Natural Products for Materials Protection: Mechanism of Corrosion

Article pubs.acs.org/JPCC

Natural Products for Materials Protection: Mechanism of Corrosion Inhibition of Mild Steel by Acid Extracts of Piper guineense Emeka E. Oguzie,*,† Chinonso B. Adindu,† Conrad K. Enenebeaku,† Cynthia E. Ogukwe,† Maduabuchi A. Chidiebere,† and Kanayo L. Oguzie‡ †

Electrochemistry and Material Science Research Laboratory, Department of Chemistry, Federal University of Technology, PM B 1526, Owerri, Nigeria ‡ Department of Environmental Technology, Federal University of Technology, PM B 1526, Owerri, Nigeria ABSTRACT: The adsorption and corrosion inhibiting effect of acid extracts of Piper guineense (PG) leaves on mild steel corrosion in 1 M HCl and 0.5 M H2SO4 was investigated using gravimetric, potentiodynamic polarization and electrochemical impedance spectroscopy techniques as well as scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The polarization and impedance results revealed that PG inhibited the cathodic and anodic partial reactions of the corrosion process via adsorption of the extract organic matter on the metal/solution interface. The mechanism of adsorption deduced from the variation of inhibition efficiency with temperature as well as kinetic and activation parameters suggest significant chemisorption of the extract constituents on the metal surface. Density functional theory calculations were performed to model the electronic structures of some extract constituents, including chemisorptive interactions with the Fe surface.

1. INTRODUCTION Excessive corrosion attack is known to occur on iron and steel surfaces deployed in service in aqueous aggressive environments. The corrosion process can be significantly suppressed by incorporation of certain substances into the environment in order to inhibit the corrosion reaction and reduce the corrosion rate. Organic compounds, particularly those containing polar functions with nitrogen, oxygen, and/or sulfur in a conjugated system have been widely used for this purpose.1−10 The inhibitors act at the interphase created by corrosion products between the metal and aqueous aggressive solution, and their interaction with the corroding metal surface, usually via adsorption, often leads to a modification in either the mechanism of the electrochemical process at the double layer or in the surface available to the process. These inhibitors are usually quite specific in action, and their efficiency depends on the mechanical, structural, and chemical characteristics of the adsorption layers formed under particular conditions. The corrosion of iron in any given environment may proceed via different mechanisms and manifest in various forms. Accordingly, a corrosion inhibitor may be required to perform more than one function in order to be considered effective. Given that acid inhibitors are known to be quite specific in action, combinations of inhibitors are often necessary in order to provide the multiple services required for effective corrosion inhibition. Again, the high costs of most common corrosion inhibitors, coupled with recent concerns about the environment provide sufficient motivation for the development of new classes of inexpensive yet effective additives, specifically designed to address future environmental and safety needs. © 2012 American Chemical Society

In recent times, considerable effort has been devoted to study the corrosion inhibiting efficacy of some natural products, particularly biomass extracts. The reason for this is not farfetchedthe abundant phytochemical constituents of biomass extracts have considerable potential as inexpensive, non toxic, readily available and renewable sources of a wide range of organic chemicals of prospective industrial significance. Unfortunately, this nature’s chemical (or phytochemical) bounty has remained largely unexploited and underutilized and the scope of applications is still rather narrow, limited more or less to medicinal and nutritional uses. Even so, a number of the phytochemical constituents of plant biomass (including alkaloids, tannins, amino acids carbohydrates, etc.) have molecular and electronic structures bearing close similarities with those of conventional corrosion inhibitors and have been found to possess the ability to inhibit metal corrosion. Lecante et al.11 reported that alkaloids isolated from Guatteria ouregou and Simira tinctoria effectively retarded the acid corrosion of carbon steel. Vrsalovic and co-workers found that phydroxybenzoic acid and protocatechuic acid found in the acidic fraction of the aqueous extract of rosemary leaves functioned as efficient corrosion inhibitors for CuNi10Fe alloy in 0.5 M NaCl solution.12,13 Other isolates from biomass extracts such as phenolic acids, chestnut and mangrove tannins, sinapinic acid, lawsone, gallic acid, D-glucose, and tannic acid have also been shown to possess corrosion inhibiting Received: January 24, 2012 Revised: May 31, 2012 Published: June 1, 2012 13603

dx.doi.org/10.1021/jp300791s | J. Phys. Chem. C 2012, 116, 13603−13615

The Journal of Physical Chemistry C

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efficacy.14−19 From the foregoing, it is reasonable to expect that crude biomass extracts, which comprise of mixtures of organic compounds, should overcome the restrictions inherent in the specificity of action of single compounds and rather provide the multiple services required for effective corrosion inhibition. Understandably, more studies seem to focus on the corrosion inhibition performance of crude biomass extracts, with very positive results; even though mechanistic insights are lacking.20−30 In the present study, the adsorption and corrosion inhibiting effect of acid extracts of Piper guineense (PG) on mild steel in hydrochloric and sulphuric acid solutions has been extensively investigated using combined experimental and theoretical techniques. Gravimetric and electrochemical techniques of corrosion monitoring were employed, while morphological changes on the corroding steel surface were visualized by scanning electron microscopy (SEM). We herein point out that the complex compositions of crude biomass extracts pose a considerable restriction to precise experimental determination of the contributions of the different constituents to the overall inhibiting effect, and understandably most investigators shy away from the subject. We propose to overcome some of these restrictions, at least mechanistically, by performing theoretical computations in the framework of the density functional theory (DFT). Current developments in DFT-based quantum chemical computations have made this powerful tool ever more accessible to corrosion scientists for theoretical investigation of corrosion and corrosion inhibition systems (see ref 31 and the references therein). Our approach involves analysis of the molecular electronic structures of key phytochemical constituents of the crude extract, probing the mechanism of their interactions with the metal surface, as well as computing the respective adsorption energies.

