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Poly(aniline-formaldehyde): A new and effective corrosion inhibitor for mild steel in hydrochloric acid
Materials Chemistry and Physics 113 (2009) 685–689
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Poly(aniline-formaldehyde): A new and effective corrosion inhibitor for mild steel in hydrochloric acid M.A. Quraishi ∗ , Sudhish Kumar Shukla Department of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi 221005, India
a r t i c l e
i n f o
Article history: Received 20 March 2008 Accepted 10 August 2008 Keywords: Mild steel Poly(aniline-formaldehyde) Electrochemical impedance spectroscopy (EIS) Atomic force microscopy (AFM)
a b s t r a c t In recent years, polyanilines have emerged as efficient class of corrosion inhibitors for mild steel in acidic media. The corrosion inhibition of poly(aniline-formaldehyde) on mild steel in 1.0N HCl has been evaluated by potentiodynamic polarization, linear polarization, electrochemical impedance spectroscopy and weight loss measurements. Results obtained show that poly(aniline-formaldehyde) is a mixed inhibitor and it inhibits mild steel corrosion through adsorption mechanism. It showed >90% inhibition efficiency at 10 ppm. AFM clearly reveals that surface roughness of inhibited mild steel sample is less than uninhibited mild steel. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Inhibition of corrosion of mild steel is a matter of theoretical as well as practical importance [1]. Acids are widely used in industries such as pickling, cleaning, descaling, etc. because of their aggressiveness; inhibitors are used to reduce the rate of dissolution of metals. Compounds containing nitrogen, sulphur and oxygen have been reported as inhibitors [2–6]. The most efficient organic inhibitors are organic compounds having -bonds in their structures [7]. The efficiency of an organic compound as an inhibitor is mainly dependent on its ability to get adsorbed on metal surface which consists of a replacement of water molecule at a corroding interface as Org(Sol) + nH2 O(ads) → Org(ads) + nH2 O(Sol) The adsorption of these compounds is influenced by the electronic structure of inhibiting molecules, steric factor, aromaticity, and electron density at donor site, presence of functional group such as –CHO, –N N, R–OH, etc., molecular area and molecular weight of the inhibitor molecule [8–11]. The earlier studies [12–18] have shown that inhibitive properties of polyaniline and its derivatives on corrosion of iron in acid chloride solution are due to the presence of -electrons, quaternary nitrogen atom and large molecular size which ensures greater coverage of the metallic surface. So the adsorption of polymer
molecules on the iron electrode surface is more, which leads to more inhibition efficiency. In this work the inhibitive behavior of poly(aniline-formaldehyde) on mild steel in 1N HCl solution has been studied. 2. Experimental 2.1. Synthesis of poly(aniline-formaldehyde) [17] Reagent grade aniline was purified by distillation under reduced pressure and formaldehyde (40%) is taken as such it is supplied. 5 ml of aniline in dil-HCl is taken in a 250 ml beaker. 5 ml of formaldehyde solution (40%) is added slowly with stirring. The mixture is then stirred-well for 15 min to form an orange-red solution. Then this solution is fed into aqueous sodium hydroxide solution (1%) and refluxed for 30–45 min. The poly(aniline-formaldehyde) will precipitate out. Filter the polymer wash with water and dry it. Characterization is done by using UV spectroscopy and FTIR-spectroscopy [Scheme 1]. 2.2. Methodology The mild steel strips having composition (wt.%): C 0.04, Mn 0.035, Si 0.17, S 0.025, P 0.03 and balance Fe were used for weight loss as well as electrochemical studies. The aggressive solution of hydrochloric acid (AR grade) is used for all studies. The weight loss studies were conducted on mild steel strips of 5.0 cm × 2.0 cm × 0.025 cm sizes. Weight loss study was carried out at 35 ◦ C temperature and 3 h time duration and 1N hydrochloric acid solution. Weight loss studies are also done on various strength of acid, various duration ranges and various temperature ranges in presence of optimum concentration of inhibitor. The inhibition efficiency (%) and surface coverage () was determined by using following equation: I.E. (%) =
∗ Corresponding author. Tel.: +91 9335636661; fax: +91 542 2368428. E-mail address: [email protected] (M.A. Quraishi). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.08.028
=
Wo − Wi × 100 Wo
Wo − Wi Wo
686
M.A. Quraishi, S.K. Shukla / Materials Chemistry and Physics 113 (2009) 685–689
Scheme 1.
