Introduction

The deterioration of materials (such as metals, alloys, and composites) through chemical or electrochemical interaction with the environment is known as corrosion. It mainly results in adverse effects such as loss of materials, plant shutdown that leads to production loss, increased maintenance cost, oil leakage due to pipeline breakage, contamination of products because of metal leaching, and environmental pollution [1]. The estimated cost of corrosion worldwide is increasing drastically, indicating the significance of corrosion control in conserving the world’s material resources. Metal matrix composites (MMCs) are novel structural materials because of their lightweight, environmental resistance, and favorable mechanical properties. MMCs composed of two phases, the matrix phase and the dispersed phase. Aluminum alloys are the commonly used metal matrix materials reinforced with various particulate or fiber materials [2]. In 6061Al-SiC composite, 6061Al alloy is the matrix phase, reinforced with SiC particulate. These composites with high specific strength, ideal for vehicles, aircraft, and military applications [3]. Adding ceramic particles, short fibers, and whiskers to aluminum alloys improves tribological characteristics significantly. A metal matrix alloy is usually less prone to corrosion than the corresponding MMC. The presence of reinforcing particles or fibers causes in-homogeneities on surfaces exposed to aggressive environments. It becomes significant in MMCs made of aluminum alloy matrix and semiconductor-type reinforcing materials such as silicon carbide. In such cases, the galvanic cell formation promotes the corrosion of the metal matrix [4]. The HCl solution is widely used in the automobile and aero industry pickling processes, where the excessive degradation of the materials should be adequately controlled.

Thus, the corrosion of MMCs such as 6061Al-SiC(p) composite in an aggressive medium like hydrochloric acid solution can be best controlled using a suitable inhibitor. Combating corrosion by the addition of inhibitors is the easiest and simplest method. As per the literature, organic compounds with heteroatoms and conjugate double bonds act as effective inhibitors to combat the corrosion of many metals [5, 6]. Amino acids are employed as environment-friendly inhibitors to combat many metals’ deterioration [7]. Glutathione is an amino acid derivative containing multiple heteroatoms (sulfur, oxygen, and nitrogen), polar groups (−CO, −NH2, −OH), and conjugated double bonds in their structures (Fig. 1), expected to show good inhibition activity. It is a non-toxic material readily soluble in an aqueous medium. Glutathione exhibited a good inhibition efficiency towards copper [8] and AA6061 [9] in the hydrochloric acid medium. This paper demonstrates the inhibition behavior of an eco-friendly inhibitor, glutathione, on the attenuation of 6061 Al- SiC(p) composite corrosion in a 0.5 M HCl solution. The surface morphology of corroded and inhibited composite specimens was analyzed. UV–visible and FTIR spectral analysis supported the adsorption of Gt molecules on the composite surface. Experimental results were compared with the results of the theoretical calculations.

Fig. 1
figure 1

The structure of glutathione (Gt)

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Materials and methods

Materials

The specimen employed in the present work is 6061Al-15% (v) SiC(p) composite (hereafter referred to as Al alloy-SiC). Its elemental composition is mentioned in Table 1.

Table 1 The composition of Al alloy-SiC composite
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The specimen was obtained by cutting a cylindrical rod of the material, and then, it was embedded in an epoxy resin, exposing the surface area of 1 cm2. The specimen’s exposed surface was abraded using different grades (200–1000) of emery paper and finally on a disc polisher. Then, the sample was washed thoroughly with water, acetone rinsed, and finally dried. The corrosive medium used in this study is 0.5 M HCl, prepared using 35% HCl (analar grade) and standardized volumetrically. Glutathione (Merck) is the inhibitor. Figure 1 depicts the structure of glutathione (Gt).

Electrochemical studies

The experimental studies were performed using a CH instrument (604D series USA model with beta software). The study was done at four different temperatures (303, 313, 323, and 333 K) using a cell system comprising an auxiliary electrode of platinum, a reference electrode of saturated calomel, and a working electrode of an Al alloy-SiC composite specimen. Initially, the cell system was immersed in a 100 ml of 0.5 M HCl solution contained in a beaker and connected to the CH instrument. The setup is kept for about 1 h to attain the steady-state condition, and then, the open circuit potential (OCP) is recorded. First, the electrochemical impedance studies (EIS) were performed in the frequency range, 10 kHz–0.01 Hz, under an applied AC signal of 10 mV amplitude by varying the concentration of Gt. The potentiodynamic polarization (PDP) studies were done by polarizing the working electrode at OCP from −250 mV (cathodic) to +250 mV (anodic) with a scan rate of 1 mVs−1. Then, the Tafel curves were recorded. Three trials were taken in each measurement, and the average value was used for further calculations.

