Bioelectrochemistry 130 (2019) 107332

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Bioelectrochemistry

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Detailed characterization of Phellodendron chinense Schneid and itsapplication in the corrosion inhibition of carbon steel in acidic media

Tao He a, Wilfred Emori b,c,⁎, Run-Hua Zhang b,c, Peter C. Okafor d, Min Yang e, Chun-Ru Cheng a,c,⁎⁎a School of Chemical Engineering, Institute of Pharmaceutical Engineering Technology and Application, Key Laboratory of Green Chemistry of Sichuan Institutes of Higher Education, SichuanUniversity of Science and Engineering, Zigong 643000, Sichuan, PR Chinab School of Materials Science and Engineering, Sichuan University of Science and Engineering, Zigong 643000, Sichuan, PR Chinac Key Laboratory of Material Corrosion and Protection of Sichuan Province, Zigong 643000, Sichuan, PR Chinad Corrosion and Electrochemistry Research Group, Department of Pure and Applied Chemistry, University of Calabar, P.M.B. 1115, Calabar, Nigeriae Centre for Characterization and Analysis, School of Physical Science and Technology, ShanghaiTech University, Shanghai, PR China

⁎ Corresepondence to: W. Emori, School of Materials ScUniversity of Science and Engineering, Zigong 643000, Sic⁎⁎ Correspondence to: C.R. Cheng, School of ChemPharmaceutical Engineering Technology and ApplicatChemistry of Sichuan Institutes of Higher Education, SicEngineering, Zigong 643000, Sichuan, PR China.

E-mail addresses: emoriental@yahoo.com (W. Emori),(C.-R. Cheng).

https://doi.org/10.1016/j.bioelechem.2019.1073321567-5394/© 2019 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 April 2019Received in revised form 12 July 2019Accepted 12 July 2019Available online 15 July 2019

Wepresent a combined experimental and theoretical study of the effective corrosion protection of carbon steel in1 M HCl solution by Phellodendron chinense Schneid (PCS) bark extract. Fourier-transform infrared spectroscopy(FTIR) and liquid chromatography tandem multi-stage mass spectrometry (LC-MSn) were employed for theextract characterization. The properties of PCS as a corrosion inhibitor were evaluated by electrochemical andgravimetric experiments. Quantum chemical calculation was used to describe the electronic and adsorptionproperties of the identified and characterized compounds found in the extract while molecular dynamics simu-lation was employed to predict the equilibrium configurations and binding energies of the compounds on thesteel surface. The electrochemical results revealed that PCS acted as a mixed-type corrosion inhibitor whose ef-ficiency increased with the extract concentration but slightly decreased with increasing temperature. Quantumchemical parameters, such as the energy difference (ΔE) and the number of transferred electrons (ΔN), wereused to predict the contribution of each characterized compound to the inhibition process while moleculardynamics simulation predicted parallel orientations for the configuration of the compounds and high binding en-ergies on the metal substrate.

© 2019 Elsevier B.V. All rights reserved.

Keywords:Carbon steelFTIRLC-MSSEMAdsorptionCorrosion inhibition

1. Introduction

Metals and alloys have been the most dependable materials formany industrial applications. Therefore one of the biggest challengesin industries is to devise methods for protecting these materials againstcorrosion as corrosion processes account for some of the various lossesexperienced in industries. Corrosion control of metals and alloys is anexpensive procedure and industries invest heavily to control this prob-lem. Due to the huge industrialization of developed countries, the costof corrosion usually takes up to 3–5% of their gross national product [1].

Many methods for corrosion reduction have been proposed withtheir effectiveness being judged on mitigation cost, labor cost and gen-eral return on investment. The use of inhibitors in controlling metal

ience and Engineering, Sichuanhuan, PR China.ical Engineering, Institute ofion, Key Laboratory of Greenhuan University of Science and

pharmaceutics@163.com

corrosion processes has been validated to be effective and achievable.Corrosion inhibitors are chemical substances which maybe either syn-thetic or natural and are very effective in small amounts by aiding thedecrease in the rate of metal deterioration in corrosive environmentswhen they are added to such environments [2].

The attention of corrosion inhibitor studies are aimed at three basicpaths: the first is to explore the potency of commonly known com-pounds, followed by the application of theoretical models and simula-tions for the synthesis of new compounds, and finally, studies of thesynergistic actions of different tested inhibitor compounds to expandthe range of inhibitor applications [3]. Until recently, most inhibitorcompounds were reported to be toxic to humans and the environmenteven when they yielded excellent inhibition data. The toxicity of theseorganic and inorganic inhibitors invoked environmental laws thatdiverted research attentions to non-toxic natural products inhibitors.This diversity has encouraged the appearance of many studies that usenatural products [4–18] as well as phytochemicals [19–21] as corrosioninhibitors.

Most of the studied plant parts as corrosion inhibitors have showngood results and these were attributed to the types and nature of or-ganicmolecules found in the inhibitormaterials, especially the reactions

2 T. He et al. / Bioelectrochemistry 130 (2019) 107332

of organic molecules containing heteroatoms such as oxygen, sulfur, ni-trogen and phosphorus as they can easily be adsorbed on the surface ofmetals, examples are nitrogen [22], sulfur [23–25], oxygen [23], phos-phorus [26], etc.

