Evaluation of corrosion inhibition effect of glycerol stearate on aluminium metal by electrochemical techniques

2.1 Materials

Glycerol stearate (GS) was purchased from Prestige laboratory supplies, South Africa. Aluminium metal sheets [grade: 8xxx and % composition: Al (94.4%), Cu (4.3%), Mn (0.7%) and Fe (0.6%)] were prepared and purchased from North West University, South Africa, and 32 % Hydrochloric acid (HCl) was purchased at Rochelle chemicals, South Africa. All reagents were used as obtained.

2.2 Gravimetric method

The blank solution prepared was 1. 0 M of hydrochloric acid. The specimen used was aluminium with 3 cm × 4 cm dimensions, prepared from emery papers of grades ranging from 800 to 1200. The gravimetric experiments were conducted in 100 ml of the solution containing various concentrations of glycerol stearate (10 × 10^-5 M, 30 × 10^-5 M, and 50 × 10^-5 M). Three thermostats at different temperatures were used 318 K, 328 K, and 338 K. Initial masses of aluminium were recorded before corrosion testing. The immersion time for the corrosion testing was 3 h, both in the absence of the inhibitor and in the presence of the inhibitor. After the corrosion testing, the metal was washed with distilled water and then left to dry. After drying the metal, final masses were recorded, and mass loss, corrosion rate, percentage inhibition efficiency, and surface coverage were calculated based on mass loss. The experiment was conducted in triplicates for each concentration, and the average weight loss (W =  $\frac{W_1+W_2+W_3}{3}$ in grams) was recorded. The corrosion rate (ρ in g. cm⁻²· h⁻¹), percentage inhibition efficiency (%I), and surface coverage (θ) were calculated from the weight loss using the equations below (Arslanoğlu et al., 2012; Suh et al., 2012; Lupi et al., 2017; Tavernier et al., 2016; Foucher and López-Martínez, 2014; Boudalia et al., 2023; Daoud et al., 2014; Peme et al., 2015):

Where ΔW denoted aluminium average weight loss, S denoted total surface area of the Al specimen (cm²), and t denoted immersion time (h), while ρ₁ and ρ₂ were corrosion rates with and without inhibitors respectively.

2.3 Characterization technique

The inhibitor was confirmed by the Spectrum II FTIR spectrometer (PerkinElmer). The spectrum was within 500 and 4000 cm⁻¹ at a resolution of 4 cm⁻¹. Scanning electron microscopy (SEM) was used to perform morphological analysis on the samples. SEM was used on a TESCAN Vega TC using TESCAN software, together with energy dispersive X-ray spectroscopy (EDXS) to determine the elemental composition of the samples (at 20 kV). The samples were gold-coated to enhance imaging by forming a conductive layer on the analytes' surfaces, which prevented charging and elemental composition interference.

2.4 Adsorption studies

During the corrosion testing in the presence of the inhibitor compound, an adsorptive film was formed on a metal surface, and an oxide layer was further characterized using the FTIR technique, which was essential in studying the protective layer formed on the metal surface. This technique allows for the analysis of both liquid and solid samples. In the FTIR technique, the infrared absorption spectrum was measured, and another advantage was that the technique is fast in processing data. Furthermore, the functional groups of the chemical inhibitor molecule were investigated using FTIR qualitative analysis.

2.5 Electrochemical measurement

Electrochemical parameters were obtained with the aid of the Bio-Logic SP150 potentiostat working station, which consisted of a three-electrode cell, namely, saturated calomel electrode (SCE), reference electrode (RE), platinum counter electrode (CE) and an Al metal (1 cm²) working electrode (WE) without and with corrosion inhibitor concentrations. Potentiodynamic polarization studies were conducted between −0.250 and + 0.250 mV scanning range at a 1.0 mV/s constant sweep rate on the open circuit potential (OCP). Both anodic and cathodic polarization graphs were achieved when stabilization was attained after the working electrode was immersed for 30 min in the test solution. Electrochemical impedance (EIS) measurements were conducted by employing a 100 kHz to 10 Hz frequency range with a 10 mV peak-to-peak voltage, using an alternating current (AC) signal at corrosion potential.

