Combined experimental and computational analysis of green Vachellia Nilotica as a mild steel corrosion inhibitor in acidic medium

Abstract

For mild steel (MS), Vachellia nilotica leaf extract (VNL) was used as a corrosion inhibitor in 1 M HCl. Mass loss (ML) and electrochemical methods were used in the inquiry. Increasing the VNL concentration and decreasing the temperature will increase the inhibition efficiency. Electrochemical techniques, including potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS), revealed that VNL effectively inhibited both cathodic and anodic corrosion reactions. With a good fit, VNL follows the Langmuir isotherm. At 150 ppm, the inhibition efficiency of the VNL inhibitor reaches 91.8% at 298 K. However, it decreased with elevating temperatures and prolonged exposure. A high activation energy (55.1 kJ/mol) for the inhibited solution than that of the blank solution (29.5 kJ/mol) and the free energy of adsorption (-19.3 to -18.4 kJ mol-1) provides evidence for physical adsorption mechanisms in the interaction of the VNL inhibitor with the steel surface. The efficiency of the extract improved with concentration, achieving optimal values of 93.9% (POD) and 92.2% (EIS) at 150 ppm. Furthermore, the negative free energy values confirm the spontaneous nature of this adsorption process. To find out how the VNL inhibitor affected the MS surface, researchers employed Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM), which confirmed the formation of a protective film on the metal surface. Optimized molecular structures of phytochemicals confirmed their inhibitory properties via Quantum chemical calculations (DFT), which showed the molecular inhibitory action of VNL.

Introduction

Metal corrosion is a major concern in both academia and industry, and it has been extensively studied. Acid solutions are commonly used in various industrial processes to remove rust and scale¹. They chose hydrochloric acid (HCl) solution as the corrosive medium because of its strong corrosive properties and broad industrial significance. In processes where metallic materials are regularly exposed to extremely acidic conditions, such as pickling metals, descaling, and acidizing oil wells, HCl is routinely utilized^2,3. Research indicates that these organic compounds exhibit strong inhibitory activity when N, S, and O are present. However, they are costly and have severe, harmful effects on aquatic and animal life, as well as the environment. Consequently, to achieve the goal of using inexpensive, ecologically friendly, plentiful, and efficient molecules with very high inhibitory efficacy and minimal environmental impact, natural products and plant extracts have been proposed. Plant extracts have been widely used to protect metals from corrosion in aggressive acidic environments, such as during acid pickling and cleaning processes⁴. Notably, plant extracts containing various organic compounds, including nitrogen (N), sulfur (S), and oxygen (O) atoms, have demonstrated effectiveness as corrosion inhibitors for metals, especially active ones^5–8.

The ability of plant extracts to reduce corrosion of metals and their alloys under various conditions has been investigated in numerous studies. Usually, the leaves, stems, seeds, or fruits of different plant species are used to make these extracts^9–13.

By forming a barrier on the metal surface, plant extracts prevent corrosion by slowing its progression. The adsorption of phytochemicals onto the metal surface, which prevents corrosive substances from entering and stabilizes the passive oxide layer, may be one of the specific mechanisms behind inhibition^14–21. An environmentally friendly substitute for synthetic inhibitors, Terminalia arjuna²² is a medicinal herb that inhibits mild steel corrosion in 0.2 M HCl, achieving 64.1% efficiency at 30 °C over 3 days. demonstrated notable corrosion inhibition properties when immersed in acidic solutions like 0.5 M HCl and 0.5 M H₂SO₄. It exhibited a remarkable 84% inhibitory efficiency at 30 °C and 0.75 g/L. This substance creates a shielding layer on the metal surface by acting as a mixed-type inhibitor.

The methanolic extract derived from the Arum dioscoridis plant leaf exhibited notable corrosion resistance properties when applied to mild steel immersed in 1 M HCl. The extract’s inhibitory effectiveness initially showed polarization resistance, measured at 761 Ω cm2 at 1000 ppm AD. The high inhibition efficiency was attributed to phytochemicals in the AD extract. A high activation energy (70.34 kJ/mol) for the inhibited solution than that of the blank solution (49.66 kJ/mol). Both physisorption and chemisorption contribute to the formation of a protective layer on the MS surface, inhibiting electrochemical reactions. The adsorption process underlying this inhibitory effect was found to be spontaneous, endothermic, and of mixed-type. The inhibitor molecules adsorbed onto the steel surface as a monolayer, in accordance with the Langmuir isotherm²³.

In a highly corrosive 1 M HCl solution at 25 °C, date palm leaf extracts showed notable corrosion inhibition of low-carbon steel when extracted with several organic solvents, including ethanol, acetone, and butanol. Butanol was the most successful of these solvents, producing a maximum inhibition rate of 82% at a dosage of 400 ppm. In general, the inhibitory efficacy increased with increasing extract dose, reaching 97% protection at 1000 ppm. between higher temperatures, however, inhibitory performance decreased somewhat, falling to 86% between 40 °C and 50 °C and then to 82% at 60 °C²⁴. Vernonia amygdalina (bitter leaf) extracts as green corrosion inhibitors for mild steel and aluminum in various aggressive environments. A survey of previous literature revealed that increasing the concentration of the extract significantly reduced the corrosion rate and enhanced the corrosion inhibition efficiency for various mild steel and aluminum samples studied under different operating conditions.

Vernonia amygdalina (bitter leaf) extracts exhibit significant efficacy in inhibiting the corrosion of mild steel and aluminum in 1 M HCl and 0.5M H₂SO₄ solutions. Physical adsorption is widely reported as the mechanism by which Vernonia amygdalina extracts adhere to the surfaces of mild steel and aluminum. Findings from most studies indicate that the extract’s surface coverage aligns with the Langmuir isotherm model²⁵. Table 1 below shows a list of extract sources used for corrosion inhibition of metals in different media:

Table 1.

