Origin of the synergistic effect between polysaccharide and thiourea towards adsorption and corrosion inhibition for mild steel in sulphuric acid

Abstract

The origin of the synergistic effect between polysaccharides and thiourea towards adsorption on mild steel in 0.5 M H₂SO₄ and their effect on corrosion inhibition has been studied. Two different polysaccharides, gum arabic and agar agar are used for this purpose. Results obtained from potentiodynamic polarization and electrochemical impedance spectroscopic studies reveal that thiourea imparts a profound synergistic effect towards the corrosion inhibition of mild steel in H₂SO₄ by the polysaccharides. The polysaccharide–thiourea system acts essentially as a mixed type corrosion inhibitor and the phenomenon of adsorption mechanism on metal surface is proposed as physisorption. Detailed FTIR studies of the surface adsorbed layers of inhibitors have been done to elucidate the origin of the synergistic effect towards the coadsorption and subsequent corrosion inhibition of the metal.

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Introduction

Growing environmental concern and enforcement of strict environmental regulations have compelled the researchers to focus on the development of eco-friendly and biodegradable corrosion inhibitors. In this regard a flurry of work is now being undertaken by various research groups involving plant extracts, drugs, bio-polymeric molecules, amino acids, medicinal products, etc.^1–9 Among bio-polymers, polysaccharides have added advantages as corrosion inhibitors by being water soluble, easily available and of low cost.^4,10–14 Chemically modified polysaccharides have been reported to impart greater corrosion inhibitory action over the parent polysaccharides.^15–17 Apart from chemical modification, halides and surfactants, together with the polysaccharides, provide better results due to a synergistic effect.^18,19 In this endeavor, we report the synergistic effect of thiourea (TU) on the corrosion inhibition of mild steel by gum arabic (GA) and agar agar (AA) in 0.5 M H₂SO₄ solution. GA is a water soluble anionic polymer comprising a complex mixture of polysaccharides (major fraction) and glucoproteins (minor fraction). Its polysaccharide component consists of L-arabinose, D-galactose, L-rhamnose, D-glucuronic acid (found in nature as magnesium, potassium, and calcium salts).^20,21 The anticorrosive behavior of GA for various metals in different corrosive conditions is observed, primarily due to the adsorption of GA on the metal surface through the carboxylate group of the glucuronic acid present in GA.^11–14,18 For AA, the main backbone is assigned to be a linear polymer structure consisting of alternating 1,3-linked-D-galactopyranose and 1,4-linked 3,6-anhydro-L-galactopyranose units. The neutral form of these polymers is called agarose and it is held responsible for the high gelation property of agar.²² However, the gelation behavior varies with the degree of substitution of the 1,3-linked-D-galactopyranose units by sulphate esters or other groups.²³ AA has been established as a potential corrosion inhibitor for aluminium alloys in alkaline media, as well as for mild steel in acid solution.^24–26 TU and substituted thioureas have long been known for their excellent corrosion inhibitory action on mild steel in acidic media.^27,28 It is the sulfur atom of TU which gets adsorbed on the metal surface and thereby decreases both the cathodic and anodic reactions.^27,28

Reports on the synergistic effect produced by thiourea, or its derivatives, towards the inhibition of corrosion by other inhibitors is very limited and the origin of such synergism has not been addressed at all.²⁹ In addition, it is reported that TU exists mostly in a protonated form in acidic media.³⁰ Thus it would be interesting to investigate the effect of any possible electrostatic interactions of the protonated TU with anionic GA or the hydroxyl group of AA towards simultaneous adsorption on the metal surface, which may impart synergism towards the inhibition of corrosion of mild steel in 0.5 M H₂SO₄.

Experimental

Materials

Test specimens were cut from a commercially available mild steel rod (wt% composition: 0.22 C, 0.31 Si, 0.60 Mn, 0.04 P, 0.06 S and the remainder iron) and the cross-sectional surface was ground with different grade emery papers (400, 600, 800, 1200 and 1600), rinsed with acetone, washed thoroughly with doubly distilled water and used as the working electrode. GA, AA and TU were obtained from Merck India and used without any further purification.