2.6 Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy was utilized for studying charge transfer resistance occurring during the corrosion of aluminium metal in a corrosive environment. This technique was used to evaluate electrochemical measurements including solution resistance (R_s), charge transfer resistance (R_ct) with the inhibitor and charge transfer resistance (R_ct⁰) without the inhibitor, double layer capacitance (d_ll), the constant phase element (CPE), and exponents, which were further investigated by employing EIS. Furthermore, according to Eq. (4) below, R_ctand R_ct⁰were used to calculate the inhibition efficiency (Çakır et al., 2013; Guo et al., 2020; Kumar et al., 2013; Liu et al., 2006):

2.7 Computational studies

The computational methodology adopted in this study was based on density functional theory dispersion corrected (DFT-D) (Burke, 2012), which was essential for the accurate description of the organic molecules within the DMol³ (Delley, 2010) code embedded in the Materials Studio 2020 version. Geometry optimizations of the inhibitor were performed to calculate the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). These were performed using the generalized gradient approximation (GGA) (Perdew et al., 2014) of Perdew-Wang 91 exchange–correlation functional (Enkovaara et al., 2020; Kannemann and Becke, 2010) (GGA-PW91). The Tkatchenko and Scheffler (TS) (Zarrouk et al., 2010) dispersion correction to the PW91 was adopted. The convergence tolerances for energy, force, and displacement were 2.0 × 10^–5 Ha, 0.004 Ha.Å⁻¹, and 0.005 Å, respectively. The double numerical plus polarization (DNP) basis set with 4.4 Basis files was set using DFT semi-core pseudopotentials. Koopmans theorem defines these parameters (Tsuneda et al., 2010):

Electronegativity (χ) was defined as the measure of the power of an electron or group of atoms to attract electrons towards itself and it was estimated using Eq. (5):

Global hardness (ᶯ) measured the resistance of an atom to a charge transfer and was estimated using Eq. (6):

The global electrophilicity index ( $\omega$ ) was estimated using a relationship between electronegativity and chemical hardness parameters in Eq. (7):

Studies showed that a high electrophilicity value showed a good electrophile and a small value showed a good nucleophile.

Global softness (σ), which showed the capacity of an atom or group of atoms to receive electrons (Tsuneda et al., 2010), was estimated through Eq. (8):

Electron affinity (A) was defined as the energy released when an electron was added to a neutral molecule. It is related to E_LUMO through Eq. (9):

Ionization potential (I) was defined as the amount of energy required to remove an electron from a molecule. It is related to E_HOMO through Eq. (10):

3.1 Structural characterization

The Fourier transform (FTIR) spectra of glycerol stearate and adsorption film were displayed in Fig. 2. The region of interest was from 3000 to 4000 cm⁻¹ high energy region (Suh et al., 2012; Lupi et al., 2017; Tavernier et al., 2016). This region highly corresponds to the OH^-stretching vibrations (Tsuneda et al., 2010; Hammouti et al., 2011). The spectrum of glycerol stearate showed a broad peak at around 3296–3233 cm⁻¹, which was attributed to the hydroxyl groups (–OH) presence in the glycerol stearate structure. Two additional notable peaks appeared in the low-energy region, at 2934 and 2865 cm⁻¹, suggested the typical CH₂ stretch of alkyl carbon chains. The characteristic peak at 1727 cm⁻¹ was attributed to the –C = O stretching vibrations. The symmetric and asymmetric stretching vibration of the –CH₂ and –CH₃ were observed at the regions of 1470 cm⁻¹. The CO bending of the glycerol stearate was observed at 1170 cm⁻¹ and the long-chain band was noticed at 720 cm⁻¹. On the other hand, the adsorption film showed the newly formed adsorption bands for Al-O and Al-OH at 1100 cm⁻¹ and 1250 cm⁻¹ respectively, which attest to the fact that the inhibitor had successfully attached to the empty d-orbital of the metal. Furthermore, glycerol stearate binds to the metal through its active sites such as lone pairs located on the oxygen atoms.

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Fig. 2

The FTIR spectra of (a) Glycerol stearate and (b) Adsorption film.

