Adsorption and corrosion inhibition performance of multi-phytoconstituents from Dioscorea septemloba on carbon steel in acidic media: Characterization, experimental and theoretical studies

Journal Pre-proof Adsorption and corrosion inhibition performance of multi-phytoconstituents from Dioscorea septemloba on carbon steel in acidic media: characterization, experimental and theoretical studies Wilfred Emori (Conceptualization) (Writing - original draft), Run-Hua Zhang (Investigation), Peter C. Okafor (Writing - review and editing) (Validation), Xing-Wen Zheng (Formal analysis), Tao He (Investigation), Kun Wei (Investigation), Xiu-Zhou Lin (Supervision), Chun-Ru Cheng (Conceptualization) (Writing - review and editing)

PII:

S0927-7757(20)30127-8

DOI:

https://doi.org/10.1016/j.colsurfa.2020.124534

Reference:

COLSUA 124534

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

14 November 2019

Revised Date:

6 January 2020

Accepted Date:

30 January 2020

Please cite this article as: Emori W, Zhang R-Hua, Okafor PC, Zheng X-Wen, He T, Wei K, Lin X-Zhou, Cheng C-Ru, Adsorption and corrosion inhibition performance of multi-phytoconstituents from Dioscorea septemloba on carbon steel in acidic media: characterization, experimental and theoretical studies, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2020), doi: https://doi.org/10.1016/j.colsurfa.2020.124534

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Adsorption and corrosion inhibition performance of multi-phytoconstituents from Dioscorea septemloba on carbon steel in acidic media: characterization, experimental and theoretical studies

Wilfred Emori

a,b

, Run-Hua Zhang

a,b

, Peter C. Okafor

c,d,

*, Xing-Wen Zheng e, Tao He f,

Kun Wei f, Xiu-Zhou Lin a,b, Chun-Ru Cheng b,f, **

a

School of Materials Science and Engineering, Sichuan University of Science and Engineering, Zigong 643000, Sichuan, PR China b

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Key Laboratory of Material Corrosion and Protection of Sichuan Province, Zigong 643000, Sichuan, PR China c

CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, CAS, Shenyang 110016, PR China d

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Corrosion and Electrochemistry Research Group, Department of Pure and Applied Chemistry, University of Calabar, P.M.B. 1115, Calabar, Nigeria e

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School of Chemistry and Environmental Engineering, Sichuan University of Science and Engineering, Zigong 643000, Sichuan, PR China f

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School of Chemical Engineering, Institute of Pharmaceutical Engineering Technology and Application, Key Laboratory of Green Chemistry of Sichuan Institutes of Higher Education, Sichuan University of Science and Engineering, Zigong 643000, Sichuan, PR China

*Corresponding Authors

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Peter C. Okafor CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, CAS, Shenyang 110016, PR China. E-mail: [email protected];

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Chun-Ru Cheng School of Chemical Engineering, Institute of Pharmaceutical Engineering Technology and Application, Key Laboratory of Green Chemistry of Sichuan Institutes of Higher Education, Sichuan University of Science and Engineering, Zigong 643000, Sichuan, PR China. E-mail: [email protected]

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Graphical abstract

Highlights:

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Extracts of Dioscorea septemloba were characterized. The extracts acted as mixed-type corrosion inhibitors Surface analysis confirmed the presence of the extracts on the metal surface. The adsorption properties were discussed by computational studies.

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   

ABSTRACT The adsorption and corrosion inhibition performance of the water phase and organic phase extracts from Dioscorea septemloba on carbon steel have been carried out in 1 M HCl solutions. Extracts characterization was accomplished by NMR spectroscopy and the generic carbohydrate test. Electrochemical techniques and gravimetry were utilized for the evaluation of corrosion inhibition behaviors. 3D surface measurement instrument, SEM, and FT-IR 2

were used to examine the surface properties of carbon steel coupons after full immersion in 1 M HCl solutions without and with the extracts. Quantum chemical calculation (QCC) and molecular dynamics simulation (MDS) were used to predict and describe the electronic and adsorption properties of some characterized compounds in the extracts. Both extracts exhibited similar corrosion inhibition behaviors acting as mixed-type inhibitors and having their efficiencies dependent on concentration and temperature. Time-dependent corrosion evaluation showed that the inhibition efficiencies of the extracts were improved with time.

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Low surface roughness data were recorded for the extracts by 3D surface measurements and the results were consistent with the observations from SEM micrographs and FT-IR spectra. Computational parameters obtained from QCC and MDS were used to predict the adsorption

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behavior of some characterized compounds from the extracts.

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Keywords: Adsorption; Corrosion inhibition; Carbon steel; Extract characterization; EIS;

1.

Introduction

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polarization

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One of the greatest limitations to the use of steel and other materials in industrial applications is corrosion [1]. The cost of corrosion is quoted to be in the range of 3–5% of the

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gross national product of developed countries [2]. Corrosion control has proven to be an expensive procedure with many methods being proposed. Researchers have reported

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materials modification [3–7], solutions modification [8], improvements of operations [9], and the application of corrosion inhibitors [10,11] and coatings [12,13], as some practicable procedures to curb corrosion. The application of corrosion inhibitors is certified to be the most effective and efficient method due to its general cost and returns on investment. Despite the emergence of a wide range of corrosion inhibitors, environmentally benign and biodegradable compounds have been the favored choices. Therefore, there have been 3

growing and intensive studies on natural products as effective inhibitors in varying environments and applications [10,14–25]. Organic compounds that generally contain pi– electrons in their structures, aromatic rings, and heteroatoms like N, S, O, and P, are mostly preferred [26–31]. This is because they readily exhibit an interaction with metals surfaces by adsorption using these functional groups and eventually offering protections to the metals against aggressive environments. The interaction of an inhibitor compound with the surface of a metal is influenced by the type of corrosion medium, the chemical structure of the

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inhibitor compound, and the type and surface properties of the metal [32,33]. Dioscorea septemloba (called bìxiè in Chinese Pinyin), a specie of the genus Dioscorea belonging to the Dioscoreaceae family, is one of over 600 species of other

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flowering plants belonging to the family [34,35]. It is widely distributed around China and

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has gained significant interest in Traditional Chinese Medicine (TCM) as a depurative, carminative, dampness-dispelling, and an anti-paralysis drug, as well as its use in the

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treatment of various diseases and health challenges, such as arthritis and kidney deficiency [35]. D. septemloba contains a myriad of phytoconstituents such as glycosides, steroids,

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sterols, fatty acids and some fatty acid derivatives, which have displayed good antioxidant properties [34–36]. It will therefore be worthwhile to investigate the antioxidant ability of D.

