Evaluation of Green Corrosion Inhibition by ... - ACS Publications

Subscriber access provided by TULANE UNIVERSITY

Article

Evaluation of green corrosion inhibition by alkaloid extracts of Ochrosia oppositifolia and isoreserpiline against mild steel in 1M HCl medium Pandian Bothi Raja, Mehran Fadaeinasab, Ahmad Kaleem Qureshi, Afidah Abdul Rahim, Hasnah Osman, Marc Litaudon, and Khalijah Awang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie401387s • Publication Date (Web): 12 Jul 2013 Downloaded from http://pubs.acs.org on July 18, 2013

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Evaluation of green corrosion inhibition by alkaloid extracts of Ochrosia oppositifolia and isoreserpiline against mild steel in 1M HCl medium

Pandian Bothi Raja,† Mehran Fadaeinasab,‡ Ahmad Kaleem Qureshi,‡ Afidah Abdul Rahim,*,† Hasnah Osman,† Marc Litaudon,‖ and Khalijah Awang‡ †

School of Chemical Sciences, Universiti Sains Malaysia, 11800 USM, P. Pinang, Malaysia Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia ‖ Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles, CNRS, 1, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France ‡

*

corresponding author, Tel: Fax: +60 46574854, Email: [email protected] (Afidah Abdul Rahim)

1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Alkaloid extracts of leaves (OOL) and bark (OOB) of Ochrosia oppositifolia as well as Isoreserpiline (ISR), the major alkaloid isolated from OOL and OOB were investigated as potential corrosion inhibitors for mild steel (MS) in 1M HCl medium. The inhibition properties of these phytoconstituents were studied using electrochemical techniques (potentiodynamic polarization measurements and electrochemical impedance - EIS) and scanning electron microscopy (SEM). Results indicated that these green inhibitors reduced the corrosion rate effectively. Polarization studies showed that these inhibitors decreased the corrosion current densities by mixed mode mechanism. EIS data were analyzed by equivalent circuit model for the electrode /electrolyte interface. SEM observations confirmed the existence of the adsorbed protective film of green inhibitors while the adsorption was found to follow Langmuir adsorption isotherm. FTIR and molecular modeling studies were also employed which revealed that the presence of ISR could be responsible for OOL and OOB’s corrosion inhibition potential.

1. Introduction The study of the corrosion behavior of iron, iron alloys and steel in corrosive media has continued to attract considerable attention because of the many important applications of the metal.1-3 The corrosion of mild steel (MS) is of such interest because it is widely used as constructional materials in many industries and this is due to its excellent mechanical properties and low cost. Acids, in particular hydrochloric acid (HCl) solutions are widely used for pickling, cleaning, descaling, etc. of iron and steel.4 Corrosion inhibitors are generally used in these processes to control the metal dissolution. Most of the well known acid corrosion inhibitors are organic compounds containing nitrogen, 2


Page 2 of 38

Page 3 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sulphur and oxygen atoms.5-9 There has been a growing interest in the use of organic compounds as inhibitors for the aqueous corrosion of metals. High corrosion inhibition potential was exhibited by many organic molecules; however they pollute the environment during their synthesis and applications.10 Due to the currently imposed environmental requirements for ecofriendly corrosion inhibitors, there is a growing interest in the use of natural products such as leaves or seeds extracts. The term “eco-friendly corrosion inhibitor” or “green inhibitor” refers to the substances that are biocompatible such as plant extracts since they are of biological origin.11 Thus, the natural products are being studied by several authors, for their corrosion inhibition potential as they are more environmentally friendly, showing good inhibition efficiency with low risk of environmental pollution.12-16 From our lab, various plant sources like green tea,17 mangrove tannins,18 Xylopia ferruginea19 and Kopsia singapurensis20 were successfully reported as effective corrosion inhibitors for metals in different corrosive media. Although, many authors reported on natural corrosion inhibitors; identification of active ingredients responsible for corrosion inhibition potential and mechanistic approach were rarely discussed. The main objective of this work is to study the inhibitory effect of crude alkaloid extract from the leaves (OOL) and bark (OOB) of Ochrosia oppositifolia on the corrosion inhibition of MS in 1M HCl solution. Further, the pure alkaloid Isoreserpiline (ISR) was isolated from the leaves (OOL) and bark (OOB) of Ochrosia oppositifolia and examined for its corrosion inhibition potential. The corrosion behavior of MS in 1M HCl with and without inhibitor is studied using potentiodynamic polarization, EIS, SEM, FTIR and molecular modeling techniques.

3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 38

Ochrosia oppositifolia belonging to Apocynaceae family is an alkaloid-rich plant. Plants of this genus find wide use in the traditional system of medicine.21 A literature search on the alkaloids of Ochrosia oppositifolia revealed the presence of isoreserpiline, reserpiline, ochroposinine, epirauvanine, bleekerine, 10-hydroxyapparicine, 10-methoxyapparacine, 10methoxydihydrocorynantheol

ochrolofuanine,

reserpinine,

isoreserpinine

and

9-

methoxyellipticine.22 2. Experimental 2. 1. Material The plant materials (bark and leaves) were collected from Pangkor Islands in 2007. The sample was identified by L.E.Teo, University Malaya and deposited at the herbarium unit (specimen no. KL 5349). 2. 2. Extraction and Isolation of Inhibitor The dried stem bark (1 kg) and leaves (1.9 kg) of O. oppositifolia were first defatted separately in hexane (10 L) for 48 hours, and the extracts were dried using the rotary evaporator, then wetted with 10 % ammonia solution and left for 2 hours. Then each of them was reextracted successively with dichloromethane (12 L). After removal of the solvents, the dichloromethane crude extract of bark (12 g) and leaves (18 g) were collected. The presence of alkaloids were tested by using Thin layer chromatography (TLC) technique and Dragendoff’s staining reagent giving orange spot on Aluminum TLC plate and by Mayer reagent giving cloudy precipitates.23

