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Nicotiana tabacum leaf extract protects aluminium alloy AA3003 from acid attack
⁎Corresponding author. emekaoguzie@gmail.com (Emeka E. Oguzie) emeka.oguzie@futo.edu.ng (Emeka E. Oguzie)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
We investigated leaf extracts of Nicotianatabacum (NT) for corrosion inhibition efficacy on aluminium alloy AA3003 corrosion in 0.1 M HCl, using a combination of experimental and computational techniques. The results revealed that NT effectively protected the alloy surface in the studied environment, particularly at higher concentrations, though this effect became subdued with prolonged exposure. Electrochemical results showed that the extract organic matter adsorbed on the Al alloy/solution interface reduced both the anodic and cathodic current densities, with the latter effect being more pronounced. Fourier transform infrared spectroscopy (FTIR) and energy dispersive X-ray spectroscopy (EDX) confirmed the adsorption of NT constituents on the Al alloy surface, while scanning electron microscopy (SEM) and atomic force microscopy confirmed the protective effect of the adsorbed inhibitor layer. Computational simulation of the adsorption of various constituents of the extract on Al (1 1 0) surface achieved within the framework of the density functional theory confirmed strong interaction of some extract constituents with Al.
Keywords
Aluminium alloy
Corrosion
Inhibition
Electrochemical technique
Adsorption
Molecular dynamics
FTIR
AFM
1 Introduction
Aluminium alloys that normally exhibit some levels of oxide film-induced corrosion resistance in varied environments have been shown to be readily susceptible to corrosion in acidic and alkaline media and need to be protected in such environments (Branzoi et al., 2002; Kowal et al., 1996; Liu and Wheat, 2009; Nguyen and Foley, 1982; Okeoma et al., 2012). Corrosion protection in aggressive fluid environments is often achieved by incorporation of suitable corrosion inhibitors into the environment in order to impede the corrosion reaction and diminish the corrosion rate (Gao et al., 2015; Obi-Egbedi et al., 2012; Oguzie et al., 2006a, 2005). As a consequence of regulatory restraints on use of the effective but toxic inorganic chromate inhibitors, attempts to develop alternative environmentally benign corrosion inhibiting additives have focused on organic compounds, particularly those containing polar hetero atoms and conjugated multiple bond systems (Zulfareen et al., 2016; Jeeva et al., 2015; Liu and Cheng, 2011; Oguzie et al., 2008). The capacity of these organic compounds to protect metal surfaces from corrosion attack has been ascribed to their tendency to become adsorbed on the substrate to form a protective layer isolating the metal from the corrosive species (Mejaha et al., 2012; Oguzie et al., 2012a, 2006b; Raja and Sethuraman, 2008; Umoren et al., 2009).
The worldwide drive towards sustainability has inspired studies aimed at developing a new class of corrosion inhibiting additives derived from biomass extracts (Abdel-Guber et al., 2006; Akalezi et al., 2013; Oguzie et al., 2012a; Okafor et al., 2011; Umoren et al., 2013). The effectiveness of such biomass extracts is often attributed to the presence of several phytochemical compounds. Some of these compounds, including alkaloids, tannins, flavonoids, saponins, have been found to possess identical electronic structures as conventional inhibitors (Oguzie et al., 2012b; Umoren et al., 2013).
There is no disputing the environmental and economic benefits of developing effective and standard corrosion inhibiting additives from biomass extracts, which possibly explains why the subject is attracting a lot of attention and will continue to do so for some time, or till significant breakthroughs are accomplished. There is as well no doubt that the complexity of the composition of biomass extracts has been a sort of hindrance to proper understanding of their corrosion inhibition performance. This challenge can be systematically overcome by developing a mechanistic/theoretical framework to promote more in-depth understanding of the mechanisms associated with the diverse findings reported in the literature.
In continuation of our ongoing effort in investigating biomass extracts for corrosion inhibiting efficacy, the present study assesses ethanol extracts of Nicotiana tabacum (NT) leaves as corrosion inhibitor for aluminium alloy AA3003 in hydrochloric acid using joint experimental and computational approach. N. tabacum (tobacco) is a perennial plant that is widely distributed in many parts of Africa including Nigeria and has been reported to contain relatively high concentrations of alkaloids, fatty acids and nitrogen-, fluorine-, sulphur- and oxygen-containing compounds as well as polynuclear aromatic hydrocarbons (Von Fraunhofer, 1995). NT has been earlier determined to be an effective corrosion inhibitor for mild steel in hydrochloric acid (Njoku et al., 2013).
