A descriptive study for corrosion control of low-alloy steel by Aloe vera extract in acidic medium

2.1 Preparation of Aloe extract

Plant material (Aloe barbedensis) in natural condition was shade dried then kept in oven for 4 h at 50°C to remove the moisture content. Five grams of the powdered and dried material, characterized by Fourier transform infrared spectroscopy (FTIR), was placed in 500 ml round bottom flask and sufficient quantity of methanol (100 ml) was added; then the methanolic solution was refluxed for 5 h. Thereafter, AE was filtered in order to remove any suspended impurities. The solvent of this filtrate was evaporated to get the dried extract (Hart & James, 2014). Further, the stock solution of the dried extract (AE) was prepared by adding 50 ml of 0.5 m HCl and kept in a clean corked bottle for further use. The Aloe leaves have a number of chemical constituents, of which aloe-emodin (A1) (C₁₅H₁₀O₅) and aloetic acid (A2) (C₁₅H₆N₄O₁₃) are major components (Sharrif & Verma, 2011), and, being electron rich as well, are expected to offer the inhibition properties (Hart & James, 2014). The chemical structures of A1 and A2 are presented in Figure 1A and B, respectively, proving that the compounds are rich in electron density also evidenced by the FTIR study (Kumar & Mathur, 2013; Sharrif & Verma, 2011; Tuaweri et al., 2015).

Figure 1:

FTIR of Aloe vera leaf powder.

Structures of (A) aloe-emodin and (B) aloetic acid.

2.2 Corrosive solutions

Test solutions of 0.5 m HCl were prepared after dilution of AR grade 35% HCl (Qualichem) with double-distilled water. The concentration of the inhibitor’s stock solution was further diluted to obtain different concentrations of AE solution. The concentration range of AE used for the corrosion studies was 0.5%, 1.0%, 1.5%, 2.0%, and 2.5% v/v.

2.3 Mild steel coupons

Gravimetric analysis and electrochemical studies were performed on MS coupons of dimensions 3 cm×1.5 cm× 0.028 cm and 1 cm², respectively. For preparation, test coupons were polished successively, using grades of emery paper of 400, 800, 1000, and 1200.

2.4 Gravimetric analysis

The MS specimens used had a rectangular shape of 3 cm× 1.5 cm×0.028 cm were abraded with a series of emery papers (400, 800, 1000, and 1200 grades) and then washed with distilled water and finally with acetone. After weighing accurately (citizen scale CX230), the specimens were immersed in conical flask containing 200 ml of 0.5 m HCl in the absence and presence of different concentrations of the inhibitor. Each experiment was performed in duplicate in order to get reproducible results, and the temperature was thermostatically controlled at 303 K in thermostat for 4 h, using freshly prepared solution. Then, the specimens were taken out, washed carefully to remove the corrosion product, dried, and weighed again accurately.

2.5 Electrochemical polarization

The electrochemical analysis was carried out in a conventional three-electrode cell using Autolab Potentiostat Galvanostat (PGSTAT204), Netherlands. A carbon electrode was used as counter electrode along with a saturated Ag/AgCl electrode coupled to a fine Luggin capillary as reference electrode, and the exposed area of the working electrode was 1 cm² during polarisation and for corrosion attack. Tafel polarization curves were obtained by changing the electrode potential automatically from -500 to +500 mV at open circuit potential with a scan rate of 1 mV s^-1. Stern-Geary method (El-Etre et al., 2005) was used for determining corrosion current by extrapolation of anodic and cathodic Tafel lines to a point which gives log i_corr and the corresponding corrosion potential (E_corr) for acidic medium.

2.6 Electrochemical impedance spectroscopy technique

The electrochemical impedance spectroscopy EIS study was performed on the same cell setup as used for polarization technique. The parameters were observed on the analyzer in a frequency range of 1.0×10⁵ to 1.0 Hz with the AC signal 5 mV peak to peak at open circuit potential, stabilized at 303±1 K for 4 h. The main parameter deduced from the analysis of Nyquist diagram is the resistance of charge transfer R_ct (diameter of high frequency loop) which is then calculated to give the double layer capacitance C_dl. The f_max values are derived as a result of EIS technique.

