Valeriana wallichii root extract as a green & sustainable corrosion inhibitor for mild steel in acidic environments: experimental and theoretical study

Abstract

Corrosion inhibition analysis and the adsorption behaviour of Valeriana wallichii root extract for mild steel in 0.5 M H₂SO₄ solution has been investigated utilizing weight loss, electrochemical impendence spectroscopy (EIS), potentiodynamic polarization, SEM, AFM UV-visible spectroscopy, FT-IR, and quantum chemical calculations. Both the electrochemical and weight loss estimations indicated that the root extract of Valeriana wallichii shows excellent inhibition performance of up to 93.47% for mild steel at 500 mg L⁻¹ concentration in 0.5 M H₂SO₄ solution at 298 K. Naphthoic acid, iridoid and an iridoid analogue are the major phytochemical constituents in the extract of Valeriana wallichii, which possess oxygen atoms (lone pair electrons) and π electrons that facilitate adsorption onto the metal surface. The adsorption of this extract obeys the Langmuir adsorption isotherm. Polarization investigations confirmed that the root extract of Valeriana wallichii acts as a mixed type of inhibitor. The surface studies confirm corrosion retardation in the presence of the extract due to the formation of a protective layer on the surface of mild steel.

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1. Introduction

The most widely used alloy in industrial processes, fabrication and construction is mild steel because of its excellent mechanical competency, its easy availability, and most importantly, its low production cost.¹ Acid cleaning processes on steel surfaces lead to corrosion problems. Corrosion can be controlled by using inhibitors.² However, most of the synthetic inhibitors are expensive, toxic and hazardous to the environment.^3,4 This has prompted researchers to find some non-toxic, eco-friendly, biodegradable, cheap and effective green inhibitors.⁵ Considering the above facts, plant extracts, which consist of various organic compounds with heteroatoms, like nitrogen and oxygen, and conjugated bonds, are a good option.^6–8 The organic molecules used as the inhibitor are adsorbed on the metal surface through these heteroatoms and their conjugated system. This adsorption of the inhibitor molecules can either be physical, by electrostatic interaction (physisorption), or chemical, through covalent bond formation (chemisorption).^9,10 The importance of the mechanism of corrosion inhibition, surface analysis and its relation to the electronic and anatomical properties of the inhibitor molecule has also attracted attention through quantum chemical calculations.^10–16 Many researchers have already reported different plants as green corrosion inhibitors for acidic media with good inhibition efficiency; for example: Salvia officinalis shows 96% inhibition efficiency at 2500 mg L⁻¹,¹⁷Osmanthus fragans shows 94% inhibition efficiency at 340 mg L⁻¹,¹⁸Musa paradisiaca shows 90% inhibition efficiency at 300 mg L⁻¹,¹⁹Mangrove tannins shows 89% inhibition efficiency at 6000 mg L⁻¹,²⁰Jasminum nudiflorum shows 92% inhibition efficiency at 1000 mg L⁻¹,²¹Lawsonia inermis shows 92% inhibition efficiency at 1200 mg L⁻¹,²²Dendrocalamus brandisii shows 90% inhibition efficiency at 1000 mg L⁻¹,²³ aqueous coffee grounds shows 83% inhibition efficiency at 400 mg L⁻¹,²⁴Phyllanthus amarus shows 81% inhibition efficiency at 4000 mg L⁻¹,²⁵ black radish shows 92% inhibition efficiency at 1000 mg L⁻¹,²⁶Ginkgo shows 80% inhibition efficiency at 100 mg L⁻¹,²⁷Kola nitida shows 78% inhibition efficiency at 1200 mg L⁻¹,²⁸ radish roots show 79% inhibition efficiency at 100 000 mg L⁻¹,²⁹Murraya koenigii shows 96% inhibition efficiency at 600 mg L⁻¹,³⁰ bamboo leaf shows 89% inhibition efficiency at 200 mg L⁻¹³¹ and Butea monosperma shows 98% inhibition efficiency at 500 mg L⁻¹.³²

Valeriana wallichii is a rhizome herb. It belongs to the Valerianaceae family and is also known as Jatamansi, Hardwickii and Tagar. It is commonly found all throughout India, Pakistan, Nepal and China. The roots of Valeriana wallichii are very easily available in the market. Valeriana wallichii contains naphthoic acids, iridoid and an iridoid analogue as the major phytochemical compounds.^33,34 The presence of various flavonoids in Valeriana wallichii root extract means that it shows an outstanding sleep-wake profile, and hence it is used for the treatment of primary insomnia.^35,36 The presence of various flavonoids, which are very rich in heteroatoms and aromatic rings, means that it also shows anti-corrosive properties. An image of Valeriana wallichii roots and their major phytochemical constituents is shown in Fig. 1.

Fig. 1 (a) Picture of Valeriana wallichii roots and their major chemical constituents: (b) naphthoic acid, (c) iridoid and (d) an iridoid analogue.

The present study is aimed at acquiring an extract from the Valeriana wallichii roots and investigating it as a corrosion inhibitor for mild steel in 0.5 M H₂SO₄ by utilizing methods like gravimetric measurements, electrochemical impedance spectroscopy, and potentiodynamic polarization measurement. In addition, the effect of structural parameters of the inhibitors on the inhibition efficiency, their adsorption mechanism on the metal surface and the correlation between the experimental data and the quantum chemical parameters have been studied.

