constant, 1S0 is the entropy of activation and 1H0 is the enthalpy of activation.
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Fig. 3. Plot of log (Rate/T) against 1/T of AMPA for the mild steel in 2 M HCl at different concentrations: (a) 0.00, (b) 1 × 10−3 M, (c) 3 × 10−3 M, (d) 5 × 10−3 M, (e) 7×10−3 M and (f) 1 × 10−2 M.
Fig. 4. Plot of log (Rate/T) against l/T of AMPA for the mild steel in 2 M HCl at different concentrations: (a) 0.00, (b) 1 × 10−3 M, (c) 3 × 10−3 M, (d) 5 × 10−3 M, (e) 7 × 10−3 M and (f) 1 × 10−2 M.
A plot of log(Rate/T) versus 1/T should give a straight line (Fig. 4) with a slope of (−1H0 /2.302R) and an intercept of (log R/Nh−1S0 /2.303R) from which the values of 1H0 and 1S0 were calculated (Table 1). It is observed that the apparent activation energy is higher in the presence of inhibitor than in its absence. This type of inhibitor retards corrosion at ordinary temperatures but inhibition is diminished at elevated temperature. The sign of 1H0 reflects the exothermic nature of the corrosion process. The values of E0 and 1H0 enhance with an increase in the concentration of AMPA suggesting that the energy barrier of corrosion reaction increases as the concentration of AMPA is increased. This means that the corrosion reaction will further be pushed to surface sites that are characterized by progressively higher values of E0 as the concentration of the inhibitor becomes greater [11]. The entropy of activation, 1S0 , in the absence and presence of AMPA is negative. This implies that, the activated complex in the rate determining step represents association rather than dissociation step, meaning that a decrease in disordering takes place on
going from reactants to the activated complex [12,13]. However, this value decreases gradually with increasing AMPA concentrations. 3.1. Adsorption isotherms The effect of temperature on the corrosion rate in the absence and presence of the inhibitors was used to elucidate the nature of adsorption of this compound on the steel surface. It was found that the experimental data obtained within the temperature range (20–60◦ C) fits Flory–Huggins adsorption isotherm which is given by [14]: θ = log K + x log (1 − θ ) (4) log C and El Awady et al. Thermodynamic–kinetic model [15,16] which is given by: θ = log K 0 + y log C (5) log (1 − θ )
Table 1 Activation parameters of the dissolution of mild steel in 2 M HCl in the absence and presence of different concentrations of AMPA obtained from weight loss and polarization methods [AMPA]/(M)
From weight loss method E0
0.000 0.001 0.003 0.005 0.007 0.010
(kJ mol−1 )
61.50 82.50 85.60 90.80 92.50 97.30
1H0 58.80 79.80 82.90 88.10 89.80 94.80
From polarization method (kJ mol−1 )
1S0
(J mol−1
−92.70 −35.40 −27.00 −11.70 −7.10 7.30
K −1 )
E0 (kJ mol−1 )
1H0 (kJ mol−1 )
1S0 (J mol−1 K −1 )
80.51
79.88
−25.54
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271
Table 2 Inhibitor binding constant (K), free energy of adsorption (1G0ads ), number of active sites (x) for AMPA in 2 M HCl at different temperatures Temp./K Kinetic–thermodynamic model
293 303 313 323 333
Fig. 5. Curve fitting of the corrosion data of mild steel in 2 M HCl in presence of AMPA to Flory–Huggins isotherm at different temperatures: (a) 20, (b) 30, (c) 40, (d) 50 and (e) 60◦ C.
where θ is the degree of coverage (θ = P %/100), x is the number of inhibitor molecules occupying one active site (or the number of water molecules replaced by one molecule of AMPA), (1/y = x), K is the equilibrium constant of the adsorption reaction and K = K 0(1/y) . Curve fitting of data to Flory–Huggins isotherm and thermodynamic–kinetic models are given in Figs. 5 and 6, respectively. These data gave straight lines.
Fig. 6. Curve fitting of the corrosion data of mild steel in 2 M HCl in presence of AMPA to the kinetic model at different temperatures: (a) 20, (b) 30, (c) 40, (d) 50 and (e) 60◦ C.
