Using electrochemical testing and modeling to assess the efficiency of a water-soluble inhibitor on the corrosion of 6061Al-15 % _(v) SiC _(p) composite

2.1 Materials

6061Al-15 % _(v) SiC _(p) composite (represented hereafter words as Al alloy-SiC composite) was used as the specimen material in this work. The composite was prepared using the stir-casting technique by reinforcing 6061Al alloy with SiC particulate (purity 99.9 % and size 23 μm). 6061Al base alloy used has the composition: Mg (1.0 wt%); Si (0.60); Cu (0.25); Cr (0.25); and Al (97.9). The test coupons were prepared using a rod of Al alloy-SiC composite and then embedded with epoxy resin under cold conditions, leaving an uncovered surface of 1 cm² at one end. The specimen’s exposed surface was abraded with emery papers (grades 200 to 1500) and finely on a disc polisher. Then the sample was washed using deionised water, and acetone was rinsed and dried.

2.2 Preparation of medium

Hydrochloric acid solutions (0.5 M) were prepared from 37 % hydrochloric acid (AR grade) on dilution using double-distilled water and standardised by a volumetric method.

2.3 Synthesis and characterisation of NMSc

N-(1-morpholinobenzyl) semicarbazide (NMSc) was synthesised as per the reported procedure (Viswanathan 2016). Semicarbazide hydrochloride (1.11 g, 10 mM) was dissolved in 20 mL ethanol and neutralised with a dropwise addition of NH₃. To this solution, morpholine (1 mL, 10 mM) and benzaldehyde (1 mL, 10 mM) were added dropwise on constant stirring. A colourless product precipitated was filtered. The separated product was recrystallised from ethanol. The molecular structure of NMSc is shown in Figure 1. The recrystallized sample’s IR spectra were recorded by a Shimadzu FTIR 8400S spectrophotometer in a 4000–400 cm⁻¹ frequency range using KBr. ¹H-NMR spectra were recorded in DMSO‑d₆ by a Bruker 400 MHz NMR spectrophotometer.

Figure 1:

Molecular structure of N-(1-morpholinobenzyl) semicarbazide (NMSc).

Colourless crystalline solid, yield: 80 %; C₁₂H₁₈N₄O₂: m.p: 216 ± 1 °C; IR (KBr) [cm⁻¹]: 1141 (C–O str.), 1600 (C=O str.), 1643 (C=N str.), 3028 (Ar. CH str.), and 3288 (NH str.). ¹H-NMR (400 MHz, DMSO‑d₆) (⸹ppm): 2.5 (4H, –N–CH₂), 3.74 (4H, –O–CH₂), 6.23 (1H, NH), 6.32–7.72 (5H, Ar. H), 6.7 (1H, –CH), 7.83 (2H, –NH₂), 10.94 (1H, –N=C–OH). Elemental analysis, Calcd. (Found) for C₁₂H₁₈N₄O₂: C, 57.14 (57.28); H, 6.88 (6.92); N, 21.62 (21.52).

2.4 Electrochemical techniques

Electrochemical experiments were conducted with CH Instruments (CH600D-series, U.S. model). A three-electrode cell containing a saturated calomel electrode, the Al alloy-SiC composite specimen’s working electrode and an auxiliary electrode of Pt were used. Experiments were conducted with three trials, and the average value was considered for the calculation. Initially, the three-electrode system dipped in 100 mL of 0.5 M HCl solution in a 250 mL beaker was allowed to attain the steady state open circuit potential (OCP). The experiment was conducted at 303, 313 and 323 K temperatures using a water bath connected to a calibrated thermostat. The electrochemical impedance tests were performed at OCP in the 10 kHz–0.01 Hz frequency range by applying an AC signal of 10 mV amplitude. Nyquist impedance data plots were recorded and analysed using the Zsimpwin program. At OCP, the potentiodynamic polarisation (PDP) tests were performed by varying potential from −250 to +250 mV with a 1 mV/s scanning rate. From the recorded Tafel plots, E_corr (corrosion potential, V), i_corr (corrosion current density, mA/cm²), the anodic (β_a) and cathodic (β_c) slopes were obtained.