bristle brush, washed, dried, and weighed. The weight loss was taken to be the difference between the weight of the coupons at a given time and its initial weight. All tests were run in triplicate, and the data showed good reproducibility. Average values for each experiment were obtained and used in subsequent calculations. Gravimetric experiments were also undertaken after 3 h of immersion, to determine the character of the inhibiting action at short exposure times as well as to assess the effect of temperature change (30−60 °C) on corrosion and corrosion inhibition processes. 2.3. Electrochemical Measurements. Metal samples for electrochemical experiments were machined into test electrodes of dimension 1 × 1 cm2 and fixed in polytetrafluoroethylene (PTFE) rods by epoxy resin in such a way that only one surface, of area 1 cm2, was left uncovered. The exposed surface was cleaned using the procedure described above. Electrochemical experiments were conducted in a conventional threeelectrode glass cell of capacity 400 mL using a VERSASTAT 400 Complete DC Voltammetry and Corrosion System, with V3 Studio software. A graphite rod was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. The latter was connected via a Luggin’s capillary. Measurements were performed in aerated and unstirred solutions at the end of 1 h of immersion at 30 ± 1 °C. Impedance measurements were made at corrosion potentials (Ecorr) over a frequency range of 100 kHz to 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 a scan rate of 0.33 mV/s. Each test was run in triplicate to verify the reproducibility of the data. 2.4. Infrared Spectroscopy. Fourier transform infrared (FTIR) spectra (KBr pellet) were recorded using a NicoletMagna-IR 560 FTIR spectrophotometer. The spectra for PG powder as well as the protective film formed on the mild steel surface after 3 h immersion in 1 M HCl and 0.5 M H2SO4 solutions containing 800 mg/L PG extract were recorded by carefully removing the film, mixing it with KBr, and making the pellet. 2.5. SEM. Morphological studies of the mild steel electrode surface were undertaken by SEM examinations of electrode surfaces exposed to different test solutions using a Shimadzu SSX-550 scanning electron microscope. Mild steel specimens of dimensions 15 × 10 × 2 mm were cleaned as previously described and immersed for 3 h in the blank solutions (1 M HCl and 0.5 M H2SO4) without and with 800 mg/L PG at 30 ± 1 ◦C, and then washed with distilled water, dried in warm air, and submitted for SEM surface examination. 2.6. Computational Details. All theoretical calculations were performed using the DFT electronic structure programs VAMP and DMol3 as contained in the Materials Studio 4.0 software.

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. Tests were performed on mild steel specimens with weight percentage compositions as follows: C − 0.05; Mn − 0.6; P − 0.36; Si − 0.3 and the balance Fe. The aggressive acid environments were, respectively, 1 M HCl and 0.5 M H2SO4 solutions. Stock solutions of the plant extract were prepared by refluxing weighed amounts of the dried and ground seeds of PG in 1.0 M HCl and 0.5 M H2SO4 solutions, respectively. The resulting solutions were cooled and then triple filtered. The amount of plant material extracted into solution was 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 (25−1000 mg/ L) by diluting the stock extract with the corresponding corrodent solution. 2.2. Weight Loss Measurements. Gravimetric experiments were conducted on test coupons of dimension 3 cm × 3 cm × 0.14 cm. These coupons were wet-polished with silicon carbide abrasive paper (from grade #400 to #1000), rinsed with distilled water, dried in acetone and warm air, weighed, and stored in moisture-free desiccators prior to use. The precleaned and weighed coupons were suspended in beakers containing the test solutions using glass hooks and rods. Tests were conducted under total immersion conditions in 300 mL of the aerated and unstirred test solutions at 30 ± 1 °C. To determine weight loss with respect to time, the coupons were retrieved at 24-h intervals progressively for 144 h, immersed in 20% NaOH solution containing 200 g/L of zinc dust, scrubbed with a

3. RESULTS AND DISCUSSION 3.1. Gravimetric Data. The inhibitive effect of the acid extracts of PG on the corrosion of mild steel in 1 M HCl and 0.5 M H2SO4 was investigated using gravimetric technique. The data presented are means of triplicate determinations, with standard deviation ranging from 0 to 0.0006. Figure 1 illustrates the weight loss of carbon steel in uninhibited and inhibited 1 M HCl (Figure 1a) and in 0.5 M H2SO4 (Figure 1b) as a function of exposure time. The results show that PG extract diminished the corrosion rates of mild steel in both acid environments. 13604



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Figure 1. Weight losses of mild steel in (a) 1 M HCl and (b) 0.5 M H2SO4 solutions without and with PG extract, as a function of immersion time.