Investigations on plant extracts as mild steel corrosion inhibitors.

Inhibitor (Extract)	Sample	Medium	% IE	Refs.
Solid waste extract from tea	Steel	1 M HCl	84.5	26
Acalypha chamaedrifolia leaf	Mild steel	HCl	85.1	27
Rhoeo discolor plant leaf	Mild steel	1 M HCl	87.7	28
Aloe vera	API 5 L steel	Seawater	88.34	29
Ocimum basilicum seed extract	Mild steel	1 M HCl	95.1	30
Myristica fragrans	Mild steel	0.5 M H2SO4	87.81	31
Tectona grandis dye	Mild steel	1 M HCl	54.37	32
Gongronema latifolium	Mild steel	5 M H2SO4	93.7	33
Flavonoids extracted from gongronema latifoliunm	Mild steel	5 M H2SO4	84.0	34
Kigelia pinnata leaf	Mild steel	1 M H2SO4 1 M HCl at 125 ppm	86.53 82.7	35
Vachellia Nilotica (VNL)	Mild steel	1 M HCl	91.8	Our results

The novelty statement for a green inhibitor (VNL extract) is the claim that the study closes a knowledge gap by offering a new, a cheap, biodegradable, economically feasible, environmentally friendly, and naturally occurring substance on the corrosion behavior of MS, and efficient solution and/or by offering previously unheard-of, comprehensive information on its protective mechanism, thus contributing both theoretically and practically to sustainable corrosion control. The extraction process is also reasonably simple and economical. This study is unique not only because it found that VNL extract is a good corrosion inhibitor at low concentration, but also because it thoroughly examined its ability to reduce corrosion rates in 1 M HCl and investigated the phytochemical components responsible for this activity.

This study aims to investigate how VNL inhibits the corrosion of mild steel in a 1 M HCl solution. VNL is considered a promising candidate for corrosion inhibition due to its containing various organic compounds that feature heteroatoms (oxygen and nitrogen) and multiple double bonds and was also selected as a corrosion inhibitor for its environmental friendliness. The present study offers an advantage by providing a detailed investigation of corrosion behavior in HCl medium, emphasizing the effectiveness of the proposed protection method under realistic acidic conditions using different methods [mass loss (ML) method, (PDP), and (EIS)]. The research incorporated various theoretical calculations, including Density Functional Theory (DFT)³⁶.

Materials and methods

Materials

“Mild steel specimens, mseasuring 2 cm x 2 cm x 0.2 cm, were prepared for mass loss experiments and surface analysis.” The mild steel coupons contained mainly iron with trace amounts of elements with different weight% (wt%) as follows: C: 0.170%, Si: 0.026%, Mn: 0.460%, P: 0.012%, Cr: 0.050%, Al: 0.023%, Cu: 0.135% and Ni: 0.050%. The corrosive media is 1 M (37%) HCl, prepared by dilution with bidistilled water. Plant material: Fresh leaf samples were collected, and the plant leaf sample was air-dried and ground into a fine powder using an electric mill. Stock solutions of the inhibitors were prepared by soaking the dried VN plant leaves (50 g) in deionized water (100 mL) for 24 h. The VN extract was filtered to obtain the desired concentrations, and then 1 gm of the VNL extract was dissolved in 1 L of distilled water to create a stock solution (1000 ppm). The prepared extract was stored at 4 °C until further use. Working solutions with concentrations of (25, 50, 75, 100, 125, and 150 ppm) were subsequently prepared by serial dilution of the stock solution, and these concentrations were tested in HCl (1 M). Accordingly, the information of GC-MS spectrum shows that it quantitatively contains most of the following compounds: area% (8.12) of benzene-1,2,3-triol (C₆H₆O₃), 19.06% of N’¹, N’²-dicyclohexylideneoxalohydrazide (C₁₄H₂₂N₄O₂), (E)-3,7,11,15-tetramethylhexadec-2-en-1-yl acetate (C₂₂H₄₂O₂) and 14.24% of (E)-6-(5-(furan-2-yl)-1H-pyrazol-3(2H)-ylidene)-4-methoxycyclohexa-2,4 dienone (C₁₄H₁₂H₂O₃), as the major chemical constituents. The following diagram shows VNL extract and the major chemical constituents (Fig. 1)³⁷.

Fig. 1.

VNL plant and the major chemical constituents.

Methods

Mass loss (ML)

For mass loss analysis, MS samples were prepared according to ASTM G31-72³⁸. To ensure a consistently smooth mild steel (MS) surface, the samples were meticulously abraded with emery sheets of varying grit sizes, from 300 to 1800. Following abrasion, they were degreased using pure acetone and then air-dried. The prepared MS coupons were subsequently stored in a vacuum desiccator to preserve their state for upcoming corrosion measurements³⁹. The MS samples were then submerged in 1 M HCl with and without several concentrations of the investigated VNL. The duration of immersion was measured at different temperatures. The duration of this experiment is merely three hours. Samples are expelled from acidic solutions and reweighted at 30-minute intervals. Every experiment was conducted in a static, aerated environment. Particularly, “the standard deviation (SD) observed among the parallel triplicate experiments was consistently below 5%, “which demonstrates a high level of reproducibility for the measurements. Lastly, the following equations can be used to determine the Corrosion rate (k_corr), inhibition efficiency (IE%), and surface coverage degree (θ) as shown in Eqs. (1, 2, and 3):

1

Where W = weight loss (mg), ρ = density of the material (g/cm³), A = exposed surface area (cm²), and t = exposure time (hours).

2

3

Where represent the corrosion rate with and without the inhibitor at various concentrations, respectively.

