African peach leaves were collected from Iroko-Ekiti, in Ijero Local Government Area of Ekiti State, Nigeria. The leaves were authenticated at the Department of Crop, Soil and Pest Management of The Federal University of Technology, Akure, Nigeria. The leaves were washed with clean running water from a tap to remove dirt and impurities. The leaves were air-dried for 21 days, pulverized, and the pulverized sample was weighed and kept in a clean specimen container until ready for use [15,16].

The ethanol extract of African peach dried sample was obtained by measuring 10 g of the powdered sample into a Soxhlet apparatus and extracted with ethanol (70-80°C) till a colour change was observed. The extract solution was then collected, concentrated over a thermostatic water bath at 45°C, and stored until use.

This study employed coupons cut from mild steel of known composition by weight percent (Fe-58.83%; Ca-0.87%; Ti-0.69%; Mg-0.59%; Mn-0.49%; Al-0.47%; Co-0.42%). The cut-out coupons with dimensions of 24 × 15 × 4 mm were polished with emery paper to remove surface particles. The dimensions of coupons for gravimetry and electrochemical studies were the same. The mild steel samples were thoroughly washed first under running water and later then with deionized water and ethanol before being dried in air [16]. The samples were stored in a desiccator afterward. A digital electronic balance was then used to take the weights of the coupons and record them in a logbook. All chemicals (solvents and reagents) used for this project were of analytical grade; they were not subjected to further treatment. In contrast, deionized water was used to prepare the samples.

Before the coupons were used for the study, they were thoroughly washed under deionized water, then ethanol, and dried with a clean handkerchief. Thereafter, the coupons were hung by thread and a glass rod in 100 ml of aqueous 1.0 M NaOH medium in a 250 ml glass beaker, containing a specific amount of ethanol extract of the African peach leaf. The concentration study was carried out by varying the concentration of the extract from 0.2 g/L to 1.0 g/L in 100 mL of separate solutions containing 1.0 M NaOH. Each coupon with known dimension and initial weight in gramme (W₁) was immersed inside the respective concentration of the blank (1.0 M NaOH) and inhibitor solution for 6 h after which it was retrieved, rinsed with distilled water (to remove corrosive residues and products) and acetone (to remove residual water, organic residues, oils, etc.), dried with clean handkerchief and re-weighed (W₂) [17]. The weight loss in gramme (g), W₃, was calculated by subtracting final weight from initial weight (W₁ − W₂). After gravimetric corrosion experiments, values of corrosion rates (CR_grav, g cm^-2 h^-1) and inhibition efficiencies (IE_grav %) were evaluated with the aid of Equations 1 and 2, respectively.

where A is the surface area of the coupon (cm²); t is the immersion time (h); and CR_b and CR_i are the respective corrosion rates in the absence (blank) and presence of extract (inhibitor) at various concentrations.

To probe the effect of immersion time on the corrosion characteristics, experiments were performed at varying times (24, 48, 72, 96, 120, 144, and 168 h). Similarly, the effect of temperature on the corrosion study was investigated at 28, 40, 50, and 60°C in a thermostatic shaker. The thermodynamic and activation parameters of the reaction were evaluated using the data from the temperature study. The activation energy, E_A, was calculated using the Arrhenius equation as shown in Equation 3, while the other thermodynamic parameters (standard entropy change, ΔSads∘, and standard enthalpy change of activation, ΔHads∘) were calculated using Equation 4.

where A is the Arrhenius pre-exponential factor, E_A is the activation energy (kJ mol^-1), T is the absolute temperature (K), R is the universal gas constant (J K^-1 mol^-1), N is Avogadro’s constant (mol^-1), h is Planck’s constant (J s), ΔSads∘ is the standard change in entropy (J mol K^-1), and ΔHads∘ is the standard enthalpy of activation (kJ mol^-1).

The concentration dependent (equilibrium) data were subjected to Langmuir, Freundlich and Temkin models which are represented in Equations 5-7, respectively.

where θ = fractional surface coverage; C = concentration of the inhibitor; K_F, K_L and K_T = Langmuir, Freundlich and Temkin equilibrium constants, respectively, n_F = Freundlich empirical constant, and a = interaction parameter.

Potentiodynamic polarisation (PDP) and open circuit potential (OCP) measurements were carried out using the electrochemical impedance spectrophotometry (EIS) technique at 28°C. The details of electrochemical studies followed our earlier procedure [16]. Using an ammeter connected in series, the OCP measurements were carried out at different cathodic and anodic potentials with varying concentrations of African peach leaf extract applied on the WE (working electrode) in the range ±250 mV at a scan rate of 1.0 mV s^-1 for 30 min to obtain the current densities.