Electrochemical measurements

Potentiodynamic polarization and electrochemical impedance measurements were done using a conventional three-electrode system (model: Gill AC, ACM Instruments, UK) consisting of a mild steel working electrode (WE) with an exposed area of 0.25 cm², platinum as the counter electrode and a saturated calomel electrode (SCE) as the reference. The potential sweep rate for the potentiodynamic polarization curves was 1 mV s⁻¹. Before polarization measurements, the WE was kept in the test solution for 30 minutes and the open circuit potential (OCP) was then monitored for another 10 minutes for confirmation of a steady state. Corrosion current density (i_corr) was determined from the intercept of extrapolated cathodic and anodic Tafel lines at the corrosion potential (E_corr). Electrochemical impedance measurements were performed in the frequency range 10 mHz to 100 kHz with an a.c. amplitude of ±10 mV (rms) at the rest potential. All the experiments have been carried out at a room temperature of around 30 °C.

Surface analysis

A scanning electron microscope (SEM, S-3000N, Hitachi) was used to study the surface morphology of the metal surface. Before the measurement, test coupons were exposed to 0.5 M H₂SO₄ solutions with different concentrations of the inhibitors for four hours, washed with distilled water and then kept in a vacuum desiccator. The surface of the dried specimen was scratched with a knife and the resultant powder was used for FTIR studies (KBr pellet method, Thermo Nicolet, model iS10).

Results and discussion

Polarization measurements

Potentiodynamic polarization curves for the mild steel in 0.5 M H₂SO₄ solution containing 10 mM TU, 1000 ppm GA, 1000 ppm AA and their mixture with 10 mM TU are shown in Fig. 1. Values of the electrochemical corrosion parameters, such as the corrosion potential (E_corr), cathodic Tafel slope (b_c), anodic Tafel slope (b_a), and corrosion current density (i_corr) for different concentrations of GA, AA, TU and both the GA–TU and AA–TU mixed systems are given in Table 1.

Fig. 1 Potentiodynamic polarization curves for mild steel in 0.5 M H₂SO₄ in the presence of (a) 1000 ppm GA, (b) 1000 ppm AA, (c) 10 mM TU, (d) 1000 ppm GA–10 mM TU and (e) 1000 ppm AA–10 mM TU.

Table 1 Data from the potentiodynamic polarization studies for mild steel in 0.5 M H₂SO₄ in various inhibitor systems

System	Ecorr (mV per SCE)	icorr (mA cm^−2)	ba (mV dec^−1)	bc (mV dec^−1)	η%P	s
Blank	517	1.7	93	114	—	—
10 mM thiourea	525	0.3	81	122	82	—
Gum arabic
100 ppm	488	1.1	90	120	35	—
250 ppm	487	0.9	80	123	47	—
500 ppm	491	0.82	86	109	52	—
750 ppm	479	0.79	91	106	54	—
1000 ppm	481	0.71	78	115	58	—
10 mM thiourea + gum arabic
100 ppm	524	0.19	88	113	88	1.33
250 ppm	520	0.15	82	101	91	1.42
500 ppm	518	0.13	72	101	92	1.46
750 ppm	518	0.1	88	116	94	1.45
1000 ppm	515	0.09	66	105	95	1.48
Agar agar
100 ppm	483	0.98	66	105	42	—
250 ppm	477	0.86	76	105	49	—
500 ppm	493	0.63	73	107	63	—
750 ppm	496	0.47	67	106	72	—
1000 ppm	488	0.42	75	105	75	—
10 mM thiourea + agar agar
100 ppm	534	0.3	87	117	82	1.51
250 ppm	521	0.13	77	112	92	1.58
500 ppm	527	0.11	67	106	93	1.41
750 ppm	515	0.08	70	110	95	1.62
1000 ppm	516	0.07	64	88	96	1.64

