Rhus verniciflua as a green corrosion inhibitor for mild steel in 1 M H₂SO₄

Abstract

The methanolic extract of the plant Rhus verniciflua was examined as a corrosion inhibitor for mild steel in 1 M H₂SO₄ through weight loss measurements, ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, electrochemical impedance spectroscopy, and potentiodynamic polarization measurements. The sample surface morphology was analyzed using scanning electron microscopy/energy-dispersive X-ray spectroscopy. The total phenolic content of the R. verniciflua leaf extract was found to be 158 mg g⁻¹, while the total flavonoid content was 19.65 mg g⁻¹. Eleven phenolic compounds, three flavonoid compounds, five derivatives of hydroxycinnamic acid, and four hydroxybenzoic acids were identified in the extract. Polarization studies showed that the R. verniciflua plant extract acts as a good mixed-type inhibitor. The inhibition efficiency was found to increase with an increase in the inhibitor concentration. On the other hand, the inhibition efficiency decreased with an increase in the temperature. The adsorption of the plant extract constituents is discussed based on the Langmuir, Temkin, and El-Awady isotherms. Moreover, the adsorption and thermodynamic parameters corresponding to extract adsorption onto mild steel were calculated and are discussed. The results show that the R. verniciflua plant extract can be employed as an effective corrosion inhibitor with good anticorrosion properties for metals in acidic environments.

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Introduction

Corrosion is one of the most devastating problems faced by most industries. Hence, scientists and engineers are devoting significant resources to corrosion research, concentrating not only on the corrosion behavior of metals in various environments but also on ways of protecting them against corrosion. In the oil and gas industry alone, the total cost of the damage done by corrosion is estimated to be $1.372 billion; this can be broken down to $463 million annually for down hole tubing, $589 million for surface pipeline and facility costs, and $320 million in capital expenditures related to corrosion.^1–7

Mild steel is used commonly in numerous applications in various industries, including for pipelines in the oil and gas industry. This is owing to its low cost, good mechanical properties, high strength and weight ratio, good environmental stability, and high thermal and electrical conductivities.⁸ Of the various mineral acids available, sulfuric acid is one of the more effective and economical ones for use in the industry for acid pickling, petrochemical and electrochemical etching, and oil acidization.^9–13 Mild steel is susceptible to localized corrosion when exposed to aggressive media. In particular, in an aggressive environment, the presence of chloride ions promotes the breakdown of the protective passive layer, leading to corrosion.^14,15 When metals or alloys react with an acidic medium, material loss occurs because of corrosion. Several methods are available for reducing the threat of corrosion. One of the simplest methods is using a corrosion inhibitor. Inorganic substances such as chromates, dichromates, phosphates, borates, silicates, molybdates, arsenates, and tungstates have been found to be effective as inhibitors of metallic corrosion. Organic substances having polar functional groups containing sulfur, nitrogen, and/or oxygen in the conjugated system have also been reported to be good corrosion inhibitors.¹⁵

One of the best methods of preventing corrosion is to use inhibitors that are eco-friendly and green, as this results in few adverse effects on the environment.¹⁶ Generally, corrosion inhibitors affect the health and cause environmental problems, owing to the fact that they are toxic in nature and nonbiodegradable. Thus, when using inhibitors, efforts must be made to choose ones that are eco-friendly, nontoxic, cheap, and easily available. Plant extracts (i.e., extracts from the leaves, root, and stem, among other parts) have been identified as exhibiting good corrosion inhibition properties. Organic inhibitors are compounds that contain nitrogen, oxygen, sulfur, and phosphorous. Further, in the case of such compounds, the molecules are adsorbed onto the active sites on the metal surface through conjugated bonds.^17–21 The use of inhibitors is one of the most promising methods of controlling corrosion in an aggressive environment.^22–25 However, while most organic and inorganic inhibitors are toxic in nature, those obtained from natural sources such as plants are usually nontoxic and eco-friendly.²⁶ These inhibitors get adsorbed on the metal surface and form a compact layer. As a result, the active sites are blocked, resulting in a decrease in the degree of corrosion.^27–30

In the present work, we used the extract from the plant Rhus verniciflua as an effective corrosion inhibitor for mild steel in 1 M H₂SO₄. R. verniciflua is a medicinal plant that belongs to the Anacardiaceae family and exhibits medicinal properties. It is a rich source of phenolic compounds and flavonoids. Further, R. verniciflua has been shown to have antibacterial, antifungal, and anti-inflammatory properties. To the best of our knowledge, the mild steel substrate in 1 M of H₂SO₄ containing R. verniciflua green corrosion inhibitors and it is naturally occurring in environment safe, cheap and for better future.