The corrosion inhibition efficiency (IE_EIS %) was calculated from EIS data using Equation 8 while corrosion rate (CR_PDP) and IE_PDP % were also calculated from PDP using respective Equations 9 and 10, respectively.

where Rct0 and R_ct = respective charge transfer resistance of the uninhibited and inhibited solutions, EW = steel’s equivalent weight, ρ = density (g cm^-3) of the mild steel, k = conversion factor (3.27 × 10^-3), I_corr and Icorrinh = respective corrosion current densities (μA cm^-2) without and with inhibitors.

The FTIR spectra of the samples (plant extract, mild steel, and mild steel + extract + 1.0 M NaOH) were obtained on Thermo Fischer Scientific (Nicolet iS5, iD7 ATR, Germany) to detect various functional groups in the samples. Surface examinations of the plant extract, mild steel, and mild steel specimen immersed in 1.0 M NaOH solutions in the presence of optimum concentration (1.0 g/L) of crude extract were done using SEM-EDX (Zeiss 121 DSM 982 Gemini) after 24 h of immersion time at room temperature (~28°C). The sample was removed from the solution, rinsed, dried and kept in a desiccator to prevent the formation of hydrofluoric acid, which may damage the sample and interfere with the EDX measurements. The coupons were coated with a conductive layer to prevent charging during SEM imaging.

Corrosion rates of mild steel and inhibition efficiencies of the inhibitors are always dependent on the concentration of inhibitors present in the corrosive media. Generally, the extracts from plants could serve as eco-friendly corrosion inhibitors, and the performance of these green inhibitors to protect metals’ surfaces improves as their concentrations in corrosive media increase due to increased adsorption and protective film formation on the metals’ surfaces [18]. Figure 1 presents the dependence of the corrosion rate of mild steel and the inhibition efficiency of African peach extract on the concentration of the extract (inhibitor) in 1.0 M NaOH at 28°C and 6 h. As the concentration of the inhibitor increases (from 0.2 g/L to 1.0 g/L), the corrosion rate of mild steel in the alkaline medium slightly decreases (from 9.8 × 10^-5 to 2.6 × 10^-5 g/cm² h). This observation could be linked to the presence of more phytochemical constituents that can be adsorbed on the surface of the mild steel [13]—this forms a protective film against corrosion on the surface of the metal. However, the inhibition efficiency of the inhibitor increases (from 38.7% to 73.2%) as the concentration of the inhibitor increases (from 0.2 g/L to 1.0 g/L). This is expected because at a low concentration of inhibitor, the mild steel surface in an alkaline medium was partly covered, which resulted in low to moderate protection. A high concentration of the inhibitor provided high adsorption active sites for coverage, and this resulted in improved corrosion resistance of the metal.

The temperature of the corrosion system is adjudged to play a significant role in corrosion experiments, and variation in temperature mostly affects the corrosion rate of mild steel in corrosive media and the inhibition efficiency of the inhibitor [10]. Figure 2 presents the combined effects of inhibitor concentration and temperature on the corrosion rates and inhibition efficiencies. Corrosion rate significantly increased as the temperature was increased from 28°C to 40°C. There were slight changes in corrosion rates between 40 and 60°C, especially in the inhibited systems. An increase in corrosion rates as temperature increased could be related to high mass loss due to an increase in mild steel dissolution in the corrosive (1.0 M NaOH) medium [19]. The inhibition efficiency of the African peach leaf extract for mild steel protection improved with increasing concentration as the temperature of the medium was increased.

The duration of immersion in a corrosion environment provides important information about the corrosion rate. Figure 3 presents the effect of immersion time of mild steel in 1.0 M NaOH in the absence and presence of inhibitor (over different concentration values) at 28°C. The corrosion rate was higher in the blank (uninhibited system) than in the other inhibited systems, and in general, the corrosion rate decreased as the concentration of the inhibitor was increased over the time range. It is noticeable in the figure that the corrosion rate increased as immersion time increased from 24 h to 72 h; after 72 h, the corrosion rate significantly decreased till 168 h of immersion. The observation could be linked to the scenario in which the mild steel surface is highly reactive at the initial stage of the experiment where there was no protective layer, and the 1.0 M NaOH corrosive medium attacked the mild steel surface leading to mild steel dissolution and the corresponding weight loss (high corrosion rate). However, as the immersion time was prolonged, there was the formation of a protective film on the surface of mild steel by the inhibitor, and this film formed a barrier (reduction of contact) between the mild steel surface and the alkaline medium; hence, a reduction in corrosion rate [20,21]. For inhibited corrosion systems, there was a negligible increase in the inhibition efficiency of the inhibitor from immersion time of 24 h to 48 h; after 48 h immersion time, the corrosion efficiency slightly decreased, and this phenomenon can be linked to partial desorption of the inhibitor from the metal surface as well as degradation of inhibitor molecules, which weakened the protective film [22,23].