where Wo and Wi are the weight loss values in absence and in presence of inhibitor. The electrochemical studies were carried out in a three electrode cell assembly at room temperature. The working electrode was a mild steel of above composition of 1 cm2 area and the rest being covered by using commercially available lacquer. A large rectangular foil was used as counter electrode and saturated calomel electrode as reference electrode. The working electrode was polished with different grades of emery papers, washed with water and degreased with acetone. The polarization and impedance studies were made after 30 min of immersion using Gamry Potentiostat (Model PC4/750). The inhibition efficiency was evaluated from the measured Icorr values using the relationship: I.E. (%) =
o i − Icorr Icorr × 100 o Icorr
o i and Icorr are the corrosion currents in absence and in presence of inhibitor. where Icorr For linear polarization resistance measurement, the potential of the electrode was scanned from −0.02 to +0.02 V vs corrosion potential a scan rate of 0.5 mV s−1 and the polarization resistance was measured from the slope of vs i curve in the vicinity of corrosion potential. From the measured polarization resistance value, the inhibition efficiency has been calculated using the relationship:
I.E. (%) =
Rpi − Rpo Rpi
× 100
where Rpo and Rpi are the polarization resistances in absence and in presence of inhibitor. The impedance studies were carried out using ac signals of 10 mV amplitude for the frequency spectrum from 100 kHz to 0.01 Hz. The charge transfer resistance values were obtained from the diameter of the semi circles of the Nyquist plots. The inhibition efficiency of the inhibitor has been found out from the charge transfer resistance values using the following equation I.E.(%) =
o i − Rct Rct i Rct
× 100
o i and Rct are the charge transfer resistances in absence and in presence of where Rct inhibitor. The interfacial double layer capacitance (Cdl ) values have been estimated from the impedance value using bode plot by the formula
Z =
1 2fCdl
and the surface coverage () by the inhibitor molecule is given by =
o i − Cdl Cdl o Cdl
3. Result and discussion It is well known fact that compounds with high molecular weight and bulky structure may cover more area on the active electrode surface [12]. If such a bulky molecule can have a chemisorptive property, it is naturally expected to inhibit corrosion more effectively. 3.1. Weight loss study The values of percentage inhibition efficiency (%I.E.) and corrosion rate (CR) obtained from weight loss method at different concentrations of inhibitor at 35 ◦ C are summarized in Table 1. It has been found that this polymer resin inhibits the corrosion of mild steel in hydrochloric acid solution, at all concentrations used in this study, i.e., 1–10 ppm. It is evident from the teble1 that the corrosion rate is decreased from 44.85 to 4.29 mmpy with the addition of 1.0 ppm of poly(aniline-formaldehyde). The variation of inhibiTable 1 Corrosion parameters for mild steel in aqueous solution of 1N HCl in absence and presence of different concentrations of inhibitor from weight loss measurements at 35 ◦ C for 3 h S. No.
Inhibitor concentration (ppm)
Weight loss (mg)
I.E. (%)
CR (mmpy)
1. 2. 3. 4. 5. 6. 7.
Blank 1 2 4 6 8 10
241.5 23.1 17.6 9.5 8.3 6.8 3.0
– 90.43 92.71 94.83 96.56 97.18 98.75
44.85 4.29 3.27 1.76 1.54 1.26 0.57
M.A. Quraishi, S.K. Shukla / Materials Chemistry and Physics 113 (2009) 685–689
687
Fig. 1. Variation of inhibitor efficiency with (a) concentration of inhibitors, (b) immersion time, (c) temperature and (d) acid concentration.
tion efficiency with increase in inhibitor concentrations is shown in Fig. 1(a). The effect of immersion time on inhibition efficiency from 2 to 6 h is shown in Fig. 1(b). It was found that the inhibition efficiency first increases with increase in immersion time and after some time it becomes constant. This showed that the formation of inhibitor film takes some times. The influence of solution temperature on inhibition efficiency is shown in Fig. 1(c). It is observed that inhibition efficiency decreases with increase in temperature 35–55 ◦ C. The decrease in inhibition efficiency with temperature may be attributed to desorption of the inhibitor molecules from metal surface at higher temperature [18]. The variation of inhibition efficiency with increase in acid concentration from 0.5 to 2.0N is shown in Fig. 1(d). It is clear that change in acid concentration from 0.5 to 2.0N did not cause any significant change in inhibition efficiency values from 0.5 to 1.0N after that it shows steady fall in inhibition efficiency. This inference suggesting that compound is effective corrosion inhibitor in acid solution over the concentration range 0.5–1.0N. The mechanism of corrosion inhibition may be explained on the basis of adsorption behavior [19]. The degrees of surface coverage () for different inhibitor concentrations were evaluated by weight loss data. Data were tested graphically by fitting to various isotherms. A straight line (Fig. 2) was obtained on plotting Cinh vs Cinh / for poly(aniline-formaldehyde). From this plot, it is observed that it obeys Langmuir adsorption isotherm through surface coverage of inhibitor adsorption on mild steel surface. The adsorption of the poly(aniline-formaldehyde) molecule with the metal surface is usually through the already adsorbed chloride ion. In acidic solutions, amines exits as cations and adsorb through electrostatic interaction between the positively charged anilinium cations and adsorbed chloride ions [20]. The higher inhibitive property of poly(aniline-formaldehyde) is also due to the presence of -electrons, quaternary nitrogen atom and the larger molec-
ular size which ensures greater coverage of the metallic surface [13–16,20]. 3.2. Tafel polarization The Tafel polarization curves for mild steel in 1.0N hydrochloric acid with the addition of various concentrations of poly(anilineformaldehyde) are shown in Fig. 3. Electrochemical parameters such as corrosion current density (Icorr ), corrosion potential (Ecorr ), Tafel constants (ba and bc ), percentage inhibition efficiency (%I.E.) and corrosion rate (CR) are calculated from the Tafel plots are given in Table 2. It is evident from the table that the corrosion current value (Icorr ) is decreased from 754 A cm−2 of the blank to 131 A cm−2 with the addition of 1.0 ppm inhibitor and it gets further reduced gradually with increasing concentration of inhibitor.