2.7 Quantum chemical analysis

This technique is a theoretical means of determining the effectiveness of any inhibitor. AM1 (Austin Model 1), a semiempirical method based on the neglect of differential diatomic overlap approximation, was used for the quantum chemical studies. The quantum parameters were calculated using HyperChem professional 8.0 package (Hypercube Inc., USA). The technique was studied to correlate the various parameters and their results with the studied experimental data.

3.1 Gravimetric analysis

The values of weight loss, inhibition efficiency (% IE), and surface coverage (θ) was obtained from gravimetric (weight loss) measurements at different concentrations (Eddy & Ebenso, 2010) of AE in 0.5 m HCl solution at 303 K temperature, which are summarized in Table 1.

The coupons were weighed before and after immersion in the test solution, where Wo and Wi are the values of corrosion weight loss (mg/cm² h) of MS in uninhibited and inhibited solutions, respectively. The increase in inhibition efficiency was observed with the inhibitor concentration. Maximum inhibition efficiency was showed at 2.5% AE. The next higher concentration is tabulated in Table 1 showing optimization of the inhibition effect by the inhibitor, and it was found that after 2.5% v/v, the inhibitor showed almost constancy in the inhibition behavior.

3.2 Electrochemical polarization

From the anodic and cathodic polarization curves of the Tafel plot (Bentiss et al., 2009) presented in Figure 2, various electrochemical parameters like corrosion current density (i_corr), corrosion potential (E_corr), and cathodic and anodic Tafel slopes (β_c and β_a, respectively) are listed in Table 2 with their computed inhibition efficiency (% IE) using the equation below.

Figure 2:

Tafel plot of AE at different concentrations on MS in 0.5 m HCl.

The i^o_corr is the current density in the absence of AE and i_corr in the presence of AE, and it was observed that the E_corr values were shifted slightly to the anodic region in comparison to the E_corr value obtained without inhibitor. However, both the anodic and cathodic Tafel slope values decreased with the increase in the concentration of the inhibitor, indicating the mixed mode of inhibition (Ahamad & Quraishi, 2010). This reveals that the green inhibitor also reduces anodic dissolution along with the reduction of cathodic hydrogen evolution.

3.3 Electrochemical impedance spectroscopy

From the Nyquist plots for MS in acid media as in Figure 3A, the plots are not perfect semicircles, attributing to the non-homogeneity of the surface and roughness of the metal (Bouklah et al., 2006). Also, these plots show the increase in impedance response on MS with addition of AE in the medium by their increasing diameters, corresponding to charge transfer resistance (R_ct) values (Musa et al., 2012). The double-layer capacitance (C_dl) values were calculated using equation given, by f_max, maximum frequency.

Figure 3:

Nyquist plots.

(A) Nyquist plots of AE at different concentrations and (B) equivalent circuit diagram for the Nyquist plot of the system.

Table 3 shows that the R_ct values increased with the increase of the concentration of AE, which shows protection of MS surface by the inhibitor, while the values of C_dl decreased with the increase in the thickness of protective layer at higher concentrations. The inhibition efficiency (% IE) and surface coverage, calculated from the values of R_ct, were both found to be maximum at a concentration of 2.5% v/v of AE, using the equation (Musa et al., 2012) below.

The findings of electrochemical studies (polarization and EIS) were in fair agreement with those of gravimetric studies along with slight deviations, tabulated in Figure 4. This deviation is due to the fact that the gravimetric study is an average technique of calculating the % IE (Bouklah et al., 2006), while an instantaneous efficiency can be calculated by the electrochemical studies performed.

Figure 4:

Comparative percent inhibition efficiency of AE on MS in 0.5 m HCl using different techniques.