2.5 Specimen’s surface analysis

Al alloy-SiC specimen’s surface morphology was analysed after immersing it in 0.5 M hydrochloric acid solution without and with NMSc for 3 h using an SEM (JEOL JSM-6380L model). Pre-and post-inhibition roughness of the specimen surface was recorded using AFM (IB342-Innova model).

2.6 UV-visible spectral analysis

UV-visible spectra of NMSc (0.01 mM) in 0.5 M HCl solution were taken by a UV-visible spectrophotometer (1800 Shimadzu). The Al alloy-SiC composite test coupon was immersed in this solution for 3 h. After removing the test coupon, the absorption spectra of the solution were recorded.

2.7 Theoretical studies

The theoretical calculations by density functional theory (DFT) were made with the help of Schrodinger’s software with B3LYP functions and a 631G** basis set. The optimised structure of the inhibitor molecule, HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) were recorded. Different theoretical parameters were obtained, including energies of HOMO and LUMO, energy band gap, the fraction of electrons transferred, hardness and softness values.

3.1 Electrochemical results

3.1.1 PDP measurements

The PDP plots for Al alloy-SiC composite in 0.5 M HCl medium in the absence and presence of NMSc at 303 K are shown in Figure 2. Polarisation parameters obtained from these plots include corrosion potential (E_corr, V), corrosion current density (i_corr, mA/cm²), anodic slope (β_a, V dec⁻¹) and cathodic slope (β_c, V dec⁻¹).

Figure 2:

The PDP plots for Al alloy-SiC in acid and inhibited acid solutions at 303 K.

The corrosion rate (CR, mmy⁻¹) of the composite specimen was given by Equation (1).

(1)CR (mmy−1)=3270×M×icorrd×Z

where 3270 is the unit conversion constant, Z is the number of electrons transferred per metal (3 for Al), M is the atomic mass (9.15) and d (2.66 g/cm³) is the density of the metal (Fontana 1987).

The inhibition efficiency (IE (%)) of NMSc was determined according to Equation (2).

(2)IE (%)=icorr−icorr(inh)icorr×100

where i_corr represents the corrosion current density in acid, while i_corr(inh) is in an inhibited acid medium (Fontanna 1987)_.

The PDP parameters obtained (Table 1) indicate the decrease in i_corr and CR of the composite specimen in the inhibited acid medium because of the NMSc’s adsorption on the composite surface. According to Table 1, the IE of NMSc increases with increasing concentration and decreases with increasing medium temperature. It presumably indicates the physisorption of NMSc molecules on the composite surface (Nagalaxmi et al. 2020). NMSc showed a maximum IE of 95.42 % at 2.56 mM concentration and 303 K. The metal oxidation process is represented by the anodic curve of the potentiodynamic polarisation plot (Figure 1), while the cathodic curve represents the hydrogen evolution phenomenon.

Table 1:

PDP results for Al alloy-SiC in the blank and inhibited medium at varying temperatures.

T (K)	C Inh (mM)	E corr (V vs. SCE)	i corr (mA/cm2)	β a (mV dec−1)	β c (mV dec−1)	CR (mm y−1)	IE (%)
303	0.0	−0.676	9.440	4.694	4.861	103.3	–
	0.01	−0.682	6.135	5.025	5.660	67.08	35.01
	0.04	−0.685	5.180	4.939	6.241	56.64	45.13
	0.16	−0.692	2.711	5.279	6.893	29.64	71.28
	0.64	−0.695	1.466	5.492	7.509	16.03	84.47
	2.56	−0.712	0.4319	8.879	7.845	4.72	95.42
313	0.0	−0.676	13.15	4.689	4.612	143.8	–
	0.01	−0.691	11.05	4.731	5.216	120.8	15.97
	0.04	−0.691	8.024	4.703	5.653	87.73	39.00
	0.16	−0.695	5.403	4.921	6.137	59.08	58.91
	0.64	−0.696	4.478	5.129	6.197	48.97	65.95
	2.56	−0.696	3.020	5.300	6.407	33.02	77.03
323	0.0	−0.692	21.30	4.653	4.007	232.9	–
	0.01	−0.704	18.93	4.599	4.543	207.1	11.13
	0.04	−0.704	16.71	4.565	4.840	182.8	21.55
	0.16	−0.704	12.59	4.626	5.208	137.6	40.89
	0.64	−0.703	10.45	4.630	5.368	114.2	50.94
	2.56	−0.705	6.017	5.032	5.984	65.8	71.75

