Inhibitory effect of Gentiana olivieri extracts on the corrosion of mild steel in 0.5 M HCl: Electrochemical and phytochemical evaluation

Evrim Baran; Ahmet Cakir; Birgül Yazici

doi:10.1016/j.arabjc.2016.06.008

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Original article

12 (

); 4303-4319

doi:

10.1016/j.arabjc.2016.06.008

Inhibitory effect of Gentiana olivieri extracts on the corrosion of mild steel in 0.5 M HCl: Electrochemical and phytochemical evaluation

Evrim Baran^{^a,⁎}, Ahmet Cakir^{^a}, Birgül Yazici^{^b}

a Department of Chemistry, Faculty of Science and Literature, Kilis 7 Aralık University, 79000 Kilis, Turkey

b Department of Chemistry, Faculty of Science and Literature, Çukurova University, 01000 Adana, Turkey

⁎Corresponding author at: Kilis 7 Aralık University, Science and Letters Faculty, Chemistry Department, 79000 Kilis, Turkey. Fax: +90 348 822 23 51. evrimbaran@kilis.edu.tr (Evrim Baran)

Received: 2016-3-26, Accepted: 2016-6-11, Published: 2016-12-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Abstract

Natural inhibitors including plant extracts and their pure metabolites have become important as environmentally acceptable, low-cost, readily available and renewable source for a wide range of corrosion inhibitors. In the current study, the inhibitory effect of the methanol extract of Gentiana olivieri and its fractions on the corrosion of mild steel in a 0.5 M HCl solution were investigated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The chemical compositions of the methanol extract and its fractions were identified by high-performance liquid chromatography (HPLC). EIS results showed maximum inhibition efficiency (93.7%) at an inhibitor concentration of 800 mg/L. Polarization curves indicated that the plant extract and its fractions are mixed-type inhibitors. Surface analyses (SEM/EDX) were carried out to establish the corrosion-inhibitive properties of the samples. Thermodynamic parameters (E_a, ΔH°, ΔS°, ΔG°) were also examined.

Keywords

Gentiana olivieri

Mild steel

EIS

Acid inhibition

SEM/EDX

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1 Introduction

Mild steel is an abundant and efficient building material. However, it is difficult to protect mild steel exposed to harsh environments against corrosion (Li et al., 2009). The use of corrosion inhibitors is one of the most practical and economical methods for diminishing corrosion rates and the protection of metal surfaces against corrosion (Palou et al., 2014). Although many inorganic, organic and polymeric compounds have performed well as corrosion inhibitors for different metals and alloys, it has been reported that many of these inhibitors are toxic to aquatic and animal life (Palou et al., 2014; Garai et al., 2012). Natural inhibitors are often nontoxic or at least have lower toxicities to species in the environment compared with the synthetic organic inhibitors (Lebrini et al., 2011). Therefore, many researchers have focused on the study of ‘‘green’’ or ‘‘eco-friendly’’ corrosion inhibitors synthesized or isolated from various aromatic herbs, spices and medicinal plants (Ansari and Quraishi, 2014). The corrosion inhibitory effects of the extracts and their pure components of various plants, such as Phaseolus vulgaris L. (Abdel-Gaber et al., 2006), henna (Ostovari et al., 2009), Aloe vera (Abiola and James, 2010), Jasminum nudiflorum Lindl (Deng and Li, 2012a), Salvia officinalis (Soltani et al., 2012), bamboo (Li et al., 2012), Ficus hispida (Muthukrishnan et al., 2019), Aniba rosaeodora (Chevalier et al., 2014), Mansoa alliacea (Suedile et al., 2014), Ginkgo species (Deng and Li, 2012b), on carbon steel, cold rolled steel, stainless steel and zinc metals have been reported, with the very promising efficiency. The inhibition performances of plant extracts are closely related to their chemical compositions, which take the form of tannins, alkaloids, carbohydrates, phenolics, and proteins, to name a few. These metabolites usually bear polar functional groups containing nitrogen, sulphur, or oxygen atoms, as well as moieties consisting of triple or conjugated double bonds or aromatic rings, which are the major adsorption centres. The adsorption of molecules on metal surfaces depends on a range of functional group properties, including steric factors and electron density (Liao et al., 2011).

Gentiana olivieri Griseb. (Afat), belonging to the Gentianaceae family (Deliorman Orhan et al., 2003), is widely used as a bitter tonic, stomachic, appetizer, antipyretic, anticonvulsant, antidiabetic, antihepatotoxic, antinociceptive, anti-inflammatory, antiulcerogenic, sedative, antioxidant, antidepressant, and antianemic (Khan et al., 2014). The plant is known to possess a number of bitter secoiridoids glycosides, flavonoids (isoorientin and its derivatives), alkaloids, xhanthones, triterpenoid acids and fats (Singh et al., 2012). Recently, among the species of the Gentianaceae family, G. olivieri have been a focus of interest due to the identification of phytochemicals such as amarogentin and sweroside as new potential drugs (Khan et al., 2014; Singh et al., 2012; Peng et al., 2005; Wang et al., 2007 ). According to a literature survey, there have been no reports on the corrosion inhibition effects of G. olivieri or its individual constituents on metals such as mild steel, aluminium, iron, zinc and nickel in acidic or alkaline solutions. Therefore, the aim of this study was to evaluate the inhibition efficiency of the methanol extract of the aerial parts of G. olivieri and the ethyl acetate, n-butanol and water fractions of the methanol extract on the corrosion of mild steel in 0.5 M HCl. High-performance liquid chromatography (HPLC) and FT-IR spectrometry were applied to identify the components in the methanol extract of G. olivieri and its separated fractions. Investigations of the corrosion parameters were performed by electrochemical polarization measurements and electrochemical impedance spectroscopy (EIS) at different concentrations (200, 400, 600 and 800 mg/L) of the extract and its fractions at different temperatures (293, 303, 313, and 323 K). The surface charge of the mild steel in inhibited solutions, the adsorption isotherms and thermodynamic parameters were determined to clarify the potential inhibition mechanism. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analyses were also carried out to establish the corrosion inhibitory properties of the samples in 0.5 M HCl solution.

2 Experimental

2.1

2.1 Preparation of methanol extract and its fractions of G. olivieri

The aerial parts of G. olivieri were collected at the flowering stage from the Gaziantep region of Turkey in June 2014, and dried. The dried and powdered plant samples (250 g) were extracted with methanol (99.9%) (5 × 1 L) at room temperature. The extract was filtered and concentrated using a rotary evaporator under reduced pressure and temperature, yielding a dark brown extract (53.11 g). The concentrated extract (40 g) was then dissolved in distilled water at 60 °C and was filtered to remove lipophilic substances and chlorophyll. Afterwards, this solution was extracted with ethyl acetate and n-butanol. The organic solvents were removed under reduced pressure using a rotary evaporator. Hence, the methanol (MeOH) extract was fractioned into three fractions, ethyl acetate fraction (EtOAc Fr., 5.20 g), n-butanol fraction (n-BuOH Fr., 14.97 g) and water Fr. (19.83 g) (Aydin et al., 2014).

2.2

2.2 Characterization of methanol extract and its fractions of G. olivieri

The FT-IR spectra of the MeOH extract of G. olivieri and its EtOAc-, n-BuOH- and water-soluble fractions were recorded within the wavelength ranging between 4000 and 400 cm⁻¹ using a FT-IR spectrometer (Thermoscientific Nicolet iS10). Chemical composition analyses of the extract and its fractions were performed using a high-performance liquid chromatographic system (Shimadzu class LC) consisting of a FCV-10 ACVP pump, a DGU-20A5 degasser, a thermostated CTO-10VP column oven compartment and a SPD-20A prominence UV/VIS detector. A reverse-phase Kromasil 100-5C18 (150 mm × 4.6 mm, 5 μm) column was used. HPLC analyses were performed at a column temperature of 30 ± 1 °C, with a mobile phase of water: acetonitrile: acetic acid (79:20:1, v/v), an injection volume of 20 μl, and a flow rate of 1.0 ml/min (Sezik et al., 2005; Toker et al., 2011). The UV absorbance of the eluent was measured at 244 nm.