Surface studies of the composite specimen

The composite specimen was subjected to surface studies to identify the surface texture of the corroded and inhibited samples. The surface pictures of the composite sample immersed in a solution of 0.5 M HCl without and with Gt, respectively, for 3 h were taken using EVO 18-5-57 model SEM. The surface roughness of these composite samples was checked using AFM (IB342 Innova model).

UV–visible and FTIR study on the adsorption of Gt

Using an 1800 Shimadzu UV–visible spectrophotometer, UV–visible spectra of 0.1 mM Gt in 0.5 M HCl solution were initially recorded. The absorption spectra of the same solution were then recorded after 2 h of immersion of the test coupon of AA-SiC composite.

FTIR spectroscopy was performed on scrapped products produced after immersion of the composite specimen in 0.1 mM Gt in 0.5 M HCl solution for 2 h using Shimadzu-IR Spirit in the frequency range 4000–400 cm−1.

Theoretical simulation studies

The experimental results of the inhibition activity of Gt were validated through the theoretical studies using density functional theory (DFT) with the help of 6-31G** as the basis set and B3LYP as the exchange–correlation function. The computed parameters were analyzed for the validation of the electrochemical results.

Result and discussion

Open circuit potential (OCP)

The OCP monitoring is essential for all corrosion studies since electrochemical techniques like EIS work at OCP and potentiodynamic polarization are accomplished by drifting the system away from its equilibrium state. In both circumstances, OCP plays a critical role, and if it were not maintained, the accuracy of the electrochemical result would affect. To establish a steady state, the freshly polished composite specimen was dipped in 0.5 M HCl in the absence and presence of Gt. The change in electrode potential with time was recorded. As shown in Fig. 2, the test specimen reached the steady-state potential in 0.5 M HCl in the presence and absence of Gt in around 1500 s. As seen in Fig. 2, the potential first changes to a critical value known as the induction time. Upon visual inspection, a dull layer could be seen on the specimen electrode surface. This dull coating could be attributable to surface corrosion products or oxide layer deposition [10]. After the initial induction period, the potential reaches a steady state, implying the attainment of a dynamic equilibrium between surface film deposition and underlying metal dissolution via surface film breakdown or dissolution. In the presence of Gt, the potential values shift towards the cathodic area, indicating that the inhibitor primarily affects the cathodic reaction. EIS and potentiodynamic polarization measurements were performed after establishing the OCP.

Fig. 2
figure 2

Potential measurements of working electrode vs. SCE with time in the absence and presence of 0.01 mM Gt in 0.5 M HCl

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Potentiodynamic polarization studies

The Tafel plots for Al alloy-SiC composite in the acid and inhibited acid medium at 303 K are depicted in Fig. 3.

Fig. 3
figure 3

Tafel plots for Al alloy-SiC composite in 0.5 M HCl without and with Gt at 303 K

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Similar curves were obtained at 313, 323, and 333 K also. The anodic curve is accountable for metal oxidation, while the cathodic curve is responsible for hydrogen evolution in the acidic medium [11]. Table 2 presents the electrochemical parameters for the composite, such as corrosion potential (Ecorr), corrosion current density (icorr), anodic Tafel slope (βa), cathodic Tafel slope (βc), and corrosion rate (CR) at various Gt concentrations and temperatures of the hydrochloric acid medium. The Tafel plots (Fig. 3) show that when the concentration of Gt rises, the anodic and cathodic curves shift to lower corrosion current densities (icorr). In the presence of Gt, the corrosion potential (Ecorr) shifts towards the cathodic region. However, the shift is less than ±85 mV, the widely accepted threshold for determining whether an inhibitor is exclusively anodic or cathodic [12]. The change in Ecorr value corresponds to inhibitors’ mixed-type behavior, controlling metal dissolution at the anode and hydrogen evolution at the cathode to various extents. However, the predominant control is on the cathodic hydrogen evolution reaction, reducing the overall rate of corrosion [13]. Besides these findings, the significant reduction in corrosion current density (icorr) shows an effective decrease in corrosion rate in an optimal Gt concentration of 0.7 mM in a 0.5 M hydrochloric acid medium. The results obtained at other temperatures are comparable to the one reported.