Electrochemical studies

To determine how MS corroded in 1 M HCl, potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS)were used. A conventional three-electrode system was employed for the electrochemical measurements. In this setup, platinum (Pt) served as the counter electrode (CE), mild steel (MS) as the working electrode (WE), and saturated calomel electrode (SCE) as the reference electrode^40–42. The electrode area in use measured 1 by 1 cm. After being rinsed and polished with various grades of emery paper, it was cleaned and degreased with acetone, then rinsed with bidistilled water. Every experiment was conducted at 298 K. To ensure the reliability and reproducibility of the outcome results, the tests were conducted in triplicate, and the average values of corrosion parameters are reported. Before starting the electrochemical measurements, the working electrode (WE) was immersed in the test solution for 30 min to achieve a stable open-circuit potential (OCP). Subsequently, key electrochemical parameters, including corrosion potential (E_corr), corrosion current density (icorr), Tafel slopes (βa, βc), and corrosion rate (kcorr), were determined using software integrated with the Gamry Potentiostat/Galvanostat instrument.

Potentiodynamic polarization (PDP) technique

Polarization measurements were conducted at the end of a 30-minute immersion period at 298 K. The electrode potential was automatically swept from − 250 mV to + 250 mV relative to OCP at a scan rate of 0.2 mV s^− 1. Inhibition efficiency (IE%) and surface coverage (θ) were subsequently calculated from the determined corrosion current density (i_corr​) values using Eq. (4):

4

Electrochemical impedance spectroscopy (EIS) method

This non-destructive method confirms the inhibitor’s adsorption on the metal surface. AC impedance measurements were performed at the corrosion potential (E_corr), in air, over a frequency range of 0.1 Hz to 10,000 Hz, with an amplitude of 10 mV. Important factors that may be inferred from the Nyquist graph (generated by the Gamry computer programmer) are the double-layer capacitance (C_dl). Additionally, bode diagrams were created. IE% and θ were subsequently calculated from Eq. (5):

5

Where the charge transfer resistances in the absence and presence of inhibitor are represented by R°_ct, R_ct, respectively.

Theoretical calculations

A quantum-chemical simulation elucidated the adsorption mechanism. The calculated quantum chemical features were correlated with the observed inhibitory effectiveness of the corrosion inhibitor under investigation. The inhibition efficiency of VNL was determined by calculating various electronic and thermodynamic parameters, including the adsorption energy, the energy of the highest occupied molecular orbital (E_HOMO), the energy of the lowest unoccupied molecular orbital (E_LUMO), and the energy gap (ΔE). The compass force field was utilized to model the behavior of the VNL molecule on the MS surface during adsorption, along with the interaction mechanisms between the molecule and the metal surface^43,44. The adsorption behavior of the four major compound extracts from the VNL inhibitor on the Fe (1 1 0) surface was investigated using Monte Carlo (MC) simulations. This computational method allows exploration of possible adsorption configurations by minimizing the system’s total energy via stochastic sampling⁴⁵.

Fourier transform infrared spectroscopy (FTIR) technique

FTIR spectra were recorded using a Perkin-Elmer 1600 spectrophotometer to identify the functional groups present in the VNL inhibitor. For this analysis, the highest concentration of VNL in 1 M HCl, with mild steel (MS) added for three hours, was utilized.

Atomic force microscope (AFM) technique

It is used to identify the morphological characteristics of the outer surface of MS, which become rough due to corrosion or inhibitor adsorption, causing the material to depart from its optimum shape. This method involves immersing MS metal in 1 M HCl for three hours, both with and without VNL. After treatment, the samples were removed, quickly rinsed with distilled water, and dried. The subsequent analysis was performed using a Nano Surf Easy Sweep2 Flex AFM instrument located at the Nanotechnology Center, Mansoura University.

Scanning electron microscopy (SEM)

JEOL JSM-6510 is used in these techniques. MS samples were dipped in 1 M HCl with and without the maximum amount of VNL for only 3 h, roughly following the same method as for AFM analysis. It is also employed to determine the morphological characteristics of the MS outer surface. A few photos of the MS surface were captured at different magnification levels.

Energy dispersive X-ray (EDX)

The microscope used (JEOL JSM-6510 LV, Japan) was equipped with an energy-dispersive X-ray (EDX) spectroscopy unit operating at 20 kV, allowing both surface features and elemental composition to be examined simultaneously.

Results and discussion

Mass loss method

The mass loss (ML) method, a gravimetric technique, was used to assess the anti-corrosion activity of VNL. Figure 2 illustrates the relationship between ML and time for MS in 1 M HCl at 298 K, both with and without varying concentrations of VNL. Table 2 presents the results obtained from different inhibitor concentrations. It is evident that 150 ppm of VNL produced the highest inhibitory efficiency, approximately 91.8%. When the inhibitor concentration exceeds 150 mg/L (the optimal concentration), the solution becomes saturated. There is also the possibility of a strong interaction between the inhibitor attached to the metal surface and the molecular inhibitor in the solution⁴⁶. This possibility can cause the coating formed from the inhibitor to be released into the solution. Furthermore, the table indicates a direct correlation: as the VNL concentration increased, the inhibition efficiency increased, while the corrosion rate decreased. This improved efficiency is attributed to the inhibitor’s formation of a protective barrier on the metal surface, which effectively reduces metal disintegration^47,48.

Fig. 2.

Mass loss vs. time for the corrosion of MS in 1.0 M HCl without and with various concentrations from VNL at 298 K.

Table 2.

Mass loss data for MS corrosion at 1 h in 1.0 M HCl without and with various concentrations from VNL at altered temperatures.