Traditional Chinese medicine has become recently popular world-wide because of their successes in curing and treating many diseases.Most of the herbs employed contain large amounts of antioxidant andbioactive phytochemical species and they function by the actions ofthese species. Since corrosion is an oxidation process, it will thereforebe interesting to explore other functions for the herbs, especially theuti-lization of the least popular parts of these herbs. Our current plan is tostudy the corrosion inhibition actions of Phellodendron chinense Schneid(PCS) extract, one of the 50 fundamental herbs in traditional Chinesemedicine. PCS is extensively cultivated in Southwest China. Some phy-tochemicals that are produced from this herb are: berberine (whichhas antibacterial and antifungal abilities) [27–29], jatrorrhizine (whichhas anti-mutagenic properties) [30], phellodendrine (which is an im-mune suppressant) [31–33] and palmitine (a vasodilator) [34,35]. Inthis research, PCS bark extract has been investigated and characterizedfor the purpose of corrosion studies in order to understand its natureand adsorption on metal surfaces. To the best of our knowledge, thereis no report relating to the use of PCS in corrosion studies. Being readilyavailable, cost effective and biodegradable, the inhibition study of PCS isexpected to benefit the corrosion protection of steel materials in acidenvironments.

2. Experimental

2.1. Materials

Corrosion experiments were performed on carbon steel specimenswith composition (wt%): C(0.23), Mn(0.79), P(0.02), S(0.03), Cu(0.29), Si(0.20), and Fe(balance). The coupons for gravimetric experi-ments and surface analysis had dimensions of 2 cm by 1 cm by 0.2 cmwith a total surface area of 5.2 cm2 while the coupons for electrochem-ical measurements were soldered to a copper wire and embedded inepoxy resin leaving an exposed area of 1 cm2. Before the experiments,the surfaces were polished with emery paper of increasing fineness,washed with distilled water, cleaned with ethanol in an ultrasonicbath, and air-dried. The corrosive mediumwas 1 M HCl which was pre-pared from 36.5% HCl (analytical grade, supplied by ChongqingChuandong Chemical Group Co., Ltd.) diluted with double distilledwater. The PCS bark was obtained from Sichuan Province, China.

2.2. Preparation of the PCS extract and the test solutions

1 kg bark of PCS was dried and finely powdered. The powder wasrefluxed three times with 75% ethanol (8 L × 3) and the extracted eth-anol solution was filtered and evaporated to dryness under vacuum toafford the crude extract (180 g). 10 g of the crude extract was digestedin 1 L of 1 M HCl and estimated to be 10 g/L. The resultant solution wasallowed to stand for 24 h, and then filtered and labeled as the stock in-hibitor solution. The inhibitor test concentrations (0.5 g/L, 1.0 g/L,2.0 g/L, and 4.0 g/L) were subsequently prepared from the stocksolution.

2.3. Techniques

2.3.1. Characterization of PCS extractThe PCS extract was first characterized by Fourier-transform infra-

red spectroscopy (FTIR) (PerkinElmer, MIT, USA) using the KBr disktechnique. The FTIR instrument was connected with Omnic software,which extended from 400 to 4000 cm−1. The main constituents of thePCS extract was then characterized by Liquid chromatography tandemmulti-stagemass spectrometry. A LTQ Orbitrap Elite mass spectrometry(Thermo Fisher Scientific, San Jose, CA, USA)was connected to the HPLC

system (Agilent 1260 G1312) via an ESI ion source as interface. Themass spectrometer was controlled by Xcalibur 2.2 SP1 software(Thermo Fisher Scientific, San Jose, CA, USA). The LTQ Orbitrap Elitewas operated in both positive and negative heating electrospray ioniza-tionmode. Themass spectrometry ion source parameterswere adjustedas follows: For the positive polarity: electrospray voltage of 3.5 KV, ionsource heater temperature of 300 °C and flow rates of 45, 5 and 0 arbi-trary units for the sheath, auxiliary and sweep gas, respectively; For thenegative polarity: electrospray voltage of 3.2 KV, ion source heater tem-perature of 300 °C and flow rates of 45, 5 and 0 arbitrary units for thesheath, auxiliary and sweep gas, respectively. The S-lens RF level wasset to 60%. Three different settings were applied. The search for molec-ular ions of the constituents was performed in full scan mode, rangingfrom m/z 100–1200 at a resolution of 60,000 FWHM. For confirmationof the identities of the chemical constituents MS2 and MS3 exact massfragmentation of the chemical constituentswere performed. The energyfor collision-induced dissociation (CID) was set at 35 eV, with an isola-tion width of m/z 2. The fragmentation spectra were collected withinm/z 100–1200 at a resolution of 30,000 FWHM. Dynamic exclusionwas enabled. The mass spectrometer was externally calibrated from100 to 1200 using a Calibration solution (Thermo Fisher Scientific).The HPLC conditions for LC/MS analysis were as follows: An AgilentZorbax SB-C18 column (rapid resolution HD 2.1 × 100 mm, 1.8 μm)was used at room temperature. A linear gradient elution of solvent A(ultra-pure water containing 0.1% formic acid) and solvent B (acetoni-trile) was applied with the following program: 0–10 min, 10–22%;10–18 min, 22–32%; 18–23 min, 32–55%; 23–28 min, 55–80%. A pre-equilibration period of 15 min was used between individual runs. Theflow rate was 0.3 mL/min, and injection volume was 2 μL.

2.3.2. Electrochemical experimentThe setup for electrochemical corrosion experiment consisted of a

250 mL glass cell, water bath with temperature controller, Potentiostat(Solartron SI 1287 Electrochemical interface/Solartron SI 1260 Imped-ance gain-phase analyzer) and a three-electrode corrosion system(counter electrode, CE: platinum foil; reference electrode, RE: saturatedcalomel reference electrode (SCE); working electrode, WE: carbonsteel). All potentials in this work were measured versus SCE. Open cir-cuit potential (OCP) measurements were conducted for 1800 s to en-sure stabilization of the systems. The electrochemical impedancespectroscopy (EIS) measurements were conducted at OCP with a sinu-soidal potential perturbation of 10 mV in a frequency range from100 kHz to 10 mHz, and the obtained EIS spectra were fitted using theZSimpWin software. The potentiodynamic polarization experimentswere performed in the potential range of ±0.25 V vs. OCP at 0.5 mV/sscan rate.