Fig. 2. Langmuir adsorption isotherm plot for the adsorption of inhibitor in 1N HCl, on the surface of mild steel.
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Table 2 Tafel polarization parameters for the corrosion of mild steel in 1N HCl in absence and presence of different concentration (0–10 ppm) inhibitor Concentration of inhibitor (ppm)
Ecorr (mV vs SCE)
ba (mV dec−1 )
bc (mV dec−1 )
Icorr (A cm−2 )
Inhibition efficiency (%)
Blank 1.0 4.0 8.0 10.0
−446 −478 −480 −460 −455
45 47 55 54 48
193 135 140 156 173
754 131 110 44.3 41.3
– 82.62 85.41 94.12 94.52
Fig. 4. Electrical equivalent circuit (R = uncompensated solution resistance, Rt = polarization resistance and Cdl = double layer capacitance).
Fig. 3. Potentiodynamic polarization behavior of mild steel in 1N HCl with the addition of inhibitor (1) blank, (2) 1 ppm, (3) 4 ppm, (4) 8 ppm and (5) 10 ppm.
It is also observed from table that Ecorr values and Tafel slope constants ba and bc do not change significantly in inhibited solution as compared to uninhibited solution. It is seen from the results that poly(aniline-formaldehyde) do not shift Ecorr values significantly thereby suggesting that they are mixed type of inhibitors. Jayaprabha et al. have reported that substituted polyanilines behave as mixed type of inhibitor in acid solution [21]. Fig. 5. Nyquist plot of mild steel in 1N HCl with different concentrations (1) blank, (2) 1 ppm, (3) 4 ppm, (4) 8 ppm and (5) 10 ppm of inhibitor.
3.3. Polarization resistance study The polarization resistance (Rp ) values of mild steel in 1N hydrochloric acid increases from 19.42 that of the blank to 215.4 with the 10 ppm concentration of inhibitor (Table 3). The increase in the Rp value suggests that the inhibition efficiency increases with the increase in the inhibitor concentration. 3.4. Electrochemical impedance studies Electrochemical impedance measurements were carried over the frequency range from 10 kHz to 100 mHz at open circuit potential. The simple equivalent Randle circuit for studies is shown in Fig. 4, where R represents the solution and corrosion product film; the parallel combination of resister, Rt and capacitor Cdl represents the corroding interface. The Nyquist representation of the impedance behavior of mild steel in 1N HCl with and without addition of various concentrations of inhibitor is given in Fig. 5. It is seen that addition of inhibitor increases the value of Rct from 19.8 to 197.6 cm2 and reduces Cdl from 5011 to 650 F cm−2 . Table 3 Polarization parameters for the corrosion of mild steel in 1N HCl in absence and presence different concentration of inhibitor
The decrease in Cdl is attributed to increase in thickness of electronic double layer [22]. The increase in Rct value is attributed to the formation of protective film on the metal/solution interface [23,24]. The observations suggests that poly(aniline-formaldehyde) molecule function by adsorption at metal surface thereby causing decrease in Cdl values and increase in Rct values. The charge transfer resistance (Rct ) and the interfacial double layer capacitance (Cdl ) derived from these curves are given in Table 4. 3.5. Surface morphology: AFM studies Surface morphology of the mild steel in 1N HCl in absence and presence of PAF polymer was investigated through atomic force microscopy (AFM) technique. The results are shown in Fig. 6. The average roughness of polished mild steel and mild steel in 1N hydrochloric acid without inhibitor was calculated as 80 and 166 nm respectively. It is clearly shown in Fig. 6(b) that mild steel Table 4 Electrochemical impedance parameters for mild steel in 1N HCl in absence and presence different concentration of inhibitor Cdl (F cm−2 )
Rp ()
Inhibition efficiency (%)
Concentration of inhibitor (ppm)
Rt ( cm2 )
Concentration of inhibitor (ppm)
Inhibition efficiency (%)
Blank 1 4 8 10
19.42 104.5 120.8 180.7 215.4
– 81.41 84.18 89.25 90.98
Blank 1 4 8 10
19.8 88.4 93.1 184.7 197.6
5011 1202 1110 677 650
– 77.6 78.7 89.3 90.0
M.A. Quraishi, S.K. Shukla / Materials Chemistry and Physics 113 (2009) 685–689
689
occurs through -electron of aromatic rings and lone pair of electrons of nitrogen atom, which decreases anodic dissolution of mild steel [25]. The high performance of the polymer is attributed to the presence of -electrons, quaternary nitrogen atom and the larger molecular size. 4. Conclusion 1. The inhibition efficiency of poly(aniline-formaldehyde) increases with increase in the inhibitor concentration. The inhibitor showed more than 90% inhibition efficiency by the addition of 1 ppm of the polymer. It showed maximum inhibition efficiency 98.75% at 10 ppm. 2. Langmuir adsorption isotherm and impedance studies showed that poly(aniline-formaldehyde) inhibits through adsorption mechanism. 3. Potentiodynamic polarization showed that it is a mixed type of inhibitor. 4. Average roughness of inhibited steel sample was reduced to 120 nm as compared to uninhibited sample (roughness 166 nm). Acknowledgement One of the authors Sudhidh Kumar Shukla is thankful to UGC New Delhi, India for research fellowship. References
Fig. 6. Atomic force micrographs of mild steel (a) Polished Mild Steel 80 nm, (b) Mild Steel in HCl 166 nm and (c) Inhibited Mild Steel 120 nm.