2 Experimental
2.1 Material preparation
Experiments were performed on aluminium alloy (AA3003) with elemental composition by weight percentage, Si – 0.363, Fe – 0.55, Mn – 1.22, Cu – 0.077, V – 0.009, Ti – 0.026, Pb – 0.064, Al – 97.67 and others 0.019. The metal sheet of thickness 0.1 cm was mechanically pressed-cut into coupons of dimension 1.5 cm × 1.5 cm × 0.8 cm (for gravimetric measurements) and 1.5 cm × 1.5 cm (for electrochemical measurements). The coupons were used as procured without further polishing, but were degreased in absolute ethanol, dried in acetone, weighed and stored in moisture free desiccators prior to use.
The chemicals and reagents used for the study were of analytical grade and were used as source without further purification. The corrodent employed was 0.1 M HCl while distilled water was used for all solution preparations.
N. tabacum (NT) leaves obtained locally were washed, dried to constant weight and ground into fine powder. Stock solutions of the plant extract were prepared by soaking weighed amounts of the dried and ground leaves of N. tabacum in ethanol as the extractant for 48 h. The resulting solution was triple filtered and the amount of plant material extracted into the ethanol solution quantified by comparing the weight of the dried residue with the initial weight of the dried plant material before extraction. Inhibitor test solutions were prepared in the desired concentration range by appropriately diluting the stock extract with the corrodent solution.
2.2 Gravimetric experiments
Gravimetric tests were conducted under total immersion conditions in 500 ml of aerated and unstirred test solutions maintained at 303 K, following the procedure described previously (Oguzie et al., 2013, 2014; Chidiebere et al., 2012). The cleaned coupons were weighed and then suspended, with complete immersion, in all-glass cells containing 1000 ml of the test solutions by means of glass hooks and rods. Tests were conducted in aerated and quiescent test solutions. Coupons were retrieved after 2 h, 6 h, 12 h, 24 h and 48 h respectively, cleaned appropriately and reweighed to obtain the weight loss. All tests were run in triplicate and the mean values used in subsequent calculations.
2.3 Electrochemical measurements
Coupons of the composition mentioned earlier were machined into specimens of dimensions 1.5 × 1.5 cm2 and employed for the electrochemical experiments. These were appropriately sealed with epoxy resin in such a way that only a square surface of the area 1.0 cm2 was left uncovered. The exposed surface area was degreased in ethanol and acetone respectively, rinsed with distilled water and dried in warm air. Electrochemical experiments were conducted as described elsewhere (Oguzie et al., 2013, 2014; Chidiebere et al., 2012) Electrochemical impedance spectroscopy measurements were carried out using VERSASTAT 400 complete DC Voltammetry and Corrosion System with Versa Studio software, whereas potentiodynamic polarization measurements employed PAR Model 263 Potentiostat/Galvanostat with Powersuite software.
Electrochemical experiments were undertaken in a three-electrode glass cell, with a graphite rod as counter electrode and a saturated calomel electrode (SCE) connected via a Luggin’s capillary as reference electrode. Experiments were carried out at the end of 1 h of immersion at 303 K. Impedance measurements were made at corrosion potentials (Ecorr) over a frequency range of 100 kHz–10 mHz, with a signal amplitude perturbation of 5 mV. Potentiodynamic polarization studies were carried out in the potential range ±250 mV versus corrosion potential at 0.333 mV/s scan rate. Each test was run in triplicate to verify the reproducibility of the data.
2.4 Surface morphological observation and analysis
Fourier transform infra red (FTIR) spectra (KBr) were recorded using a Nicolet Magna-IR 560 FTIR spectrophotometer. The frequency ranged from 4000 to 400 cm−1. The Test samples include (i) NT extract, (ii) Al alloy corrosion product film in uninhibited 0.1 M HCl and (iii) Al alloy corrosion product film in 0.1 M HCl containing NT extract, representing the corrosion inhibitor film. The corrosion products were grown over a 48 h period in 0.1 M HCl without and with 1200 mg/L NT. The films were scraped off and ground thoroughly with KBr and subjected to IR analysis as reported elsewhere (Chidiebere et al., 2015a, 2015b).