2. Experimental

2.1. Preparation of the working electrode

The mild steel rod (1 cm diameter) used in this study has the nominal (wt%) composition presented in Table 1. The mild steel was incised and sectioned with a Clarke power hacksaw and ESM 700 excel shaping machine to give mild steel specimens with an average length of 1 cm to use as the test specimens for gravimetric measurements and for electrochemical studies. The same steel specimens were soldered to coat Cu-wires for electrical connections and after that mounted over epoxy resin with a laid open zone of 1 cm². Before any measurements, different grades of emery paper, 100, 320, 600, 800, 1000 and 1200, were used to abrade the surface of the working electrode mechanically. After that, the mild steel specimens were rinsed with distilled water and acetone, and finally dried in warm air flow.

Table 1 Nominal composition (wt%) of mild steel

Element symbol	C	Mn	Si	P	Cu	Ni	Cr	Fe
% Composition	0.08	0.43	0.39	0.12	0.43	0.27	0.45	97.60

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2.2. Preparation of the inhibitor and electrolytes

The raw product of Valeriana wallichii roots was purchased from a market and was validated by Dr Arbeen Ahmad Bhatt under accession number HERB/BOT/LPU-480 in the Department of Biotechnology, Lovely Professional University, Punjab, India. First, the roots of Valeriana wallichii were cleaned with distilled water to remove any remains of mud, and then the roots were dried for 48–72 h in a thermostat at 60 °C, ground and converted to powder form. Then, 100 g of powdered Valeriana wallichii was refluxed with ethanol at 75 °C for 24 h. The refluxed products were filtered, and the pH of the aqueous solution was found to be 9. The filtered solution was evaporated to 100 ml of dark brown residue and then dried in a vacuum drying oven at 60 °C for 48 h. At the point of dark brown colour, the deposit (around 5 g of gummy material) was procured after complete drying and saved in a vacuum desiccator. Sulphuric acid was diluted to 0.5 M H₂SO₄ as the corrosive medium. The test solutions with different concentrations of Valeriana wallichii root extract (100, 200, 300, 400 and 500 mg L⁻¹) were obtained by diluting the extract with 0.5 M H₂SO₄. The corrosive solution (0.5 M H₂SO₄) was prepared from reagent grade, 98% H₂SO₄ (Sigma-Aldrich) and distilled water. The solutions of the inhibitor in 0.5 M H₂SO₄ were prepared with various concentrations (100–500 mg L⁻¹). The greatest solvency of the Valeriana wallichii extracts in 0.5 M H₂SO₄ was observed up to 500 mg L⁻¹.

2.3. Gravimetric measurements

Weight loss estimations of the mild steel specimens were performed as described in ASTM G 31-72.³⁷ Gravimetric measurements (weight loss) were done for 24 h at 298 K, 308 K and 318 K. The blank acid 0.5 M H₂SO₄ solutions that contained 100, 200, 300, 400 and 500 mg L⁻¹Valeriana wallichii root extract were prepared, and then pre-weighed metal specimens were immersed in the test solutions. Following a drenching period, the metal specimens were removed, deliberately rinsed with acetone, dried under nitrogen flow and weighed using a Shimadzu BL-220H/D455006313 electronic balance. The temperature of the experiments was kept constant by keeping the beakers in a PPI Fini X48 water circulated thermostat (±0.5 K exactness). In this study, the average values of three replicates are reported.

2.4. Electrochemical measurements

Electrochemical studies were conducted using a computer-controlled CH instrument electrochemical workstation (Model: CHI-760) in a corrosion cell kit with three electrode configurations. Platinum was used as a counter electrode, mild steel was used as the working electrode and a saturated calomel electrode (SCE) was used as a reference electrode. The temperature remained constant (298 K) for all the experiments in 0.5 M H₂SO₄. The inhibitor at a concentration of 100, 200, 300, 400 or 500 mg L⁻¹ was added to the corrosive medium. The mild steel specimens were inserted into epoxy resin leaving a working zone of 1 cm². The electrode potential was measured against the SCE and the polarization curves were recorded at an examination rate of 1 mV s⁻¹ with various potentials from the detected open circuit potential (OCP), following 60 minutes of immersion. The cathodic and anodic curves of the linear Tafel segments were extrapolated to the corrosion potential to achieve the corrosion current densities. Electrochemical impedance spectroscopy (EIS) estimations were carried out in a frequency range of 100 kHz to 0.01 Hz with a single amplitude perturbation of 5 mV at OCP with a scan rate of 1 mV s⁻¹. Potentiodynamic polarization curves were recorded with a scan rate of 1 mV s⁻¹ starting at −250 mV with respect to the OCP and continuing up to +250 mV vs. SCE. All the experiments were performed in quiescent conditions, while the temperature was maintained constant (298 K) using a water bath. Reliable values were achieved by repeating all electrochemical measurements three times. By applying both techniques, the inhibition efficiency was determined.

2.5. UV-visible spectroscopy

With the help of a Shimadzu UV-1800 UV-Visible absorption spectrophotometer, the UV spectra of Valeriana wallichii in 0.5 M H₂SO₄ were recorded. The spectra were observed in two different situations, i.e. the acidic solution of the inhibitor in which the mild steel specimens were not immersed and the solution in which the steel specimens were immersed for 24 h at 298 K. Both these spectra were compared to explain the mechanism of inhibition.