Corrosion rate increased with immersion time for both inhibited and uninhibited systems but decreased steadily with increasing PG extract concentration. The inhibiting effect of PG on mild steel corrosion was quantified by evaluating the inhibition efficiency IE (%) as follows: ⎛ ΔWinh ⎞ IE% = ⎜1 − ⎟ × 100 ΔWblank ⎠ ⎝

Figure 2. Relationship between inhibition efficiency of PG extracts and immersion period for mild steel corrosion in (a) 1 M HCl and (b) 0.5 M H2SO4 solutions.

inhibiting effect of the 1 M HCl extract was tested only in 1 M HCl solution (as corrodent) the 0.5 M H2SO4 extracts tested in 0.5 M H2SO4 solution, the inhibiting effects of both the 1 M HCl extract and 0.5 M H2SO4 extract have been investigated in both corrodents using electrochemical techniques. 3.2.1. Electrochemical Impedance Spectroscopy Data. Impedance measurements were undertaken to provide information on the kinetics of the electrochemical processes at the mild steel/acid interface and how this is modified by the presence of PG extract. Nyquist and Bode phase plots for mild steel corrosion in 1 M HCl in the absence and presence of 900 mg/L of the extracts of PG are given in Figure 3a,b, respectively. Figures 4a,b show the equivalent plots for mild steel corrosion in 0.5 M H2SO4. The Nyquist plots show single semicircles for all systems over the frequency range studied, corresponding to one time constant in the Bode plots. The high frequency intercept with the real axis in the Nyquist plots is assigned to the solution resistance (Rs) and the low frequency intercept with the real axis ascribed to the charge transfer resistance (Rct). The impedance spectra were analyzed by fitting to the equivalent circuit model Rs(QdlRct), which has been used previously to adequately model the mild steel/acid interface.20,32 In this equivalent circuit, the solution resistance is

(1)

where ΔWinh and ΔWblank represent the weight losses in inhibited and uninhibited solution, respectively. Figure 2 illustrate the corresponding trend of inhibition efficiency for various concentrations of PG extract and as expected, efficiency increased steadily with PG concentration in both acid media, with efficiency (particularly at higher PG concentrations) exceeding 90% within the first few days of immersion, and did not fall below 80% throughout the entire period of immersion. The evident stability of the corrosion inhibiting action of PG extracts makes them rather attractive for actual practical applications. 3.2. Electrochemical Measurements. Electrochemical impedance spectroscopy and potentiodynamic polarization measurements were thus undertaken to understudy the inhibiting effect of PG extract (900 mg/L) on the electrochemical corrosion behavior of mild steel in 0.5 M H2SO4 and 1 M HCl from an electrochemical perspective. In addition, unlike what was obtained in the gravimetric experiments, wherein the 13605



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Figure 4. Electrochemical impedance spectra of mild steel in 0.5 M H2SO4 solution without and with PG extract: (a) Nyquist and (b) Bode phase angle plots. Inset in panel a is the Nyquist plot in uninhibited acid.

Figure 3. Electrochemical impedance spectra of mild steel in 1 M HCl solution without and with PG extract: (a) Nyquist and (b) Bode phase angle plots.

Table 1. Impedance Results for Mild Steel Corrosion in 1 M HCl and 0.5 M H2SO4 without and with 900 mg/L of PG Extracts

shorted by a constant phase element (CPE) that is placed in parallel to the charge transfer resistance. The CPE is used in place of a capacitor to compensate for deviations from ideal dielectric behavior arising from the inhomogeneous nature of the electrode surfaces. The impedance of the CPE is given by ZCPE = Q−1(jω)−n

Rct (Ω cm2)

system

Qdl (μΩ−1 Sn cm−2)

N

η%

0.9195 0.8945 0.8850

88.4 92.1

0.8879 0.9002 0.9022

96.1 97.7

1 M HCl

(2)

1 M HCl (blank) PG (HCl extract) PG (H2SO4 extract)

where Q and n stand for the CPE constant and exponent, respectively, j = (−1)1/2 is an imaginary number, and ω is the angular frequency in rad s−1 (ω = 2πf), where f is the frequency in Hz. The corresponding electrochemical parameters are presented in Table 1 and reveal that the PG extracts increased the magnitude of Rct, with corresponding decrease in the double layer capacitance (Qdl). The increase in Rct values in inhibited systems, which corresponds to an increase in the diameter of the Nyquist semicircle and in the magnitude of the phase angles in the Bode plots, confirms the corrosion inhibiting effect of the PG extracts. The observed decrease in Cdl values, which normally results from a decrease in the dielectric constant and/or an increase in the double-layer thickness, can be attributed to the adsorption of the extract organic matter (with lower dielectric constant compared to the displaced adsorbed water molecules) onto the metal/electrolyte interface, thereby protecting the metal from corrosive attack. Inhibition efficiency from the impedance data (IER%) was estimated by comparing the values of the charge transfer

0.5 M H2SO4 (blank) PG (HCl extract) PG (H2SO4 extract)

23.5 13.5 204.8 3.1 295.3 4.2 0.5 M H2SO4 9.8 12.9 251.7 4.0 431.1 2.93

resistance in the absence (Rct,bl) and presence of inhibitor (Rct,inh) as follows: ⎛ R ct(inh) − R ct ⎞ ⎟⎟ × 100 IE R % = ⎜⎜ ⎝ R ct(inh) ⎠

