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Inhibitive Effect of Chlorophytum Borivilianum Root Extract on Mild Steel Corrosion in HCl and H2SO4 Solutions gopal ji, Priyanka dwivedi, Shanthi Sundaram, and Rajiv Prakash Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie4008387 • Publication Date (Web): 02 Jul 2013 Downloaded from http://pubs.acs.org on July 20, 2013

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Industrial & Engineering Chemistry Research

Inhibitive Effect of Chlorophytum Borivilianum Root Extract on Mild Steel Corrosion in HCl and H2SO4 Solutions Gopal Ji, Priyanka Dwivedia, Shanthi Sundarama, Rajiv Prakash* School of Materials Science and Technology, Indian Institute of Technology, Banaras Hindu University, Varanasi 221 005, India a

Centre of Biotechnology, Allahabad University, Allahabad 211002, India

*Corresponding Author E-mail: [email protected]; Phone: +91-9935033011

ACS Paragon Plus Environment

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Abstract Inhibition effect of Chlorophytum borivilianum root extract (CBRE) on mild steel corrosion in 1 M HCl and 0.5 M H2SO4 media has been studied using various techniques viz. weight loss, electrochemical impedance spectroscopy, Tafel polarization and scanning electron microscopy. Results indicate that inhibition efficiency of CBRE increases with increase in inhibitor concentration in both the acid media. Furthermore, the effect of immersion time (120 hours), temperature (35-55˚C) and acid concentration on inhibition potential of CBRE has been investigated by weight loss method. Characterization of CBRE is carried out using FTIR, Uv-visible spectroscopy and preliminary phytochemical screening tests. Langmuir isotherm model is proposed as most suitable adsorption isotherm in both acid solutions. Uv-vis study and SEM images have confirmed the molecular adsorption of the extract on mild steel surface. Results of the techniques used are in good agreement and reflect potential of the extract for corrosion inhibition of mild steel in acidic environments.

Keywords: Acid Corrosion; Mild Steel; Natural Product; Green Inhibitor; Polarization; Chlorophytum borivilianum.


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1. Introduction Corrosion of the metals and alloys has been a serious problem for the scientists since ancient years. The risk involved in the management of corrosion losses has grown aggressively with time and requires huge investment of money and utmost safety concerns for effective handling. There are several methods invented to reduce the hazardous effect of corrosion but use of inhibitors

1-6

is one of

the most successful, efficient and easy methods adopted for protection of metals. In many industries like textiles, paper, sugar mill, chemical etc., acid solutions are commonly consumed for removing contaminating substances and unwanted corrosion products from the surface of machines and its parts. Among all mineral acids, use of hydrochloric acid and sulfuric acid are considered more effective and economical for these purposes. Many times during cleaning and descaling process, acid solutions damage the metals and increase corrosion rates violently along with loss of materials. To avoid such type of undesirable losses of metals and alloys from corrosion, organic inhibitors are commonly used for its high inhibition efficiency in acidic medium

7-11

. Several chemical compounds

have been tested till now for corrosion inhibition of metals and alloys, however, the compounds having N,S,O hetero atoms, incorporated in aromatic system, in their structure have been found to posses excellent anticorrosion potential. In recent years, due to environmental issues, researchers have been working on the concept of negligible harmful effects to the environment (green inhibitors) to avoid the toxic effect of synthetic corrosion inhibitors. This new class of inhibitors is found highly efficient in acidic medium

12-18

. For the same purpose, various plant extracts are also studied to control the

corrosion of metals in acidic medium

19-21

. High inhibition efficiency of the plant’s extracts is

acknowledged due to adsorption of the organic moieties (present in the extract) on the metals surface, which effectively reduced exposed surface area

22, 23

of the metals in corrosive solutions.

