Evaluation of vateria indicia leaves extract as a green source of potential corrosion inhibitor against mild steel corrosion in 1M HCl solution: electrochemical, and surface characterisation studies

Abstract

The components of aqueous vateria indica leaves extract (VILE) were formulated as a green corrosion inhibitor for mild steel (MS) in 1 M HCl medium by weight change and electrochemical routes at 298 K. Nyquist plots indicated, as an increase the concentration of VILE, increases the resistance to charge transfer and lowers the value of electrical double layer capacitance. VILE affected both anodic and cathodic potentiodynamic curves decided that VILE components were mixed type. The addition of VILE concentration increases, progressively increasing the percentage inhibition efficiency and reaching a maximum of 93.22% at 150 ppm. The inhibition efficiency of the VILE was evaluated at elevated temperatures. Activation energy values were found to be higher in the presence of VILE compared to blank. The corrosion inhibition process was correlated through the Langmuir adsorption isotherm model using the Arrhenius equation. SEM and AFM images in the occurrence of VILE visualised that, MS surface was least affected compared to the corroded surface in the absence of an inhibitor. FT- IR spectral peaks suggested that the active groups present in the extract strongly interacted over the MS surface and formed a protective layer, this layer decreased the rate of corrosion to a greater extent.

Keywords:

• Aqueous extract of vateria indicia
• mild steel
• corrosion inhibitors
• adsorption isotherm
• tafel plots
• inhibition efficiency

1. Introduction

MS materials are largely utilised in storage tanks, petrochemical industries, and so on. The major difficulty of using MS is easily attacked by acid surroundings. In many industrial operations aqueous acids being extensively applied in support of taking away unwanted scale and rust from the metal surfaces. Favourably HCl solution was treated one to eliminate the scale and rust during industrial process. Chemical compounds are usually added as inhibitors in such operations to oppose both the metal dissolution and acid intake process (Chauhan et al. 2018; Praveen et al. 2021; Addoun et al. 2019; Aljourani, Raeissi, and Golozar 2009).

Many chemical compounds like organic & inorganic compounds particularly these compounds contains heteroatoms, unsaturation bonds, cyclic rings, aromatic groups have been developed as inhibitors for many metals in different corrosive environment (Ahamad et al. 2010; El Ibrahimi et al. 2021; Singh et al. 2018; Hebbar et al. 2020; Dehghani, Bahlakeh, and Ramezanzadeh 2019; Qiang et al. 2017). Schiff base synthesised by the condensation reaction between C=O and –NH₂ groups tend to became potent inhibitor. The most benefit of many Schiff bases are suitably and simply synthesise with comparatively inexpensive materials. Several researchers investigated, Schiff bases possessed –C=N–, hetero atoms such as N, S or O atoms shown significantly and effectively protected the MS corrosion from acid solutions (Bentiss, Lebrini and Lagrenee 2005; Jacob and Parameswaran 2010; Wang et al. 2016; Kalkhambkar and Rajappa 2022; Hosseini and Azimi 2009; Ahamad, Prasad, and Quraishia 2010; Bentiss et al. 2009; Behpour et al. 2010). Many of the synthesised organic inhibitor are possessed toxic and harm to environment. Although various synthetic organic and inorganic compounds can be acts as good corrosion inhibitors in acidic environment as they are highly efficient but they are of more expensive and ecological risks due to the their hazardous environmental effects. The bark, resin, and leaves are used in Ayurvedic, Siddha, Unani, and folk medicine for the treatment of leprosy, eczema, rheumatism, diarrhoea, and ulcers. Various components contained in the plants extracts such as such as alkaloids, pigments, tannins, amino acids etc., are well known and demonstrate good inhibiting achievement towards corrosion of MS in acid media. In the current scenario, the researchers are promoted to utilise the potentiality of the natural products extracts of plant roots, leaves, flowers, seeds, grains, etc., to combat metals corrosion in different corrosive environment. These extracts being are ecologically acceptable, inexpensive, cheaper, biodegradable, non-toxic and readily available (Kalkhambkar et al. 2022; Muthukumarasamy et al. 2020; Guruprasad and Sachin 2021; Farzana, Banu, and Ahamed 2021).