2.6. FT-IR and ¹H NMR spectroscopy

For a better understanding of the inhibition mechanism, the FTIR spectra of the Valeriana wallichii root extract were recorded. The Valeriana wallichii extract, blended with KBr powder, was transformed into a pellet for FTIR characterization in a Shimadzu FTIR 8400S spectrophotometer at a wavenumber of 500–4000 cm⁻¹. ¹H NMR (CDCl₃) spectra of the inhibitor were recorded on a Bruker Advance II 400 MHz system for identification of the major phytochemical constituents present in the Valeriana wallichii extract.

2.7. Quantum chemical studies

On examining the impact of the molecular structure and electronic properties of the inhibitor molecules on the inhibition efficiency, quantum chemical calculations were carried out utilizing density functional theory (DFT) adopting the B3LYP function with the 6-31G+(d,p) basis set for all molecules, using the Hyperchem 8.0 project programming bundle. Key parameters, for example, the energy of the LUMO (E_LUMO) and the HOMO (E_HOMO), the energy gap (ΔE) between the LUMO and the HOMO, the dipole moment (μ), the electronegativity (χ), the softness (σ) and the hardness (η) of the molecules, the energy distinction between electron exchange of the molecule and back-donation starting with the atom (ΔE Back Donation), the chemical potential (π) and the number of transferred electrons (ΔN) were obtained from the optimized structures in the gas phase. It should be noted that doing the theoretical calculations in the gas phase is a useful method because the results obtained in the gas phase show no significant variation from those obtained in the aqueous phase, while at the same time the time required for the calculation can be significantly decreased.¹⁸

2.8. Surface morphology studies (SEM & AFM analysis)

The morphology of the mild steel surface was observed under a scanning electron microscope (Model: LEO-435VP) and then an atomic force microscope (Model: NT-MDT-INTEGRA) instrument after immersing the pre-treated mild steel specimens in 0.5 M H₂SO₄ solution without and with 500 mg L⁻¹Valeriana wallichii root extract for 24 hours at 298 K, rinsing the surface of the mild steel specimen with double-distilled water and acetone, and then drying. This gives important information about the morphology of the surface of the mild steel.

3. Results and discussion

3.1. Weight loss measurements

The weight loss method, being the simplest technique, is considered as the “gold standard” of corrosion testing. However, since the mass can essentially only be measured to an accuracy of 0.1 mg at best, the reliability of the weight loss estimations is limited because of their variability. As a result, these estimations were performed three times at 298 K, 308 K and 318 K with various inhibitor concentrations (100, 200, 300, 400 and 500 mg L⁻¹) for 24 h, so that they can provide an average rate.²

The corrosion rate was determined in the absence and presence of inhibitor using the following formula:

where W is the weight loss of the mild steel strip (g), C_RW is the corrosion rate (mm year⁻¹), t is the immersion time (h), K is a constant equal to 8.76 × 10⁴, ρ is the density in g cm⁻³, which is 7.86 g cm⁻³ for mild steel according to ASTM G 31-72,³⁷ and A is the surface area (cm²) of mild steel, which is calculated using eqn (2).The inhibition efficiency (IE%) and surface coverage (θ) values were calculated with the help of the following formulae:where C⁰_RW and Cⁱ_RW are the corrosion rate of mild steel in the absence and presence of inhibitor, respectively. θ is the surface coverage value. In eqn (2), D and l are the diameter and the thickness of the specimen, respectively, while d is the diameter of the hole for holding.⁹

The weight loss measurement values were used to plot the Langmuir adsorption isotherm and to calculate the thermodynamic parameters. The results are presented in Table 2. The inhibition efficiency increases with an increase in inhibitor concentration. This phenomenon is caused by the adsorption of active molecules of the Valeriana wallichii extract on the mild steel surface. The highest value of inhibition efficiency is obtained at 500 mg L⁻¹ inhibitor concentration at 298 K.

Table 2 Corrosion parameters for mild steel in 0.5 M H₂SO₄ without and with various concentrations of Valeriana wallichii extract for 24 h at different temperatures

Temperature (K)	Inhibitor concentration (mg L^−1)	C R (mm year^−1)	Efficiency (IE%)	θ
298	0	11.33	—	—
	100	2.48	78.09	0.7809
	200	2.14	81.10	0.8110
	300	1.95	82.77	0.8277
	400	1.16	89.68	0.8968
	500	0.96	91.45	0.9145
308	0	15.29	—	—
	100	3.82	74.96	0.7496
	200	3.60	76.45	0.7645
	300	3.08	79.80	0.7980
	400	2.78	81.78	0.8178
	500	2.44	84.01	0.8401
318	0	19.14	—	—
	100	7.14	62.67	0.6267
	200	5.89	69.20	0.6920
	300	5.40	71.78	0.7178
	400	4.98	73.96	0.7396
	500	4.58	76.03	0.7603

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3.2. UV-visible spectroscopy analysis

The UV spectra of the Valeriana wallichii extract with 0.5 M H₂SO₄ prior to and after immersion of the mild steel specimens in the solution were compared and are shown in Fig. 2. The spectrum of the Valeriana wallichii extract shows adsorption peaks at 278 nm and 321 nm. These peaks can be assigned as π–π* and n–π* transitions, respectively. From the absorption spectra, the absorbance of the solution in which the steel specimen was not immersed is higher in contrast with the solution into which the mild steel specimen was immersed for a period of 24 h. Shifting of the value of the absorption maximum (λ_max) or a change in the value of the absorbance point demonstrates the creation of a complex between Fe²⁺ particles and inhibitor molecules. In conclusion, the results clearly indicate that when the mild steel specimen was immersed in the acidic solution of Valeriana wallichii extract, a few molecules from the solution are adsorbed on the metal surface.³⁸

Fig. 2 UV spectra of the Valeriana wallichii extract after and before immersion of mild steel strips for 24 h at 298 K.

