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Effect of some ethoxylated fatty acids on the corrosion behaviour of mild steel in sulphuric acid solution
Materials Chemistry and Physics 60 (1999) 286±290
Materials Science Communication
Effect of some ethoxylated fatty acids on the corrosion behaviour of mild steel in sulphuric acid solution E.E. Foad El Sherbini Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt Received 6 October 1998; received in revised form 11 April 1999; accepted 23 April 1999
Abstract The inhibition of the corrosion of mild steel in 1.0 M sulphuric acid solution by some ethoxylated fatty acids OL[EO]20, OL[EO]40 and OL[EO]80 has been studied in relation to the concentration of the inhibitors as well as the temperature using chemical (weight loss) and electrochemical (potentiodynamic polarization) techniques. The inhibition ef®ciency increases with increasing the concentration and the chain length of the inhibitor but decreases with temperature. The inhibition was assumed to occur via adsorption of the fatty acid molecules on the metal surface. The thermodynamic functions of dissolution and adsorption processes were calculated. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Fatty acids; Mild steel corrosion; Inhibition
1. Introduction Study of organic corrosion inhibitor is an attractive ®eld of research due to its usefulness in various industries. Acid solutions are generally used for the removal of undesirable rust in several industrial processes. Inhibitors are usually used in these processes to control the corrosion of the metal. Most of the well-known acid inhibitors are organic compounds containing nitrogen, sulphur and oxygen [1±7]. Many organic acids or their salts are used to inhibit the corrosion rate of metal in aqueous media [8,9]. The aim of this work is to investigate the role played by some ethoxylated fatty acids on the corrosion behaviour of mild steel in 1.0 M sulphuric acid. Interacting the corrosion of steel in sulphuric acid employed the weight loss and potentiodynamic polarization methods. The thermodynamic parameters for both dissolution and adsorption processes were calculated and discussed. 2. Experimental details Samples of polyoxyethylene (n) monooleate [OL(EO)n], H3C±(CH2)±CH=CH±(CH2)8±COO±(CH2CH2O)nH were *Corresponding author.
used as inhibitors, where n20, 40 and 80, these compounds were prepared [10]. They were well-dissolved in aqueous sulphuric acid solutions. The electrodes used were machined from mild steel sheets with the following composition: 0.09% (C), 0.07% (Si), 0.37% (Mn), 0.017% (S), 0.028% (P), 0.005% (Al), 0.015% (Ni), 0.011% (Cr), 0.004% (Mo), 0.006% (Cu) and 0.007% (V). The electrodes were polished with emery papers of 1/0, 2/0, 3/0 and 4/0 grade for ®ne polishing. They were washed thoroughly with doubly distilled water then degreased with acetone and ®nally dried. The dimension of each specimen was 2.31.2 cm. 1.0 M H2SO4 was the working solution used in all cases. For weight loss measurements, each run was carried out in a glass vessel containing 20 ml test solution A clean weighed mild steel electrode was completely immersed at an inclined position in the vessel. After 1 h of immersion, the electrode was withdrawn, rinsed with doubly distilled water, washed with acetone, dried and weighed. The weight loss was used to calculate the corrosion rate in milligrams per square centimetre per hour. For potentiodynamic polarization measurements, the cell was a conventional three electrodes Pyrex glass with a platinum counter electrode, the mild steel sheet as working electrode and saturated, calomel electrode (SCE) as a reference electrode. The potentiodynamic current-potential curves were recorded by changing the electrode potential
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 0 9 3 - 0
E.E. Foad El Sherbini / Materials Chemistry and Physics 60 (1999) 286±290
287
automatically from ÿl000 mV to l000 mV (SCE) with scanning rate of 100 mVsÿ1 using potentiostat±galvanostat (EG and G model 273). All measurements were carried out in freshly prepared deaerated with nitrogen solution at constant temperatures, 308C, 408C, 508C and 608C0.58C with the help of an air thermostat. 3. Results and discussion The weight loss of steel strips in 1.0 M H2SO4 in the absence and the presence of various concentrations of OL[EO]20, OL[EO]40 and OL[EO]80 were determined after 1 h of immersion period. The weight losses as a function of concentrations of each surfactant studied at 308C are plotted in Fig. 1. In all cases, the weight loss decreases with increasing concentration of each surfactant and inhibits the corrosion of steel in 1.0 M H2SO4 solution. Fig. 2 shows the effect of temperature on the weight loss of steel in 1.0 M H2SO4 with 5.010ÿ4 M of each surfactant. The weight loss increases with increasing temperature in the absence and the presence of the inhibitors. These results indicate that the corrosion rates of steel increases with increasing temperature. The protection ef®ciencies (P%) of the three inhibitors in 1.0 M H2SO4 solution were calculated according to the following equations:
Fig. 2. Variation of weight loss with temperatures at concentration 510ÿ4 M inhibitors.