2.3

2.3 Isolation procedures and characterization

In order to isolate the metabolites responsible for the inhibitory effect of the extract and its fractions, the n-BuOH Fr. (14.97 g) was fractioned on silica gel (250 g, 70–230 mesh) using different EtOAc-EtOH solutions (9:1, 8:2, 7:3, 6:4, 4:6, 2:8, and 0:10) as the eluents. In the end of this process, five major fractions (coded as A, B, C and D) were obtained in the following amounts: 0.46, 8.87, 1.88 and 3.18 g, respectively. Fraction B was submitted silica gel CC (100 g, 230–400 mesh) and eluted with CH₂Cl₂-EtOH (7:3). The fraction was allowed the isolation of two compounds coded as 1 (3.90 g) and 2 (200 mg) (Fig. 1). To isolate another major component, C Fr. (1.88 g) was eluted with CHCl₃-EtOH (6:4) on silica gel (60 g) yielding compound 3 (70 mg). Compound 3 was crystallized over methanol. The ¹H NMR and ¹³C NMR spectra of the isolated compounds were recorded on a Bruker 400 Avance III spectrometer (¹H: 400 MHz, ¹³C NMR: 100 MHz). Dimethyl sulfoxide (DMSO)-d₆ was used as solvent, and tetramethylsilane (TMS) was used the internal standard for NMR analyses. The spectral data of the characterized compounds are given below (Peng et al., 2005; Wang et al., 2007, 2013; Jianga et al., 2011; Aydin et al., 2014 ).

The chemical structures of the gentiopicroside (1), isoorientin (2), and sucrose (3).

Compound 1 (gentiopicroside): ¹H NMR: (δ values, DMSO-d₆): 5.00 (m, H-3), 5.64 (m, H-4), 3.30 (m, H-5), 5.59 (d, J = 3.14 Hz, H-6), 7.42 (d, J = 1.25 Hz, H-8), 5.72 (ddd, J₁ = 17.37 Hz, J₂ = 10.10 Hz, J₃ = 6.77 Hz, H-9), 5.22 (m, H-C10), 4.48 (J = 7.92 Hz, H-1′), 3.01 (dd, J₁ = 9.07 Hz, J₂ = 5.37 Hz, 5.49 Hz, H-C2′), 3.15 (m, H-3′), 2.94 (m, H-4′), 3.15 (m, H-5′), 3.68 (Ha, dd, J₁ = 11.29 Hz, J₂ = 7.04 Hz, H-6′), 3.42 (Hb, dd, J₁ = 12.85 Hz, J₂ = 6.42 Hz, H-C6′). ¹³C NMR ((DMSO)-d₆): 162.7 (C-1), 69.1 (C-3), 116.2 (C-4), 124.9 (C-4a), 44.3 (C-5), 96.3(C-6), 148.7 (C-8), 103.2 (C-8a), 133.9 (C-9), 117.9 (C-10), 98.7 (C-1′), 69.0 (C-2′), 76.5 (C-3′), 72.7 (C-4′), 77.3 (C-5′), 61.0 (C-6′).

Compound 2 (isoorientin): ¹H NMR: (δ values, DMSO-d₆): 6.67 (s, H-3), 6.67 (s, H-8), 7.47 (br. s, H-2′), 6.96 (d, J = 8.25 Hz, H-5′), 7.42 (dd, J₁ = 7.40 Hz, J₂ = 1.29 Hz, H-6′), 4.60 (d, J = 9.77 Hz, H-C-1″). ¹³C NMR ((DMSO)-d₆): 163.6 (C-2), 102.7 (C-3), 181.8 (C-4), 160.6 (C-5), 108.8 (C-6), 163.6 (C-7), 93.6 (C-8), 156.1 (C-9), 103.2 (C-10), 121.3 (C-1′), 113.3 (C-2′), 145.9 (C-3′), 149.8 (C-4′), 116.2 (C-5′), 118.9 (C-6′), 72.5 (C-1″), 70.1(C-2″), 79.0 (C-3″).

Compound 3 (sucrose): ¹H NMR: (δ values, DMSO-d₆): 5.18 (d, J = 8.29 Hz, H-1), 3.88 (t, J = 8.13 Hz H-3′), 3.77 (m, H-4′), 3.65 (m, H-5), 3.56 (m, H-5′), 3.48 (m, H-3), 3.39 (m, H-1′), 3.18 (m, H-2), 3.14 (m, H-4). ¹³C NMR ((DMSO)-d₆): 104.0 (C-2′), 91.7 (C-1), 82.5 (C-5′), 76.9 (C-3′), 74.2 (C-4′), 72.8 (C-3), 72.7 (C-5), 71.6 (C-2), 69.8 (C-4), 62.1 (C-6′), 62.0 (C-1′), 69.0 (C-2′), 60.4 (C-6).

2.4

2.4 Electrodes and electrolytes

The mild steel (MS) sample used in the inhibitory assays had the following chemical composition: 0.173 wt.% C, 0.046 wt.% Si, 0.435 wt.% Mn, 0.026 wt.% P and 99.32 wt.% Fe. The electrodes were prepared by embedding mild steel rods in epoxy resin. The surface area of the mild steel exposed to the test solution was 0.785 cm². Before the measurements, the surface of mild steel was mechanically abraded using different grades of sand papers up to the 1200 grade, and then polished mechanically with 1 μm Al₂O₃ paste until a mirror bright surface was obtained. Then, the electrode was cleaned with distilled water using an ultrasonic bath and quickly immersed into the test solution. The corrosion tests were performed in a 0.5 M HCl solution in the absence and presence of 200, 400, 600 and 800 mg/L concentrations of the MeOH extract and its EtOAc-, n-BuOH- and water-soluble fractions. Also, the electrochemical tests were carried out at different temperatures (293, 303, 313, 323 K) without stirring. The temperatures of the solutions were controlled thermostatically.

2.5

2.5 Electrochemical measurements

The electrochemical measurements were carried out using a CHI 604E A.C. electrochemical analyser with computer control. For these tests, an electrochemical cell with a three-electrode configuration was used. A platinum sheet (with 2 cm² total surface area) and an Ag/AgCl (3 M KCl) system were used as the auxiliary and the reference electrodes, respectively. All potential values were referred to the Ag/AgCl (3 M KCl) electrode. EIS and potentiodynamic polarization measurements were carried out establishing a steady-state open circuit potential (OCP), after which the mild steel electrodes were immersed into the corrosive solutions for 1 h, in a 0.5 M HCl solution with and without 200, 400, 600 and 800 mg/L concentrations of the MeOH extract and its fractions at different temperatures (293, 303, 313, and 323 K). Potentiodynamic polarization curves were obtained from −0.8 V (Ag/AgCl) to −0.2 V (Ag/AgCl) at a sweep rate of 1 mV s⁻¹. To obtain corrosion current densities (I_corr), the linear Tafel segments of cathodic curves were extrapolated to corrosion potential. EIS experiments were carried out in a frequency range from 100 kHz to 10⁻² Hz at OCP with 5 mV amplitude. For the measurement of the potential of zero charge (PZC), electrochemical impedance spectra were recorded by scanning frequencies from 10⁵ Hz to 10⁻² Hz at an AC amplitude of 5 mV, and by applying different potentials. The double layer capacitance values obtained were plotted against the applied potentials to determine the PZC.

2.6

2.6 Surface characterization

Mild steel surfaces exposed for 24 h to immersion in 0.5 M HCl and inhibited solutions (MeOH extract of G. olivieri and its EtOAc, n-BuOH and water fractions) were examined by a Zeiss/Supra 55 Scanning Electron Microscope (SEM) and by energy dispersive X-ray spectroscopy (EDX) under high vacuum at an accelerating voltage of 15.00 kV and with 5 K X magnification. The average percentages of the elements on the mild steel surface were determined by an EDX detector coupled to the SEM.