In present work, we have worked on the problem of mild steel protection in two frequently encountered environments (chloride and sulphate ions) in the industries. Mild steel is one of the most utilized iron alloys for numerous engineering and industrial applications due to its cost effectiveness and excellent functional properties. So, we have carried out corrosion inhibition studies on the mild steel and on the basis of achieved results, we are reporting aqueous root extract of Chlorophytum borivillianum (Liliaceae family) as a green corrosion inhibitor for mild steel in acid media. In India, Chlorophytum borivillianum is popular as ‘Safed Musli’ and roots of this plant have been used since many years for medical purposes24. Chlorophytum borivillianum roots are a rich source of saponins,


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flavonoids and other effective biodegradable organic compounds, which contain hetero atoms (arranged in aromatic system) and fulfill the general norms of an effective inhibitor. This fact attracted our attention towards corrosion studies using extract of the roots of this plant. To the best of our knowledge, this extract has not been used for the purpose of corrosion inhibition studies so far. We have demonstrated anticorrosion potential of this root extract using weight loss, Tafel polarization, electrochemical impedance spectroscopy, SEM, UV-visible and FTIR spectroscopy techniques. We have also studied the effect of immersion time (120 hours), temperature (35-55˚C) and acid concentration (1-5 M HCl and 0.5-2.5 M H2SO4) on inhibition ability of the extract using weight loss method. Our investigation shows great potential of the Chlorophytum borivilianum root extract for corrosion inhibition of mild steel in acidic environment.

2. Experimental 2.1 Extraction Procedure The roots of Chlorophytum borivilianum plant were collected from the field and extract was prepared 24

according to method described in Kenjale et al. . The roots were dried in sun light for one week and powdered for extraction. Powder of Chlorophytum borivilianum roots (100 g) was added to distilled water (600 mL) and kept on stirring for 3 hr. This aqueous solution was filtered and the process was repeated two times for exhaustive extraction. All filtrates were collected and dried in oven for one day. 1 g of obtained dried powder was dissolved in 10 mL distilled water and used for the corrosion study without any further purification.

2.2 Materials The following composition of the mild steel was used for all the experiments (wt %): C-0.16, Mn-0.032, Si0.18, S-0.026, P-0.03 and rest Fe. Weight loss and electrochemical studies were performed on mild steel coupons of dimensions 1×5×0.03 cm3 and 1×1×0.03 cm3 respectively. For preparation, test coupons were abraded with emery paper of grade 1/0 to 6/0 successively. Afterwards, cleaning and degreasing of the samples were done with AR grade acetone. 2.3 Corrosive Solutions Test solutions of HCl and H2SO4 were prepared by required dilution of AR grade 35 % HCl and AR grade 98% H2SO4 respectively with distilled water. Concentration of inhibitor’s stock solution (dark brown) was 100


4

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-1

mg L , which was further diluted with distilled water to obtain different concentrations of extract solution. The concentration range of CBRE used for corrosion studies was 100-600 mg L-1.

2.4 Techniques Used 2.4.1 Weight Loss Measurements Prepared mild steel coupons were immersed in 100 ml of 1 M HCl and 0.5 H2SO4 with different concentrations of CBRE at room temperature (26±1° C). The total time for acid exposure was optimized and 5 h is uniformly used for weight loss method. Triplicate measurements were collected (three set for one concentration) for higher accuracy and validity of the results. After 5 h of immersion, test coupons were taken out and washed with distilled water followed by drying in a vacuum oven. Weights of the specimens, before and after immersion, were measured by METTLER TOLEDO electronic balance having sensitivity of ±0.1 mg .The inhibition efficiency (µwL) and surface coverage (θ) values were calculated by following equations: (1)

(2)

Where, Wo and W i is used for weight loss (mg) of mild steel in absence and presence of inhibitor. Corrosion rates (Cr) at different concentrations of CBRE were also computed using equation:

Cr ( mmpy) =

87.6W Atd

(3) 2

Where, W = weight loss in mg, A = area of specimen in cm exposed in acidic solution, t = immersion -3

time in hours and d= density of material used (7.86 g cm ).