(3)

The magnitude and trend of the obtained values presented in Table 1 agree with those determined from gravimetric measurements. In addition, there are no pronounced differences in the inhibition efficiencies of the sulphuric acid extract of PG and the hydrochloric acid extract in either acid corrodent, although the former appears to have a slight edge. 13606



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3.2.2. Potentiodynamic Polarization Data. Potentiodynamic polarization experiments were undertaken to distinguish the effect of PG extract on the anodic dissolution of mild steel and cathodic hydrogen ion reduction. Typical potentiodynamic polarization curves for mild steel in 1 M HCl and 0.5 M H2SO4 in the absence and presence of 900 mg/L of the PG extracts are shown in Figure 5a,b, respectively, while the electrochemical

densities and the corresponding corrosion current density (icorr). This indicates that the extracts functioned as mixed-type inhibitors in both acid solutions. The anodic inhibiting effect of either of the PG extracts is slightly more pronounced in 1 M HCl, whereas the cathodic inhibiting effect is more pronounced in 0.5 M H2SO4. The values of the corrosion current densities in the absence (icorr,bl) and presence of inhibitor (icorr,inh) were used to estimate the inhibition efficiency from polarization data (IEi%) as follows: ⎛ icorr,inh ⎞ ⎟⎟ × 100 IEi% = ⎜⎜1 − icorr,bl ⎠ ⎝

(4)

The obtained values presented in Table 2 follow the same trend as the gravimetric and impedance data, again highlighting the corrosion inhibiting efficacy of acid extracts of PG. 3.3. Adsorption Considerations. Our impedance data provide direct experimental evidence of the adsorption of the organic constituents of PG extract on the corroding mild steel surface. Further characterization of the adsorption involved fitting the experimental data to some frequently used adsorption isotherms. Such fitting procedure is derived from the direct relationship between inhibition efficiency and the degree of surface coverage (θ) [η% = 100 × θ] for different inhibitor concentrations as determined from gravimetric measurements at short immersion periods (3 h). Figure 6

Figure 5. Potentiodynamic polarization curves of mild steel in (a) 1 M HCl and (b) 0.5 M H2SO4 solution without and with PG extract.

parameters derived from the polarization curves are summarized in Table 2. Addition of the extracts is seen to affect the anodic as well as the cathodic partial reactions, shifting the corrosion potential (Ecorr) slightly toward more positive (anodic) values and reducing the anodic and cathodic current

Figure 6. Relationship between weight loss and PG extract concentration for mild steel in 1 M HCl and 0.5 M H2SO4 after 3 h of immersion.

presents the plots of weight loss versus inhibitor concentration after 3 h of immersion, while Figure 7 illustrates the corresponding trend of inhibition efficiency. Corrosion rates in both acid environments decreased, while inhibition efficiency increased steadily with increasing concentration of PG extract. What is immediately obvious from Figure 7 is the wide variation in the effectiveness of PG extract in 1 M HCl and 0.5 M H2SO4, which was not observed at longer immersion periods (t ≥ 24 h). Efficiency is generally lower at short exposure times, which means that the adsorption of the extract organic matter is gradual and time dependent, a characteristic feature of the chemisorption process. According to the Langmuir adsorption isotherm;

Table 2. Potentiodynamic Polarization Polarization Parameters for Mild Steel in 1M HCl and 0.5 M H2SO4 in the Absence and Presence of 900 mg/L PG Extract system 1 M HCl (blank) PG (HCl extract) PG (H2SO4 extract) 0.5 M H2SO4 (blank) PG (HCl extract) PG (H2SO4 extract)

Ecorr (mV vs SCE) 1 M HCl −509 −472 −473 0.5 M H2SO4 −502 −457 −474

icorr (μA/cm2)

η%

2745.6 232.2 216.9

91.5 92.1

4065.0 179.9 162.6

95.6 96.0

C/θ = 1/b + C 13607

(5)



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Figure 7. Trend of inhibition efficiency with PG extract concentration for mild steel corrosion in 1 M HCl and 0.5 M H2SO4 after 3 h of immersion.

Figure 9. FTIR spectra of PG powder and the surface film on mild steel specimens immersed in 1 M HCl and 0.5 M H2SO4 solutions containing PG extract.

The plot of C/θ versus C is shown in Figure 8 to be linear for PG extract in both 1 M HCl and 0.5 M H2SO4; with slopes of