where (Wuninh.) and (Winh.) are the corrosion rate of steel due to the dissolution in 1.0 M H2SO4 in the absence and the presence of de®nite concentration of inhibitors, respec-
tively. Fig. 3 and Table 1 show that the protection ef®ciencies (P%) increase with increasing concentration of the inhibitors. This may suggest that these compounds inhibit the acid dissolution of steel by adsorption at the steel/acid solution interface. The adsorption process takes place vie ion pair and ion exchange mechanism by their ethylene oxide groups while their hydrophobic chains are oriented towards the aqueous media [11]. The protection ef®ciency of these surfactants decreases in the order: OL(EO)80>OL(EO)40>OL(EO)20 due to the difference in the chain length. The increase in the length of the hydrocarbon chain causes an increase in the bulk of the groups attached to the absorption centre [9], and hence reduce the rate of corrosion.
Fig. 1. Variation of weight loss with concentration of inhibitors in 1.0 M H2SO4.
Fig. 3. Variation of the protection efficiency with the logarthmic concentrations of the inhibitors in 1.0 M H2SO4 at 308C.
P%
Wuninh: ÿ Winh: 100; Wuninh:
(1)
288
E.E. Foad El Sherbini / Materials Chemistry and Physics 60 (1999) 286±290
Table 1 The effect of the inhibitor concentration on the protection efficiency
Table 2 The effect of temperature on the protection efficiency
C (mol lÿ1)
Temperature (8C)
ÿ3
1.510 1.010ÿ3 5.010ÿ4 5.010ÿ5 1.010ÿ5
Log C
ÿ2.82 ÿ3.00 ÿ3.30 ÿ4.3 ÿ5.00
P% n20
n40
n80
92.27 91.38 89.10 87.50 86.03
93.32 92.50 91.98 89.10 88.11
94.95 94.25 93.06 92.37 91.50
Fig. 4 and Table 2 show the effect of temperature on the protection ef®ciency. The data reveal that the protection ef®ciency decreases with increasing temperature. This effect is due to desorption of some adsorbed inhibitor molecules from the metal surface. The activation parameters for the corrosion process were calculated from Arrhenius type plot according to the following equation
30 40 50 60
P% n20
n40
n80
89.10 87.03 84.98 82.11
91.98 89.62 87.12 84.84
93.06 91.25 88.36 86.60
Table 3 The values of activation parameters Ea, and H0, and S0 for mild steel in 1.0 M H2SO4 in the absence and the presence of the inhibitors Sample
Ea (kJ molÿ1)
H0 (kJ molÿ1)
S0 (J Kÿ1 molÿ1)
000 n20 n40 n80
36.76 52.84 51.08 50.29
36.95 53.23 55.29 56.48
ÿ64.89 ÿ30.61 ÿ22.76 ÿ21.15
where Ea is the apparent activation energy, R the universal gas constant, h is Planck's constant, N is Avogadro's number, S0 the entropy of activation, H0 the enthalpy of activation and T is the absolute temperature. A plot of rate vs. 1/T gives straight lines with slope of Ea/2.303R. The intercept will be A. Fig. 5 represents the relation between log (rate) and reciprocal of the absolute temperature of mild steel in 1.0 M H2SO4 in the presence of different inhibitors at concentration (5l0ÿ4 M). The data are summarized in Table 3. Inspection of this table shows