3.3. FT-IR and ¹H NMR studies

FTIR analysis has been performed to obtain some understanding of the possible interactions between the adsorbed molecules and the mild steel surface in an acidic environment. Generally, FTIR provides us with a basic idea of the functional groups present in an inhibitor molecule. Fig. 3 shows the FTIR spectrum of the root extract of Valeriana wallichii, which was investigated to evaluate the main functional groups. It was observed that the peak at 3312.37 cm⁻¹ is due to O–H stretching of alcohols. The additional peak at 2943.34 cm⁻¹ is due to C–H stretching and the peak at 1130.23 cm⁻¹ indicates C–O stretching. In addition, the major phytochemical constituents of the Valeriana wallichii extract were confirmed by ¹H NMR (Fig. S1, ESI†). ¹H NMR (CDCl₃, 400 MHz); δ = 6–7 ppm (H aromatic rings), δ = 1.27 (–CH₃), δ = 5.40 (–OH phenolic), δ = 7.4 (–COOH). The presence of almost all the characteristic peaks as previously reported verifies the presence of phenolic compounds in the Valeriana wallichii root extract.^33,34 The FT-IR and ¹H NMR spectra of the Valeriana wallichii extract show that the anticorrosive properties of the Valeriana wallichii extract are due to the presence of oxygen atoms and aromatic rings.

Fig. 3 FT-IR spectra of the Valeriana wallichii extract.

3.4. Adsorption isotherm & thermodynamic parameters

The interaction between the inhibitor molecules and the mild steel surface is better demonstrated by adsorption isotherms. The inhibition effectiveness of an inhibitor depends on how well the inhibitor particles adsorb on the mild steel surface. The adsorption procedure includes reinstatement of water particles at the metal solution interface as stated by this equation:³⁹

Org_(sol) + nH₂O_(ads) → Org_(ads) + nH₂O_(sol) (5)

where Org_(sol) is the inhibitor molecules in the solution, Org_(ads) is the inhibitor molecules adsorbed on the metal surface, and n is the amount of water molecules replaced by inhibitor molecules. It is already known that the 1st stage of the corrosion restraint of metals is the adsorption of natural inhibitor particles between the metal and the solution. The surface coverage depends on the inhibition effectiveness, described as (θ = IE (%)/100), and the concentration of the inhibitor. The portion of the surface covered by the adsorbed molecules increases with increasing inhibitor concentration. To evaluate the thermodynamic and activation parameters of the corrosion process of mild steel in 0.5 M H₂SO₄, weight loss estimations were carried out at temperatures of 298 K, 308 K and 318 K for 24 h. The obtained information shows that the inhibition effectiveness decreases with increasing temperature. This is because of the dissolution process of the mild steel and the partial desorption of inhibitor molecules from the mild steel surface. The log of C_R is a straight-line function with 1/T (Arrhenius eqn (6)).where E_a is the viable activation energy, R is the gas constant, and λ is the Arrhenius pre-exponential factor. The graph of log C_Rvs. 1/T is a straight line as indicated in Fig. 4(b) with a slope of −E_a/2.303R.

Fig. 4 Adsorption isotherm plots for (a) log C_Rvs. 1/T, (b) log(C_R/T) vs. 1/T and (c) the Langmuir adsorption isotherm (C/θ vs. C) based on weight loss measurements of the Valeriana wallichii extract on a mild steel surface in 0.5 M H₂SO₄ for 24 h.

The values of E_a recorded in Table 3(a) indicate that the thermodynamic activation function (E_a) of the corrosion of mild steel in 0.5 M H₂SO₄ in the presence of Valeriana wallichii extract is higher compared to that of the pure acid solution, demonstrating that the extract shows low IE (%) at raised temperature.

Table 3 (a) Standard thermodynamic parameters for the adsorption of the Valeriana wallichii extract on the mild steel surface in 0.5 M H₂SO₄. (b) Equilibrium adsorption parameters for the adsorption of the Valeriana wallichii extract on the mild steel surface in 0.5 M H₂SO₄ at 298 K

(a)
Inhibitor concentration (mg L^−1)	E a (kJ mol^−1)	ΔH^0 (kJ mol^−1)	ΔS^0 (J K^−1 mol^−1)
Blank	20.67	44.80	−218.66
Valeriana wallichii (500 mg L^−1)	61.37	193.38	56.35

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(b)
Temperature (K)	K ads (L g^−1)	ΔG^0ads (kJ mol^−1)
298	28.62	−25.42
308	38.88	−27.06
318	25.89	−26.86

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An elective plan for Arrhenius mathematical statement 7:

where h is Plank's constant, N is Avogadro's number, ΔS⁰ is the entropy of activation and ΔH⁰ is the enthalpy of activation. A chart of log(C_R/T) vs. 1/T offered a straight line, as shown in Fig. 4(c), with a slope of −ΔH⁰/2.303R and an intercept of log(R/Nh) + ΔS⁰/2.303R, from which ΔS⁰ and ΔH⁰ were ascertained and recorded in Table 3(a).