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septemloba in the corrosion mechanisms of metals since corrosion is an oxidation process. Some literatures have reported the corrosion inhibition of metals by the action of starch (a

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polysaccharide extracted from a wide range of plant materials, including the Dioscoreaceae family) [10,37,38], but to the best of our knowledge, no comprehensive report exists on the corrosion inhibition characteristics of extracts from D. septemloba. Due to its central nature in TCM and because of the vast phytoconstituents that have been identified and proven viable as antioxidants, D. septemloba is an interesting choice for investigation in corrosion inhibition studies. 4

In this research, two extract types from the rhizome of D. septemloba have been characterized and comparatively investigated for their potency in the inhibition of carbon steel corrosion in 1 M HCl solutions with an aim to understanding their nature and adsorption on the steel surface. While one extract represents the organic phase consisting primarily of constituents such as steroidal glycosides, diarylheptanoids etc., the other is the water phase extract of D. septemloba which contains water soluble compounds such as glucose, fructose, etc. The comparative inhibition study offers to describe the general behavior of the plant in

Experimental

2.1.

Materials

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corrosion studies as well as the distinctive properties of each extract.

Corrosion experiments were performed on carbon steel specimen with composition as

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shown (wt.%): C(0.23), Mn(0.79), P(0.02), S(0.03), Cu(0.29), Si(0.20), and Fe(balance). The

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coupons for weight loss experiments and surface analyses were of dimensions 2 cm by 1 cm by 0.2 cm with 5.2 cm2 as total surface area while the coupons for electrochemical tests were

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soldered to a copper wire and submerged in epoxy resin leaving an exposed area of 1 cm2. Prior to the measurements, the surfaces were abraded with silicon carbide abrasive paper of

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increasing fineness, rinsed with double distilled water, cleaned with ethanol in an ultrasonic bath, and air-dried. The corrosive medium was 1 M HCl solution which was prepared from

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36.5% HCl (analytical grade, supplied by Chongqing Chuandong Chemical Group Co., Ltd.) diluted with double distilled water. The rhizome of D. septemloba was obtained from Sichuan Province, China.

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2.2.

Extracts preparation Dried rhizomes of D. septemloba (4.5 kg) were pulverized into fine powder, which

was then refluxed three times with 75% ethanol (36 L×3). The extracted ethanol solution was filtered, and concentrated under vacuum until dryness, and then applied to a D101 macroporous resin column, which was eluted with water (32 L), and 100% ethanol (32 L), successively. The water eluted solution was evaporated to produce the water phase extract (431 g). Accordingly, the 100% ethanol eluted solution was also evaporated to afford the

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organic phase extract (169 g). 10 g each of the crude extracts were digested in 1 L of 1 M HCl solution and estimated to be 10 g/L. The resultant solutions were allowed to stand for 24 hours, and then filtered and used as the stock inhibitor solutions. The inhibitor test

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concentrations (0.1 g/L, 0.5 g/L, 1.0 g/L, and 2.0 g/L) were subsequently prepared from the

Techniques

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2.3.

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stock solutions.

2.3.1. Characterization of the extracts of D. septemloba

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The extracts of D. septemloba were characterized by Fourier-transform infrared spectroscopy (FTIR) (PerkinElmer, MIT, USA) using the KBr disk technique. The FTIR instrument was connected with Omnic software, which extended from 400 to 4000 cm-1. 1H

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NMR (600 MHz), 13C NMR (150 MHz), HMQC, HMBC spectra were recorded on a Bruker

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Avance III 600 MHz spectrometer (Bruker). 2.3.2. Electrochemical experiment The setup for electrochemical experiment consisted of a 250 ml glass cell,

temperature-controlled water bath, Potentiostat (Solartron SI 1287 Electrochemical interface/Solartron SI 1260 Impedance gain-phase analyzer) and a conventional threeelectrode corrosion cell (counter electrode: platinum foil; reference electrode: saturated 6

calomel reference electrode (SCE); working electrode: carbon steel). All potentials reported in this work were measured versus SCE. For all tests, open circuit potential (OCP) measurement was first conducted for 1800 seconds to achieve stabilization of the systems. The electrochemical impedance spectroscopy (EIS) measurements was conducted at OCP with AC signals of 10 mV amplitude in a frequency range from 100 kHz to 10 mHz, and the obtained EIS spectra were fitted using the ZSimpWin software. The potentiodynamic polarization experiments were performed in the potential range of ±0.25 V vs. OCP at 0.5

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mV/s scan rate. To affirm the reproducibility, all experiments were carried out in triplicates with standard deviations ranging from 2 – 8 Ωcm2 and 1 – 4 μA/cm2 for the impedance and polarization data, respectively.

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2.3.3. Weight loss experiment

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In addition to depicting the corrosion inhibition trend, weight loss experiment was used to evaluate the time-dependent effect of the extracts of D. septemloba on carbon steel

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corrosion in 1 M HCl solutions. Clean carbon steel coupons were weighed and completely immersed in 250 ml beakers containing unstirred test solutions (solutions without and with

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0.5 g/L extracts). They were allowed to stand in the test solutions at 303±1 K for the different test durations (24, 48, 72 and 120 hours). After the exposure times, the coupons were

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retrieved from the test solutions and transferred into a pickling acid solution (containing 500 ml HCl + 500 ml double distilled water + 3.5 g hexamethylenetetramine) placed in an

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ultrasonic bath for 600 s to chemically remove all surface corrosion products [39]. The cleaned coupons were then washed with double distilled water, dipped in acetone, air-dried, and reweighed. The weight loss of the coupon was calculated as the difference in weight before and after exposure to the test solution. Mean weight loss values were recorded after triplicate experiments. The corrosion rate of the carbon steel coupon was calculated from equation (1) [40]: 7

87600 × 𝑊𝑒𝑖𝑔ℎ𝑡 𝑙𝑜𝑠𝑠 (𝑔)

𝐶𝑜𝑟𝑟𝑜𝑠𝑖𝑜𝑛 𝑟𝑎𝑡𝑒, 𝐶𝑅 (𝑚𝑚𝑦 −1 ) = 𝐴𝑟𝑒𝑎 (𝑐𝑚3 ) × 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 (𝑔𝑐𝑚−3 ) × 𝑡𝑖𝑚𝑒 (ℎ)