2.3.3. Gravimetric experimentClean carbon steel couponswereweighed and completely immersed

in 250 mL beakers containing unstirred test solutions. They wereallowed to stand in the test solutions for 24 h. To study the effect ofPCS concentration on the corrosion process of carbon steel, the test sys-temswere maintained at 303 ± 1 Kwhile for the evaluation of temper-ature effect, the system was maintained at 0.5 g/L PCS concentration.After the exposure time, the coupons were retrieved from the test solu-tions and transferred into a pickling acid solution (containing 500 mLHCl + 500 mL double distilled water +3.5 g hexamethylenetetramine)placed in an ultrasonic bath for 600 s to chemically remove all surfacecorrosion products. The cleaned coupons were then washed with dou-ble distilled water, dipped in acetone, air-dried, and reweighed. Theweight loss of the carbon steel couponswas calculated as the differencein weight before and after exposure to the test solutions. The reporteddata are the mean values obtained after triplicate experiments. Usingexpressions from literature, the weight loss was used to calculate the

3T. He et al. / Bioelectrochemistry 130 (2019) 107332

corrosion rate [36] and inhibition efficiency [37] as follows:

Corrosion rate;CR mmy−1� �

¼ 87600�Weight loss gð ÞArea cm3ð Þ � density gcm−3ð Þ � time hð Þ

ð1Þ

Inhibition efficiency;η %ð Þ ¼ 1−CRPCS

CRblank

� �� 100 ð2Þ

where CRPCS and CRblank are the calculated corrosion rates for theinhibited and uninhibited solutions, respectively.

2.3.4. Surface analysisScanning electron microscope (SEM) examination (TESCAN 3SBU)

was used for the morphological studies of the surfaces of carbon steelcoupons unexposed and exposed to 1 M HCl solutions without andwith 0.5 g/L PCS. The coupons were exposed to the test solutions for24 h at 303±1 K. On retrieval, theywere gently rinsedwith double dis-tilled water, dipped in acetone followed by drying with warm air.

3. Results and discussion

3.1. Characterization of PCS extract

3.1.1. Characterization of PCS extract by FTIRThe FTIR spectrum of PCS extract is shown in Fig. S1. The absorption

peak at 3400 cm−1 could be attributed to the O\\H stretching vibrationwhile the peak at 3012 cm−1 could be associated with the C\\Hstretching vibration of aromatic rings. The absorption peaks at2854 cm −1 and 2926 cm−1 could be attributed to the C\\H stretchingvibration of CH3. The absorption peak at 1698 cm−1 could be due toC_O stretching vibration. The peaks at 1602 cm−1 and 1510 cm−1

could be assigned to C_C stretching vibration. 1602 cm−1 could alsobe associated with C_N+. The peaks at 819 cm−1 and 770 cm−1 arepossibly due to C\\H or C_C bending vibrations of benzene ring and

Fig. 1. Chemical structures of the compounds

aromatic compound. The FTIR results illustrate that the extract of PCScontains oxygen and nitrogen atoms in functional groups (such asC_O, O\\H, C_N) and aromatic rings. These are generally assumed toplay important roles in corrosion inhibition as described in literature[22,23].

3.1.2. Characterization of the main constituents of PCS extract by Liquidchromatography-tandem multi-stage mass spectrometry

The LC-MSn spectra of the PCS extract is presented in Fig. S2.The mass spectra were obtained in both negative (mainly forphenylpropanoids) and positive (mainly for alkaloids) ionizationmode. Based on the comparison of the accurate MS, MS2, and MS3

data with those reported in literature [38,39], 14 main compoundswere unambiguously identified. The chemical structures of the identi-fied compounds are shown in Fig. 1. Among them, neo-chlorogenicacid, 3-O-feruloylquinic acid, and 5-O-feruloylquinic acid, which arephenylpropanoids, were analyzed in negative ion mode. The othercompounds identified were alkaloids, which are very typical in thePCS. Detailed data of the compounds are listed in Table 1. For simplicityin further discussions, short codes were assigned to each compound asshown in the table. All of these compounds were organic compoundscontaining aromatic ring and C_O, O\\H, C\\O, and C_C functionalgroups, all of which were also observed in the FTIR test. Therefore, theadsorption of these compounds on themetal surfacewould be assumedto be mostly responsible for the inhibition of the corrosion reaction.

3.2. Electrochemical measurements

3.2.1. Electrochemical impedance spectroscopy measurementsBefore all electrochemical measurements, the carbon steel speci-

menswere immersed in 1MHCl without andwith PCS for 1800 s to at-tain stable OCPs. The importance of studying the OCPs was to assess thestability of the carbon steel specimen in the corrosive environment. Itcan be observed from Fig. S3 depicting the potential-time curves thatthe specimen achieved stable potentials after 1800 s. The introduction

of PCS characterized by LC-MS analysis.

Table 1Identified compounds contained in PCS extract by LC-MS analysis.