The morphological changes and surface roughness of the Al alloy surface after immersion in 0.1 M HCl, in the absence and presence of 1200 mg/L NT for different exposure periods were visualized via scanning electron microscopy (SEM, XL-30FEG) and atomic force microscopy (AFM Picoplus 2500). The corroded coupons were retrieved from the test solutions, cleaned carefully and dried before surface imaging. The SEM images were obtained at a voltage of 25 kV, working distance of 10 mm and magnification of 5000× for the various surface morphological observations. For the AFM analysis, the scanning probe was operated in contact mode and the image scanning area was 2000 × 2000 nm.
2.5 Computational studies
Materials Studio 4.0 software (BIOVIA Inc.) was employed for the computational studies. The DFT electronic structure program DMol3 was used for electronic structure calculations, as described in Mejaha et al. (2012), using a Mulliken population analysis. Simulation parameters include a restricted spin polarization, with the DND basis set and Perdew–Wang (PW) local-correlation-density functional. Before electronic structure modelling, the molecular structures of the selected extract constituents were initially subjected to geometry optimization in a COMPASS force field, using the Smart minimization method by high-convergence criteria.
Molecular dynamics simulation of the adsorption of the optimized structures of selected extract constituents on an Al (1 1 0) surface was undertaken using Forcite quench molecular dynamics as described in Mejaha et al. (2012). Calculations were carried out in a 12 × 9 × 8 super cell in the COMPASS force field, using the Smart algorithm, with NVE ensemble. Simulation temperature was 350 K, and simulation time was 5 ps, with time step of 1 fs.
3 Results and discussion
3.1 Corrosion studies
3.1.1 Gravimetric data
To ascertain the ability of NT extract to mitigate the free corrosion of aluminium alloy in the acidic medium, gravimetric experiments were conducted over a total immersion period of 48 h in 0.1 M HCl solution without and with NT extract. Fig. 1 shows the weight loss with respect to time. The plots reveal two major relationships: an increase in corrosion rates with exposure time for all systems (especially in the uninhibited acid) and a pronounced mitigation of corrosion rates due to addition of NT extract, signifying that the extract inhibited the corrosion of the aluminium alloy immersed in 0.1 M HCl. The effectiveness of inhibition was aptly quantified by comparing weight losses in uninhibited (W0) and inhibited (WI) solutions to obtain the inhibition efficiency (IE%), given by the following:
Weight loss of aluminium alloy in 0.1 M HCl with and without different concentrations of NT for various immersion periods.
Fig. 2 compares inhibition efficiency values for various concentrations of NT as a function of exposure time. From the plots, efficiency increased steadily with NT extract concentration, but however showed no steady relationship with exposure time, increasing up to 6 and 12 h and decreasing thereafter. Maximum inhibition efficiency of 87% was obtained with the highest concentration of NT.
Variation of the inhibition efficiency with NT extract concentration in 0.1 M HCl for different exposure times.
3.1.2 Polarization data
Potentiodynamic polarization experiments were carried out to establish to what extent NT extract modified the anodic and cathodic reactions. Polarization curves for aluminium alloy in 0.1 M HCl solution without and with NT (200 mg/L and 1200 mg/L) are presented in Fig. 3, with corresponding electrochemical parameters in Table 1. The plots for all systems show active dissolution with no evidence of passivation within the studied potential range, which suggest that NT did not alter the corrosion mechanism. Nevertheless, introduction of NT extract into the acid medium caused Ecorr to shift towards more negative values as well as decreased the magnitude of the anodic and cathodic current densities, with a more pronounced cathodic effect. NT extract can thus be considered a mixed-type inhibitor, with predominant cathodic effect (Akalezi et al., 2013; Okafor et al., 2011). This inhibiting effect was more pronounced at high NT concentration. The values of the corrosion current density (icorr) in the absence and presence of NT extract were used to calculate the inhibition efficiency from polarization data as follows:
Potentiodynamic polarization curves for aluminium alloy in 0.1 M HCl in the absence and presence of different concentrations of NT extract.