Recently many authors have been working on the use of several extracts of natural plant sources as green source of inhibitors for the corrosion of different metals in acidic environment (Abuelela et al. 2023; Haque et al. 2021; Bahlakeh, Dehghani, et al. 2019; Khan et al. 2015; Lgaz et al. 2015; Oukhrib et al. 2017; Sharma, Peter, and Obot 2015; Asadi et al. 2019).

In the current investigations, an aqueous extract of vateria indicia leaves is chosen to study the anticorrosion behaviour in 1 M HCl solution for MS. The inhibition ability was determined employing weight change and electrochemical methods. Further the investigations are focused on interaction of components of plant extract on MS surface and modification of MS surface during corrosion and inhibition by AFM, SEM and FT-IR analyses.

2. Experimental procedure

2.1. Vateria indica leaves extraction

The image of the Vateria indica leaves is shown in Figure 1. Vateria indica leaves are washed through running water followed by dried out at an ambidient temperature and crushed to powder. About 30 g powder was added to 500 ml double de-ionised water and the whole mixture is then taken in a Soxhlet extraction apparatus and boiled for 3h. The extraction was separated by filtration further reduces volume through water evaporation and dried in microwave oven. By weighing the dried VILE extract, 500 ppm stock solution in 1 M HCl was prepared. Further, the different concentration of inhibitors such as 25, 50, 75, 125 and 150 pmm were prepared using stock solution by dilution method in 1 M HCl solution. The same concentrations were used for weight change and electrochemical methods. Vateria indica leaves extract is labelled as VILE.

Figure 1. Vateria indicia leaves.

2.2. Corrosive solution

About 35.5–36% assay of Analytical grade hydrochloric acid was used to prepare 1 M HCl corrosive solution for MS.

2.3. Mild steel specimens

Rectangular mild steel (MS) sheets of 0.068% C, 0.26% Mn, 0.007% S, 0.014% P and the remaining % of Fe composition was employed for experimental object. These MS sheets mechanically well-polished with different emery papers from grades 400, 800, 1000, 1500 and 2000 and later washed with distilled water and dried. The well dried MS specimens were taken and record the weight of each specimen using a digital weighing balance of 0.01 mg accuracy.

2.4. Weight change measurements

The method is based on the measurement of the weight change of the specimen, which is immersed in 1 M HCl test solution in addition and absence of the extract. The well-polished previously weighted MS coupons were immersed in hanging position in various concentrations of inhibitor solution for different time of interval at 298 ± 1K temperature. After specified time, change in weight in ‘mg’ was noted. The values of weight changes used to determine the rate of corrosion ($V_{\mathit{\text{corr}}}$) in ‘mm/year using Equation (1)(1) $$V_{\mathit{\text{corr}}}\left(\mathit{\text{mm}}{/\mathit{\text{year}}}^{}\right)=\frac{87.6x\mathrm{\hspace{0.17em}}W}{\mathit{\text{Atd}}}$$(1) and followed by percentage inhibition efficiencies using Equation (2)(2) $$\text{\% IE}=1-\frac{V_{\mathrm{\text{corr}}}^1}{V_{\mathrm{\text{corr}}}}\text{x}100$$(2) . (1) $$V_{\mathit{\text{corr}}}\left(\mathit{\text{mm}}{/\mathit{\text{year}}}^{}\right)=\frac{87.6x\mathrm{\hspace{0.17em}}W}{\mathit{\text{Atd}}}$$(1) where w = weight loss in mg, A = exposed surface area in sq.cm, t = Duration of specimen immersed in hour and d = Density of the specimen (g/cm³) (2) $$\text{\% IE}=1-\frac{V_{\mathrm{\text{corr}}}^1}{V_{\mathrm{\text{corr}}}}\text{x}100$$(2) where V_corr = corrosion rate of MS in 1 M HCl and V¹_corr = corrosion rate in presence of extract.

