Corrosion inhibition of copper in 0.5 M NaCl solutions by aqueous and hydrolysis acid extracts of olive leaf

Journal of Electroanalytical Chemistry 859 (2020) 113834

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Corrosion inhibition of copper in 0.5 M NaCl solutions by aqueous and hydrolysis acid extracts of olive leaf ⁎

Philippe Refait a, , Chahla Rahal b, Mohamed Masmoudi b a Laboratoire des Sciences de l'Ingénieur pour l'Environnement (LaSIE), UMR 7356 CNRS, La Rochelle Université, Bât. Marie Curie, Av. Michel Crépeau, 17042 La Rochelle cedex 01, France b Laboratory of Electrochemistry and Environment (LEE), Sfax National Engineering School (ENIS) BPW, 3038 Sfax, University of Sfax, Tunisia

A R T I C L E

I N F O

Article history: Received 17 September 2019 Received in revised form 13 December 2019 Accepted 7 January 2020 Available online xxxx Keywords: Olive leaf extract Copper Corrosion HPLC Voltammetry EIS

A B S T R A C T

Aqueous extracts and acid hydrolysates from olive leaf, rich in various phenolic compounds, were prepared under different experimental conditions. Their inhibitive action on the corrosion of copper in 0.5 M NaCl solutions was studied by electrochemical impedance spectroscopy and voltammetry. High performance liquid chromatography showed that the extracts were rich in oleuropeine, hydroxytyrosol, and elenolic acid. The acid hydrolysis extracts obtained at high temperature mainly contained hydroxytyrosol and elenolic acid and led to the highest inhibition efficiency (95%). The polarization curves were modelled on a large potential range (e.g. from −500 mV to −100 mV/SCE) using electrochemical kinetic laws to obtain a maximum of reliable information. Elenolic acid and oleuropein acted as cathodictype corrosion inhibitors.

1. Introduction Though copper is a rather noble metal, it may suffer severe corrosion, in particular in chloride-rich media such as seawater [e.g. 1–5]. The extensive use of copper and copper alloys in marine industrial applications led to the necessary development of anticorrosion processes and corrosion inhibitors were in particular used successfully [6]. Many of the inhibiting species initially developed proved highly toxic, which mostly orientated the new investigations towards environmentally friendly organic compounds. In general, the inhibiting action of these compounds is due to their adsorption on the copper surface, a process mainly governed by polar functional groups. For copper, it has been demonstrated long ago that nitrogen, sulfur and aromatic-containing organic compounds can act as efficient corrosion inhibitors [7–9]. Numerous non-toxic organic inhibitors were then considered and studied successfully such as sodium heptanoate [10], imidazole [11], trithiocyanuric acid [12], 4-aminoantipyrine [13], aminoacids [14,15], phytic acid [16], ⁎ Corresponding author at: Laboratoire des Sciences de l'Ingénieur pour l'Environnement (LaSIE), UMR 7356 CNRS, La Rochelle Université, Bât. Marie Curie, Av. Michel Crépeau, 17042 La Rochelle cedex 01, France E-mail addresses: [email protected], [email protected]. (P. Refait).

http://dx.doi.org/10.1016/j.jelechem.2020.113834 1572-6657/© 2020 Elsevier B.V. All rights reserved.

© 2020 Elsevier B.V. All rights reserved.

chitosan [17], guanine derivative [18], etc. High inhibition efficiency (IE) was often reported, e.g. up to 90% for 4-aminoantipyrine [13], ~93% for chitosan [17], 95% for trithiocyanuric acid [12] or through the combination of imidazole and polyaspartic acid [11]. Plant extracts were in particular considered as they constitute a rich source of naturally synthesized chemical compounds that can be extracted by simple procedures, and some of them proved efficient as corrosion inhibitors of copper in different media [19–22]. IE up to 83% could be obtained with aqueous extracts of Morinda tinctoria [19] and up to 93% with Jujube extract [21] for copper in 0.5 M HCl solution. Myrrh extract led to an IE of 67% for α-brass in 3.5% NaCl solution polluted with sulfide [23]. Finally, a first study devoted to olive leaf extracts led to IE as high as 90% in 0.5 M NaCl solutions [20]. The idea of studying olive leaf extract as a possible copper corrosion inhibitor originated from its phenolic content. Various biophenols were actually detected in olive leaf, including oleuropein, hydroxytyrosol, tyrosol, and there secoiridoid derivatives [24–26]. In the first study devoted to the possible inhibiting action of olive leaf extracts [20], only one extraction process was considered: powdered leaves were mixed with 1 L of de-ionized water under stirring for 1 h at 75° and hydrolyzed another hour at 25 °C. Oleuropein was the main phenolic compound extracted, which led to a maximal IE of 90% after 24 h of immersion with the highest concentration used, i.e. 2.42 mmol L−1. Oleuropein was found a cathodic-type inhibitor, actually