System
Ecorr (mV vs SCE)
icorr (μA/cm2)
IE%
0.1 M HCl
−760.098
5.2520
–
200 mg/L
−773.073
1.2170
77
1200 mg/L
−831.264
1.0066
80
3.1.3 Impedance data
Impedance experiments enabled deduction of any modifications to electrochemical processes on the Al/acid interface due to addition of NT extract. In the present study, we investigated the interfacial phenomena at different immersion times (1 and 24 h). Fig. 4(a–c) presents the impedance spectra for aluminium alloy in 0.1 M HCl without and with NT extract after 1 h of immersion. The Nyquist plots consist of a large capacitive loop at high frequencies followed by an inductive loop at low frequencies. The diameter of the capacitive loop corresponds to the charge transfer resistance (Rct) at the metal/solution interface and determines the ability of the corrosion product and/or any surface active additive to hinder the corrosion process. The low frequency inductive loop is often associated with bulk relaxation of adsorbed intermediates, which is more pronounced when the intermediates are strongly adsorbed (Studuan and Xiangbong, 2012; De Wit and Len drink, 1996; Oguzie et al., 2010). Similar results have been reported for aluminium corrosion in hydrochloric acid media (Studuan and Xiangbong, 2012; De Wit and Len drink, 1996). A close look at the low frequency response in uninhibited acid reveals a tendency towards formation of another capacitive loop at low frequencies, which is not present in the impedance response in the presence of NT extract. The influence of NT extract on the impedance response is also clearly obvious from the increased diameter of the Nyquist plot (Fig. 4a), higher impedance modulus (Fig. 4b) and greater phase angle (Fig. 4c).
Impedance spectra for aluminium alloy in 0.1 M HCl solution without and with NT after 1 h of immersion: (a) Nyquist, (b) Bode modulus, and (c) Bode phase angle plot.
Fig. 5 illustrates the impedance responses in uninhibited and inhibited systems after 24 h of immersion. The plots show comparable features and trends as observed after 1 h of immersion. The Nyquist and Bode plots all indicate that the extract retained its inhibiting effect even after 24 h immersion. Interestingly, it can be seen that the magnitude of the impedance response for each system increased remarkably, going from 1 h to 24 h of immersion. This trend, which is more pronounced for the uninhibited system implies that the corrosion product on the Al surface became more compact and protective with prolonged immersion, providing some sort of self-inhibition of corrosion.
Impedance spectra of aluminium alloy in 0.1 M HCl solution without and with NT after 24 h of immersion: (a) Nyquist, (b) Bode modulus, and (c) Bode phase angle plot.
According to Nguyen and Foley (1982), the dissolution rate of aluminium in aqueous corrosive media is controlled by the complexation reaction between the hydrated aluminium ion and the anion present in the solution, corresponding to ([AlOH]2+ + Cl− → [AlOHCl]+) in chloride media. Consistent with this mechanism, the complex ion is further hydrolysed to yield stable hydrous oxides, which possibly contributes to the enhancement of corrosion resistance with immersion time as observed in this study.
The impedance response of each system was appropriately modelled using the equivalent circuits depicted in Fig. 6a (for uninhibited acid) and b (in the presence of NT extract), as contained in the Zsimpwin software. Each model contains a solution resistance (Rs), a series combination of an inductance (L) and inductive resistance (RL), which is in parallel with charge transfer resistance (Rct) and a constant phase element (CPE1). In order to accommodate the observed low frequency capacitive response in uninhibited acid, the model also includes another constant phase element (CPE2) which is placed in parallel with another charge transfer resistance component (Rct2).
Equivalent circuit models for describing aluminium alloy corrosion in 0.1 M HCl for without (a) and with NT extract (b).
The constant phase element (CPE) used in place of the capacitive elements in each time constant was introduced to accommodate deviations from ideal capacitive response associated with the depression of capacitive loops. The impedance of the CPE (ZCPE) corresponds to Njoku et al. (2013).