3 Results and discussion

3.1

3.1 Chemical composition of the extract and its fractions

The chemical structures of compounds 1, 2 and 3 isolated from the n-BuOH soluble fraction of the MeOH extract of G. olivieri were characterized by IR, ¹H NMR, ¹³C NMR, DEPT, HMQC and ¹H,¹H-COSY spectroscopic methods, and these compounds were assigned as gentiopicroside, isoorientin and sucrose, respectively (Fig. 1). Gentiopicroside and isoorientin are characteristic metabolites of the Gentiana genus (Ko et al., 1998; Wang et al., 2007). Their chemical structures were confirmed by comparison with previous published spectroscopic data (Peng et al., 2005; Wang et al., 2007, 2013; Jianga et al., 2011; Aydin et al., 2014 ). The NMR data of the gentiopicroside, isoorientin and sucrose are shown in Supplementary material.

3.1.1

3.1.1 HPLC and Fourier transform infrared (FT-IR) spectroscopy of the MeOH extract and its EtOAc, n-BuOH and water fractions

According to HPLC analyses (Fig. 2), the extract and its fractions contain approximately 7–10 major peaks, along with a few small peaks belonging to minor constituents. HPLC analysis showed that the extract and its fractions contained a major compound, gentiopicroside (1), with a retention time of 3.42 min. Gentiopicroside (1) is characteristic compound of the Gentiana species, and constituted to 56.10%, 41.52%, 54.18% and 37.18% of the MeOH extract, the EtOAc-soluble fraction, the n-BuOH-soluble fraction and the water-soluble fraction, respectively (Table 1). Sucrose was found to be the second major component of the MeOH extract (19.48%), the water fraction (28.19%) and the n-BuOH fraction (13.16%). Isoorientin (2) was detected at low levels in the MeOH extract, constituting only 4.13% of total extract (Table 1).

HPLC chromatograms of the MeOH extract of G. olivieri and its fractions.

Table 1 The chemical composition of G. olivieri MeOH extract and its fractions obtained from the HPLC results.

Components	R_t	MeOH extract (%)	Water Fr. (%)	n-BuOH Fr. (%)	EtOAc Fr. (%)
Gentiopicroside	3.42	56.10	37.18	54.18	41.52
Isoorientin	3.77	4.13	0.10	9.75	3.56
Sucrose	1.86	19.48	28.19	13.16	1.95
Total identified (%)	–	79.71	65.47	77.09	47.03

FT-IR was used to identify the functional groups of the components in the extract and its fractions. The absorbance bands observed the FT-IR spectra of the extract and its fractions (Fig. 3) are in accordance with the chemical structures of the components (gentiopicroside (1) isoorientin (2) and sucrose (3)). In the IR spectra, the absorption bands belonging to the O—H stretching vibration, α-β unsaturated carbonyl groups, C—H stretching vibrations, C⚌C vibrations and C—O—C bonds vibrations were observed at the around 3100–3500 cm⁻¹, 1705 cm⁻¹, 2900–3000 cm⁻¹, 1400–1600 cm⁻¹ and 1050 cm⁻¹, respectively. As can been seen in Table 1, the amounts of gentiopicroside and isoorientin were found as high amount in the MeOH extract, and its EtOAc and n-BuOH fractions. However, their amounts were detected at low amount in the water fraction. These results are in accordance with the absorbance bands in the FT-IR spectra of water fraction. In this spectrum, intensity of C⚌O absorption band was low, whereas O—H vibration band was observed to be high as compared with the other spectra.

FT-IR spectrums of the MeOH extract of G. olivieri (a) and its fractions (EtOAc Fr. (b), n-BuOH Fr. (c), and water Fr. (d)).

3.2

3.2 Electrochemical tests

3.2.1

3.2.1 Open circuit potential (OCP) curves

In the absence and presence of 800 mg/L concentration, which exhibited the highest inhibition effect based on the EIS results of the MeOH extract of G. olivieri and its fractions, the change of open circuit potential of the mild steel in 0.5 M HCl with time is depicted in Fig. 4. It can be seen from the curves in the presence of the inhibitor solutions, OCP values obtained for MeOH extract and its fractions shift towards the positive potential. However, the MeOH extract has more positive OCP than that of n-BuOH, water and EtOAc fractions. HPLC results show that the MeOH extract contains the relatively high amount of the major compound, gentiopicroside (1) when compared with other fractions (Table 1). Therefore, the inhibition effect of the extracts and its fraction can be attributed to gentiopicroside (1) adsorbing on the metal surface (Yıldız, 2015). As seen from OCP-time curves, the OCP values become almost constant after 2000 s. So, 1 h is chosen as immersion time for the electrochemical measurement.

The change of open circuit potential as a function of exposure time in 0.5 M HCl solution and containing 800 mg/L of MeOH extract of G. olivieri and its fractions.

3.2.2

3.2.2 Potentiodynamic polarization curves

The potentiodynamic polarization curves obtained for the mild steel in 0.5 M HCl with and without various concentrations (800, 600, 400 and 200 mg/L) of the MeOH extract and its fractions at 293 K are shown in Fig. 5. The values of i_corr, E_corr and cathodic Tafel slopes (β_c) obtained by the extrapolation of linear Tafel segments of cathodic curve and inhibition efficiency (η) calculated using Eq. (1) are listed in Table 2.

(1)

η = [\frac{i_{{corr}^{0}} - i_{corr}}{i_{{corr}^{0}}}] \times 100

where

i_{{corr}^{0}}

and i_corr are the corrosion current density values in the absence and presence of the MeOH extract of G. olivieri and its fractions, respectively. At all inhibitor concentrations, the values of corrosion current density (i_corr) were reduced, compared with the value of inhibitor free. The corrosion current density values of the MeOH extract and n-BuOH fractions decreased with increasing the concentration of the inhibitor. But, the i_corr values calculated for EtOAc and water fractions do not change regularly with varying of inhibitor concentration and the inhibitors showed better efficiency even at low concentration values. This may explain the relatively low amount of gentiopicroside (1) in the EtOAc and water fractions (Table 1). As seen in Fig. 5, the current densities of the anodic and cathodic branches decreased in the presence of the G. olivieri methanol extract and its fractions which suggest that the inhibitory molecules constitute a protective layer on the metal surface (Zheng et al., 2014). The parallel cathodic curves in the presence of various concentrations of the MeOH extract and its fractions and the almost constant cathodic Tafel slopes (β_c) indicate that the presence of the inhibitory molecules does not change the mechanism of hydrogen evolution and the hydrogen evolution reaction is activation controlled (Su et al., 2014; Biswas et al., 2015). In the presence of the MeOH extract and its fractions, the values of corrosion potential (E_corr) shifted to positive values (9–30 mV) as compared to the blank. If the displacement in corrosion potential is less than 85 mV with respect to corrosion potential (E_corr) at the blank, an inhibitor can be classified as mixed-type inhibitor (Zhang et al., 2015; Verma et al., 2015; Rajeswari et al., 2014; Umoren et al., 2010 ). Thus, the MeOH extract and its fractions can be classified as mixed-type inhibitors.

Polarization curves for mild steel in 0.5 M HCl without and with different concentrations of MeOH extract of G. olivieri and its fractions (immersion time is 1 h).

Table 2 Electrochemical parameters obtained from potentiodynamic polarization plots of the mild steel immersed in 0.5 M HCl with different concentrations of the MeOH extract and its fractions (immersion time is 1 h).