The positive signs of enthalpy (ΔH⁰) in both systems reflect the endothermic route for the dissolution process, and the shift towards a positive value of entropy (ΔS⁰) indicates that disorder increases on going from the reactants to the activated complex. To further dissect the adsorption nature of the Valeriana wallichii extract on the mild steel surface, the Langmuir adsorption isotherm was utilized to fit the experimental data obtained from the weight loss estimations. The Langmuir adsorption isotherm is a plot of C/θ vs. C, and is shown in Fig. 4(a). The adsorption of inhibitor molecules on the mild steel surface according to the Langmuir adsorption isotherm is provided by the following equations:

where θ is the surface coverage, C is the inhibitor concentration and K_ads is the equilibrium adsorption constant.⁴⁰

The straight fitted lines with slopes of nearly 1 in Fig. 4(a) demonstrate that the absorption of the inhibitor molecules on the mild steel surface obeys the Langmuir adsorption isotherm and the inhibitor particles cover a monolayer on the surface of the mild steel.⁴⁰ The higher value of K_ads calculated from eqn (8) indicates the better adsorption of the inhibitor molecules on the surface of mild steel. Right away the K_ads can be correlated with the standard free energy of adsorption by the following mathematical statement:

ΔG⁰_ads = −R × T ln(C_water × K_ads) (10)

where R is the gas constant, T is the supreme temperature, ΔG⁰_ads is the standard free energy of adsorption and C_water is the concentration of water in the solution. It should be noted that the unit of C_water depends on the K_ads. As per Table 3(b), the unit of K_ads is L g⁻¹, which in turn shows that the unit of C_water is g L⁻¹ with a value of approximately 1 × 10³. The related outcomes are obtained and listed in Table 3(b). It has been shown that if the value of ΔG⁰_ads is −40 kJ mol⁻¹ or more negative, then a coordinate bond type metallic bond may be formed between the inhibitor molecules and the metal surface. This bond is formed due to chemisorption wherever charge sharing, or charge exchange, happens from the natural species to the metal surface. Different to this, if the value is around −20 kJ mol⁻¹ or less negative, than physisorption happens due to electrostatic attraction between the inhibitor and the charged metal surface. Here, in the present study, the value of ΔG⁰_ads lies in the range of −20 kJ mol⁻¹ to −40 kJ mol⁻¹, which most likely indicates that each physisorption and chemisorption occurs inside the framework.

3.5. Potentiodynamic polarization measurements

The impact of the Valeriana wallichii extract concentration on the polarization behaviour of mild steel in 0.5 M H₂SO₄ at 298 K was analysed and the Tafel plots were recorded for different inhibitor concentrations, as shown in Fig. 5. The corrosion current densities were calculated by extrapolation of the straight parts of the anodic and cathodic curves to the points of intersection of the corresponding corrosion potential. The Tafel curves show a decrease in the current densities of the anodic and cathodic extensions in the presence of Valeriana wallichii extract. This ascertained development is the effect of the coating of adsorbed inhibitor molecules on the surface of the mild steel and the decreased dissolution of the steel surface zone. A higher concentration results in a lower current density (i_corr) at 100–500 mg L⁻¹. Surface coverage increases with increasing inhibitor concentration. From the Tafel plots, we obtained the values of the anodic and cathodic Tafel slope (β_a & β_c), the corrosion current density (i_corr), the corrosion potential (E_corr) and the corrosion inhibition efficiency (IE%) with the help of eqn (11), which are listed in Table 4.where iⁱ_corr and i⁰_corr represent the corrosion current density values with and without inhibitor, respectively.

Fig. 5 Potentiodynamic polarization curves for mild steel in 0.5 M H₂SO₄ with 0–500 mg L⁻¹ concentrations of Valeriana wallichii extract at 298 K with a scan rate of 1 mV s⁻¹.

Table 4 Polarization parameters for mild steel in 0.5 M H₂SO₄ without and with different concentrations of Valeriana wallichii extract at 298 K

Inhibitor concentration (mg L^−1)	−E corr (mV vs. SCE)	i corr (μA cm^−2)	β a (mV dec^−1)	−βc (mV dec^−1)	Efficiency (IE%)	θ
0	465	890.90	141.66	164.26	—	—
100	582	195.10	60.17	111.15	78.10	0.7810
200	465	183.30	58.70	111.14	79.42	0.7942
300	477	177.30	75.76	111.64	80.10	0.8010
400	467	62.51	45.02	109.79	92.98	0.9298
500	319	58.15	36.29	114.63	93.47	0.9347

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From Fig. 5, every anodic metal disintegration and cathodic hydrogen evolution reaction was inhibited on the addition of inhibitors to the corrosive medium. The inhibition of these reactions was increased on increasing the inhibitor concentration. The fact that the anodic and cathodic Tafel slopes are almost unaffected before and after including the Valeriana wallichii extract demonstrates that the anodic and cathodic reaction mechanisms are controlled by charge transfer.⁴¹ Thus, the principle constituents of the Valeriana wallichii extract were adsorbed onto the steel surface and inhibit the corrosion process of mild steel without any change in the reaction mechanism. The Valeriana wallichii extract contains a number of natural compounds, which are not only adsorbed on the steel surface through electrostatic interaction, but can also form strong coordination bonds with iron via their heteroatoms. The blending of physisorption and chemisorption means that the active molecules of the Valeriana wallichii extract are strongly adsorbed onto the steel surface, keeping the steel safe from corrosion, which can be verified by the reduction in the corrosion current density values. The values in Table 4 verify that on increasing the concentration of the Valeriana wallichii extract, the corrosion current density decreases.