1/y
K
1G0ads
1.9 3.6 3.7 4.0 4.3
33808 30269 25475 4786 3909
−35.2 −36.1 −36.9 −33.5 −34.0
(kJ mol−1 )
Flory–Huggins Isotherm x
K
1G0ads (kJ mol−1 )
1.8 3.1 3.9 4.3 4.4
12087 10023 7055 2087 1412
−32.7 −33.3 −33.5 −34.2 −34.5
The calculated values of K, x and l/y are given in Table 2. Inspection of the data of this Table shows the following adsorption characteristics of AMPA in 2 M HCl: (i) values of (1/y) and x are more than unity indicating that each molecule of the inhibitor is attached to one active site of the steel surface; (ii) The value of the equilibrium constant K, decreases with temperature suggesting that this inhibitor is physically adsorbed on the metal surface and desorption process enhances with elevating the temperature; (iii) There is a good agreement between the values of l/y, obtained from the fit of thermodynamic–kinetic model and the values of, x, from the fit of Flory–Huggins isotherm. The thermodynamic functions of adsorption reaction 0 and 1S 0 were calculated from the dependence of 1Hads ads the equilibrium constant K on the temperature by using the equation: log K =
0 0 1Sads −1Hads + 2.303RT 2.303R
(6)
Fig. 7 shows the plot of log K versus l/T which gives straight 0 /2.303R) and intercepts of lines with slopes of (−1Hads
Fig. 7. Plot of the logarithmic value of the inhibitor binding constant, Log K, against l/T.
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Fig. 8. Polarization curves of mild steel in 2 M HCl in the presence of different concentrations of AMPA: (1) 0.00, (2) 1 × 10−3 , (3) 3 × 10−3 , (4) 5 × 10−3 , (5) 7 × 10−3 and (6) 1 × 10−2 M at 30◦ C.
Fig. 9. Polarization curves of mild steel in 2 M HCl in presence of 0.003 M AMPA at different temperatures. (1) 20, (2) 30, (3) 40, (4) 50 and (5) 60◦ C.
0 /2.303R). The standard free energy of adsorption re(1Sads action, −1G0ads was calculated using the equation:
cathodic processes, i.e. acts as a mixed-type inhibitor. On the other hand, the shift in anodic and cathodic polarization decreases with an increase in temperature. The various electrochemical parameters were calculated from Tafel plots and the results are shown in Table 3. The approximate values of Tafel slopes (Bc and Ba ) suggest that the inhibition mechanism involves a single reaction site blocking without modifying the corrosion mechanism [18,19]. Since corrosion rate is directly related to corrosion current, Icorr , the inhibition efficiency, P%, at different inhibition concentrations and temperatures can be calculated from the equation, Icorr 0 × 100 (8) P % = Icorr − 0 Icorr
0 0 − T 1Sads 1G0ads = 1Hads
(7)
0 and 1S 0 are the enthalpy and entropy of where 1Hads ads the adsorption process, respectively. The calculated values 0 is −49.6 kJ mol−1 , 1S 0 is −79.3 J K−1 mol−1 for 1Hads ads 0 and 1G0ads is −28.0 kJ mol−1 . The negative value 1Hads indicate that the adsorption of inhibitor molecules is an exothermic process [17].