(1)

Consequently, the inhibition efficiency of the extracts was estimated from the corrosion rate calculations as shown [41]: 𝐼𝑛ℎ𝑖𝑏𝑖𝑡𝑖𝑜𝑛 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦, 𝜀 (%) = (1 −

𝐶𝑅𝑒𝑥𝑡𝑟𝑎𝑐𝑡 𝐶𝑅𝑏𝑙𝑎𝑛𝑘

) × 100

(2)

where CRextract and CRblank are the calculated corrosion rates for the inhibited and uninhibited

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solutions, respectively. 2.3.4. Surface analysis

Optical surface profile and surface roughness of carbon steel coupons were measured

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by 3D surface measurement instrument (Bruker Nano GmbH: Contour GTK-18) for coupons unexposed and exposed to 1 M HCl solutions without and with 0.5 g/L of extracts of D.

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septemloba. The surface roughness analysis was aided by the use of Vision64 Map software.

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Scanning electron microscope (SEM) examination (TESCAN 3SBU) was also used to observe the surfaces of the coupons. Furthermore, the inhibitive layers formed in the presence

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of the extracts were analyzed by means of FTIR. For the 3D surface measurement test, the exposed coupons were fully immersed in the test solutions for 1 hour and 2 hours at 303±1 K,

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while they were immersed for 24 hours for the SEM and FTIR tests. After immersion, the coupons were carefully rinsed with double distilled water, dipped in acetone and air-dried.

Computational studies

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2.4.

Quantum chemical calculations were performed using Gaussian 09 W package. The

molecular structures of the compounds present in the extract were fully optimized by density functional theory (DFT) using RB3LYP calculation method with 6-311++G (d, p) basis set in gas phase. Quantum chemical parameters such as the energy of highest occupied molecular

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orbital (EHOMO), energy of lowest unoccupied molecular orbital (ELUMO), energy gap (ΔE), ionization energy (I), electron affinity (A), electronegativity (χ), global hardness (γ) and the number of transferred electrons (ΔN) were calculated. I and A were calculated as the negative values of EHOMO and ELUMO, respectively, and they were applied to equations (3) and (4) to obtain values of χ and γ [42]. 𝜒=

𝐼+𝐴

𝜎=

𝐼−𝐴

(3)

2

(4)

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The obtained values were useful in the application of the Pearson method to calculate ΔN

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[11,43]: 𝜙−𝜒𝑖𝑛ℎ

ΔN = 2(𝜎

(5)

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𝐹𝑒 +𝜎𝑖𝑛ℎ )

where ϕ is the work function of Fe (110) which is equal to 4.82 eV, and is reported to exhibit

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a high stabilization energy and packed surface [44]. The local reactivity nucleophilic and electrophilic attacks) of the molecules were obtained by the Fukui functions using Mulliken

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population analysis according to the following equations [45]:

𝑓𝑟− = 𝑞𝑟 (𝑁) − 𝑞𝑟 (𝑁 − 1)

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𝑓𝑟+ = 𝑞𝑟 (𝑁 + 1) − 𝑞𝑟 (𝑁)

where qr (N-1), qr (N+1), qr (N) are the cationic, anionic, and neutral charges of the molecules, respectively.

Molecular dynamics simulation (MDS) was employed to investigate the adsorption behavior of the extracts compounds using the Forcite module of Material Studio software. The simulation process involved cutting the Fe (1 1 0) surface followed by the setup of 6 x 6

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x 6 super unit with no boundary effect to permit the complete accommodation of 300 water molecules and each studied inhibitor molecule. MDS was then performed after optimizing the structure of the entire system. The simulation time was set at 1000 s to guarantee full adsorption of each molecule on Fe (1 1 0) surface.

Results and discussion

3.1.

Extract characterization

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3.

The chemical structures of the pure compounds from the organic phase extract (OP) were unambiguously identified by interpreting the 1H NMR,

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C NMR, HMBC, HMQC

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spectra, and comparing with references. The main compounds (Fig. 1) identified include: dioscin [46], prosapogenin A of dioscin [46], methyl protodioscin [47], prosapogenin B of

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dioscin [48], β-sitosterol [49], palmitic acid [49], oct-1-yn-4-ol [49], 4,8-dimethyl-1,7nonadiene [49], methyl laurate [49], stigmasterol [50], daucosterol [50], (+)-syringaresinol

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[35], polyphyllin V [51], diosgenin-3-O-{α-L-rhamnopyranosyl(1→2)- [β-D-glucopyranosyl (1→3)]}-β-D-glucopyranoside [52], spongipregnoloside A [53], spongipregnoloside D [53],

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dioscoroside E1 [54], dioscoroside E2 [54], 3,5-dihydroxy- 1,7-bis(4-hydroxyphenyl)heptane [54], diospongin A [55], diospongin B [55], diospongin C [55], dioscorone A [55], [56],

5,6,2-trihydroxy-3,4-me-thoxy-

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dihydroxy-2,4dimethoxy-9,10-dihydrophenanthrene

5,6-

9,10-dihydrophenanthrene [56], oleamide [57], 4,4',7,7'- tetrahydroxy-2,2',6,6'-tetramethoxy-

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1,1'-biphenanthrene [58], dioscorone B [58]. The constituents in the water phase extract (WP) were high polar compounds which

proved very difficult to separate from each other by current chromatographic methods; therefore, this was a limitation in identifying pure compounds from WP, and subjecting them to NMR test. Nonetheless, upon treatment with Tollens' reagent (ammoniacal silver (I)

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nitrate), WP showed a positive result (characteristic silver mirror). Also, on treatment with Fehling's reagent (basic solution of bistartratocuprate (II) complex), WP exhibited a positive result (formation of a brick-red precipitate). From the results of the generic tests, the main chemical constituents in WP were possibly the saccharides such as glucose, fructose, arabinose, etc.

3.2.