It is clear from Table 1 that there is a slight variation in the Tafel slopes (β_a and β_c) in inhibited acid medium, indicating the NMSc’s adsorption, which blocks the surface available for corrosion reducing the corrosion rate without altering the corrosion mechanism (Pinto et al. 2011; Yurt et al. 2006). However, a significant variation in the β_a value of the anodic curve corresponding to a 2.56 mM concentration of NMSc at 303 K is observed (Table 1). The plateau of this anodic curve is unclear (Figure 1). It may be due to the kinetic barrier effect occurring because of either the partial breakdown or the formation of porosity on the passivated layer. Because of the passivation of the metal in the range of −0.7 to −0.6 V, a considerable change in the anodic Tafel slope (β_a) was observed (Charitha and Rao 2018b). Since the anodic current plateau is unclear, the corrosion current densities were calculated by extrapolating a cathodic slope.

As reported elsewhere (Li et al. 2008), an inhibitor can be classified as an anodic or cathodic type if the shift in E_corr value in the inhibited medium is more than ±85 mV than that in the blank medium. In this work, the observed change in E_corr value is – 0.36 mV indicating that NMSc acts as a mixed-type inhibitor showing predominant control on cathodic reaction.

3.1.2 EIS measurements

The impedance plots (Figure 3a) obtained are depressed semi-circular, indicating the charge transfer nature of the corrosion phenomenon (Lenderink et al. 1993). There is an increase in the diameter of impedance curves in the inhibited acid medium, which indicates that corrosion inhibition occurs through NMSc adsorption at the composite surface. The impedance plot (Figure 3a) contains an inductive loop and a depressed capacitive loop at the lower and higher frequency regions. The in-homogeneities on the composite surface results in frequency dispersion (Metikos-Hukovic et al. 2002). A similar impedance plot was reported earlier on Al alloy/composite corrosion in HCl solution (Kumari et al. 2020; Noor 2009).

Figure 3:

Al alloy-SiC composite’s (a) impedance plot at 303K and (b) stimulating plot with the equivalent circuit.

The capacitive loop at the higher frequency regions could be accountable for the charge transfer during corrosion and oxide layer formation covering the composite surface (Metikos-Hukovic and Babic 1998). Brett 1992, reported that the capacitive loop attributes to interfacial reactions. At the metal oxide/electrolyte interface, Al⁺ ions are formed, which migrate through the interface and oxidise to Al³⁺ ions with simultaneous formation of OH⁻ or O²⁻ ions. The inductive loop observed at a lower frequency may be due to the surface relaxation process, indicating the inhibitor adsorption on the oxide layer (Abd El Rehim et al. 2002). The capacitive loop’s diameter increases as the concentration of NMSc rises, revealing the reduction in the relaxation process and an increase in the corrosion resistance of the composite due to NMSc adsorption (Umoren et al. 2010).