that the apparent activation energy of mild steel in 1.0 M sulphuric acid in the absence of inhibitors was 36.8 kJ molÿ1. Data of this table reveal high activation energies for the inhibition process by the different inhibitors at given temperatures indicating their higher protective ef®ciency. The activation energy Ea values decrease with increasing the chain length of the ethylene oxide unit in the oleate structure. By using the transition state Eq. (3) a plot of log (rate/T) vs. (1/T) gives straight lines as shown in Fig. 6. The slopes of these lines equal H0/2.303R and the intercept will be (log RT/NhS0/2.303R) and their values are given in Table 3. The data show that high negative values of enthalpies of activation (H0) are obtained. The negative signs re¯ect the exothermic nature of the dissolution process. Also, the values of entropy of activation (S8) are large and
Fig. 4. Variation of the protection efficiency with temperatures at concentration 510ÿ4 M inhibitors.
Fig. 5. Arrhenius plots of the corrosion rate of mild steel in 1.0 M H2SO4 in presence of inhibitors.
Log rate ÿEa =2:303 RT A
(2)
and from transition state plot according to the following equation rate RT=Nh exp S0 =R exp H 0 =RT;
(3)
E.E. Foad El Sherbini / Materials Chemistry and Physics 60 (1999) 286±290
Fig. 6. Transition state plots of the corrosion rate of mild steel in 1.0 M H2SO4 in the presence of inhibitors.
negative. This implies that the activated complex in the ratedetermining step represents association rather than dissociation meaning that a decrease in disordering takes place on going from reactants to activated complex [12,13]. The corrosion data of mild steel in 1.0 M H2SO4 with different additives were ®tted by adsorption Langmuir isotherm equation = 1 ÿ K: C; Wuninh: ÿ Winh: ; Wuninh:
(4)
289
Fig. 8. Curves fitting of corrosion data of mild steel in 1.0 M H2SO4 in the presence of inhibitors to Frumkin isotherm.
ideal equation that should be applied to the ideal case of the physical and chemical adsorption on a smooth surface with no interaction between adsorbed molecules. Fig. 7 gives the result of Langmuir's plot for corrosion inhibition data of these compounds. The same data were ®tted by the Frumkin isotherm equation [14]. Ln= 1 ÿ : C Ln k 2a;
(6)
where is the surface coverage function, K the equilibrium constant of the adsorption reaction and C is the inhibitor concentration in the bulk of the solution. This equation is the
where a is the lateral interaction term describing the molecular interactions in the adsorbed layer and the heterogeneity of the surface. If this relation gives a straight line then Frumkin isotherm is applicable as in Fig. 8. The difference between the slopes of the three oleate structures may be taken as an indication for the presence of lateral interaction
Fig. 7. Curves fitting of the corrosion data of mild steel in 1.0 M H2SO4 in the presence of inhibitors to Langmuir isotherm.
Fig. 9. Potentiodynamic anodic and cathodic polarization curves for mild steel in 1.0 M H2SO4 in the presence of different concentrations of inhibitor.