3.2. Potentiodynamic polarization An example of the anodic and cathodic potentiodynamic polarization curves for mild steel in 2 M HCl in the absence and presence of different concentrations of AMPA at 30◦ C is represented in Fig. 8. However, Fig. 9 represents the effect of temperature on the anodic and cathodic polarization curves for mild steel in 2 M HCl containing 3.0 × 10−3 M AMPA. The curves show that the inhibitor causes anodic and cathodic overpotential and that the magnitude of the displacement of Tafel plots is proportional to its concentration. This result devotes that AMPA affects the anodic and
0 and Icorr are the corrosion currents in the abwhere Icorr sence and presence of the inhibitor. The inhibition efficiencies calculated from polarization measurements at 30◦ C are given in Table 3. It is seen that the inhibition efficiencies obtained from weight loss method are somewhat different from those obtained from polarization method. The difference can be attributed to the fact that the first method gives average corrosion rates, whereas the second method gives
Table 3 Electrochemical parameters for mild steel with AMPA at 30◦ C in 2 M HCl and P% obtained from weight loss and potentiodynamic polarization tests AMPA Concentration (M)
−Ecorr (V)
Icorr (mA cm−2 )
−Bc (V per decade)
Ba (V per decade)
P (%) Tafel data
P (%) gravimetric data
0.000 0.001 0.003 0.005 0.007 0.010
0.555 0.532 0.531 0.529 0.522 0.521
3.79 0.93 0.88 0.71 0.62 0.41
0.15 0.15 0.15 0.14 0.15 0.12
0.17 0.13 0.12 0.12 0.13 0.11
– 75.5 76.7 81.3 83.6 89.3
– 85.9 89.4 92.5 93.1 95.7
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instantaneous corrosion rates. This difference may also be expected to arise because of the difference in the time required to form an adsorbed layer of inhibitor on metal surface that can inhibit corrosion [16]. By application of Eqs. (2) and (3), we obtained the thermodynamic parameters for corrosion of mild steel in 2 M HCl in the presence of 0.003 M AMPA, the data are given in Table 1. These results show that at this concentration, the activated parameters E0 , 1H0 and 1S0 obtained from polarization measurements are lower than those obtained from weight loss measurements.
4. Conclusion 4-(20 -amino-50 -methylphenylazo) antipyrine (AMPA) was found to be an inhibitor for mild steel corrosion in HCl solution. The inhibition decreases with the increase of the temperature. The inhibition is due to the adsorption of inhibitor molecules on the metal surface and blocking its active sites. The inhibitor acts as a mixed-type inhibitor. The data obtained fit well Flory–Huggins adsorption isotherm and El Awady thermodynamic–kinetic model. References [1] A. El-Sayed, J. Appl. Electrochem. 27 (1997) 193. [2] G. Schmitt, Br. Corros. J. 19 (1984) 165.
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[3] S.S. Abd El Rehim, M.A.M. Ibrahim, K.F. Khalid, J. Appl. Electrochem. 29 (1999) 593. [4] J.M. Sykes, Br. Corros. J. 25 (1990) 175. [5] P. Chatterjee, M.K. Banerjee, K.P. Mukherjee, Indian J. Technol. 29 (1991) 191. [6] S. Rengamani, S. Muralidharan, M. Anbu Kulamdainathan, S. Venkatakrishna Iyer, J. Appl. Electrochem. 24 (1994) 355. [7] G.K. Gomma, M.H. Wahdan, Bull. Chem. Soc. Jpn. 67 (1994) 2621. [8] M. Ajmal, A.S. Mideen, M.A. Quraishi, Corros. Sci. 36 (1994) 79. [9] A. El-Sayed, J. Appl. Electrochem. 27 (1992) 193. [10] Orgata, Nakagawa, Inaishi, Bull. Chem. Soc. Jpn. 54 (1981) 2853. [11] M.K. Gomma, M.H. Wahdan, Mater. Chem. Phys. 39 (1995) 209. [12] J. Marsh, Advanced Organic Chemistry, 3rd Edition., Wiley Eastern, New Delhi, 1988. [13] E. Khamis, A. Hosney, S. El-Khodary, Afinidad (Rev. Quim. Teor. Aplic.) 456 (1996) 95. [14] A. El-Awady, A. Abd El-Naby, S. Aziz, M. Khlifa, Al-Ghamdey, Int. J. Chem. 1(1990) 169. [15] A. El-Awady, A. Abd El-Naby, S. Aziz, J. Electrochem. Soc. 139 (1992) 2149. [16] S. Muralidharan, M.A. Quraishi, S.V.K. Lyer, Corros. Sci. 37 (11) (1995) 1739. [17] B.A. Abd El Naby, E. Khamis, M.S. Ramadan, A. El Gindy, in: Proceedings of the Eighth Annual European Symposium on Corrosion Inhibitors, Univ. Ferraram N.S., Sez, Vol. 10, 1995, p. 299. [18] G. Moretti, G. Quartarone, A. Tassan, A. Zingales, Electrochim. Acta 41 (199) 1971. [19] T.P. Hoar, R.P. Khera, in: Proceedings of the First European Symposium on Corrosion Inhibitor, Vol. 73, University of Ferrara, Ferrara, Italy, 1960.