Temp., K	Conc., ppm	kcorr., mg cm2min− 1	kcorr., mmy− 1	θ	% IE
298	Blank	0.0931 ± 0.0012	62.3354	--	--
	25	0.0117 ± 0.0016	7.8337	0.874	87.4
	50	0.0103 ± 0.0022	6.8964	0.889	88.9
	75	0.0097 ± 0.0020	6.4946	0.896	89.6
	100	0.0090 ± 0.0018	6.0259	0.903	90.3
	125	0.0082 ± 0.0015	5.4903	0.912	91.2
	150	0.0076 ± 0.0012	5.0886	0.918	91.8
303	Blank	0.1180 ± 0.0013	79.0073	--	--
	25	0.0163 ± 0.0014	10.9137	0.862	86.2
	50	0.0153 ± 0.0017	10.2441	0.87	87
	75	0.0143 ± 0.0021	9.5746	0.879	87.9
	100	0.0131 ± 0.0023	8.7711	0.889	88.9
	125	0.0122 ± 0.0008	8.1685	0.897	89.7
	150	0.0107 ± 0.0022	7.1642	0.909	90.9
308	Blank	0.1460 ± 0.0016	97.7549	--	--
	25	0.0270 ± 0.0015	18.0779	0.815	81.5
	50	0.0253 ± 0.0013	16.9397	0.827	82.7
	75	0.0232 ± 0.0018	15.5336	0.841	84.1
	100	0.0210 ± 0.0017	14.0606	0.856	85.6
	125	0.0196 ± 0.0011	13.1232	0.866	86.6
	150	0.0177 ± 0.0010	11.8511	0.879	87.9
313	Blank	0.1790 ± 0.0012	119.8502	--	--
	25	0.0360 ± 0.0014	24.1039	0.799	79.9
	50	0.0329 ± 0.0023	22.0283	0.816	81.6
	75	0.0308 ± 0.0021	20.6222	0.828	82.8
	100	0.0294 ± 0.0020	19.6848	0.836	83.6
	125	0.0269 ± 0.0017	18.0110	0.85	85
	150	0.0234 ± 0.0015	15.6675	0.869	86.9
318	Blank	0.2010 ± 0.0018	134.5804	--	--
	25	0.0420 ± 0.0016	28.12127	0.791	79.1
	50	0.0388 ± 0.0015	25.9787	0.807	80.7
	75	0.0376 ± 0.0014	25.1752	0.813	81.3
	100	0.0348 ± 0.0017	23.3004	0.827	82.7
	125	0.0314 ± 0.0011	21.0240	0.844	84.4
	150	0.0271 ± 0.0012	18.1449	0.865	86.5

Effect of temperature

Using the mass-loss method, the impact of temperature on the corrosion rates of MS in 1 M HCl, with gradually increasing inhibitor levels, was examined over the temperature range of 298 to 318 K. One feature of physisorption is that the IE% of the added inhibitor decreases slightly as the temperature rises. Temperature has an impact on the rate of corrosion of the reaction; Arrhenius indicates that the Eq. (6)⁴⁹:

6

where A is the Arrhenius constant, E*_a is the activation energy, T is the absolute temperature and R is the gas constant. Figure 3 plots log k_corr vs. 1000/T given straight lines. E*_a can be computed from the slopes of the lines.

Fig. 3.

Arrhenius curves (Log k_corr vs. 1000/T) for MS in 1 M HCl with and without different concentrations of VNL.

According to transition state theory, using Eq. (7):

7

Where the enthalpy of activation () and the entropy are the respective values. Figure 4, a plot of log (k_corr/T) versus 1000/T, shows straight lines. From this graph, ΔH* and ΔS* were derived from the slope (−ΔH* / 2.303 R) and the intercept (log (R / Nh) + ΔS*/2.303 R), respectively. From the data in Table 3, it’s evident that the activation energy (E*_a) for the inhibited solution is higher than that of the uninhibited solution. This trend indicates that the adsorption process is predominantly physical, as higher values of E*_a typically reflect weak electrostatic interactions between the inhibitor molecules and the MS surface. This increase in the energy barrier could be attributed to physical adsorption as an initial step⁵⁰ or to an increase in the adsorption of inhibitor molecules on the MS surface with decreasing temperature. A similar trend is observed for the enthalpy values (ΔH*). Lower enthalpy values in the inhibited solution compared to the reference electrolyte suggest a higher protection efficiency and less aggressive mild steel dissolution when the inhibitor is present. Furthermore, the positive ΔH* values in both the inhibited and uninhibited solutions confirm the endothermic nature of MS dissolution. In this study, the ΔS* values for the inhibited solutions were higher than those for the uninhibited solutions, which suggests an increase in disorder during the transition state of the corrosion process⁵¹. This consistency with the existing literature reinforces the idea that our results support the notion that inhibitors not only increase the activation energy but also enhance activation entropy, thereby effectively mitigating corrosion.

Fig. 4.

Transition-state for MS corrosion in 1 M HCl in the absence and presence of altered concentration of VNL.

Table 3.

Activation thermodynamic parameters for MS dissolving in 1 M HCl with and without different VNL concentrations at different temperatures.

Conc., ppm	Activation parameters
	E*a, kJ mol–1	∆H*, kJ mol–1	-∆S*, J mol–1K–1
Blank	29.5	27.1	173
25	50.2	47.7	121
50	51.8	49.4	116
75	52.5	50.0	114
100	53.1	50.7	113
125	54.2	51.6	110
150	55.1	52.7	108

Adsorption isotherm

The surface coverage parameter was utilized to analyze the adsorption and interaction behavior of the inhibitor on the metal’s surface⁵². The adsorption characteristics were further determined by graphically fitting the data to various adsorption isotherms. It was found that adsorption accurately followed the Langmuir isotherm, as evidenced by a correlation coefficient (R²) close to 1. This indicates the formation of a thin monolayer on the MS surface. By using the Langmuir equation, one can evaluate the adsorption equilibrium constant (K_ads) and observe the relationship between the inhibitor equilibrium concentration (C) and the surface coverage (. The Langmuir adsorption isotherm was derived from the linear correlation between C/θ and concentration (Fig. 5), following the Eq. (8)⁵³:

8

Fig. 5.