Table 2 PDP results for Al alloy-SiC composite in 0.5 M HCl without and with Gt at varying temperatures
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The inhibition efficiency, % IE, was calculated using Eq. (1).

$$\%\;IE=\frac{{i}_{corr}-{i}_{corr(inh)}}{{i}_{corr}}\times 100$$
(1)

icorr and icorr(inh) represent the corrosion current density in the corrosive medium without and with Gt, respectively [14].

In the presence of Gt, the values of icorr decrease, increasing IE. It is presumably due to the film formation by Gt molecules adsorption, thereby protecting the composite from corrosion [15]. The observed anodic (βa) and cathodic (βc) slope values do not vary significantly in the inhibited solution and the acid medium, suggesting no change in the corrosion mechanism either in the acid or inhibited acid medium [16].

Electrochemical impedance spectroscopy (EIS) studies

EIS measurements were performed to access the Al alloy-SiC composite’s deterioration in 0.5 M HCl containing varying concentrations of Gt. Figure 4 shows impedance plots for Al alloy-SiC composites at 303 K in varying Gt concentrations. From the impedance plot, it is clear that the depressed semicircle diameter increases on increasing the Gt concentrations, suggesting a decrease in corrosion rate. Due to inhomogeneity, such as the roughness of the specimen surface, the deposition of corrosion product, or the adsorption of Gt molecules, the impedance plots capacitive loops are depressed [17]. Similar plots were obtained at other temperatures also. Similar impedance plots have been reported in the literature on the control of corrosion of Al alloy and its composite in the hydrochloric acid media [18,19,20]. At a higher frequency, the impedance plot shows a capacitive loop and an inductive loop at a lower frequency. The higher frequency (HF) loop corresponds to the resistance offered for the charge transfer of the corrosion process, and the lower frequency (LF) loop to the relaxation process of hydrogen ions and the adsorption of corrosive chloride ions onto the oxide film [21]. The metal Al is oxidized to Al+ ions and migrates to the oxide/electrolyte interface forming Al3+ ions. At the same time, O2− or OH− ions formed at the oxide/electrolyte interface. Due to the overlapping processes or one process dominating the others, all of these processes may be represented by a single capacitive loop [22]. In the presence of Gt, the area covered by the HF loop and LF loop combined is more than in its absence, which might be due to Gt forming a barrier film on the composite surface, influencing the processes as mentioned earlier [23]. In the presence of Gt, Fig. 4 clearly shows expanded capacitive and inductive loops, indicating increased corrosion resistance. Due to ionic conduction in the oxide film, the aluminum oxide film is considered a parallel circuit of a resistor and a capacitor due to its dielectric characteristics [24, 25].

Fig. 4
figure 4

Impedance plots for Al alloy-SiC composite in the acid and inhibited acid medium at 303 K

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An appropriate equivalent circuit was obtained by simulating the EIS data for the composite in the presence of Gt in the acid medium using ZSimpWin software (version 3.21). The Nyquist plot was simulated using an equivalent electric circuit shown as an inset in Fig. 5 to fit the impedance obtained theoretically. The representative simulated and fitted curves for corrosion of the alloy composite in the presence of 0.75 mM Gt in 0.5 M HCl at 303 K are shown in Fig. 5.

Fig. 5
figure 5

The simulated plot and the equivalent circuit

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In the equivalent circuit (Fig. 5), Rs represents the solution resistance, Rct indicates the charge transfer resistance, and Q is the constant phase element (CPE) that corresponds to the capacitance (C) of the double layer. L is an inductive element, and RL is the equivalent resistance. The elements Rs and Rct are parallel in the circuit. Because the impedance plots show depressed semicircles, Q is substituted for actual capacitance C in simulation. Because of surface inhomogeneity, the frequency dispersion is accounted for during impedance analysis. Equation (2) was used to compute the double-layer capacitance (Cdl).

$${C}_{\mathit{dl}}={Q}_{dl}{(2\pi {f}_{max})}^{ n-1}$$
(2)

where Qdl is the CPE constant, fmax represents the frequency where the imaginary part of the impedance is maximum, and n is the CPE exponent related to the surface morphology of the composite electrode surface and lies in the range −1 ≤ n ≤ 1. When n = 1, CPE acts as an ideal capacitor [26].