The inhibition of 4-(20-amino-50-methylphenylazo) antipyrine on corrosion of mild steel in HCl solution S.S. Abd El Rehim a , Magdy A.M. Ibrahim a,∗ , K.F. Khalid b a b
Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt Chemistry Department, Faculty of Education, Ain Shams University, Cairo, Egypt
Received 25 February 2000; received in revised form 10 May 2000; accepted 3 August 2000
Abstract The effect of 4-(20 -amino-50 -methylphenylazo) antipyrine (AMPA) on the corrosion of mild steel in a 2 M HCl solution was studied using weight loss and potentiodynamic polarization techniques. All of the data reveal that AMPA acts as an inhibitor in the acid environment; furthermore, polarization data show that the compound behaves as a mixed-type inhibitor. It was found that the inhibition efficiency increases with an increase in AMPA concentration but decreases with an increase in temperature. Flory–Huggins adsorption isotherm and El Awady thermodynamic–kinetic model fit the experimental data of the studied compound. Thermodynamic parameters for corrosion and adsorption processes were obtained from experimental data of the temperature studies. © 2001 Published by Elsevier Science B.V. Keywords: 4-(20 -amino-50 -methylphenylazo) antipyrine; Corrosion inhibition; Mild steel; Adsorption isotherms; Hydrochloric acid solution
1. Introduction
2. Experimental
Since aggressive acid solutions are widely used for industrial purposes, inhibitors are commonly used to control the metal dissolution as well as acid consumption. Most of the well known acid inhibitors are organic compounds containing nitrogen, sulphur and/or oxygen atoms. Moreover, many N-heterocyclic compounds have been proved to be effective inhibitors for the corrosion of metals and alloys in aqueous media [1–3]. The inhibition of steel corrosion by acids was previously studied by some nitrogen containing organic compounds [3–9]. These compounds can adsorb on the metal surface and block the active sites on the surface and thereby reduce the corrosion rate. AMPA has not been investigated as corrosion inhibitor. Accordingly, this work deals with the study of the corrosion inhibition properties of this compound. The aim of this study was to determine the inhibition efficiency of AMPA as an inhibitor for the corrosion of mild steel in 2 M HCl. Two techniques were employed to carry out the measurements: (a) chemical (weight loss) and (b) electrochemical (potentiodynamic polarization).
The chemical composition of mild steel rods used in the present work is: C = 0.12%; Mn = 0.85%; S = 0.055%; P = 0.05%; Si = 0.09% and the remainder iron. The electrodes were polished with a series of emery papers, starting from a coarse one 500 and proceeding in steps to fine grade 1500. The electrodes were washed thoroughly with distilled water and degreased with acetone. AR grade HCl (MERK) was used for preparing solutions. Double distilled water was used to prepare solutions of 2 M HCl for measurements. The inhibitor 4-(20 -amino-50 -methylphenylazo) antipyrine was synthesized in our laboratory [10]. The structure of the compound is given below:
∗ Corresponding author. Fax: +20-2483-1836. E-mail address: [email protected] (M.A.M. Ibrahim).
0254-0584/01/$ – see front matter © 2001 Published by Elsevier Science B.V. PII: S 0 2 5 4 - 0 5 8 4 ( 0 0 ) 0 0 4 6 2 - 4
Weight loss measurements were carried out using the mild steel rods, each of size 2 cm length and 0.5 cm diameter (surface area = 6.5 cm2 ). The time of immersion was 120 min. For potentiodynamic polarization measurements, the cell used is a conventional three electrode Pyrex glass cell with a platinum wire counter electrode and a saturated
S.S. Abd El Rehim et al. / Materials Chemistry and Physics 70 (2001) 268–273
269
calomel electrode (SCE) as a reference. The working mild steel rod electrode was embedded in Araldite so that its cross sectional area (0.126 cm2 ) was in contact with the solution. The potentiodynamic current–potential curves were recorded by changing the electrode potential automatically from −800 to 1000 mV (SCE) with scan rate of 100 mV s−1 . A potentiostat/Galvanostat (EG&G model 273) connected with PC, was used to perform these experiments. In all experiments, the temperature of solutions was controlled by using a water thermostat.