4


Page 5 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2. 3. ISR from OOB The dichloromethane crude extract of bark (4 g) was subjected to a silica gel 60 (40-63 μm) column (CH2Cl2/MeOH, 100:0 → 0:100), 24 fractions were collected. Fraction 12 (CH2Cl2/ MeOH, 90:10) was subjected to a preparative silica gel TLC (CH2Cl2 / MeOH, 88:12) to ISR compound (120 mg, 0.012 %) which showed an orange spot on TLC upon spraying with Dragendorff reagent. 2. 4. ISR from OOL The dichloromethane crude extract of leaves (18 g) was fractionated via silica gel column chromatography (CC) by eluting with dichloromethane - methanol (CH2Cl2/MeOH, 100:0 → 0:100) to afford 8 fractions. The 3rd fraction (CH2Cl2/MeOH, 90:10, 2.2 g) was chromatographed over silica gel 60 (40-63 μm), eluted with dichloromethane- methanol (100:0 → 80:20) to give four sub-fractions while the sub-fraction 3 (CH2Cl2: MeOH, 89:11, 0.43g) was further purified by a preparative thin layer chromatography (PTLC), (CH 2Cl2/MeOH, 86:14) to give isoreserpiline 1 (42 mg, 0.0022%) which showed an orange spot on TLC upon spraying with Dragendorff reagent. 2. 5. Characterization of Isoreserpiline Brownish amorphous solid; [ 299 (11500) nm; IR (CHCl3) NaCl cell

24 D

−36 (c 0.05, CHCl3); UV (MeOH) max

max

3361, 2928, 1702, 655 cm-1; ESIMS gave a pseudo

molecular ion peak at m/z 413 (M+ H)+); 1H-NMR (400MHz, CDCl3) δ: 7.73 (1H, s, NH-1), 7.54 (1H, s, H-17), 6.88 (1H, s, H-9), 6.79 (1H, s, H-12), 4.40 (1H, m, H-19), 3.90 (3H, s, OMe-10), 3.87 (3H, s, OMe-11), 3.46 (3H, s, OMe-22), 3.30 (1H, br d, J= 11.48 Hz, H-3), 3.05 (1H, m, H5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

21), 2.90 (1H, m, H-5), 2.81 (1H, m, H-6), 2.80 (1H, m, H-15), 2.72 (1H, m, H-6), 3.10 (1H, dd, J=1.6 Hz, 12.2 Hz, H-21), 2.60 (1H, m, H-14), 2.88 (1H, m, H-5), 1.50 (1H, m, H-5), 1.55 (1H, q, J=9.76 Hz, H-14), 1.41 (1H, d, J=6.8 Hz, H-18). 13C-NMR (100MHz, CDCl3) δ: 168.22 (C22), 155.81 (C-17), 146.53 (C-11), 144.90 (C-10), 133.31 (C-2), 130.23 (C-13), 120.0 (C-8), 109.60 (C-16), 107.91 (C-7), 100.42 (C-9), 94.91 (C-12), 72.62 (C-19), 60.0 (C-3), 56.51 (C-21), 56.50 (OMe-10), 56.42 (OMe-11), 53.73 (C-5), 50.81 (OMe-22), 38.50 (C-20), 34.41 (C-14), 31.40 (C-15), 21.91 (C-6), 18.62 (C-18). 2. 6. Specimen preparation Mild steel (MS) specimens (C=0.205 wt%, Si=0.06 wt%, Mn=0.55 wt%, S=0.047 wt%, P=0.039 wt% and Fe remaining) with an exposed area of 3.14 cm2 were used for the electrochemical study and specimens of size 2.3  0.2  2.3 cm were used for the SEM analysis. The surface preparation of the mechanically abraded specimens was carried out using different grades (350, 500, 800, 1000, 1200 and 1500) of emery papers. 2. 7. Electrochemical studies Electrochemical studies were carried out using Gamry Instruments reference 600 (Potentiostat / galvanostat / ZRA). A conventional three electrode system was used for this purpose. MS specimen was used as a working electrode. Pt electrode and saturated calomel electrode (SCE) served as auxiliary and reference electrodes, respectively. All polarization and impedance curves were recorded at room temperature (30 ± 2º C) at pH range of 0.7 – 0.8. The working electrode was immersed in the test solution during 30 minutes until a steady state open circuit potential was attained. GAMRY Echem Analyst software package 5.50 was used for fitting impedance data in an equivalent circuit as well as for extrapolating Tafel slopes. 6


Page 6 of 38

Page 7 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AC impedance measurements were carried out at potential amplitude of 10 mV, peak-topeak (AC signal) in open-circuit, with 10 points per decade and the frequency ranging from 10000 Hz to 0.1 Hz. The impedance diagrams obtained are given as Nyquist plots. IE is calculated from the charge transfer resistance (Rct) values by using equation 1.