(5)
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E.E. Foad El Sherbini / Materials Chemistry and Physics 60 (1999) 286±290
Table 4 Electrochemical parameters and protection efficiency for mild steel in 1.0 M H2SO4 in the absence and the presence of different concentrations of OL(EO)40 at 308C C (mol lÿ1)
ÿEcorr/V (SCE)
Icorr (mA cmÿ2)
Ba (V decÿ1)
Bc (V decÿ1)
P%
000 110ÿ5 510ÿ5 510ÿ4 110ÿ3
0.517 0.487 0.461 0.458 0.450
7.063 0.695 0.609 0.351 0.250
0.295 0.130 0.112 0.110 0.105
0.310 0.147 0.146 0.136 0.129
000 90.1 91.5 95.0 96.4
of the adsorbed molecules [6]. For this reason, we can favour the Frumkin isotherm. These results show that the ®ning of corrosion data are in agreement with Langmuir and Frumkin's isotherms and they show that the action of an inhibitor is assumed to be due to its adsorption at the metal/ solution interface [15]. Also, adsorption of these additives leads to the formation of a monolayer of the adsorbate on the metal surface [16]. Anodic and cathodic potentiodynamic polarization of mild steel electrode in 1.0 M H2SO4 in the absence and the presence of various concentrations (110ÿ5 to 110ÿ3 M) of OL(EO)40 at 308C as an example has been studied. The polarization curves are shown in Fig. 9 and the electrochemical parameters are summarized in Table 4. The addition of OL(EO)40 to 1.0 M H2SO4 shifts the anodic polarization to less negative values, and the cathodic polarization to more negative values. From results given in Table 4, a decrease of corrosion current densities (Icorr) with a slight shift of corrosion potential (Ecorr) towards more positive values when the concentration of the inhibitor increases was observed. These results indicate that this compound predominates as anodic inhibitor. The approximately constant values of the Tafel slopes (near to 0.14 V decÿ1 for Bc and 0.11 V decÿ1 for Ba, suggest that the inhibition is related to a single reaction site blocking without modifying the corrosion mechanism [17,18]. However, the protection ef®ciency can be also calculated from the electrochemical relation P%
Iuninh: ÿ Iinh: ; Iuninh:
(7)
where (Iuninh.) is the corrosion current density in the absence of the inhibitor and (Iinh.) corrosion current density in the presence of the inhibitor, determined by extrapolation of cathodic and anodic Tafel lines to the corrosion potentials. The data show that P% increases with increasing concentration of the inhibitor. These results are comparable with those calculated from weight loss measurements in Table 1 but there is a little difference. This observation was also reported by several authors [19±21] and they attributed this difference between electrochemical (potentiodynamic polarization) and chemically determined rates (weight loss) to the operation of a separate potential independent (chemical dissolution) process which co-exists with the electrochemical process but not measured by the polarization curve.
4. Conclusion 1. Some ethoxylated fatty acids act as inhibitors for corrosion of mild steel in 1.0 M H2SO4. 2. The inhibition is due to adsorption of the inhibitor molecules on the steel surface and blocking its active sites. 3. The inhibition efficiency enhances with increasing the concentration of each inhibitor but decreases with temperature. 4. The increase in the ethylene oxide units in the molecular structure leads to more inhibition efficiency. 5. The adsorption of the inhibitor compounds on steel surface obeys Langmuir and Frumkin isotherms. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