	C	E_corr	i_corr	β_c	η
	(mg/L)	(V vs. Ag/AgCl)	(mA cm⁻²)	(mV dec⁻¹)	(%)
0.5 M HCl	–	−0.524	0.302	−105.4	–

MeOH ext. of G. olivieri	200	−0.510	0.048	−103.3	84.11
	400	−0.505	0.038	−106.4	87.42
	600	−0.499	0.028	−98.6	90.73
	800	−0.511	0.030	−99.5	90.07

EtOAc Fr.	200	−0.515	0.055	−111.7	81.79
	400	−0.502	0.030	−100.6	90.07
	600	−0.522	0.042	−99.2	86.09
	800	−0.512	0.031	−97.2	89.74

n-BuOH Fr.	200	−0.512	0.052	−111.2	82.78
	400	−0.511	0.048	−109.7	84.11
	600	−0.511	0.042	−107.2	86.09
	800	−0.519	0.033	−112.6	89.07

Water Fr.	200	−0.483	0.036	−104.0	88.08
	400	−0.494	0.033	−109.0	89.07
	600	−0.504	0.038	−104.1	87.42
	800	−0.504	0.040	−102.1	86.75

The effect of temperature on the corrosion of mild steel in 0.5 M HCl in the absence and presence of 800 mg/L MeOH extract and its fractions were investigated at different temperatures (293, 303, 313 and 323 K) (Supplementary material). The electrochemical parameters obtained from potentiodynamic polarization plots are given in Table 3. In the presence of the MeOH extract and its fractions, the values of inhibitor efficiency were nearly constant in the studied temperature range and the values of current density increase with increasing temperature due to increase in the desorption rate of the adsorbed molecules on the metal surface, that previously blocked active sites against attacks by acid (Zhang et al., 2009; Li et al., 2014; Daoud et al., 2014 ). To determine the activation energy (E_a) of corrosion and the inhibition mechanism in the absence and presence of the extract and its fractions, the following Arrhenius equation was used:

(2)

i_{corr} = A \exp (\frac{- E_{a}}{RT})

Table 3 Activation and polarization parameters for the steel immersed in 0.5 M HCl containing 800 mg/L of MeOH extract and its fractions at 20–50 °C (immersion time is 1 h).

	T	E_corr	i_corr	β_c	η	E_a
	(°C)	(V vs. Ag/AgCl)	(mA cm⁻²)	(mV dec⁻¹)	(%)	(kJ mol⁻¹)
0.5 M HCl	20	−0.524	0.302	−105.4	–	42.59
	30	−0.505	0.510	−109.9	–
	40	−0.500	0.892	−121.1	–
	50	−0.498	1.522	−131.7	–

MeOH ext. of G. olivieri	20	−0.511	0.030	−99.5	90.07	41.52
	30	−0.497	0.040	−103.7	92.25
	40	−0.486	0.064	−114.8	92.82
	50	−0.514	0.150	−113.7	90.14

EtOAc Fr.	20	−0.512	0.031	−97.2	89.74	48.99
	30	−0.505	0.050	−110.1	90.19
	40	−0.499	0.079	−120.0	91.12
	50	−0.505	0.214	−130.1	85.94

n-BuOH Fr.	20	−0.519	0.033	−112.6	89.07	40.69
	30	−0.503	0.053	−113.2	89.58
	40	−0.497	0.076	−118.1	91.53
	50	−0.488	0.165	−114.3	89.13

Water Fr.	20	−0.504	0.040	−102.1	86.75	32.08
	30	−0.497	0.062	−112.4	87.84
	40	−0.492	0.095	−117.6	89.38
	50	−0.486	0.135	−120.9	91.14

In Eq. (2), i_corr is the corrosion current density obtained from the polarization curves, A is the Arrhenius pre-exponential factor, T is the absolute temperature, and R is the universal gas constant. The values of E_a calculated from the slopes of line graphs in Fig. 6 are presented in Table 3. As seen from the Table 3, while the E_a values do not show remarkable change in the presence of MeOH extract and n-BuOH fraction, the activation energy is slightly increased in the presence of EtOAc fraction and decreased in the presence of water fraction, compared to that in the inhibitor free. These results indicate that the inhibitors do not lead to effective change in the corrosion mechanism of steel in HCl environment (Sığırcık et al., 2015). Considering the E_a values and the changes in η (%) with rise in temperature, the results suggest that the strong physical adsorption or weak chemical bonding occurs between the molecules of inhibitor and the mild steel surface (Garai et al., 2012; Soltani et al., 2014).

Arrhenius plots for mild steel corrosion rate in 0.5 M HCl in the absence and presence of 800 mg/L of MeOH extract of G. olivieri and its fractions.

3.2.3

3.2.3 Electrochemical impedance spectroscopy (EIS)

The Nyquist and bode plots for the mild steel in 0.5 M HCl solutions in the absence and presence of different concentrations of the MeOH extract and its fractions after 1 h are shown in Fig. 7. As can be seen, the Nyquist plots of the mild steel exhibit single capacitive semi-circles indicating a corrosion process mainly controlled by charge transfer (Li et al., 2012; Rosliza et al., 2008). The increase in the diameter of the capacitive semicircles with increasing concentrations of the MeOH extract and its fractions during 1 h of immersion time points to the formation of a strong protective film on the surface of the mild steel (Li et al., 2014). When compared with the 0.5 M HCl solution, the shape of the EIS curves obtained from solutions containing the MeOH extract and its fractions were maintained. These results demonstrate that the corrosion mechanism was not changed by the addition of inhibitors (Deng and Li, 2012a,b; Rosliza et al., 2008 ). The Nyquist plots deviating from a perfect semicircle can be attributed to frequency dispersion and inhomogeneities of metal surface, resulting in non-uniform charge loading between the metal and OHP (outer Helmholtz plane) (Ansari and Quraishi, 2014; Soltani et al., 2014). Therefore, a constant phase element CPE was employed instead of a pure double layer capacitor in the equivalent circuit model (Fig. 8) to achieve a better fit in the analysis of the EIS results. R_s and R_ct in the equivalent circuit represent the solution resistance and charge transfer resistance, respectively. The CPE is defined by the following mathematical expression (Muthukrishnan et al., 2019; Torres et al., 2014):

(3)

Z_{CPE} = \frac{1}{Y_{0} {(j ω)}^{n}}

Nyquist and bode plots of the mild steel in 0.5 M HCl without and with different concentrations of MeOH extract of G. olivieri and its fractions at 20 °C (immersion time is 1 h).

The CPE consists of two parameters, namely the admittance magnitude (Y₀) and the exponent (n) (Mehdipour et al., 2015). The exponential parameter n is used to estimate surface irregularity resulting from surface roughness, inhibitor adsorption and porous layer formation (Deng and Li, 2012b). The double layer capacitance (C_dl) was calculated from the following equation, using parameters (Y₀, n) of the CPE and the frequency (f_max) at which the imaginary impedance reaches a maximum value (Torres et al., 2014; Mehdipour et al., 2015):

(4)

C_{dl} = Y_{0} {(2 π f_{max})}^{n - 1}

The electrochemical parameters R_ct, Y₀, n and C_dl were obtained by fitting of experimental data with the ZView software package. The inhibition efficiency (η, Table 4) was calculated utilizing the following equation:

(5)

η = [\frac{R_{ct (inh)} - R_{ct}}{R_{ct (inh)}}] \times 100

Table 4 EIS parameters and inhibition efficiency for mild steel in 0.5 M HCl in the absence and presence of different concentrations of MeOH extract and its fractions.

	C (mg/L)	R_ct (Ω cm²)	Y₀ (sⁿ Ω⁻¹ cm⁻²)	n	C_dl (μF cm²)	η (%)
0.5 M HCl	–	94.0	4.08 · 10⁻⁴	0.84	223.69	–

MeOH extract	200	627.3	1.07 · 10⁻⁴	0.85	66.40	85.0
	400	722.8	1.02 · 10⁻⁴	0.83	59.40	87.0
	600	915.3	1.07 · 10⁻⁴	0.81	60.64	89.7
	800	1200.0	7.40 · 10⁻⁵	0.84	45.87	92.2

EtOAc Fr.	200	846.4	7.22 · 10⁻⁵	0.84	42.09	88.9
	400	897.1	7.09 · 10⁻⁵	0.85	44.00	89.5
	600	1119.0	6.35 · 10⁻⁵	0.85	39.41	91.6
	800	1159.0	5.77 · 10⁻⁵	0.87	38.16	91.9

n-BuOH Fr.	200	889.9	6.53 · 10⁻⁵	0.85	39.37	89.4
	400	1037.0	7.93 · 10⁻⁵	0.85	50.65	90.9
	600	1169.0	6.84 · 10⁻⁵	0.85	43.69	92.0
	800	1481.0	5.78 · 10⁻⁵	0.86	39.07	93.7

Water Fr.	200	663.6	8.78 · 10⁻⁵	0.86	54.76	85.8
	400	876.5	6.99 · 10⁻⁵	0.86	44.78	89.3
	600	955.2	5.18 · 10⁻⁵	0.86	32.31	90.2
	800	1277.0	4.99 · 10⁻⁵	0.85	30.97	92.6