Effect of extract addition

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3.2.1. Electrochemical impedance spectroscopy measurements EIS experiments permitted the acquisition of information on the surface properties of the carbon steel working electrode, as well as the kinetics of the electrode processes in the

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uninhibited and inhibited systems. Fig. 2 illustrates the impedance data as Bode and Nyquist plots for carbon steel in 1 M HCl solutions in the absence and presence of different

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concentrations of OP and WP at 303 K. The magnitude of impedance from the Bode plots

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show noticeable increases for the inhibited systems compared to the uninhibited system. As OP and WP were added, the impedance magnitude resultantly increased. In all cases, the Nyquist plots showed a clear deviation from an ideal semi-circle as all the plots depicted

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depressed semi-circle loops having one shoulder and a one-time constant over the range of studied frequencies. The plots revealed that with the addition of OP and WP, the diameter of

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the semi-circles increased and the increase was concentration-dependent. For both extracts,

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the diameter increased steadily as the concentration increased, reaching a maximum equivalent to 2.0 g/L. The increase of impedance magnitude with the addition of an inhibitor compound is related to the formation of a protective film layer by the inhibitor on the carbon steel surface [23], whereas the corresponding increase in diameter of the semi-circle is attributed to an increase in the resistance to charge transfer by the inhibitor film layer [23,59,60]. OP generally showed larger increases in both the impedance magnitude and

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semicircle diameter than WP. These suggest that the protectiveness offered by OP was more than that by WP. The detected depression in the semi-circle loops is a consequence of the heterogeneity and roughness of the carbon steel surface together with the geometric nature of current distribution [14]. This is characteristic of carbon steel electrodes undergoing corrosion process [61]. Accordingly, a constant phase element (CPE) was employed to fit the non-ideal capacitive behavior in the equivalent circuit (inset of Fig. 2b and d; Fig. 4b and e) used for

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the fitting of impedance data. From the equivalent circuit, Rs is the resistance of the solution, Rct is the charge transfer resistance, Rf stands for the film resistance, while CPE1 and CPE2 represent the constant phase elements of the double layer and film capacitance, respectively.

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The fitted impedance parameters for carbon steel in 1 M HCl solutions without and with

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different concentrations of OP and WP are listed in Table 1. η represents the inhibition efficiency, which was calculated from the polarization resistance, Rp (Rp = Rct + Rf) based on

𝑅𝑜

𝜂 = (1 − 𝑅𝑝𝑖 ) × 100

(3)

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𝑝

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the following expression:

where 𝑅𝑝𝑜 and 𝑅𝑝𝑖 are the corresponding Rp values for the solution without and with inhibitors,

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respectively. Although the plots in Fig. 2 show one large capacitive loop at high frequency region, the fitting results reveal the existence of a second capacitive loop, albeit

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inconspicuous, at the low frequency region as contained in the electrical equivalent circuit. The thin nature of the adsorbed inhibitor films could have led to the overlapping of the two capacitive loops, thus responsible for the inconspicuous behavior of the second loop. Table 1 shows that the addition of the extracts increased both the Rct and Rf values, with Rp increasing accordingly. This effect was improved with increase in the concentration

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of the extracts, from which it can be inferred that OP- and WP-adsorption films were formed on the surface of carbon steel, and consequently inhibiting the charge transfer process. Therefore, η increased with increasing concentration of the extracts and reached 72.1% for OP, and 65.3% for WP, at 2.0 g/L. 3.2.2. Potentiodynamic polarization measurements Potentiondynamic polarization curves for carbon steel in 1 M HCl solutions in the absence and presence of different concentrations of the two extracts (OP and WP) of D.

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septemloba at 303 K are shown in Fig. 3 while the corresponding fitted data of the linear Tafel segments of the potentiodynamic polarization curves are presented in Table 2. The Polarization experiment was carried out to assess the influence of the adsorbed compounds of

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OP and WP extracts on the anodic and cathodic corrosion processes of carbon steel. From the

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plots, the curves for the uninhibited and inhibited systems look similar; indicating that the addition of the extracts did not change the mechanism of carbon steel corrosion, but only

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retarded the corrosion reaction rate [62]. It is also observed that all the curves shifted to lower current densities as the extracts were added, and the shift continued with increase in the

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concentration of the extracts. Calculated values of η which were evaluated from corrosion current densities using equation (4) show that 0.1 g/L of OP and WP offered inhibition

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efficiencies of 81.7% and 73.1%, respectively. The values increased progressively reaching 89.2% and 82.8%, respectively at 2.0 g/L.

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𝑖

𝑖

𝜂 = (1 − 𝑖 𝑐𝑜𝑟𝑟 ) × 100 𝑐𝑜𝑟𝑟0

(4)

where icorri and icorro are the corrosion current densities in the presence and absence of the extract, respectively.

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Furthermore, a noticeable displacement (to the more positive direction) of the corrosion potential was observed when the extracts were added to the corrosion medium, signifying that they have the tendency for inhibition of carbon steel corrosion. A corrosion potential of -466.8 mV was initially recorded for the uninhibited system but the introduction of 0.1 g/L OP and WP shifted the potential to -428.5 mV and -428.2 mV, respectively. Further increment in extracts concentration did not induce large potential displacements as the potential maintained a fairly stable range for both extracts. The maximum potential

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displacements of 51.7 mV for OP and 54.4 mV for WP are less than 85 mV. An inhibitor is effectively classified as either anodic- or cathodic-type when the corrosion potential displacement is greater than 85 mV [16,63]. These observations, together with the lowering

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of current density values, suggest that both OP and WP induced a geometric blocking effect [64] and retarded the oxidation of oxide-free ions and evolution of hydrogen reactions,

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making them act as mixed-type inhibitors. Similar observations were reported for inhibition

3.3.

Effect of temperature

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studies involving other carbohydrate-based materials such as exudates [65–68].

