ARTICLE pubs.acs.org/IECR
Inhibitive Effect of Argemone mexicana Plant Extract on Acid Corrosion of Mild Steel Gopal ji,† Sudhish Kumar Shukla,‡ Priyanka Dwivedi,§ Shanthi Sundaram,§ and Rajiv Prakash*,† †
School of Materials Science and Technology, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India Department of Chemistry, School of Mathematical and Physical Sciences, North West University (Mafikeng Campus), South Africa § Centre of Biotechnology, Allahabad University, Allahabad, India ‡
ABSTRACT: The effect of plant extract of Papaveraceae family Argemone mexicana is studied for use as a low cost and efficient corrosion inhibitor for mild steel in acidic environment. This plant extract is selected for the study of corrosion inhibition in view of its rich source of organic inhibiting molecules as proteins, amino acids, tannins, phenolic compounds, saponins, and flavonoids and nonalkaloids organic compounds such as fused benzene rings, hetero N atom rings, OCH3, and OH groups. A simple extraction method is adopted to obtain water-based plant extract. Argemone mexicana extract is for the first time used as an efficient inhibitor for mild steel in 1 M HCl. Weight loss and electrochemical methods are used to study the corrosion. Nearly 80% corrosion inhibition is observed at around 200 mg L1 inhibitor concentration and maximum (92.5%) for 500 mg L1 extract concentration in 1 M HCl. Inhibition mechanism is studied using UVvis, electrochemical, and surface imaging techniques.
1. INTRODUCTION Metals and alloys react with corrosive meda to form a stable compound, which may be termed as corrosion as the loss of metal occurs and the metal surface gets corroded. Various methods are used to reduce the corrosion rate of metals and alloys, among which the use of corrosion inhibitor is very popular.16 Corrosion inhibitors act by adsorption of ions or molecules over metal surfaces. They reduce the corrosion rate mainly by increasing or decreasing the anodic and/or cathodic reactions, decreasing the diffusion rate for reactants to the surface of the metal and the electrical resistance of the metal surface. For industrial and largescale use cost of the inhibitor, its toxicity, availability, and environment friendliness are very important. Therefore, apart from various organic inhibitors,711 natural inhibitors, also called “green inhibitors”, have received much more attention in recent years.1218 Plant extracts as natural organic inhibitors have been tested for many years.1921 However, mainly medicinal and edible herbs, well-known plants, and seed extracts are used for possible inhibitors. High inhibitor efficiency of these extracts is mainly explained by the occurrence of alkaloids erysotrine, erysodine, erythraline, hyponine, erythroidine, and erbydine in the seeds.22,23 Extracts and oils from wild bushes and plants having such alkaloids may be better inhibitors due to low cost and availability. In the present work we report Papaveraceae family Argemone mexicana (AM) plant’s extract as an efficient corrosion inhibitor for low carbon steel in HCl solution. The choice of Argemone mexicana is due to its low cost and easy availability. From a literature survey we found that only those plants which had some medicinal properties and were rich in alkaloids, flavonoids, and different natural organic compounds were reported as efficient inhibitors. Such compounds also had N, O, and S hetero atoms in their molecular structure. Argemone mexicana is one of the similar plants that has been used in Indian Ayurvedic system from many years to cure skin diseases and other problems. It is also famous as r 2011 American Chemical Society
Svarnakshiri in Ayurvedic system. Because alkaloids are not found in the aqueous extract of this plant, it does not show toxicity.24 We found most of such effective organic constituents (good for corrosion inhibition) in this plant extract but at low cost. This motivated us to determine the inhibitive effect of the plant’s extract. Corrosion inhibition efficiency is examined by weight loss measurements and electrochemical techniques, i.e., Tafel polarization and electrochemical impedance spectroscopy techniques using various amounts of extract in 1 M HCl. The surface morphology was examined by scanning electron microscopy (SEM).