somewhat subdued intensities in the adsorbed surface films (CO stretching frequency around 1700 cm−1, the O−H stretching frequencies around 3350 cm−1 and 2800 cm−1, the C−O stretch around 1100 cm−1, and the C−N stretching frequency around 1250 cm−1) suggest that these functional groups are directly involved in metal-inhibitor interactions, thus confirming the proposed adsorption of some organic constituents of PG extract on the mild steel. 3.5. SEM Surface Examination. Morphological studies of the surfaces of mild steel specimens in uninhibited and inhibited acid were followed by SEM after immersion in the different test solutions for 3 h at 30 °C. Figure 10 shows the SEM images of the mild steel surface after 3 h immersion in uninhibited 0.5 M H2SO4 (Figure 10a) and the presence of 800 mg/L PG extract (Figure 10b), while Figures 11a and 11b present similar images for mild steel in uninhibited and inhibited 1 M HCl. A severely corroded surface morphology is observed after immersion in the uninhibited systems, due to corrosive attack of the acid solutions (Figures 10a and 11a). Corrosion was relatively general with no evidence of localized attack. With addition of PG extract (Figures 10b and 11b), the corrosion damage is visibly reduced, and there is slight evidence of the adsorbate presence on the metal surface. Moreover, the corrosion product layer on the metal surface in uninhibited 0.5 M H2SO4 (Figure 10a) is clearly very loose and porous and would thus offer insignificant corrosion protection. On the other hand, a distinct, compact, and highly ordered corrosion product layer is obvious in the presence of PG extract (Figure 10b), which possibly accounts for the high inhibition efficiency of PG extract in 0.5 M H2SO4, even at short immersion periods. The morphology of the mild steel surface in uninhibited 1 M HCl (Figure 11a) reveals blisters of soluble corrosion product, with inherent fractures. Addition of PG extract into the acid solution hindered formation of the corrosion product blisters (Figure 11b), leaving a comparatively smoother and more protective corrosion product layer, which is not, however, as compact as that formed by PG extract in 0.5 M H2SO4. The difference in inhibition efficiency of PG extract in 1 M HCl and 0.5 M H2SO4 at short immersion periods is therefore interconnected with the extent to which the extract modifies the morphology of the corrosion product layer in the respective acid media.

Figure 8. Langmuir adsorption isotherms for PG extract on mild steel in (a) 1 M HCl and (b) 0.5 M H2SO4 solution.

1.51 (r2 = 0.992) and 1.15 (r2 = 0.999), respectively, suggesting that the experimental data follows the Langmuir isotherm. Deviation of the slopes from the anticipated value of unity can be attributed to interactions between extract species adsorbed on the metal surface as well as changes in the adsorption heat with increasing surface coverage. Nonetheless, the data fit to the Langmuir isotherm confirms adsorption of the extract species on the corroding metal surface. 3.4. Infrared Spectroscopy Analysis. The FTIR spectra of PG powder as well as scrapings from the inhibitor films on the surface of mild steel specimens immersed for 3 h in 1 M HCl and 0.5 M H2SO4 containing 800 mg/L of PG extract are presented in Figure 9. The multiplicity of peaks in the spectra for PG powder implies the presence of a complex mixture of compounds. Again, almost all the peaks observed for PG powder are also noticed for the surface film of the extract on the mild steel surface, which means that most of the functional groups (or compounds) within PG powder are also present in the adsorbed surface film. Moreover, some of the peaks for the adsorbed film diminished or even vanished; the peaks with 13608



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Figure 10. SEM images of the mild steel surface after 3 h immersion at 30 °C in 0.5 M H2SO4: (a) without additive and (b) with 800 mg/L PG extract. The images beneath depict the selected areas at higher magnification (5000×).

3.6. Effect of Temperature. To assess the effect of temperature on corrosion and corrosion inhibition processes, gravimetric experiments were performed at 30, 40, 50, and 60 °C in uninhibited acid (1 M HCl and 0.5 M H2SO4) and in inhibited solutions containing 25 mg/L and 800 mg/L PG extract. The extract concentrations were selected to properly reflect the temperature effects at low and high surface coverage. The results obtained for a 3 h immersion period are given in Figure 12 and show that corrosion rates in both uninhibited and inhibited acids increased with rise in temperature. Also, the PG extracts can be seen to maintain their inhibiting effect at all temperatures. Figure 13a reveals that inhibition efficiency in 1 M HCl increased with rise in temperature at both low and high PG extract concentrations. On the other hand, inhibition efficiency in 0.5 M H2SO4 decreased drastically from 76% to 22% with rise in temperature for low PG extract concentration, but is somewhat stable at higher extract concentrations (Figure 13b). The adsorption of an organic inhibitor can affect the corrosion rate by either decreasing the available reaction area (geometric blocking effect) or by modifying the activation energy of the anodic or cathodic reactions occurring in the inhibitor-free surface in the course of the inhibited corrosion process. The Arrhenius-type relationship between the corrosion rate (k) of mild steel in acidic media and temperature (T) (eq 6) was used to determine the activation energies (Ea): k = A exp( −Ea /RT )

A is the preexponential factor, and R the universal gas constant. The variation of logarithm of corrosion rate with reciprocal of absolute temperature is shown in Figure 14 for 1 M HCl and 0.5 M H2SO4 without and with PG extract, while the calculated values of Ea are given in Table 3. Addition of PG extract is seen to decrease Ea at low and high concentrations for the corrosion reaction in 1 M HCl as well as at high concentrations in 0.5 M H2SO4, while a low concentration of PG extract increased the activation energy in 0.5 M H2SO4. An increase in inhibition efficiency with rise in temperature, including lower corrosion activation energies in the presence of an inhibitor as observed in 1 M HCl is often attributed to chemisorption of inhibitor molecules on the metal surface.33,34 Accordingly, some constituents of the PG extract are chemically adsorbed on the mild steel surface and exert a controlling influence on the corrosion inhibition performance of the extract. This explains the observed slow rate of adsorption of the extract in 1 M HCl. Chemisorbed molecules are thought to provide more effective protection since they reduce the inherent reactivity of the metal at the sites where they are attached. This effect is enhanced with rise in temperature, leading to greater surface coverage and hence inhibition efficiency. Inhibition efficiency in 0.5 M H2SO4 decreased drastically from 76% to 22% with rise in temperature for low PG extract concentration, and the obtained Ea is higher than that in uninhibited acid. A decrease in inhibition efficiency with rise in