In this study, we investigated the extract from R. verniciflua as a corrosion inhibitor for mild steel in 1 M of H₂SO₄ using weight loss measurements, potentiodynamic (Tafel) polarization measurements, and electrochemical impedance spectroscopy (EIS). The surface morphologies of the test samples were examined using scanning electron microscopy (SEM), while their elemental compositions were identified using energy-dispersive X-ray spectroscopy (EDX). Ultra-high-performance liquid chromatography (UHPLC) was used to identify the individual phenolic and flavonoid compounds present in the plant extract.

Experimental

Preparation of plant extract

Leaves of R. verniciflua were washed thoroughly with tap water in order to remove the unwanted impurities and then dried in sunlight for one week. They were then crushed completely in a grinder and turned into powder form. Next, 100 g of this powdered material was soaked in 3 L of methanol at room temperature (303 ± 1 K) for 72 h. The solvent was changed thrice in 72 h at intervals of 24 h. The extract was filtered by using Whatman® Grade 1 filter paper. The residue was then dried and the above-described procedure was repeated again. After filtration, the filtrate was evaporated and dried in a rotatory evaporator. The stock solution was prepared by dissolving a known amount of the R. verniciflua extract in 1 M H₂SO₄ and using the kinetic study.³¹

Preparation of mild steel specimens

Sheets of a mild steel (purity of 99%) with dimensions of 3 cm × 1 cm × 0.5 cm (thickness) were mechanically polished using various grades (1/0, 2/0, 3/0, 4/0, 5/0, 6/0, and 7/0) of fine emery paper. Finally, the specimens were cleaned with deionized water, degreased with acetone, and dried at room temperature (303 ± 1 K).

Determination of total phenolic content and total flavonoid content

The total phenolic content (TPC) and total flavonoid content (TFC) of the R. verniciflua plant methanolic extract were analyzed using an Optizen 2120 ultraviolet (UV) spectrometer (Mecasys, Korea) using a previously reported procedure.^32,33 The TPC and TFC values were used to determine the calibration curve.

Extraction of phenolic compounds for UHPLC analysis

1 g of dried samples was extracted using a previously reported procedure.³³ The filtrate was subjected to UHPLC analysis using an Accela UHPLC system (Thermo Scientific, Rochester, NY, USA). The test sample was separated using a HALOC18 (2.7 μm, 2.1 mm × 100 mm) column; the absorbance peak was observed at 280 nm. During the UHPLC analysis, the mobile phases used were the following: solvent A was 0.1% glacial acetic acid in distilled water and solvent B was 0.1% glacial acetic acid in acetonitrile. Before analyzing the plant extract sample, a standard sample was dissolved in methanol, and the various compounds present in it were identified.

Gravimetric weight loss measurements

During the gravimetric measurements, which were performed to study the effect of the concentration of the R. verniciflua extract on its corrosion inhibition efficiency, previously weighed mild steel samples were immersed in 1 M H₂SO₄ with and without the R. verniciflua extract added in various concentrations. After immersion for 3 h, the specimens were removed, washed with distilled water carefully, dried, and again weighed. On the basis of the decrease in the weights of the samples, the extent of surface coverage, inhibition efficiency, and corrosion rate were determined using the following equations.

η (%) = (W₀ − W_i)/(W₀) × 100 (1)

θ = η (%)/100 (2)

C_r = 87.6W/(A × t × d) (3)

where W₀ and W_i are the weight loss values in the absence and presence of the inhibitor, θ is the degree of surface coverage of the inhibitor, A is the area of the mild steel sample (1 cm²), t is the immersion period (in h) and d, the density of mild steel in g cm⁻³. The same procedure was followed for an immersion time of 1 h, in order to determine the effect of temperature; the investigated temperatures were 303, 313, 323, and 333 ± 1 K.