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3.2 Gravimetric analysis

3.2.1

3.2.1 Glycerol stearate concentration and temperature effects on corrosion rate

The results for the rate of corrosion through weight loss measurements for Al in 1 mol. L^-1 HCl at various GS concentrations from 10 × 10^-5- 50 × 10^-5 M and at 318 K, 328 K, and 338 K are shown in Fig. 3 (a), and the data were summarized in Table 1. In addition, when 10 x10^-5 M of inhibitor was added the rate of corrosion decreased as compared to that for the uninhibited solution at three various temperatures, and there was a decrease in the rate of corrosion when 30 x10^-5 M and 50 × 10^-5 M of the inhibitor were added as noticed in Table 1. The rate of corrosion decreased as GS concentration increased since surface coverage (θ) of GS increased on the Al metal surface [39]. It was also noticed that the rate of corrosion, ρ was high for GS at 338 K as compared to 318 K and 328 K. In addition, the corrosion rate decreased as GS concentration increased as shown in Table 1.

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Fig. 3

(a) Efficiency (%I) versus GS concentration plot for GS, (b) Langmuir isotherm of adsorption GS inhibitor on Al metal sheet at 318 K, 328 K, and 338 K, (c) Arrhenius graphs for Al metal in 1 M HCl with and without GS, and (d) Transition state graphs at differing GS.

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Table 1 Corrosion rate (ρ), the efficiency of inhibition, (%I), and surface coverage (θ) of GS at 318, 328, and 338 K for aluminium metal.

Inhibitor	Temperature (K)	Concentration (x 10-5 M)	Weight loss (g)	Corrosion rate (x 10-3 g. cm−2. hr-1)	The inhibition efficiency (I)	Surface coverage (ϴ)	C/ϴ
GS	318	0	0.67	37.22	_	_	_
		10	0.03	1.67	95.52	0.96	10.47
		30	0.02	1.11	97.01	0.97	30.92
		50	0.01	0.56	98.51	0.99	50.76
	328	0	0.73	40.56	_	_	_
		10	0.26	1.44	96.44	0.96	10.37
		30	0.19	1.06	97.40	0.97	30.80
		50	0.04	0.22	99.45	0.99	50.28
	338	0	0.74	41.11	_	_	_
		10	0.38	2.11	94.86	0.95	10.54
		30	0.35	1.94	95.27	0.95	31.49
		50	0.33	1.83	95.54	0.955	52.334

Fig. 3 (a) showed the percentage efficiency of inhibition against GS concentrations at 318 K, 328 K, and 338 K. At 50 × 10^-5 M, the highest inhibition efficiency values were obtained. In addition, the %I of GS increased with increasing inhibitor concentration. Moreover, it was observed that the percentage inhibition efficiency at 50 × 10^-5 M from 318 K to 328 K increased, this behaviour was due to the wax-like nature of the glycerol stearate and its solubility effect in different conditions and medium. The donation of lone pairs from the O atoms into the unfilled orbit of the Al metal formed a coordinate bond. Also, Cl^- ions were adsorbed on Al/solution interface (Daoud et al., 2014; Peme et al., 2015). The rate of corrosion (ρ in g. cm⁻²· h⁻¹), the efficiency of inhibition (%I), and surface coverage (θ) were determined from the weight loss data (Boudalia et al., 2023; Daoud et al., 2014) using Eqs. (1–3).

At 50 x10^-5 M percentage inhibition efficiency of GS ranged between 96 and 99%. At 30 × 10^-5 M, percentage inhibition efficiency was between 95 and 97%. Finally, at 10 × 10^-5 M percentage inhibition efficiency of GS ranged from 95 to 96%. The adsorption isotherm results were shown in Fig. 3 (b). Langmuir adsorption isotherm and inhibitor surface coverage (θ) on Al surface related to GS concentration (C_in) according to Eq. (11):

The linear form is given by:

(11)

$$\frac{C_{\mathit{in}}}{\theta }=\frac{1}{K_{\mathit{ads}}}+C_{\mathit{in}}$$

Where C_in denoted GS concentration, θ denoted surface coverage degree, and K_ads denoted constant of equilibrium. Fig. 3 (b), the plot of C_in/θ against C_in showed a linear graph with a positive slope. In Table 2, K_ads values were presented and there was no regularity from obtained values of K_ads at 318 K, 328 K, and 338 K (Peme et al., 2015). In this project, larger values of K_ads for GS indicated the process of adsorption which was favoured by larger values of K_ads (Kumar et al., 2013).

Table 2 Measurements of adsorption from Langmuir isotherm for GS on aluminium.