 Rct  % IE  1  (o)   100  Rct (i)  

(1)

where, Rct(o) is the charge transfer resistance of MS without inhibitor. Rct(i) is the charge transfer resistance of MS with inhibitor. Potentiodynamic polarization measurements were carried out by scanning the electrode potential from -800 mV to -200 mV (vs SCE) with a scan rate of 5 mVs-1. The linear Tafel segments of the anodic and cathodic curves were extrapolated to corrosion potential to obtain the corrosion current densities (icorr). Equation 2 shows the calculation of IE from the icorr values.

 icorr(i)   100 % IE  1   icorr  (o)  

(2)

where, icorr(o) is the corrosion current density of MS without inhibitor. icorr(i) is the corrosion current density of MS with inhibitor. 2. 8. Scanning Electron Microscope (SEM) analysis SEM LEO SUPRA 50VP - Scanning Electron Microscope was used at an accelerating voltage of 15 keV for monitoring the surface morphological changes. For this study, MS plates were carefully polished to a 2500 grit surface finish using silicon carbide paper and immersed in 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1M HCl medium in the presence and absence of 25 mg L-1 of inhibitors for 6 hours. Then the specimens were cleaned with distilled water, dried in cold air blaster and used for the analysis. 2. 9. FTIR analysis The isolated inhibitor (Isoreserpiline) as well as the protective film formed over the MS surface by the inhibitor molecule was analyzed separately by FTIR spectroscopy using the KBr pellet method.

For this purpose, MS specimens were immersed in the corrosive medium

consisting of 25 mg L-1 of Isoreserpiline for 120 hours which resulted in the formation of a fine protective film over the MS surface. Further, the film was carefully scratched out from the MS surface and analyzed. The study was carried out by using the Perkin Elmer System 2000 FTIR instrument. 2. 10. Molecular modeling Quantum chemical calculations have already proven to be very useful in determining the molecular structure as well as elucidating the electronic structure and reactivity.24 Thus, it becomes a common practice to carry out quantum chemical calculations in corrosion inhibition studies. Highest occupied molecular orbital energy (EHOMO) and lowest unoccupied molecular orbital energy (ELUMO) are very popular quantum chemical parameters. These orbital, also called the frontier orbital which determine the way the molecule interacts with other species. The structure optimization of ISR as well as the molecular orbital calculations viz., HOMO and LUMO was performed using Gaussian 03 program.25 The global minimum energy structure of inhibitor was obtained by using Hartree – Fock method with default spin and 3 - 21G basis set. The same level of theory was used to compute the molecular orbital of different

8


Page 8 of 38

Page 9 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


oxidation states of Fe (0, +2 and +3). The molecular orbitals were visualized using Gassview (4. 2) program. 3. Results and Discussion 3. 1. Electrochemical impedance study Electrochemical impedance study provides details about the kinetics of the electrode processes and simultaneously about the surface properties of the investigated systems. From the shape of the impedance diagram mechanistic information can be derived.26 The impedance results obtained in the presence and absence of green inhibitors viz., OOB, OOL and ISR were given as Nyquist plots in Fig. 2 (a, b and c) respectively. It is evident from Fig. 2, that MS exhibited typical impedance behavior in 1M HCl medium for all the green inhibitors screened and displayed marked changes in impedance response for each concentration studied. It is worth noting that the changes in concentration of green inhibitors did not alter the profile of the impedance behavior, suggesting similar mechanism for the corrosion inhibition of mild steel by green inhibitors. From Fig. 2, it is clear that the Nyquist plots do not yield perfect semicircles as expected from the theory of EIS but showed depressed semicircles. The observed deviations from perfect semicircles are often referred to the frequency dispersion of interfacial impedance. This anomalous phenomenon is interpreted by the in-homogeneity of the electrode surface arising from surface roughness or interfacial phenomena.27 – 29 The existence of a single semicircle (nearly) depicts the presence of single charge - transfer process during dissolution which is unaffected by the presence of green inhibitors. An equivalent circuit (Fig. 3) was used to consider all the processes involved in the electrical response of the system, which is a parallel combination of the charge-transfer resistance (Rct) and the constant phase element (CPE), both in series with the solution resistance 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 38

(Rs). The CPE element is used to explain the depression of the capacitance semi-circle, which corresponds to surface heterogeneity resulting from surface roughness, impurities, dislocations, grain boundaries, adsorption of inhibitors, formation of porous layers30,

31

etc. The CPE

impedance (ZCPE) is obtained by,

ZCPE 

1 1  Q ( j ) n

(3)

where, Q is the CPE coefficient, n is the CPE exponent (phase shift), ω is the angular frequency (ω = 2πf , where f is the AC frequency), and j here is the imaginary unit. When the value of n is 1, the CPE behaves like an ideal double - layer capacitance (Cdl).32, 33 The various impedance parameters namely Rct, CPE, n and goodness of fit (chi square) were obtained by fitting the equivalent circuit (Fig. 3) and are listed in Table 2. It can be seen from Table 2 that, Rct values in the presence of the inhibitor were always greater than their values in the absence of the inhibitor which indicates that these compounds were acting as adsorption inhibitors. Further, Rct values reached maximum at concentration of 25 mg L-1 for all the green inhibitors (4 to 70, 34 and 59 Ω cm2 for OOB, OOL and ISR respectively), which indicate the reduction of MS corrosion rate. The increase of Rct values with increasing inhibitor concentration suggests the formation of a protective layer on the electrode surface. This layer acts as a barrier for mass and charge transfer.34, 35 Addition of green inhibitors reduced the CPE values (625 to 76, 153 and 143 Fcm-2 for OOB, OOL and ISR, respectively) as well, which was caused by a reduction in local dielectric constant and /or by an increase in the thickness of the electrical double layer. This fact suggests that the inhibitor molecules acted by adsorption at the metal/solution interface.36 The “n” values seem to be associated with the non-uniform distribution of current as a result of roughness and 10