3. Results and discussion The loss in weight of the mild steel samples in 2 M HCl in the absence and presence of different concentrations (0.001–0.01 M) of AMPA were determined. The weight loss (in gm cm−2 h−1 ) as a function of AMPA concentration and at different temperatures (20–60◦ C) is plotted in Fig. 1. The weight loss decreases with increasing the concentration of AMPA at any given temperature. This indicates that the presence of AMPA in the solution inhibits the corrosion of mild steel by HCl and that the extent of corrosion inhibition depends on the amount of AMPA present. From the determined weight loss values, the inhibition efficiencies, P%, were calculated using the following equation [11]: w (1) P % = w 0 − 0 × 100 w where w 0 and w are the weight loss in the absence and presence of inhibitor, respectively.
Fig. 2. Variation of the inhibition efficiency with the logarithmic concentration of AMPA for the mild steel in 2 M HCl at different temperatures (a) 20, (b) 30, (c) 40, (d) 50 and (e) 60◦ C.
Fig. 2 illustrates the variation of the inhibition efficiency, P%, versus the logarithmic concentration of AMPA at different temperatures. Inspection of these data reveal that the inhibition efficiency increases with increasing the concentration of the inhibitor. The corrosion inhibition can be attributed to the adsorption of AMPA molecules at the steel acid solution interface. However, the inhibition efficiency decreases with an increase in temperature. Such behaviour can be interpreted on the basis that an increase in temperature resulted in the desorption of some adsorbed AMPA molecules from the metal surface. The apparent effective energy, E0 , for the corrosion reaction of mild steel in 2 M HCl in the presence and absence of AMPA was calculated from Arrhenius-type equation log(Rate) = −
E0 +A 2.303RT
(2)
where E0 is the apparent activation energy, R is the universal gas constant and A is Arrhenius factor. A plot of log corrosion rate (gm cm−2 h−1 ) of the mild steel obtained from weight loss measurements versus l/T gives straight lines as shown in Fig. 3. The values of the activation energy obtained from the lines are given in Table 1. An alternative formulation of Arrhenius equation is the transition state equation: 1S 0 1H 0 RT exp exp (3) Rate = Nh R RT Fig. 1. Variation of the corrosion rate with the concentration of AMPA for the mild steel in 2 M HCl at different temperatures: (a) 20, (b) 30, (c) 40, (d) 50 and (e) 60◦ C.
where N is Avogadro’s number, h is Planck’s constant, 1S0 is the entropy of activation and 1H0 is the enthalpy of activation.
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Fig. 3. Plot of log (Rate/T) against 1/T of AMPA for the mild steel in 2 M HCl at different concentrations: (a) 0.00, (b) 1 × 10−3 M, (c) 3 × 10−3 M, (d) 5 × 10−3 M, (e) 7×10−3 M and (f) 1 × 10−2 M.
Fig. 4. Plot of log (Rate/T) against l/T of AMPA for the mild steel in 2 M HCl at different concentrations: (a) 0.00, (b) 1 × 10−3 M, (c) 3 × 10−3 M, (d) 5 × 10−3 M, (e) 7 × 10−3 M and (f) 1 × 10−2 M.