2.4.2 Electrochemical Methods To know the electrochemical aspects of corrosion process, an Electrochemical Work Station CHI 7041C (CH Instrument, USA) was used to monitor corrosion reactions occurring in a conventional 3

three electrode cell assembly. This electrochemical cell include mild steel coupons (1×1×0.03 cm ) as working, Ag/AgCl electrode as reference

and

a

large

area

platinum mesh

as

counter

electrode. All electrochemical measurements were taken at room temperature using 100 ml 1 M HCl


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and 0.5 M H2SO4. Prior to conduct experiments, system was left for 10 minutes to attain stable open circuit potential (ocp). Tafel polarization test was carried out by allowing the electrode (test coupon) to corrode on potential of 5 mV, applied from cathodic to anodic direction. The polarization was carried out from -0.25 V vs. Ag/AgCl (cathodic potential) to + 0.25 V vs. Ag/AgCl (anodic potential) with respect to the open circuit -1

potential at a sweep rate 0.5 mVs . The linear segments of anodic and cathodic polarization curves were extrapolated to obtain the corrosion current densities (Icorr) and equilibrium potentials (Ecorr). All the data from polarization curves were computed with the help of Chi7041C software. The corrosion inhibition efficiency (µp %) was evaluated from the obtained Icorr values using the relationship: (4)

o i Where, I corr and I corr 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 5 mV amplitude in the frequency spectrum from 100 kHz to 10 mHz. The corrosion inhibition efficiency (µRt %) of the inhibitor was calculated from the charge transfer resistance values using the following equation: (5)

Where,

and

are the charge transfer resistance in absence and in presence of inhibitor.

2.5 Surface Morphology Surface morphology of the mild steel test coupons was recorded with the help of Scanning electron 3

microscope SUPRA 40 (Carl Zeiss, Germany). To capture images of mild steel, samples (1×1×0.03 cm ) were immersed for 5 hours in 1 M HCl and 0.5 M H2SO4 alone and surface morphology was compared with the obtained surface images in the presence of maximum inhibitor concentration used in the study.

2.6 FT-IR Spectroscopy Thermo scientific FTIR Instrument (Nicolet 6700) was used to identify major functional groups present in the root extract. For the study, pallet of extract material was prepared with the KBr and thus obtained pallet


6

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was examined by the FTIR instrument in transmittance mode. Presence of functional groups was confirmed by comparing the results with standard FT-IR data.

3. Results and Discussion 3.1 Characterization of extract 3.1.1 FTIR and Uv-Visible Spectroscopy FT-IR spectrum of the CBRE is shown in Figure 1. A strong and broad band observed at 3370 cm-1 can be assigned to N-H or O-H stretching frequencies. Peak at 3271 cm-1 corresponds to N-H stretching of the primary amines. A small band noticed at 2984 cm-1 is observed due to C-H stretching -1

vibration. Strong absorption peak at 1616 cm can be attributed to C=C and C=N stretching or N-H -1

bending vibration. Peak at 1435 cm correspond to O-H bending. Strong band obtained at 1387 cm

-1

is related with CH3 or C-H in plane bending vibration. There are 4 peaks found in the range of 1000 cm-1 to 1300 cm-1 which correspond to C-N or C-O stretching. The absorption peaks obtained below 1000 cm-1 are due to aliphatic and aromatic C-H functional groups. It can be concluded on the basis of FT-IR spectrum that Chlorophytum borivilianum aqueous root extract contains functional groups having C-N, C-O, O-H, N-H, C=N linkages as well as aromatic rings.

Figure 1. FTIR spectrum of Chlorophytum borivilianum root extract.


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-1

Figure 2 shows Uv-Visible spectrum of the pure extract and the solutions containing 500 mg L of CBRE in 1 M HCl and 0.5 M H2SO4. In both the acid media, obtained spectrum shows almost same absorption curve as obtained for the root extract, which suggests that inhibitor molecules (present in the CBRE) were adsorbed on the mild steel surface successfully. In the absorption curve of the extract, two sharp bands can be clearly observed at 275 nm and 280 nm along with small peaks in the range of 300-400 nm, which indicates presence of aromatic rings in

-1

25

the extract.

-1

Figure 2. Uv-visible spectra of a) 500 mg L CBRE, b) 500 mg L CBRE in 1 M HCl and c) 500 mg L-1 CBRE in 0.5 M H2SO4.