Compound t(min)

Identification(with short codes)

[M ± H]±;[M]+(m/z)

Elementcomposition

MS2 data (m/z) (% base peak) MS3 data (m/z) (% base peak)

1 3.00 Phellodendrine(PDD)

342.1713 C20H24NO4 177.0553 (5), 163.0396 145.0289, 135.0445 (20)

2 3.65 Neo-chlorogenic acid(NCA)

353.0879⁎ C16H18O9 271.0614, 245.0821, 205.0508 (30), 179.0351 (28) 227.0715 (28), 203.0715, 187.0401(30), 161.0609 (20)

3 4.08 Magnoflorine(MFL)

342.171 C20H24NO4 311.1288 (20), 299.1287 (20), 297.1131, 279.1024(10), 265.0868 (25)

282.0896 (10), 265.0868

4 7.74 3-O-feruloylquinicacid(3FQA)

367.1033⁎ C17H20O9 264.7128 (10), 191.0563, 173.0458 (5) 173.0456 (75), 171.0300 (5), 127.0402,85.0297 (95)

5 8.02 Menisperine(MSR)

356.1868 C21H26NO4 265.0791 (10), 264.0791, 248.0941 (20) 247.0761 (10), 236.0839, 234.0682 (20)

6 9.28 5-O-feruloylquinicacid(5FQA)

367.1034⁎ C17H19O9 193.0507 (5), 191.0562 173.0457 (75), 171.0300 (25), 127.0402(70), 85.0297

7 10.28 Tetrahydropalmatine(THP)

356.1866⁎⁎ C21H26NO4 192.1025 177.0789

8 10.52 Demethyleneberberine(DMB)

324.1237 C19H18NO4 309.1006, 308.0928 (10), 280.0977 (15) 306.0768 (40), 292.0947 (5), 280.0947

9 10.89 Oxyberberine(OBB)

352.1190⁎⁎ C20H18NO5 337.0958, 336.0881 (5), 308.0930 (10) 336.0877 (40), 318.0771 (10), 308.0927

10 12.85 Columbamine(CLB)

338.1396 C20H20NO4 338.1397 (15), 323.1161, 322.1084 (10), 294.1134(10)

322.1081 (30), 307.0846 (60),294.1132, 279.0896 (5)

11 12.91 Berberrubine(BBRB)

322.1086 C19H16NO4 307.0851 307.0843 (90), 279.0893

12 13.14 Jatrorrhizine(JTRZ)

338.1395 C20H20NO4 338.1394 (15), 323.1158, 322.1082 (10), 294.1131(15)

322.1084 (35), 307.0805 (50), 294.1135

13 15.20 Berberine(BBR)

336.1241 C20H18NO4 321.1008, 320.0931 (10), 292.0981 (15) 320.0928 (45), 292.0978

14 15.50 Palmatine(PMT)

352.1554 C21H22NO4 337.1318, 336.1239 (10), 308.1290 (10) 336.1244 (50), 321.1008 (40), 308.1294

The bold fonts in the MS2 data column indicate the base peaks of the spectrum.⁎ Quaimolecular ion peaks at m/z [M – H] − (negative ESI mode).⁎⁎ Quaimolecular ion peaks at m/z [M+ H] +(positive ESI mode).

4 T. He et al. / Bioelectrochemistry 130 (2019) 107332

of PCS caused a shift of the potential towards the positive directionwhen compared with the curve for the uninhibited system. Further-more, the magnitude of the potential shift to positive values dependedon the concentration of PCS. The potential increased progressivelyuntil a maximum corresponding to 4.0 g/L PCS (the highest concentra-tion of the present study). The maximum potential displacement was77 mV. These preliminary observations suggest that PCS can restrainboth the oxidation reaction of oxide-free ions and evolution of hydrogenions leading to the formation of hydrogen gas on the carbon steel sur-face. Furthermore, the positive potential shift with increasing PCS

Fig. 2. Impedance spectra for carbon steel in 1 M HCl solution without andwith PCS: (a) Bode a1.0 g/L, blue down triangle – 2.0 g/L, purple diamond – 4.0 g/L, black line – fitting line).

concentration indicates the solidness of the surface layer in thepresenceof inhibitors [40].

The electrochemical processes at the interface between the carbonsteel specimen and the solution without and with PCS are describedby the EIS measurements. The data are presented as Bode and Nyquistplots in Fig. 2. The Bode plots show increasingmagnitude of impedancewith PCS concentration which tends to zero at high frequencies. Thephase angle plots show the existence of a one-time constant and alsoportray an increasing relationship with PCS concentration with maxi-mum values recorded for 4.0 g/L PCS. These observations are related

nd (b) Nyquist plots. (Legend: black square – 0 g/L, red circle – 0.5 g/L, green up triangle –

Fig. 3. Potentiodynamic polarization plots for carbon steel in 1MHCl solutionwithout andwith PCS. (Legend: black square – 0 g/L, red circle – 0.5 g/L, green up triangle – 1.0 g/L, bluedown triangle – 2.0 g/L, purple diamond – 4.0 g/L).

Table 3Potentiodynamic polarization parameters for carbon steel in 1 M HCl without and withPCS.

C (g/L) Ecorr (mV/SCE) Icorr (μA/cm2) βa (mV/dec) βc (mV/dec) η (%)

0.0 −466.8 ± 2.0 93.2 ± 2.1 47.1 ± 2.9 213.0 ± 3.90.5 −487.6 ± 1.2 15.9 ± 1.1 45.4 ± 2.1 149.4 ± 4.3 82.81.0 −444.6 ± 2.6 11.3 ± 1.4 51.3 ± 3.2 157.5 ± 2.0 88.22.0 −436.6 ± 3.0 9.0 ± 0.9 99.6 ± 2.8 207.8 ± 3.1 90.34.0 −410.5 ± 4.9 7.1 ± 1.0 109.3 ± 3.7 253.2 ± 4.5 92.5

5T. He et al. / Bioelectrochemistry 130 (2019) 107332

to the resistance against penetration of corrosive media by the blockingof the active sites by PCS molecules [20,41]. The Nyquist plots (Fig. 2b)show a depressed semi-circle with one shoulder and a one-time con-stant over the range of studied frequencies as reported for the phaseangle plots. To determine the number of time constants in an electro-chemical system, the number of distinguishable maxima (from thephase angle plots) and related shoulders (from the Nyquist plots) isusually taken into consideration [42]. As with the Bode plots, theNyquist plots also depict a continuous increase in corrosion resistancewith PCS concentration by the increase in the diameter of the semi-circle loops.