As seen in the Table 4, charge transfer resistance (R_ct) increased with increasing concentrations of the MeOH extract and its fractions. The greatest inhibitive effect was observed at 800 mg/L of the MeOH extract, the EtOAc fraction, the n-BuOH fraction and the water fraction, giving R_ct values of 1200 Ω cm², 1159 Ω cm², 1481 Ω cm², and 1277 Ω cm², respectively. This is because the total amount of adsorption and the coverage of inhibitor on the steel surface increase with increasing inhibitor concentration (Li et al., 2014). The increase in the charge resistance leads to an increase in inhibition efficiency. The inhibition efficiency values of the fractions of the MeOH extract were 93.7, 91.9 and 92.6 for 800 mg/L concentrations of the n-BuOH, EtOAc and water fractions, respectively. According to these results, there were no significant differences in the inhibition effects provided by the MeOH extract or its fractions. There were several investigations on corrosion inhibitors of some plant extracts in various acidic media and their optimum concentrations. For instance, the alkaloid extract from Oxandra asbecki plant (OAPE) has been studied as a possible source of green inhibitor for the corrosion of C38 steel in 1 M HCl and the maximum inhibition efficiency of 92% is achieved at 100 ppm OAPE (Lebrini et al., 2011). Use of some plant extracts such as Chamomile, Halfabar, Black cumin and Kidney bean plants for the corrosion of steel in 1 M H₂SO₄ resulted in 90.2%, 87.1%, 87.2% and 83.5% of inhibition efficiency at the optimum plant concentrations of 7.56, 2.52, 1.14 and 2.4 g/L, respectively (Abdel-Gaber et al., 2006). The highest inhibition efficiency exhibited by the Pennisetum purpureum extract for mild steel in 1 M HCl solution was reported to be above 95% (Alaneme et al., in press). Alcohol and water extracts of the leaves of Pimenta dioica (PD) were reported as non-toxic corrosion inhibitor for mild steel in acidic medium and the maximum of 98% and 97% percentage inhibition efficiency has been exhibited by PD-water and alcoholic extract in 0.5 M HCl, respectively (Anupama et al., 2015). These data and our results for the corrosion of mild steel in 0.5 M HCl in the presence of the G. olivieri extract and its fractions suggest that the plant extracts could serve as effective corrosion inhibitors. The excellent inhibition effects of the extract and its fractions can be attributed to its components, including gentiopicroside (1) and isoorientin (2) (Table 1). As seen from the Table 1, the extract and its fractions have similar qualitative and quantitative chemical compositions and they all contained gentiopicroside (1) as the major component.

The double layer capacitance (C_dl) decreased with increasing concentrations of the MeOH extract and its EtOAc, n-BuOH and water fractions. At an inhibitor concentration of 800 mg/L, C_dl declined from 223.69 (inhibitor-free medium) to 45.87 (MeOH), 38.16 (EtOAc), 39.07 (n-BuOH), and 30.97 (water) μF cm⁻². The C_dl value according to the Helmholtz model is expressed in the following equation (Li et al., 2012; Mehdipour et al., 2015; Bothi Raja et al., 2013 ):

(6)

C_{dl} = \frac{ε \cdot ε^{0}}{d} A

In Eq. (6), ε⁰ is the permittivity of air, ε is the local dielectric constant, d is the thickness of the film and A is the surface area of the electrode. Accordingly, C_dl values lower than found in the absence of the inhibitors can be associated with a decrease in the dielectric constant and/or increase in the electrical double layer thickness due to displacement of ions and water molecules originally adsorbed on the surface by inhibitor molecules (Umoren et al., 2010). Increasing values of n, defined as a measure of the surface heterogeneity, compared with an inhibitor-free medium, may be explained by the formation of a more uniform surface as a result of adsorbed inhibitors (Abiola and James, 2010; Umoren et al., 2010; Bothi Raja et al., 2013 ). According to the data presented in Table 4, lower n values were found in environments containing the MeOH extract and its fractions as compared to inhibitor-free 0.5 M HCl solutions. These n values showed irregular trends with the increase in inhibitor concentrations. This situation may be the result of the random dispersion of organic molecules physically adsorbed on the metal surface. In agreement with our results, irregular trends in n values have been recorded in many studies (Umoren et al., 2010; Khaleda, 2010).

EIS experiments were also carried out for the mild steel immersed in 0.5 M HCl containing 800 mg/L of the MeOH extract and its fractions at 20–50 °C (Fig. 9). EIS parameters were calculated using the equivalent circuit model (Fig. 8). The inhibition efficiencies of 800 mg/L concentrations of the MeOH extract and its fractions at 20–50 °C on mild steel in 0.5 M HCl are given in Table 5. The decrease in the charge transfer resistances (R_ct) at high temperatures indicates an increase in the corrosion rates of mild steel. The increasing rate of metal dissolution and the shift of the adsorption/desorption equilibrium towards inhibitor desorption led to a decrease in the degree of the surface coverage (Lebrini et al., 2011; Umoren et al., 2010). The values of n lie between 0.88 and 0.82, and the inhibition efficiency ranges between 86.4% and 93.7% in 0.5 M HCl solutions containing 800 mg/L of the samples, indicating the formation of protective layers at the electrode surfaces. These results confirm that the MeOH extract and its fractions remain effective across all analysed temperatures.

Nyquist and bode plots of the mild steel in 0.5 M HCl and 800 mg/L MeOH extract of G. olivieri and its fractions at 20–50 °C (immersion time is 1 h).

Table 5 EIS parameters and inhibition efficiency for mild steel in 0.5 M HCl in the absence and presence of 800 mg/L concentrations of MeOH extract and its fractions at different temperatures (immersion time is 1 h).

	Temperature (°C)	R_ct (Ω cm²)	Y₀ (sⁿ Ω⁻¹ cm⁻²)	n	C_dl (μF cm²)	η (%)
0.5 M HCl	20	94.0	4.08 · 10⁻⁴	0.84	223.69	–
	30	59.0	5.48 · 10⁻⁴	0.84	291.37	–
	40	36.8	4.90 · 10⁻⁴	0.84	230.43	–
	50	24.2	4.23 · 10⁻⁴	0.88	229.40	–

MeOH extract	20	1200.0	7.40 · 10⁻⁵	0.84	45.87	92.2
	30	831.5	7.96 · 10⁻⁵	0.83	44.87	92.9
	40	392.7	7.73 · 10⁻⁵	0.83	38.24	90.6
	50	211.5	9.13 · 10⁻⁵	0.81	35.94	88.6

EtOAc Fr.	20	1159.0	5.77 · 10⁻⁵	0.87	38.16	91.9
	30	933.2	4.52 · 10⁻⁵	0.87	28.44	93.7
	40	502.7	6.11 · 10⁻⁵	0.86	35.16	92.7
	50	246.1	7.37 · 10⁻⁵	0.88	42.83	90.2

n-BuOH Fr.	20	1481.0	5.78 · 10⁻⁵	0.86	39.07	93.7
	30	771.0	7.90 · 10⁻⁵	0.85	47.64	92.4
	40	480.9	6.09 · 10⁻⁵	0.87	35.55	92.3
	50	220.4	8.26 · 10⁻⁵	0.86	42.68	89.0

Water Fr.	20	1277.0	4.99 · 10⁻⁵	0.85	30.97	92.6
	30	614.5	8.37 · 10⁻⁵	0.84	47.32	90.4
	40	384.6	8.70 · 10⁻⁵	0.83	43.04	90.4
	50	177.6	9.03 · 10⁻⁵	0.82	37.33	86.4

3.2.4

3.2.4 Adsorption isotherms and thermodynamic characterization

Adsorption isotherms are a very useful for understanding the interactions between inhibitor molecules and steel surfaces. As such, Langmuir, Freundlich and Temkin adsorption isotherms were tested in hopes of describing the adsorption behaviour of the MeOH extract and its fractions on mild steel in 0.5 M HCl. Through the use of the correlation coefficient (R²), the results were found to be best fit by a Langmuir adsorption isotherm (Fig. 10):

(7)

\frac{C_{inh}}{θ} = \frac{1}{K_{ads}} + C_{inh}

Langmuir adsorption plots for mild steel in 0.5 M HCl containing different concentrations of MeOH extract of G. olivieri and its fractions.