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The inhibition efficiency (η_%P) values are calculated from the following equation:

where, i_corr and i_corr(inh) are the values of the corrosion current densities of uninhibited and inhibited specimens, respectively. It can be seen from the table that GA and AA exhibit low inhibition efficiency, but in the presence of thiourea, a substantial increase in the inhibition efficiency is observed. With respect to the uninhibited sample, the corrosion potential is seen to shift towards a negative direction in the presence of TU, indicating a decrease in the rate of the cathodic hydrogen evolution reaction (cathodic inhibitor). Polysaccharides, on the contrary, shift the corrosion potential towards a noble direction and thus act as anodic inhibitors. In the polysaccharide–TU mixed system, the corrosion potential remains in the negative direction at low concentrations of polysaccharides, which gradually changes towards the value of the uninhibited sample with an increase in the polysaccharide concentration. The results indicate that the polysaccharide–TU system acts as a cathodic inhibitor at lower polysaccharide concentrations, while in the presence of higher concentrations of polysaccharides, it behaves as a mixed type inhibitor, suppressing both the cathodic and anodic reactions.

Electrochemical impedance measurements (EIS)

The corrosion behaviour of mild steel in 0.5 M H₂SO₄ solution and in the presence of various concentrations of inhibitors is investigated by EIS. Nyquist plots for the mild steel in 0.5 M H₂SO₄ solution containing 1000 ppm GA, 1000 ppm AA, 10 mM TU and their mixture are shown in Fig. 2. The complex plane plots obtained in the presence of various concentrations of inhibitors show only one time constant corresponding to one capacitive loop. The diameter of the capacitive loop increases with an increase in inhibitor concentration, indicating an increased charge transfer resistance at the metal–electrolyte interface.^31–35 Closer observations of these plots also reveal that the capacitive loops are depressed with their centre under the real axis, which may be related to the surface heterogeneity due to the microscopic roughness of the electrode surface and inhibitor adsorption on it.^31–35 Accordingly, we have fitted the observed Nyquist plots with an equivalent circuit containing a parallel combination of a charge transfer resistance-constant phase element (R_ct-CPE) as depicted in Fig. 3. R_s corresponds to the electrolyte resistance.

Fig. 2 Nyquist plots for mild steel in 0.5 M H₂SO₄ in the presence of (a) 1000 ppm GA, (b) 1000 ppm AA, (c) 10 mM TU, (d) 1000 ppm GA–10 mM TU and (e) 1000 ppm AA–10 mM TU.

Fig. 3 Equivalent circuit used to fit the impedance data shown in Fig. 2.

The constant phase element (CPE) is related to the double layer capacity (C_dl) and its impedance is given by:^31–35

Z_CPE = Q⁻¹(iω)⁻ⁿ (2)

where, Q is a proportionality coefficient, i is the imaginary number, ω is the angular frequency, and n is a measure of surface irregularity. For whole numbers of n = 1, 0, −1, CPE is reduced to the classical lumped element capacitor (C), resistance (R), and inductance (L), respectively. The fitted parameters using the above model are presented in Table 2.

Table 2 Impedance parameters for the corrosion of mild steel in 0.5 M H₂SO₄ in various inhibitor systems

System	Rct (Ω cm^2)	Q (μΩ^−1 s^n cm^−2)	n	Cdl (μF cm^−2)	η%Z	s
Blank	3.7	740	0.83	221	—	—
10 mM thiourea	51	109	0.88	54	93	—
Gum arabic
100 ppm	8.5	463	0.84	161	56	—
250 ppm	11	327	0.85	121	66	—
500 ppm	12	293	0.82	85	69	—
750 ppm	12.5	250	0.83	77	70	—
1000 ppm	13	235	0.83	72	71	—
10 mM thiourea + gum arabic
100 ppm	85	99	0.85	43	96	1.56
250 ppm	95	92	0.85	40	96	1.66
500 ppm	108	81	0.86	37	96.5	1.68
750 ppm	157	71	0.86	34	97.6	1.68
1000 ppm	163	57	0.87	28	97.7	1.69
Agar agar
100 ppm	7.2	572	0.85	217	48	—
250 ppm	11.3	457	0.84	167	67	—
500 ppm	18.6	388	0.85	162	80	—
750 ppm	22	288	0.85	118	83	—
1000 ppm	24	154	0.86	62	84.5	—
10 mM thiourea + agar agar
100 ppm	53.5	89	0.9	49	93	1.52
250 ppm	121	81	0.9	48	97	1.65
500 ppm	139	77	0.9	46	97	1.78
750 ppm	154	76	0.89	44	97.6	1.81
1000 ppm	173	71	0.89	41	97.8	1.82