Langmuir plots (C/ vs. C) for VNL at different temperatures.

The adsorption equilibrium constant (K_ads) directly indicates the strength of the interaction between inhibitor molecules and the metal surface. In our study, the K_ads values derived from the Langmuir adsorption isotherm at various temperatures consistently showed strong adsorption of VNL on MS, confirming its effective inhibitory action. We determined the standard Gibbs free energy (ΔG°_ads) by using Eq. (9):

9

The standard enthalpy (ΔH_ads°) was determined from the slope resulting from graphing log K_ads vs. 1000/T (Fig. 6) as well as using van’t Hoff Eq. (10):

10

Fig. 6.

Plots of log K_ads vs. 1000/T of VNL inhibitor at various temperatures.

Then, the standard entropy (ΔS°_ads) can be calculated from Eq. (11):

11

Table 4 presents the computed thermodynamic parameters for VNL adsorption. The standard free energy of adsorption (ΔG°_ads​) values, ranging from − 19.3 to -18.4 kJ mol⁻¹, indicate a physical adsorption mechanism. The negative ΔG°_ads value confirms the spontaneity of the adsorption process and the stability of the adsorbed film⁵⁴. Furthermore, negative values of the standard enthalpy of adsorption (ΔH°_ads) indicate an exothermic reaction, typical of systems in which physical adsorption predominates. The positive entropy changes (ΔS°₍ads₎) imply increased disorder at the metal–solution interface, likely due to the replacement of adsorbed water molecules by the inhibitor species as they attach to the steel surface^55,56.

Table 4.

Langmuir adsorption parameters of the examined VNL at different temperature.

Temp., K	Kads,	--ΔG°ads, kJ mol− 1	-ΔH°ads, kJ mol− 1	-ΔS°ads, J mol− 1 K− 1
298	43.8	19.3	33.4	64.7
303	35.4	19.1		62.9
308	29.3	18.9		61.3
313	22.5	18.6		59.2
318	19.0	18.4		57.8

Electrochemical methods

Open circuit potential (OCP)

As a preliminary diagnostic tool for assessing corrosion behavior and inhibition efficiency of the VNL inhibitor (E_OCP), measurements offer basic insights into the electrochemical processes at the steel-electrolyte interface. The change in E_OCP with time for MS in a 1 M HCl solution, with and without varying doses of the VNL inhibitor at 298 K, is shown in Fig. 7. As a result, when inhibitors are present at concentrations between 25 and 150 ppm, the potential increases rapidly towards a more positive potential. After 100 s, the potential stabilizes to reach the stationary value throughout the immersion period. The disintegration of the oxide coating and the formation of a protective film on the metallic surface can both explain this phenomenon⁶⁴.

Fig. 7.

The variation in E_OCP over time for MS in a 1 M HCl solution, both with and without various VNL concentrations at 298 K.

PDP measurements

Figure 8 shows the PDP curves for MS in a 1.0 M HCl solution, both in the absence and presence of various VNL concentrations at 298 K. The corrosion current density (i_corr) was determined by extrapolating the linear portions of the cathodic and anodic Tafel slopes. Table 5 summarizes the key electrochemical parameters obtained, including the inhibition efficiency of VNL (IE %, β_a and β_c), corrosion potential (E_corr), surface coverage (θ), and corrosion current density (i_corr) for each inhibitor concentration. Table 5 data clearly indicate that the corrosion current density (i_corr) for both the cathodic and anodic processes decreases with increasing VNL concentration. This reduction in i_corr directly correlates with an increase in VNL’s inhibition efficiency. Significantly, the Tafel slopes (β_a, β_c​) remain largely unchanged across different VNL concentrations. This suggests that the fundamental corrosion mechanism itself is not altered by VNL. Instead, inhibition primarily occurs because VNL molecules block active sites on the electrode surface through adsorption⁵⁷. Furthermore, the relatively constant corrosion potential (E_corr) values observed suggest that VNL acts as a mixed-type inhibitor⁵⁸. In summary, these potentiodynamic results are consistent with the findings from the gravimetric method, reinforcing VNL’s effectiveness as a corrosion inhibitor.

Fig. 8.

PDP curves of MS dissolution with and without altered doses of VNL inhibitor at 298 K.

Table 5.

Corrosion parameters obtained from PDP for MS in one molar HCl with various VNL inhibitor doses.

Conc, ppm	icorr. µA cm− 2	-Ecorr. mV vs. SCE	βa mV dec− 1	βc mV dec− 1	kcorr mpy	Θ	% IE
0.0	946 ± 0.2017	452 ± 0.1728	106	147	823	--	--
25	258 ± 0.2145	453 ± 0.1625	91	145	209	0.727	72.7
50	215 ± 0.1741	447 ± 0.1514	88	140	98	0.773	77.3
75	149 ± 0.1654	452 ± 0.1101	93	133	67	0.842	84.2
100	104 ± 0.1912	459 ± 0.1524	89	130	48	0.890	89.0
125	73 ± 0.1841	447 ± 0.1647	98	128	35	0.923	92.3
150	58 ± 0.1914	448 ± 0.1210	96	119	26	0.939	93.9