(6) 13609



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Figure 11. SEM images of the mild steel surface after 3 h immersion at 30 °C in 1 M HCl: (a) without additive and (b) with 800 mg/L PG extract. The images beneath depict the selected areas at higher magnification (5000×).

predict the adsorption characteristics of selected constituents of PG extract, with molecular structures analogous to conventional organic corrosion inhibitors. The phytochemical constituents present in PG have been characterized and discussed elsewhere.36 The selected constituents include piperine (an amide alkaloid), safrole (a phenylpropene) and dihydrocubebin (a naturally occurring lignan). We herein build on our previous efforts investigating the noncovalent adsorption of these molecules on Fe (110),36 but have, however, performed additional geometry optimization of the molecules, this time using the NDDO (neglect of diatomic differential overlap) method with the AM1 Hamiltonian as contained in the semi empirical molecular orbital package VAMP (MS Studio 4.0).37−39 The geometry optimization was without any symmetry constraints, and we have also included a solvation scheme (COSMO: conductor-like screening model) in the present computation to mimic the aqueous environment. The solvent molecules are modeled as a continuum of uniform dielectric constant (ε = 78.39 for water), and the solute is placed in a cavity within it.40 The three highest occupied molecular orbitals (HOMO, HOMO-1, and HOMO-2) and the lowest unoccupied molecular orbitals (LUMO, LUMO+1, LUMO+2) of the piperine, safrole, and dihydrocubebin are imaged in Figures 15, 16, and 17, respectively. The phenyl and dioxole groups are common to all the molecules, and the HOMO, HOMO-1, and HOMO-2 have π-type and σ-type contributions around the phenyl ring and dioxole rings, respectively. It can be seen that the HOMO for each molecule shows pronounced contributions

temperature implies that the surface active constituents of the extract are physically adsorbed on the mild steel surface at low concentrations and function via a geometric blocking effect. Physisorption involves weak metal−inhibitor interactions, which can be disrupted by any slight perturbation to the system. Such perturbations include the enhanced rates of hydrogen gas evolution at higher temperatures, which increasingly agitates the interface and also promotes dispersal of adsorbed inhibitor. The high temperature disruption of the system is sufficiently minimized at high extract concentration and the inhibiting effect is more or less stable (Figure 13b). This is probably because some of the extract constituents become present in sufficient amounts to chemically interact with the metal surface, hence improving the stability of the inhibiting surface layer and modifying the corrosion activation energy. 3.6. Theoretical Considerations. Our experimental results show that the corrosion inhibiting efficacy of PG extract is accomplished to a large extent via chemisorption of some organic constituents of the extract on the corroding metal surface. This normally results from Lewis acid−base interactions in which the metal and inhibitor act as Lewis acid and Lewis base respectively and their interaction is accomplished by favorable overlap of frontier orbitals: the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Accordingly, we have utilized basic molecular reactivity indicators within the theoretical framework of the hard/soft acid/base (HSAB) principle35 to elucidate the electronic structures and hence 13610



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Figure 12. Effect of temperature on the corrosion rates of mild steel in (a) 1 M HCl and (b) 0.5 M H2SO4 without and with PG extract.

Figure 13. Effect of temperature on the inhibition efficiency (%) of PG extract for mild steel corrosion in (a) 1 M HCl and (b) 0.5 M H2SO4.

from the dioxole function, which represents the preferred site for electrophilic attack, whereas the lower energy HOMO-2 and HOMO-1 feature increasingly subdued contributions from the dioxole function. Table 4 provides some quantum-chemical parameters related to the molecular electronic structure of the most stable conformation of the molecules. The tendency of a molecule to donate electrons to the vacant d orbitals within the Fe atom is reflected by high values of EHOMO. In the same way, low values of the energy of the gap ΔE = ELUMO−HOMO means that the energy required to remove an electron from the last occupied orbital will be minimized, resulting in improved inhibition efficiencies. The HOMO and LUMO energies are related to the ionization potential (I) and the electron affinity (A), respectively, as follows:41−43 I = −E HOMO

(7)

A = −E LUMO

(8)

ΔN values presented in Table 4 were computed using theoretical values of 7 eV/mol and 0 eV/mol for χm and ηm, respectively.45 ΔN has been reported to correlate remarkably with adsorption energy for metal−inhibitor interactions, with larger ΔN values corresponding to stronger adsorption.43 χ and η are related to A and I as follows: I+A χ= (10) 2 I−A (11) 2 Chemical hardness (or softness) has been shown to be an important indicator of a molecule’s propensity toward covalent interaction, since softness facilitates close contact between the molecule and the surface as well as hybridization with the metal states.46 The values of our computed quantum chemical indicators presented in Table 4 suggest that the reactivity order (in terms of electronic transitions) for the different molecules in aqueous phase is piperine > dihydrocubebin > safrole. Accordingly, molecular dynamics simulation of the covalent interactions between piperine molecule and Fe surface was undertaken to model the chemisorption of constituents of PG extract on the mild steel surface. Our intention here is not necessarily to η=