Page 11 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


possible oxide surface defects. When n = 1, CPE is an ideal capacitor while a true capacitive behavior is rarely obtained. The “n” values close to 1 (Table 1) represent the deviation from the ideal capacitor. A constant phase element (CPE) is utilized for data fitting instead of an ideal capacitor, since the “n” values obtained were in the range of 0. 9, the value obtained from the data fitting was taken as the capacitance. The quality of fitting to the equivalent circuit was judged by chi square value.37 The obtained chi square values (0.000067 to 0.000234) in Table 1 indicate a good fitting to the proposed circuit. The inhibition efficiency calculated from the Rct values obtained in the absence and presence of green inhibitors varied from 86 to 94% over a concentration range of 20 - 25 mg L-1. 3. 2. Potentiodynamic polarization measurements Potentiodynamic polarization curves were recorded in order to study the corrosion inhibition effect of green inhibitors on the electrochemical behavior of MS in 1M HCl. Results were displayed as Tafel plots in Fig. 4 a, b and c for OOB, OOL and ISR, respectively. The Tafel parameters which include corrosion potential (Ecorr), cathodic and anodic Tafel slops (bc and ba), corrosion current density (Icorr) and the protection efficiency (% IE) were derived from polarization curves and are summarized in Table 2. It should be noted from Fig. 4 that cathodic polarization curves gave rise to parallel Tafel lines indicating that the hydrogen evolution reaction is activation controlled.38 Parallel Tafel lines obtained suggest that addition of green inhibitors does not modify the mechanism of the hydrogen reduction (i.e.) proton discharge reaction. The values of Ecorr, bc and ba do not change appreciably with the addition of inhibitors, indicating that the inhibitor is not interfering the anodic dissolution or cathodic hydrogen evolution reactions independently but acts as mixedtype of inhibitor.39 Further, addition of green inhibitors reduced the corrosion current density 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

from 2110 to 228, 193 and 195 Acm-2 at maximum concentration of 25 mg L-1 signifying that corrosion rate has reduced and resulted in enhanced IE of 89, 90 and 91 % for OOB, OOL and ISR, respectively. The IE values are in good agreement with EIS studies while showed slight variation in the lowest concentration (5 mg L-1) for all the green inhibitors. This may due to shorter experimental period for the impedance study in comparison with the polarisation measurement that may reduce the adsorption of inhibitor molecules at lower concentrations. Consequently, less inhibitor molecules were adsorbed onto the MS surface and were not sufficient to form a time-resistant layer,40 leading to low Rct values which caused a decrease in IE values at low concentrations of inhibitors. 3. 3. SEM analysis The formation of a surface protective film by the green inhibitors on the MS surface was confirmed by SEM observations. Fig. 5 shows an array of SEM images recorded for MS that was exposed for 6 hrs in 1 M HCl solutions with and without green inhibitors of maximum concentration (25 mg L-1). Parallel features on the polished MS surface before exposure to the corrosive solution, which are associated with polishing scratches (Fig. 5a) were observed. The morphology of MS surface in Fig. 5b reveals that in the absence of inhibitors, the surface is highly damaged with areas of typical uniform corrosion on the MS surface.41 However, in presence of the green inhibitors (micrographs c – e), the rate of corrosion is suppressed; due to the formation of an adsorbed film of the inhibitor on the MS surface. The protective nature of this film is reflected in the inhibition efficiency measurements obtained from electrochemical methods. Thus, SEM examinations of MS surface support the results of the electrochemical methods that OOB, OOL and ISR reduced the corrosion rate by adsorption process.

12


Page 12 of 38

Page 13 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3. 4. Adsorption isotherm Corrosion inhibition of MS in acidic solutions by the inhibitors can be explained on the basis of molecular adsorption. Thus, the application of adsorption isotherm is very useful to study the mechanism of corrosion inhibition. The value of surface coverage (θ) can be obtained from the following equation for both electrochemical techniques.

 Rct (i) - Rct (o)     R ct (i)  

(4)

 icorr(o) - icorr(i)     i corr (o)  

(5)

 

 

Data were tested graphically by fitting to various isotherms and found to fit well with Langmuir adsorption isotherm accordingly with the following equation.

C





1 C K ads

(6)

where, C is the concentration of inhibitor, and K the equilibrium constant. The values of C / θ were plotted against C for different concentrations of green inhibitors. The straight line obtained in Fig. 6 (a, b and c for OOB, OOL and ISR, respectively) and the linear regression coefficients (R2) that are almost equal to 1 indicates that the adsorption of green inhibitors on the MS surface follows Langmuir adsorption isotherm.42 3. 5. FT IR analysis Pure alkaloid (ISR), and its scratched protective film formed over MS was screened by FT IR spectroscopy technique and the results obtained are depicted in Fig. 7 a and b, respectively. Characteristic IR bands were found for ISR43 in Fig. 7 a; at 3395 cm-1 (N-H), 1702 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 38