3.1.2 Preliminary Phytochemical Screening of CBRE Some chemical tests

26

were performed on Chlorophytum borivlianum extract to identify possible

organic compounds and results are summarized in Table 1. Positive sign ‘+’ shows presence of compound whereas negative sign ‘-‘is used to indicate absence of compound. Numbers of positive sign assigned to particular compound shows intensity of presence of that compound. From the analysis of results, it was found that saponins were present in majority in the extract and could be 27

considered as a chief constituent. It is reported somewhere that presence of furostanol saponins can -1

-1

be verified with peaks obtained in the frequency range of 1600 -1700 cm and 3350-3400 cm in IR region. On the basis of this fact presence of saponins in root extract can be confirmed with strong peak obtained at 3370 cm-1 and 1616 cm-1 (Figure 1).


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Table 1: Preliminary test showing different compounds present in the Chlorophytum borivilianum aqueous root extract. Test

Water extract of root of Chlorophytum borivilianum

Flavanoid a. Ferric chloride test

+

b. NaOH + HCl

+

Carbohydrate

++

Ethanol+ glacial acetic acid+H2SO4+ ethanolic water Cardiac glycoside a.FeCl3+ glacial acetic

-

acid+H2SO4 b.pyridine + sodium

-

nitroprusside + NaOH Saponins

+++

Frothing test Tannins

++

H2SO4 + HCl

3.2 Weight Loss Measurements 3.2.1 Effect of CBRE Concentration on Inhibition Efficiency and Corrosion rate Weight loss method is a useful, reliable and widely used technique to investigate inhibition potential of inhibitors. This method was used to determine inhibition efficiency of CBRE and it was found from the results that significant loss in the weights of specimens occurred when it was immersed in acid solutions. But presence of CBRE retarded dissolution of mild steel in both acidic medium and increase in inhibition efficiency was observed with increase in amount of inhibitor. Figure 3 shows corrosion rate values and inhibition efficiencies obtained at room temperature (RT- 26±1° C) in presence of different concentrations of CBRE in 1 M HCl and 0.5 M H2SO4. Inspection of Figure 3 revealed that in presence of CBRE, resistance of mild steel against corrosion increased drastically and attained maximum value at inhibitor concentration of 500 mg L-1 in both acid solutions. On addition of 500 mg


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-1

L inhibitor, corrosion rate significantly decreased from 36.30 mmpy to 3.10 mmpy in 1 M HCl and from 42.80 mmpy to 7.2 mmpy in 0.5 M H2SO4 (Figure 3a) . Showing similar behavior inhibition efficiency increased with inhibitor concentration and maximum value was acknowledged as 90% in HCl while it was 83 % in H2SO4. The changed behavior of mild steel could be attributed to effective blanketing of mild steel surface in corrosive solutions. With the increasing amount of inhibitor, exposed surface area was reduced due to increased number of adsorbed inhibitor molecules, however, after reaching maximum concentration no significant effect of CBRE was observed.

Figure 3. Showing a) Corrosion rate and b) Inhibition efficiency at different concentrations of inhibitor in 1 M HCl and 0.5 M H2SO4 at 26±1° C for 5h.

3.2.2 Adsorption Characteristics of Inhibitor Generally, inhibition efficiency of inhibitors depends upon degree of adsorption of its constituents on metal surface. But stability of adsorbed molecules (inhibition period) varies with type of adsorption, chemical/physical/both, to a great extent. So, it becomes necessary to study metal-inhibitor interaction through adsorption isotherms. There are many isotherms which are being used to depict adsorption mechanism like Langmuir, Temkin, Frumkin, Flory-Huggins, Freundlich and El-awady. Efforts were made to fit the results according to these popular isotherms but in our case, we found Langmuir isotherm most suitable to explain adsorption behavior of inhibitor molecules in both acidic medium. 28

According to Langmuir isotherm , surface coverage θ is related to concentration of inhibitor C by following relation:


10

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(6) Where Kads is adsorption coefficient showing degree of metal/inhibitor interaction.