The surface coverage (Ɵ) was calculated using the following Equation (3)(3) $$\mathrm{Ɵ}=1-\frac{V_{\mathrm{\text{corr}}}^1}{V_{\mathrm{\text{corr}}}}$$(3) . (3) $$\mathrm{Ɵ}=1-\frac{V_{\mathrm{\text{corr}}}^1}{V_{\mathrm{\text{corr}}}}$$(3)

2.5. Electrochemical tests

Potentiodynamic polarizations and electrochemical impedance spectroscopic tests were conducted using AUTOLAB model PGSTAT204 linked with software NOVA and associated with FRA32M potentiostat/galvanostat through frequency response analyser controlled by Autolab computer. The ASTM glass cell assembly was used to conduct all the electrochemical tests. The cell equipped with 3 mm MS cylindrical rod of 1 cm exposed area taken as a working electrode, platinum as auxiliary electrode and saturated Ag/AgCl used as a reference electrode. All the electrochemical determinations were recorded at 298 ± 1K and even at different temperature regions from 308-328 ± 1K. In the beginning working MS electrode was suspended half an hour to ensure steady open circuit potential. The EIS studies were performed over a frequency range of 100 kHz–0.1 Hz with signal amplitude 5 mV sine wave as excitation signal and ZSimp Win 3.21 software employed to analysed impedance data. Further the percentage inhibition efficiency ${(ƞ}_{\mathit{\text{EIS}}}$%) was calculated by the Equation (4)(4) $${(\mathit{ƞ}}_{\mathit{\text{EIS}}}\mathit{\%})=\frac{R_p-R_p^0}{R_p}\times 100$$(4) . (4) $${(\mathit{ƞ}}_{\mathit{\text{EIS}}}\mathit{\%})=\frac{R_p-R_p^0}{R_p}\times 100$$(4) where R_p = polarisation resistance in 1 M HCl with addition of VILE and $R_{ p}^0$ = polarisation resistance in 1 M HCl.

Tafel polarisation plots were taken at a scan rate 0.5 mV/S from -200 to +200 mV potential range relative to open circuit potential value. The corresponding corrosion current density (I_corr), corrosion potential (E_corr.), anodic and cathodic tafel slopes were measured by tafel extrapolation system. The ${\% ƞ}_{\mathit{\text{pol}}}$ was calculated using the Equation (5)(5) $${(\mathrm{ƞ}}_{\mathit{\text{pol}}}\mathrm{\%})=\frac{\mathrm{ }{I}_{\mathit{\text{corr}}}^1-I_{\mathit{\text{corr}}}}{\mathrm{ }{I}_{\mathit{\text{corr}}}^1}\times 100\mathrm{ }$$(5) . (5) $${(\mathrm{ƞ}}_{\mathit{\text{pol}}}\mathrm{\%})=\frac{\mathrm{ }{I}_{\mathit{\text{corr}}}^1-I_{\mathit{\text{corr}}}}{\mathrm{ }{I}_{\mathit{\text{corr}}}^1}\times 100\mathrm{ }$$(5) where $I_{\mathit{\text{corr}}}^1$ = corrosion current density in 1 M HCl and $I_{\mathit{\text{corr}}}$ = corrosion current density in 1 M HCl with the addition of VILE. Further from polarisation curve corrosion rate (CR) can be calculated by Equation (6)(6) $$\mathit{\text{CR}}=\frac{I_{\mathit{\text{corr}}}\times T\times M}{F\times S\times d}\times 10$$(6) . (6) $$\mathit{\text{CR}}=\frac{I_{\mathit{\text{corr}}}\times T\times M}{F\times S\times d}\times 10$$(6)

Where T = time (sec.), M = molar weight of iron (g/mol), F = Faraday constant (96,500 C/mol), S = the exposed surface area of electrode (cm²), d = density of metal (g/cm³) and 10 = constant convert cm to mm (Döner et al. 2011; Praveen et al. 2018; Rathod and Rajappa 2021).

2.6. Adsorption isotherms and thermodynamic parameters

To evaluate the mode of corrosion inhibition through adsorption phenomenon, the experimental values were closely correlated with several adsorption isotherms, which include Langmuir, Bockris-Swinkels, Temkin, Flory-Huggins, and Freundlich. Temperature effect determine the activation parameters for corrosion process that causes VILE to demonstrate up on the MS surface. Thermodynamic parameters such as, enthalpy of activation (ΔH*) and entropy of activation (ΔS*) were evaluated.

2.7. FT-IR analysis

FT-IR spectra of pure VILE and the scratched corrosion product collected from the immersed MS surface containing an optimum concentration of VILE were analysed by Nicolet 5700 spectroscope in 4000–400 cm⁻¹ frequency range.