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To obtain a direct correlation between the charge-transfer resistance (R_ct) and the double layer capacitance (C_dl), the latter has been recalculated using the following equation:^34,35

C_dl = (QR_ct¹⁻ⁿ)^1/n (3)

and the values are shown in Table 2. It is seen that with an increase in the concentration of GA and AA in their TU mixed system, the R_ct values gradually increase with a concomitant decrease in C_dl values. This reflects a gradual increase in the thickness of the adsorbed inhibitor layer with inhibitor concentration. The R_ct values for both the mixed system are found to be several times greater than the sum of the corresponding values when the inhibitors are used separately. This is a clear indication of the existence of a positive interaction towards co-adsorption in the polysaccharide–TU mixed systems on the metal surface.

The inhibition efficiencies, η_%Z, in terms of R_ct are calculated through the following equation:

where, R_ct and R⁰_ct are the values of the charge transfer resistance observed in the presence and absence of an inhibitor. The inhibition efficiencies, η_%Z, are seen to increase considerably in the mixed inhibitor systems (Table 2) showing the same trend as those obtained from the polarisation measurements (Table 1).

Synergism parameter

To determine the extent of synergism between GA–TU and AA–TU towards adsorption on the metal surface and the subsequent inhibition of corrosion, a synergism parameter (s), as originally proposed by Aramaki and Hackerman³⁶ and used subsequently by many authors,^18,37 has been calculated using the following equation:

s = [1 − (η₁ + η₂)]/(1 − η₁₊₂) (5)

where, η₁ is the inhibition efficiency of TU, η₂ is that of the polysaccharide and η₁₊₂ is the measured inhibition efficiency for the polysaccharide in combination with TU. Normally, s < 1 indicates an antagonistic effect, s = 1 means no interaction, whereas s > 1 represents a synergistic interaction between the two inhibitors towards adsorption on the metal surface. Using the inhibition efficiency values from the polarization measurements and the EIS experiment, synergistic parameters have been calculated (Tables 1 and 2). Values of s for all concentrations of the polysaccharides studied are greater than unity, reflecting a strong synergism between the polysaccharide–TU towards co-adsorption on the mild steel surface in 0.5 M H₂SO₄. It is noteworthy that in 1 M HCl, R_ct values of the polysaccharide–TU mixed systems do not vary much from the total R_ct values for TU and the polysaccharide (Fig. 4). This rules out the existence of any significant synergistic effect between the polysaccharide–TU towards inhibition of corrosion for mild steel in HCl.

Fig. 4 Nyquist plots for mild steel in 1 N HCl in the presence of (a) 500 ppm GA, (b) 500 ppm AA, (c) 10 mM TU, (d) 500 ppm GA–10 mM TU and (e) 500 ppm AA–10 mM TU.

Adsorption isotherm

To elucidate the nature of the interaction involving the polysaccharide–TU mixed inhibitor systems with the mild steel surface, we have assessed several types of adsorption isotherms. Excellent fitting of the experimental values was obtained following the simplest Langmuir monolayer adsorption model where equivalency of all the adsorption sites are considered (correlation coefficient, R² = 0.999). According to this model, the degree of surface coverage θ (θ = η_%Z/100) is related to the concentration of the inhibitor (C) as per the equation:

C/θ = 1/K_ads + C (6)

where, K_ads is the adsorption constant. The plot of C/θ with C at 30 °C (Fig. 5) yields a straight line with a slope value of 0.999 and 0.983 for the GA–TU and AA–TU mixed systems, respectively. From the values of the adsorption constant, K_ads, the standard free energy ΔG⁰_ads of adsorption for both the systems are determined using the following equation:

ΔG⁰_ads = −RT ln(1 × 10⁶K_ads) (7)

where, 1 × 10⁶ is the concentration of water molecules expressed in mg L⁻¹, R is the universal gas constant and T is the temperature. From the above equation, ΔG⁰_ads is calculated as −25.36 kJ mol⁻¹ and −24.38 kJ mol⁻¹ for the GA–TU and AA–TU systems, respectively at 303 K (30 °C). The negative value of ΔG⁰_ads is indicative of the spontaneous adsorption of the inhibitor system on the mild steel surface. The observed range of free energy values suggests that the adsorption is electrostatic in nature, i.e., the physisorption provides a major contribution towards the whole adsorption process.³⁸

Fig. 5 Langmuir adsorption plot for mild steel in 0.5 M H₂SO₄ in the presence of 1000 ppm GA–10 mM TU (-■-) and 1000 ppm AA–10 mM TU (-○-).