Table 2: Various parameters obtained from straight lines drawn between C/θ and C. Corrosive medium

Slope

Intercept (g L-1)

Kads (L g-1)

1 M HCL

0.910

0.118

8.50

0.5 M H2SO4

0.902

0.172

5.81

Attempts were made to plot a graph between C/θ and concentration of inhibitor C (Figure S1supporting information) which showed good agreement with Langmuir isotherm with high regression 2

coefficient (R = 0.9984 and 0.9972) in both the cases. Parameters obtained from Langmuir isotherm fitting are listed in Table 2. Slopes of the curves are nearly 1, which also validated the approach of selecting Langmuir isotherm for explanation. From careful investigation, it was observed that CBRE was more efficient in 1 M HCl than 0.5 M H2SO4. The reason could be explained on the basis of -1

-1

isotherm model. High value of Kads (8.50 L g ) was acknowledged in 1 M HCl while it was 5.81 L g in 0.5 M H2SO4, which advocated that adsorption of CBRE on mild steel surface was more favored in HCl.

3.2.3 Effect of Immersion Time on Corrosion Inhibition Variation in corrosion rate with immersion time was studied for period of 120 hours (at RT 26±1° C) -1

using 500 mg L concentration of inhibitor in 1 M HCl and 0.5 M H2SO4. It is evident from Figure S2 (supporting information) that corrosion rate in both acid solutions increased significantly up to 78 hours and remained almost constant for rest of the period. Initially (6 Hours), it was 36.3 mmpy in 1 M HCl and 42.8 mmpy in 0.5 M H2SO4, which was drastically increased to 70.6 mmpy and 86 mmpy in HCl and H2SO4 respectively (at the end of the experiment). But, in the presence of CBRE corrosion rate did not change appreciably showing high stability of inhibitor in acidic environment. After 6 hours it was 3.1 mmpy in 1 M HCl and 7.2 mmpy in 0.5 M H2SO4 where as it was 5.60 mmpy and 11.7 mmpy in HCl and H2SO4 respectively after 120 hours. Increased resistance of mild steel in acid solutions could be attributed to the high stability of the inhibitor molecules, accumulated at the metal-


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acid interface, which acted as a barrier for corroding molecules and successfully reduced exposed surface area of the samples for long time. However, slight increase in corrosion rate was observed with time which might be the result of degradation of inhibitor molecules in acidic environments leaving larger area available for corrosion.

3.2.4 Effect of Acid Concentration on Corrosion Inhibition Figure S3 (supporting information) illustrates change in inhibition ability of CBRE with acid concentration. Figure S3a revealed that corrosion rate gradually increased up to 3 M HCl but increased sharply for higher concentration of acid. On the other side, inhibition efficiency decreased at lower rate up to 3 M HCl whereas significant acceleration in diminishing rate was observed for further increase in concentration of HCl. Similar pattern was observed in the case of H2SO4 (Figure S3b). In any case, higher corrosion inhibition was acknowledged in HCl than H2SO4 media. Decreased corrosion inhibition effect of CBRE in either media with increase in concentration could be attributed to the higher desorption rate/lower adsorption rate of inhibitor molecules at the metal-acid interface14.

3.2.5 Effect of Temperature on Corrosion Inhibition Effect of temperature on inhibition ability of CBRE is depicted in Figure S4 (supporting information). It was revealed from inspection of Figure S4 that significant protection of mild steel was achieved in either acid media using 500 mg L-1 of extract. At 35˚ C, corrosion rate in 1 M HCl and 0.5 M H2SO4 was 41 and 50 mmpy respectively. But, increase in temperature caused high damage to the mild steel and higher corrosion rate of 59.5 and 76.8 mmpy was observed at 55˚ C in HCl and H2SO4 correspondingly. In contrast, CBRE significantly retarded the corrosion rate in both acid media; however, inhibition ability was decreased with increase in temperature. The reason for decreased inhibition effect of CBRE at higher temperature may be given as higher degradation rate of the adsorbed inhibitor molecules, due to which, CBRE could not resist access of the corrosive molecules towards mild steel surface and enhanced corrosion occurred with increase in temperature.

3.3 Polarization Test Figure 4 shows polarization curves obtained for mild steel with various concentrations of CBRE in acid solutions (at 26±1° C). Inspection of Figure 4a and 4b revealed that presence of inhibitor (500 mg L-1)


12

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lowered the corrosion current significantly with respect to corrosion current obtained in 1 M HCl and 0.5 M H2SO4 respectively. It was also clearly observed from polarization curves that both anodic and cathodic reactions were inhibited by CBRE (Figure 4), however, more pronounced effect was seen on anodic reactions (mild steel dissolution) than cathodic reactions (hydrogen evolution).Tafel polarization parameters like free corrosion potential (Ecorr), corrosion current density (Icorr), anodic Tafel slope (ba) and cathodic Tafel slope (bc) are calculated and summarized in Table 3.