Inhibitor	T (K)	Kads (x105 L.mol−1)	R2	-ΔGoads (kJ.mol−1)
GS	318	2.00	0.99	42.89
	328	1.81	0.99	43.97
	338	9.04	1	49.83

In Fig. 3 (b), Langmuir adsorption isotherm was followed by GS observed from data given by R² values closer to and at unity in Table 2. Furthermore, from slope values, monolayer adsorption was obtained (Arslanoğlu et al., 2012; Peme et al., 2015; Kumar et al., 2013). From K_ads values obtained, the free energy of adsorption (ΔG°_ads) was determined using Eq. (12):

(12)

$$\mathrm{\Delta }{G}^{°}=-2.303RT\mathrm{l}\mathrm{o}\mathrm{g}(55.5K_{\mathit{ads}})$$

ΔG°_ads denoted adsorption Gibbs energy, 55.5 value represented the molar concentration of water in the solution, denoted absolute temperature was denoted by T, and the constant of equilibrium for the adsorption process was denoted by K_ads. In Table 2 it was observed that ΔG°_ads values for GS inhibitor were above −40 kJ. mol⁻¹, which indicated a chemisorption mechanism of adsorption (Arslanoğlu et al., 2012; Çakır et al., 2013; Guo et al., 2020; Kumar et al., 2013; Peme et al., 2015). Furthermore, the K_ads values were observed to decrease from 318 K to 328 K and increase from 328 K to 338 K, this trend owes to the wax-like nature of the inhibitor and its solubility was more enhanced at increased temperature. It was noticed that the solubility of the inhibitor enhanced inhibition efficiencies and introduced consistency in the corrosion kinetics (Guo et al., 2020).

3.3 Potentiodynamic polarization

Aluminium potentiodynamic polarization curves in 1 M HCl in uninhibited and inhibited systems were presented in Fig. 4. In Table 4, shown are values of inhibitor concentrations, E_corr, I_corr, Tafel slopes (b_a and b_c) anodic and cathodic respectively, and percentage inhibition efficiencies (%I_PDP). Obtained current densities by extrapolating curves of tafel slopes to corrosion potential.

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Fig. 4

Potentiodynamic polarization plot for aluminium in 1 M HCl in the uninhibited and inhibited solutions of GS different concentrations.

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Table 4 revealed that inhibition efficiency increased as inhibitor concentration increased and as corrosion current densities decreased. This was a result of the adsorption of GS on the aluminium surface. The potentiodynamic polarization study showed that GS inhibition efficiencies increased in the order: of 10 × 10^-5 M (51.37%), 30 × 10^-5 M (78.80%), and 50 × 10^-5 M (81.06%). The corrosion potential value of inhibited solutions against the blank played an essential role in the sense that if E_corr was greater than 85 mV was given to either anodic or cathodic type inhibitors and if E_corr was less than 85 mV was accredited to a mixed type inhibitor mechanism (Liu et al., 2006). Furthermore, Table 4 showed that E_corr values were less than 85 mV with the cathodic inhibitor mechanism dominating inferring from the tafel slopes.

3.4 Electrochemical impedance spectroscopy

The corrosion behavior of aluminium in 1 M HCl and the presence of 10 × 10^-5 M, 30 × 10^-5 M, and 50 x10^-5 M GS concentrations was studied with the aid of the EIS. The Nyquist plot of aluminium without and with GS concentrations was presented in Fig. 5. Table 5 showed that R_ct values increased in inhibited solutions and a decrease in C_dl values was observed as the inhibitor concentration increased due to the formation of a barrier at the metal-solution interface. Similar behavior showed by Chi-Ucán et al (Chi-Ucán et al., 2014).

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Fig. 5

Nyquist plot for aluminium in 1 M HCl in the uninhibited and inhibited solution with different GS concentrations.