The observed depressed semi-circle is a clear deviation from idealityand could be the result of the surface inhomogeneity and roughness. Inthis regard, a constant phase element (CPE) was used to depict the non-ideal capacitive behavior in the equivalent circuit used to fit the exper-imental data (inset of Figs. 2b and 5b). From the equivalent circuitmodel, R(Q(R(QR))), Rs is the resistance of the solution, Rct is the chargetransfer resistance, Rf is the film resistance, CPE1 and CPE2 are the con-stant phase element of the double layer and film capacitance, respec-tively. The fitting data are presented in Table 2 and the validity of thedata was confirmed by the chi-square values. From the table, n is asso-ciated with the degree of surface inhomogeneity of the interface, RT isthe sum total of Rct and Rf, and η is the calculated inhibition efficiencybased on the following expression:

Inhibition efficiency;η %ð Þ ¼ 1−RT;blank

RT;PCS

� �� 100 ð3Þ

where RT, blank and RT, PCS are the corresponding RT values for the solu-tion without and with PCS, respectively. Table 2 reveals an increase inRT with the introduction of PCS and the increase continued with in-crease in PCS concentration. This means that PCS increased the protec-tiveness of the metal surface by increasing the resistance and thuscaused a reduction of carbon steel corrosion in the medium. Also, thetable shows a decreasing trend of CPE values with PCS concentration.Deng and Li [43] explained that the value of CPE is related to both thedielectric constant and the thickness of the electric double layer. Thesefactors give insights on the potential presence of water molecules andtheir displacement by organic molecules. In their view, decrease inCPE is a consequence of either the decrease in the dielectric constantor the increase in the electric double layer and sometimes, a combinedeffect of both factors. The displacement of water molecules is possibleduring adsorption of PCS due to the lower dielectric effect of organicspecies [44]. Therefore, from the data in Table 2, PCS caused a corrosioninhibition of carbon steel in the electrolyte medium and the inhibitionefficiency increased with concentration up to 92% corresponding to4.0 g/L PCS.

3.2.2. Potentiodynamic polarization measurementsThe results of potentiodynamic polarization measurements con-

ducted for carbon steel in 1 M HCl solution without and with PCS arepresented in Fig. 3. Consequently, the values of anodic Tafel slope (βa),cathodic Tafel slope (βc), corrosion current density (Icorr), and corrosionpotential (Ecorr) were extrapolated from the fitting of the linear Tafelsegments of the potentiodynamic polarization curves and presented inTable 3. From Fig. 3, the depicted corrosion potentials for carbon steel

Table 2Electrochemical parameters for carbon steel in 1 M HCl without and with PCS obtained from th

C (g/L) Rs (Ωcm2) Rct (Ωcm2) CPE1 (μFcm−2) n1 Rf (Ωcm2)

0.0 1.40 ± 0.04 10.6 ± 1.0 74.0 ± 5.6 0.88 ± 0.04 144.8 ± 3.50.5 1.85 ± 0.13 46.6 ± 2.1 9.9 ± 2.1 0.98 ± 0.07 776.8 ± 2.91.0 1.63 ± 0.08 114.9 ± 4.6 18.2 ± 2.2 0.90 ± 0.05 998.6 ± 7.22.0 1.57 ± 0.17 122.1 ± 2.0 7.9 ± 1.5 0.98 ± 0.05 1076.7 ± 3.4.0 1.42 ± 0.11 478.7 ± 6.8 13.8 ± 1.8 0.90 ± 0.08 1468.0 ± 5.

in the uninhibited and inhibited systems did not change to a large ex-tent as the potential remained relatively around the same value.Table 3 shows that the variation between the potentials with respectto that of the uninhibited system is less than ±85 mV as described byliterature [45–47]. It is also clear that the anodic curve (representingthe iron dissolution reaction) and cathodic curve (representing the hy-drogen evolution reaction) were affected by the addition of PCS with ashift to more negative directions. The inhibitor caused a reduction inthe current densities of both reactions and this reduction increasedwith PCS concentration. These observations show that the inhibitionof carbon steel corrosion by PCS was due to geometric blocking effect[48] and PCS exhibited a mixed-type inhibition mechanism. Inhibitionefficiency was calculated from Icorr using the following expression:

Inhibition efficiency;η %ð Þ ¼ 1−icorr;PCSicorr;blank

� �� 100 ð4Þ

where icorr, PCS and icorr, blank represent corrosion current densities in thepresence and absence of PCS, respectively. The calculated values showgood inhibition effects up to a maximum value of 91.4% for 4.0 g/L PCSconcentration. These results are in agreement with the general trenddescribed by the EIS measurements.

e fitting of EIS data.

CPE2 (μFcm−2) n2 Chi-square (×10−3) RT (Ωcm2) η (%)

155.0 ± 4.8 0.89 ± 0.06 2.0 ± 1.4 155.471.7 ± 5.1 0.73 ± 0.04 4.0 ± 0.8 823.4 81.159.0 ± 3.3 0.73 ± 0.07 4.0 ± 1.1 1113.5 86.0

3 44.3 ± 1.9 0.67 ± 0.05 3.0 ± 0.9 1198.8 87.04 40.6 ± 2.1 0.60 ± 0.08 0.4 ± 0.1 1946.7 92.0

Fig. 4. Temperature dependence of corrosion rate by gravimetric experiments for carbonsteel in 1 M HCl without and with PCS. (Legend: White – 0 g/L, red – 4.0 g/L).

Fig. 6. SEMmicrographs for carbon steel unexposed to (a) exposed to uninhibited (b) andexposed to inhibited (c) 1 M HCl solution.