3.6. Electrochemical impedance spectroscopy measurements

To study the impedance parameters of the mild steel specimens in 0.5 M H₂SO₄ with various concentrations of Valeriana wallichii inhibitor, EIS measurements were performed at 298 K. Before every test, the working electrode was immersed in the test solution for 60 minutes at 298 K to attain a steady potential. It is clear from Fig. 6(a) that E_OCP remains almost constant after 90 minutes of immersion, which is indicative of a steady state. The temperature remained constant (298 K) for all the tests. To determine the impedance parameters from the experimental outcomes, the data were fitted to the electrical equivalent circuit. Fig. 6(b) shows the electrical equivalent circuit in the presence and absence of inhibitor. Astounding fit outcomes were acquired utilizing these circuits. In the equivalent circuit, R_s is the solution resistance, R_ct is the charge exchange resistance and CPE refers to the consistent phase element.

Fig. 6 (a) OCP versus time curves for mild steel in 0.5 M H₂SO₄ solution in the absence and presence of 500 mg L⁻¹Valeriana wallichii extract. (b) Equivalent circuit used to fit the impedance data of mild steel in 0.5 M H₂SO₄ solution in the absence and presence of the Valeriana wallichii extract. (c) Nyquist plots for mild steel in 0.5 M H₂SO₄ with 0–500 mg L⁻¹ concentrations of Valeriana wallichii extract at OCP and 298 K. (d) Bode-Z and Bode-phase plots for mild steel in 0.5 M H₂SO₄ with 0–500 mg L⁻¹ concentrations of Valeriana wallichii extract at OCP and 298 K.

The results are shown in Table 5. The EIS curves (Nyquist and Bode plots) for mild steel with and without Valeriana wallichii extract are shown in Fig. 6(c) and (d). An unfaltering potential (vs. SCE) was promptly accomplished, corresponding to the free E_corr (vs. SCE) of the mild steel. In the Nyquist graph, due to the charge transfer resistance, the semicircle in each curve corresponds to the time constant. Increasing the Valeriana wallichii extract concentration increases the diameter of the capacitive loop from 100 to 500 mg L⁻¹, which implies a promotion of the inhibition effect. The inhibition efficiency based on the impedance study can be calculated using the following formula:

where R_ct and R⁰_ct are the charge transfer resistance with and without the inhibitor, respectively.

Table 5 EIS parameters for mild steel in 0.5 M H₂SO₄ without and with different concentrations of Valeriana wallichii extract at 298 K

Concentration of inhibitor (mg L^−1)	R ct (Ω cm^2)	f max (Hz)	R s (Ω cm^2)	CPE (μF cm^−2)	Efficiency (IE%)	θ
0	15.71	37.60	1.22	269.48	—	—
100	89.10	5.48	0.70	325.40	82.36	0.8236
200	104.14	3.74	1.60	408.70	84.91	0.8491
300	123.09	4.54	1.30	284.83	87.23	0.8723
400	153.49	4.54	0.68	228.42	89.76	0.8976
500	221.48	4.54	1.99	158.31	92.90	0.9290

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The inhibition efficiency is improved on increasing the concentration of the Valeriana wallichii extract, obtaining the highest value of 92.90% at 500 mg L⁻¹. For the EIS curves of the blank solution, the inductive loops exist at low frequency, which results from the relaxation of the adsorbed intermediate products from the steel surface.⁴² For the rest of the concentrations, the disappearance of the inductive loop is usually regarded as a phenomenon of “degradation” for EIS.⁴³ In the Bode plot, the variation trend of the impedance value with Valeriana wallichii extract concentration in the Bode modulus curves is consistent with that of the semicircle diameter in the Nyquist plot. All phase angle-frequency curves show a single wave, which confirms the one-time constant obtained from the Nyquist plot. According to the literature, if the phase angle equals 90° or 0°, the electrochemical behaviour at the steel solution interface is capacitive or resistive, respectively.^44,45 It has been reported that the extract of Valeriana wallichii contains high levels of naphthoic acid, iridoid and an iridoid analogue.^33,34 These molecules can be adsorbed onto the surface of the mild steel by donating π-electrons from the aromatic rings or lone-pairs of electrons from the heteroatoms to the vacant orbital of iron. These compounds can form a protective coating to block the mild steel surface from the corrosive medium by obstructing the transfer of charges and ions. The consequence is plainly that the Valeriana wallichii inhibitor inhibits the corrosion of mild steel at any concentration utilized and on increasing the inhibitor concentration, the inhibition effectiveness builds.²⁹