The EIS data obtained for the Al alloy-SiC composite in the presence of an inhibitor was checked for fitment to an appropriate equivalent circuit (Figure 3b) utilizing Zsimpwin software (version 3.21). It is evident from Table 2 that the values of χ² are in the order of 10⁻³, which authenticates the selected equivalent circuit. The equivalent circuit depicted in Figure 3b comprises elements, R_ct (charge transfer resistance), Q (constant phase element, CPE), R_s (solution resistance) and R_L and L (inductive elements). The elements Q, R_ct and R_L are parallel in the equivalent circuit, but L and R_L are in series. CPE (Q) is utilised as a substitute for the ideal capacitive element (C) in simulation analysis to account for depressed capacitive loops. The non-uniformity of the composite electrode surface leads to frequency dispersion, resulting in depressed semicircle impedance plots and deviations from the ideal behaviour (Jüttner 1990). The real value of C_dl (double layer capacitance) in the existence of frequency dispersion can be calculated using Equation (3) (Babic et al. 1998).

(3)Cdl=Qdl(ωmax)n−1

In this relation, Q_dl is the CPE constant, ω_max = 2πf_max, f_max indicates the frequency where the −Z″ reaches the maximum and n is a CPE exponent determined by the morphology of the composite electrode surface. Its value lies in the range −1 ≤ n ≤ 1, and CPE shows an ideal behaviour as a capacitor when n = 1.

Table 2:

EIS results for Al alloy-SiC in the blank and inhibited solutions at different temperatures.

T (K)	C Inh (mM)	R L (Ω cm2)	R ct (Ω cm2)	R p (Ω cm2)	n	χ 2 (×10−3)	C dl (μF cm−2)	IE (%)
303	Blank	19.27	7.62	5.45	0.85	4.789	3056	–
	0.01	25.64	12.86	8.56	0.83	4.436	1361	36.33
	0.04	27.45	17.40	10.65	0.82	2.396	1113	48.83
	0.16	75.34	37.46	25.02	0.81	3.655	376	78.22
	0.64	85.98	61.19	35.75	0.81	6.066	179	84.76
	2.56	222.18	98.78	68.38	0.80	2.602	91.23	92.03
313	Blank	15.67	6.35	4.52	0.85	8.226	5069	–
	0.01	17.98	9.85	6.36	0.82	5.348	3638	28.93
	0.04	20.12	13.19	7.97	0.82	3.676	2285	43.29
	0.16	25.67	20.95	11.54	0.81	3.693	1355	60.83
	0.64	35.96	28.79	15.99	0.81	3.547	938	71.73
	2.56	48.56	45.75	23.56	0.81	3.248	555	80.81
323	Blank	11.45	4.93	3.44	0.83	4.300	10,339	–
	0.01	13.98	6.00	4.20	0.82	4.930	8104	18.09
	0.04	16.54	6.76	4.80	0.82	4.954	6756	28.33
	0.16	21.57	8.77	6.24	0.82	6.626	3895	44.87
	0.64	26.65	10.54	7.55	0.82	5.427	2993	54.44
	2.56	34.54	19.14	12.31	0.82	7.589	1291	72.05

The relation, Equation (4), was used to calculate the polarisation resistance (R_p) value (Noor 2009)

(4)Rp=RL RctRL+Rct

The inhibition efficiency of NMSc (IE, %) were obtained using Equation (5) (Noor 2009)

(5)IE (%)=Rp(inh)− RpRp(inh)×100

R _p(inh) refers to polarisation resistance in the inhibited solution and R_p in the acid solution. Table 2 displays the parameters obtained from the EIS measurement for the Al alloy-SiC composite in a 0.5 M HCl medium. It is evident from Table 2 that polarisation resistance (R_p) and charge transfer resistance (R_ct) increase with a rise in NMSc concentration, indicating that the formation of adsorbed inhibitor film resists the charge transfer process, controlling corrosion of the composite in the acid medium. These results suggest that NMSc undergoes physisorption. The electrical double layer developed at the charged composite surface-medium interface could be taken as a capacitor. By replacing the initially adsorbed water molecules and other ions on the surface, NMSc adsorption on the composite electrode reduces its electrical capacity. The decreased capacity could be attributed to forming of a protective film on the composite electrode surface (Kedam et al. 1981). The decrease in C_dl values with increasing NMSc concentration is primarily due to the improved thickness of the electrical double-layer at the composite/acid medium interface (Bessone et al. 1992).