Adsorption isotherms are usually used to describe the relation between inhibitor molecule coverage on the metal surface and inhibitor concentration in the bulk solution [24]. To deduce the mechanisms of corrosion inhibition, the values of surface coverage (θ), corresponding to different studied concentrations of African peach leaf extract in the selected temperature range (301 to 333 K), were tested by fitting isotherm (equilibrium) data into Langmuir, Freundlich, and Temkin adsorption models, and the plots of these models are presented in Figure 4. Table 1 presents the parameters of these models. It is evident that the Langmuir model suitably described the adsorption of the inhibitor onto the mild steel surface because this model gave the highest values of Radj2 (adjusted correlation coefficient) across all the experimental temperatures. The Langmuir isotherm is a quantitative model useful for describing the adsorption of the inhibitor onto the metal surface, assuming a monolayer coverage of the inhibitor on homogeneous sites [24]. For the isotherm data to follow the Langmuir model in this study signifies that the adsorption characteristics features are close to ideal monolayer adsorption. It implies that there was adsorption of inhibitor molecules to form a protective layer that mitigates the dissolution of mild steel. The value of K_L is ≈1 across temperatures, indicating strong interaction and high inhibition effectiveness of African peach leaf extract in preventing corrosion of mild steel [24].

To probe the characteristics of the mild steel-African peach leaf extract thermodynamic system, the temperature-dependent data were subjected to thermodynamic inquisition, such as Arrhenius and Eyring equations. Arrhenius and Eyring plots are presented in Figure 5, while Table 2 and Table 3 summarize the thermodynamic parameters. As presented in Table 2, the values of standard Gibb’s free energy (ΔGads∘), which were calculated from Equation 11, are negative which shows that the thermodynamics of the corrosion and adsorption process is spontaneous. This observation means that there was an interaction between the molecules of African peach leaf extract and the mild steel, leading to the formation of a good protective film that reduced corrosion density. When the value of ΔGads∘ is ca. -20 kJ mol^-1 or less negative, the process is termed physisorption (electrostatic interaction), the value of ΔGads∘ ca. -40 kJ mol^-1 or more negative connotes chemisorption (chemical bonding) while the value of ΔGads∘ that lied between -20 and -40 kJ mol^-1 represents mixed adsorption [25]. The values of ΔGads∘ (between -10.202 and -10.881 kJ/mol) obtained in this study over the experimental temperature range (301-333 K) indicate spontaneous physisorption of molecular extract of the African peach on mild steel coupons [15].

The positive values of E_A (Table 3) for inhibited solutions (0.2 g/L-0.8 g/L) are lower than those of the uninhibited solution. In contrast, those of the 1.0 g/L inhibited solution are higher than those of the uninhibited solution. At values less than 40 kJ mol^-1, the calculated activation energies, E_A, for the adsorption on mild steel in 1.0 M NaOH imply a physisorption process, which means weak electrostatic interaction [15,26].

Positive values of ΔHads∘ ranging between 5 and 40 kJ/mol, indicate an endothermic adsorption process and this implies that the adsorption process is favoured at low temperatures [27]. The negative values of ΔSads∘ signifies that adsorption of the extract (inhibitor) onto mild steel surface was orderly. The adsorption mechanism as observed in this study is best described as a physisorption process that involves weak electrostatic intermolecular van der Waals interactions between the molecules of inhibitor and the surface of mild steel [25,28].

Electrochemical investigations in corrosion experiments enable the prediction of the lifespan of mild steel, the diagnosis of failure nodes, the optimization of mild steel composition, the interaction of inhibitor with metal surface, as well as provision of a mechanistic explanation of corrosion data [29,30]. The electrochemical impedance spectroscopy (EIS) data are presented in Figure 6 (Nyquist plots, Randle circuit used to fit the EIS data, and Bode plots) and Table 4. Electrochemical impedance spectroscopy, via Nyquist analysis, is a non-destructive technique used to determine corrosion mechanisms and rates as well as to evaluate inhibitors’ performance and efficiency. The Nyquist plots (Figure 6A) exhibited nearly linear behavior rather than semicircles, which may be attributed to a non-ideal capacitor arising from surface roughness, electrode porosity, or non-uniform current distribution [31]. These factors may lead to the formation of a protective layer of corrosion products, which causes slow diffusion control on the mild steel surface [32]. The Bode plots (Figure 6C) appeared as imperfect sinusoids because of the following reasons: roughness of the mild steel surface; inconsistent build-up of charges at the metal-inhibitor interface as a result of frequency dispersion; or the surface of the mild steel was non-linear or non-stationary [33].