In Eq. (7), C_inh is the concentration (mg/L) of the inhibitor, K_ads (L/mg) is the adsorptive equilibrium constant, and θ is the surface coverage calculated by using the η/100 ratio obtained from the EIS measurements at different inhibitor concentrations. The linear curves of Langmuir isotherm model for all the samples are illustrated in Fig. 10. The adsorptive equilibrium constants (K_ads) were determined from the intercepts of the straight lines and K_ads was also related to the standard adsorption free energy (ΔG°) using a following equation (Daoud et al., 2014).

(8)

Δ G^{0} = - RT \ln (C_{w} K_{ads})

In Eq. (8), R is the gas constant, T is the temperature, and C_w is the concentration of water (1.0 × 10⁶ mg/L), as the molecular masses of the extract components are not known (Li et al., 2010). In the current study, the high K_ads values obtained for the MeOH extract and its fractions (Table 6) showed that the metabolites in all the samples strongly adsorbed on the mild steel surface. As a general rule, values of ΔG⁰ less than −20 kJ mol⁻¹ indicate physical adsorption, which results from electrostatic interactions between the charged inhibitor molecules and the charged metal, while those more negative than −40 kJ mol^–1 indicate sharing or transfer of electrons from inhibitor molecules to the metal surface in the formation of co-ordinate bonds (chemisorption) (Soltani et al., 2014; Ozcan et al., 2008). The values of ΔG° obtained for the samples are between 25.27 and 26.79 kJ/mol, indicating that the interaction between the inhibitor molecules and the surface of the mild steel involves both physisorption and chemisorption (El-Shamy et al., 2015; Xu et al., 2014). Combined with the results from values of E_a, therefore, it may be said that firstly, the formation of an adsorptive film with electrostatic character occurs due to the adsorbed water molecules on the surface of mild steel, and then the removal of water molecules from the surface is accompanied by chemical interaction between the metal surface and inhibitor molecules (Li et al., 2010; Xu et al., 2014; Donahue and Nobe, 1965 ).

Table 6 The thermodynamic parameters of adsorption of MeOH extract and its fractions on the mild steel surface at 20 °C.

	K_ads (L/g)	ΔG° (kJ mol⁻¹)	ΔH° (kJ mol⁻¹)	ΔS° (J mol⁻¹ K⁻¹)
MeOH extract	31.91	−25.27	−21.25	13.73
EtOAc Fr.	45.23	−26.12	−19.47	22.70
n-BuOH Fr.	59.38	−26.79	−14.39	42.27
Water Fr.	39.84	−25.81	−15.96	33.62

The other thermodynamic parameters, standard adsorption enthalpy (ΔH°) and entropy (ΔS°), were calculated by the following equations:

(9)

(\frac{θ}{1 - θ}) = A \cdot C \exp (- \frac{Δ H^{0}}{2303 RT})

(10)

Δ S^{0} = \frac{Δ H^{0} - Δ G^{0}}{T}

All of the calculated thermodynamic parameters are presented in Table 6. As shown in Table 6, the negative values of ΔH⁰ imply that the adsorption of the inhibitor molecules on the mild steel surface is exothermic processes. The positive sign of ΔS⁰ values can be explained by a quasi-substitution process between the organic inhibitor(s) in the aqueous phase (Org_(sol)) and water molecules at the electrode surface (H₂O(ads)) (Li et al., 2009; Soltani et al., 2014). In this situation, the adsorption of organic inhibitors is enabled by desorption of water molecules from the surface. Thus, while the adsorption process for the inhibitor is believed to be associated with a decrease in entropy of the solute, the opposite is true for the solvent. Therefore, the gain in entropy is attributed to the increase in solvent entropy (Zhang et al., 2015). The positive values of ΔS⁰ imply that entropy is the driving force for the adsorption of the molecules in the MeOH extract and its fractions on the mild steel surface. Based on the thermodynamic results, the calculated ΔG° and ΔH° values show that adsorption mechanism is not completely physical or chemical and a combination of physisorption and chemisorption exists between the inhibitor molecules and metal surface.

3.2.5

3.2.5 Determination of surface charge of the metal

The results discussed thus far indicate that the adsorption of the extract and its fractions on the mild steel surface are dominated by electrostatic interactions (physical adsorption). Adsorption of organic inhibitors on the metal surfaces is primarily associated with the surface charge of metal, chemical structure of inhibitor, the inhibitor’s dipole moment or charge, the chemical composition of the solution, the nature of metal surface and the pH (Amin et al., 2007; Xu et al., 2013). EIS measurements carried out at the different potentials were utilized to determine the surface charge of metal (E_pzc). To obtain more information about the electrostatic adsorption mechanism, plots of the calculated charge transfer resistances (R_ct) versus applied potential were obtained (Fig. 11). The maxima of the parabola curves are termed the PZC of the electrode. The surface charge of the mild steel at open-circuit potential (OCP) was acquired using the following equation:

(11)

E_{r} = E_{ocp} - E_{pzc}

The plot of Rp vs. applied potential in 0.5 M HCl in the absence and presence of 800 mg/L of MeOH extract of G. olivieri and its fractions (MeOH extract: , n-BuOH Fr.: , EtOAc Fr.: , water Fr.: ).

In Eq. (11), E_r is the Antropov’s ‘‘rational’’ corrosion potential (Mallaiya et al., 2011; Döner et al., 2013; Solmaz et al., 2008a ).

The potentials (E_pzc) corresponding to the maximum values of R_ct, were −0.457, −0.452, −0.445, −0.452 V (vs. Ag/AgCl) for 0.5 M HCl solutions containing 800 mg/L concentrations of the MeOH extract, the n-BuOH fraction, the EtOAc-soluble fraction, and water-soluble fraction, respectively. The negative E_r (−10, −10, −20, and −20 mV for the extract and its n-BuOH, EtOAc and water fractions, respectively) indicated that the surfaces of electrodes in the experimental conditions were negatively charged.

According to these results, molecules in the extract and its fractions that act as inhibitors can be directly absorbed by electrostatic forces onto the metal surface. Both the extract and its fractions contain large amounts of secoiridoids and phenolic compounds (Singh et al., 2012). As seen from the Table 1, HPLC analyses show that the methanol extract, the EtOAc-soluble fraction, the n-BuOH-soluble fraction, and the water-soluble fraction contained mainly gentiopicroside (1), representing 56.10%, 41.52%, 54.18% and 37.18%, respectively, of total extract and its fractions. These compounds, as well as others, in the extract and its fractions may exist in the protonated form in 0.5 M HCl. The protonated molecule can be absorbed at cathodic sites in competition with hydrogen ions, and thereby reduce the rate of hydrogen evolution (Döner et al., 2013; Solmaz et al., 2008b; Hamani et al., 2014 ). The effects of the extract and its fractions on the hydrogen evolution reaction were expressly displayed by the polarization measurements. Gentiopicroside (1) and other phenolic constituents, such as isoorientin (2) and sweroside, contain one or two benzene rings and α,β-unsaturated carbonyl carbons. The presence of these groups and the delocalization in the molecules biosynthesized by G. olivieri may contribute to the electrostatic interactions between mild steel surfaces and the inhibitor molecules. Hence, the inhibitory effects of the extract and its fractions can be attributed to the generation of a protective adsorbed film on the steel surface, acting as a barrier against corrosion of the mild steel.

3.3

3.3 Scanning Electron Microscope (SEM) and EDX

The SEM analysis results of the mild steel after 24 h of immersion are presented in Fig. 12. Rough, pitted surface formations and corrosion products were observed (Fig. 12a), resulting from excessive metal dissolution in the aggressive solution. However, the SEM images (Fig. 12(b–d)) of the mild steel corroded in solutions containing 800 mg/L of the extract and its fractions displayed smooth surfaces due to the formation of a protective film on the metal surface. Through examination of the SEM images of the steel treated in the presence of the extract and its fractions, it was clear that the MeOH extract and its n-BuOH fraction, containing a large amount of gentiopicroside (1), gave a material without obvious corrosion defects, pits and cracks on the mild steel surface. The SEM images of material treated in the presence of the EtOAc and water fractions of the extract showed more irregularities.