Electrochemical investigations

Tafel polarization measurements and EIS were carried out in a 1 M H₂SO₄ solution, in order to analyze the corrosion process and the corrosion inhibition reaction. The electrochemical experiments (CHI 760C, USA) were performed in a three-electrode cell assemblage, in which the mild steel specimen, a platinum electrode, and a saturated calomel electrode (SCE) were used as the working electrode, counter electrode, and reference electrode, respectively. The potential was measured between the working electrode, which had a surface area of 1 cm², and the SCE, which was kept in contact with the test solution. The degree of Tafel polarization was measured at a scan rate of 1 mV s⁻¹. The corrosion current densities (I_corr) were obtained by extrapolating the linear anodic and cathodic Tafel segments to the corrosion potential. The corrosion inhibition efficiency, η (%), was evaluated from the measured I_corr values using the following relationship:

η (%) = (I′_corr − I_corr)/(I′_corr) × 100 (4)

The EIS measurements performed at an open-circuit potential for frequencies of 10 mHz to 100 kHz at an amplitude of 5 mV. The inhibition efficiency, η (%), of the R. verniciflua plant extract was calculated using the following equation:

η (%) = (R_ct − R′_ct)/R_ct (5)

where R_ct and R′_ct are the charge transfer resistances in the absence and presence of the inhibitor, respectively.

Wide-angle X-ray diffraction analysis

The X-ray diffraction (XRD) analysis was performed to confirm the nature of the R. verniciflua plant extract using a Shimadzu XRD 600 X-ray diffractometer, which was controlled at 40 kV and 30 mA. Nickel-filtrated Cu-Kα radiation with a wavelength 1.54 nm was used for the measurements for the angular range 100 < θ < 900.³⁴

Fourier transform infrared spectroscopy

Fourier transform infrared (FT-IR) spectroscopy was used to identify the major functional groups present in the plant extract as well as the corrosion products abraded from a mild steel specimen immersed in 1 M H₂SO₄ containing the optimal concentration of the plant extract; the frequency range used was 4000–400 cm⁻¹. To record the FT-IR spectra, pellets of the plant extract and the corrosion products were formed using KBr and examined using an FT-IR spectrometer (ATR-IR Affinity-1, Shimadzu, Japan).³⁵

UV-visible spectroscopy

The UV-visible (UV-vis) absorption spectroscopic technique was employed to identify the complex ions in the 1 M H₂SO₄ solution containing the plant extract. The UV-vis spectra of 1 M H₂SO₄ solutions containing 500 ppm of the R. verniciflua extract with and without a mild steel sample immersed for 3 h were recorded with a Perkin-Elmer UV-Visible Lambda 2 spectrometer (Model 3000+).

Surface morphology analysis

Mild steel specimens immersed in 1 M H₂SO₄ with and without 500 ppm of the R. verniciflua plant extract were examined using an SEM system (JEOL, Model JSM 6390) equipped with an EDX attachment, in order to study their morphologies and analyze the predominant elements present in the corrosion product.

Results and discussion

TPC and TFC contents

The TPC and TFC values of the methanolic extract of R. verniciflua are depicted in Fig. 1 in the form of absorbance versus concentration plots. Fig. 1 indicates that the R. verniciflua plant extract exhibited TPC and TFC values of 158 mg g⁻¹ and 19.65 mg g⁻¹, respectively.

Fig. 1 Total phenolic and flavonoid contents of the R. verniciflua extract.

UHPLC analysis of phenolic compounds in R. verniciflua extract

The data obtained from the UHPLC analysis of the R. verniciflua plant extract are shown in Fig. 2. It can be seen that the R. verniciflua extract was composed of three flavonoids, eleven phenolic compounds, five derivatives of hydroxycinnamic acid, and four hydroxybenzoic acids. The average concentrations of the three flavonoids were as follows: 1985.33 μg g⁻¹ of myricetin, 81.02 μg g⁻¹ of quercetin, and 42.65 μg g⁻¹ of kaempferol. With regards to the five hydroxycinnamic acid derivatives, the plant extract contained 29.58 μg g⁻¹ of ferulic acid and 4.02 μg g⁻¹ of chlorogenic acid as well as o, m, and p-coumaric acids in concentrations of 49.87 μg g⁻¹, 1.55 μg g⁻¹, and 5.68 μg g⁻¹, respectively.