Figure 4. Tafel curve plot for mild steel in a) 1M HCl and b) 0.5 M H2SO4 with different concentrations of inhibitor at Room Temperature.

Careful inspection of Table 3 revealed that increase in CBRE concentration decreased corrosion current densities remarkably and maximum inhibition efficiency of 91 % and 84 % was achieved in 1 M and 0.5 M H2SO4 respectively. In both acid solutions, free corrosion potential (Ecorr) was found shifted in presence of inhibitor; however, shift was not observed in a particular direction. This fact 29, 30

suggested that CBRE acted in a mixed mode of inhibition

. Further, it was found that there was no

definite pattern in the change of slope values (ba and bc ) with inhibitor concentration, which also supported the fact that CBRE behaved as a mixed type inhibitor in both HCl and H2SO4 solutions. Reason for effective inhibition could be given as increased resistance of mild steel against polarization 2

in presence of inhibitor (Table 3). Initially, in 1 M HCl resistance was 28 Ω cm while in 0.5 M H2SO4 it 2

2

2

-1

was18 Ω cm , which was considerably increased up to 200 Ω cm and 99 Ω cm using 500 mg L

inhibitor concentration in HCl and H2SO4 respectively. Probably, organic moieties present in the


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extract gathered at metal-acid interface and resisted further polarization of the mild steel, which was well reflected by increasing polarization resistance values with the inhibitor concentration.

Table 3: Tafel polarization parameters obtained at different concentration of inhibitor for 1 M HCl and 0.5 M H2SO4 at RT. Acid solution

Concentration -Ecorr -1

(mg L )

I corr

ba -2

bc -1

-1

(mV ,Ag/AgCl) (µA cm ) (mV dec ) (mV dec )

µp

Linear

%

polarization resistance 2

(Ω cm ) 1 M HCl

Blank

521

1095

79

62

-

28

300

508

255

103

74

77

96

400

511

188

111

78

83

121

500

517

102

113

84

91

200

534

1646

86

59

-

18

300

511

479

121

81

71

45

400

529

394

109

61

76

64

500

521

265

101

65

84

99

0.5M H2SO4 Blank

3.4 Electrochemical Impedance Spectroscopy Impedance behavior of mild steel in 1 M HCl and 0.5 M H2SO4 with various concentrations of CBRE is shown in Figure 5. In both acid solutions, impedance spectra showed single capacitive loop (one time constant) which indicated that inhibition of mild steel was performed by CBRE reducing charge transfer process at metal acid interface. Shape of Nyquist plots followed nearly same pattern at every concentration used in the study, which suggested that addition of inhibitor caused no change in the mechanism of corrosion inhibition. Figure 5a and 5b illustrated that the impedance spectra were not perfect semi circles, which reflected deviation of electrode (mild steel) from ideal capacitor behavior. This type of deviation (depressed capacitive loops) is often shown by the systems due to surface heterogeneity and irregularity, which may be the result of surface roughness, distribution of the active sites or adsorption of the inhibitor molecules

31,32

. It was observed from the Nyquist plots that

diameters of these capacitive loops were increasing with increasing inhibitor concentration. This fact revealed that impedance of mild steel against corrosion was increased in accordance with the CBRE amount in acid solutions. Different values of frequency at each inhibitor concentration are shown in Figure 5a and 5b, in which, the point of maximum imaginary impedance is indicated by an arrow. It


14

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was observed that the value of frequency was decreasing with addition of inhibitor, which supported the fact discussed above i.e. increased charge transfer resistance with increase in inhibitor concentration.33 Further It was analyzed from Figure 5c and 5d that phase angle enhanced with amount of CBRE used. This fact indicated that inhibitor reduced the roughness of mild steel surface which was extremely rough in case of uninhibited samples due to aggressive attack of acid solutions.