6 T. He et al. / Bioelectrochemistry 130 (2019) 107332

3.3. Gravimetric results

Weight loss measurements were used to calculate the corrosionrates of carbon steel in 1MHCl without andwith PCS to evaluate the ef-fect of PCS concentration. From the calculated corrosion rates, the inhi-bition efficiencies were calculated. Fig. S4 shows the plot for corrosionrate and inhibition efficiency against concentration and it reveals thatthe corrosion rate was highest in the uninhibited solution.With the ad-dition of 0.5 g/L PCS, the corrosion rate was reduced by about seventimes the value for the uninhibited solution. Although to smaller de-grees, increase in PCS concentration led to a continuous decrease inthe corrosion rates of carbon steel. Resultantly, the inhibition efficiencyranged between 86.4% and 92.6%, corresponding to 0.5 g/L and 4.0 g/LPCS, respectively. Table S1 displays the data obtained from gravimetricexperiments. The fraction of the surface covered by the adsorbed PCSmolecules (θ) was calculated from the inhibition efficiency by the fol-lowing expression:

θ ¼ ηPCS

100ð5Þ

Fig. 5. Impedance spectra for carbon steel in 1 M HCl solution containing 4.0 g/L PCS at differen313 K, green up triangle – 323 K, blue down triangle – 333 K, black line – fitting line).

ηPCS denotes the inhibition efficiency of PCS. The values of θ increasedwith PCS concentration. This means that as the concentration of PCS in-creased, the available corrosion sites on the surface of the carbon steelcoupons were covered by the inhibitor molecules, thus limiting the

t temperatures: (a) Bode and (b) Nyquist plots. (Legend: black square – 303 K, red circle –

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

7T. He et al. / Bioelectrochemistry 130 (2019) 107332

attack of the corrosion species from the aggressive environment to themetal surface.

3.4. Effect of temperature

To evaluate the effect of temperature on the corrosion inhibition ofPCS, weight loss data for carbon steel in 1 M HCl solution without andwith 4.0 g/L PCS were obtained for test carried out in the temperaturerange 303–333 K. Fig. 4 shows that corrosion rate increased steadily astemperature was raised for both the uninhibited and inhibited systems

but at greater magnitudes for the uninhibited system. Moreover, whencompared with data obtained for the uninhibited system at differenttemperatures, PCS displayed good inhibition properties with values of86.4, 84.7, 85.1, and 82.1% for 303, 313, 323, and 333 K, respectively.

Corresponding evaluations were carried out by electrochemicalmeasurements at the same temperature range for the solution contain-ing 4.0 g/L PCS. The polarization measurements show that both the an-odic and cathodic parts of the polarization curves increased to morepositive values as temperature was raised (Fig. S5) and the observationwas validated with increase in icorr values from Table S2, hence,

Fig. 7 (continued).

8 T. He et al. / Bioelectrochemistry 130 (2019) 107332

suggesting that temperature initiates a reduction in the inhibitionability of PCS and increases corrosion rate of carbon steel. The Bodeand Nyquist plots of EIS measurements also gave indications of thecorrosion inducing effect of temperature (Fig. 5). The magnitude ofimpedance, phase angle value and the diameter of the Nyquist semi-circle decreased progressively as temperature was raised. These obser-vations point to the direction of a reduction in the resistance of theworking electrode to corrosion with increase in temperature. Thetrend of increasing CPE values and decreasing RT values with temper-ature from the fitting results of the EIS data (Table S3) confirms theseobservations.

3.5. Surface analysis

The surface morphology of carbon steel coupons unexposed and ex-posed to uninhibited and inhibited 1 M HCl solution after 24 h immer-sion at 303 ± 1 K were investigated using SEM and the results arepresented in Fig. 6. The figure shows that the surface was uniformlycorroded in the uninhibited HCl solution and the attack was severe.However, a layer was noticeably present on the surface of carbon steelfrom the inhibited HCl solution. This significantly reduced the corrosiveattack of the solution. This could possibly be the protective layer offeredby the inhibition of PCS.

Table 4Calculated quantum chemical parameters of the characterized compounds in PCS extract.

Compound EHOMO (eV) ELUMO (eV) ΔE (eV) I = −EHOMO A = −ELUMO χ (eV) ϕ (eV) σ (eV−1) ΔN110

THP −4.594 −2.451 2.143 4.594 2.451 3.523 1.071 0.933 0.605PMT −4.483 −1.774 2.709 4.483 1.774 3.129 1.355 0.738 0.624BBRB −4.654 −1.900 2.754 4.654 1.900 3.277 1.377 0.726 0.560JTRZ −4.731 −1.794 2.937 4.731 1.794 3.263 1.469 0.681 0.530PDD −4.864 −1.679 3.185 4.864 1.679 3.272 1.592 0.628 0.486BBR −5.104 −2.757 2.347 5.104 2.757 3.930 1.173 0.852 0.379MFL −5.482 −2.134 3.348 5.482 2.134 3.808 1.674 0.597 0.302DMB −5.111 −2.739 2.372 5.111 2.739 3.925 1.186 0.843 0.377OBB −5.472 −1.626 3.846 5.472 1.626 3.549 1.923 0.520 0.330CLB −5.684 −1.538 4.146 5.684 1.538 3.611 2.073 0.482 0.2925FQA −6.049 −1.706 4.344 6.049 1.706 3.878 2.172 0.460 0.217NCA −5.876 −1.068 4.808 5.876 1.068 3.472 2.404 0.416 0.2803FQA −6.542 −1.548 4.994 6.542 1.548 4.045 2.497 0.400 0.155MSR −6.607 −1.155 5.452 6.607 1.155 3.881 2.726 0.367 0.172