2. EXPERIMENTAL SECTION 2.1. Plant Extract. Plant of Argemone mexicana (100 g of only leaves) was washed thoroughly and dried on tissue paper. It was crushed in the grinder properly and 100 mL of water was added and kept for 48 h with constant stirring. The extract was filtered and residue was again suspended in double distilled water and the above procedure was repeated three times for exhausted extraction. All the filtrates were pooled and dried in a rotatory evaporator. The stock was prepared by dissolving 1 g of the dried residue in 1 mL of distilled water and used for kinetic study.24 2.2. Preparation of Mild Steel Specimen (Test Coupon). Mild steel strips having the composition (wt %) C 0.16, Mn 0.032, Si 0.18, S 0.026, P 0.03, and balance Fe were used for weight loss and electrochemical studies. Mild steel coupons of dimensions 1 5 0.09 cm3 and 1 1 0.09 cm3 were used for corrosion studies using weight loss and electrochemical methods, respectively. Before taking any measurements the test coupons were abraded using emery papers of grade 2/0 to 4/0 successively. Received: July 7, 2011 Accepted: September 26, 2011 Revised: August 23, 2011 Published: September 26, 2011 11954
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Table 1. Corrosion Parameters for Mild Steel in 1 M HCl in Absence and Presence of Different Concentrations of AM Extract Obtained from Gravimetric Method (Weight Loss Measurements) at 27 ( 1 °C for 5 h concentration of inhibitor (mgL‑1)
weight loss mg cm‑2
blank
15.40
50 100
11.97 8.63
22.27 43.96
200
3.65
300
2.05
400
inhibition efficiency μWL (%)
surface coverage (θ)
Cinh/θ (mg/L)
26.7 19.2
0.2227 0.4396
224.5 227.5
76.30
8.1
0.7630
262.2
86.69
4.6
0.8669
346.1
1.88
87.79
4.2
0.8779
455.6
500
1.15
92.53
2.6
0.9253
540.4
600
1.15
92.53
2.6
0.9253
648.4
34.3
After polishing, degreasing was done by AR grade acetone and washing thoroughly with double distilled water followed by drying in vacuum oven. 2.3. Techniques Used. 2.3.1. Weight Loss Measurement. Weight loss method is the easiest way to find the corrosion rate and inhibition efficiency. For this study, mild steel test coupons were immersed in 32 mL of 1 M HCl alone and with the addition of different concentrations of AM extract at room temperature (27 ( 1 °C). The total time for acid exposure was optimized and 5 h was uniformly used for weight loss method. Triplicate experiments were conducted for each concentration of the inhibitor for the reproducibility. After exposure, test coupons were cleaned with distilled water and dried in a vacuum oven. The inhibition efficiency (μWL%) surface coverage (θ) were determined by following equations: μWL ð%Þ ¼ θ¼
Wo Wi 100 Wo
Wo Wt Wo
ð2Þ
87:6W Atd
anodic potential of +0.25 V vs SCE with respect to the open circuit potential at a sweep rate 0.5 mVs1 to study the effect of inhibitor on mild steel corrosion. The linear Tafel segments of anodic and cathodic curves were extrapolated to corrosion potential to obtain the corrosion current densities (Icorr). The corrosion inhibition efficiency (μp %) was evaluated from the measured Icorr values using the relationship μp ð%Þ ¼
ð3Þ
where W = weight loss, A = area of specimen in cm2 exposed in acidic solution, t = immersion time in hours, and d = density of mild steel (7.86 g cm3). 2.3.2. Electrochemical Measurements. A three-electrode cell assembly with mild steel coupons (1 1 0.09 cm3) as working, Ag/AgCl electrode as reference, and a large area platinum mesh as counter electrode was used for corrosion studies using Electrochemical Workstation, CHI 7041C (CH Instrument, USA). All electrochemical experiments were conducted at room temperature using 100 mL of 1 M HCl. Triplicate experiments were conducted for each concentration of the inhibitor for reproducibility. Electrochemical impedance spectroscopy was carried out at open circuit potential (ocp) (left the system for equilibrium until it is ocp) in the frequency range of 10 mHz to 100 kHz at an amplitude of 10 mV. For potentiodynamic polarization (Tafel) the electrode (test coupon) was allowed to corrode on applying potential from cathodic to anodic direction. The polarization was carried out from cathodic potential of 0.25 V vs SCE to an
o i Icorr Icorr 100 o Icorr
ð4Þ
where Iocorr and Iicorr are the corrosion current densities in absence and in presence of various concentrations of the 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 inhibition efficiency of the inhibitor was calculated from the charge transfer resistance values using the equation
ð1Þ
where Wo and Wi are the weight loss value in absence and in presence of inhibitor, respectively. Corrosion rate (Cr) values at different concentrations were calculated by using the formula reported below Cr ðmm=yÞ ¼
corrosion rate (mm/y)
μRt ð%Þ ¼
Rti Rto 100 Rti
ð5Þ
where Rot and Rit are the charge transfer resistance in absence and in presence of inhibitor. 2.4. Surface Morphology. Surface morphology of mild steel samples was studied before and after corrosion and in presence of optimum concentration of inhibitor using scanning electron microscope (SEM) Supra 40, Carl Zeiss, Germany.