A. El Sayed, J. Appl. Electrochem. 27 (1992) 193. G. Schmitt, Br. Corros. J. 19 (1984) 165. M. Sykes, Br. Corros. J. 25 (1990) 175. P. Chatterjee, M.K. Banerjee, K.P. Mukherjee, Ind. J. Technol. 29 (1991) 19. S. Rengamani, S. Muralidharan, M.A. Kulandainathan, S. Venkatakrishna Iyer, J. Appl. Electrochem. 24 (1994) 355. G.K. Gomma, M.H. Wahdan, Bull. Chem. Soc. Jpn. 67 (1994) 2621. M. Ajmal, A.S. Mideen, M.A. Qurainhi, Corros. Sci. 36 (1994) 79. M.N. Desai, R.R. Patel, D.K. Shah, J. Ind. Chem. Soc. 341 (1973). M.M. Osman, S.S. Abd El Rehim, Mater. Chem. Phys. 53 (1998) 34. M.N. Shalaby, Ph.D. Thesis, Am Shams University, 1996. S. Fujii, K. Aramaki, Corros. Eng. (Jpn.) 7 (1958) 162. G.K. Gomma, M.H. Wahdan, Mater. Chem. Phys. 30 (1995) 209. J. Marsh, Advanced Organic Chemistry, 3rd ed., Wiley Eastern, New Delhi, 1988. A.N. Frumkin, Z. Phys. Chem. 116 (1925). F. Mansfeld, Corrosion 36(5) (1981) 301. M.N.H. Moussa, F.I.M. Taha, M.M. Gouda, G.M. Singan, Corros. Sci. 16 (1976) 379. G. Moretti, G. Quartarone, A. Tassan, A. Zingoles, Electrochim. Acta. 41 (1996) 1971. T.P. Hoar, R.P. Khera, Proceedings of the First European Symposium on Corrosion Inhibitor, Ferrara, Italy, vol. 73, University of Ferrara, 1960. G.M. Florianovich, Ya.M. Kolotyrikin, L.A. Sokolova, Third International Conference on Metallic Corrosion, Moscow, vol. 1, 1966, p. 192. G. Oakes, J.M. West, Br. Corros. J. 4 (1969) 66. N. Hackerman, R.M. Hard, R.R. Annand, Corrosion 18 (1962) 37.
Materials Science Communication
Effect of some ethoxylated fatty acids on the corrosion behaviour of mild steel in sulphuric acid solution E.E. Foad El Sherbini Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt Received 6 October 1998; received in revised form 11 April 1999; accepted 23 April 1999
Abstract The inhibition of the corrosion of mild steel in 1.0 M sulphuric acid solution by some ethoxylated fatty acids OL[EO]20, OL[EO]40 and OL[EO]80 has been studied in relation to the concentration of the inhibitors as well as the temperature using chemical (weight loss) and electrochemical (potentiodynamic polarization) techniques. The inhibition ef®ciency increases with increasing the concentration and the chain length of the inhibitor but decreases with temperature. The inhibition was assumed to occur via adsorption of the fatty acid molecules on the metal surface. The thermodynamic functions of dissolution and adsorption processes were calculated. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Fatty acids; Mild steel corrosion; Inhibition
1. Introduction Study of organic corrosion inhibitor is an attractive ®eld of research due to its usefulness in various industries. Acid solutions are generally used for the removal of undesirable rust in several industrial processes. Inhibitors are usually used in these processes to control the corrosion of the metal. Most of the well-known acid inhibitors are organic compounds containing nitrogen, sulphur and oxygen [1±7]. Many organic acids or their salts are used to inhibit the corrosion rate of metal in aqueous media [8,9]. The aim of this work is to investigate the role played by some ethoxylated fatty acids on the corrosion behaviour of mild steel in 1.0 M sulphuric acid. Interacting the corrosion of steel in sulphuric acid employed the weight loss and potentiodynamic polarization methods. The thermodynamic parameters for both dissolution and adsorption processes were calculated and discussed. 2. Experimental details Samples of polyoxyethylene (n) monooleate [OL(EO)n], H3C±(CH2)±CH=CH±(CH2)8±COO±(CH2CH2O)nH were *Corresponding author.