Surface analysis

Scanning electron micrographs (SEM) of the surface of the mild steel immersed in 0.5 M H₂SO₄, with and without the inhibitors are shown in Fig. 6(a–d). The mild steel surface immersed in 0.5 M H₂SO₄ shows a very rough surface owing to severe uniform corrosion. TU is seen to provide a perforated adsorbed layer. However, in the presence of mixed inhibitors, a compact adsorbed layer on the metal surface is observed, which acts as a barrier between the metal surface and the corrosive media, and thereby reduces the rate of corrosion.

Fig. 6 SEM images of mild steel after immersion in 0.5 M H₂SO₄ with (a) no inhibitor, (b) 10 mM TU (c) 1000 ppm GA–10 mM TU and (d) 1000 ppm AA–10 mM TU.

To get some insight into any possible interactions between the polysaccharides (GA, AA) and TU in acidic medium, which can play an important role for the synergism towards adsorption on the mild steel sample, FTIR spectra of pure GA, AA, TU, polysaccharides adsorbed on the metal surface, TU adsorbed on the metal surface and the GA–TU and AA–TU mixed systems adsorbed on the metal sample have been recorded and the relative intensities of the major vibrational bands are compared (Fig. 7). For TU, the bands at 1413 cm⁻¹ and 730 cm⁻¹ are assigned to the stretching vibration of C=S, whereas the bands around 3100–3400 cm⁻¹ are due to N–H stretching.^39,40 Bands at 1470 cm⁻¹ and 630 cm⁻¹ are assignable to different modes of NCN vibrations.^39,40 In the TU–steel complex, the intensity of the 1413 cm⁻¹ band is decreased and shifts to 1380 cm⁻¹, the N–H stretching bands, on the contrary, are seen to be shifted towards higher wavenumbers. The band at 730 cm⁻¹ shifts to a lower wavenumber and overlaps with that of the NCN vibration to give a broad band at 675 cm⁻¹. This clearly indicates that the complex is formed through the S atom of TU.³⁹ For pure GA, the band at 1626 cm⁻¹ is assigned to the stretching vibrations of the C=O bond of the carboxylate group and the stretching vibration of the C–O bond is located at 1070 cm⁻¹ and 1424 cm⁻¹.¹⁴ On adsorption of GA on steel, all these bands are observed with reduced relative intensity and with a shift to a lower frequency for the vibrational modes of the C–O bond (1424 cm⁻¹ band shifts to 1380 cm⁻¹) suggesting bonding of GA with the oxide surface of steel through the carboxylate group.¹⁴ In the GA–TU–steel complex, a relatively broad peak centered at 1350 cm⁻¹ is seen to develop, which may be interpreted as discussed below.

Fig. 7 FT-IR spectra of (a) TU, (b) TU–steel complex, (c) GA, (d) GA–steel complex, (e) GA–TU–steel complex, (f) AA, (g) AA–steel complex and (h) AA–TU–steel complex.

Among the three canonical forms of TU, as shown below, contribution from II and III increases when TU is bonded to the metal surface through its sulfur atom.⁴¹ Complete protonation of the NH₂ group may also occur in an acidic solution, as used in the present study.³⁰ This cationic or protonated NH₂ group of TU interacts electrostatically with the carboxylate group of GA which will increase the contributions from the canonical forms II and III even further. It is already reported that in comparison to HCl medium, a greater number of GA molecules remain free and dispersed in H₂SO₄ medium.¹⁴ Due to GA–TU adduct formation, most of the GA molecules are now activated to get adsorbed on the metal surface, which will in turn help to make the TU–steel bond stronger. This cooperative adsorption of TU and GA is responsible for the observed synergism towards inhibition of corrosion of mild steel in H₂SO₄. A similar type of cooperative adsorption of TU and ClO₄⁻ on iron electrodes in both acidic and neutral media is already documented.^30,41 As a result of such interaction, the 1380 cm⁻¹ bands observed both for the GA–steel and TU–steel complexes overlap with each other, and shift further towards a lower wavenumber at 1350 cm⁻¹ in the GA–TU–steel complex.