Surface morphology and EDX results of mild steel in 0.5 M HCl in the absence and presence of 800 mg/L of MeOH extract of G. olivieri and its fractions.

In the current study, EDX spectra were used to determine the elements present on the mild steel surface after 24 h of immersion in the blank and inhibitor-containing solutions. The elemental compositions obtained through EDX are displayed in Fig. 12. The EDX data obtained from mild steel immersed in the 0.5 M HCl solution showed that the electrode surface consisted of excess oxygen due to the formation of iron oxides in the corrosion process. Chlorine is additionally present. Furthermore, the lower percentage of iron present on the mild steel surface exposed to 0.5 M HCl indicated severe corrosion damage. However, the oxygen percentages were significantly reduced by the presence of the extract and its n-BuOH fraction, and there was no Cl detected on the surface of the material treated in the presence of the extract or its n-BuOH fraction. These findings suggested that the high efficiency of the extract and its n-BuOH fraction can be attributed to high contents of gentiopicroside (1) and isoorientin (2), as compared to the other fractions. In accord with these findings, SEM and EDX confirmed that gentiopicroside (1), isoorientin (2) and/or other metabolites contribute to the formation of a protective film on the mild steel. When compared with the other fractions, the corrosion protection provided by the water fraction, containing high proportion of sucrose, decreased after 24 h

4 Conclusions

In the present study, EIS results show that the MeOH extract of G. olivieri and its fractions are effective inhibitors of the corrosion of mild steel. The highest inhibition efficiency calculated from the EIS results was obtained for the n-BuOH-soluble fraction (93.7% at 20 °C). The adsorption of the G. olivieri extract and its fractions on the mild steel surface were found to obey the Langmuir model isotherm. Anodic and cathodic polarization curves indicated that the extract and its fractions are mixed-type inhibitors. Both the thermodynamic parameters and the PZC results demonstrated that the adsorption mechanism is not completely physical or chemical and a combination of chemisorption and predominantly physisorption exists between the inhibitor molecules and metal surface. The thermodynamic parameters show that the SEM and EDX studies also showed that the EtOAc and water fractions partially lose their inhibitory effect after an immersion time of 24 h, which is not the case for the MeOH extract and its n-BuOH fraction. According to these results, the MeOH extract and its n-BuOH fraction, containing the high amounts of gentiopicroside (1) and isoorientin (2), can be used as effective inhibitors against the corrosion of mild steel.

Acknowledgements

The authors are greatly thankful to Çukurova University and Kilis 7 Aralık University research fund. The authors also thank to The Scientific and Technical Research Council of Turkey (TUBITAK).

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Ostovari A., Hoseinieh S.M., Peikari M., Shadizadeh S.R., Hashemi S.J., . Corrosion inhibition of mild steel in 1 M HCl solution by henna extract: A comparative study of the inhibition by henna and its constituents (Lawsone, Gallic acid, a-d-Glucose and Tannic acid) Corros. Sci.. 2009;51:1935-1949.
[Google Scholar]
Ozcan M., Solmaz R., Kardas G., Dehri I., . Adsorption properties of barbiturates as green corrosion inhibitors on mild steel in phosphoric acid. Colloids Surf. A: Physicochem. Eng. Aspects. 2008;325:57-63.
[Google Scholar]
Palou R.M., Olivares-Xomelt O., Likhanova N.V., . Aliofkhazraei M., ed. Environmentally friendly corrosion inhibitors, developments in corrosion protection. InTech; 2014. p. :432-465.
Peng J., Fan G., Hong Z., Chai Y., Wu Y., . Preparative separation of isovitexin and isoorientin from Patrinia villosa Juss by high-speed counter-current chromatography. J. Chromatogr. A. 2005;1074:111-115.
[Google Scholar]
Rajeswari V., Kesavan D., Gopiraman M., Viswanathamurthi P., Poonkuzhali K., Palvannan T., . Corrosion inhibition of Eleusine aegyptiaca and Croton rottleri leaf extracts on cast iron surface in 1 M HCl medium. Appl. Surf. Sci.. 2014;314:537-545.
[Google Scholar]
Rosliza R., Wan Nik W.B., Senin H.B., . The effect of inhibitor on the corrosion of aluminum alloys in acidic solutions. Mater. Chem. Phys.. 2008;107:281-288.
[Google Scholar]
Sezik E., Aslan M., Yesilada E., Ito S., . Hypoglycaemic activity of Gentiana olivieri and isolation of the active constituent through bioassay-directed fractionation techniques. Life Sci.. 2005;76:1223-1238.
[Google Scholar]
Alaneme K.K., Olusegun S.J., Alo A., . Corrosion inhibitory properties of elephant grass (Pennisetum purpureum) extract: effect on mild steel corrosion in 1 M HCl solution. Alexandria Eng. J.. 2016;xxx xxx–xxx (in press)
[Google Scholar]
Sığırcık G., Tüken T., Erbil M., . Inhibition efficiency of aminobenzonitrile compounds on steel surface. Appl. Surf. Sci.. 2015;324:232-239.
[Google Scholar]
Singh S., Cps Y., Noolvi M.N., . Immunomodulatory activity of butanol fraction of Gentiana olivieri Griseb. on Balb/C mice. Asian Pac. J. Trop. Biomed.. 2012;2:433-437.
[Google Scholar]
Solmaz R., Kardas G., Yazıcı B., Erbil M., . Investigation of adsorption and inhibitive effect of 2-mercaptothiazoline on corrosion of mild steel in hydrochloric acid media. Electrochim. Acta. 2008;53:5941-5952.
[Google Scholar]
Solmaz R., Kardas G., Yazıcı B., Erbil M., . Adsorption and corrosion inhibitive properties of 2-amino-5-mercapto-1,3,4-thiadiazole on mild steel in hydrochloric acid media. Colloids Surf. A: Physicochem. Eng. Aspects. 2008;312:7-17.
[Google Scholar]
Soltani N., Tavakkoli N., Kashani M.K., Mosavizadeh A., Oguzie E.E., Jalali M.R., . Silybum marianum extract as a natural source inhibitor for 304 stainless steel corrosion in 1.0 M HCl. J. Ind. Eng. Chem.. 2014;20:3217-3227.
[Google Scholar]
Soltani N., Tavakkoli N., Khayatkashani M., Jalali M.R., Mosavizade A., . Green approach to corrosion inhibition of 304 stainless steel in hydrochloric acid solution by the extract of Salvia officinalis leaves. Corros. Sci.. 2012;62:122-135.
[Google Scholar]
Su W., Tian Y., Peng S., . The influence of sodium hypochlorite biocide on the corrosion of carbon steel in reclaimed water used as circulating cooling water. Appl. Surf. Sci.. 2014;315:95-103.
[Google Scholar]
Suedile F., Robert F., Roos C., Lebrini M., . Corrosion inhibition of zinc by Mansoa alliacea plant extract in sodium chloride media: extraction, characterization and electrochemical studies. Electrochim. Acta. 2014;133:631-638.
[Google Scholar]
Toker G., Edis M., Yeşilada E., . Quantitative analysis of isoorientin in several Turkish Gentiana species by high performance liquid chromatography. FABAD J. Pharm. Sci.. 2011;36:149-154.
[Google Scholar]
Torres V.V., Rayol V.A., Magalhães M., Viana G.M., Aguiar L.C.S., Machado S.P., Orofino H., D’Elia E., . Study of thioureas derivatives synthesized from a green route as corrosion inhibitors for mild steel in HCl solution. Corros. Sci.. 2014;79:108-118.
[Google Scholar]
Umoren S.A., Li Y., Wang F.H., . Electrochemical study of corrosion inhibition and adsorption behaviour for pure iron by polyacrylamide in H₂SO₄: synergistic effect of iodide ions. Corros. Sci.. 2010;52:1777-1786.
[Google Scholar]
Verma C., Singh A., Pallikonda G., Chakravarty M., Quraishi M.A., Bahadur I., Ebenso E.E., . Aryl sulfonamidomethylphosphonates as new class of green corrosion inhibitors for mild steel in 1 M HCl: electrochemical, surface and quantum chemical investigation. J. Mol. Liq.. 2015;209:306-319.
[Google Scholar]
Wang D., Xu M., Zhu H.T., Chen K.K., Zhang Y.J., Yang C.R., . Biotransformation of gentiopicroside by asexual mycelia of Cordyceps sinensis. Bioorg. Med. Chem. Lett.. 2007;17:3195-3197.
[Google Scholar]
Wang Y.M., Xu M., Wang D., Yang C.R., Zeng Y., Zhang Y.J., . Anti-inflammatory compounds of “Qin-Jiao”, the roots of Gentiana dahurica (Gentianaceae) J. Ethnopharmacol.. 2013;147:341-348.
[Google Scholar]
Xu B., Liu Y., Yin X., Yang W., Chen Y., . Experimental and theoretical study of corrosion inhibition of 3-pyridinecarbozalde thiosemicarbazone for mild steel in hydrochloric acid. Corros. Sci.. 2013;74:206-213.
[Google Scholar]
Xu B., Yang W., Liu Y., Yin X., Gong W., Chen Y., . Experimental and theoretical evaluation of two pyridinecarboxaldehyde thiosemicarbazone compounds as corrosion inhibitors for mild steel in hydrochloric acid solution. Corros. Sci.. 2014;78:260-268.
[Google Scholar]
Yıldız R., . An electrochemical and theoretical evaluation of 4,6-diamino-2-pyrimidinethiol as a corrosion inhibitor for mild steel in HCl solutions. Corros. Sci.. 2015;90:544-553.
[Google Scholar]
Zhang X.L., Jiang Zh.H., Yao Zh.P., Song Y., Wu Zh.D., . Effects of scan rate on the potentiodynamic polarization curve obtained to determine the Tafel slopes and corrosion current density. Corros. Sci.. 2009;51:581-587.
[Google Scholar]
Zhang Z., Tian N., Li X., Zhang L., Wu L., Huang Y., . Synergistic inhibition behavior between indigo carmine and cetyl trimethyl ammonium bromide on carbon steel corroded in a 0.5 M HCl solution. Appl. Surf. Sci.. 2015;357:845-855.
[Google Scholar]
Zheng X., Zhang S., Li W., Yin L., He J., Wu J., . Investigation of 1-butyl-3-methyl-1H-benzimidazolium iodide as inhibitor for mild steel in sulfuric acid solution. Corros. Sci.. 2014;80:383-392.
[Google Scholar]