used as inhibitors, where n20, 40 and 80, these compounds were prepared [10]. They were well-dissolved in aqueous sulphuric acid solutions. The electrodes used were machined from mild steel sheets with the following composition: 0.09% (C), 0.07% (Si), 0.37% (Mn), 0.017% (S), 0.028% (P), 0.005% (Al), 0.015% (Ni), 0.011% (Cr), 0.004% (Mo), 0.006% (Cu) and 0.007% (V). The electrodes were polished with emery papers of 1/0, 2/0, 3/0 and 4/0 grade for ®ne polishing. They were washed thoroughly with doubly distilled water then degreased with acetone and ®nally dried. The dimension of each specimen was 2.31.2 cm. 1.0 M H2SO4 was the working solution used in all cases. For weight loss measurements, each run was carried out in a glass vessel containing 20 ml test solution A clean weighed mild steel electrode was completely immersed at an inclined position in the vessel. After 1 h of immersion, the electrode was withdrawn, rinsed with doubly distilled water, washed with acetone, dried and weighed. The weight loss was used to calculate the corrosion rate in milligrams per square centimetre per hour. For potentiodynamic polarization measurements, the cell was a conventional three electrodes Pyrex glass with a platinum counter electrode, the mild steel sheet as working electrode and saturated, calomel electrode (SCE) as a reference electrode. The potentiodynamic current-potential curves were recorded by changing the electrode potential
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 0 9 3 - 0
E.E. Foad El Sherbini / Materials Chemistry and Physics 60 (1999) 286±290
287
automatically from ÿl000 mV to l000 mV (SCE) with scanning rate of 100 mVsÿ1 using potentiostat±galvanostat (EG and G model 273). All measurements were carried out in freshly prepared deaerated with nitrogen solution at constant temperatures, 308C, 408C, 508C and 608C0.58C with the help of an air thermostat. 3. Results and discussion The weight loss of steel strips in 1.0 M H2SO4 in the absence and the presence of various concentrations of OL[EO]20, OL[EO]40 and OL[EO]80 were determined after 1 h of immersion period. The weight losses as a function of concentrations of each surfactant studied at 308C are plotted in Fig. 1. In all cases, the weight loss decreases with increasing concentration of each surfactant and inhibits the corrosion of steel in 1.0 M H2SO4 solution. Fig. 2 shows the effect of temperature on the weight loss of steel in 1.0 M H2SO4 with 5.010ÿ4 M of each surfactant. The weight loss increases with increasing temperature in the absence and the presence of the inhibitors. These results indicate that the corrosion rates of steel increases with increasing temperature. The protection ef®ciencies (P%) of the three inhibitors in 1.0 M H2SO4 solution were calculated according to the following equations:
Fig. 2. Variation of weight loss with temperatures at concentration 510ÿ4 M inhibitors.
where (Wuninh.) and (Winh.) are the corrosion rate of steel due to the dissolution in 1.0 M H2SO4 in the absence and the presence of de®nite concentration of inhibitors, respec-
tively. Fig. 3 and Table 1 show that the protection ef®ciencies (P%) increase with increasing concentration of the inhibitors. This may suggest that these compounds inhibit the acid dissolution of steel by adsorption at the steel/acid solution interface. The adsorption process takes place vie ion pair and ion exchange mechanism by their ethylene oxide groups while their hydrophobic chains are oriented towards the aqueous media [11]. The protection ef®ciency of these surfactants decreases in the order: OL(EO)80>OL(EO)40>OL(EO)20 due to the difference in the chain length. The increase in the length of the hydrocarbon chain causes an increase in the bulk of the groups attached to the absorption centre [9], and hence reduce the rate of corrosion.
Fig. 1. Variation of weight loss with concentration of inhibitors in 1.0 M H2SO4.
Fig. 3. Variation of the protection efficiency with the logarthmic concentrations of the inhibitors in 1.0 M H2SO4 at 308C.