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The increase in R_ct values was due to the adsorption of GS molecules on aluminium metal thus retarding metal dissolution and hydrogen evolution which are oxidation and cathodic processes occurring at the metal solution interface (Chi-Ucán et al., 2014). Furthermore, Nyquist plots in Fig. 5 showed a single depressed capacitive arc or semicircle due to metal surface roughness and this revealed that aluminium dissolution was by a single transfer process (Solmaz, 2014). The impedance nature of GS was studied with the use of an electrical circuit comprising R_s, R_ct, and C_dl shown in Fig. 7. In addition, corresponding Bode plots in Fig. 6 showed that the high-frequency limit corresponded to the solution resistance Rs represented on the electric circuit shown in Fig. 6. Furthermore, it was observed that in Bode plots the low-frequency limit (R_s + R_ct) from Nyquist and Bode plots were in good agreement. Bode absolute impedance plot (Fig. 6a) showed that the impedance value increased greatly over the whole frequency range with the incremental concentration of GS, and a larger log |Z| represented a better protection performance. In the corresponding Bode phase plot (Fig. 6b), large values of the phase angle indicated that superior inhibition behavior is obtained by increasing the concentration of GS (Zheng et al., 2015).

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Fig. 6

Bode plots of aluminium in 1. 0 M HCl with and without glycerol stearate.

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Fig.7

The suggested electrical circuit for studied GS.

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3.5 Scanning electron microscopy

Fig. 8 (a), showed the smooth surface of aluminium before corrosion testing, any inhomogeneity revealed was due to abrasion with emery papers. Nonetheless in Fig. 8 (c), after immersion in 1 M HCl aluminium surface showed a rougher nature. These observations were substantiated by corresponding EDS spectra revealing the absence and presence of Cl^- which led to aluminium surface roughness (Zheng et al., 2015).

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Fig. 8

(a) SEM image, (b) EDS spectrum of pristine aluminium metal; (c) SEM micrograph, (d) EDS spectrum of aluminium in 1 M HCl solution; (e) SEM micrograph, and (f) EDS spectrum of aluminium in 1 M HCl and glycerol stearate inhibitor.

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However, introducing glycerol stearate inhibitor minimized more surface roughness when SEM micrographs in the absence and presence of inhibitor in 1 M HCl solution were compared. In Fig. 8 (e). Fig. 8 (b), (d) and (f) illustrated were the EDS spectrums of pristine aluminium, aluminium in the presence of HCl and aluminium in the presence of HCl/ GS, respectively. It was observed that the compositions for aluminium, copper, manganese, and iron elements were identified in a pristine material signifying that the material was an alloy. Fig. 8 (d) showed a decrease in aluminium composition as a result of the presence of chloride ion from the corrosive medium. However, Fig. 8 (f) showed a restored aluminium composition due to the presence of the inhibitor which in turn reduced the chloride ion composition.

3.6 Computational studies

The data collected from weight loss measurements and electrochemical techniques were further substantiated with chemical quantum calculations which played an essential role in studying the reactivity and selectivity parameters of glycerol stearate as the inhibitor molecule in this study (Tsuneda et al., 2010; Hammouti et al., 2011; Hegazy et al., 2013). The need to study the inhibitor's reactivity and selectivity emanated due to different regions within the inhibitor which interacted with the metal surface. There were ample electronic properties on which the reactivity of the inhibitor depended, such as dipole moment, partial charges, and electronic density to mention a few. In addition, the nature of functional groups within the inhibitor influenced the electronic properties. In Fig. 9 (a), displayed was the optimized geometry of glycerol stearate with the atom numbering used in this study. The reason why the geometry of the inhibitor was of utmost importance was due to the dependence of the inhibition efficiency on the geometry of the inhibitor molecule. Among many, inhibitor compounds with planar geometry were most preferred due to their inhibition efficiencies as compared to non-planar geometries (Zheng et al., 2015). This was due to the high possibility of a planar inhibitor molecule which resulted in a larger surface coverage on the metal surface through the inhibitor's most reactive atomic sites.

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Fig. 9

(a) The optimized geometry and the atom numbering of the studied glycerol stearate, (b) Relaxed geometries and HOMO (isosurface generation iso value = 0.05) of glycerol stearate, (c) Relaxed geometries and LUMO (isosurface generation iso value = 0.05) isosurfaces of glycerol stearate.