cm-1 (conjugated carbonyl ester group stretching), 1629 cm-1 (carbonyl group), C = C 1206 cm-1 (C – N stretching) and 1083 cm-1 (C=C – phenyl ring torsion). Almost identical IR spectra of ISR and ISR protective film (Fig. 7 a and b), clearly evidenced that ISR is simply adsorbed over MS surface and protect it from aggressive acid attack. However, existence and broadening of NH band at 3395 cm-1 as well as phenyl ring band at 1083 cm-1 were observed in Fig. 7b, an indication that ISR may be involved in coordination with MS surface through the lone pair electrons of N – H group (pyrrole ring) and π – electron clouds of phenyl ring. Thus, FT IR studies have paved the way to identify the active sites of coordination in a multi ring molecule. 3. 6. Molecular modeling studies Quantum calculations are used to emphasize experimental results of electrochemical techniques. The geometrical configuration with optimized minimum energy of ISR is shown in Fig. 8. HOMO and LUMO of ISR and Iron (Fe0, Fe+2 and Fe+3) are depicted in Fig. 9 and Fig. 10, respectively. The HOMO is the orbital that could act as an electron donor, since it is the outermost (highest energy) orbital containing electrons and higher EHOMO values enhances electron donation. The LUMO is the orbital that could act as the electron acceptor, since it is the innermost (lowest energy) orbital that has room to accept electrons and lower ELUMO values enhances electron acceptance. According to the frontier molecular orbital theory, the formation of a transition state is due to an interaction between the frontier orbitals (HOMO and LUMO) of reactants.44 From Fig. 9 (a and b), it is clear that electron density (HOMO and LUMO) of ISR is located within the vicinity of the aromatic indole moiety. This observation indicates that N-H of indole ring and π – electron clouds of the phenyl ring have high electron donating ability to MS surface; which is in good agreement with the FT IR results that shows the coordination of ISR 14


Page 15 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with the MS surface through indole moiety. An ideal corrosion inhibitor has a greater tendency to donate electrons to the metal surface through HOMO as well as receive electrons from metal surfaces (during Fe to Fe 2+, 3+ conversion) through LUMO or bind strongly to the metal surface45 which suggests that ISR may donate and accept electrons through the indole moiety. On the other hand, Table 3 represents calculated energy levels viz., EHOMO, ELUMO and ∆E (EHOMO – ELUMO) for ISR and Iron. EHOMO value for ISR was found to be -0.3083 eV, which was higher (less negative) than that of Fe2+ and Fe3+ (-1.2178 and -2.0712 eV); while ELUMO values were found to be in the order ISR > Fe > Fe2+ > Fe3+. This result indicates that ISR may donate electrons to MS surface but may not receive the donated electron from MS surface in return. Further, higher energy gap (∆E) values for ISR (-0.3033 eV) than Fe2+ (-0.6815eV) and Fe3+ (1.1285 eV) indicated that ISR preferred electron donation to Fe2+ and Fe3+ rather than inter electron transfer within the molecule. 3. 7. Mechanism of inhibition The corrosion inhibition of MS in 1M HCl by OOB, OOL and ISR can be explained on the basis of an adsorption process. Polarization results evidenced that all inhibitors reduced corrosion by controlling both the anodic and cathodic reactions. Since, these inhibitors are basic in nature, in acidic solutions all of them may exist predominantly as protonated species. These protonated species may adsorb on the cathodic sites of the MS and decrease the evolution of hydrogen. Adsorption on anodic sites occurs through the π - electrons of aromatic rings and the lone pair electrons of nitrogen, which decreases anodic dissolution of mild steel.46 The inhibitory properties of all inhibitors are very similar, thus implying that the leaves and bark extracts may contain similar constituents and ISR is the major contributor to the activity. Possible mode of interaction between ISR and MS surface is given in Fig. 11. 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 38

4. Conclusions Corrosion inhibition efficiency of Ochrosia oppositifolia bark, leaves extract and isoreserpiline on MS in 1M HCl medium was determined by polarization, impedance, SEM and molecular modeling analysis. Results evidenced that all the samples (OOB, OOL and ISR) respectively displayed good inhibitive properties and showed excellent performance (more than 85 % at 20 – 25 mg L-1) as corrosion inhibitors. Results of potentiodynamic polarization curves indicated that all inhibitors act through mixed - type mechanism which affected both the anodic and cathodic reactions by simple blocking of the active metal sites, i.e. inhibitors did not change the corrosion mechanism of the MS in 1M HCl medium. They inhibit corrosion through adsorption process and were found to follow Langmuir adsorption isotherm. The equivalent circuit was selected based on the properties of the EIS - Nyquist diagrams and fitted the experimental data well. The changes in the impedance parameters confirmed the strong adsorption of the inhibitors on the steel surface, which prevented anodic dissolution of the metal by blocking active metal surface sites. The IE of both electrochemical techniques is in good agreement with slight deviations in numerical values. The SEM results showed the formation of a protective and dense layer on the steel surface in the presence of the inhibitors. FTIR analysis suggested that the indole skeleton of ISR is involved in the coordination with the MS surface. Molecular modeling studies supported well the FTIR findings and evidenced the possibility of electron transfer from inhibitor to metal surface. Acknowledgment The authors gratefully acknowledged the financial support provided b y University

Sains

Malaysia

(304/PKIMIA/6311087

and

16


FRGS-203/PKMIA/671030),

Page 17 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


University of Malaya Research Grants (UMRG RG011/09BIO and UMRP 001- 2012A) and Centre National de la Recherche Scientifique (CNRS- Grant 57-02-03-1007). Supporting Information NMR, Mass and UV spectra of Isoreserpiline (ISR). This material is available free of charge via the Internet at http://pubs.acs.org.