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Journal of Electroanalytical Chemistry 859 (2020) 113834

NaCl was added so that a 0.5 M concentration was obtained. For comparison with the results obtained in S and SH solutions, the pH was adjusted at 5.8 by adding NaOH. The electrochemical behavior of copper in the 0.5 M NaCl solutions containing olive leaf extracts was compared to that occurring in a 0.5 M NaCl solution of similar pH (“blank”).

close to the frontier between a cathodic-type and a mixed-type inhibitor, bound to the copper surface via a physisorption mechanism [20]. The present article describes the second part of the study of olive leaf extracts. The first objective was to test various extraction processes, and identify and quantify the extracted phenolic compounds. However, a “green” inhibitor has to be obtained via a “green” industrial process. Besides, it has to be economically viable which implies that this industrial process has to be simple and inexpensive. This explains while the first study [20] was achieved with extracts obtained with water heated for only 1 h. However, only oleuropein could be extracted this way. According to previous studies, the compounds of interest could all be extracted with a mixture of methanol and water [26,27]. The use of acidified water (pH = 2 with HCl) leads to the same qualitative phenolic profile, with a higher amount of extracted compounds by comparison with the extraction using methanol [27]. The decomposition rate of oleuropein could also be increased via an increase of the temperature of the extraction process. Consequently, the temperature of the hydrolysis stage was increased from 25 °C to 100 °C. Moreover, the extraction was also performed in acidic solutions of pH = 2. As a result, two other phenolic compounds could be extracted. The second objective was to determine the inhibitive efficiency of the various extracts to confirm definitely the potentiality of olive leaf extracts as green inhibitors for copper corrosion. It was achieved using a combination of electrochemical methods, i.e. voltammetry and electrochemical impedance spectroscopy (EIS). In the first part of the study [20], an innovative approach was used to obtain a maximum of accurate and reliable information from voltammetry: voltammograms obtained on a short range (± 50 mV) of potential around OCP were computer fitted using various electrochemical kinetic laws, i.e. not only Tafel law. This is a crucial point if one of the involved electrochemical reaction does not obey Tafel law, which is often the case for the reduction of dissolved oxygen. However, this method proved in some cases unreliable because one voltammogram may be adequately fitted in many ways leading to an important error on the corrosion current density [28]. In the present study, this approach was used for the first time with voltammograms acquired on a large potential range. It proved actually possible to model reliably some of the curves in a potential range as large as 400 mV, while the cathodic process, i.e. O2 reduction, was under mixed activation/diffusion control. A focus was then made on this specific part of the electrochemical analysis (Section 3.3).