Fig. 2 Primary phenolic constituents identified in the R. verniciflua extract by UHPLC analysis.

Further, the R. verniciflua plant extract contained four hydroxybenzoic acids, of which gentisic acid (75.68 μg g⁻¹) and gallic acid (51.23 μg g⁻¹) were present in larger proportions and protocatechuic acid (28.65 μg g⁻¹) and syringic acid (20.54 μg g⁻¹) were present in smaller proportions. The eleven phenolic compounds included rutin (44.65 μg g⁻¹), vanillin (9.57 μg g⁻¹), resveratrol (25.62 μg g⁻¹), naringenin (17.80 μg g⁻¹), formononetin (12.23 μg g⁻¹), biochanin A (4.63 μg g⁻¹), p-hydroxybenzoic acid (10.32 μg g⁻¹), naringin (8.65 μg g⁻¹), trans-cinnamic acid (13.89 μg g⁻¹), catechin (25.36 μg g⁻¹), and hesperetin (10.87 μg g⁻¹). These compounds were present in the plant extract and were adsorbed onto the metal surface when the extract was used as a corrosion inhibitor. During the UHPLC analysis, the R. verniciflua plant extract showed higher concentrations of rutin, gentisic acid, o-coumaric acid, and myricetin are used to study the mild steel specimen inhibit the corrosion inhibitor in 1 M of H₂SO₄.^33,36

Gravimetric measurements

The results obtained from the gravimetric measurements are listed in Table 1. Fig. 1 shows clearly the effect of the inhibitor concentration (100–500 ppm) on the corrosion of mild steel in 1 M H₂SO₄. It can be seen that there was a decrease in the weight loss and the corrosion rate as the concentration of R. verniciflua was increased. This was because an increase in the concentration of the plant extract resulted in the adsorption of a greater amount of the inhibitor onto the mild steel specimen exposed to the aggressive acid medium. Hence, the inhibition efficiency, η (%), increased with the R. verniciflua extract concentration.

Table 1 Inhibition efficiency at various concentrations (C) of the inhibitor with respect to the corrosion of mild steel in 1 M H₂SO₄, as determined by weight loss measurements at 303 ± 1 K

C (ppm)	W (mg cm^−2)	Cr (mmpy)	(θ)	η (%)
Blank	0.4364	75.7585	—	—
100	0.2616	45.4134	0.4005	40.05
200	0.2178	37.8098	0.5309	50.09
300	0.1472	25.5537	0.6626	66.26
400	0.0417	7.239	0.9044	90.44
500	0.0392	6.805	0.9101	91.01

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An increase in the temperature accelerates the hydrogen gas evolution process and hence the rate of metal dissolution.^37,38 The R. verniciflua extract showed an efficiency of 86–50% over the studied temperature range (Table 2). The decline in the inhibitor efficiency might be due to the decreased adsorption and enhanced desorption of the adsorbed inhibitor molecules from the metal surface. This would signify physisorption, which is likely related to the weak van der Waals force between the inhibitor and the metal.

Table 2 Corrosion-related parameters of mild steel in 1 M H₂SO₄, as determined by weight loss measurements at high temperatures

T (K)	W (mg cm^−2)	Cr (mmpy)	η (%)
303	0.0194	10.1034	86.44
313	0.0539	28.0709	71.16
323	0.1057	55.0482	67.31
333	0.2849	148.3748	50.94