P%
Wuninh: ÿ Winh: 100; Wuninh:
(1)
288
E.E. Foad El Sherbini / Materials Chemistry and Physics 60 (1999) 286±290
Table 1 The effect of the inhibitor concentration on the protection efficiency
Table 2 The effect of temperature on the protection efficiency
C (mol lÿ1)
Temperature (8C)
ÿ3
1.510 1.010ÿ3 5.010ÿ4 5.010ÿ5 1.010ÿ5
Log C
ÿ2.82 ÿ3.00 ÿ3.30 ÿ4.3 ÿ5.00
P% n20
n40
n80
92.27 91.38 89.10 87.50 86.03
93.32 92.50 91.98 89.10 88.11
94.95 94.25 93.06 92.37 91.50
Fig. 4 and Table 2 show the effect of temperature on the protection ef®ciency. The data reveal that the protection ef®ciency decreases with increasing temperature. This effect is due to desorption of some adsorbed inhibitor molecules from the metal surface. The activation parameters for the corrosion process were calculated from Arrhenius type plot according to the following equation
30 40 50 60
P% n20
n40
n80
89.10 87.03 84.98 82.11
91.98 89.62 87.12 84.84
93.06 91.25 88.36 86.60
Table 3 The values of activation parameters Ea, and H0, and S0 for mild steel in 1.0 M H2SO4 in the absence and the presence of the inhibitors Sample
Ea (kJ molÿ1)
H0 (kJ molÿ1)
S0 (J Kÿ1 molÿ1)
000 n20 n40 n80
36.76 52.84 51.08 50.29
36.95 53.23 55.29 56.48
ÿ64.89 ÿ30.61 ÿ22.76 ÿ21.15
where Ea is the apparent activation energy, R the universal gas constant, h is Planck's constant, N is Avogadro's number, S0 the entropy of activation, H0 the enthalpy of activation and T is the absolute temperature. A plot of rate vs. 1/T gives straight lines with slope of Ea/2.303R. The intercept will be A. Fig. 5 represents the relation between log (rate) and reciprocal of the absolute temperature of mild steel in 1.0 M H2SO4 in the presence of different inhibitors at concentration (5l0ÿ4 M). The data are summarized in Table 3. Inspection of this table shows
that the apparent activation energy of mild steel in 1.0 M sulphuric acid in the absence of inhibitors was 36.8 kJ molÿ1. Data of this table reveal high activation energies for the inhibition process by the different inhibitors at given temperatures indicating their higher protective ef®ciency. The activation energy Ea values decrease with increasing the chain length of the ethylene oxide unit in the oleate structure. By using the transition state Eq. (3) a plot of log (rate/T) vs. (1/T) gives straight lines as shown in Fig. 6. The slopes of these lines equal H0/2.303R and the intercept will be (log RT/NhS0/2.303R) and their values are given in Table 3. The data show that high negative values of enthalpies of activation (H0) are obtained. The negative signs re¯ect the exothermic nature of the dissolution process. Also, the values of entropy of activation (S8) are large and
Fig. 4. Variation of the protection efficiency with temperatures at concentration 510ÿ4 M inhibitors.
Fig. 5. Arrhenius plots of the corrosion rate of mild steel in 1.0 M H2SO4 in presence of inhibitors.
Log rate ÿEa =2:303 RT A
(2)
and from transition state plot according to the following equation rate RT=Nh exp S0 =R exp H 0 =RT;
(3)
E.E. Foad El Sherbini / Materials Chemistry and Physics 60 (1999) 286±290
Fig. 6. Transition state plots of the corrosion rate of mild steel in 1.0 M H2SO4 in the presence of inhibitors.
negative. This implies that the activated complex in the ratedetermining step represents association rather than dissociation meaning that a decrease in disordering takes place on going from reactants to activated complex [12,13]. The corrosion data of mild steel in 1.0 M H2SO4 with different additives were ®tted by adsorption Langmuir isotherm equation = 1 ÿ K: C; Wuninh: ÿ Winh: ; Wuninh:
(4)
289
Fig. 8. Curves fitting of corrosion data of mild steel in 1.0 M H2SO4 in the presence of inhibitors to Frumkin isotherm.
ideal equation that should be applied to the ideal case of the physical and chemical adsorption on a smooth surface with no interaction between adsorbed molecules. Fig. 7 gives the result of Langmuir's plot for corrosion inhibition data of these compounds. The same data were ®tted by the Frumkin isotherm equation [14]. Ln= 1 ÿ : C Ln k 2a;
(6)
where is the surface coverage function, K the equilibrium constant of the adsorption reaction and C is the inhibitor concentration in the bulk of the solution. This equation is the
where a is the lateral interaction term describing the molecular interactions in the adsorbed layer and the heterogeneity of the surface. If this relation gives a straight line then Frumkin isotherm is applicable as in Fig. 8. The difference between the slopes of the three oleate structures may be taken as an indication for the presence of lateral interaction
Fig. 7. Curves fitting of the corrosion data of mild steel in 1.0 M H2SO4 in the presence of inhibitors to Langmuir isotherm.