3.2 Kinetic parameters

The inhibition efficiency of NMSc decreased with a temperature rise of the medium, probably because of the short time lag between the adsorption/desorption process of NMSc molecules on the surface of the composite that occurs with the temperature rise. It reveals the physisorption of NMSc molecules on the composite surface (Nagalaxmi et al. 2020). A maximum IE of 95.42 % was displayed at 2.56 mM NMSc and 303 K. The effect of temperature on the extent of NMSc adsorption on the composite surface helps to obtain kinetic data like E_a (activation energy), ΔH_a (enthalpy of activation) and ΔS_a (entropy of activation).

E _a value in the acid and the inhibited acid solution was obtained from Equation (6) (Schorr and Yahalom 1972)

B indicates the Arrhenius constant, R refers to the gas constant (8.314 J K⁻¹ mol⁻¹) and T denotes the temperature in K.

The Arrhenius plot for Al alloy-SiC composite in the blank and inhibited solution (Figure 4a) depicts a straight line, and its slope equals (−E_a/R). Hence, E_a values in acid and inhibited acid solutions were calculated. With the help of the transition state equation [Equation (6)], ΔH_a (Enthalpy of activation) and ΔS_a (Entropy of activation) values were calculated (Abd El-Rehim et al. 1999).

where h refers to Planck’s constant and N denotes Avogadro’s number.

Figure 4:

Al alloy-SiC composite’s  (a) Arrhenius plot and  (b) ln (CR/T) vs. 1/T plot.

Figure 4b depicts a straight-line graph of ln(CR/T) versus 1/T for Al alloy-SiC composite. The slope of the straight line is (−ΔH_a/R), and the intercept is [ln(R/Nh) + ∆S_a/R]. As a result, ΔH_a and ΔS_a values were computed. Table 3 shows the activation parameters for Al alloy-SiC composite in acid and inhibited acid solutions.

In the presence of NMSc, activation energy increased, resulting in a decrease in corrosion rates. It has been proposed that organic inhibitor adsorption can influence corrosion rate by either decreasing the available reaction area (geometric blocking effect) or changing the activation energy of anodic or cathodic reactions occurring in the inhibitor-free surface during the inhibited corrosion process (Fouda et al. 2010).

The increase in the E_a value in the inhibited acid medium indicates the possibility of NMSc physisorption on the composite surface (Obot et al. 2009). The negative ∆S_a value suggests that the activated-complex state of the rate-determining step involves association rather than dissociation, resulting in less disorder in the reaction system (Gomma and Wahdan 1995).

3.3 Adsorption and thermodynamic behaviour

Knowing the adsorption mode of NMSc on the composite surface helps to understand the inhibition mechanism for Al alloy-SiC composite corrosion. The surface coverage (θ) values at various NMSc concentrations were computed from Equation (8).

The θ values were applied to Langmuir, Temkin and Freundlich adsorption isotherms to confirm the fitment. The finest fitment of the data was obtained with the Langmuir adsorption model. Based on the Langmuir adsorption isotherm, θ is related to the inhibitor concentration (C_inh) (Obot et al. 2009),

where K_ads indicates the adsorption equilibrium constant.

Figure 5a represents the plot of Langmuir adsorption isotherm for NMSc on Alalloy-SiC composite in 0.5 M HCl at different temperatures. The C_inh/θ versus C_inh plot gives a straight line with an intercept at 1/K_ads and regression coefficients (R²) close to one, suggesting that NMSc follows the adsorption isotherm of Langmuir (Jeeva et al. 2017). It reveals the potential adsorption of NMSc molecules, creating a protective barrier on the composite surface. Besides, the Langmuir isotherm shows a protective monolayer film formation on the composite surface with a fixed number of equivalent adsorption sites. Each site holds an adsorbate with no interaction between the inhibitor molecules. The higher value of K_ads (Table 4) indicates the stronger adsorption of MBS, particularly at a lower temperature (Tang et al. 2006).