The impedance parameters derived from the various models for the different systems are given in Table 2 and reveal that introduction of NT into the system enhanced the charge transfer resistance values and decreased the values of double layer capacitance. These observations all imply that NT extract modified processes at the Al alloy/acid interface in such a way as to improve Rct, hence corrosion resistance. The decrease in Cdl values in the presence of NT has been previously reported for biomass corrosion inhibitors (Oguzie et al., 2014, 2012a) and attributed mainly to growth in the thickness of the double layer due to adsorption of the extract organic matter onto the metal/electrolyte interface. The adsorbed organics also contribute to the decrease in Cdl by further lowering the dielectric constant at the interface. The double layer capacitance values (Cdl) were calculated as follows:
System
Time (h)
Rs (Ω cm2)
n
Rct (Ω cm2)
Cdl (μF/cm2)
IE%
Blank
1
17.46
0.879
395.2
6.99
–
1200 mg/L NT
1
22.38
0.9097
2592
0.96
84.80
Blank
24
49.53
0.931
1403
1.82
–
1200 mg/L NT
24
30.03
0.8449
3658
0.66
61.65
The self-inhibition demonstrated by the aluminium alloy specimen after 24 h of immersion is justified by the comparatively lower Cdl value, which corresponds to the formation of a more compact and protective corrosion product layer. Interestingly, Cdl decreased by a factor of 3.84 going from 1 h to 24 h, while Rct increased by a factor of 3.55, thus signifying the close correlation between both parameters. The formation of a more compact and stable corrosion product layer is bound to restrict further adsorption of corrosion inhibiting species on the interface. Accordingly, Table 2 lists that Rct in the presence of NT extract increased by a factor of 1.44, while Cdl decreased by a factor of 1.45. Inhibition efficiency, quantified by comparing the Rct values in the absence and presence of NT extract using Eq. (5), revealed that efficiency dropped from 84.8% (after 1 h of immersion) to 61.7% (after 24 h). This however does not mean that the aluminium surface became less protected after 24 h. Rather, the formation of a more stable, self-inhibiting corrosion product layer diminished the need for the inhibitor, or more appropriately, the affinity of the aluminium surface for the corrosion inhibiting species contained in the extract.
3.2 Surface morphological observation and analysis
3.2.1 Infrared spectroscopy
Fig. 7 shows the FTIR spectra for the NT extract powder and the adsorbed films formed on Al alloy immersed in 0.1 M HCl without [Al film (Blank) and with 1200 mg/L NT extract [Al film (Inh)]. The spectrum for NT powder reveals the presence of numerous organic functional groups, associated with the organic constituents of the extract. The spectrum for Al alloy surface immersed in the acid solution produced a band around 729 and 713 cm−1, which has been attributed to Al—O bond vibration (Kalnynya et al., 1976) and a band around 3400 cm−1, attributed to hydroxyl group (Muthukrishnan et al., 2014).
FTIR spectra for NT powder and surface films on aluminium alloy specimens immersed in 0.1 M HCl solution without and with 1200 mg/l NT.
It is obvious that most of the peaks identified in NT powder spectrum were also present with the adsorbed film obtained in the presence of NT extract, with some peaks shifting towards higher or lower wave numbers, while some other peaks disappeared. For instance, the peak around 1461 cm−1 shifted to 1465.67 cm−1 while the peaks at 1433 and 1311 cm−1 shifted towards shorter wave numbers 1382.69 and 1062.7 cm−1 respectively. Some peak positions remained almost the same; such as the peak for hydroxyl around 3400 cm−1 and the peaks for the stretching modes of aliphatic and aromatic CH groups around 2925 and 2854 cm−1 respectively (Muthukrishnan et al., 2014; Prabhu et al., 2013). The peaks at 2925 cm−1 and 2854 cm−1 correspond to the hydrophobic or non-reactive ends of NT constituents. The peak at 1631 cm−1 which is attributed to carbonyl C⚌O stretching mode (Himmelsbach et al., 2002; Naik et al., 2010) and the peak at 1311 cm−1 shifted to 1643 and 1062 cm−1 respectively. These results collectively confirm adsorption of NT extract on the Al alloy surface.