13

Figure 5. Showing Nyquist plot for mild steel in a) 1 M HCl , b) 0.5 M H2SO4 and corresponding bode plot c) 1 M HCl, d) 0.5 M H2SO4 at different concentrations of inhibitor at room temperature. e) Proposed electrochemical circuit for fitting.


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An electrical circuit was used to explain the results obtained by EIS which could simulate same response as given by electrochemical system in real conditions (Figure 5 e). In this circuit, Rs is solution resistance, Rt is charge transfer resistance and CPE is a constant phase element which was considered to compensate ideal capacitive behavior. The impedance of the CPE can be described as follows: (7) where, Y is the magnitude of the CPE, ω is the angular frequency and n is a parameter which indicate degree of deviation from ideal capacitor behavior and reflect surface fluctuations

34

. From equation 7 it

is evident that for (a) n=0, CPE will correspond to resistance with magnitude Y= 1/R (b) n=1, CPE will represent ideal capacitor with Y= C and (c) n=-1 a case of inductance with Y=1/L. Generally, in iron/acid interface systems, ideal capacitor behavior is not observed because of frequency dispersion due to roughness or random distribution of current on electrode surface 35,36.

Table 4: Impedance parameters for mild steel in 1 M HCl and 0.5 M H2SO4 in absence and presence of different concentrations of inhibitor at RT. Acid solution

Concentration -1

1 M HCl

0.5 M H2SO4

Rs

Rt 2

n 2

(mg L )

(Ω cm )

(Ω cm )

Blank

1.03

32

300

0.82

400

Y0

Cdl -6

-1

-2

µRt -2

(10 Ω cm )

(µF cm )

%

0.802

147.00

41.70

-

122

0.816

102.40

37.80

76

1.92

225

0.825

80.45

34.12

87

500

0.65

287

0.832

64.30

28.60

90

Blank

1.16

27

0.820

164.80

49.90

-

300

1.23

79

0.831

126.00

39.30

66

400

1.30

109

0.839

89.00

36.80

82

500

1.25

179

0.840

81.20

35.96

85

The EIS parameters like Rs, Rt, Cdl, Y0, and n were calculated by ZSimDemo (version 3.22 d) software. The value of Double layer capacitance (Cdl) can also be calculated by the formula

37

as

given below: (8)

It was observed from Table 4 that in 1 M HCl, charge transfer resistance was increased from 32 ohm cm2 (blank) to 287 ohm cm2 (500 mg L-1) whereas Cdl value decreased from 41.70 µF cm-2 to 28.60 µF


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-2

cm for the same inhibitor concentration. Similar trend was noticed in case of 0.5 M H2SO4 i.e. Rt values were noticed to increase with decrease in Cdl values. The reason for increased Rt can be attributed to decreased heterogeneity of the surface (reflected in n values), which was observed probably due to molecular adsorption of the inhibitor at most active sites

38

. Decrease in double layer

capacitance reflects effective reduction in corrosion rate which can be further explained with the help of following equation

39

: (9)

where, ε is the local dielectric constant of the protective layer, εo is the permittivity of the free space, A is exposure area and d is the thickness of layer formed at steel-inhibitor interface. From equation 9 it is evident that decrease in double layer capacitance correspond to decrease in local dielectric constant and/or increase in thickness of layer formed at metal-acid interface. Probably, adsorbed inhibitor molecules gathered at metal-acid interface forming a protective layer which was further increased (thickness) with increasing inhibitor concentration. However, no appreciable effect was observed after a certain thickness (concentration) due to saturation of layer.

3.5 Surface Morphology Analysis Figure 6 shows surface images of the mild steel samples, captured in both HCl and H2SO4 solutions, without and with maximum concentration of inhibitor. Figure 6a illustrates surface morphology of mild steel before immersion in acid solutions. Some scratches can be seen on the surface, which was the result of uneven grinding of the surface by emery paper during preparation of test specimen. Due to aggressive attack of acid solutions mild steel corroded at higher rate, which made the surface more rough and unpleasant with respect to mild steel specimen. It is clear from Figure 6 b and 6 d that surface damage of mild steel was more destructive in H2SO4 than damage in HCl. In case of inhibited samples, drastic change in morphology of mild steel surface was observed (Figure 6 c and 6 e) which correspond to retarded corrosion rate in both acidic medium. On the basis of above discussions, it can be concluded that film formed at metal-acid interface efficiently inhibited corrosion of mild steel in 1 M HCl and 0.5 M H2SO4.