Figure 5:

Al alloy-SiC-NMSc adsorption system’s (a) Langmuir  adsorption isotherm plot and (b) ΔG°ads vs. T plot.

The standard free energy of adsorption (ΔG°_ads) was obtained from Equation (10) (Umoren et al. 2010).

where 55.5 mol/L is the water concentration.

For the adsorption process, ΔG°_ads values are linked to ΔH°_ads (standard enthalpy) and ΔS°_ads (standard entropy) as per Equation (11).

Figure 5b depicts a straight line obtained from the ΔG°_ads versus T plot. The slope provided the ΔS°_ads value for the adsorption process, and the intercept provided the ΔH°_ads value. Table 4 displays the thermodynamic outcomes.

Generally, ΔG°_ads ≤ −20 kJ mol⁻¹ indicate the inhibitor’s electrostatic interactions with the metal surface leading to physisorption. When ΔG°_ads ≥ −40 kJ mol⁻¹, the charge transfer from the inhibitor to the metal surface occurs, leading to chemisorption (Li et al. 2008). In this work, the values of ΔG°_ads (Table 4) lie between 34 and 38 kJ mol⁻¹, implying physico-chemical adsorption of NMSc. The ΔG°_ads (Table 4) are negative, indicating that NMSc adsorption is spontaneous. Quraishi et al. 2000 reported that ΔH°_ads value is less than −41.86 kJ mol⁻¹ implying electrostatic interaction and that approaching −100 kJ mol⁻¹ means electron-pair interaction of the inhibitor. The ΔH°_ads value for NMSc is −92.82 kJ mol⁻¹, which is closer to −100 kJ mol⁻¹, suggesting its chemisorption. The negative sign of ΔS°_ads value indicates the decrease in disordering occurs during the inhibition process (Bentiss et al. 2001).

3.4 Inhibition mechanism for Al alloy-SiC composite corrosion

NMSc’s inhibition performance on Al alloy-SiC composite corrosion primarily depends on the extent of its adsorption. The strength of inhibitor adsorption depends on its structure, electrochemical potential and temperature. The chemical structure of the inhibitor will play a major, including the number and charge density of the active adsorption centres, the size of the molecules and the type of adsorption. As per one concept, the organic inhibitor molecules could adsorb at the metal/medium interface by replacing the previously adsorbed water molecules, as indicated below (Solmaz 2014)

In this equation, Inh _(aq) and Inh _(ads) referred to inhibitors in an aqueous medium and adsorbed on the metal, respectively, and ‘n’ denotes the number of replaced water molecules. As a result, the adsorption of NMSc molecules creates a protective film, inhibiting Al alloy-SiC composite corrosion. The thermodynamic results showed that NMSc adsorption occurs through the physico-chemical adsorption process.

The aluminium metal undergoes dissolution reactions in the hydrochloric acid medium as follows (Bereket and Pinarbasi 2004):

In the anodic region:

In the cathodic region:

The Al alloy-SiC composite surface is positively charged in an acidic medium because the pH_ZCh value (pH at zero charge potential) equals 9.1 for Al (Bereket and Pinarbasi 2004). As a result of the attraction of electrostatic forces, the Cl⁻ ions in the acid medium can adsorb at the composite/acid solution interface. The amino group in NMSc is typically protonated (NMSc-H⁺) in a strongly acidic medium, such as the one used in this study. Furthermore, NMSc-H⁺ can be attracted towards the negatively charged chloride ions that are primarily adsorbed on the composite electrode surface. As a result, the physical adsorption of protonated NMSc can occur on the composite surface. The chemisorption of NMSc may also happen by the donor-acceptor kind interactions of N and O atoms, or π bonds of the aromatic ring in NMSc with empty d-orbitals of Al, resulting in coordination bond formation (Mansfeld 1987). The protective layer formed by NMSc can protect the composite surface by isolating it from direct contact with the corrosive acid medium, thereby controlling the composite’s deterioration.