Fig. 9. Potentiodynamic anodic and cathodic polarization curves for mild steel in 1.0 M H2SO4 in the presence of different concentrations of inhibitor.
(5)
290
E.E. Foad El Sherbini / Materials Chemistry and Physics 60 (1999) 286±290
Table 4 Electrochemical parameters and protection efficiency for mild steel in 1.0 M H2SO4 in the absence and the presence of different concentrations of OL(EO)40 at 308C C (mol lÿ1)
ÿEcorr/V (SCE)
Icorr (mA cmÿ2)
Ba (V decÿ1)
Bc (V decÿ1)
P%
000 110ÿ5 510ÿ5 510ÿ4 110ÿ3
0.517 0.487 0.461 0.458 0.450
7.063 0.695 0.609 0.351 0.250
0.295 0.130 0.112 0.110 0.105
0.310 0.147 0.146 0.136 0.129
000 90.1 91.5 95.0 96.4
of the adsorbed molecules [6]. For this reason, we can favour the Frumkin isotherm. These results show that the ®ning of corrosion data are in agreement with Langmuir and Frumkin's isotherms and they show that the action of an inhibitor is assumed to be due to its adsorption at the metal/ solution interface [15]. Also, adsorption of these additives leads to the formation of a monolayer of the adsorbate on the metal surface [16]. Anodic and cathodic potentiodynamic polarization of mild steel electrode in 1.0 M H2SO4 in the absence and the presence of various concentrations (110ÿ5 to 110ÿ3 M) of OL(EO)40 at 308C as an example has been studied. The polarization curves are shown in Fig. 9 and the electrochemical parameters are summarized in Table 4. The addition of OL(EO)40 to 1.0 M H2SO4 shifts the anodic polarization to less negative values, and the cathodic polarization to more negative values. From results given in Table 4, a decrease of corrosion current densities (Icorr) with a slight shift of corrosion potential (Ecorr) towards more positive values when the concentration of the inhibitor increases was observed. These results indicate that this compound predominates as anodic inhibitor. The approximately constant values of the Tafel slopes (near to 0.14 V decÿ1 for Bc and 0.11 V decÿ1 for Ba, suggest that the inhibition is related to a single reaction site blocking without modifying the corrosion mechanism [17,18]. However, the protection ef®ciency can be also calculated from the electrochemical relation P%
Iuninh: ÿ Iinh: ; Iuninh:
(7)
where (Iuninh.) is the corrosion current density in the absence of the inhibitor and (Iinh.) corrosion current density in the presence of the inhibitor, determined by extrapolation of cathodic and anodic Tafel lines to the corrosion potentials. The data show that P% increases with increasing concentration of the inhibitor. These results are comparable with those calculated from weight loss measurements in Table 1 but there is a little difference. This observation was also reported by several authors [19±21] and they attributed this difference between electrochemical (potentiodynamic polarization) and chemically determined rates (weight loss) to the operation of a separate potential independent (chemical dissolution) process which co-exists with the electrochemical process but not measured by the polarization curve.
4. Conclusion 1. Some ethoxylated fatty acids act as inhibitors for corrosion of mild steel in 1.0 M H2SO4. 2. The inhibition is due to adsorption of the inhibitor molecules on the steel surface and blocking its active sites. 3. The inhibition efficiency enhances with increasing the concentration of each inhibitor but decreases with temperature. 4. The increase in the ethylene oxide units in the molecular structure leads to more inhibition efficiency. 5. The adsorption of the inhibitor compounds on steel surface obeys Langmuir and Frumkin isotherms. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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