AA adsorbs on the metal surface mainly through its hydroxyl groups, as evident from a prominent shift of the –OH stretching vibration band from 3440 cm⁻¹ in its pure state,^42,43 to 3395 cm⁻¹ in its adsorbed state. The 1380 cm⁻¹ band for the C=S stretching vibration in the TU–steel complex is seen to be shifted to a lower wavenumber, and is overlapped with the 1370 cm⁻¹ band of AA belonging to C–C bending,⁴³ resulting in a relatively broad band at around 1345 cm⁻¹ in the AA–TU–steel complex. This suggests that the cationic or protonated NH₂ of TU interacts with the hydroxyl group of AA (ion–dipole interaction), which subsequently results in a further decrease in C=S bond order by forming a stronger TU–steel complex formation. Such ion–dipole interaction is reported to act during the adsorptive removal of cationic methylene blue by agar in acidic medium.⁴²

Conclusions

Potentiodynamic polarization (Tafel extrapolation) and an EIS method have established the existence of a strong synergism between polysaccharide–TU towards their adsorption on a mild steel surface in H₂SO₄ medium and the subsequent inhibition of corrosion.

From potentiodynamic polarization studies, it is observed that at lower polysaccharide concentrations, the polysaccharide–TU predominantly retards the cathodic reduction reaction, while at higher polysaccharide concentrations it acts as a mixed inhibitor.

The calculated standard free energy of adsorption indicates the physical adsorption of the polysaccharide–TU system on the metal surface.

FTIR studies reveal that TU gets adsorbed onto the metal surface through the S atom, whereas the carboxylate group present in GA and the hydroxyls of AA are found to be mostly responsible for adsorption. The cationic or protonated NH₂ group of TU interacts with the carboxylate group of the non-adsorbed GA molecules (ion–ion interactions) and the hydroxyl group of AA (ion–dipolar interaction). This helps to activate a greater number of polysaccharide molecules to be adsorbed on the metal surface, which in turn results in a stronger metal–TU bond. Such cooperative adsorption of polysaccharide–TU onto the steel surface seems to be responsible for the observed synergistic effect towards inhibition of mild steel corrosion in H₂SO₄ medium.

Acknowledgements

PI thanks the Department of Science and Technology, Govt of India for supporting a research project under the Fast Track Scheme for Young Scientists (no. SR/FT/CS-110/2010, dt. 20.09.2011).