The polarization resistance (Rp) is computed from Eq. (3) [18].

$${R}_{p}=\frac{{R}_{ct} {R}_{L}}{{R}_{ct}+{R}_{L}}$$
(3)

Based on the polarization values in the blank (Rp) and inhibited medium (Rp(inh)), the percentage IE can be calculated using Eq. (4) [18].

$$\%\;IE=\frac{{R}_{p(inh)}-{R}_{p}}{{R}_{p(inh)}}\times 100$$
(4)

From Table 3, it is clear that the value of Rp increases with the Gt concentrations, resulting in improved corrosion resistance in the presence of Gt. Because of film formation by Gt on the composite surface, higher resistance is offered for the flow of electrons, resulting in the reduction of corrosion rate and increase of IE [27]. The decrease in Cdl value (Table 3) observed in increasing Gt concentrations is probably due to the rise in the electrical double-layer thickness at the composite/solution interface [28]. These findings are supported by an increase in the percentage of IE with increasing Gt concentrations. The experimental results obtained at other temperatures showed similar patterns.

Table 3 EIS parameters for Al alloy-SiC composite in 0.5 M HCl without and with Gt at varying temperatures
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Influence of temperature

The kinetic and thermodynamic data controlling the corrosion and inhibition process could be evaluated by studying the effect of temperature on IE. With the help of these parameters, one can follow the adsorption pattern of Gt molecules on the composite. As the temperature of the medium rises, the icorr also increases because of the rise in the conductivity of the medium. In the presence of Gt, there is an increase in IE, revealing that the composite’s deterioration is under control. PDP and EIS results (Tables 2 and 3) indicate the decline in IE observed with a temperature rise, showing the physisorption of Gt on the composite at higher temperatures [29].

The activation energy (Ea) for the corrosion of the composite specimen in 0.5 M HCl was computed as per the Arrhenius equation [30].

$$\mathrm{ln}(CR)=B-\frac{{E}_{a}}{RT}$$
(5)

The “B” indicates the Arrhenius constant, “R” is the gas constant (8.314 J K−1 mol−1), and “T” is the temperature.

The slope of the straight line obtained (Fig. 6) by plotting ln (CR) versus 1/T is equal to −Ea /R. Thus, Ea values can be computed for the composite in the corrosive and inhibited corrosive medium.

Fig. 6
figure 6

Arrhenius plot for Al alloy-SiC composite in 0.5 M HCl without and with Gt

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The change in entropy (ΔS#) and enthalpy (ΔH#) of activation for the corrosion process in the acid and inhibited acid solution was computed as per the transition state equation [31].

$$CR=\frac{RT}{Nh}\mathit{exp}\left(\frac{\Delta {S}^{\#}}{R}\right)\mathit{exp}\left(\frac{-\Delta {H}^{\#}}{RT}\right)$$
(6)

In this relation, “N” is Avogadro’s number (6.023 × 1023 mol−1), and “h” denotes Planck’s constant (6.626 × 10−34 J s).

The ln (CR/T) vs. 1/T plot (Fig. 7) gives a straight line with the slope and intercept value equal to (−ΔH#/R) and [ln (R/Nh) + ΔS#/R], respectively. The activation parameters are recorded in Table 4.

Fig. 7
figure 7

ln(CR/T) versus 1/T plot for Al alloy-SiC in 0.5 M HCl without and with Gt

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Table 4 The activation results for Al alloy-SiC composite in 0.5 M HCl without and with Gt
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Ea values in the inhibited acid medium were higher than in the acid medium (Table 4). The observed increase in Ea value in the inhibited acid medium suggests the physical adsorption of Gt on the composite [30]. Hence, the physically adsorbed Gt molecules can increase the physical energy barrier for the corrosion process. The surge in Ea value with an increase in Gt concentration may be due to the protective film formation by Gt molecules on the composite [15, 32]. The positive value of ∆H# indicates that the inhibition is an endothermic process. The ∆S# values are negative, showing the decrease in randomness occurring during the inhibition, favoring the adsorption of Gt molecules [33].