sample is getting cracks due to the acid attack on steel sample. However in presence of low concentration (10 ppm) of polymer the average roughness was reduced to 120 nm. 3.6. Mechanism of inhibition Corrosion inhibition of mild steel in hydrochloric acid solution by poly(aniline-formaldehyde) can be explained on the basis of molecular adsorption. The compound inhibits corrosion by controlling both the anodic and cathodic reactions. In acidic solutions the polymer inhibitor exists as protonated species. These protonated species adsorb on the cathodic sites of the mild steel and decrease the evolution of hydrogen. The adsorption on anodic sites
[1] S.A. Ali, M.T. Saeed, S.V. Rahman, Corros. Sci. 45 (2003) 253–266. [2] M. Lagrenee, B. Mernari, M. Bouanis, M. Traisnel, F. Bentiss, Corros. Sci. 44 (2002) 573–588. [3] M.A. Quraishi, R. Sardar, Corrosion 58 (2002) 748–755. [4] M.A. Quraishi, M. Athar, H. Ali, Br. Corros. J. 37 (2002) 155–158. [5] M.A. Quraishi, F.A. Ansari, J. Appl. Electrochem. 33 (2003) 233–238. [6] M.A. Quraishi, S. Khan, J. Appl. Electrochem. 36 (2006) 539–544. [7] G. Schmitt, Br. Corros. J. 19 (1984) 165–176. [8] F. Bentiss, M. Lagrenee, M. Traisnel, J.C. Hornez, Corros. Sci. 41 (1999) 789–803. [9] F.B. Growcock, W.W. Frenier, P.A. Andreozzi, Corrosion 45 (1989) 1007–1015. [10] I. Lukovits, E. Kalman, G. Palinkas, Corrosion 51 (1995) 201–205. [11] R.C. Ayers Jr., N. Hackerman, J. Electrochem. Soc. 110 (1963) 507–513. [12] T. Grchev, M. Cvetkovska, J.W. Schultze, Corros. Sci. 32 (1991) 103–112. [13] P. Manivel, G. Venkatachari, J. Appl. Polym. Sci. 104 (2007) 2595–2601. [14] P. Manivel, G. Venkatachari, J. Met. Mater. Sci. 46 (2004) 165. [15] P. Manivel, G. Venkatachari, J. Mater. Sci. Technol. 22 (2006) 301–305. [16] S. Sathiyanarayanan, S.K. Dhawan, D.C. Trivedi, K. Balakrishnan, Corros. Sci. 33 (1992) 1831–1841. [17] Practicals in Polymer Science, 1st ed., Siddaraman, CBS Publishers Bangalore 2005, pp. 43–44. [18] M. Schorr, J. Yahalom, Corros. Sci. 12 (1972) 867–868. [19] S.T. Hirozawa, in: Proc. 8th Eur. Symp. Corros. Inhib., Ann. University, Ferrara, Italy 1 (1995) 25. [20] M.A. Quraishi, M.A.W. Khan, M. Ajmal, S. Muralidharan, S.V. Iyer, Br. Corros. J. 32 (1997) 72–76. [21] C. Jayaprabha, S. Sathiyanarayan, K.L.N. Phani, G. Venkatachari, J. Electroanal. Chem. 358 (2005) 250–255. [22] M.G. Hosseini, M. Ehteshamzadeh, T. Shahrabi, Electrochem. Acta 52 (2007) 3680–3685. [23] F. Bentiss, M. Traisnel, M. Lagrenee, Corros. Sci. 42 (2000) 127–146. [24] S. Murlidharan, K.L.N. Phani, S. Pitchumani, S. Ravichandran, S.V.K. Iyer, J. Electrochem. Soc. 142 (1995) 1478–1483. [25] M.A. Quraishi, J. Rawat, M. Ajmal, J. Appl. Electrochem. 30 (2000) 745–751.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Poly(aniline-formaldehyde): A new and effective corrosion inhibitor for mild steel in hydrochloric acid M.A. Quraishi ∗ , Sudhish Kumar Shukla Department of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi 221005, India
a r t i c l e
i n f o
Article history: Received 20 March 2008 Accepted 10 August 2008 Keywords: Mild steel Poly(aniline-formaldehyde) Electrochemical impedance spectroscopy (EIS) Atomic force microscopy (AFM)