3.7. Theoretical studies

3.7.1. Quantum chemical calculations. As is already known, a plant extract is an intricate mixture of different phytochemical components. Valeriana wallichii contains naphthoic acid, iridoid and an iridoid analogue as the major chemical constituents. To examine the impact of electronic and molecular structure on the inhibition effectiveness of the inhibitor molecules, quantum chemical calculations have been performed. The optimized structures, lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the major phytochemical compounds of Valeriana wallichii are shown in Fig. 7. The quantum chemical parameters, E_HOMO, E_LUMO and ΔE (energy gap between HOMO and LUMO) were computed. A lower value of E_LUMO describes a propensity towards the acceptance of electrons from the metal surface, while a higher value of E_HOMO shows a tendency of the molecule to give electrons to the metal with empty molecular orbitals. Moreover, a lower value of the energy gap ΔE (E_LUMO − E_HOMO) will result in good inhibition efficiencies, since the energy required to remove an electron from the most recently involved orbital will be minimized.⁴⁶ A higher value of the number of transferred electrons (ΔN) demonstrates a higher capacity to give electrons to the metal surface. The electronegativity (χ) parameter can be used to illustrate the electron holding ability of the molecule. The higher the value of χ, the lower the electron transfer by the inhibitor molecule. Electron donation from the inhibitor to the metal increases with increasing softness (σ) and decreases with increasing hardness (η) of the inhibitor. A higher value of π demonstrates better performance for an inhibitor. All of the computed parameters were computed separately according to the following equations:⁹

ΔE = E_LUMO − E_HOMO (13)

π = −χ (19)

where χ_inh and χ_Fe represent the electronegativity of the inhibitor molecule and iron, respectively. η_inh and η_Fe represent the hardness of the inhibitor molecule and iron, respectively. A value of 7 eV mol⁻¹ has been taken for χ_Fe, and at the same time 0 eV mol⁻¹ has been taken for η_Fe, as per Pearson's electronegativity scale.³⁸η is the softness value of the inhibitor molecule and σ is the chemical hardness of the inhibitor molecule. ΔE_{Back Donation} (eqn (18)) indicates that, at η > 0 and ΔE_{Back Donation} < 0, the charge transfer to a molecule, via back-donation starting with the molecule, is favoured energetically, and π is the chemical potential.^47,48

Fig. 7 Optimized structures and frontier molecular orbital density distributions (HOMO & LUMO) obtained by the DFT/B3LYP/6-31G+(d,p) method of naphthoic acid, iridoid and the iridoid analogue.

Table 6 reveals the key parameters of the significant phytochemical components of Valeriana wallichii, which are thought to be important because of their direct impact on the electronic interaction between the inhibitor molecule and the surface of the mild steel. As per Table 6, the value of E_HOMO of the studied molecules follows the order: naphthoic acid > iridoid > iridoid analogue, which indicates the order of donation of electrons to the vacant orbital of Fe. Naphthoic acid has the lowest E_LUMO (−0.39 eV), which shows that it has the highest capability to accept electrons from the d-orbital of Fe, and also naphthoic acid has the lowest ΔE (0.79 eV), which shows the good stability of the [naphthoic acid–Fe] complex. The fact that it has the highest value of ΔN (7.86 e) shows that naphthoic acid has the greatest ability to donate electrons to the metal surface. The higher value of ΔE_{Back Donation} of naphthoic acid shows the higher stability of the [Fe–naphthoic acid] complex. The higher value of χ of naphthoic acid (0.78 eV) shows that it has the lowest tendency of electron donation to the vacant d-orbital of Fe. Furthermore, inspection of the obtained data shows that the value of χ decreases with increasing softness (σ) and decreasing hardness (η) of the inhibitor molecule.³⁶ Taking every computed parameter and the molecular structures into account, the inhibition effect follows the order: naphthoic acid > iridoid > iridoid analogue. Naphthoic acid can be assumed to be the most essential component in the inhibition performance of the Valeriana wallichii extract. However, since the molecules change extensively under specific condition, any of them might exert a dominant impact; for example, if the examination conditions change, specific characteristics might in turn be upgraded or alternatively suppressed, which may result in a change in the inhibition performance of the Valeriana wallichii extract. In addition, it should be noted that all of the determined parameters were procured in the gas phase and the dissolution effects were neglected. In spite of the fact that a couple of analysts have shown that the registered parameters in the gas phase exhibit no critical difference compared to those in the vicinity of a solvent, it should be pointed out that this strategy can't be exact.¹⁸ As per the literature, specific isolated phytochemical constituents from plant extracts exhibit comparatively low inhibition efficiencies as compared to mixtures of various phytochemical constituents, which means that a mixture of naphthoic acid, iridoid and the iridoid analogue will show better results than each component separately.

Table 6 Calculated quantum chemical parameters of naphthoic acid, iridoid and the iridoid analogue obtained from the DFT/B3LYP/6-31G+(d,p) method in the gas phase

Molecule	E HOMO (eV)	E LUMO (eV)	ΔE (eV)	μ (D)	ΔN (e)	ΔEBack-Donation (eV)	η (eV)	σ (eV^−1)	χ (eV)	π (eV)
Naphthoic acid	−1.18	−0.39	0.79	1.80	7.86	−0.09	0.39	2.53	0.78	−0.78
Iridoid	−1.40	−0.11	1.29	1.20	4.84	−0.16	0.64	1.55	0.75	−0.75
Iridoid analogue	−1.50	−0.37	1.13	1.30	5.36	−0.14	0.56	1.76	0.93	−0.93

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3.8. SEM analysis

SEM micrographs of the mild steel surface following 24 h of immersion in 0.5 M H₂SO₄ at 298 K with (500 mg L⁻¹) and without inhibitor are shown in Fig. 8. As is apparent from the figure, the smooth surface of the polished mild steel (Fig. 8(a)) is determinedly damaged in the absence of inhibitors (Fig. 8(b)) because of the intensive metal dissolution, resulting in an exceedingly porous surface with extensive and profound holes/cracks. In contrast, in the presence of inhibitor (Fig. 8(c)), the steel surface is significantly improved.