Adsorption behavior of Gt

Organic inhibitors control metal corrosion by adsorption on the metal/medium interface, forming a protective coating that isolates the metal surface from the corrosive medium. The adsorption approach is commonly described as the process of displacement of water molecules adsorbed on the metal surface [H2O(ads)] by the organic inhibitor molecule in the aqueous medium [Inh(aq)] as indicated below [31]:

$${\mathrm{lnh}}_{(\text{aq})}+{\mathrm{nH}}_2{\mathrm O}_{(\mathrm{ads})}\rightleftharpoons{\mathrm{lnh}}_{(\mathrm{ads})}+{\mathrm{nH}}_2{\mathrm O}_{(\mathrm{aq})}$$
(7)

“n” is the number of water molecules displaced by the inhibitor molecules.

Adsorption mainly depends on the nature of the metal, the corrosive medium, the temperature of the medium, and the concentration and molecular structure of the inhibitor. The adsorption isotherm can provide the mode of interaction of inhibitors at the composite surface. The surface coverage (θ) values of Gt at different concentrations were obtained from PDP measurements. These results were applied to various isotherms. The adsorption isotherm with a linear regression coefficient (R2) value close to one was carefully chosen as the best fit. In this case, the best linear fit was obtained with the Langmuir adsorption isotherm.

The Langmuir adsorption isotherm can be expressed by Eq. (8) [15]:

$$\frac{{C}_{Inh}}{\theta }=\frac{1}{{K}_{ads}}+{C}_{Inh}$$
(8)

where CInh denotes the inhibitor concentration, Kads indicate the equilibrium constant for adsorption (mM−1), and θ is the surface coverage, which is obtained by Eq. (9)

$$\theta =\frac{\%\,IE}{100}$$
(9)

% IE denotes the percentage inhibition efficiency obtained from Eq. (1).

The CInh/θ vs. CInh plot (Fig. 8) is a straight line whose intercept equals 1/Kads. The straight-line plot’s regression coefficient (R2) is much closer to unity, indicating that Langmuir’s adsorption isotherm obeyed for the adsorption of Gt on the surface of the composite (Table 5). It suggests that the adsorbed Gt molecules form a monolayer film on the composite surface, and there is no interaction between the adsorbed Gt molecules.

Fig. 8
figure 8

CInh/θ vs. CInh plot for the adsorption of Gt on the composite surface

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Table 5 Thermodynamic results for the adsorption of Gt on the composite surface
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The standard free energy of adsorption (ΔG0ads) value of Gt is related to its Kads value according to Eq. (10) [34].

$${K}_{ads}=\frac{1}{{C}_{water}}\mathrm{exp}\left(\frac{{-\Delta G}_{ads}^{0}}{RT}\right)$$
(10)

Cwater indicates the molar concentration of water in the solution (55.5 mol/L).

The higher Kads (Table 5) value reveals a strong interaction of Gt molecules at the composite surface, resulting in a higher IE [35]. Figure 9 depicts the ΔG0ads vs. T plot, representing a straight line. The standard adsorption value of enthalpy (ΔH0ads) and entropy (ΔS0ads) was equal to the slope and intercept of the straight line. The value of ΔG0ads is given by Eq. (11).

Fig. 9
figure 9

ΔG0ads vs. T plot for the adsorption of Gt on the composite surface

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$${\Delta G}_{ads }^{0}={\Delta H}_{ads}^{0}-T{\Delta S}_{ads}^{0}$$
(11)

The ΔG0ads value less than or equal to −20 kJ mol−1 suggests that the physisorption, while greater than or equal to −40 kJ mol−1, indicates the chemisorption. ΔG0ads values between these threshold limits reveal the inhibitor’s mixed adsorption on the composite surface [36]. As shown in Table 5, ΔG0ads values for Gt are in the range of −21 kJ mol−1, and the values are closer to −20 kJ mol−1, suggesting the mixed adsorption with stronger physisorption of Gt on the composite surface. A negative value of adsorption enthalpy indicates the inhibitor molecules’ physical adsorption [37]. As reported, the ΔH0ads value for physisorption is lower than −41.86 kJ mol−1, and for chemisorption, it must approach −100 kJ mol−1 [38]. The estimated value of ΔH0ads for Gt (Table 5) is −30.17 kJ mol−1. It indicates that Gt molecules preferably undergo physisorption on the composite surface. The observed decrease in IE with the rise in temperature (Tables 2 and 3) supports the physisorption of Gt molecules [29]. Therefore, it may be concluded that the inhibitor follows mixed adsorption, with physisorption playing a significant role. The value of ΔS0ads is negative, showing decreased disorderliness during adsorption [39].