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Figure 6. SEM images obtained for surface morphology of (a) mild steel sample before immersion (b) after corrosion in 1 M HCl (c) in presence of inhibitor (500 mg L-1) in 1 M HCl -1

(d) after corrosion in 0.5 M H2SO4 (e) in presence of inhibitor (500 mg L ) in 0.5 M H2SO4

3.6 Corrosion Inhibition Inhibition of mild steel corrosion in 1 M HCl and 0.5 M H2SO4 can be explained on the basis of molecular interaction of inhibitor with mild steel. From characterization of the plant extract, it was revealed that CBRE was rich in various organic compounds having saponins (furostanol saponins

27

,

FTIR) as a major constituent among them. Saponins are the class of chemical compounds, secondary metabolites, which contain fused benzene rings, hetero N, O atoms and -OH functional group in their structure (Figure 7). Similarly, tannins and flavanoids are rich with aromatic rings and hetero cycles. These molecules may be adsorbed on mild steel surface via (a) Physical adsorption or (b) Chemical


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adsorption or (c) combination of both type of adsorption. It is a well established fact that chloride ions/sulphate ions, being adsorbed on metal surface, promote adsorption of positively charged ions on metals by making a bridge between cations and positively charged metals8. Probably inhibitor molecules (protonated form of organic moieties) interacted with the mild steel through above stated mechanism and formed a layer at metal acid interface, due to which, corrosion of mild steel was inhibited in HCl and H2SO4 solutions. It is also possible that the layer at acid-steel interface was formed due to chemical bonding between pi electrons or lone pair electrons (from N,O atoms or aromatic rings of organic moieties) and vacant d-orbit electrons of iron. In addition, CBRE might interact with Fe2+ to make metal inhibitor complexes which could be adsorbed on the steel surface through weak chemical bonding. Adsorption of extract constituents on mild steel surface is well supported by the results of techniques used and SEM images although exact method of adsorption can’t be predicted due to unknown molecular weight of extract. Furthermore, higher inhibition efficiency of inhibitor was observed in HCl than the efficiency in H2SO4. The reason behind this fact was smaller degree of hydration of Chloride ions with respect to sulphate ions, which created more favorable conditions for inhibitor molecules to be adsorbed on mild steel surface.

40

Figure 7. Showing chemical structure of Saponins.

4. Conclusions In present work, Chlorophutum borivilianum root extract was tested to explore its corrosion inhibition potential for mild steel in HCl and H2SO4 solutions. Results indicated that CBRE performed very well in both acidic solutions, however, more pronounced effect was acknowledged in HCl medium. Inhibition potential of the extract was found to vary with inhibitor concentration in both acid media (1 M HCl and 0.5 M H2SO4). Polarization study revealed that CBRE acted as a mixed type inhibitor in acid solutions diminishing corrosion activities at both anodic and cathodic sites. Impedance analysis suggested that corrosion inhibition was achieved due to successful adsorption of the chemical constituents of CBRE,


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Page 20 of 25

which caused enhancement in Rt values along with reduction in Cdl values. Interaction of inhibitor with mild steel was also supported by the Uv- visible spectra and SEM images. In the view of above discussions, it can be concluded that CBRE effectively retarded mild steel corrosion in 1 M HCl and 0.5 M H2SO4 solutions; however, decrease in inhibition ability was observed with increase in immersion time, temperature of the experiment and acid concentration. This eco-friendly root extract has potential to be used on wide scale, although, we are working on the purification of CBRE by removing the inactive or less active components in order to further reduce the amount of inhibitor.

Acknowledgement One of the authors, Gopal ji, acknowledges IIT BHU Varanasi for providing financial support during this work. Supporting Information Details of the results of Langmuir isotherm fitting (Figure S1), change in inhibition potential with time (Figure S2), effect of acid concentration (Figure S3) and effect of temperature (Figure S4) are given in supporting information. This material is available free of charge via the internet at http://pubs.acs.org.


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