2.8. Surface analysis

The change of characteristics of MS substrates after corrosion in blank and corrosion in presence of plant extract was examined through SEM and water contact angle (WCA) techniques.

3. Results and discussion

3.1. Weight change method

The corrosion parameters measured from weight change method at varying the strength of VILE in 1 M HCl solution with 24 h immersion period are listed in Table 1. It was observed that, the specimens immersed in varying amounts of VILE solutions, showed decrease in weight loss and reduces corrosion rate to significant extent from 5.8125 mm/year to 0.5201 mm/year and thereby showing highest inhibition efficiency of 91.05% at 150 ppm VILE concentration. The concentration of the VILE enhanced, there is a progressive improvement on the surface coverage of the MS surface and thereby increasing the % IE of the plant extract. Further hike in VILE strength did not cause any appreciable change in the inhibition extent of the inhibitor. The reported higher inhibition efficiencies of the VILE inhibitor due to the presence of active components which shows higher bonding ability of the inhibitor and further adsorbs on the MS alloy surface and acts as adsorption inhibitor (Guruprasad et al. 2019).

Table 1. Corrosion rate, surface coverage and % IE of MS in 1 M HCl with different strength of VILE at 298 ± 1 K.

Conc. of VILE (ppm)	Vcorr (mm/year)	Surface coverage (Ɵ)	% IE
Blank	5.8125	–	–
25	1.3722	0.7639	76.39
50	1.0496	0.8194	81.94
75	0.9225	0.8412	84.12
125	0.7018	0.8792	87.92
150	0.5201	0.9105	91.05

3.2. Electrochemical studies

3.2.1. Electrochemical impedance studies

Typical set of Nyquist plots generated for MS in 1 M HCl solution with and without addition of VILE inhibitor at 298 ± 1K was represented in Figure 2. It has been seen that, all the impedance plots shows one depressed semicircle capacitive loop shifted along the real impedance axis at higher frequency and such type of behaviours treated as frequency dispersion. This frequency dispersion indicated that roughness and heterogeneities of the solid surfaces. Hence, to get a more precise fit to the experimental data set a CPE (constant phase element) was used. Figure 3 shows the Randles equivalent circuit consists of parallel combination to resistance polarisation (R_p) and the constant phase element (CPE), both which are in series with the solution resistance (R_s). Figure 4 implies closeness of the obtained experimental data with standard data. The addition of VILE markedly increases the impedance of MS in acid solution, which is further increased with increased VILE concentration. From the slope of the current– potential curves, R_p values were calculated according to the Equation (7)(7) $${\mathrm{ }R}_P=\frac{S\times \mathit{\text{dE}}}{\mathit{\text{di}}}$$(7) . (7) $${\mathrm{ }R}_P=\frac{S\times \mathit{\text{dE}}}{\mathit{\text{di}}}$$(7)

Figure 2. MS nyquist plots in 1 M HCl solution with varying amount of VILE. Where (R_p) is the resistance polarisation, (R_s) is the solution resistance and (CPE) is the constant phase element.

Figure 3. Electrical equivalent standard randals circuit used to fit the EIS data of the interface MS in 1 M HCl solution with varying amount of VILE.

Figure 4. Nyquist plot fitted with standard equivalent circuit.

Where S is the electrode surface area, dE represents the difference in applied potential and difference in current indicated by ‘di’. Calculated R_p values and corresponding inhibition efficiencies are reported in Table 2. It is found that, R_p values increases with increasing VILE strength suggesting the enhancement of adsorption of active components on the MS surface by blocking the active sites from MS surface efficiently and thereby retard the extent of MS corrosion (Morad, Kamal, and El-Dean 2006).

Table 2. Electrochemical impedance parameters data of MS without and with varying amount of VILE at 298 ± 1 K.

VILE (ppm)	Rp (Ώ/cm^2)	CPE	N	% ƞpol
1M HCl	8269	5.657 $\times$ 10^−11	0.998	–
25	32,530	1.951 $\times$ 10^−11	0.995	74.58
50	44,580	3.875 $\times$ 10^−11	0.989	81.45
75	56,650	2.961 $\times$ 10^−11	0.982	85.40
100	76,850	4.565 $\times$ 10^−11	0.966	89.24
150	97,800	6.873 $\times$ 10^−11	0.927	91.54

The data show that the solution resistance R_S, which measures the total resistance of electrode and material, is practically unchanged in 1 M HCl solution with and without VILE extract. On the other hand, the values of resistance polarisation R_p are shown as semicircles with centres on the real part of the Nyquist plot. It was found that the value of resistance polarisation increased significantly after the addition of different amounts of VILE.