3. RESULTS AND DISCUSSION In 1 M HCl, mild steel corrodes rapidly but a significant corrosion inhibition was observed with the increasing concentration of AM extract. This was studied by weight loss and electrochemical techniques. 3.1. Weight Loss Study. Initially mild steel corroded very fast in 1 M HCl and showed very high corrosion rate with heavy weight loss. The corrosion rate decreased and the inhibition efficiency increased appreciably by addition of AM extract, and inhibition efficiency reached up to 92.5% at the addition of 500 mg L1 of AM extract. Inhibition efficiency was found to be increased with increase in AM extract concentration up to 500 mg L1 in acid solution. After this concentration, no appreciable change in inhibition efficiency was observed. Detailed data regarding the increase in corrosion rate are given in Table 1 and the change in inhibition efficiency is also clearly indicated. 3.1.1. Adsorption Isotherm. Adsorption depends mainly on the charge and the nature of metal surface, electronic characteristics of metal surface, adsorption of solvent and other ionic species, on the electrochemical potential at solution interface. The adsorption isotherm study describes the adsorptive behavior 11955
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Table 2. Potentiodynamic Polarization Parameters for Mild Steel without and with Different Concentrations of AM Extract in 1 M HCl Solution concentration of inhibitor (mgL‑1)
Ecorr
Icorr
ba
bc
μp
(V vs Ag-AgCl) (μA cm‑2) (mV dec‑1) (mV dec‑1) (%)
blank
0.500
1220
60
114
200
0.513
264
97
160
78.4
300 400
0.526 0.549
213 140
103 112.2
171 182
82.5 88.5
108
131.4
174
91.2
500
528
Figure 1. Langmuir adsorption isotherm for mild steel in 1.0 M HCl containing different concentrations of AM extract.
Figure 3. Nyquist plot of mild steel in 1 M HCl without and with different concentrations of AM extract.
Figure 2. Tafel plot of mild steel in 1 M HCl with different concentrations of inhibitor in 1 M HCl solution.
of organic inhibitor which is important in order to know the adsorption mechanism of inhibitor to the metal surface. The most frequently used adsorption isotherms are Langmuir, Temkin, Frumkin, and Freundluich isotherms. The Langmuir adsorption isotherm was found to provide the best description of the adsorption behavior (Figure 1) with the regression coefficient R2 almost unity (0.98424). Inhibitor molecules form a thin layer on the metal surface which isolates the metal from the corrosive environment and opposes further dissolution of the metal. The above discussions clearly suggest that the inhibition of the mild steel in acid solution in presence of various concentrations of AM extract depends upon molecular adsorption which follows the Langmuir adsorption isotherm. 3.2. Electrochemical Studies. 3.2.1. Potentiodynamic Polarization Study. The potentiodynamic studies were carried out in order to know the kinetics of cathodic and anodic reactions. Figure 2 shows the results of the effect of AM extract on the cathodic and anodic polarization curves of mild steel in 1 M HCl. It is observed from the figure that both the reactions were suppressed with the addition of the AM extract. This suggests that the AM extract reduced the anodic dissolution reactions as well as retarded the hydrogen evolution reactions on the cathodic sites. Electrochemical corrosion kinetics parameters, i.e., corrosion potential (Ecorr), corrosion current density (Icorr), anodic and cathodic Tafel slopes (ba and bc) obtained from the extrapolation of the polarization curves are listed in Table 2. It is evident from Table 2 that the value of bc changed with increase in inhibitor concentration, indicating the influence of
Figure 4. Electrochemical equivalent circuit used to fit the impedance spectra.