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 38

Figure captions: Fig. 1. Structure of Isoreserpiline Fig. 2. Nyquist plots for MS in 1M HCl medium in the presence and absence of a) OOB b) OOL and c) ISR Fig. 3. The electrical equivalent circuit for AC impedance measurement Fig. 4. Tafel plots for MS in 1M HCl medium in the presence and absence of a) OOB b) OOL and c) ISR Fig. 5. Langmuir plot of a) OOB b) OOL and c) ISR on MS corrosion in 1M HCl medium Fig. 6. SEM images of a) abraded MS, b) MS immersed in 1M HCl medium; MS immersed in 1M HCl medium containing 25 mg L-1 of c) OOB d) OOL e) ISR Fig. 7. IR absorption spectra of a) Isoreserpiline and b) protective film formed by Isoreserpiline Fig. 8. Optimized structure of Isoreserpiline Fig. 9. Electron density distribution on Isoreserpiline molecule a) HOMO b) LUMO Fig. 10. Electron density distribution on Iron a) Fe (HOMO and LUMO together), b) Fe2+ (HOMO), c) Fe2+ (LUMO), d) Fe3+ (HOMO), e) Fe3+ (LUMO) Fig. 11. Possible mode of interaction of Isoreserpiline on MS surface in 1M HCl medium

18


Page 19 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References (1)

Bentiss, F.; Lagrenee, M.; Traisnel, M. 2, 5-bis(n-Pyridyl)-1,3,4-Oxadiazoles as corrosion inhibitors for mild steel in acidic media: Corros. 2000, 56, 733 - 742.

(2)

Atia, A.; Saleh, M. M. Inhibition of acid corrosion of steel using cetylpyridinium chloride: J. Appl. Electrochem. 2003, 33, 171 - 177.

(3)

Saeed, M. T.; Ali, S. A.; Rahman, S. U. The cyclic hydroxylamines: a new class of corrosion inhibitors of carbon steel in acidic medium: Anti - Corros. Methods and Materials. 2003, 50, 201 - 207.

(4)

Kandemirli, F.; Sagdinc, S. Theoretical study of corrosion inhibition of amides and thiosemicarbazones: Corros. Sci. 2007, 49, 2118 – 2130.

(5)

Zarrouk, A.; Hammouti, B.; Dafali, A.; Bentiss, F. Inhibitive properties and adsorption of purpald as a corrosion inhibitor for copper in nitric acid medium: Ind. Eng. Chem. Res. 2013, 52, 2560 - 2568.

(6)

Loganathan, S.; Kumar, A.; Gopiraman, M.; Kumar, M. S.; Sreekanth, A. 2Acetylpyridine-N(4)-morpholine thiosemicarbazone (HAcpMTSc) as a corrosion inhibitor on mild steel in HCl: Ind. Eng. Chem. Res. 2011, 50, 7824 - 7832.

(7)

Kamal, C.; Sethuraman, M. G. Caulerpin—A bis-Indole alkaloid as a green inhibitor for the corrosion of mild steel in 1 M HCl solution from the marine alga Caulerpa racemosa: Ind. Eng. Chem. Res. 2012, 51, 10399 - 10407.

(8)

Yadav, D. K.; Quraishi, M. A. Application of some condensed uracils as corrosion inhibitors for mild steel: gravimetric, electrochemical, surface morphological, UV– visible, and theoretical investigations: Ind. Eng. Chem. Res. 2012, 51, 14966 - 14979.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(9)

Page 20 of 38

Chidiebere, M. A.; Ogukwe, C. E.; Oguzie, K. L.; Eneh, C. N.; Oguzie, E. E. Corrosion inhibition and adsorption behavior of Punica granatum extract on mild steel in acidic environments: Experimental and theoretical studies: Ind. Eng. Chem. Res. 2012, 51, 668 – 677.

(10)

Raja, P. B.; Sethuraman, M. G. Natural Products as corrosion inhibitor for metals in corrosive media – a review: Mater. Lett. 2008, 62, 113–16.

(11)

Kesavan, D.; Gopiraman, M.; Sulochana, N. Green inhibitors for corrosion of metals: A review: Che. Sci, Rev. Lett. 2012, 1, 1 – 8.

(12)

Orubite, K. O.; Oforka, N. C. Inhibition of the corrosion of mild steel in hydrochloric acid solutions by the extracts of leaves of Nypa fruticans Wurmb: Mater. Lett. 2004, 58, 1768 - 1772.

(13)

Fares, M. M.; Maatya, A. K.; Al – Mustafa, J. A. Corrosion inhibition of iota carrageenan natural polymer on aluminum in presence of zwitter ion mediator in HCl media: Corros. Sci. 2012, 65, 223 - 230.

(14)

Bothi Raja, P; Sethuraman, M. G. Inhibitive effect of black pepper extract on the sulphuric acid corrosion of mild steel: Mater. Lett. 2008, 62, 2977 - 2979.

(15)

Sethuraman, M. G.; Bothi Raja, P. Corrosion inhibition of mild steel by Datura metel (leaves) in acidic medium: Pigm. Resin. Tech. 2005, 34, 327 - 331.

(16)

Bothi Raja, P.; Sethuraman, M. G. Strychnos nux-vomica an eco – friendly corrosion inhibitor for mild steel in 1M Sulphuric acid medium: Mater. Corros. 2009, 60, 22 28.

(17)

Nofrizal, S.; Afidah Rahim, A.; Saad, B.; Bothi Raja, P.; Shah, A. M.; Yahya, S. Elucidation of the corrosion inhibition of mild steel in 1M HCl by catechin monomers 20


Page 21 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


from commercial green tea extracts: Metall. Mater. Trans. A. 2011, 43 A, 1382 1393. (18)

Rahim, A. A.; Rocca, E.; Steinmetz, E, J.; Kassim, M. J. Inhibitive action of mangrove tannins and phosphoric acid on pre-rusted steel via electrochemical methods: Corros. Sci. 2008, 50, 1546 – 1550.