Appendix A

Supplementary material

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.arabjc.2016.06.008.

Appendix A

Supplementary material

Supplementary Figs. 1–4

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Introduction

Experimental

Preparation of methanol extract and its fractions of G. olivieri
Characterization of methanol extract and its fractions of G. olivieri
Isolation procedures and characterization
Electrodes and electrolytes
Electrochemical measurements
Surface characterization

Results and discussion

Chemical composition of the extract and its fractions
Electrochemical tests
Scanning Electron Microscope (SEM) and EDX

Conclusions

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[Google Scholar]

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[Google Scholar]

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[Google Scholar]

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[Google Scholar]

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[Google Scholar]

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[Google Scholar]

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[Google Scholar]

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[Google Scholar]

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[Google Scholar]

[47] Suedile F., Robert F., Roos C., Lebrini M., . Corrosion inhibition of zinc by Mansoa alliacea plant extract in sodium chloride media: extraction, characterization and electrochemical studies. Electrochim. Acta. 2014;133:631-638.
[Google Scholar]

[48] Toker G., Edis M., Yeşilada E., . Quantitative analysis of isoorientin in several Turkish Gentiana species by high performance liquid chromatography. FABAD J. Pharm. Sci.. 2011;36:149-154.
[Google Scholar]

[49] Torres V.V., Rayol V.A., Magalhães M., Viana G.M., Aguiar L.C.S., Machado S.P., Orofino H., D’Elia E., . Study of thioureas derivatives synthesized from a green route as corrosion inhibitors for mild steel in HCl solution. Corros. Sci.. 2014;79:108-118.
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[50] Umoren S.A., Li Y., Wang F.H., . Electrochemical study of corrosion inhibition and adsorption behaviour for pure iron by polyacrylamide in H₂SO₄: synergistic effect of iodide ions. Corros. Sci.. 2010;52:1777-1786.
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[51] Verma C., Singh A., Pallikonda G., Chakravarty M., Quraishi M.A., Bahadur I., Ebenso E.E., . Aryl sulfonamidomethylphosphonates as new class of green corrosion inhibitors for mild steel in 1 M HCl: electrochemical, surface and quantum chemical investigation. J. Mol. Liq.. 2015;209:306-319.
[Google Scholar]

[52] Wang D., Xu M., Zhu H.T., Chen K.K., Zhang Y.J., Yang C.R., . Biotransformation of gentiopicroside by asexual mycelia of Cordyceps sinensis. Bioorg. Med. Chem. Lett.. 2007;17:3195-3197.
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[Google Scholar]

[54] Xu B., Liu Y., Yin X., Yang W., Chen Y., . Experimental and theoretical study of corrosion inhibition of 3-pyridinecarbozalde thiosemicarbazone for mild steel in hydrochloric acid. Corros. Sci.. 2013;74:206-213.
[Google Scholar]

[55] Xu B., Yang W., Liu Y., Yin X., Gong W., Chen Y., . Experimental and theoretical evaluation of two pyridinecarboxaldehyde thiosemicarbazone compounds as corrosion inhibitors for mild steel in hydrochloric acid solution. Corros. Sci.. 2014;78:260-268.
[Google Scholar]

[56] Yıldız R., . An electrochemical and theoretical evaluation of 4,6-diamino-2-pyrimidinethiol as a corrosion inhibitor for mild steel in HCl solutions. Corros. Sci.. 2015;90:544-553.
[Google Scholar]

[57] Zhang X.L., Jiang Zh.H., Yao Zh.P., Song Y., Wu Zh.D., . Effects of scan rate on the potentiodynamic polarization curve obtained to determine the Tafel slopes and corrosion current density. Corros. Sci.. 2009;51:581-587.
[Google Scholar]

[58] Zhang Z., Tian N., Li X., Zhang L., Wu L., Huang Y., . Synergistic inhibition behavior between indigo carmine and cetyl trimethyl ammonium bromide on carbon steel corroded in a 0.5 M HCl solution. Appl. Surf. Sci.. 2015;357:845-855.
[Google Scholar]

[59] Zheng X., Zhang S., Li W., Yin L., He J., Wu J., . Investigation of 1-butyl-3-methyl-1H-benzimidazolium iodide as inhibitor for mild steel in sulfuric acid solution. Corros. Sci.. 2014;80:383-392.
[Google Scholar]

Inhibitory effect of Gentiana olivieri extracts on the corrosion of mild steel in 0.5 M HCl: Electrochemical and phytochemical evaluation

Abstract

Abstract

Keywords

Gentiana olivieri

Mild steel

EIS

Acid inhibition

SEM/EDX

1 Introduction

2 Experimental

2.1 Preparation of methanol extract and its fractions of G. olivieri

2.2 Characterization of methanol extract and its fractions of G. olivieri

2.3 Isolation procedures and characterization

2.4 Electrodes and electrolytes

2.5 Electrochemical measurements

2.6 Surface characterization

3 Results and discussion

3.1 Chemical composition of the extract and its fractions

3.1.1 HPLC and Fourier transform infrared (FT-IR) spectroscopy of the MeOH extract and its EtOAc, n-BuOH and water fractions

3.2 Electrochemical tests

3.2.1 Open circuit potential (OCP) curves

3.2.2 Potentiodynamic polarization curves

3.2.3 Electrochemical impedance spectroscopy (EIS)

3.2.4 Adsorption isotherms and thermodynamic characterization

3.2.5 Determination of surface charge of the metal

3.3 Scanning Electron Microscope (SEM) and EDX

4 Conclusions

Acknowledgements

References

Appendix A

Supplementary material

Appendix A

Supplementary material

Supplementary Figs. 1–4

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