the inhibitor on the kinetics of the hydrogen evolution. The shift in the anodic Tafel slope ba may be due to the chloride ion or inhibitor molecules adsorbed on the metal surface. The corrosion current density (Icorr) decreases with the increase in the adsorption of the inhibitor by increasing the inhibitor concentration. Decrease in the corrosion current density with increase in the inhibitor concentration can also be observed from the data reported in Table 2. According to Ferreira et.al.25 and Li et. al.,26 if the displacement in corrosion potential is more than 85 mV with respect to the corrosion potential of blank solution, the inhibitor can be consider as a cathodic or anodic type. In the present study the maximum displacement was 49 mV, indicating that the studied inhibitor is a mixed type inhibitor. 3.2.2. Impedance Analysis. Corrosion behavior of mild steel in 1.0 M hydrochloric acid solution was investigated with and without AM extract by electrochemical impedance spectroscopic measurements. Figure 3 shows the impedance spectra of mild steel corrosion in the form of Nyquist plots. A single semicircle has been observed at high frequency which can be attributed to charge transfer of the corrosion process and the diameter of the semicircle increased with increasing inhibitor concentration. Figure 3 clearly shows that the impedance spectra are not a perfect semicircle. They seem to be depressed with center under real axis and resemble depressed 11956
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Table 3. Impedance Parameters for Mild Steel in 1.0 M HCl in Absence and Presence of Different Concentrations of AM Extract concentration (mgL1)
Rs (Ω cm2)
Rt (Ω cm2)
N
Yo (106 Ω1 cm2)
Cdl (μF cm2)
blank
1.23
30
0.802
147
41.7
200
0.95
66
0.809
120
39.0
54.5
300
1.01
84
0.815
100
36.9
64.3
400
0.89
124
0.869
99
33.4
75.8
500
0.83
209
0.885
83
29.1
85.7
μRt (%)
Figure 5. SEM images for surface morphology of (a) abraded mild steel sample before immersion, (b) after corrosion in 1 M HCl, and (c) in presence of AM extract (500 mg L1) in 1 M HCl.
capacitive loops. Such phenomenon often corresponds to surface heterogeneity which may be the result of surface roughness, dislocations, distribution of the active sites, or adsorption of the inhibitor molecules.2729 An equivalent circuit was introduced to explain the EIS data as shown in Figure 4. This circuit is generally used to describe the iron/acid interface model.30 In this circuit Rs is solution resistance,
Rt is charge transfer resistance, and CPE is a constant phase element. The impedance function of the CPE is as follows: ZCPE ¼ Y 1 ðjωÞn
ð6Þ
where Y is the magnitude of the CPE, ω is the angular frequency, and the deviation parameter n is a valuable criterion of the nature of the 11957
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Figure 6. UVvisible spectrum of the AM extract (500 ppm) and the washing solution of the steel strip incubated in the AM extract (at pH 7) for 5 h followed by dip washing.