2.3. High-performance liquid chromatography (HPLC) analysis The identification and quantification of the phenolic compounds were carried out using HPLC. The chromatograph was an Agilent Technologies series 1100 liquid chromatography system (HPLC, Agilent Technologies, Karlsruhe, Germany) equipped with a LPG-3400 SD pump, a column oven and a DAD-3000 RS diode array detector. An Eclipse XDB-C18 column (250 × 4.6 mm, i.d., 5 μm particle size; Waters Co., Milford, MA) was used at RT with an injection volume of 10 μL. The mobile phase used was 0.25% acetic acid in water (A) versus methanol (B) at a flow rate of 0.6 mL min−1, with the following steps: 0 min, 5% B; 7 min, 35% B; 12 min, 45% B; 17 min, 50% B; 22 min, 60% B; 25 min, 95% B; 27 min, 5% B, and finally a conditioning cycle of 5 min at the same conditions for the subsequent analysis. Detection and quantification were performed at 254 nm near the maximum absorption of most phenols [24]. The peaks of the various chromatograms were interpreted by comparison with available standards. Each phenolic compound was quantified in comparison with its standard when it was available while the other detected compounds were quantified by other equivalent compounds. 2.4. Electrochemical measurements All experiments were carried out at RT with a BioLogic VSP potentiostat and a classical three-electrode cell. Pure copper, platinum foil and saturated calomel electrode (SCE, E = +0.244 V/SHE at 25 °C) were used as working, counter and reference electrodes, respectively. The open-circuit potential (OCP) was monitored continuously except during voltammetry and EIS experiments. The EIS measurements were carried out after 24 h of immersion in the solution. Impedance diagrams were obtained over a frequency range of 100 kHz to 10 mHz with ten points per decade using a 10 mV peak-topeak sinusoidal voltage. Electrical equivalent circuits were used to obtain a maximum of information from the impedance data, and the experimental spectra were fitted with EC-Lab software (Bio-Logic). After the EIS experiment, a polarization curve was acquired at a sweep rate dE/dt = 0.5 mV min−1 from −0.55 V/SCE to +0.6 V/SCE. This large range of potential was considered so as to study the influence of the various inhibiting species on all the phenomena, and in particular the diffusioncontrolled dissolution of copper observed at high potential (E > 0.05 V/ SCE). Moreover the OCP values were observed between −0.450 V/SCE and −0.200 V/SCE depending on the considered olive leaf extract, which implied to investigate also a large cathodic region. Each experiment was performed at least three times and the values given for the various determined parameters are the average of the various measurements. The accurate determination of the corrosion current density from the voltammetry experiment was obtained through an innovative approach: a large part of the polarization curves around the OCP was computer fitted using electrochemical kinetic laws as described in previous works [20,28]. If both cathodic and anodic reactions are controlled by charge transfer, the mathematical expression of the current density j is:

2. Experimental 2.1. Electrodes and electrolytes The work electrodes were prepared from a pure copper (99.99%) rod of 6 mm in diameter. The surfaces were abraded with silicon carbide (from grade 240 to grade 1200), washed with distilled water and degreased with acetone. The electrolyte chosen for the corrosion study was a 0.5 M NaCl solution prepared using de-ionized water (resistivity 18.2 MΩ cm) and 98% min. Purity NaCl. The aerated solutions were kept stagnant during the experiments. The influence of the extraction process was studied by electrochemical methods at room temperature (RT = 25 ± 2 °C) after 24 h of immersion. 2.2. Extraction of phenolic compounds For all experiments, the olive leaves were dried at RT for 30 days while sheltered from light. They were ground to powder and mixed with deionized water at a concentration of 50 g/L. The obtained mixture was stirred for 1 h at 75 °C before to be filtered through a filter paper. An amount of 150 mL of the obtained solution was hydrolyzed at 25 °C (S) or 100 °C (SH) for 1 h. Subsequently, NaCl was added to 150 mL of each solution to reach a concentration of 0.5 M. The pH of the final solution was measured at 5.8 and used as it was for the corrosion study. Alternatively, the solution obtained after 1 h of stirring at 75 °C was hydrolyzed in a HCl solution of pH = 2. The extracted solutions at 25 °C and 100 °C were called SA and SAH, respectively. As for solutions S and SH,

h i jE ¼ ja þ jc ¼ jcorr eβa ðE−Ecorr Þ −eβc ðE−Ecorr Þ

ð1Þ

In this expression, ja and c are the anodic and cathodic current densities, jcorr is the corrosion current density, βa and βc are the anodic and cathodic Tafel coefficients (in V−1), Ecorr is the corrosion potential and E is the potential of the electrode. The Tafel coefficients are linked to the Tafel slopes ba,c 2