(19)

Raja, P. B.; Rahim, A. A.; Osman, H.; Awang, K. Investigation of the inhibitive effect of Xylofia ferrungina extract on the corrosion of mild steel in 1 M HCl medium: Int. J. Min. Met. Mater. 2011, 18, 413 – 418.

(20)

Raja, P. B.; Rahim, A. A.; Osman, H.; Awang, K. Inhibitory effect of Kopsia singapurensis extract on the corrosion behavior of mild steel in acid media: Acta. Phys. Chim. Sin. 2010, 26, 2171 – 2176.

(21)

Jordan, W.; Scheuer, P. Hawaiian plant studies—XIV alkaloids of Ochrosia sandwicensis A.Gray: Tetrahedron, 1965, 21, 3731-3740.

(22)

Locou. N. P.; Koch, M.; Plat, M. Alcaloïdes des écorces d'Ochrosia oppositifolia: Phytochem. 1972, 11, 2109 - 2111.

(23)

Ching, Y. L.; Knochel, P. Preparation of Polyfunctional Aryl Azides from Aryl Triazenes. A New Synthesis of Ellipticine, 9-Methoxyellipticine, Isoellipticine, and 7-Carbethoxyisoellipticine: J. Org. Chem. 2007, 72, 7106 - 7115.

(24)

Kraka, E.; Cremer, D. Diastereoselective Intermolecular Cyclopropanation of Simple Alkenes by Fischer Alkenyl and Heteroaryl Carbene Complexes of Chromium: Scope and Limitations: J. Am. Chem. Soc. 2000, 122, 8245–8264.

(25)

Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; et al. Gaussian 03, Revision B. 05, Gaussian Inc., Pittsburgh, PA, 2003. 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(26)

Page 22 of 38

Popova, A.; Sokolova, E.; Raicheva, S.; Christov, M. AC and DC study of the temperature effect on mild steel corrosion in acid media in the presence of benzimidazole derivatives: Corr. Sci. 2003, 45, 33 - 58.

(27)

Aljourani, J. Benzimidazole and its derivatives as corrosion inhibitors for mild steel in 1M HCl solution: Corros. Sci. 2009, 51, 1836 –1843.

(28)

Shih, H.; Mansfeld, H. A fitting procedure for impedance data of systems with very low corrosion rates: Corros. Sci. 1989, 29, 1235 - 1240.

(29)

Martinez, S.; Hukovic, M. M. A nonlinear kinetic model introduced for the corrosion inhibitive properties of some organic inhibitors: J. Appl. Electrochem., 2003, 33, 1137 –1142.

(30)

Growcock, F. B.; Jasinski, J. H. Time resolved impedance spectroscopy of mild steel in concentrated hydrochloric acid: J. Electrochem. Soc. 1989, 136, 2310 - 2314.

(31)

Rammet, U.; Reinhart, G. The influence of surface roughness on the impedance data for iron electrodes in acid solutions: Corros. Sci. 1987, 27, 373 - 382.

(32)

Gohr, H.; Schaller, J.; Schiller, C. A. Impedance studies of the oxide layer on zircaloy after previous oxidation in water vapour at 400°C: Electrochim. Acta. 1993, 38, 1961 - 1964.

(33)

Kliskic, M.; Radosevic, J.; Gudic, S.; Katalinic, V. Aqueous extract of Rosmarinus officinalis L. as inhibitor of Al–Mg alloy corrosion in chloride solution: J. Appl. Electrochem. 2000, 30, 823 - 830.

(34)

Khaled, K. F. Application of electrochemical frequency modulation for monitoring corrosion and corrosion inhibition of iron by some indole derivatives in molar hydrochloric acid: Mater. Chem. Phys. 2008, 112, 290 – 300. 22


Page 23 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(35)

Behpour, M.; Ghoreishi, S. M.; Mohammadi, N.; Soltani, N.; Salavati-Niasari, M. Investigation of some Schiff base compounds containing disulfide bond as HCl corrosion inhibitors for mild steel: Corros. Sci. 2010, 52, 4046 – 4057.

(36)

Sorkhabi, H. A.; Shaabani, B.; Seifzadeh, D. Effect of some pyrimidinic Shciff bases on the corrosion of mild steel in hydrochloric acid solution: Electrochim. Acta. 2005, 50, 3446–3452.

(37)

Lavos - Valereto, I. C., Wolynec, S.; Ramires, I.; Guastaldi, A. C.; Costa, I. Electrochemical impedance spectroscopy characterization of passive film formed on implant Ti–6Al–7Nb alloy in Hank's solution: J. Mater. Sci. Mater. Med. 2004, 15, 55 – 59.

(38)

Bouklah, M.; Ouassini, A.; Hammouti, B.; El Idrissi, A. Corrosion inhibition of steel in 0.5 M H2SO4 by [(2-pyridin-4-ylethyl)thio]acetic acid: Appl. Surf. Sci. 2005, 250, 50 – 56.

(39)

Sathiyanarayanan, S.; Marikkannu, C.; Palaniswamy, N. Corrosion inhibition effect of tetramines for mild steel in 1M HCl: Appl. Surf. Sci. 2005, 241, 477 – 484.

(40)

Moretti, G.; Guidi, F.; Grion, G. Tryptamine as a green iron corrosion inhibitor in 0.5 M deaerated sulphuric acid: Corros. Sci. 2004, 46, 387 – 403.