SEM and AFM analysis

Figure 10a depicts the SEM image of the Al alloy-SiC composite dipped in 0.5 M HCl, indicating a rough surface containing many cavities due to the aggressive action of the acid medium. The SEM image of the composite specimen (Fig. 10b) dipped in the corrosive medium added with 0.75 mM Gt showed a surface with fewer cavities. It may presumably be due to the adsorption of Gt molecules on the composite leading to a protective film. It blocks the further attack of the acid medium, controlling corrosion of the composite material.

Fig. 10
figure 10

The SEM pictures of composite samples dipped in 0.5 M HCl a without and b with 0.75 mM Gt

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The AFM pictures of the composite sample dipped in the corrosive medium without and with 0.75 mM Gt, respectively, are shown in Fig. 11a, b. The measured average values of surface roughness (Ra) and root mean square roughness (Rq) of the corroded and inhibited specimens are shown in Table 6. The measured roughness values, Ra and Rq, for the inhibited composite sample are lower than the corroded composite sample. The drastic reduction in the surface roughness on the inhibited specimen reveals the adsorption of Gt, controlling the deterioration of AA6061 in the HCl medium.

Fig. 11
figure 11

The AFM images of the composite samples dipped in 0.5 M HCl a without and b with 0.75 mM of Gt

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Table 6 The surface roughness results for Al alloy-SiC composite samples
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The proposed corrosion inhibition mechanism

Generally, an inhibitor controls the deterioration of a metal in contact with an acid medium by adsorbing its molecules over the metal surface. The inhibitor’s adsorption behavior is governed by its nature, the inhibitor structure, the type of electrolyte, and temperature. In the HCl medium, aluminum corrosion occurs due to the depletion of the protective oxide layer. Aluminum with higher negative potential usually reacts with the hydrochloric acid medium and deteriorates following the anodic reactions [40]:

$$\mathrm{AI}+{\mathrm{CI}}^\text{-}\rightleftarrows {{\mathrm{AICI}}^\text{-}}_{\mathrm{ads}}$$
(12)
$${{\mathrm{AICI}}^\text{-}}_{\mathrm{ads}}+{\mathrm{CI}}^\text{-}\longrightarrow{{\mathrm{AICI}}^\text{+}}_{2}+3{\mathrm{e}}^\text{-}$$
(13)

The evaluation of hydrogen takes place at the cathode.

$${\mathrm{H}}^\text{+}+{\mathrm{e}}^\text{-}\longrightarrow{\mathrm{H}}_{\mathrm{ads}}$$
(14)
$${\mathrm{H}}_{\mathrm{ads}}+{\mathrm{H}}_{\mathrm{ads}}\longrightarrow{\mathrm{H}}_{2}$$
(15)

Organic inhibitors can adsorb over the composite via physisorption, chemisorption, or both. As discussed earlier, Gt follows mixed-type adsorption predominantly with physisorption. In a strongly acidic medium, Al alloy-SiC composite surface can attain a positive charge since the value of pHzch (i.e., pH at zero charge potential) for aluminum is 9.1 [41]. As a result, the chloride ions first adsorb by the electrostatic force of attraction at the composite/HCl solution interface. Hence, the composite surface acquires a negative charge. Being an amino acid derivative, Gt can undergo protonation in an acid medium similar to the one used in the present work [40]. The negatively charged composite surface can easily attract the protonated inhibitor, leading to physisorption (Fig. 12). The chemisorption of Gt can occur by sharing electron pairs on N, S, O atoms, and π-electrons of carbonyl groups in Gt with the empty d-orbitals of aluminum, leading to a coordination bond (Fig. 12) [42]. As reported elsewhere, electron-donating groups (−OH, −NH2) can improve the inhibitive performance of Gt [43].