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Electrochemical experiments were employed to assess the effect of temperature on the corrosion behavior of carbon steel in 1 M HCl solutions in the absence and presence of the

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concentration of the extracts with the average Icorr and RT values (0.5 g/L) in 1 M HCl

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solutions. The tests were carried out in the temperature range 303 – 323 K. The impedance data are presented as Bode (Fig. 4a and d) and Nyquist (Fig. 4b and e) plots for OP and WP, respectively, while their corresponding fitted impedance parameters are displayed in Table 3. As evident in the figures, the diameter of the Nyquist semi-circle and the magnitude of the Bode impedance decreased steadily with increasing temperature. Interestingly, the impact of temperature was more obvious in the system with OP. Table 3 shows that RT decreased from

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740.6 to 256 .4 Ωcm2 when temperature was raised from 303 to 313 K, while the difference in RT for WP at the same temperature range was 282.2 Ωcm2. This observation suggests that the compounds present in OP showed good adsorption on carbon steel leading to high resistance values at low temperature, but the adsorption is easily lost when temperature is raised. On the other hand, although WP offered a lower resistance than OP, its compounds were more resilient to the initial temperature rise. The polarization tests reveal that the anodic and cathodic parts of the potentiodynamic

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polarization curves for both extracts increased to more positive directions with increase in temperature (Fig. 4c and f). This indicates that temperature increment encouraged both the iron dissolution reaction and the evolution of hydrogen reaction. Table 4 representing the

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fitted data of the potentiodynamic polarization curves shows that temperature increment

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resulted in the increase in icorr. This supports the observation that temperature induced more corrosiveness for the systems. Furthermore, the influence of temperature on the extracts

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performances was more noticeable in OP which showed larger differences in icorr for each temperature change compared with those of WP. This observation is consistent with the

Time-dependent corrosion test

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3.4.

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results of the impedance measurements.

The time-dependent effect of the extracts of D. septemloba on the corrosion of carbon

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steel in 1 M HCl solutions was determined by weight loss measurements. While equation (1) permitted the calculation of corrosion rate, equation (2) was useful for the calculation of the corresponding inhibition efficiency. In all cases, the corrosion rate increased with time (Fig. 5a), and the uninhibited solution showed more marked increments. It is evident from the figure that the presence of 0.5 g/L OP and 0.5 g/L WP caused significant reductions in the corrosion rate of carbon steel, and this was maintained all through the immersion duration. 15

Although the corrosion rate values of both extracts were close, OP generally offered superior inhibition efficiencies than WP (Fig. 5b). Values for initial immersion (24 h) were 75.4% and 74.6 % for OP and WP, respectively. The values increased steadily up to 85.3% and 84.9% at the end of the immersion test (120 h). The results revealed that the inhibition efficiency of the extracts of D. septemloba increased with time, and OP is a better inhibitor than WP. A Similar result of improved corrosion inhibition with time was reported by Brindha et al. [69] in their study of the synergistic effect between starch and substituted 2,6-diphenyl-3-

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methylpiperidin-4-one ( DPMP ) on the corrosion inhibition of mild steel in HCl. The report further reported that the inhibition efficiency of starch was enhanced when DPMP was introduced into the test medium. Generally, molecular starch exhibits poor adhesion strength

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and reduced solubility which limits its application in corrosion inhibition studies, and the majority of reports on starch are based on additional chemical modifications to improve its

Morphological studies

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3.5.

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anti-corrosion ability [10].

The 3D surface profile is shown in Fig. S1. From the figure, the corrosion damage to

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the coupons increased with prolonged exposure to HCl solution while the presence of OP and WP significantly reduced the damage to the surface of coupons. The surface roughness (Sq)

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data presented in Table 5 reveals that carbon steel showed an increased roughness on

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exposure to HCl solution with Sq increasing to about 5 times the value for the unexposed coupon (64.6 nm) after 1 hour immersion. The roughness further increased to 360.5 nm when the immersion duration was 2 hours, indicating the severity of acid attack with prolonged exposure. However, in the presence of 0.5 g/L OP and 0.5 g/L WP, the values of Sq decreased adequately. The decrease in roughness in the solution with the extracts was probably due to

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the formation of adsorbed thin protective layers of OP and WP molecules on the metal surface. Assessment by SEM of the surface morphologies of carbon steel coupons unexposed and exposed to uninhibited and inhibited 1 M HCl solutions after total immersion for 24 h at 303 ± 1 K are presented in Fig. 6. The surface of the coupon exposed to the uninhibited solution was severely damaged (Fig 6b) compared to that of the unexposed coupon. This was likely a consequence of the active dissolution caused by the attack of the corrosive medium.

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The corrosion was noticed to be uniform with no indication of a localized attack. With the addition of 0.5 g/L WP (Fig 6d), the extent of damage on the carbon steel coupon was significantly reduced. Furthermore, the least visible corrosion attack corresponds to the

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addition of 0.5 g/L OP (Fig 6c). Polishing marks can be observed on the surface of the

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specimen signifying that the attack from the corrosive medium was most hindered in the presence of OP. On close examination of the coupons from the solution containing OP and

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WP, it was observed that they each exhibited a layer on their surface. These layers may have obstructed mass and charge transfer on the carbon steel surface and could be accountable for

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the restriction of the attack of the corrosive medium which was evidenced by the reduced corrosion of the carbon steel coupons. The results of SEM were in agreement with those of

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the 3D surface measurements.

To confirm the adsorption of the chemical compounds on the metal surface, FT-IR

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spectroscopy, widely reported as a formidable technique to ascertain the formation of complexes between corrosion inhibitors and the surfaces of the metal [70–75], was used to characterize the extracts (OP and WP) as well as their adsorbed films on the carbon steel surface. The spectra obtained are shown in Fig. 6e and f. For the spectrum of OP (Fig 6e), the absorption peak at 3404 cm-1 corresponds to the stretching vibration of O-H, and the peak at 2933 cm-1 can be assigned to the stretching vibration of C-H. The peak at 1655 cm-1 can be 17

caused by the stretching vibration of C=O and the stretching vibration of C=C. The bending vibration of C-H and the in-plane bending vibration of O-H can be found at 1481 cm-1. The peak at 1040 cm-1 can be attributed to the stretching vibration of C-O. The out-of-plane bending vibration of O-H can be found at 636 cm-1. On the other hand, the spectrum of WP depicted in Fig 6f shows a broad peak at 3434 cm-1 which can be credited to the stretching vibration of O-H. The peaks 2928 cm-1 and 2867 cm-1 correspond to the stretching vibration of CH2 while the peak at 1617 cm-1 can be assigned to the stretching vibration of C=O and

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C=C. The absorption peak at 1392 cm-1 can be attributed to the bending vibration of C-H. The absorption peak at 1085 cm-1 can be attributed to the stretching vibration of C-O. FTIR results show that both extracts of D. septemloba contain oxygen atoms in functional groups

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(such as C=O, O-H,), which have been reported to play vital roles in corrosion inhibition [26,27]. Additionally, the FTIR results of the adsorbed OP and WP films on carbon steel

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surface show similar spectra and consistent absorption peaks with those of the pure extracts.