metal surface and reflects microscopic fluctuations of the surface. For n = 0 ZCPE represents a resistance with R = Y1; n = 1 an inductance with L = Y‑1 and n = 1 an ideal capacitor with C = Y.31 In iron/acid interface systems, ideal capacitor behavior is not observed due to the roughness or uneven current distributions on the electrode surface which results in frequency dispersion.28,32,33 The electrochemical parameters (Rs, Rt, Yo, and n) obtained from the fitting the recorded data using the equivalent circuit of Figure 4 are listed in Table 3. Cdl values listed in Table 3 were derived from CPE parameters by use of the following equation:34 Cdl ¼ ðYo , Rt1 n Þ1=n
ð7Þ
From Table 3 it is evident that the Rt values increases with the increase in AM extract concentration. The increase in Rt value is attributed to the formation of protective film on the metal/ solution interface. The increase in the values of n of the inhibited samples in comparison with uninhibited samples can be explained by decrease of the surface heterogeneity, due to the adsorption of the inhibitor on the most active adsorption sites.35 The values of double layer capacitance (Cdl) decreased with increasing AM extract concentration. The thickness of the protective layer (d) is related to Cdl according to the following equation:36 Cdl ¼
εεo d
ð8Þ
where ε is the dielectric constant of the protective layer and εo is the permittivity of the free space. It is obvious from the results that the AM extract inhibited the corrosion of mild steel in 1 M HCl solution at all the concentrations used in the study and μRt% was increased continuously with increasing inhibitor concentration (Table 3). The inhibition efficiencies calculated from EIS show the same trend as those obtained from weight loss and Tafel polarization data. 3.3. Surface Study. Scanning electron microscope images of the surface of the samples were taken to study the morphology before and after the inhibition process. Cleaned steel sample surfaces showed a few defects and submicrometer cracks over the surface as shown in Figure 5a. Deep corrosion and considerable loss of the surface material was observed when the steel sample was treated in 1 M acid as it is shown in Figure 5b. However, a significant decrease in the loss of surface material is observed when the corrosion was carried out in presence of inhibitor (500 mg L1) for same time period. Surface morphology was retained up to
certain extent and pits were seen probably due to attack of acid at defect and crack sides of the steel (Figure 5c). 3.4. Mechanism of Inhibition. Corrosion inhibition of mild steel in hydrochloric acid solution by AM extract is explained on the basis of molecular adsorption. Adsorption of organic molecules was found stable as supported by UVvis studies (Figure 6). UVvis suggests that the compounds present in the AM extract were adsorbed on the mild steel surface. A simple experiment was carried out in order to observe the adsorption of the organic molecules present in the AM extract over the steel samples. A steel strip was incubated in the AM extract at pH 7 for 5 h and then taken out followed by thorough dip washing in water. After this the strip was washed in fresh water by rubbing the surface and this washing solution was used for UVvis study. The diluted AM extract (500 mg L1) showed two peaks and the same also appeared in the washings of the steel strip incubated in the extract solution. Adsorption of inhibitor molecules can also be verified by the surface coverage parameter θ which was found to be increased with the increasing concentration of inhibitor (Table 1). Inhibitor molecules increased the surface coverage (from 0.22 to 0.925) so that the HCl could not react effectively with the mild steel and the metal was inhibited. Corrosion is taking place through two reactions: cathodic (hydrogen evolution) reaction and anodic (metal dissolution) reaction. Organic compounds present in the extract inhibited corrosion probably by controlling both the anodic and cathodic reactions. Constituents of AM extract are carbohydrates, gums, mucilages, proteins, amino acids, tannins, phenolic compounds, saponins, and flavonoids.24 Organic compounds in the extract containing fused benzene rings and OCH3, hetero N atoms in the rings, OH molecules. These macromolecules have strong affinity toward metals and in acidic environment protonation takes place, which probably shows better adsorption over the metal surface. These protonated species adsorbed on the cathodic sites of the mild steel and decreased the evolution of hydrogen. The adsorption on anodic sites probably occurred through long π-electrons of aromatic rings and lone pair of electrons of nitrogen atoms, which decreased anodic dissolution of mild steel.37
4. CONCLUSION Argemone mexicana leaves extract is used as inhibitor for mild steel corrosion in 1 M HCl. Weight loss and electrochemical techniques were used for the study and surface morphology was studied using SEM. The inhibitor showed maximum inhibition efficiency up to 92.5% at 500 mg L1 inhibitor concentration. Langmuir adsorption isotherm and impedance studies showed that AM extract inhibited mild steel through adsorption mechanism. It was further supported by UVvis studies. AM extract is proved as an efficient organic inhibitor having mixed type of inhibitor properties due to presence of saponins, flavonoids, and organic compounds having fused benzene rings, hetero N atom rings, OCH3 and OH groups. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: +91-9935033011.
’ ACKNOWLEDGMENT G.j. and S.K.S. acknowledge SMST, IT, BHU (India) and North West University (South Africa) respectively, for financial support (scholarships). 11958
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