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In the case of the acid corrosion of carbon steel, the logarithm of the corrosion rate is a linear function of 1/T, as per the Arrhenius equation:

log(C_r) = −E_a/2.303RT + log K (6)

where K is the Arrhenius pre-exponential factor, R is the molar gas constant, T is the absolute temperature, and E_a is the activation energy for the corrosion process. Morad et al.³⁹ reported that if the E_a value of an inhibited system remains unchanged or decreases, it indicates chemisorption, whereas a higher E_a value suggests physisorption. The E_a values, as calculated from the slope of the Arrhenius plot (Fig. 3) for the inhibitor-containing and inhibitor-free systems were 38.96 kJ mol⁻¹ and 72.58 kJ mol⁻¹, respectively, signifying physical adsorption. To gain more insight into the reason for the change in the entropy (ΔS) and enthalpy of activation (ΔH) related to activation complex formation in the transition state, the log corrosion rate/T was plotted against 1/T (see Fig. 3) using the following equation:

C_r = (RT/Nh)exp(ΔS°/R)exp(ΔH°/RT) (7)

where h is Planck's constant and N is the Avogadro number. The ΔH values, as computed from the slope and intercept of the plots for the inhibitor-treated (30.57 kJ mol⁻¹) and untreated (15.97 kJ mol⁻¹) mild steel specimens indicated an increase in the enthalpy of activation for the metal dissolution process in the inhibitor-containing solution. The positive sign of ΔH suggested that the metal dissolution process was an endothermic one, meaning that metal dissolution did not occur readily.⁴⁰ The ΔS values as determined from the intercept of the linear transition state plots corresponding to the inhibitor-free and inhibitor-containing systems were −22.44 kJ mol⁻¹ and 18.90 kJ mol⁻¹, respectively. The increase in the entropy of activation in the presence of the inhibitor was probably due to an increase in the randomness of the adsorbed molecules on the metal surface.⁴¹

Fig. 3 (a) Arrhenius and (b) transition plots for the corrosion of mild steel in 1 M H₂SO₄ solutions in the absence and presence of the inhibitor in different concentrations.

The inhibition efficiency performance of the present R. verniciflua is better or comparable to many other green inhibitors including Phyllanthus fraternus (85.80%),⁴² Sida acuta (85%),⁴³ Polyalthia longifolia (74%)⁴⁴ and Cryptostegia grandiflora (87.54%)³³ (Table 3).

Table 3 Comparison of present green inhibitor over other green inhibitors of mild steel in 1 M H₂SO₄ at room temperature^a

S. no	Inhibitor	η (%)
a [$] = present work.
1	Phyllanthus fraternus	85.80
2	Sida acuta	85.00
3^[$]	Rhus verniciflua	91.01
4	Polyalthia longifolia	74.00
5	Cryptostegia grandiflora	87.54

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Adsorption isotherms

Generally, the mechanism underlying the inhibition of corrosion by an inhibitor can be explained on the basis of the corresponding adsorption isotherm, which can provide the necessary information regarding the interaction occurring at the metal–inhibitor interface. The inhibition efficiency of the R. verniciflua plant extract mainly depended on the degree of adsorption of its constituents on the metal surface. The stability of the adsorbed molecules varied with the type of adsorption, which was either chemical or physical or both. Hence, it was necessary to study the adsorption isotherm(s). Of the various types of isotherms explored using the surface coverage values obtained from the weight loss measurements, the best fits were observed using the Langmuir, Temkin, and Frumkin isotherms and the El-Awady kinetic–thermodynamic model. The Langmuir adsorption isotherm can be expressed as follows:⁴⁵

C/θ = 1/K_ads + C (8)

where K_ads is the equilibrium constant for the adsorption process, C is the concentration, and θ is the extent of surface coverage (i.e., the fractional inhibition efficiency). The linear plot (Fig. 4a) of C/θ versus C had a correlation coefficient, R², of 0.9497, confirming the suitability of the Langmuir adsorption isotherm and indicating that the monolayer adsorption of the inhibitor molecules did indeed occur on the metal surface.

Fig. 4 Adsorption isotherm plots for mild steel in 1 M H₂SO₄ solutions in the absence and presence of the inhibitor in various concentrations at 303 ± 1 K: (a) Langmuir (b) Temkin, and (c) El-Awady isotherms.