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Journal of Electroanalytical Chemistry 859 (2020) 113834

that appears in Tafel law via the expression: βa,c = ln(10)/ba,c. The experimental polarization curve can then be computed fitted using Eq. (1), leading to the determination of βa, βc and jcorr. In general, Ecorr is previously determined form the log|j| vs E curve and is not computer fitted. This approach is only a refinement of the so-called “Tafel method”. However, the reactions may not be controlled by charge transfer. This is often the case for the reduction of dissolved O2 that may be totally or partially controlled by diffusion. The polarization curve can however be fitted using the corresponding kinetic law. If the cathodic process is entirely controlled by diffusion then: jc ðEÞ ¼ jlim ¼ −jcorr

ð2Þ

In this equation, jlim is the limiting current density. Dissolved O2 reduction is however often under mixed activation/diffusion control. The expression of jc can then be derived from the Koutecky-Levich equation [29], which leads to: jc E ¼

1 1 jlim

−e−βc ðE−Ecorr Þ :



1 jcorr

þ

1



ð3Þ

jlim

In this case, the mathematical expression of the current density is given by the following equation: jE ¼ jcorr :eβa E−Ecorr þ

1 1 jlim

−βc ðE−E corr Þ

−e

 :

1 jcorr

þ

1



ð4Þ

jlim

Consequently, the approach based on the detailed analysis of the polarization curve can give information on the process that controls the anodic and cathodic reactions. It also leads to a more accurate determination of the corrosion current density because it is not restricted, in contrast with the “Tafel method”, to reactions controlled by charge transfer. 3. Results and discussion 3.1. HPLC determination of phenolic compounds The identification and quantification of the major phenolic compounds of the olive leaf extract was carried out by HPLC. The chromatograms are displayed in Fig. 1 and the main results of the analysis are listed in Table 1. The analysis was focused on three phenolic compounds: oleuropein (peak 2), the main phenolic compound of the extract obtained after 1 h of hydrolysis at 25 °C (solution S, Fig. 1a), hydroxytyrosol (peak 1), and elenolic acid (peak 3), two compounds detected in the extracts obtained at higher hydrolysis temperature and/or lower pH (Figs. 1b-1d). In agreement with previous works [20,26,30], oleuropein (peak 2) is observed as the major component of solution S. Seven other phenolic compounds are also identified in the olive leaf extract, namely, tyrosol (A), caffeic acid (B), p-coumaric acid (C), luteolin 7-glucoside (D), apigenin 7glucoside (E), verbascoside (F) and hydroxytyrosol (1). They were also already reported [20,24–26,30]. Elenolic acid could not be detected and the amount of hydroxytyrosol proved too small to be quantified. As indicated in Table 1, the concentration of oleuropein was estimated at 1.21 mmol L−1 in solution S. When the temperature of the 1 h hydrolysis period was increased to 100 °C (SH solution), the amount of hydroxytyrosol increased and that of oleuropein decreased, as it can clearly be seen in Fig. 1b. The hydroxytyrosol concentration was estimated at 0.109 mmol L−1 and that of oleuropein at 0.926 mmol L−1. These changes are due to the increase of temperature because high temperature, especially above 60 °C, causes the degradation of polyphenols [31]. Oleuropein was then partially converted to hydroxytyrosol and possibly also hydrolysed to other compounds (not determined during this work) [32].

Fig. 1. HPLC chromatograms at 254 nm of solutions S (a), SH (b), SA (c) and SAH (d). Peak 1 = Hydroxytyrosol, peak 2 = Oleuropein, peak 3 = Elenolic acid, peaks A-F: see text.

Table 1 Quantities of phenolic compounds detected in olive leaf extract in various solutions S, SH, SA and SAH. Solution

Oleuropein (mmol L−1)

Hydroxytyrosol (mmol L−1)

Elenolic acid (mmol L−1)

S SH SA SAH

1.21 0.926 0.092 Not detected

Traces 0.109 0.384 0.413

Not detected Traces 1.683 2.118

The HPLC analysis also shows an increase in hydroxytyrosol concentration due to the acid treatment (Fig. 1c, solution SA). The hydroxytyrosol concentration reaches here 0.384 mmol L−1 while that of oleuropein drops down to 0.092 mmol L−1. It was previously observed that acid hydrolysis led to more complex phenolic molecules [33]. As a result, another compound, elenolic acid, is now identified as revealed by peak 3. This 3