3.5 Surface analysis of Al alloy-SiC specimen

SEM images of the Al alloy-SiC specimen immersed in acid and inhibited acid solutions are presented in Figure 6a and b. Many micro pits/holes were observed on the corroded specimen surface immersed in 0.5 M HCl solution (Figure 6a). However, a uniform surface with very few micro pits/holes were observed on the specimen surface, immersed in 0.5 M HCl containing 2.56 mM NMSc (Figure 6b). It could be the result of NMSc’s adsorption on the composite surface.

Figure 6:

SEM pictures of Al alloy-SiC specimen immersed in (a) acid and (b) inhibited acid solution.

The AFM images of the Al alloy-SiC composite dipped in acid medium and inhibited acid medium with 2.56 mM NMSc are shown in Figure 7a and b. The R_a (average surface roughness) and R_q (root mean square roughness) values for the corroded composite specimens are 709 nm and 914 nm, respectively. Similarly, these values for inhibited composite samples are 248 nm and 176 nm, respectively. These results reveal that inhibited surface is more uniform/smoother than the fully corroded surface, suggesting the NMSc adsorption, which leads to a protective film formation. Thus, the composite material’s deterioration was kept under control.

Figure 7:

AFM images of Al alloy-SiC sample dipped in (a) acid and (b) inhibited acid solution.

2.6 UV-visible spectral analysis on NMSc adsorption

UV-visible spectrum is one of the valuable tools to support NMSc adsorption onto Al alloy-SiC composite. The UV-visible spectra of the inhibited acid solution containing 0.01 mM NMSc were recorded (Figure 8). The spectra showed two absorption peaks at 226 nm and 256 nm associated with π→π* and n→π* transitions. Then, the composite specimen was immersed in the above solution for 3 h, and spectra of the left solution were recorded. Figure 8 shows that the second spectra showed the same two peaks with a reduced intensity compared to the first spectra. The decrease in intensity of the second spectra reveals the adsorption of NMSc at the surface of the composite specimen.

Figure 8:

Absorption spectra of the inhibited acid solution containing 0.01 mM NMSc before and after the immersion of composite coupon.

3.7 Quantum chemical calculations

The theoretical studies using density functional theory (DFT) were carried out to find the influence of the inhibitor structure on its inhibition activity. Electron-donating or accepting properties of organic inhibitor molecules at the metal surface mainly control its inhibition activity. The amino group in NMSc can undergo protonation in an acid medium like the one (0.5 M HCl) used in this investigation. As a result, the participation of neutral and protonated NMSc in the inhibition of Al alloy-SiC composite corrosion in a 0.5 M HCl medium could be influenced. Therefore, the DFT studies were performed on neutral and protonated NMSc using the 631G** basis set. The parameters calculated were E_HOMO (energy of highest occupied molecular orbital), E_LUMO (energy of lowest unoccupied molecular orbital), ∆E_g (energy gap), η (hardness), σ (softness), ΔN (the fraction of electron transferred) and ω (electrophilicity index) (Chang-Guo et al. 2003).

The Mulliken charge indicates the active sites for adsorption in the inhibitor molecule and the possible interactions with the metal surface. The heteroatoms with high negative Mulliken charges on atoms in neutral and protonated NMSc (Figure 9a and b) are the preferable active sites for the donor–acceptor interactions. In the neutral NMSc, atoms such as N3 (−0.953), O4 (−0.65), N7 (−0.523), N2 (−0.478) and O11 (−0.466), while in protonated NMSc atoms like N3 (−0.628), N7 (−0.477), O11 (−0.471) and N2 (−0.378) showed higher negative charges (Figure 9a and b). These high negative charge atoms in neutral and protonated NMSc can act as active adsorption centres.

Figure 9:

Mulliken charges on (a) neutral and (b) protonated NMSc.

The optimised structures of neutral and protonated MBS and the corresponding HOMO and LUMO are depicted in Figure 10.