Electron charge transfer, (ΔN) from Lewis base (the inhibitor molecule) to Lewis acid (metal surface) was quantified using the equation43−45 χm − χi ΔN = 2(ηm + ηi) (9) where χm and χi denote the absolute electronegativity of the metal and the inhibitor molecule, respectively; ηm and ηi denote the absolute hardness of the metal and the inhibitor. 13611



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Figure 15. Electronic properties of piperine. Only the three highest relevant occupied molecular orbitalsHOMO, HOMO-1, and HOMO-2and lowest unoccupied molecular orbitalsLUMO, LUMO+1, and LUMO+2are shown. (Atom legend: white = H; gray = C; red = O; blue = N.) The blue and yellow isosurfaces depict the electron density difference, the blue regions show electron accumulation, while the yellow regions show electron loss.

Figure 14. Arrhenius plots for mild steel corrosion in (a) 1 M HCl and (b) 0.5 M H2SO4 without and with PG extract.

Table 3. Activation Energies for Mild Steel Corrosion in 1 M HCl and 0.5 M H2SO4 without and with PG Extract system

Ea (kJ/mol)

system

Ea (kJ/mol)

1 M HCl (blank) 25 mg/L PG 800 mg/L PG

110.2 55.1 64.7

0.5 M H2SO4 (blank) 25 mg/L PG 800 mg/L PG

63.6 97.7 51.4

calculate chemisorption, but rather to model the primary stages of the covalent interactions. We used the DMol3 code to study the chemisorption of a single piperine molecule on the Fe (110) surface, using a simulation box (7.45 Å × 7.45 Å × 22.03 Å) with periodic boundary conditions. The Fe (110) was first built and relaxed by minimizing its energy via molecular mechanics using the Discover molecular simulation program (MS Studio 4.0). The surface area was increased, and its periodicity changed by constructing a 5 × 4 super cell, with a vacuum slab of thickness 20 Å. Structure optimizations and corresponding total energy calculations of the most stable geometries are based on the generalized-gradient approximation (GGA) function with the Perdew−Burke−Ernzerhof (PBE) correction.47 For core electrons in the lowest lying atomic orbitals, the DFT semicore pseudopotentials (DSPP) core treatment, which replaces core electrons by a single effective potential, was implemented for relativistic effects.48 The Dmol3 electronic options were adjusted as follows: Monkhorst-Pack k-point mesh parameters were set to 2 × 2 × 1, with k-point separation 0.05 1/Å. Self-

Figure 16. Electronic properties of safrole. Only the three highest relevant occupied molecular orbitalsHOMO, HOMO-1, and HOMO-2and lowest unoccupied molecular orbitalsLUMO, LUMO+1, and LUMO+2are shown. (Atom legend: white = H; gray = C; red = O; blue = N.) The blue and yellow isosurfaces depict the electron density difference, the blue regions show electron accumulation, while the yellow regions show electron loss.

consistent field (SCF) procedures were carried out with a convergence criterion of 10−5, using direct inversion in an iterative subspace (DIIS) and an orbital occupancy smearing parameter of 0.005 Ha to speed up SCF convergence. We have neglected solvent and charge effects in all our simulations and performed the calculations at the metal/vacuum interface. Although this is clearly an oversimplification of the factual situation, it is adequate to qualitatively illustrate the trend of the covalent interactions of interest. First, we explored different adsorption configurations to search out the most stable adsorption geometry for covalent interactions between piperine and Fe (110). Figure 18a−h 13612



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Fe−O bond undergoes a change from the low-spin state ((1.32 Å + 0.66 Å) to the high-spin state (1.52 Å + 0.66 Å). Spin-crossovers have been reported for coordination compounds of some third row transition metals50−52 and are often accompanied by metal−ligand bond length variations. Such spin transitions arise from variations in the donor/ acceptor properties of coordinated ligands, in response to some external stimuli, which may be physical or chemical. Physicalinduced spin-crossover could be caused by temperature change, light irradiation; magnetic fields etc., whereas chemical induced spin-crossover can result from solvation, bond breaking, ligand exchange, or isomerization. 52 According to Meier and Hanusa,53 steric effects arising from certain ligand orientations can also influence the metal−ligand bond distances, with longer bonds creating weaker ligand fields, which stabilize high-spin or spin-crossover complexes. This reasoning is consistent with our computational results, i.e., significant distortion of the piperine molecule and subsequent cleavage of the Fe−O bond induced the spin transition. Interestingly, Huse et al.54 using timeresolved soft X-ray spectroscopy, observed significant reduction in orbital overlap between the 3d orbital of Fe and the ligand 2p orbital during low to high spin-crossover, which corroborate the resultant increase in bond length. Detailed discussion of the spin-crossover phenomenon is beyond the intended scope of this paper, but can be found in refs 50−55 and the references therein. The theoretically predicted low spin to high spin transition for the Fe−piperine couple, which has not been widely reported in the literature, could provide useful information in understanding the mechanisms of corrosion inhibition and thus requires further investigation and adequate experimental validation. Moreover, reliability of predicting spin state differs among common computational approaches, whether ab initio Hartree−Fock theory or pure DFT functional.55 The adsorption energy (E ads ) for the most stable configuration of piperine on the Fe (110) surface was computed using the relationship below:56