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Journal of Electroanalytical Chemistry 859 (2020) 113834

rate, which is typical of a cathodic-type inhibitor. The inhibiting effect obtained with solution S was already reported and described previously [20]. From this detailed study, it was concluded that the inhibiting species, i.e. mainly oleuropein, indeed acted as a (slightly) cathodic inhibitor adsorbed on the metal surface via a physisorption mechanism [20]. The polarization curve (c) obtained in solution SH is very close to that obtained in solution S, which can be attributed to the fact that oleuropein is still the predominant phenolic compound. The corrosion potential decreased slightly further and is now equal to −0.289 V vs SCE. The current density in the cathodic region has also decreased further. However, the anodic branch appears slightly modified. The slope of the curve after Ecorr is smaller than that observed in the two previous solutions and changes at point E1 where the polarization curve almost superimposes with that obtained in the blank solution. This phenomenon can be attributed to hydroxytyrosol because it is more pronounced in solutions SA and SAH characterized by the highest hydroxytyrosol concentrations. This is clearly illustrated by curves (d) and (e). In these two solutions, Ecorr is shifted to the very low value of −0.453 V vs SCE, which indicates that elenolic acid, the main species in these two extracts, acts as a cathodic-type inhibitor. After Ecorr, the anodic curve bends progressively, its slope decreasing until point E1 found at −0.25 V vs SCE for SAH and −0.18 V vs SCE for SA (and SH). After point E1, the slope of the curve increases very rapidly to reach the value typical of the blank solution. The increasing hydroxytyrosol and elenolic acid concentrations also led to a decrease of the cathodic current, as revealed by the shift of the cathodic branch of curve (e) to lower current densities. Actually, the very low Ecorr value obtained in SA and SAH solutions is rather typical of deaerated conditions [4,34]. In agreement with this assumption, a linear Tafel behavior is observed in the cathodic part of the curve, which indicates that the cathodic process is mainly water reduction. This would mean that O2 reduction is almost totally inhibited, more likely because O2 diffusion is strongly hindered by the layer of organic molecules adsorbed on the metal surface. The comparison of curves (d) and (e) shows that the increase of hydroxytyrosol and elenolic acid concentrations led to a similar decrease of both cathodic and anodic components of current density, which explains why Ecorr remained constant. This shows that the inhibiting organic species involved here are blocking, through their adsorption of the metal surface, both anodic and cathodic sites. At point E1, around −0.2 V vs SCE, the anodic sites are no more blocked and the anodic reaction rate becomes identical to that observed in the blank solution. This may be due to a partial desorption of the organic molecules. Note that some beneficial effect of the inhibiting species is also observed for the highest potentials. The limiting current density is the lowest here with SA and SAH solutions. This shows that the organic molecules still interact with the electrode surface, hindering the diffusion of CuCl− 2 .

compound is the main component of the olive leaf extract and its concentration was estimated at 1.683 mmol L−1. Finally, the combination of acid hydrolysis and increased temperature (SAH solution) leads to the highest hydroxytyrosol and elenolic acid amounts, as revealed by Fig. 1d. Oleuropein could not even be detected in this case. Elenolic acid remains the main component, with a concentration estimated at 2.118 mmol L−1. The hydroxytyrosol concentration peaks now at 0.413 mmol L−1. 3.2. Voltammetry study All the polarization curves are displayed in Fig. 2 as log|j| vs E. They were acquired with copper electrodes immersed 24 h at RT in the various solutions. Curve (a) was obtained in the 0.5 M NaCl aqueous solution without olive leaf extract (blank). In the cathodic domain, the slope of the curve is very small and tends towards a horizontal asymptote, indicating that the cathodic reaction is (at least partially) controlled by diffusion. In the potential range considered here, from −0.55 V vs SCE to Ecorr (−0.205 V vs SCE), the cathodic reaction is the reduction of dissolved oxygen (5) and a partial control by diffusion could be expected, in agreement with previous studies [2,4]: O2 þ 2H2 O þ 4e− →4OH−

ð5Þ

The anodic region is more complex. First, a linear Tafel region is observed, corresponding to the dissolution of copper as CuCl− 2 [1,2,4], according to the global reaction (6): Cu þ 2Cl− →CuCl2 − þ e−