Fig. 12
figure 12

Adsorption pattern for Gt on the composite surface

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UV–visible and FTIR analysis on adsorption of Gt

The adsorption of Gt molecules on the Al alloy-SiC composite surface can be confirmed using UV–visible spectra. The UV–vis spectra of 0.1 mM Gt in 0.5 M HCl solution (Fig. 13) revealed two absorption peaks (228 and 290 nm), corresponding to the π → π* and n → π* transitions, respectively. The composite specimen was immersed for 2 h in 0.1 mM Gt in 0.5 M HCl solution, and UV–vis spectra were retaken. The second recorded spectra had the same two peaks as the first, but their intensity was lower. The decrease in the intensity of the second spectra supports the adsorption of Gt molecules on the composite surface.

Fig. 13
figure 13

UV–visible spectra of 0.1 mM Gt in 0.5 M HCl before and after immersion of composite specimen

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The FTIR spectra of Gt are shown in Fig. 14. The observed FTIR absorption peaks are (ATR, cm−1) around 3200 (OH, NH2, NH str.), 2520 (SH, str.), 1700 (C=O str. of COOH), and 1514 (C=O str. of amide). The scraped corrosion product obtained from the specimen composite after immersing in 0.5 M HCl solution containing 0.1 mM Gt for 2 h showed the same peaks but with less intensity, confirming the physisorption of Gt molecules on the composite surface.

Fig. 14
figure 14

FTIR spectra of Gt and scraped corrosion product

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Theoretical DFT analysis

As per the DFT-based theoretical calculations, different parameters were evaluated to understand the interaction of the inhibitor on the composite metal surface. Gt can exist in the protonated form in an aqueous acid medium [41, 44]. Since the composite is exposed to an acidic medium (0.5 M HCl), the participation of protonated Gt could not be ruled out along with neutral Gt in the corrosion inhibition process. Hence, the DST calculations using the 6-31G** basis set were carried out on neutral and protonated Gt.

Figures 15a–f depict the optimized structure and the frontier molecular orbital structures of neutral and protonated Gt.

The EHOMO and ELUMO values are interrelated to the value of I (ionization potential) and A (electron affinity), respectively, as per the relations, Eqs. (16) and (17) [45].

$$I=- {E}_{HOMO}$$
(16)
$$A=- {E}_{LUMO}$$
(17)

The values of χ, η, and σ were computed using Eqs. (18–20), respectively [45, 46].

$$\chi =\frac{I+A}{2}$$
(18)
$$\eta =\frac{I -A }{2}$$
(19)
$$\sigma =\frac{1}{\eta }$$
(20)

The ΔN involved in the inhibitor and metal surface interaction was obtained using Eq. (21) [47].

$$\Delta N=\frac{{X}_{Al}-{X}_{Inh}}{2({\eta }_{Al}+ {\eta }_{Inh} ) }=\frac{{\Phi }_{Al}-{X}_{Inh}}{2 {\eta }_{Inh}}$$
(21)

In these relations, χAl and χInh represent the electronegativity, while ηAl and ηInh indicate the hardness of Al and inhibitor, respectively, and ɸAl is the work function. In the present work for aluminum, the theoretical value of electronegativity, ϕAl, is taken as 4.28 eV [48] and ηAl as zero [47].

As per Parr et al. [49], the electrophilicity index (ω) is given by Eq. (22):

$$\omega =\frac{{\mu }^{2}}{2\eta }$$
(22)

In the above relation, µ indicates the electronic chemical potential, defined as:

$$-\mu =\frac{I+A}{2}=\chi$$
(23)

The parameters like energies of the highest occupied molecular orbital (EHOMO) and lowest unoccupied molecular orbital (ELUMO), energy gap (ΔEg), hardness (η) and softness (σ), the fraction of transferred electrons (ΔN), and the electrophilicity index (ω) are summarized in Table 7.

Table 7 Quantum chemical parameters for neutral and protonated Gt
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The Gt molecule has multiple heteroatoms (O, N, S). Therefore, the heteroatoms in Gt with high negative Mulliken charges can preferably act as the active sites of adsorption. Figure 16a, b reveals that the heteroatoms such as N6 (−0.58), O10 (−0.55), N14 (−0.53), N10 (−0.53), O15 (−0.52), N9 (−0.52), O1 (−0.47), O18 (−0.47), O19 (−0.46), and O4 (−0.46) in neutral Gt molecule show high negative charges. Similarly in protonated Gt heteroatoms such as N6 (−0.56), O10 (−0.52), N14 (−0.51), N9 (−0.52), O19 (−0.52), N14 (−0.51), O15 (−0.51), O1 (−0.45), O4 (−0.45), and O18 (−0.43) show high negative Mulliken charges. These heteroatoms can provide active sites for adsorption.