3.2.2 Scanning electron microscopy results
The surface morphologies of Al alloy immersed in 0.1 M HCl solution without and with 1200 mg/L of NT for different exposure times are presented in Fig. 8(a, b, d and e), while the associated EDX spectra are shown in Fig. 8(e–f). The images for the Al alloy surface in the uninhibited solution reveal highly a defective corrosion product, which thickened with prolonged immersion. This should be responsible for the observed increase in the value of charge transfer resistance recorded after 24 h immersion by the EIS technique. On the other hand, the presence of NT resulted in a comparatively smoother and intact surface layer with fewer defects, even after 24 h immersion time (Fig. 8d and e). The compositions of the films formed on Al alloy immersed in the uninhibited and inhibited solutions were estimated by EDX analysis. The results are presented in Fig. 8c and e respectively. The EDX result for the Al alloy immersed in the uninhibited system showed peaks for oxygen, Al and other constituents of the alloy. This indicates that the film layer formed on Al alloy surface in the uninhibited solution is composed mainly of oxides and maybe hydroxides of Al. Interestingly, the spectrum for the inhibited solution showed identical peaks as the uninhibited system, with an additional peak for carbon, which confirms that organic constituents of NT extract actually interacted/adsorbed on the Al alloy surface.
SEM images for aluminium alloy immersed in 0.1 M HCl at 25 °C without (a and b) and with 1200 mg/L NT (d and e) after 1 h and 24 h respectively. Associated EDX spectra are in the absence and presence of NT after 24 h shown in c and f respectively.
3.2.3 Atomic force microscopy (AFM) observation
Further characterization of the corrosion morphology of Al alloy in 0.1 M HCl without and with 1200 mg/L NT extract was undertaken by AFM analysis on the samples after 24 h immersion. The samples were pre-cleaned with ethanol and dried in warm air to remove the adsorbed films, thereby exposing the bare metal surface for morphological evaluation of the surface roughness. Fig. 9(a and b) shows the three dimensional (3D) AFM images for the samples immersed in uninhibited (a) and inhibited (b) acid. The presence of NT extract in the 0.1 M HCl solution reduced the average surface roughness of Al alloy from about 619 Å as a result of corrosion attack, to 113.4 Å due to the protective effect of organic constituents of NT adsorbed on the Al alloy surface.
AFM micrographs for aluminium alloy immersed in 0.1 M HCl at 25 °C without NT (A) and with 1200 mg/L NT (B) after 24 h.
4 Adsorption considerations
The adsorption of organic constituents of NT extract was approximated to the Langmuir adsorption isotherm as follows:
Langmuir adsorption isotherm plot for NT extract on aluminium alloy corrosion in 0.1 M HCl.
The complex composition of biomass extracts poses a major hindrance to precise identification of the actual species responsible for their corrosion inhibiting efficacy. Nonetheless, electronic and molecular structure analysis based on computer simulations using the density functional theory (DFT) can be adapted to theoretically ascertain the likely contribution of selected organic constituents of the extracts. For NT extract, some of such constituents possessing characteristic features of conventional organic corrosion inhibitors (extensive electron delocalization, heterocyclic rings and the presence of heteroatoms) have been identified in an earlier report (Njoku et al., 2013) to include nicotine (PYDM), ethanol, 2-(octadecyloxy) (ETHO), 2-chloropropionic acid octadecyl ester (CAOE), tetracosyl heptafluorobutyrate (THBT), octatriacontyl pentafluoropropionate (OPTF) and tricontyl trifluoroacetate (TTFP). The optimized molecular and electronic structures of the molecules highlighted in Figs. 11–13, show desirable features of electron rich centres as well as extensive hydrophobic carbon chains. The corresponding electronic structure parameters are listed in Table 3.
Electronic properties of Nicotine (PYDM): (a-i) optimize structure; (a-ii) HOMO orbital; (a-iii) LUMO orbital; (a-iv) Fukui (f−) function; (a-v) Fukui (f+) function (a-vi) total electron density.
Electronic properties of (b) ETHO, (c) CAOE, (d) THFB: (i) optimize structure; (ii) HOMO orbital; (iii) LUMO orbital; (iv) Fukui (f−) function; (v) Fukui (f+) function and (vi) total electron density.
Electronic properties of (e) OPFP, and (f) TTFA: (i) optimize structure; (ii) HOMO orbital; (iii) LUMO orbital; (iv) Fukui (f−) function; (v) Fukui (f+) function and (vi) total electron density.