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Through the analysis of the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO), the reactive sites of glycerol stearate could be studied. Furthermore, the study of reactivity parameters such as the HOMO energy (E_HOMO), the LUMO energy (E_LUMO), global softness (σ), global hardness (η), electron affinity (EA), ionization potential (IP), and electronegativity (χ) was essential. In Fig. 9 (b) and 9(c), shown are the HOMO and LUMO of the studied glycerol stearate respectively. The area where an electrophilic attack mostly emanated at the HOMO while the area where the nucleophilic attack likely occurred from the LUMO (Solmaz, 2014). From this study, the highest HOMO densities of glycerol stearate occur at carbon atoms on C1, C2, C3, and C4, on hydroxide oxygen atoms (O1 and O2), on the ether functional group O3 and lastly on the carbonyl O4. Since the LUMO densities entailed information concerning the nucleophilic attack, however, this was mainly applicable provided the metal surface back donated electrons to glycerol stearate.

Table 6 showed the molecular quantum chemical parameters which relate to the reactivity of glycerol stearate used in this study. The parameters included are E_HOMO, E_LUMO, and the energy gap (ΔE), among others. The interaction between the HOMO and LUMO of reacting glycerol stearate was responsible for electron transition between the metal-inhibitor interfaces and was informed by Frontier Molecular Orbital Theory (FMO) (Tsuneda et al., 2010; Hammouti et al., 2011; Hegazy et al., 2013; Solmaz, 2014; Zheng et al., 2015). In addition, at the E_HOMO glycerol stearate donated electrons to the empty d-orbital of the metal, thus higher E_HOMO values were appreciated since they catered to the electron-deficient species according to studies (Obot et al., 2015). Furthermore, a chemical compound possessing higher E_HOMO values showed appreciable inhibition efficiencies and enhanced an effective adsorption process at the metal-inhibitor interface (Zarrouk et al., 2013). The degree to which a chemical compound could receive electrons was shown by E_LUMO values. Lower E_LUMO values showed a high probability to a compound could have accepted electrons from some electron-rich chemical species (Zarrouk et al., 2013).

Further information with regards to glycerol stearate reactivity, was through the study of the energy gap. From ΔE, the stability and reactivity of the inhibitor molecule could be studied. Thus, a higher ΔE value was associated with high stability and less reactivity, meanwhile lower ΔE value was associated with low stability and more reactive to other species (Zarrouk et al., 2013). Regarding the polarity of the molecule, a dipole moment was studied. In addition, from other projects dipole moments were reported to increase with increasing inhibition efficiencies meanwhile in other reports a different trend was observed (Tsuneda et al., 2010; Hammouti et al., 2011; Hegazy et al., 2013; Solmaz, 2014; Zheng et al., 2015; Obot et al., 2015; Zarrouk et al., 2013). The other crucial reactivity parameter shown in Table 6 was electronegativity (x) which gave information regarding the electron density and the ability of an electron or atom to attract more electrons to itself (Tsuneda et al., 2010; Hammouti et al., 2011; Hegazy et al., 2013; Solmaz, 2014; Zheng et al., 2015; Obot et al., 2015; Zarrouk et al., 2013). The complementing parameter to electronegativity was the global electrophilicity index (ω), chemical inhibitors with higher ω value were named good electrophiles meanwhile those with lower ω value were named good nucleophiles (Tsuneda et al., 2010; Hammouti et al., 2011; Hegazy et al., 2013; Solmaz, 2014; Zheng et al., 2015; Obot et al., 2015; Zarrouk et al., 2013; Murulana et al., 2012).

Furthermore, information regarding the resistance of an atom to charge transfer was given by global hardness (ɳ) and higher ɳ values signified a higher resistance for the inhibitor to transfer charge to the metal surface. Thus, inhibitors with a lower ɳ value were most appreciated as this enhanced a better adsorption process at the metal-inhibitor interface. Global softness (σ) relates to the softness of the inhibitor compound and at the highest σ inhibitor region an enhanced inhibitor-metal adsorption was observed (Tsuneda et al., 2010; Hammouti et al., 2011; Hegazy et al., 2013; Solmaz, 2014; Zheng et al., 2015; Obot et al., 2015; Zarrouk et al., 2013). Electron affinity (A) revealed an electron deficiency region hence it associated with the E_LUMO. Moreover, another crucial molecular reactivity parameter was ionization potential (I) which revealed the amount of energy required to remove an electron from the molecule (Tsuneda et al., 2010; Hammouti et al., 2011; Hegazy et al., 2013; Solmaz, 2014; Zheng et al., 2015; Obot et al., 2015; Zarrouk et al., 2013) and this helped investigate the amount of energy it took reactive atoms within glycerol stearate to transfer electrons to the metal's empty orbital.