Fig. 15
figure 15

Neutral Gt—a optimized structure, b HOMO, and c LUMO. Protonated Gt—d optimized structure, e HOMO, and f LUMO

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Fig. 16
figure 16

Mulliken charges on atoms of a neutral and b protonated Gt

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Generally, the value of EHOMO indicates the inhibitor’s electron-releasing tendency. A larger EHOMO value of the inhibitor indicates its higher electron-releasing and lower electron-accepting capability. The value of ELUMO reveals the electron-accepting property. The inhibitors with lower values of ELUMO exhibit poor electron-realizing properties and a more robust electron-accepting nature [50]. It is evident (Table 7) that neutral Gt shows a higher EHOMO value (−6.6472 eV) compared to protonated Gt (−9.1653 eV). It reveals that neutral Gt exhibits better electron-releasing properties than its protonated species. ELUMO value of protonated Gt is lower (−4.2472 eV) than neutral Gt (−0.2373 eV), which indicates the former’s better electron-accepting tendency.

The energy bandgap (∆Eg) between the HOMO and LUMO can predict inhibitor species’ relative adsorption order. In general, higher chemical reactivity is shown by the inhibitor species with lower ΔEg value and, hence, exhibits high inhibition activity [51]. In this work, the ΔEg value for protonated Gt (4.9181 eV) is lower than neutral Gt (6.4099 eV). Hence, protonated Gt can show higher reactivity and good inhibition performance. It supports the physisorption of Gt on the composite surface, as revealed by the experimental results. According to the HSAB principle, chemical hardness and softness parameters influence the inhibition performance of the inhibitor molecule. The bulk metals are chemically soft and preferentially interact strongly with soft inhibitor species. The inhibitor species with a lower η value and higher σ value imply stronger chemical reactivity and contribute more towards IE [52]. In the present case, protonated Gt showed this behavior, indicating its good inhibition activity. Once again, this confirmed the physisorption of protonated Gt.

The value of ΔN ˃ 0 reveals the greater electron-donating tendency of inhibitor species, while ΔN ˂ 0 shows the higher electron-accepting trend [53]. Table 7 indicates that neutral Gt can readily donate electrons, and protonated Gt can accept electrons. A good nucleophile exhibits a low value of ω, whereas a good electrophile exhibits a high value of ω [54]. The protonated Gt showed the highest ω value (9.1445), indicating its higher tendency to accept electrons from composite metal. It results in stronger adsorption of protonated Gt on the composite, improving the IE.

Conclusions

The following key features can be summarized based on experimental, theoretical, spectral, and surface analysis results.

The electrochemical results proved Gt as a potential green inhibitor for attenuating Al alloy-SiC composite corrosion in a 0.5 M HCl medium. The inhibitor, Gt, showed a mixed inhibitor behavior that significantly influenced the cathodic reactions. Gt evinced inhibition efficiency of 80% by PDP and EIS methods at a concentration of 0.75 mM and 303 K temperature. Thermodynamic results revealed the mixed adsorption of Gt with stronger physisorption on the composite surface. The adsorption of Gt on the composite surface follows the Langmuir adsorption isotherm. The electrochemical results show that the inhibition efficiency increases with a rise in medium temperature, suggesting the physical adsorption mode of Gt on the composite. The surface morphology analysis by SEM and AFM studies provides evidence for the protective film formation by the adsorption of Gt molecules on the composite surface. The results of the EIS technique support the PDP results. The spectral (UV–visible and FTIR) analysis confirmed the adsorption of Gt molecules on the composite surface. The theoretical studies using DFT validate the experimental results. Gt acts as an efficient inhibitor for Al alloy-SiC composite in HCl medium owing to its potential attributes like bulky molecular size, multiple donor atoms (N, S, and O), water solubility, stability, and ready availability with reasonable cost and non-toxic nature.