3.2.2. Potentiodynamic polarisation measurement

The PDP curves of MS in 1 M HCl solution with and without addition of varying amount of VILE are shown in Figure 5. The various parameters such as corrosion potential (E_corr), corrosion current density (I_corr), Cathodic and anodic tafel slopes (${‐\mathrm{\beta }}_{\mathrm{c}}$ and ${\mathrm{\beta }}_{\mathrm{a}}$) and corrosion rate in (mm per year) are calculated and the results are depicted in Table 3.

Figure 5. Polarisation curves in 1 M HCl electrolyte containing 0 ppm to 150 ppm of VILE.

Table 3. PDP parameters for MS with different concentrations of VILE in 1M HCl at 298 ± 1K.

VILE (ppm)	$E_{\mathit{\text{corr}}}$ (mV)	$I_{\mathit{\text{corr}}}$ (µA $/{\mathrm{\text{cm}}}^2$)	Corrosion rate (mm/year)	${-\beta }_c$ (mV/dec)	${\mathrm{\beta }}_{\mathrm{a}}$ (mV/dec)	${\mathrm{ƞ}}_{\mathrm{p}}$ (%)
Blank	−0.4965	202.02	3.385	8.65	12.89	–
25	−0.4658	49.26	0.840	−7.5	11.0	75.61
50	−0.4580	34.97	0.615	−7.19	11.5	82.68
75	−0.4674	24.72	0.441	−8.8	8.78	87.76
100	−0.4965	20.20	0.340	−6.75	13.97	90.00
150	−0.4710	13.69	0.240	−8.8	8.5	93.22

Figure 5 indicated that the addition of 25 ppm −150 ppm concentration of VILE to the corrosive electrolyte, VILE significantly alters both dissolution of iron anode and cathodic hydrogen evolution reactions through masking reactive sites on the sample surface. Hence corrosion current density and corrosion rate of MS considerably reduces in the presence of the VILE than those of the inhibitor-free solution. In this manner, the percentage inhibition efficiencies (ƞ_pol %) found to be enhanced with rising inhibitor concentrations. The maximum inhibition efficiency 93.22% was reported at 150 ppm concentration of VILE inhibitor (Olasunkanmi, Mashuga, and Ebenso 2018).

3.3. Effect of temperature

EIS and PDP were applied to investigate the effect of temperature ranges between 298-328 ± 1 K in the presence and absence of optimised VILE inhibitor concentrations. Figures 6 and 7 show the Nyquist plots and polarisation curves of MS with 0 ppm and 150 ppm of VILE in 1 M HCl at various temperatures respectively. The electrochemical studies data such as corrosion rate and percentage inhibition efficiencies are located in Tables 4 and 5. The data suggested that, the percentage inhibition efficiencies are found to decrease from 93.22% to 69.56% in case of tafel polarisation studies, whereas 91.66% - 69.33% in case of EIS studies with an increase of temperature. These decreases of efficiencies are attributed that, the adsorbed VILE molecules get progressively detached from the MS surface at elevated temperatures. As the temperature of the test solution increased, the transport action of the H⁺ ions towards the MS surface increases and subsequent cathodic reduction reaction rate markedly enhanced both in the presence and absence of VILE. As a result, the rate of corrosion of MS increased at elevated temperatures.

Figure 6. Nyquist graphs of MS with 0 ppm to 150 ppm of VILE in 1 M HCl at A) 308 ± 1 K B) 318 ± 1 K C) 328 ± 1 K.

Figure 7. Tafel graphs of MS with 0 ppm to 150 ppm of VILE in 1 M HCl: (A) 308 ± 1 K; (B) 318 ± 1 K; (C) 328 ± 1 K.

Table 4. EIS parameters of MS in the presence of 0 ppm and 150 ppm VILE in 1 M HCl at 308-328 ± 1 K.