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Noticeably, the peak intensities appeared weaker and slightly shifted after the adsorption of OP and WP on carbon steel surface. This may be the direct outcome of the interactions between some active constituents of the extracts and the active sites of the carbon steel

Computational studies

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3.6.

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surface, leading to the formation of protective film layers.

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Quantum chemistry has become increasingly popular in the field of corrosion protection, supplying quick and reliable methods for corrosion studies to predict the performance of corrosion inhibitors. Four compounds were randomly chosen to represent each of the major chemical groups of compounds characterized in OP. They are dioscin, βsitosterol, dioscorone A, and palmitic acid. Dioscin represents the glycosides whose other compounds are methyl protodioscin, prosapogenin B, prosapogenin A, daucosterol,

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polyphyllin V, diosgenin-3-O-{α-L-rhamnopyranosyl(1→2)-[β-D- glucopyranosyl(1→3)]}β-D-glucopyranoside, spongipregnoloside A, and spongipregnoloside D. β-sitosterol represents the steroids which also includes stigmasterol. The phenathrene group is represented by dioscorone A, with other compounds being Dioscorone B, and 4,4',7,7'tetrahydroxy-2,2',6,6'-tetramethoxy- 1,1'-biphenanthrene, while palmitic acid represents the fatty acids and their derivatives. The optimized molecular orbitals of the selected compounds, as well as their highest occupied molecular orbitals (HOMO) and lowest unoccupied

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molecular orbitals (LUMO), are presented in Fig. 7. The HOMO and LUMO illustrate the capacity of molecules to donate and receive electrons, respectively, and they are useful to depict the donor-acceptor adsorption relationship on metal surfaces. Table 5 shows the

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calculated quantum chemical parameters which include the energy of HOMO (EHOMO), the energy of LUMO (ELUMO), dipole moment (μ), and the number of transferred electrons (ΔN).

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High electron donating and accepting capacities of molecules which describe the relationship

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between an inhibitor and a metal surface are usually correlated with high EHOMO and low ELUMO values [76]. Therefore the difference (ΔE) between EHOMO and ELUMO is a meaningful

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parameter for describing the adsorption behavior of molecules on metal surfaces. Obot and Gasem [76] explained that smaller values of ΔE are related softer molecules with high

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reactivity and low kinetic stability while larger values of ΔE relate to harder molecules with low reactivity and high kinetic stability. Also, high dipole moments are typically linked to

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high corrosion inhibition effects [16]. Therefore, from values of ΔE and μ (Table 5), the trend of reactivity of the molecules is dioscin > dioscorone A > β-sitosterol > Palmitic acid. This means that the glycoside group contributed more to the corrosion inhibition performance of OP while the fatty acids and their derivatives were the least contributors. In addition, Cao et al. [77] and Saha et al. [78] explained that the likelihood for transfer of electrons between an inhibitor molecule and a metal surface is increased when 0 < ΔN < 3.6, and Lukovits [79] 19

explained that superior inhibition performances are associated with molecules having high ΔN when the values are less than 3.6 . From Table 5, all values of ΔN are positive and <3.6, meaning that all the molecules showed an ability to donate electrons to the metal surface, and dioscin with the highest ΔN value has the highest inhibition ability while palmitic acid with the least value has the lowest inhibition ability. Although ΔE revealed the reactive strength of each molecule, ΔN showed that all the molecules were involved in the inhibition performance of OP. This is true because plant extracts are composed of myriads of phytoconstituents

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which can either lead to the inhibition or acceleration of corrosion processes. Therefore, the overall effect (both antagonistic and synergistic) of the phytoconstituents of the extracts is what is actually recorded as their inhibition efficiencies and not the ability of specific

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phytoconstituents [20].

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Fukui function analysis was carried out to gain more understanding into the adsorption sites on the characterized compounds. This analysis is effective in the assessment

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of the most active sites available for nucleophilic (fr+) and electrophilic (fr-) attacks [80] where high values fr+ and fr- denote high abilities of the atoms to receive and donate electrons,

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respectively. The calculated condensed Fukui indices for the compounds are presented in Table S1 and as expected, the oxygen heteroatom and π-electrons were observed to be the

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most susceptible sites for acceptance and donation of electrons. This means that the inhibitive ability of the extract is mainly caused by the reactive sites on the characterized compounds.

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Molecular dynamics simulation was beneficial for the determination of the

orientations of the absorbed inhibitor molecules on Fe (1 1 0) surface (Fig. 8), and the energy parameters associated with the interactions between the adsorbed molecules and the Fe surface. The figure reveals that the molecules were parallelly adsorbed on the Fe surface, suggesting the prospects of donating electrons to the empty orbitals of the metal substrate, thereby forming coordination bonds. This decreases the exposed area available for corrosion 20

attack by the acidic environment. Furthermore, the binding energy (Ebinding), which describes the minimum energy of interaction (Eint) between the molecules and the Fe surface, was obtained as follows: −𝐸𝑖𝑛𝑡 = 𝐸𝑡𝑜𝑡 − (𝐸𝑠𝑢𝑏 + 𝐸𝑖𝑛ℎ ) = 𝐸𝑏𝑖𝑛𝑑𝑖𝑛𝑔

(5)

where Etot, Esub, and Einh represent the total energy (which comprises the energy of the inhibitor, water molecules and metal substrate), the energy of metal substrate (including

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water molecules), and the energy of the inhibitor, respectively. The calculated Ebinding values were 1440.6 KJ/mol, 1085.7 KJ/mol, 402.5 KJ/mol, and 256.1 KJ/mol for dioscin, dioscorone A, β-sitosterol, and palmitic acid, respectively. This

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means that while dioscin will be the most adsorbed molecule on the Fe surface, palmitic acid will be the least adsorbed. The high binding energies for dioscin and dioscorone A could be

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due to the following reasons: (a) the large molecular sizes of the compounds which will

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ensure greater surface coverage and improved metal-inhibitor interactions; (b) the abundant potential active sites available for bonding as depicted by QCC; (c) the constituent oxygen

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heteroatom (16 atoms for dioscin and 6 atoms for dioscorone A) and π-electrons can participate in the inhibitor-Fe surface interaction either by back bonding from the d-orbital of

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Fe or by the formation of coordinate covalent bonds with the vacant d-orbitals of surface Fe atoms [81]; (d) the adsorption configurations from MDS show complete parallel orientations

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of the molecules on Fe (110) surface, and the extent of parallelity corresponds to the magnitude of interactions between the molecules and the Fe surface [82]. The molecular structure of β-sitosterol reveals the presence of methyl groups that did not align parallelly with the other groups on the surface of Fe. Therefore, the steric hindrance generated by the methyl group, thereby affecting its conformation on Fe surface, may have induced an unfavorable effect on the donor-acceptor relationship of β-sitosterol. Finally, although Fig. 8 21

shows the parallel alignment of palmitic acid on Fe surface, less presence of the basic functional groups enhancing inhibition reactions (such as the presence of heteroatoms, aromaticity and π-electrons) may have ensured its least interaction with the metal surface. The results of molecular dynamics simulation are consistent with the investigations by quantum chemical calculations.