The impedance spectra were analysed, and EIS parameters (R_s, R_ct, Y_o, n and C_dl representing single charge resistance, charge transfer resistance, admittance magnitude, phase shift relating to the surface homogeneity, and double-layer capacitance, respectively) obtained are summarised in Table 4. The R_ct value of the uninhibited solution is the lowest, and the R_ct value increases as the concentration of the inhibitor increases. The observation, which later translates to an increase in IE_EIS %, is a result of the formation of a protective film on the mild steel surface that increased the resistance of the metal to charge transfer [15]. The extreme increase in value of R_ct with increasing concentration of African peach leaf extract signifies resistance of the mild steel against corrosion with near-total efficiency in the system, that is, active blockage of all corrosion sites from the corrosive medium. Similarly, the values of the double-layer capacitance (C_dl) of the uninhibited solutions are higher than those of the inhibited solutions. This observation is connected to the increase in the thickness of the adsorbed layer of inhibitor molecules on the surface of the metal, which ultimately causes faster displacement of water molecules and hydrogen ions from the surface of the metal [34].

Tafel plots (polarisation curves) are presented in Figure 6D, while the Tafel parameters are summarised in Table 5. It is evident from the table that corrosion rate (CR_PDP) decreases while inhibition efficiency (IE_PDP %) increases as the concentration of the inhibitor in the solution increases; this is due to adsorption of inhibitor molecules on the mild steel surface that increases surface coverage and formation of a protective barrier that blocks active corrosion sites [35]. The I_corr values in the table decrease as the concentration of inhibitor molecules in the solution increases, which is attributed to an increasing number of inhibitor molecules that reduce the dissolution of mild steel in the alkaline solution [36]. The anomalous phenomenon occurring in the range of -0.5 to -0.9 eV on the Tafel plot (Figure 6D) often results from the formation of surface bubble or diffusion-controlled adsorption mechanism [37], which were similarly observed in the Nyquist (Figure 6A) and Bode (Figure 6C) plots. Although the same experimental conditions were used for EIS and PDP, the inhibition efficiencies obtained by the two techniques differ. This is because the techniques used different electrochemical processes, signal perturbations, mathematical models, and measurement timeframes to evaluate the corrosive medium. Electrochemical impedance spectroscopy evaluates the charge transfer resistance under the steady-state conditions, but PDP evaluates cathodic and anodic polarisation behaviors under dynamic potential scanning.

The FTIR analysis was carried out to elucidate the functional groups present in the samples, as shown in Figure 7. The FTIR spectra of the extract and mild steel after immersion in inhibited solution (1.0 M NaOH + 1.0 g/L of extract + mild steel) are similar—only showing slight band variations, which could be as a result of interaction of mild steel with the extract solution. The FTIR spectrum of the mild steel shows only a few FTIR bands. The peak at ca. 3331 cm^-1 is assigned to O–H (broad stretching vibration); the bands around 2927 cm^-1 and 2858 cm^-1 are assigned to asymmetric stretching of methylene –CH₂– groups (C–H asymmetric stretching vibration), 1700 cm^-1 and 1650 cm^-1 are linked to C=O; 1370 cm^-1 is for CH₃, C–O stretching vibration (aliphatic alcohols, ethers, etc.) is visible at 1020 cm^-1 while 610 cm^-1 is linked to C–H bending vibration of an alkyne. The peak around 1510 cm^-1 is assigned to NO_2, while the band at 592 cm^-1 in mild steel FTIR is assigned to Fe–O.

The SEM images show the surfaces of the extract (Figure 8A), bare mild steel (Figure 8B), and mild steel after immersion in inhibited solution (Figure 8C). The corresponding EDX spectra that show the relative surface elemental composition are presented in Figure 9. The SEM image of the extract (Figure 8A) shows a smooth, compact surface. The SEM images of the mild steel before immersion (Figure 8B) and after immersion in the inhibited solution (Figure 8C) differ markedly; before immersion in the inhibitor solution, the mild steel surface was rough. However, after immersion in the inhibited solution, the SEM image of the mild steel appears slightly smoother and more compact due to the protective film formed by its inhibitor molecules on its surface. The homogeneous morphology in Figure 8C confirms adsorption of extract molecules on the surface of mild steel, thereby minimizing corrosive attack and enhancing surface protection. The comparison of the EDX spectra of the mild steel and mild steel after immersion in an inhibited solution showed that Fe is still relatively present after corrosion tests. These surface characteristics data agreed closely with the electrochemical data. The reduction in corrosion current density (I_corr) and the increase in IE_PDP % observed in polarisation studies, as well as the increased R_ct from EIS measurements, indicate acceptable corrosion inhibition performance of the African peach extract. The improved surface features of mild steel as observed in the SEM image and the presence of Fe after immersion, as inhibited by EDX analysis, support the conclusion that the African peach extract, as a corrosion inhibitor, protects the mild steel from dissolution by blocking active corrosion sites responsible for the corrosion process [15,38].