Temperature (K)	VILE	Rp (Ώ/cm^2)	CPE	N	IE %
298	Blank	8269	5.657 × 10^−11	0.9985	–
	150	97,800	6.873 × 10^−11	0.927	91.66
308	Blank	7800	5.678 × 10^−11	0.9981	–
	150	45,000	6.578 × 10^−11	0.9439	82.66
318	Blank	6400	5.712 × 10^−11	0.9981	–
	150	30,000	2.718 × 10^−11	0.9812	78.66
328	Blank	4600	1.27 × 10^−11	0.9455	–
	150	15,000	2.495 × 10^−11	0.9959	69.33

Table 5. Electrochemical tafel polarisation parameters of MS with 0 ppm to 150 ppm of VILE in 1 M HCl at 298-328 ± 1 K.

Temperature (K)	VILE (ppm)	−Ecorr (V)	Icorr (µA cm^−2)	Corrosion rate (mm/year)	βa (V dec^−1)	−βc (V dec^−1)	IE %
298	Blank	−0.4965	202.02	3.385	8.65	12.89
	150	−0.4710	13.69	0.240	8.8	8.5	93.22
308	Blank	0.4715	427.66	4.0491	10.05	12.88	–
	150	0.4644	69.26	0.71	9.30	11.60	83.80
318	Blank	0.4623	354.24	5.0664	9.51	10.09	–
	150	0.4577	78.20	1.1539	10.8	11.00	77.22
328	Blank	0.4684	257.03	6.9722	8.86	8.33	–
	150	0.4575	78.23	2.1050	11.1	8.92	69.56

3.4. Kinetic and thermodynamic activation parameters

The results of electrochemical studies at higher temperatures are considered to evaluate the different activation parameters. Arrhenius Equation (8)(8) $$\mathrm{\text{Log}}\mathit{\text{ CR}}=-\frac{E_a^{*}}{2.303 \mathit{\text{RT}}}+A$$(8) and transition state Equation (9)(9) $$\mathrm{C}R=\frac{\mathit{\text{RT}}}{\mathit{\text{Nh}}}\mathrm{\text{exp}}\hspace{0.17em}\left(-\frac{\Delta H*}{\mathit{\text{RT}}}\right)\hspace{0.17em}\mathrm{\text{exp}}\hspace{0.17em}\left(\frac{\Delta S*}{R}\right)$$(9) are applied to evaluate the parameters like activation energy ($E_a^{*}$), enthalpy of activation (ΔH*) and entropy of activation (ΔS*) for the dissolution of MS in 1 M HCl in the presence of 0 ppm and 150 ppm of VILE. (8) $$\mathrm{\text{Log}}\mathit{\text{ CR}}=-\frac{E_a^{*}}{2.303 \mathit{\text{RT}}}+A$$(8) (9) $$\mathrm{C}R=\frac{\mathit{\text{RT}}}{\mathit{\text{Nh}}}\mathrm{\text{exp}}\hspace{0.17em}\left(-\frac{\Delta H*}{\mathit{\text{RT}}}\right)\hspace{0.17em}\mathrm{\text{exp}}\hspace{0.17em}\left(\frac{\Delta S*}{R}\right)$$(9)

Where corrosion rate is indicated as CR, A in Equation (7)(7) $${\mathrm{ }R}_P=\frac{S\times \mathit{\text{dE}}}{\mathit{\text{di}}}$$(7) represents the Arrhenius pre-exponential factor, h refers to Plank’s constant, N indicated as Avogadro’s number, the molar gas constant indicated as R, and T is the absolute temperature. Figure 8A shows the straight lines in the Arrhenius graph of Logarithm of corrosion rate verses 1/T with regression coefficient R² = 0.989 for MS corrosion in 1 M HCl electrolyte with the presence of 25 ppm to 150 ppm of VILE. The $E_a^{*}$ values were calculated by multiplying the molar gas constant with the slope of the straight line in the Arrhenius graph and the values are listed in Table 6. The $E_a^{*}$ values in presence of VILE in 1 M HCl solution were found to be higher than those with 1 M HCl alone. This higher $E_a^{*}$ values in presence of VILE contrast to blank solution noticed effective adsorption of VILE molecules and creates an energy barrier on the MS surface. It suggested that for dissolution of MS in 1 M HCl medium required more energy to overcome the molecular energy barrier from the MS surface.