3.7.

Inhibition mechanism

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The corrosion inhibition process of carbon steel by OP and WP is principally ascribed to the adsorption (physisorption and/or chemisorption) of the extract compounds on the steel surface, with the adsorption process being dependent on different conditions such as the

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electron density of the donor atoms, the functional groups involved, and the interactions between the p and d orbitals of the extract compounds and the steel surface, respectively. In

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this case, the interactions are between the lone pair of electrons on oxygen and the metal surface, the π-electrons and double bonds in the compounds and the metal surface, and the

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electrostatic interactions between the compounds and the metal surface. Understanding that chemisorption mainly occurs by inhibitor molecules displacing water molecules from metal

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surfaces, the adsorption route follows [24,83]:

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𝑌(𝑠𝑜𝑙) + 𝑥𝐻2 𝑂(𝑎𝑑𝑠) → 𝑌(𝑎𝑑𝑠) + 𝑥𝐻2 𝑂(𝑠𝑜𝑙)

Y represents the molecules of both extracts from D. septemloba and the number of displaced

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water molecules is expressed as x. During anodic dissolution process, the released Fe2+ ions combine with the adsorbed extract molecules to form a metal-inhibitor complex as shown [83]:

𝐹𝑒 → 𝐹𝑒 2+ + 2𝑒 − 𝐹𝑒 2+ + 𝑌(𝑎𝑑𝑠) → [𝐹𝑒 − 𝑌]2+ (𝑎𝑑𝑠)

22

Depending on the solubility of the adsorbed metal-inhibitor complex, [Fe-Y]2+ can either inhibit or encourage metal dissolution reaction. The experimental results revealed that the presence of the extracts produced predominantly more insoluble adsorbed complexes whose strengths improved with time and decreased with temperature. Irrespective of the extract type involved, increasing the concentration resultantly increased the inhibition efficiency. This means that more molecules were available for complex formation leading to the evolution of more insoluble surface layers. The superior inhibition potency of OP

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compared to WP is possibly the consequence of their molecular structures and the presence (and number) of electron donating groups. While WP was assumed to contain mainly the saccharides with a general molecular formula of (CH2O)n, OP contained myriad of higher

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molecular weight compounds having multiple aromatic rings and greater number of oxygen

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atoms, which offer stronger interactions with metal surfaces.

4. Conclusions

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This research reported the adsorption and corrosion inhibition behavior of two extract types from D. septemloba, namely: water phase extract (WP) and organic phase extract (OP)

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obtained by water and ethanol elution, respectively. 28 compounds were unambiguously characterized from OP by NMR analysis. They included some steroidal glycosides,

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diarylheptanoids, phenanthrenes, dihydrophenanthrenes, fatty acids and their derivatives,

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steroids, etc. Generic carbohydrate tests suggested that WP mainly contained the saccharides. Electrochemical and weight loss measurements proved that the extracts are efficient inhibitors for carbon steel corrosion. They both acted as mixed-type inhibitors with their effectiveness depending on the extract concentration and the temperature of the media. Inhibition efficiencies increased when concentration was increased and decreased with temperature increment. Time-dependent corrosion experiment showed that both extracts

23

exhibited lower corrosion rates with time, relative to those of the uninhibited solution. 3D surface measurements revealed the decreased surface roughness of carbon steel coupons retrieved from 1 M HCl solutions containing WP and OP compared to the coupons from the uninhibited solution. SEM and FTIR tests confirmed the presence of adsorbed molecules of WP and OP on carbon steel surface, thereby accounting for their corrosion inhibition abilities. Although both extracts exhibited similar inhibition behaviors to carbon steel corrosion, OP presented better inhibition results than WP across the entire tests conducted. This is possibly

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due to the existence of multiple aromatic rings and greater number of oxygen atoms existing as heteroatoms in the chemical structures of the characterized compounds in OP. These have been reported to exhibit greater interactions with metal surfaces. Quantum chemical

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calculation and molecular dynamics simulation were conducted on four selected compounds characterized from OP, with each compound representing one major chemical group. They

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are dioscin (representing the glycosides), β-sitosterol (representing the steroids), dioscorone

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A (representing the phenathrenes), and palmitic acid (representing the fatty acids and their derivatives). From the investigations, ΔN values suggested that all the phytoconstituents were involved in the inhibition process, while ΔE, μ, and Ebinding values showed that the reactivity

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of the phytoconstituents follow the trend: dioscin > dioscorone A > β-sitosterol > Palmitic acid, meaning that the glycosides played the most prominent inhibition roles while the fatty

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acids and their derivatives were the least effective.

Credit Author Statement Wilfred Emori:

Conceptualization, Writing - Original draft preparation

Run-Hua Zhang:

Investigation

24

Writing - Review & Editing, Validation

Xing-Wen Zheng:

Formal analysis

Tao He:

Investigation

Kun Wei:

Investigation

Xiu-Zhou Lin:

Supervision

Chun-Ru Cheng:

Conceptualization, Writing - Review & Editing

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Peter C. Okafor:

Declaration of interests

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Acknowledgements

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This work was supported by the Talent Introduction Funds of Sichuan University of Science

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and Engineering (No. 2018RCL13, and No. 2016RCL11), the Open Fund Research of Key Laboratory of Corrosion and Protection of Materials in Sichuan Province (No. 2018CL02),

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Chinese Academy of Sciences Visiting Professorship for Senior International Scientists (No. 2019VEA0030), the Scientific Research Fund of Sichuan Provincial Education Department

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(No. 18ZA0359), the Open Project of Key Laboratory of Green Chemistry of Sichuan Institutes of Higher Education (LZJ18202) and Key Laboratories of Fine Chemicals and Surfactants in Sichuan Provincial Universities (No. 2019JXZ02).