References

1. P. B. Raja and M. G. Sethuraman, Mater. Lett., 2008, 62, 113.
2. D. Kesavan, M. Gopiraman and N. Sulochana, Chem. Sci. Rev. Lett., 2012, 1, 1.
3. G. Gece, Corros. Sci., 2011, 53, 3873.
4. S. A. Umoren, Open Corros. J., 2009, 2, 175.
5. K. P. Vinod Kumar, M. S. N. Pillai and G. R. Thusnavis, J. Mater. Sci. Technol., 2011, 27, 1143.
6. G. M. A. El-Hafez and W. A. Badawy, Electrochim. Acta, 2013, 108, 860.
7. A. R. H. Zadeh, I. Danaee and Md. H. Maddahy, J. Mater. Sci. Technol., 2013, 29, 884.
8. J.-J. Fu, S.-N. Li, Y. Wang, X.-D. Liu and L.-D. Lu, J. Mater. Sci., 2011, 46, 3550.
9. J. Fu, H. S. Zang, Y. Wang, S. N. Li, T. Chen and X. D. Liu, Ind. Eng. Chem. Res., 2012, 51, 6377.
10. M. Abdallah, Port. Electrochim. Acta, 2004, 22, 161.
11. S. A. Umoren, I. B. Obot, E. E. Ebenso, P. C. Okafor, O. Ogbobe and E. E. Oguzie, Anti-Corros. Methods Mater., 2006, 53, 277.
12. S. A. Umeron, Cellulose, 2008, 15, 751.
13. S. A. Umeron, Port. Electrochim. Acta, 2009, 27, 565.
14. M. A. Abu-Dalo, A. A. Othman and N. A. F. Al-Rawashdeh, Int. J. Electrochem. Sci., 2012, 7, 9303.
15. S. Banerjee, V. Srivastava and M. M. Singh, Corros. Sci., 2012, 59, 35.
16. S. Cheng, S. Chen, T. Liu, X. Chang and Y. Yin, Mater. Lett., 2007, 61, 3276.
17. A. M. Fekry and R. R. Mohamed, Electrochim. Acta, 2010, 55, 1933.
18. S. A. Umoren, O. Ogbobe, I. O. Igwe and E. E. Ebenso, Corros. Sci., 2008, 50, 1998.
19. M. Mobin and M. Alam Khan, J. Dispersion Sci. Technol., 2013, 34, 1496.
20. H. A. Swenson, H. M. Kaustinen, O. A. Kaustinen and N. S. Thompson, J. Polym. Sci., Part A-2, 1968, 6, 1593.
21. Y. Dror, Y. Cohen and R. Yerushalmi-Rozen, J. Polym. Sci., 2006, 44, 3265.
22. M. Sekkal, J.-P. Huvenne, P. Legrand, B. Sombret, J.-C. Mollet, A. M. Givernaud and M.-C. Verdus, Microchim. Acta, 1993, 112, 1.
23. S. Arnott, A. Fullmer, W. E. Scott, J. C. M. Dea, R. Moorhouse and D. A. Rees, J. Mol. Biol., 1974, 90, 269.
24. J. D. Talati and R. M. Modi, Anti-Corros. Methods Mater., 1976, 23, 6.
25. A. N. Mukherji, I. Singh and V. A. Altekar, Br. Corros. J., 1975, 10, 155.
26. W. A. Siddiqui, V. M. Chaubey and M. S. Ahmad, Mater. Sci. Res. India, 2007, 4, 175.
27. M. Metikoš-Huković and R. Babić, J. Appl. Electrochem., 1996, 26, 443.
28. R. T. Loto, C. A. Loto and A. P. I. Popoola, J. Mater. Environ. Sci., 2012, 3, 885.
29. D. Gopi, N. Bhuvaneswaran, S. Rajeswarai and K. Ramadas, Anti-Corros. Methods Mater., 2000, 47, 332.
30. Z. Q. Tian, Y. Z. Lian and M. Fleischmann, Electrochim. Acta, 1990, 35, 879.
31. T. Pajkossy, Solid State Ionics, 2005, 176, 1997.
32. S. John and A. Joseph, RSC Adv., 2012, 2, 9944.
33. J.-B. Jorcin, M. E. Orazem, N. Pébère and B. Tribollet, Electrochim. Acta, 2006, 51, 1473.
34. X. Wu, H. Ma, S. Chen, Z. Xu and A. Sui, J. Electrochem. Soc., 1999, 146, 1847.
35. M. Lebrini, F. Robert and C. Roos, Int. J. Electrochem. Sci., 2010, 5, 1698.
36. K. Aramaki and N. Hackerman, J. Electrochem. Soc., 1969, 116, 568.
37. S. A. Umoren, Y. Li and F. H. Wang, Corros. Sci., 2010, 52, 1777.
38. F. Bentiss, M. Lebrini and M. Lagrenee, Corros. Sci., 2005, 47, 2915.
39. S. Selvakumar, S. M. Ravi Kumar, G. P. Joseph, K. Rajarajan, J. Madhavan, S. A. Rajasekar and P. Sagayaraj, Mater. Chem. Phys., 2007, 103, 153.
40. G. B. Aitken, J. L. Duncan and G. P. McQuillan, J. Chem. Soc. A, 1971, 2695.
41. P. Cao, J. Yao, B. Ren, R. Gu and Z. Tian, J. Phys. Chem. B, 2002, 106, 10150.
42. B. Samiey and F. Ashoori, Chem. Cent. J., 2012, 6, 14.
43. M. F. Nazarudin, A. A. Shamsuri and M. N. Shamsudin, Int. J. Pure Appl. Sci. Technol., 2011, 3, 35.

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