3.4 Quantum chemistry

Adsorption of these organic inhibitors is also attained from interactions between metal (lowest unoccupied molecular orbitals – LUMO) and non-participant electrons of heteroatoms or π electrons of the inhibiting compound (highest occupied molecular orbitals – HOMO) (Sastri & Perumareddi, 1997). The energy gap between the two frontier orbitals (LUMO and HOMO) facilitates the extent of interaction between the metal and the inhibitor and hence a good adsorption.

The quantum chemical parameters (Lukovits et al., 2001), namely, the frontier molecular orbitals, E_HOMO and E_LUMO (HOMO and LUMO, respectively), the energy difference (ΔE) between E_HOMO and E_LUMO, dipole moment (μ), electron affinity (A), ionization potential (I), the absolute electronegativity (χ), absolute hardness (η), softness (σ), and the fraction of electrons (ΔN) transferred from inhibitor components, were determined and correlated with the experimental data, given in Table 4. Values of χ_inh, σ_inh, and η_inh were calculated by using the values of I and A obtained from quantum chemical calculation, where I=-E_HOMO and A=-E_LUMO

and ΔN, the fraction of electrons transferred from inhibitor to the iron molecule, was calculated, using a theoretical χ_Fe value of 7 eV/mol and η_Fe value of 0 eV/mol for iron atom in the equation as below:

Figure 5:

(A) HOMO and (B) LUMO of A1 and A2 components of AE.

The quantum analysis (Figure 5) predicts adsorption centers of the inhibitor molecules responsible for the interaction with metal atoms (Fang & Li, 2002). The study provides the spatial distribution of electronic density of the molecules on formation of chelating centers. The structure of the adsorbing components is influenced by the electron density on the active centers of the inhibitor. The components A1 and A2 have greater electron density on O and N heteroatoms present on the planar conjugated ring system of the molecules. These atoms are expected to contribute to the interaction with the MS surface. Any compound playing a role of outstanding corrosion inhibitor not only offers electrons to the unoccupied 3D orbitals of Fe but also accepts freely available electrons present on the metal resulting in back-bonding (Fang & Li, 2002; Sastri & Perumareddi, 1997). This back-bonding may also take place in the present study advocating its high degree of efficiency.

3.5 Adsorption isotherm

The interactions of the inhibitor and the MS surface can be examined by the adsorption isotherm. The degree of surface coverage (θ) values for various concentrations (c) of the inhibitors in the solution have been estimated from the polarization measurements. A straight line with linear correlation coefficient (R²) is >0.9999 and suggests that the adsorption is following the isotherm studied. The AE is found to follow Langmuir isotherm (Aisha et al., 2013) based upon the equation below:

The plot (Figure 6A) is drawn between C/θ vs. C with R²=1.000, suggesting that the adsorption of AE on MS is monolayer.

Figure 6:

(A) Langmuir adsorption isotherm and (B) Dubinin-Radushkevich (D-R) isotherm of AE on MS.

Further from the isotherm, Gibbs free energy (ΔG) can be deduced using the equation

The value of ΔG is calculated as -18.05 kJ/mol, indicating that the adsorption is physical and confirming the experimental details. Supplementary to the Langmuir adsorption, the Dubinin-Radushkevich (D-R) isotherm (Amer et al., 2015) was also studied, using equation (11)

where ε can be correlated as ε=RTln(1+1C); where θ and β were obtained from the intercept and slope of the D-R isotherm between lnθ and ε² (Figure 6B); the E was then calculated using equation (12) and defined as the mean free energy change of the inhibitor molecule transferred to the MS surface from the medium.

The value of E ascertains the kind of adsorption as physical adsorption when its value lies below 8 kJ/mol and chemical adsorption when this value is found to be in the range of 8–16 kJ/mol (Amer et al., 2015). The value of E from the present study is 0.267 kJ/mol, proving the adsorption as physical in nature.