EIS measurements

This experiment includes Nyquist plots (Fig. 9a) of MS immersed in 1 M HCl, with and without different concentrations of the VNL inhibitor. This diagram shows a perfect semicircle. Figure 9a presents distinct impedance patterns, characterized by a prominent capacitive loop at high frequencies and a smaller inductive loop at low frequencies. Notably, the semicircular diameter in the Nyquist plots increases with higher VNL concentrations. This expansion is attributed to the adsorption of VNL onto the mild steel (MS) surface, which forms a protective film^59,60. This film effectively hinders corrosion of the electrode surface in the aggressive test medium. Additionally, the Nyquist plots deviate from perfect semicircles, a phenomenon often linked to surface heterogeneities, roughness, mass transport effects, and the dynamic adsorption-desorption behavior of VNL molecules at the MS/electrolyte interface^61–63. The Bode plots (Fig. 9b) reveal that increasing VNL concentration results in higher total impedance (Z) and a shift in the phase angle towards higher values. This shift is attributed to the adsorption of VNL onto the MS surface. To thoroughly analyze the EIS spectra, a suitable electrical equivalent circuit (shown in Fig. 10) was employed. This circuit’s components include the solution resistance (R_s), the charge-transfer resistance of the interfacial corrosion reaction (R_ct), and the double-layer capacitance (C_dl). Analysis of the EIS data in Table 6 reveals a clear trend: R_ct​ values increase while C_dl​ values decrease with higher concentrations of the VNL inhibitor. This observation indicates that VNL molecules are adsorbing onto the MS surface, effectively replacing water molecules at the metal/electrolyte interface. This adsorption forms a protective layer, which boosts the charge transfer resistance and reduces the double layer capacitance. The excellent agreement with the equivalent circuit is confirmed by the low goodness (chi-squared) χ² values presented in Table 6. The IE_EIS % calculated from these EIS measurements is consistent with the results obtained from both PDP and gravimetric methods, confirming VNL’s effectiveness as a corrosion inhibitor. VNL was adsorbed on the MS surface by displacing the water molecules that were already adsorbed on the metal surface^64,65. The following Eq. (12) is used to calculate double-layer capacitance (C_dl):

12

Fig. 9.

The Nyquist (a) and Bode (b) curves for MS dissolution in one molar HCl in with and without different doses of VNL at 298 K.

Fig. 10.

Equivalent circuit utilized for EIS examination.

Table 6.

EIS parameters for MS after dipping in one molar HCl in the presence of various VNL doses at 298 K.

Conc., ppm	Y0 (µ Ω−1 sn cm−2)	n	Rct Ω cm−2	Cdl µF cm−2	θ	% IE	Goodness of Fit2)
0	408	0.881	41 ± 0.1621	235 ± 0.1317	--	--	16.71 × 10− 3
25	197	0897	148 ± 0.1328	131 ± 0.1784	0.723	72.3	15.81 × 10− 3
50	160	0.899	237 ± 0.1187	111 ± 0.1987	0.827	82.7	12.98 × 10− 3
75	142	0.919	268 ± 0.1387	106 ± 0.1532	0.847	84.7	18.41 × 10− 3
100	127	0.927	301 ± 0.1214	98 ± 0.1478	0.864	86.4	17.83 × 10− 3
125	110	0.938	380 ± 0.1467	89 ± 0.1957	0.892	89.2	15.95 × 10− 3
150	97	0.943	527 ± 0.1399	81 ± 0.1564	0.922	92.2	17.87 × 10− 3

where f_max is the angular frequency and is the double layer capacitance.

Surface examination

FTIR spectroscopy

FTIR spectra, as shown in Fig. 11, provide remarkably accurate representations of the functional groups in the solutions and how they behave on the metal surface after adsorption. This technique involves immersion of MS in 1 M HCl with 150 ppm of VNL inhibitor for 3 h. The inhibitors have been adsorbed onto the MS surface because the spectra of the inhibitor solution and the MS surface during immersion are identical⁶⁶. The spectrum of the free extract displays a broad absorption band at 3290 cm^− 1, attributed to O–H and/or N–H stretching vibrations, characteristic of hydroxyl and amine functionalities. Following interaction with the MS surface, this band shifts to 3280 cm⁻¹, a change that indicates participation of these polar groups in the adsorption process through hydrogen bonding and/or coordination with iron sites on the metal surface. The band at 1628 cm⁻¹ in the free extract, attributed to C = O stretching of conjugated carbonyl or amide functionalities, shifts to 1630 cm⁻¹ upon adsorption. This shift points to an interaction between the oxygen lone pairs and the empty d-orbitals of iron atoms, consistent with donor–acceptor chemisorption. Similarly, the band around 1038 cm⁻¹, associated with C–O and/or C–N stretching vibrations, changes in both position and intensity after adsorption, confirming coordination between the extract’s functional groups and iron centers. As a whole, the observed spectral changes of –OH, –NH, –C = O, and –C–O/C–N regions confirm that VNL molecules are adsorbed onto the MS surface. The outcomes show how the inhibitor and MS surface interfere with one another. After immersion, the peaks in the spectrum shifted and disappeared, indicating that the inhibitor interacted with the surface of MS through functional groups.

Fig. 11.

FTIR spectrum of pure VNL and on MS.

AFM analysis

AFM provides excellent microscopic images of the surface topography of MS, allowing for evaluation of the metal’s roughness⁶⁷. Figure 12 shows the 3D AFM morphologies for the outer.

Fig. 12.

AFM 3D images of: (A) is a free sample, (B) is a blank sample, and (C) MS in 1 M HCl with the highest concentration of VNL inhibitor.

surface of pure MS and MS in 1 M HCl with and without the highest concentration of VNL inhibitor, which is 150 ppm for 3 h. The blank sample of MS is clearly extensively corroded due to corrosive attacks as seen by the greatest roughness (687.5 nm) compared to the free sample (17.1 nm) of MS. Acknowledgments to the efficiency of the adsorbed layer of inhibitor on the outer surface, the acquired roughness of the inhibited MS reached 53.2 nm, so, due to the presence of VNL the roughness was decreased to low values. Hence, limiting the corrosion of MS.