Property
PYDM
ETHO
TTFA
CAOE
THFB
OPFP
EHOMO (eV)
−5.015
−6.105
−6.359
−6.812
−6.812
−6.563
ELUMO (eV)
−4.063
0.765
−2.423
−1.633
−2.182
−2.260
ELUMO-HOMO
1.012
6.87
3.936
4.631
4.630
4.303
f−max
0.187
0.170
0.226
0.276
0.043
0.025
f+max
0.218
0.475
0.015
0.258
0.220
0.217
Mol. surf. area (Å2)
213.001
504.480
670.839
523.26
752.14
868.19
Interaction energy (kcal/mol)
−36.65
−83.55
−88.03
92.86
−97.87
−155.4
Going further, molecular dynamic (MD) simulations were employed to understudy the interactions between the different constituent molecules and the Al surface and to quantify the interaction energies. This was achieved using the Forcite quench molecular dynamics, following the protocols described elsewhere (Mejaha et al., 2012; Chidiebere et al., 2012; Oguzie et al., 2014). Simulations were performed using optimized structures of PYDM, ETHO, TTFA, CAOE, THFB, OCPF and the Al surface. The resulting low energy adsorption structures for each extract constituents on the Al (1 1 0) surface are illustrated in Fig. 14.
Adsorption structures of the different NT extract constituents on Al (1 1 0) surface: (ETHO), ethanol-2-(octadecylloxy)-; (CAOE), 2-chloropropionic acid, octadecyl ester; (THFB), tetracosyl heptafluorobutyrate; (OPFP), octatriacontyl pentafluoropropionate; and (TTFA), tricontyl trifluoroacetate.
The Interaction energy (Einteraction) between each molecules and the Al surface was calculated using the following equation:
Emol, EAl and Etotal correspond, respectively, to the total energies of the molecule, Al (1 1 0) slab and the adsorbed Mol/Al (1 1 0) couple. Mol/Al (1 1 0) couple represents the Al (1 1 0) surface with adsorbed molecule. The corresponding Einteraction values are also reflected in Fig. 14. The large negative values of the computed interaction energies suggest that the molecules strongly adsorb on the Al (1 1 0) surface, leading to the observed corrosion inhibiting action of NT extract. The trend of Einteraction correlates well with molecular size as envisaged for non-covalent adsorption, with the larger molecules with longer hydrophobic carbon skeletons exhibiting stronger interactions with the Al surface. Close inspection of Fig. 14 reveals that the adsorption orientation of some of the individual molecules coincided with the carbon skeleton within the inter-atomic spaces in the Al lattice, which is probably responsible for the high interaction energy values. Such interstitial adsorption orientations have previously been reported for the strong adsorption of some amino acids (Oguzie et al., 2011) as well constituents of Capsicum frutescens extract (Oguzie et al., 2013) on Fe (1 1 0) lattice.
5 Conclusions
This study has revealed that leaf extracts of N. tabacum (NT) effectively inhibited aluminium alloy corrosion in 0.1 M HCl. Inhibition efficiency increased with concentration but decreased with increasing exposure time. Polarization measurements show that the NT extract functioned via mixed-type mechanism, with a predominant cathodic effect. Impedance results show that the corrosion reaction was inhibited by adsorption of the extract organic matter on the corroding aluminium alloy surface and the adsorption behaviour was approximated by the Langmuir adsorption isotherm. The corrosion product on the aluminium alloy surface in uninhibited solution became more protective at longer immersion times, which improved corrosion resistance, but hindered adsorption of the extract. FTIR and EDX results reveal the formation of inhibitor adsorbed layer on Al alloy surface, while the protective effect of the adsorbed layer was confirmed by SEM and AFM. Molecular dynamics modelling of the adsorption of selected extract constituents on Al (1 1 0) surface revealed strong interactions, attributed to the alignment of the hydrophobic carbon skeletons of the molecules along the interatomic spaces on the metal lattice.
Acknowledgments
This project is supported by the Nigeria Tertiary Education Trust Fund (TETFund); under 2011/2012 TETFund Research Allocation for the Federal University of Technology Owerri. The authors are grateful to Prof. Y. Li and Prof. F.H. Wang (IMR-CAS, Shenyang, China) for providing facilities for SEM, AFM, XRD and FTIR measurements.
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