a b s t r a c t In recent years, polyanilines have emerged as efficient class of corrosion inhibitors for mild steel in acidic media. The corrosion inhibition of poly(aniline-formaldehyde) on mild steel in 1.0N HCl has been evaluated by potentiodynamic polarization, linear polarization, electrochemical impedance spectroscopy and weight loss measurements. Results obtained show that poly(aniline-formaldehyde) is a mixed inhibitor and it inhibits mild steel corrosion through adsorption mechanism. It showed >90% inhibition efficiency at 10 ppm. AFM clearly reveals that surface roughness of inhibited mild steel sample is less than uninhibited mild steel. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Inhibition of corrosion of mild steel is a matter of theoretical as well as practical importance [1]. Acids are widely used in industries such as pickling, cleaning, descaling, etc. because of their aggressiveness; inhibitors are used to reduce the rate of dissolution of metals. Compounds containing nitrogen, sulphur and oxygen have been reported as inhibitors [2–6]. The most efficient organic inhibitors are organic compounds having -bonds in their structures [7]. The efficiency of an organic compound as an inhibitor is mainly dependent on its ability to get adsorbed on metal surface which consists of a replacement of water molecule at a corroding interface as Org(Sol) + nH2 O(ads) → Org(ads) + nH2 O(Sol) The adsorption of these compounds is influenced by the electronic structure of inhibiting molecules, steric factor, aromaticity, and electron density at donor site, presence of functional group such as –CHO, –N N, R–OH, etc., molecular area and molecular weight of the inhibitor molecule [8–11]. The earlier studies [12–18] have shown that inhibitive properties of polyaniline and its derivatives on corrosion of iron in acid chloride solution are due to the presence of -electrons, quaternary nitrogen atom and large molecular size which ensures greater coverage of the metallic surface. So the adsorption of polymer
molecules on the iron electrode surface is more, which leads to more inhibition efficiency. In this work the inhibitive behavior of poly(aniline-formaldehyde) on mild steel in 1N HCl solution has been studied. 2. Experimental 2.1. Synthesis of poly(aniline-formaldehyde) [17] Reagent grade aniline was purified by distillation under reduced pressure and formaldehyde (40%) is taken as such it is supplied. 5 ml of aniline in dil-HCl is taken in a 250 ml beaker. 5 ml of formaldehyde solution (40%) is added slowly with stirring. The mixture is then stirred-well for 15 min to form an orange-red solution. Then this solution is fed into aqueous sodium hydroxide solution (1%) and refluxed for 30–45 min. The poly(aniline-formaldehyde) will precipitate out. Filter the polymer wash with water and dry it. Characterization is done by using UV spectroscopy and FTIR-spectroscopy [Scheme 1]. 2.2. Methodology The mild steel strips having composition (wt.%): C 0.04, Mn 0.035, Si 0.17, S 0.025, P 0.03 and balance Fe were used for weight loss as well as electrochemical studies. The aggressive solution of hydrochloric acid (AR grade) is used for all studies. The weight loss studies were conducted on mild steel strips of 5.0 cm × 2.0 cm × 0.025 cm sizes. Weight loss study was carried out at 35 ◦ C temperature and 3 h time duration and 1N hydrochloric acid solution. Weight loss studies are also done on various strength of acid, various duration ranges and various temperature ranges in presence of optimum concentration of inhibitor. The inhibition efficiency (%) and surface coverage () was determined by using following equation: I.E. (%) =
∗ Corresponding author. Tel.: +91 9335636661; fax: +91 542 2368428. E-mail address: [email protected] (M.A. Quraishi). 0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2008.08.028
=
Wo − Wi × 100 Wo
Wo − Wi Wo
686
M.A. Quraishi, S.K. Shukla / Materials Chemistry and Physics 113 (2009) 685–689
Scheme 1.