According to the Temkin adsorption isotherm,

KC = exp^−2aθ (9)

where C is the concentration of the inhibitor in the bulk solution, θ is the extent of surface coverage, K is the adsorption equilibrium constant, and a is the lateral interaction parameter. Fig. 4b shows the plot of log C versus θ, which is a straight line. The fact that the Temkin isotherm was applicable in this case supported the hypothesis that corrosion inhibition of mild steel by the plant extract was caused by the adsorption of the extract molecules onto the metal surface. This also confirmed that the monolayer adsorption of the inhibitor molecules occurred on a uniform, homogeneous metal surface and that interactions took place at the adsorption layer. The El-Awady adsorption isotherm is by given the following equation:

log[θ/(1 − θ)] = log K′ + y log C_inh (10)

where θ is the extent of fractional coverage, K is the equilibrium (binding) constant for the adsorption process, C_inh is the concentration of the inhibitor during the adsorption process, and K_ads = K^1/y (y is the area of the metal surface occupied by the molecules of inhibitor, resulting in a number of active sites).

A 1/y value of less than unity indicates the formation of multiple layers of molecules of the extract on the metal surface, while values greater than unity indicate that the inhibitor molecules occupied more than one active site.^46–49 Fig. 4c shows that the data could be fitted to a kinetic–thermodynamic model using a straight line. A 1/y value of 1.6 (Table 4) indicated that each inhibitor molecule occupied more than one active site on the metal surface. The values of the free energy of adsorption, ΔG, and the equilibrium adsorption constant, K, as calculated from the isotherm plots, are listed in Table 4. The ΔG values were lower than −40 kJ mol⁻¹, suggesting physisorption.^50,51

Table 4 Values of the parameters corresponding to the linearized Langmuir, Temkin, and El-Awady adsorption isotherms for the R. verniciflua extract

Isotherms	R^2	K	ΔG	1/y
Langmuir	0.9497	214.68	−23.67	—
Temkin	0.9807	1083.66	−27.72	—
El-Awady	0.9791	1416.17	−28.40	1.6

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Potentiodynamic polarization measurements

The potentiodynamic polarization measurements were carried out in acidic solutions in the presence and absence of the inhibitor. Fig. 5 shows the effect of the R. verniciflua plant extract on the anodic and cathodic reactions of a mild steel specimen in the form of polarization curves. The electrochemical kinetics parameters such as the corrosion potential (E_corr), corrosion current (I_corr), anodic Tafel slope (b_a), and cathodic Tafel slope (b_c), as determined from the polarization curves, are listed in Table 5. A marked decrease in the I_corr value was observed after the addition of the inhibitor; this was because of the blocking of the surface of the mild steel specimen owing to the adsorption of the inhibitor.^52,53 The Tafel plots suggested that the addition of the plant extract reduced the rate of anodic metal dissolution as well as retarded the cathodic hydrogen evolution reaction. Both b_a and b_c were affected; this was indicative of the mixed-mode inhibitive nature of the inhibitor. This was also evident from the change in the E_corr value in the presence of the inhibitor. For changes in E_corr greater than 85 mV in the case of the anode or cathode with respect to the blank, an inhibitor is categorized either as anodic or cathodic, respectively. Otherwise, the inhibitor is considered as a mixed-type one. In the present study, the maximum change in E_corr was 26 mV, confirming the mixed nature of the R. verniciflua plant extract.

Fig. 5 Polarization curves for mild steel in 1 M H₂SO₄ without and with the inhibitor in different concentrations.

Table 5 Corrosion parameters for mild steel at selected concentrations of the inhibitor in 1 M H₂SO₄, as determined by potentiodynamic polarization measurements at 303 ± 1 K

C (ppm)	Tafel slopes (mV dec^−1)		Ecorr	Icorr	η (%)
	ba	bc
Blank	69	121	−473.6	3108	—
100	58	106	−499.7	1451	53.31
200	49	105	−498.5	1268	59.20
300	55	143	−468.4	514.1	83.46
400	55	150	−464.8	402.7	87.04
500	48	156	−461.7	217.2	93.01

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Electrochemical impedance spectroscopy

The Nyquist and Bode plots of mild steel samples immersed in 1 M H₂SO₄ solutions containing the R. verniciflua plant extract in different concentrations are shown in Fig. 6 and Table 6. The Bode plots contained a single well-resolved peak attributable to the charge-transfer process; this was represented by the depressed semicircle in the Nyquist plot. The EIS results were simulated using the Randles equivalent circuit, which is shown in Fig. 7. The circuit allowed one to determine the solution resistance (R_s) and the charge-transfer resistance (R_ct) values. The R_ct values were obtained from the difference in the impedances at low and high frequencies. It is clear that the addition of the inhibitor caused a significant change in the impedance response of the mild steel specimens immersed in the inhibitor-free solution.^54–56

Fig. 6 Bode plots for mild steel in 1 M H₂SO₄ solutions containing the extract in various concentrations.