9T. He et al. / Bioelectrochemistry 130 (2019) 107332

3.6. Computational studies

3.6.1. Quantum chemical calculationAn examination of themolecular properties of the compounds iden-

tified in PCS has been carried out by quantum chemical calculations. Theaim of the calculations was to provide theoretical support to the exper-imental results by revealing the relation between the molecular struc-tures of the PCS molecules and their electronic properties [49]. Densityfunctional theory (DFT) calculations were done by the Gaussian 09 Wpackage. Fig. 7 presents the optimized molecular structure, highest oc-cupiedmolecular orbital (HOMO), and lowest unoccupiedmolecular or-bital (LUMO) of the identified compounds in PCS. Since the adsorptionof organic species on metallic surfaces is reported to be through adonor-acceptor relationship [50,51], it is necessary to discuss theHOMO and LUMO which represent the ability of a molecule to donateand accept electrons, respectively. The values of the quantum chemicalparameters consisting of the energy of highest occupied molecular or-bital (EHOMO), energy of lowest unoccupied molecular orbital (ELUMO),energy difference (ΔE), electronegativity (χ), global hardness (γ), soft-ness (σ), and the number of transferred electrons (ΔN) are presentedin Table 4. High values of EHOMO and low values of ELUMO are associatedwith higher electron donating and accepting abilities of the molecules,respectively, which describes the bonding between the inhibitor mole-cules and the metal surface [52]. The energy difference (ΔE) betweenthe EHOMO and ELUMO is a significant parameter for characterizing the re-activity of inhibitormolecules. It describes the ability of themolecules toattach to the surface of metal. The larger the value of ΔE, the harder theinhibitor molecule and the lower its reactivity while small values of ΔEare associated with soft molecules which translates to a higher reactiv-ity and lower kinetic stability [52]. Therefore, it is expected that themol-ecules with lower values of ΔE in Table 4 contributed more to thegeneral inhibition efficiency of PCS. Saha et al. [53] showed that ioniza-tion potential (obtained from EHOMO), and electron affinity (obtainedfrom ELUMO) were important parameters needed to calculate the valuesof χ, γ, andσ, which are useful in obtainingΔN. In depicting the numberof transferred electrons, ΔN shows the ability of the molecules to inter-actwith the d-orbitals of themetal. Calculation ofΔNwas by applicationof the Pearson method as shown:

ΔN ¼ W−χinh

2 γFe þ γinhð Þ ð6Þ

where W represent the DFT derived work function of Fe (110) with avalue of 4.82. This is the preferred Fe surface for this study because it of-fers a higher stabilization energy and packed surface [53]. Electrontransfer takes place between an inhibitor molecule and a metal surfacewhen ΔN N 0. Also, the likelihood of electron donating ability of an in-hibitor molecule is increased when ΔN b 3.6 [54,55]. All the calculatedvales of ΔN are shown to be both positive and b3.6 (Table 4). This

implies that the molecules have high tendencies to donate electrons tothe metal surface. From the observed trend of ΔE and ΔN, we can as-sume that THP, PMT, BBRB, and JTRZ were the largest contributors tothe inhibition ability of PCS for carbon steel in 1 M HCl. Nevertheless,the table revealed that the inhibition of PCS was as a result of contribu-tions from all the compounds. Table 4 depicts the chemical constituentsin order of their predicted reactivity as suggested by the values of theΔE.

3.6.2. Molecular dynamics simulation (MDS)To obtain clearer theoretical insights on the interactions between

the compounds of PCS and the iron surface, Molecular dynamics simu-lation (MDS)was performed on some selected compounds based on re-sults fromTable 4.MDSmethodology is effective in thedetermination ofthe vertical and horizontal representations of an inhibitor configuration,as well as the energy parameters associated with the interaction be-tween the inhibitor at the metal substrate [56–60]. Consequently, theequilibrium adsorption configurations of the compounds of PCS on theiron surface are presented in Fig. 8. By careful examination, it is ob-served that all the compounds adsorbed in a parallel orientation onthe iron surface, signifying the tendency of steel protection by the inhib-itor compounds. This is possible by the decrease in the contact areaavailable for attack by the aggressive corrosion medium. Additionally,the energy parameters presented in Table 5 were obtained from the fol-lowing expressions [61]:

Einteraction ¼ Etotal− Esubstrate þ Einhibitorð Þ ð7Þ

Ebinding ¼ −Einteraction ð8Þ

where Einteraction is the interaction energy between the each compoundof PCS and the iron substrate, Etotal defines the total energy of the sys-tem, Esubstrate is the energy of iron substrate, inclusive of water mole-cules, Einhibitor gives the inhibitor energy of each PCS compound, Ebindingis the binding energy of each compound. Table 5 shows high negativeinteraction energies and their corresponding positive binding energiesfor the compounds, which signifies spontaneous interactions [52],therefore good adsorption properties. The general trend for the bindingenergies is in agreement with the results of quantum chemical calcula-tions (Table 4), except for slight changes between the pairs of JTRZ andPDD, andMFL andDMB. Although all the tested compounds show inhib-itive tendencies, the results predict that THP may possibly be the mostfirmly adsorbed with Ebinding value of 266.10 kJmol−1 while OBB with208.39 kJmol−1 would be the least adsorbed. Most studies argue a spe-cific compound in a plant extract as exclusively responsible for the inhi-bition ability of the extract. This in the real sense is not true, since mostplant extracts are composed of numerous organic compounds capableof either inhibiting or promoting the corrosion process. The net (antag-onistic and synergic) action of thephytoconstituents of the plant iswhat

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

10 T. He et al. / Bioelectrochemistry 130 (2019) 107332

Fig. 8 (continued).