ð6Þ

This Tafel behavior is only “apparent” because a mixed charge transfer and mass transport controlling kinetic is usually assumed [1,2,4]. The current density increases up to a maximum, denoted as jmax, before to decrease to a local minimum, denoted as jmin, a phenomenon attributed to the formation of a CuCl film [4]. At the highest potential, the current density remains constant; the dissolution of CuCl film and metal is controlled by the diffusion of CuCl− 2 [4]. For solution S that mainly contains oleuropein, the polarization curve (b) shows similar features. The corrosion potential is shifted to a more negative value of −0.275 V vs SCE while the corrosion current density is significantly decreased. The anodic branch remains almost unchanged, excepted for the highest potentials where the limiting current density due to the diffusion of CuCl− 2 appears lower. In contrast, the cathodic branch is shifted towards lower current densities. This shows that the decrease of the corrosion current density is mainly due to the decrease of the cathodic reaction

3.3. Modelling of the polarization curves The polarization curves of Fig. 2 were used to determine the corrosion current density (jcorr) and the inhibition efficiency (IE) in each case. To obtain additional information on the kinetic of the cathodic and anodic reactions, and to increase the accuracy of the determination of jcorr, the part of the curve surrounding Ecorr was modelled as described briefly in Section 2.4 and in more details in previous works [20,28]. The case of the blank solution can be considered first, as the corrosion process of copper in aerated NaCl solutions has already been extensively studied (e.g. [1–5],[34–36]. The anodic reaction can be modelled by Tafel law, even though it is partially controlled by mass transport (“apparent” Tafel behavior [1,2,4]). The cathodic reaction is, in the potential range considered here, under mixed diffusion/activation control. The diffusion plateau, i.e. the potential range where O2 diffusion is totally controlled by charge transfer is found at lower potentials, for instance E < −0.75 V vs SCE according to [2]. The mathematical expression of j(E) is then that of Eq. (4) given in Section 2.4. It was used to fit the experimental polarization curve.

Fig. 2. Polarization curves of copper obtained after 24 h in 0.5 M NaCl solution without (blank) and with olive leaf extract (S, SH, SA and SAH) at RT. 4

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Journal of Electroanalytical Chemistry 859 (2020) 113834

The results of the computer fitting procedure are displayed in Fig. 3 and the data are listed with those obtained for the other polarization curves in Table 2. The part of the curve considered for the computer fitting starts at −0.5 V vs SCE and ends at −0.1 V vs SCE. It includes the most part of the cathodic branch and the linear part of the anodic region. Fig. 3 clearly shows that the experimental curve could be adequately fitted (R2 coefficient = 0.9997). The anodic branch is perfectly fitted with Tafel law, as it was expected from previous works [1,2,4]. The determined Tafel coefficient βa of 39 V−1 corresponds to an apparent Tafel slope ba of 59 mV/decade, consistent with previously determined values found between 59 and 60 mV/decade [1,2,4]. The cathodic part, except for the oscillations visible at −0.29 V and − 0.38 V that were not taken into account, could also be successfully fitted with the expression of jc given by Eq. (3) and derived from the Koutecky-Levich equation (see Section 2.4). This demonstrates that this equation is valid for a mixed diffusion/activation controlled kinetic in the experimental conditions considered here. The polarization curve obtained in solution S, containing oleuropein as the main phenolic compound, was also fitted using Eq. (4) because it was similar to that obtained in the blank solution. The results are illustrated by Fig. 4. The accuracy of the computer fitting is even better in this case, with a R2 coefficient of 0.9999. The anodic Tafel coefficient βa of 38 V−1 (i.e. ba = 60.6 mV/decade) is almost unchanged and thus remains typical of copper in 0.5 M NaCl solutions. The inhibitor, in this case mainly oleuropein, does not affect the anodic reaction. In contrast, the kinetic parameters of the cathodic reaction are modified by the inhibitor, the cathodic Tafel coefficient βc decreasing from −10 V−1 to −17 V−1 and the limiting current density jlim decreasing (in absolute value) from 46 to 36 μA cm−2. As already discussed above and in previous study [20], the inhibitor is in this case of cathodic-type. The decrease of Ecorr measured here is however small (70 mV) but was found equal to 85 mV [20] with a higher inhibitor concentration, which is sufficiently high to classify oleuropein as a cathodic-type inhibitor rather than a mixed-type inhibitor [37]. Note that the βc values determined here, corresponding to cathodic Tafel slopes bc of −230 and −135 mV/decade, are also consistent with previous studies [4]. The inhibiting action of the organic species issued from the olive leaf extract is illustrated by the decrease of jcorr. It can be quantified via the inhibition efficiency (IE), calculated according to the following equation: IEð%Þ ¼