Figure 17. Electronic properties of dihydrocubebin. Only the three highest relevant occupied molecular orbitalsHOMO, HOMO-1, and HOMO-2and lowest unoccupied molecular orbitalsLUMO, LUMO+1, and LUMO+2are shown. (Atom legend: white = H; gray = C; red = O; blue = N.) The blue and yellow isosurfaces depict the electron density difference, the blue regions show electron accumulation, while the yellow regions show electron loss.

shows representative cross-section snapshots of the key variations in adsorption orientation during optimization, while Figure 19 illustrates the energy evolution for different stages of optimization. Analysis of the frozen configurations taken from the snapshots reveals that piperine bonds covalently with Fe through an O atom of the dioxole nucleus. The optimization steps appear configurationally diverse, and the difference in energy between the initial and final structures is about 753 kcal/mol. The initial configuration (Figure 18a) shows the Fe− O covalent bond with length 1.98 Å, which coincides with the sum of the covalent radii of Fe (low spin: 1.32 Å) and O (0.66 Å).49 In the following frames (Figures 18b,c), the Fe−O bond length decreases gradually to about 1.96 Å as one of the H atoms in α position to the O atoms of the dioxole moiety bonds with the surface Fe atom to form one Fe−H bond of length 1.83 Å (6th optimization step). Optimization progresses with pronounced distortions in the orientation of the piperine molecule. For instance, the Fe−H bond length initially decreased to 1.63 Å, then increased to about 1.85 Å and is subsequently broken (Figure 18d: 10th optimization step). The distortions in adsorbate orientation also lead to cleavage of the Fe−O bond by the 20th optimization step (Figure 18e,f). Such fluctuations in the orientation often result from competition between Hund’s rule for the adsorbate and the formation of adsorbate−adsorbent chemical bonds, which ultimately modifies the magnetic properties of the adsorbate.50 The Fe−O bond is subsequently re-established by the 35th optimization step, with bond length 2.26 Å. The bond length decreased steadily to about 2.18 Å at the 50th optimization step, which corresponds to the low energy minima and hence the most stable chemisorption orientation. The length of the reestablished Fe−O bond suggests that the Fe is at its highspin state, with covalent radius 1.52 Å.49 This spin-crossover favors reformation of the Fe−O bond, with corresponding increase in bond length. In other words, the Fe atom of the

Eads = Etotal − (Epip + E Fe)

(12)

Epip, EFe, and Etotal correspond respectively to the total energies of the piperine molecule, Fe (110) slab, and the adsorbed piperine/Fe (110) couple in the gas phase. The resulting chemisorption energy (−715.1 kcal/mol) is more exothermic than the physisorption energy (−167.5 kcal/mol) as computed in an earlier study,36 which indicates that the piperine molecule chemisorbs strongly to the Fe (110) surface, in agreement with our experimental findings. Interestingly several other alkaloidal constituents of PG such as safrole and dihydrocubebin also bear the dioxole moiety in their molecules, which should augment the chemisorption ability of the extract.

■

CONCLUSIONS This study has revealed that acid extracts of leaves of PG effectively inhibit mild steel corrosion in both 1 M HCl and 0.5 M H 2SO4 . Polarization measurements show that they

Table 4. Calculated Quantum Chemical Properties for the Most Stable Conformation of Piperine, Safrole, and Dihydrocubebin molecule

EHOMO (eV)

ELUMO (eV)

ELUMO−HOMO (eV)

X

η

ΔN

piperine safrole dihydrocubebin

−8.89 −9.18 −9.19

−0.95 −0.12 −0.18

7.94 9.06 9.01

4.92 4.65 4.69

3.97 4.53 4.51

0.262 0.259 0.256

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Figure 18. Perspective view of representative snapshots showing changes in adsorption orientation during chemisorption of piperine on Fe (110), showing in different stages the formation, cleavage, and re-establishment of the Fe−O bond. The structures become energetically more favorable going from a to h with energy difference of 753 kcal/mol. The increase in the Fe−O bond distance suggests low to high-spin crossover.

adsorption energy confirms strong chemisorption of the piperine molecule, thus validating our experimental findings.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Tel: +2348037026581. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This project is supported by TWAS, the Academy of Sciences for the developing World, under the TWAS Grants for Research Units in Developing Countries Program (TWASRGA08-005) and the Education Trust Fund (ETF), under batch 1 of ETF 2009/2010 research projects intervention 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 and FTIR measurements.

Figure 19. Energy evolution during optimization of the chemisorption geometry of piperine on Fe (110).

■

functioned via mixed-type mechanism, inhibiting the rates of both the anodic metal dissolution and cathode hydrogen ion reduction reactions. Impedance data, SEM, and FTIR results all indicate that the corrosion reaction was inhibited by adsorption of the extract organic matter on the corroding mild steel surface. The trends of inhibition efficiency with temperature as well as values of kinetic and activation parameters for corrosion and corrosion inhibition processes point toward significant chemisorption of the extract constituents on the mild steel surface. DFT-based quantum chemical computation was used to theoretically model the chemisorptive interactions between the piperine molecule, which is the active component of the extract, and Fe (110) surface. The magnitude of the obtained

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Natural Products for Materials Protection: Mechanism of Corrosion

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