(41)

Li, Y.; Zhao, P.; Liang, Q.; Hou, B. Berberine as a natural source inhibitor for mild steel in 1 M H2SO4, Appl. Surf. Sci. 2005, 252, 1245 – 1253.

(42)

Ismail, K. M. Electrochemical and quantum chemical study of purines as corrosion inhibitors for mild steel in 1 M HCl solution: Electrochim. Acta. 2008, 53, 5953 – 60.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(43)

Page 24 of 38

Cancelieri, N. M.; Vieira, I. J. C.; Schripsema, J.; Mathias, L.; Braz-Filho, R. Darcyribeirine - a novel pentacyclic indole alkaloid from Rauvolfia grandiflora Mart: Tetrahedron Lett. 2002, 43, 1783-1787.

(44)

Fukui, K. Theory of Orientation and Stereoselection, Springer-Verlag, New York, 1975.

(45)

Solmaz, R.; Kardaş, G.; Culha, M.; Yazici, B.; Erbil, M. Investigation of adsorption and inhibitive effect of 2-mercaptothiazoline on corrosion of mild steel in hydrochloric acid media: Electrochim. Acta. 2008, 53, 5941-5952.

(46)

Quraishi, M. A.; Sardar, R. Hector bases – a new class of heterocyclic corrosion inhibitors for mild steel in acid solutions: J. Appl. Electrochem. 2003, 33, 1163 – 1168.

24


Page 25 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Corrosion inhibition effect of Ochrosia oppositifolia extract on MS in HCl medium (Impedance studies) S. No. Inhibitor

Concentration of Inhibitor

Rct

n

CPE

Ω cm2 Fcm-2

Chi

% IE

Square

(mg L-1) 1.

2.

3.

4.

-

OOB

OOL

ISR

0

4.37

625.6

0.9027 0.000067

-

5

6.68

451.4

0.9081 0.000046

35

10

10.89

273.3

0.9142 0.000131

60

15

13.75

203.3

0.9170 0.000244

68

20

41.61

97.0

0.9148 0.000459

89

25

70.44

76.3

0.9129 0.000402

94

5

10.48

202.6

0.903

0.000067

58

10

28.35

193.1

0.968

0.001512

84

15

28.88

169.2

0.912

0.000280

85

20

31.84

164.7

0.912

0.000317

86

25

34.11

153.6

0.906

0.000273

87

5

6.83

312.7

0.921

0.003939

36

10

8.89

217.0

0.951

0.001007

51

15

12.02

183.4

0.943

0.001820

64

20

37.80

171.5

0.918

0.000264

88

25

59.01

143.8

0.932

0.000234

93

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 38

Table 2. Corrosion inhibition effect of Ochrosia oppositifolia extract on MS in HCl medium (Polarization studies)

Inhibitor

Concentration of Inhibitor

ba

bc

mVdec-1 mVdec-1

( mg L-1) -

OOB

OOL

ISR

Ecorr

i corr

mV

Acm-2

(vs SCE)

(±0.5 x 10-4)

% IE

0

98

113

-466

2110

-

5

78

116

-462

943

55

10

72

119

-458

444

79

15

83

130

-452

345

84

20

78

135

-463

276

87

25

77

143

-462

228

89

5

73

126

-457

687

67

10

101

148

-454

519

75

15

100

145

-458

492

77

20

65

122

-453

246

88

25

65

130

-447

193

90

5

70

104

-470

535

75

10

67

105

-475

390

82

15

67

110

-476

330

84

20

58

116

-461

260

88

25

55

115

-461

195

91

26


Page 27 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Calculated quantum chemical parameters for Isoreserpiline and Iron (Fe, Fe2+ and Fe3+) Compound

∆E (eV)

EHOMO (eV) ELUMO (eV)

(EHOMO – ELUMO) Isoreserpiline

-0.3083

-0.0050

-0.3033

Fe

-0.2464

-0.0412

-0.2052

Fe2+

-1.2178

-0.5363

-0.6815

Fe3+

-2.0712

-0.9427

-1.1285

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

H

N

H3CO

Page 28 of 38

H CH3 O

H3CO

N H

H H H3COOC

Fig. 1. Structure of Isoreserpiline

28


Page 29 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 2. Nyquist plots for MS in 1M HCl medium in the presence and absence of a) OOB b) OOL and c) ISR

Fig. 3. The electrical equivalent circuit for AC impedance measurement

30


Page 30 of 38

Page 31 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4. Tafel plots for MS in 1M HCl medium in the presence and absence of a) OOB b) OOL and c) ISR 31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 5. Langmuir plot of a) OOB b) OOL and c) ISR on MS corrosion in 1M HCl medium

32


Page 32 of 38

Page 33 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 6. SEM images of a) abraded MS, b) MS immersed in 1M HCl medium; MS immersed in 1M HCl medium containing 25 mg L-1 of c) OOB d) OOL e) ISR 33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 7. IR absorption spectra of a) Isoreserpiline and b) protective film formed by Isoreserpiline

34


Page 34 of 38

Page 35 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 8. Optimized structure of Isoreserpiline

35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 9. Electron density distribution on Isoreserpiline molecule a) HOMO b) LUMO

36


Page 36 of 38

Page 37 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 10. Electron density distribution on Iron a) Fe (HOMO and LUMO together), b) Fe2+ (HOMO), c) Fe2+ (LUMO), d) Fe3+ (HOMO), e) Fe3+ (LUMO)

37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 11. Possible mode of interaction of Isoreserpiline on MS surface in 1M HCl medium

38


Page 38 of 38

Evaluation of Green Corrosion Inhibition by ... - ACS Publications

Recommend Documents