where Wo and Wi are the weight loss values in absence and in presence of inhibitor. The electrochemical studies were carried out in a three electrode cell assembly at room temperature. The working electrode was a mild steel of above composition of 1 cm2 area and the rest being covered by using commercially available lacquer. A large rectangular foil was used as counter electrode and saturated calomel electrode as reference electrode. The working electrode was polished with different grades of emery papers, washed with water and degreased with acetone. The polarization and impedance studies were made after 30 min of immersion using Gamry Potentiostat (Model PC4/750). The inhibition efficiency was evaluated from the measured Icorr values using the relationship: I.E. (%) =
o i − Icorr Icorr × 100 o Icorr
o i and Icorr are the corrosion currents in absence and in presence of inhibitor. where Icorr For linear polarization resistance measurement, the potential of the electrode was scanned from −0.02 to +0.02 V vs corrosion potential a scan rate of 0.5 mV s−1 and the polarization resistance was measured from the slope of vs i curve in the vicinity of corrosion potential. From the measured polarization resistance value, the inhibition efficiency has been calculated using the relationship:
I.E. (%) =
Rpi − Rpo Rpi
× 100
where Rpo and Rpi are the polarization resistances in absence and in presence of inhibitor. The impedance studies were carried out using ac signals of 10 mV amplitude for the frequency spectrum from 100 kHz to 0.01 Hz. The charge transfer resistance values were obtained from the diameter of the semi circles of the Nyquist plots. The inhibition efficiency of the inhibitor has been found out from the charge transfer resistance values using the following equation I.E.(%) =
o i − Rct Rct i Rct
× 100
o i and Rct are the charge transfer resistances in absence and in presence of where Rct inhibitor. The interfacial double layer capacitance (Cdl ) values have been estimated from the impedance value using bode plot by the formula
Z =
1 2fCdl
and the surface coverage () by the inhibitor molecule is given by =
o i − Cdl Cdl o Cdl
3. Result and discussion It is well known fact that compounds with high molecular weight and bulky structure may cover more area on the active electrode surface [12]. If such a bulky molecule can have a chemisorptive property, it is naturally expected to inhibit corrosion more effectively. 3.1. Weight loss study The values of percentage inhibition efficiency (%I.E.) and corrosion rate (CR) obtained from weight loss method at different concentrations of inhibitor at 35 ◦ C are summarized in Table 1. It has been found that this polymer resin inhibits the corrosion of mild steel in hydrochloric acid solution, at all concentrations used in this study, i.e., 1–10 ppm. It is evident from the teble1 that the corrosion rate is decreased from 44.85 to 4.29 mmpy with the addition of 1.0 ppm of poly(aniline-formaldehyde). The variation of inhibiTable 1 Corrosion parameters for mild steel in aqueous solution of 1N HCl in absence and presence of different concentrations of inhibitor from weight loss measurements at 35 ◦ C for 3 h S. No.
Inhibitor concentration (ppm)
Weight loss (mg)
I.E. (%)
CR (mmpy)
1. 2. 3. 4. 5. 6. 7.
Blank 1 2 4 6 8 10
241.5 23.1 17.6 9.5 8.3 6.8 3.0
– 90.43 92.71 94.83 96.56 97.18 98.75
44.85 4.29 3.27 1.76 1.54 1.26 0.57
M.A. Quraishi, S.K. Shukla / Materials Chemistry and Physics 113 (2009) 685–689
687
Fig. 1. Variation of inhibitor efficiency with (a) concentration of inhibitors, (b) immersion time, (c) temperature and (d) acid concentration.
tion efficiency with increase in inhibitor concentrations is shown in Fig. 1(a). The effect of immersion time on inhibition efficiency from 2 to 6 h is shown in Fig. 1(b). It was found that the inhibition efficiency first increases with increase in immersion time and after some time it becomes constant. This showed that the formation of inhibitor film takes some times. The influence of solution temperature on inhibition efficiency is shown in Fig. 1(c). It is observed that inhibition efficiency decreases with increase in temperature 35–55 ◦ C. The decrease in inhibition efficiency with temperature may be attributed to desorption of the inhibitor molecules from metal surface at higher temperature [18]. The variation of inhibition efficiency with increase in acid concentration from 0.5 to 2.0N is shown in Fig. 1(d). It is clear that change in acid concentration from 0.5 to 2.0N did not cause any significant change in inhibition efficiency values from 0.5 to 1.0N after that it shows steady fall in inhibition efficiency. This inference suggesting that compound is effective corrosion inhibitor in acid solution over the concentration range 0.5–1.0N. The mechanism of corrosion inhibition may be explained on the basis of adsorption behavior [19]. The degrees of surface coverage () for different inhibitor concentrations were evaluated by weight loss data. Data were tested graphically by fitting to various isotherms. A straight line (Fig. 2) was obtained on plotting Cinh vs Cinh / for poly(aniline-formaldehyde). From this plot, it is observed that it obeys Langmuir adsorption isotherm through surface coverage of inhibitor adsorption on mild steel surface. The adsorption of the poly(aniline-formaldehyde) molecule with the metal surface is usually through the already adsorbed chloride ion. In acidic solutions, amines exits as cations and adsorb through electrostatic interaction between the positively charged anilinium cations and adsorbed chloride ions [20]. The higher inhibitive property of poly(aniline-formaldehyde) is also due to the presence of -electrons, quaternary nitrogen atom and the larger molec-
ular size which ensures greater coverage of the metallic surface [13–16,20]. 3.2. Tafel polarization The Tafel polarization curves for mild steel in 1.0N hydrochloric acid with the addition of various concentrations of poly(anilineformaldehyde) are shown in Fig. 3. Electrochemical parameters such as corrosion current density (Icorr ), corrosion potential (Ecorr ), Tafel constants (ba and bc ), percentage inhibition efficiency (%I.E.) and corrosion rate (CR) are calculated from the Tafel plots are given in Table 2. It is evident from the table that the corrosion current value (Icorr ) is decreased from 754 A cm−2 of the blank to 131 A cm−2 with the addition of 1.0 ppm inhibitor and it gets further reduced gradually with increasing concentration of inhibitor.