Figure 10:

Neutral NMSc’s (a) optimized structure, (b) HOMO, (c) LUMO, and protonated NMSc’s (d) optimized structure, (e) HOMO, (f) LUMO.

E _HOMO value indicates an inhibitor molecule’s probability of donating electrons, whereas E_LUMO values express the ability to accept electrons. The neutral NMSc showed a higher E_HOMO value (−6.2662 eV) than the protonated NMSc (−8.9071 eV). Hence, the neutral NMSc can readily donate electrons to the Al atom’s empty d-orbital, leading to a coordinate-type bond. The E_LUMO value of protonated NMSc is lower (−4.4787 eV) than its neutral form (−0.3379 eV). It suggests that the protonated NMSc can easily accept electrons from the Al atom leading to a back-donating type bond (Obot and Obi-Egbedi 2008).

Generally, the lower the energy bandgap (ΔE_g) between the HOMO and LUMO higher is chemical reactivity shown by the inhibitor and hence exhibits good inhibition activity (Arslan et al. 2009). In this work, the observed ΔE_g value for protonated NMSc (4.4284 eV) is lower than its neutral form (5.9283 eV). Therefore, the protonated form can show higher reactivity compared to neutral NMSc. Thus, protonated NMSc contributes more towards the inhibition performance. These results support the physisorption of NMSc on the composite, as explained earlier under the experimental results. The protonated NMSc showed a lower η value (2–2142 eV) and higher σ value (0.4516 eV) compared to its neutral form (η = 2.9641 eV and σ = 0.3374 eV). It reveals the stronger chemical reactivity of protonated NMSc, contributing more towards the inhibition performance (Masoud et al. 2010). It again confirms the possible physisorption of protonated NMSc on the composite surface.

The fraction of electrons transferred (ΔN) from the inhibitor to the composite electrode surface was computed by Equation (17) (Pearson 1990).

where η_Al is the hardness of Al and η_Inh is the hardness of the inhibitor, while χ_Al and χ_Inh are the electronegativity values of Al and inhibitor. The theoretical electronegativity value of Al (ɸ_Al) is taken as 4.28 eV (Kokalj and Kovacevic 2011), and the hardness of Al (η_Al) as zero (Pearson 1990). The positive value of ΔN indicates the higher electron-donating nature, whereas the negative value implies the stronger electron-accepting trend of the inhibitor involved (Lukovits et al. 2001). Therefore neutral NMSc (ΔN = 0.1650) readily donates electrons, whereas protonated NMSc (ΔN = −0.5449) readily accepts electrons from the metal surface.

The electrophilicity index (ω) value for the inhibitor is given by Equation (18) (Parr et al. 1999).

The electronic chemical potential (µ) is given by Equation (19).

An inhibitor with a low value of ω acts as a good nucleophile, whereas a high value of ω shows a good electrophile nature (Obot and Obi-Egbedi 2010). The protonated form of NMSc showed a higher ω value (10.1041 eV), indicating its higher tendency to accept electrons. Its neutral form exhibited a lower ω value (1.8392 eV), showing a higher electron-donating ability. It results in stronger adsorption of both neutral and protonated NMSc on the composite, displaying a good inhibition performance.

3.8 Comparison of reported Al alloy-SiC composite inhibitors with NMSc

The corrosion inhibition performance of NMSc is compared to that of other reported inhibitors against Al alloy-SiC composite corrosion (Table 5). Among the reported inhibitors of Al alloy-SiC composite corrosion in a hydrochloric acid medium, NMSc evinced the highest IE of 95 % at 2.56 mM and 303 K. Most of the inhibitors showed maximum IE at 303 K, whereas others at 323 K. Some inhibitors require organic solvent (Ethanol/Acetone) to prepare their solution (Kini et al. 2010, 2011; Shetty et al. 2020), and others are soluble in the aqueous hydrochloric acid media (Charitha and Rao 2017, 2018a, 2018b, 2020). However, some of these inhibitors are effective only at lower media concentrations (0.025/0.05 M).