11T. He et al. / Bioelectrochemistry 130 (2019) 107332

is actually recorded as the inhibition efficiency of the plant extracts [62].From the characterization results, ethanolic extract from the bark ofPhellodendron chinense Schneid contains myriads of organic com-pounds. Therefore it is difficult to assign the observed corrosion effectto a particular constituent. However, from themolecular dynamics sim-ulation, it is assumed that THP, being themost firmly adsorbed, contrib-utes significantly to the adsorbed organic compounds responsible forthe inhibitive effects of the extract.

4. Conclusions

The corrosion inhibition potency of the extract of Phellodendronchinense Schneid (PCS), a well-known traditional Chinese medicine,

has been evaluated on carbon steel in 1 MHCl solution.While FTIR per-mitted the identification of the functional groups present in the extract,LC-MSn was useful for the specific identification of the 14 main com-pounds present in the extract. These compounds were mainlyphenylpropanoids and alkaloids. Electrochemical andweight loss corro-sion experiments revealed that the inhibition ability of PCSwas concen-tration and temperature dependent with the maximum inhibitionefficiency recorded for the highest concentration of the study whilethe efficiency was reduced when temperature increased. However, re-sults comparison between the uninhibited system and the system for4.0 g/L PCS at different temperatures shows a good inhibition trend.PCS acted as amixed-typed corrosion inhibitor affecting both the anodicand cathodic reactions as revealed by the potentiodynamic polarization

Table 5Interaction and binding energies for some active compounds in PCS extract on Fe (110)surface.

Compound Einteraction (kJmol−1) Ebinding (kJmol−1)

THP −266.10 266.10PMT −259.10 259.10BBRB −249.05 249.05JTRZ −236.47 236.47PDD −236.87 236.87BBR −216.88 216.88MFL −210.78 210.78DMB −211.58 211.58OBB −208.39 208.39

12 T. He et al. / Bioelectrochemistry 130 (2019) 107332

results. Micrographs of SEM indicated the presence of adsorbed specieson the surface of carbon steel coupons. Quantum chemical calculationsperformed on the identified compounds revealed their electron donat-ing and accepting properties necessary for corrosion inhibition whileMDS showed that the tested compoundswere absorbed in a parallel ori-entation on the surface of the metal and exhibited high binding ener-gies. In general, the results of the computational studies were inagreement with those from experiments and they confirm that PCScan be applied as a corrosion inhibitor for steel in acidic environments.

Acknowledgements

This work was supported by the Talent Introduction Funds ofSichuan University of Science and Engineering (No. 2018RCL13 andNo. 2017RCL61), the Scientific Research Fund of Sichuan ProvincialEducation Department (18ZA0360), the Open Fund Research of KeyLaboratory of Corrosion and Protection of Materials in Sichuan Province(No. 2018CL02), the Open Project of Key Laboratory of Green Chemistryof Sichuan Institutes of Higher Education (LZJ18202) and the NationalNatural Science Foundation of China (No. 81373965).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.bioelechem.2019.107332.

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TaoHe received her Bachelor's degree in Pharmaceutical En-gineering from Sichuan University of Science and Engineer-ing in 2016. She is currently a postgraduate student in thesame university under the supervision of Professor ChunruCheng.Her research interests are: the discovery of active nat-ural products in plant, aswell as the quantitative and qualita-tive analyses of active natural products in plants.

Wilfred Emori is a lecturer in the School ofMaterials Scienceand Engineering, Sichuan University of Science and Engi-neering. He obtained his Ph.D. inMaterials Science and Engi-neering (majoring in Corrosion Science and Protection) fromthe University of Chinese Academy of Sciences in 2017, andhis research direction is in the investigation ofmetals and al-loys behaviour in a wide range of environments. He is amember of some renowned associations and has presentedseveral conference papers in scientific meetings around theworld. He has about 10 articles in peer-reviewed journalscredited to his name.

Run-Hua Zhang is a Postgraduate student in the School ofMaterials Science and Engineering, SichuanUniversity of Sci-ence and Engineering where he is conducting research onthe corrosionmechanisms of aluminium in alkalinemetal or-ganic acid salts for aircraft applications: a National NaturalScience Foundation of China Project. He is a Member of Si-chuan Society of Corrosion and protection and has presentedseveral conference papers within China.

Peter C. Okafor is the Group Leader of the Corrosion andElectrochemistry Research Group and a Professor of PhysicalChemistry in theDepartment of Pure and Applied Chemistry,University of Calabar, Calabar, Nigeria. He obtained his Ph.D.in Physical Chemistry from theUniversity of Calabar, Calabar,Nigeria. His research efforts is focused on understanding themechanism, inhibition and modeling of corrosion processes,and environmental pollution as it relates to corrosion. He haspublished over 50 articles in both national and internationalpeer-reviewed journals. He was the Lead Guest Editor of aSpecial Issue of International Journal of Corrosion titled,Green Approaches to Corrosion Mitigation.

emis

MinYangwas born inHubei Province, China, in 1977. He ob-tained his Ph.D. in 2004 from Lanzhou University and ob-tained postdoctoral training at the School of PharmaceuticalSciences, Peking University from 2004 to 2006. He thenmoved to Shanghai Institute of Materia Medica,ChineseAcademy of Sciences and worked for ten years asan assistant and associate professor. In 2016, he joinedShanghaiTech University. His research focuses on structuralelucidation of organic products and analysis of complex sys-tem using chromatography coupled with mass spectrome-try.

14 T. He et al. / Bioelectroch

Chun-Ru Cheng was born in Zigong, China, in 1981. He re-ceived his B.Sc. from the Sichuan University in 2003. He ob-tained his Ph.D. from Chinese Academy of Sciences in 2012under the direction of Professor De-An Guo. He joined the Si-chuan University of Science & Engineering in 2012. His re-search interests include discovery of active natural productsfrom plant, structuremodification of natural products, quan-titative and qualitative analysis of active natural products inplants.

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