A plot of log(Rate/T) versus 1/T should give a straight line (Fig. 4) with a slope of (−1H0 /2.302R) and an intercept of (log R/Nh−1S0 /2.303R) from which the values of 1H0 and 1S0 were calculated (Table 1). It is observed that the apparent activation energy is higher in the presence of inhibitor than in its absence. This type of inhibitor retards corrosion at ordinary temperatures but inhibition is diminished at elevated temperature. The sign of 1H0 reflects the exothermic nature of the corrosion process. The values of E0 and 1H0 enhance with an increase in the concentration of AMPA suggesting that the energy barrier of corrosion reaction increases as the concentration of AMPA is increased. This means that the corrosion reaction will further be pushed to surface sites that are characterized by progressively higher values of E0 as the concentration of the inhibitor becomes greater [11]. The entropy of activation, 1S0 , in the absence and presence of AMPA is negative. This implies that, the activated complex in the rate determining step represents association rather than dissociation step, meaning that a decrease in disordering takes place on
going from reactants to the activated complex [12,13]. However, this value decreases gradually with increasing AMPA concentrations. 3.1. Adsorption isotherms The effect of temperature on the corrosion rate in the absence and presence of the inhibitors was used to elucidate the nature of adsorption of this compound on the steel surface. It was found that the experimental data obtained within the temperature range (20–60◦ C) fits Flory–Huggins adsorption isotherm which is given by [14]: θ = log K + x log (1 − θ ) (4) log C and El Awady et al. Thermodynamic–kinetic model [15,16] which is given by: θ = log K 0 + y log C (5) log (1 − θ )
Table 1 Activation parameters of the dissolution of mild steel in 2 M HCl in the absence and presence of different concentrations of AMPA obtained from weight loss and polarization methods [AMPA]/(M)
From weight loss method E0
0.000 0.001 0.003 0.005 0.007 0.010
(kJ mol−1 )
61.50 82.50 85.60 90.80 92.50 97.30
1H0 58.80 79.80 82.90 88.10 89.80 94.80
From polarization method (kJ mol−1 )
1S0
(J mol−1
−92.70 −35.40 −27.00 −11.70 −7.10 7.30
K −1 )
E0 (kJ mol−1 )
1H0 (kJ mol−1 )
1S0 (J mol−1 K −1 )
80.51
79.88
−25.54
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271
Table 2 Inhibitor binding constant (K), free energy of adsorption (1G0ads ), number of active sites (x) for AMPA in 2 M HCl at different temperatures Temp./K Kinetic–thermodynamic model
293 303 313 323 333
Fig. 5. Curve fitting of the corrosion data of mild steel in 2 M HCl in presence of AMPA to Flory–Huggins isotherm at different temperatures: (a) 20, (b) 30, (c) 40, (d) 50 and (e) 60◦ C.
where θ is the degree of coverage (θ = P %/100), x is the number of inhibitor molecules occupying one active site (or the number of water molecules replaced by one molecule of AMPA), (1/y = x), K is the equilibrium constant of the adsorption reaction and K = K 0(1/y) . Curve fitting of data to Flory–Huggins isotherm and thermodynamic–kinetic models are given in Figs. 5 and 6, respectively. These data gave straight lines.
Fig. 6. Curve fitting of the corrosion data of mild steel in 2 M HCl in presence of AMPA to the kinetic model at different temperatures: (a) 20, (b) 30, (c) 40, (d) 50 and (e) 60◦ C.
1/y
K
1G0ads
1.9 3.6 3.7 4.0 4.3
33808 30269 25475 4786 3909
−35.2 −36.1 −36.9 −33.5 −34.0
(kJ mol−1 )
Flory–Huggins Isotherm x
K
1G0ads (kJ mol−1 )
1.8 3.1 3.9 4.3 4.4
12087 10023 7055 2087 1412
−32.7 −33.3 −33.5 −34.2 −34.5
The calculated values of K, x and l/y are given in Table 2. Inspection of the data of this Table shows the following adsorption characteristics of AMPA in 2 M HCl: (i) values of (1/y) and x are more than unity indicating that each molecule of the inhibitor is attached to one active site of the steel surface; (ii) The value of the equilibrium constant K, decreases with temperature suggesting that this inhibitor is physically adsorbed on the metal surface and desorption process enhances with elevating the temperature; (iii) There is a good agreement between the values of l/y, obtained from the fit of thermodynamic–kinetic model and the values of, x, from the fit of Flory–Huggins isotherm. The thermodynamic functions of adsorption reaction 0 and 1S 0 were calculated from the dependence of 1Hads ads the equilibrium constant K on the temperature by using the equation: log K =
0 0 1Sads −1Hads + 2.303RT 2.303R
(6)
Fig. 7 shows the plot of log K versus l/T which gives straight 0 /2.303R) and intercepts of lines with slopes of (−1Hads
Fig. 7. Plot of the logarithmic value of the inhibitor binding constant, Log K, against l/T.
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Fig. 8. Polarization curves of mild steel in 2 M HCl in the presence of different concentrations of AMPA: (1) 0.00, (2) 1 × 10−3 , (3) 3 × 10−3 , (4) 5 × 10−3 , (5) 7 × 10−3 and (6) 1 × 10−2 M at 30◦ C.