Table 6 AC impedance parameters for mild steel for selected concentrations of the inhibitor in 1 M H₂SO₄

C (ppm)	Rct (Ω cm^2)	Cdl × 10^5 (μF)	η (%)
Blank	13.79	31.3	—
100	23.04	28.6	53.04
200	28.33	14.9	61.81
300	50.93	14.7	78.76
400	58.95	14.1	81.65
500	99.47	9.25	89.12

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Fig. 7 Equivalent circuit model used for the electrochemical impedance measurements.

The Nyquist semicircles were not perfect and were depressed into the real axis, owing to the roughness and homogeneities of the metal surface (Fig. 8). The diameter of the semicircles increased with the inhibitor concentration and so did the R_ct value. A large R_ct value is generally associated with slower corroding systems. The decrease in the C_dl value of the mild steel specimens due to the decrease in the local dielectric constant and/or an increase in the thickness of the electrical double layer indicated the adsorption of the inhibitor molecules at the metal/solution interface.^57,58

Fig. 8 Nyquist diagram for mild steel in 1 M H₂SO₄ solutions without and with the inhibitor in different concentrations.

FT-IR spectroscopy

The FT-IR spectra of the R. verniciflua plant extract and the protective layer of the extract molecules adsorbed onto a mild steel specimen surface are shown in panels a and b of Fig. 9, respectively. In Fig. 9a, the broad band at 3345.12 cm⁻¹ represented O–H stretching. The small band noticed at 2923.75 cm⁻¹ was due to the C–H stretching vibrations. The bands at 1606.85 cm⁻¹ and 1051.91 cm⁻¹ corresponded to C=C and C–O–C stretching. In the spectrum of the corrosion products scratched from a mild steel specimen immersed in 1 M H₂SO₄ containing 500 ppm of the inhibitor (Fig. 9b), the spectral peaks were shifted to 3376.16 cm⁻¹ and 1077.65 cm⁻¹; these were characteristics of N–H or O–H stretching and O–H stretching, respectively. Thus, it was concluded that the plant extract molecules were adsorbed onto the metal surface at the active sites.^59,60

Fig. 9 FT-IR spectra of mild steel specimens (a) free of the inhibitor and (b) with an adsorbed layer of the inhibitor.

UV-visible spectroscopy

A strip specimen of the mild steel was incubated in the inhibitor for 3 h and taken out and washed with demineralized water. After the strip surface had been washed in fresh water repeatedly, the UV-vis spectrum of the washings was recorded.⁶¹ As can be seen from Fig. 10, the UV-vis spectrum exhibited a change in the absorbance peak positions as well as a change in the maximum absorbance values, indicating the formation of complex ions. Fig. 10a shows that the absorption maxima occurred at 240 nm, 450 nm, and 660 nm. In Fig. 10b, the maximum absorption band occurred at 251 nm and could be assigned to the (n → π*) transition in the plant extract molecules adsorbed onto the mild steel specimen.⁶²

Fig. 10 UV-visible spectra of mild steel specimens (a) free of the inhibitor and (b) with an adsorbed layer of the inhibitor.

Wide X-ray diffraction spectroscopy

XRD analysis was used to confirm further that a protective film formed over the mild steel specimen immersed in the 1 M H₂SO₄ solution containing the inhibitor. Fig. 11a shows XRD peaks at 32.2° and 59.5°; these confirmed the crystalline nature of the plant extract. In Fig. 11b, which shows the XRD pattern of the plant extract adsorbed on the steel specimen, a high-intensity peak can be seen at 44.7° and so can an additional peak at 81.3°. These results also confirmed the adsorption of the protective inhibitor onto the surface of the mild steel specimen.⁶³ In addition FeO and Fe₂O₃ peaks conformed the formation of oxide passivation films.