Fig. 2. Langmuir adsorption isotherm plot for the adsorption of inhibitor in 1N HCl, on the surface of mild steel.
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M.A. Quraishi, S.K. Shukla / Materials Chemistry and Physics 113 (2009) 685–689
Table 2 Tafel polarization parameters for the corrosion of mild steel in 1N HCl in absence and presence of different concentration (0–10 ppm) inhibitor Concentration of inhibitor (ppm)
Ecorr (mV vs SCE)
ba (mV dec−1 )
bc (mV dec−1 )
Icorr (A cm−2 )
Inhibition efficiency (%)
Blank 1.0 4.0 8.0 10.0
−446 −478 −480 −460 −455
45 47 55 54 48
193 135 140 156 173
754 131 110 44.3 41.3
– 82.62 85.41 94.12 94.52
Fig. 4. Electrical equivalent circuit (R = uncompensated solution resistance, Rt = polarization resistance and Cdl = double layer capacitance).
Fig. 3. Potentiodynamic polarization behavior of mild steel in 1N HCl with the addition of inhibitor (1) blank, (2) 1 ppm, (3) 4 ppm, (4) 8 ppm and (5) 10 ppm.
It is also observed from table that Ecorr values and Tafel slope constants ba and bc do not change significantly in inhibited solution as compared to uninhibited solution. It is seen from the results that poly(aniline-formaldehyde) do not shift Ecorr values significantly thereby suggesting that they are mixed type of inhibitors. Jayaprabha et al. have reported that substituted polyanilines behave as mixed type of inhibitor in acid solution [21]. Fig. 5. Nyquist plot of mild steel in 1N HCl with different concentrations (1) blank, (2) 1 ppm, (3) 4 ppm, (4) 8 ppm and (5) 10 ppm of inhibitor.
3.3. Polarization resistance study The polarization resistance (Rp ) values of mild steel in 1N hydrochloric acid increases from 19.42 that of the blank to 215.4 with the 10 ppm concentration of inhibitor (Table 3). The increase in the Rp value suggests that the inhibition efficiency increases with the increase in the inhibitor concentration. 3.4. Electrochemical impedance studies Electrochemical impedance measurements were carried over the frequency range from 10 kHz to 100 mHz at open circuit potential. The simple equivalent Randle circuit for studies is shown in Fig. 4, where R represents the solution and corrosion product film; the parallel combination of resister, Rt and capacitor Cdl represents the corroding interface. The Nyquist representation of the impedance behavior of mild steel in 1N HCl with and without addition of various concentrations of inhibitor is given in Fig. 5. It is seen that addition of inhibitor increases the value of Rct from 19.8 to 197.6 cm2 and reduces Cdl from 5011 to 650 F cm−2 . Table 3 Polarization parameters for the corrosion of mild steel in 1N HCl in absence and presence different concentration of inhibitor
The decrease in Cdl is attributed to increase in thickness of electronic double layer [22]. The increase in Rct value is attributed to the formation of protective film on the metal/solution interface [23,24]. The observations suggests that poly(aniline-formaldehyde) molecule function by adsorption at metal surface thereby causing decrease in Cdl values and increase in Rct values. The charge transfer resistance (Rct ) and the interfacial double layer capacitance (Cdl ) derived from these curves are given in Table 4. 3.5. Surface morphology: AFM studies Surface morphology of the mild steel in 1N HCl in absence and presence of PAF polymer was investigated through atomic force microscopy (AFM) technique. The results are shown in Fig. 6. The average roughness of polished mild steel and mild steel in 1N hydrochloric acid without inhibitor was calculated as 80 and 166 nm respectively. It is clearly shown in Fig. 6(b) that mild steel Table 4 Electrochemical impedance parameters for mild steel in 1N HCl in absence and presence different concentration of inhibitor Cdl (F cm−2 )
Rp ()
Inhibition efficiency (%)
Concentration of inhibitor (ppm)
Rt ( cm2 )
Concentration of inhibitor (ppm)
Inhibition efficiency (%)
Blank 1 4 8 10
19.42 104.5 120.8 180.7 215.4
– 81.41 84.18 89.25 90.98
Blank 1 4 8 10
19.8 88.4 93.1 184.7 197.6
5011 1202 1110 677 650
– 77.6 78.7 89.3 90.0
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occurs through -electron of aromatic rings and lone pair of electrons of nitrogen atom, which decreases anodic dissolution of mild steel [25]. The high performance of the polymer is attributed to the presence of -electrons, quaternary nitrogen atom and the larger molecular size. 4. Conclusion 1. The inhibition efficiency of poly(aniline-formaldehyde) increases with increase in the inhibitor concentration. The inhibitor showed more than 90% inhibition efficiency by the addition of 1 ppm of the polymer. It showed maximum inhibition efficiency 98.75% at 10 ppm. 2. Langmuir adsorption isotherm and impedance studies showed that poly(aniline-formaldehyde) inhibits through adsorption mechanism. 3. Potentiodynamic polarization showed that it is a mixed type of inhibitor. 4. Average roughness of inhibited steel sample was reduced to 120 nm as compared to uninhibited sample (roughness 166 nm). Acknowledgement One of the authors Sudhidh Kumar Shukla is thankful to UGC New Delhi, India for research fellowship. References
Fig. 6. Atomic force micrographs of mild steel (a) Polished Mild Steel 80 nm, (b) Mild Steel in HCl 166 nm and (c) Inhibited Mild Steel 120 nm.