Fig. 9. Polarization curves of mild steel in 2 M HCl in presence of 0.003 M AMPA at different temperatures. (1) 20, (2) 30, (3) 40, (4) 50 and (5) 60◦ C.
0 /2.303R). The standard free energy of adsorption re(1Sads action, −1G0ads was calculated using the equation:
cathodic processes, i.e. acts as a mixed-type inhibitor. On the other hand, the shift in anodic and cathodic polarization decreases with an increase in temperature. The various electrochemical parameters were calculated from Tafel plots and the results are shown in Table 3. The approximate values of Tafel slopes (Bc and Ba ) suggest that the inhibition mechanism involves a single reaction site blocking without modifying the corrosion mechanism [18,19]. Since corrosion rate is directly related to corrosion current, Icorr , the inhibition efficiency, P%, at different inhibition concentrations and temperatures can be calculated from the equation, Icorr 0 × 100 (8) P % = Icorr − 0 Icorr
0 0 − T 1Sads 1G0ads = 1Hads
(7)
0 and 1S 0 are the enthalpy and entropy of where 1Hads ads the adsorption process, respectively. The calculated values 0 is −49.6 kJ mol−1 , 1S 0 is −79.3 J K−1 mol−1 for 1Hads ads 0 and 1G0ads is −28.0 kJ mol−1 . The negative value 1Hads indicate that the adsorption of inhibitor molecules is an exothermic process [17].
3.2. Potentiodynamic polarization An example of the anodic and cathodic potentiodynamic polarization curves for mild steel in 2 M HCl in the absence and presence of different concentrations of AMPA at 30◦ C is represented in Fig. 8. However, Fig. 9 represents the effect of temperature on the anodic and cathodic polarization curves for mild steel in 2 M HCl containing 3.0 × 10−3 M AMPA. The curves show that the inhibitor causes anodic and cathodic overpotential and that the magnitude of the displacement of Tafel plots is proportional to its concentration. This result devotes that AMPA affects the anodic and
0 and Icorr are the corrosion currents in the abwhere Icorr sence and presence of the inhibitor. The inhibition efficiencies calculated from polarization measurements at 30◦ C are given in Table 3. It is seen that the inhibition efficiencies obtained from weight loss method are somewhat different from those obtained from polarization method. The difference can be attributed to the fact that the first method gives average corrosion rates, whereas the second method gives
Table 3 Electrochemical parameters for mild steel with AMPA at 30◦ C in 2 M HCl and P% obtained from weight loss and potentiodynamic polarization tests AMPA Concentration (M)
−Ecorr (V)
Icorr (mA cm−2 )
−Bc (V per decade)
Ba (V per decade)
P (%) Tafel data
P (%) gravimetric data
0.000 0.001 0.003 0.005 0.007 0.010
0.555 0.532 0.531 0.529 0.522 0.521
3.79 0.93 0.88 0.71 0.62 0.41
0.15 0.15 0.15 0.14 0.15 0.12
0.17 0.13 0.12 0.12 0.13 0.11
– 75.5 76.7 81.3 83.6 89.3
– 85.9 89.4 92.5 93.1 95.7
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instantaneous corrosion rates. This difference may also be expected to arise because of the difference in the time required to form an adsorbed layer of inhibitor on metal surface that can inhibit corrosion [16]. By application of Eqs. (2) and (3), we obtained the thermodynamic parameters for corrosion of mild steel in 2 M HCl in the presence of 0.003 M AMPA, the data are given in Table 1. These results show that at this concentration, the activated parameters E0 , 1H0 and 1S0 obtained from polarization measurements are lower than those obtained from weight loss measurements.
4. Conclusion 4-(20 -amino-50 -methylphenylazo) antipyrine (AMPA) was found to be an inhibitor for mild steel corrosion in HCl solution. The inhibition decreases with the increase of the temperature. The inhibition is due to the adsorption of inhibitor molecules on the metal surface and blocking its active sites. The inhibitor acts as a mixed-type inhibitor. The data obtained fit well Flory–Huggins adsorption isotherm and El Awady thermodynamic–kinetic model. References [1] A. El-Sayed, J. Appl. Electrochem. 27 (1997) 193. [2] G. Schmitt, Br. Corros. J. 19 (1984) 165.
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