Fig. 11 XRD spectra of (a) free of the inhibitor and (b) with an adsorbed layer of the inhibitor.

Surface analysis

Scanning electron microscopy. SEM observations were performed to confirm the formation of a protective film of the plant extract on the mild steel specimen surface. Fig. 12 shows SEM images of the mild steel specimens exposed for 3 h to 1 M H₂SO₄ solutions in the presence and absence of 500 ppm of the plant extract. After exposure to the corrosive acid media, the mild steel specimens exhibited a few cracks and irregularities on their surfaces, owing to acid corrosion. In the presence of the inhibitor, however, the rate of corrosion was suppressed, because of the formation of a protective layer of the adsorbed extract molecules on the specimen surface.⁶⁴

Fig. 12 SEM photographs and EDX spectra of mild steel dipped in (a) 1 M H₂SO₄ and (b) 1 M H₂SO₄ with 500 ppm of the inhibitor.

Energy-dispersive X-ray spectroscopy

EDX was performed in order to determine the elements present in the mild steel specimens exposed to the inhibitor-containing and inhibitor-free 1 M H₂SO₄ solutions. Fig. 12a shows the EDX spectrum of the mild steel specimen immersed in the acid solution in the absence of inhibitor; it contained 68.79% Fe, 7.81% C, 0.57% Si, 5.35% N and 17.48% O, while the specimen exposed to the inhibitor (see Fig. 12b) contained 78.74% Fe, 8.81% C, 6.78% N, 1.07% S and 4.60% O. These results also indicated that the specimen exposed to the inhibitor was protected by the adsorbed layer of the inhibitor molecules.

Mechanism of inhibition

The corrosion inhibition of mild steel in 1 M H₂SO₄ by R. verniciflua extract can be explained on the basis of adsorption nature of the inhibitor on the mild steel. The adsorption of inhibitor molecules mainly depends upon the chemical composition and structure of the inhibitor, corrosion medium and nature of the metal surface. The polarization results indicate that the R. verniciflua extract functions as mixed mode of inhibition by controlling both the anodic and cathodic reactions. This might be due to the organic compounds present in the extract such as carbohydrates, proteins, amino acids, tannins, phenolic compounds, saponins and flavonoids contain aromatic rings, π-bonds, hetero atoms in the ring, –OH, –OCH₃ groups etc. In acidic environment the organic constituents of the extract exists in the protonated form or as neutral molecules. Protonated species decrease the hydrogen evolution by adsorption at the cathodic sites of the mild steel. At the anode, the metal dissolution is reduced by the adsorption of neutral molecules through the π-electrons of aromatic rings and lone pair of electrons over hetero atoms.⁶⁵

Conclusions

In the present work, the corrosion inhibition efficiency of an extract of the plant R. verniciflua with respect to mild steel specimens immersed in a 1 M H₂SO₄ solution was studied. The plant extract acted as a good corrosion inhibitor for the mild steel. During the weight loss measurements, the inhibition efficiency increased with an increase in the inhibitor concentration. On the other hand, an increase in the temperature decreased the efficiency. Polarization measurements showed that the plant extract acted as a mixed-type inhibitor in acidic solutions, diminishing both anodic and cathodic corrosion. Impedance analysis suggested that the inhibition of corrosion by the plant extract was owing to charge transfer through the passive layer. The inhibitor molecules adsorbed onto the metal surface could be represented by the Langmuir and Temkin isotherms and obeyed the El-Awady kinetic thermodynamic model. Surface morphology studies based on SEM analysis confirmed the formation of a protective inhibitor molecule layer on the surface of the treated mild steel specimens. Finally, EDX was used to determine the elemental compositions of the treated and untreated metal surfaces. The TPC and TFC values of the plant extract were found to be high, suggesting that the extract, in addition to being a good anticorrosion agent for mild steels, is also eco-friendly.

Acknowledgements

This research was supported by the Basic Science Research Program of the National Research Foundation of Korea (NRF), which is funded by the Ministry of Science ICT and Future Planning, Korea (2015-A423-0057).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09637a

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