SEM analysis

For only 3 h, MS samples were submerged in a corrosive solution, with or without the addition of 150 ppm of VNL inhibitor. In contrast, a free MS sample was not exposed to either corrosive solution or inhibitor (Fig. 13). The presence of the inhibitor protects the MS surface from corrosion and cracking due to exposure to a corrosive medium by forming a protective coating⁶⁸.

Fig. 13.

SEM photos of (a) free MS, (b) MS after 3 h immersion in 1 M HCl only, (c) MS after 3 h immersion in 1 M HCl with 150 ppm of VNL. (a) Free, (b) Blank, (c) VNL.

EDX analysis

Energy-dispersive X-ray (EDX) analysis was performed to assess the elemental composition of the MS samples and to provide further evidence for the adsorption mechanism underlying the inhibition process. Figure 14 shows the EDX spectra of the specimens before exposure and after 3 h of immersion in 1 M HCl, both with and without the inhibitor at 150 ppm, which represents the optimum concentration. The atomic percentages of the detected elements are summarized in Table 7. The EDX examination of the exposed surface, with acid and without the inhibitor (Fig. 14a), shows that the surface composition is primarily iron, with a significant chlorine (Cl⁻) signal. The elemental analysis supported the SEM data (Table 7), with Fe and C peaks intensifying and the O signal decreasing. In contrast, the sample treated with the VNL extract exhibits a lower iron signal, along with Nitrogen (N) and oxygen (O) peaks; the Cl peaks disappeared after the addition of VNL. The presence of these additional elements provides clear evidence for the adsorption of VNL constituents from the extract onto the MS surface. Their incorporation suggests the formation of a protective, inhibitor-derived film that limits iron dissolution and suppresses corrosion in the acidic medium.

Fig. 14.

EDX spectra for MS surface dipping in 1 M HCl solution for 3 h in the non-presence (a) and presence of VNL inhibitor (b).

Table 7.

The EDX analysis of MS was performed after 3 h of immersion in the absence and presence of 150 ppm of the examined VNL.

Samples	Wt. %
	Fe	Mn	O	Si	N	Cl	C
MS in HCl	86.77	0.31	7.08	0.18	----	2.25	3.41
MS + HCl + 150 ppm extract	50.43	0.05	14.25	0.24	18.75	0.06	16.22

Theoretical calculations

Quantum chemical calculations

The VNL inhibitor contains the major four chemical compounds: benzene-1,2,3-triol (C₆H₆O₃), N’¹, N’²-dicyclohexylideneoxalohydrazide (C₁₄H₂₂N₄O₂), (E)-3,7,11,15-tetramethylhexadec-2-en-1-yl acetate (C₂₂H₄₂O₂) and (E)-6-(5-(furan-2-yl)-1H-pyrazol-3(2H)-ylidene)-4-methoxycyclohexa-2,4 dienone (C₁₄H₁₂N₂O₃), which subjected to quantum chemical calculations using the density functional theory (DFT) method in order to evaluate its effectiveness as corrosion inhibitor for MS in 1 M HCl solution. The adsorption process occurs in two ways: either the inhibitor donates electrons to the metal’s unoccupied d-orbital or receives electrons from the metal. The molecule’s ability to give electrons is represented by E_HOMO, and its ability to receive electrons is represented by E_LUMO. The difference between E_HOMO and E_LUMO refers to E (Fig. 15). The electronic characteristics, including HOMO, LUMO, energy gap (ΔE), dipole moment (µ in debye), ionization potential, electron affinity, chemical potential, electronegativity, chemical hardness, softness, electrophilicity, and nucleophilicity, were calculated and presented in Table 8. A significant indicator of the compound’s stability is its HOMO-LUMO energy gap (ΔE). The ionization potential of molecules serves as a measure of their chemical reactivity. A lower ionization energy signifies higher reactivity for atoms and molecules, whereas a higher value indicates greater molecular stability⁶⁹, The order of % IE: (E)-6-(5-(furan-2-yl)-1H-pyrazol-3(2H)-ylidene)-4-methoxycyclohexa-2,4 dienone > N′¹, N′²-dicyclohexylideneoxalohydrazide > (E)-3,7,11,15-tetramethylhexadec-2-en-1-yl acetate > (benzene-1,2,3-triol). As shown in Table 8, the four major compounds exhibit strong inhibitory activity, attributed to their low ionization energies. Furthermore, a high value for softness (σ) and a low value for hardness (η) suggest a significant interaction between the compound and the MS (Corrosion Substrate)⁷⁰. Quantum simulations further confirm that this inhibitor has effective adsorption sites, enabling it to readily adsorb onto the MS surface and form a protective film. Molecular electrostatic potential (MEP) map: Fig. 15 shows the MEP profile of the inhibitor used in this investigation. The area of extremely negative MEP (nucleophilic interaction) and high electron density. Conversely, the highest positive zone (electrophilic attraction)⁷¹.

Fig. 15.

Optimized orbital, Mulliken atomic, and MEP surface for four major compounds extracted from VNL.

Table 8.

Quantum calculation parameters for major active compounds extracted from VNL.