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n1

Rf (Ωcm2)

CPE2 (μFcm-2)

1.40 1.41 1.67 1.91 1.71 1.20 1.12 0.92 1.16

20.6 222.0 272.5 380.6 398.5 76.3 169.0 194.8 245.8

74.2 28.8 36.4 25.3 12.4 46.0 31.2 43.1 32.7

0.9 0.9 0.8 0.8 0.8 0.9 0.9 0.8 0.9

226.1 443.3 468.1 409.8 486.4 403.5 421.4 445.3 464.8

pr 1.6 35.3 18.7 26.2 35.2 14.5 26.2 10.0 17.8

n2

0.9 0.8 0.8 0.8 0.9 0.7 0.8 0.8 0.8

RT (Ωcm2)

246.7 665.3 740.6 790.4 884.9 479.8 590.4 640.1 710.6

η (%)

62.9 66.7 68.8 72.1 48.6 58.2 61.5 65.3

χ2 (x10-4)

6.6 4.7 2.1 0.3 0.7 3.2 3.2 0.4 0.3

na l

WP

CPE1 (μFcm-2)

Jo ur

OP

0.1 0.5 1.0 2.0 0.1 0.5 1.0 2.0

Rct (Ωcm2)

e-

0.0

Rs (Ωcm2)

Pr

C (g/L)

oo

f

Table 1 Electrochemical parameters for carbon steel in 1 M HCl solution without and with extracts of D. septemloba obtained from the fitting of EIS data.

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steel in 1 M HCl solution without and βa (mV/dec) 51 76 66 70 60 66 72 76 63

βc (mV/dec) 102 201 190 183 172 173 169 167 199

η (%) 81.7 84.9 87.1 89.2 73.1 78.5 80.6 82.8

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Table 2 Potentiodynamic polarization parameters for carbon with extracts of D. septemloba. Ecorr Icorr C (g/L) (mV/SCE) (μA/cm2) 0.0 -466.8 93 0.1 -428.5 17 0.5 -415.1 14 OP 1.0 -424.5 12 2.0 -422.7 10 0.1 -428.2 25 0.5 -442.1 20 WP 1.0 -443.6 18 2.0 -412.4 16

34

Rct (Ωcm2)

CPE1 (μFcm-2)

n1

Rf (Ωcm2)

CPE2 (μFcm-2)

n2

RT (Ωcm2)

χ2 (x10-4)

303 313 323 303 313 323

1.67 1.03 1.02 1.12 0.92 0.83

272.5 182.6 75.9 169.0 156.6 84.4

36.4 39.7 68.7 31.2 38.5 51.0

0.8 0.9 0.9 0.9 0.8 0.9

468.1 74.3 40.5 421.4 151.6 31.4

18.7 34.1 33.4 26.2 31.6 42.1

0.8 0.7 0.9 0.8 1.0 0.8

740.6 256.9 116.4 590.4 308.2 115.8

2.1 2.0 2.6 3.2 1.2 3.8

Pr

e-

pr

R

na l

WP

s T (K) (Ωcm 2 )

Jo ur

OP

oo

f

Table 3 Electrochemical parameters for carbon steel in 1 M HCl solution without and with 0.5 g/L extracts of D. septemloba obtained from the fitting of EIS data at different temperatures.

35

Uninhibited solution Solution with OP

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Solution with WP

64.6 205.0 360.5 83.7 86.7 96.3 96.9

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0 1 2 1 2 1 2

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Table 4 Potentiodynamic polarization parameters for carbon steel in 1 M HCl solution without and with 0.5 g/L extracts of D. septemloba at different temperatures. Ecorr Icorr βa βc T (K) 2 (mV/SCE) (μA/cm ) (mV/dec) (mV/dec) 303 -415.1 14 66 190 OP 313 -449.3 35 57 117 323 -449.0 66 59 112 303 -442.1 20 72 169 WP 313 -431.8 33 60 110 323 -451.4 42 55 94 Table 5 Surface roughness of carbon steel unexposed and exposed to 1 M HCl solution without and with extracts of D. septemloba at different times. System Time (h) Sq (nm)

36

EHOMO (eV)

ELUMO (eV)

ΔE (eV)

Μ (Debye)

I

Dioscin Dioscorone A β-sitosterol Palmitic acid

-4.980 -6.635 -6.413 -6.902

-0.932 -0.965 -0.349 -0.355

4.048 5.669 6.064 6.546

7.094 5.301 2.198 1.525

4.980 6.635 6.413 6.902

f

A

χ (eV)

σ (eV)

ΔN110

0.932 0.965 0.349 0.355

2.956 3.800 3.381 3.629

2.024 2.835 3.032 3.273

0.460 0.180 0.237 0.182

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na l

Pr

e-

pr

Molecule

oo

Table 5 Calculated quantum chemical parameters of some characterized compounds in OP.

37

Figure caption Fig. 1 Chemical structures of the compounds characterized from the organic phase extract of D. septemloba. Fig. 2 Impedance spectra for carbon steel in 1 M HCl solution without and with extracts of D. septemloba: (a) Bode plots of OP (b) Nyquist plots of OP (c) Bode plots of WP and (d) Nyquist plots of WP. Fig. 3 Potentiodynamic polarization plots for carbon steel in 1 M HCl solution without and with extracts of D. septemloba: (a) OP and (b) WP.

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Fig. 4 Temperature effect on the behavior of carbon steel in 1M HCl without and with extracts of D. septemloba: (a) Bode plots of OP (b) Nyquist plots of OP (c) polarization plots of OP (d) Bode plots of WP and (e) Nyquist plots of WP and (f) polarization plots of WP. Fig. 5 Time dependence of (a) corrosion rate and (b) inhibition efficiency for carbon steel in 1 M HCl solution without and with 0.5 g/L extracts of D. septemloba by gravimetric experiments.

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Fig. 6 SEM micrographs for carbon steel in 1 M HCl solution: (a) unexposed (b) uninhibited (c) inhibited by OP and (d) inhibited by WP. FTIR spectra for carbon steel in 1 M HCl with (e) OP and (f) WP.

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Fig. 7 (a) The optimized molecular structures (b) highest occupied molecular orbitals and (c) lowest unoccupied molecular orbitals of some characterized compounds in OP

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Fig. 8 (a) Side view and (b) top view of the equilibrium adsorption configurations of some compounds in OP on Fe (110) surface.

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