Fig. 8 SEM micrographs of the mild steel surface after 24 h of immersion at 298 K in 0.5 M H₂SO₄: (a) abraded mild steel, (b) without inhibitor and (c) with the inhibitor (500 mg L⁻¹).

3.9. AFM analysis

Three-dimensional AFM images of the polished mild steel, the uninhibited mild steel and the inhibited mild steel in 0.5 M H₂SO₄ with Valeriana wallichii extract are shown in Fig. 9(a)–(c). Fig. 9(a) shows that the average roughness value of the polished mild steel is 2.99 nm, with a maximum height of 60 nm. Fig. 9(b) shows that in the absence of the Valeriana wallichii extract, the surface of the mild steel is strongly damaged due to the dissolution of the metal in the acid solution. The average surface roughness in this case is 138.81 nm, with a maximum height of 2.1 μm (2100 nm). Meanwhile, Fig. 9(c) shows the appearance of the mild steel surface with the Valeriana wallichii extract, which has an average surface roughness value of 45.50 nm and a maximum height of 0.12 μm (120 nm). The decrease in height and average surface roughness of the mild steel specimen is due to the adsorption of the Valeriana wallichii inhibitor on the metal surface. It can be said that a protective layer is formed on the surface of the metal.

Fig. 9 AFM images of the mild steel surface after 24 h of immersion at 298 K in 0.5 M H₂SO₄: (a) abraded mild steel, (b) without inhibitor and (c) with the inhibitor (500 mg L⁻¹).

3.10. Proposed mechanism of action

The corrosion inhibition of mild steel in 0.5 M H₂SO₄ solution by the Valeriana wallichii root extract can be explained based on molecular adsorption. In most of the inhibition studies, the formation of donor–acceptor surface complexes between the lone pair/π-electrons of the inhibitor molecules and the empty d-orbitals of the metal was suggested. As mentioned above, the investigated inhibitor has major phytochemical constituents of naphthoic acid, iridoid and an iridoid analogue, which possess a number of heteroatoms and aromatic rings, and thus act as Lewis bases and form coordinate bonds with the empty d-orbitals of the metal, consequently being adsorbed on the metal surface with the creation of a protective layer on the metal surface, which protects the metal from corrosion. The inhibitor has a free energy of adsorption of −25.42 kJ mol⁻¹ at 298 K, which demonstrates that the mechanism of adsorption of the inhibitor molecule on the mild steel surface is a mixture of both chemical and physical adsorption. The adsorption of the inhibitor molecule on the surface of the mild steel occurs through the displacement of adsorbed water molecules from the mild steel surface and the transfer of electrons between the heteroatoms and Fe. Fig. 10 shows a schematic representation of the adsorption behaviour of naphthoic acid on a mild steel surface.

Fig. 10 Schematic representation of the adsorption behaviour of the naphthoic acid molecule on a mild steel surface.

4. Conclusion

The inhibition behaviour of a root extract of Valeriana wallichii was scrutinized using weight loss, electrochemical studies, AFM and SEM surface investigation, UV-visible, FTIR and ¹H NMR spectroscopy procedures, and quantum chemical calculations. As per the theoretical calculations, the inhibition efficiencies of the major phytochemical constituents of Valeriana wallichii towards the corrosion of mild steel in 0.5 M H₂SO₄ decreases in the order of naphthoic acid > iridoid > iridoid analogue. The potentiodynamic polarization estimations demonstrated that this natural product is a mixed-type inhibitor and acts as an adsorptive inhibitor, so by increasing the inhibitor concentration (up to 500 mg L⁻¹), the inhibition effectiveness can be increased up to 93.47%. Electrochemical impedance spectroscopy indicated that the CPE of the electrical double layer decreased, and also the charge transfer resistance increased on increasing the inhibitor concentration. These conclusions additionally confirm that the Valeriana wallichii extract acts just via adsorption at the metal/solution interface. The experimental data were fit well by the Langmuir adsorption isotherm model, and ΔG⁰_ads confirms that these inhibitor molecules are strongly adsorbed onto the mild steel surface via both physisorption and chemisorption. Quantum chemical calculations predict that the naphthoic acid is the best inhibitor for preventing corrosion.

The inhibition behaviour of the Valeriana wallichii extract was additionally confirmed by taking SEM micrographs and AFM pictures of the surfaces exposed to the inhibited solution, which was in extraordinary agreement with the electrochemical tests and weight loss estimations. The most extreme inhibition effectiveness of 93.47% was achieved utilizing an inhibitor concentration of 500 mg L⁻¹ in 0.5 M H₂SO₄ at 298 K.

Conflicts of interest

There is no conflict of interest.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qm00120k

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