Parameters (Variable)	benzene-1,2,3-triol	N’1,N’2-dicyclohexylideneoxalohydrazide	(E)-3,7,11,15-tetramethylhexadec-2-en-1-yl acetate	(E)-6-(5-(furan-2-yl)-1 H-pyrazol-3(2 H)-ylidene)-4-methoxycyclohexa-2,4 dienone
EHOMO (eV)	-5.260	-5.631	-4.717	-4.157
ELUMO (eV)	-1.545	-3.947	-2.895	-3.018
∆E, (eV) (EL-EH) (energy gap)	3.715	1.684	1.822	1.139
IP (eV), (Ionization potential) IP = − EHOMO	5.260	5.631	4.717	4.157
EA (eV), (Electron affinity) EA = − ELUMO	1.545	3.947	2.895	3.018
η (eV), (Global hardness) (η = (ELUMO − EHOMO) / 2)	1.858	0.842	0.911	0.57
σ (eV− 1), (softness) σ = 1/ η	0.538	1.188	1.098	1.756
µ (debye), (Dipole moment)	5.58	2.03	2.98	11.87
Molecular surface area, Å2	161.111873	343.520955	460.134148	297.640546

Monte Carlo Simulation (MC)

The calculated energies show that the four major compound extracts from the VNL inhibitor exhibit strongly negative adsorption energies on the Fe (1 1 0) surface, confirming spontaneous adsorption. A higher adsorption energy indicates greater reactivity of atoms and molecules, whereas a higher value indicates greater molecular stability. Figures 16 provide visual confirmation of the adsorption geometries of four major compound extracts from VNL inhibitors. The adsorption energy () was calculated as Eq. (13):

13

Fig. 16.

MC of adsorption of four compound extracts from VNL inhibitors on MS.

Where is the total energy of the surface–inhibitor complex, is the energy of the clean Fe (110) surface, and is the energy of the isolated inhibitor molecule in the corresponding phase. A more negative ” indicates stronger adsorption. The first compound (E)-6-(5-(furan-2-yl)-1H-pyrazol-3(2H)-ylidene)-4-methoxycyclohexa-2,4 dienone shows an adsorption energy of − 3578.74 kcal·mol⁻¹. Similarly, N’¹, N’²-dicyclohexylideneoxalohydrazide shows enhanced adsorption (–3457.21 kcal·mol⁻¹), while (E)-3,7,11,15-tetramethylhexadec-2-en-1-yl acetate shows an adsorption energy of − 3214.47 kcal·mol⁻¹, while (benzene-1,2,3-triol) the value decreases further to − 3024.96 kcal·mol⁻¹. The order of % IE: (E)-6-(5-(furan-2-yl)-1H-pyrazol-3(2H)-ylidene)-4-methoxycyclohexa-2,4 dienone > N’¹, N’²-dicyclohexylideneoxalohydrazide > (E)-3,7,11,15-tetramethylhexadec-2-en-1-yl acetate > (benzene-1,2,3-triol). The four major compounds exhibit strong inhibitory activity, attributed to their low ionization energies. These results show that the inhibitors significantly enhance adsorption strength by increasing electrostatic interactions between heteroatoms (N, O) and the metal surface. Figures 16 provide visual confirmation of the adsorption geometries. Overall, the results demonstrate that these four compound extracts from VNL inhibitors are more effective in acidic corrosion environments.

Mechanism of corrosion inhibition

The VNL adsorption on the MS surface produced a coating that protected the MS’s surface from the corrosive medium, as demonstrated by “ML, PDP, EIS, theoretical investigations, and other surface analysis techniques”. The properties of the inhibitor’s active sites, the electron density, and the charge of the metal surface all affect the adsorption of inhibitors onto a metallic surface. As demonstrated by combined experimental investigations and theoretical computations, adsorption behavior is governed by physicochemical properties (including electron density distributions and functional group composition) and steel surface charge characteristics. We observed the formation of a film that was randomly distributed over the entire surface of the MS. Table 4 shows that the calculated ΔG°_ads values, ranging from − 19.3 to -18.4 kJ mol⁻¹, indicate that the VNL extract binds to the MS in a 1 M HCl solution via physisorption. This conclusion is further supported by the observation that inhibition efficiency decreases as temperature rises and by an increase in activation energy (E*_a) in the presence of VNL compared with its absence. Chloride ions (Cl⁻) promote metal corrosion because they are small, highly mobile, and aggressive anions that destabilize protective films and accelerate electrochemical reactions at the metal surface. The ability of the VNL extract to interact with the MS surface is likely due to the presence of nitrogen (N) and oxygen (O) atoms, along with pi (π) electrons, within its molecular structure^72,73.

When VNL is placed in an acidic solution, the inhibitor molecules become easily protonated. This protonated form is subsequently adsorbed onto the MS surface through electrostatic interactions. Although the MS surface is anticipated to be positively charged, the simultaneous presence of an excess of negatively charged Cl- ions adjacent to this positively charged MS surface is also instrumental in promoting the inhibitor’s adsorption onto the metal. In a corrosive environment, VNL molecules and chloride ions form a protective film on the MS surface through physisorption (Fig. 17). This film reduces the contact area between the metal and the corrosive medium, thereby suppressing further iron oxidation^74–76.

Fig. 17.

Mechanism of Corrosion Inhibition.

Conclusions

Based on the results obtained, the following conclusions can be obtained:

• The VNL extract tested proves to be an excellent mixed-type inhibitor for mild steel corrosion in hydrochloric acid solutions.

• Inhibition efficiencies rise with increases in inhibitor concentration and lower temperature, 298 K.

• The inhibition efficiency consistently increases with concentration, and the additive’s adsorption follows the Langmuir adsorption isotherm. This indicates that the inhibitor’s adsorption on mild steel is physical adsorption.

• These findings were corroborated by the micrographs and spectra from the AFM, SEM, and FT-IR analyses, respectively.

• Furthermore, DFT results confirmed that the VNL extract strongly bonded to the mild steel (MS) surface.

• The calculated thermodynamic parameters of adsorption, including activation energy (ΔE*), (ΔH*), and (ΔS*), demonstrated that the inhibitor used effectively reduced the rate of corrosion.

Author contributions

H.M.H. analyzed all measurement data and then wrote the manuscript the text discussed the results and reviewed the manuscript. Author contributions all parts.

Data availability

Data well be made available on request.

Declarations

Competing interests

The authors declare no competing interests.