Figure 8. Arrhenius plots: (A) log CR against 1/T, (B) log (CR/T) against 1/T for the adsorption of inhibitors in the MS surface.

Table 6. Activation parameters for MS dissolution in 1 M HCl with 0 ppm and 150 ppm of VILE.

Inhibitor	R^2	$E_a^{*}$ (kJ mol^−1)	R^2	ΔH* (kJ mol^−1)	ΔS* (J mol^−1 K^−1)
1 M HCl	0.989	22.774	0.986	20.125	−179.68
VILE	0.989	45.571	0.987	42.931	−120.18

Figure 8B represents the transition graph of Log (CR/T) verses 1/T, which gives straight line and regression coefficient R² = 0.987 with slope = $\frac{{\Delta H}^{\mathrm{*}}}{2.303\mathrm{ }R}\mathrm{ }$ and intercepts = log $\frac{R}{\mathit{\text{Nh}}}$ + $\frac{{\Delta S}^{\mathrm{*}}}{2.303\mathrm{ }R}$ from which ${\Delta H}^{*}$ and ${\Delta S}^{*}$ parameters were calculated for MS corrosion in 1 M HCl solution with 25 ppm and 150 ppm of VILE respectively. The various activation parameters are tabulated in Table 6, these parameters suggesting that, in presence of VILE, a positive sign of ${\Delta \mathrm{H}}^{\mathrm{*}}$ increased as compared to 1 M HCl alone indicating that, the process is endothermic in nature which implies dissolution of MS is a slow process might be due to the formation of active energy barrier over the MS surface. Again, ΔS* values increases with the addition of VILE, which suggested that, the rate determining step at the activated complex indicates a favours of dissociation, it means that disordering decrease moving from reactants to the activated complex. Hence physical adsorption is favour at low temperature and follows chemical adsorption at higher temperature (Fouda, Mahmoud, and Abdul Mageed 2016; Jeeva et al. 2015).

3.5. Scanning electron microscopic analysis

Figure 9 provides the surface characteristics of adherent film formed on the MS test samples placed in 1 M HCl with 0 ppm and 150 ppm of VILE for 4 hours. Figure 9a is the SEM image taken for a freshly polished MS surface, which is not subjected to corrosion test and appears smooth image. Figure 9b is the SEM image of a corroded MS surface in a corrosive medium alone. The surface seems to be severely damaged and several cavities and deep pits (marked in red circles) are distributed over the entire surface. Further, no accumulation of corrosion products on the surface during corrosion, which is further induces the acid to attach on MS surface. Figure 9c is the SEM image of the corroded MS surface in addition of 150 ppm of VILE to the corrosive medium. The surface image reflected that, there is an accumulation of layers of Fe-VILE compounds strongly adhered protective layers over the surface (marked in yellow circles), these layers guard the attack of acid solution. Further greatly disappear of cavities and deep pits over the surface, it indicate VILE components cover the MS surface through adsorption process and protect the surface from the corrosive medium (Bahlakeh, Ramezanzadeh, et al. 2019).

Figure 9. SEM images of MS surface taken with the scale of 10 µm and 10.0 kV: a) polished surface b) 4 h placed in 1 M HCl c) 4 h placed in 1 M HCl with 150 ppmVILE.

3.6. Atomic force microscope studies [AFM]

Figure 10 shows the 3D AFM micrographs of polished, corroded and protected MS specimen surfaces. The calculated corresponding average surface roughness (S_a) values are listed in Table 7. Figure 10a is the polished MS surface shows least surface roughness, which was not exposed to corrosion test, the surface appears uniform which is taken as a bench mark for comparison. Figure 10b is the MS surface placed 4h in 1 M HCl exhibits high surface roughness value of 606 nm suggested that the MS surface is drastically affected by the acid solution. Figure 10c is the MS surface placed 4h in 1 M HCl with 150 ppm VILE appeared lesser value of surface roughness compare to HCl alone. The decrease in S_a value reflecting that, effective protective layer is formed on the MS surface which protects from the metal dissolution and surface damage (Hossam et al. 2023).

Figure 10. 3D AFM images of MS surface: a) polished surface b) 4 h placed in 1 M HCl c) 4 h placed in 1 M HCl with 150 ppm VILE.