sample is getting cracks due to the acid attack on steel sample. However in presence of low concentration (10 ppm) of polymer the average roughness was reduced to 120 nm. 3.6. Mechanism of inhibition Corrosion inhibition of mild steel in hydrochloric acid solution by poly(aniline-formaldehyde) can be explained on the basis of molecular adsorption. The compound inhibits corrosion by controlling both the anodic and cathodic reactions. In acidic solutions the polymer inhibitor exists as protonated species. These protonated species adsorb on the cathodic sites of the mild steel and decrease the evolution of hydrogen. The adsorption on anodic sites
[1] S.A. Ali, M.T. Saeed, S.V. Rahman, Corros. Sci. 45 (2003) 253–266. [2] M. Lagrenee, B. Mernari, M. Bouanis, M. Traisnel, F. Bentiss, Corros. Sci. 44 (2002) 573–588. [3] M.A. Quraishi, R. Sardar, Corrosion 58 (2002) 748–755. [4] M.A. Quraishi, M. Athar, H. Ali, Br. Corros. J. 37 (2002) 155–158. [5] M.A. Quraishi, F.A. Ansari, J. Appl. Electrochem. 33 (2003) 233–238. [6] M.A. Quraishi, S. Khan, J. Appl. Electrochem. 36 (2006) 539–544. [7] G. Schmitt, Br. Corros. J. 19 (1984) 165–176. [8] F. Bentiss, M. Lagrenee, M. Traisnel, J.C. Hornez, Corros. Sci. 41 (1999) 789–803. [9] F.B. Growcock, W.W. Frenier, P.A. Andreozzi, Corrosion 45 (1989) 1007–1015. [10] I. Lukovits, E. Kalman, G. Palinkas, Corrosion 51 (1995) 201–205. [11] R.C. Ayers Jr., N. Hackerman, J. Electrochem. Soc. 110 (1963) 507–513. [12] T. Grchev, M. Cvetkovska, J.W. Schultze, Corros. Sci. 32 (1991) 103–112. [13] P. Manivel, G. Venkatachari, J. Appl. Polym. Sci. 104 (2007) 2595–2601. [14] P. Manivel, G. Venkatachari, J. Met. Mater. Sci. 46 (2004) 165. [15] P. Manivel, G. Venkatachari, J. Mater. Sci. Technol. 22 (2006) 301–305. [16] S. Sathiyanarayanan, S.K. Dhawan, D.C. Trivedi, K. Balakrishnan, Corros. Sci. 33 (1992) 1831–1841. [17] Practicals in Polymer Science, 1st ed., Siddaraman, CBS Publishers Bangalore 2005, pp. 43–44. [18] M. Schorr, J. Yahalom, Corros. Sci. 12 (1972) 867–868. [19] S.T. Hirozawa, in: Proc. 8th Eur. Symp. Corros. Inhib., Ann. University, Ferrara, Italy 1 (1995) 25. [20] M.A. Quraishi, M.A.W. Khan, M. Ajmal, S. Muralidharan, S.V. Iyer, Br. Corros. J. 32 (1997) 72–76. [21] C. Jayaprabha, S. Sathiyanarayan, K.L.N. Phani, G. Venkatachari, J. Electroanal. Chem. 358 (2005) 250–255. [22] M.G. Hosseini, M. Ehteshamzadeh, T. Shahrabi, Electrochem. Acta 52 (2007) 3680–3685. [23] F. Bentiss, M. Traisnel, M. Lagrenee, Corros. Sci. 42 (2000) 127–146. [24] S. Murlidharan, K.L.N. Phani, S. Pitchumani, S. Ravichandran, S.V.K. Iyer, J. Electrochem. Soc. 142 (1995) 1478–1483. [25] M.A. Quraishi, J. Rawat, M. Ajmal, J. Appl. Electrochem. 30 (2000) 745–751.