Table 7. AFM results of MS surface roughness (Ra) values at 298 ± 1 K.

Specimens	Average roughness (Sa) nm
MS polished	1.46
MS in 1 M HCl	606
MS in 150 ppm	294

3.7. FT-IR analysis

The FT-IR spectrum of pure VILE are shown in Figure 11A. The stretching vibration peak 1626 cm⁻¹ corresponds to C=O group and 1460 cm⁻¹ peak is related to C-C stretching vibrations in aromatic ring. Broad peak stretching vibrations observed at 3424 cm⁻¹ is connected to O-H functional group. Figure 11B shows the FT-IR spectrum of adsorbed protective layer formed on MS surface treated in 150 ppm of VILE in 1 M HCl solution. Several peaks are disappeared and modified compare to pure VILE FT-IR spectrum. This change in frequency domains reveals that, oxygen atoms from C = O and O–H group are acts as active centres during the adsorption process and are enable to develop protective layer on the MS surface (Rajappa, Venkatesha, and Praveen 2008).

Figure 11. (A) FT-IR spectrum of pure VILE; (B) FT-IR spectrum of scratched VILE after corrosion of MS surface.

3.8. Contact angle studies

Figure 12 represents the images obtained of the surface contact angle for MS, without as well as with the presence of 150 ppm of VILE in 1 M HCl solution. The measured contact angle values are given in Table 8. The VILE contains phytochemicals organic constituents and also includes polar and non-polar segments. After the adsorption of the components of inhibitors on MS surface, the surface becomes hydrophobic due to non-polar segments of VILE that can promote the water repellence on MS surface (Figure 12b). In the absence of the VILE components, the contact angle is less due to the MS surface exposed with 1 M HCl alone the surface fully occupied by corrosive components which forms hydrogen bond with aggressive HCl ions and hence the water contact angle was less observed and surface becomes more hydrophilic (Figure 12b). Thus in the presence of VILE, the adsorption of an active components over the mild steel surface blocks the active sites and enhances the hydrophobic nature of the MS (Minagalavar et al. 2023; Rathod et al. 2022).

Figure 12. Contact angle images of MS measured after 2 h immersed in (a) 1 M HCl solution (b) 150 ppm VILE in 1 M HCl solution.

Table 8. Variation of contact angle on MS surface in the presence and absence of inhibitor.

Concentration of Inhibitor	Measured contact angle (deg.)
	Left angle	Right angle
1 M HCl	53.00	53.00
150 ppm	97.69	96.93

4. Conclusions

The present study reveals that VILE developed as an efficient eco-friendly and biodegradable inhibitor for MS in 1 M HCl solutions. The percentage inhibition efficiency was extensively evaluated by weight change method, potentiodynamic and electrochemical impedance techniques. According to weight loss measurement, 91.50%η_p is observed for the acid solution containing 150 ppm of VILE extract. The effectiveness of the inhibitor improves with increasing concentration, reaching a maximum of 93.22 at 150 ppm extract. The Tafel polarisation studies inferred that VILE acted as a mixed-type inhibitor and control the corrosion rate of both cathodic and anodic reactions.

The corrosion inhibition was correlated through adsorption mechanism of VILE components on the MS surface, which follows the Langmuir adsorption isotherm model. The FT-IR, SEM, and AFM, techniques revealed that VILE protects the MS surface by forming a protective layer over the exposed metal surface, which is consistent with the electrochemical results. The findings of this study demonstrate the significant advantages of adopting natural-based corrosion inhibitors to shield metals and alloys from rust. VILE act as a corrosion inhibitor for MS has been demonstrated to have effective corrosion inhibition ability in hydrochloric acid solution.

Authors’ contributions

A. G. K.: Investigation, Methodology, Visualisation, Data curation, Writing- Original draft preparation, S. K. R.: Conception and design of the study, Supervision, Writing- Reviewing and Editing, J. M.: Software, Data curation.

Acknowledgments

Authors are thankful to, DST-SAIF University Scientific and Instruments Center, Karnatak University Dharwad for providing SEM, AFM and CA spectroscopic studies and Rani Channamma University, Belagavi for providing electrochemical workstation (potentiostat/galvanostat PGSTAT204 model) and FT-IR techniques.

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.

Data availability statement

Data will be made available on request.