j0 corr −jcorr x 100 j0 corr

Table 2 Electrochemical kinetic parameters and inhibition efficiency obtained from polarization curves (Figs. 2-5) at RT. The accuracy of the determined parameters is about ±0.5 mV for Ecorr, ±10% for jcorr, ±1 V−1 for the Tafel coefficients and ± 20% for jlim. Solution

Blank S SH SA SAH

Ecorr

βa

jcorr

(V vs SCE)

(μA cm

−0.205 −0.275 −0.289 −0.453 −0.453

10.1 1.8 1.0 1.9 0.26

−2

)

(V

βc −1

39 38 25 13 13

)

(V

jlim −1

−10 −17 −18 −11 −13

)

(μA cm −46 −36 −14 – –

IE −2

)

(%) – 82.2 90.1 81.2 97.4

Fig. 4. Experimental polarization curve of copper obtained after 24 h at RT in solution S, computed curve and corresponding anodic ja and cathodic jc components of the current density.

In this equation, j0corr is the reference corrosion current density, measured in the considered aggressive medium without inhibitor. In contrast, jcorr is the corrosion current density obtained with inhibitor and consequently jcorr < j0corr. For solution S, IE is determined at 82.2%. The polarization curve obtained in solution SH was also fitted using Eq. (4). Only the part ranging from −0.5 V vs SCE to −0.2 V vs SCE was considered (curve not presented). Actually, as discussed above, the slope of the anodic curve changes at point E1, i.e. at −0.18 V vs SCE. The presence of the inhibiting species led in this case to a change, before E1, of the anodic Tafel coefficient, as it was already visible in Fig. 2. βa is now equal to 25 V−1, which corresponds to ba = 92 mV/decade. Such an increase of the anodic Tafel slope, from 60 mV/decade to 90 mV/decade or even higher values has already been observed [4]. This modification of the apparent Tafel slope is due to a change in the balance between charge transfer and mass transport control [4]. In our case, due to the increased inhibiting effect (IE = 90.1% in this case), the kinetics of the charge transfer has decreased to a level where mass transport no longer (or lesser) influences the anodic reaction rate. These changes are more likely mainly due to hydroxytyrosol, the phenolic compound generated (from oleuropein) by the increase of the hydrolysis temperature. As hydroxytyrosol affects the anodic reaction, it cannot be, in contrast with oleuropein, a cathodic-type inhibitor. Actually, the apparition of hydroxytyrosol in the solution, which is the main difference between S and SH, did not lead to a significant variation of Ecorr (only 14 mV). Hydroxytyrosol may then rather be a mixedtype inhibitor, influencing both anodic and cathodic reactions. The cathodic Tafel coefficient (βc = 18 V−1) is similar to that measured in solution S (βc = 17 V−1) but the limiting current density has decreased (in absolute value) significantly, from 36 μA cm−2 in solution S to

ð7Þ

Fig. 3. Experimental polarization curve of copper obtained after 24 h at RT in 0.5 M NaCl solution (blank), computed curve and corresponding anodic ja and cathodic jc components of the current density. 5

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14 μA cm−2 in solution SH. This confirms that hydroxytyrosol also affects the cathodic reaction, i.e. is rather a mixed-type inhibitor. This confirms that the strong decrease of Ecorr observed with SA and SAH extracts is due to elenolic acid that is clearly a cathodic-type inhibitor. The polarization curves obtained in solutions SA and SAH that share similar features were treated similarly. The results obtained for SAH are displayed as an example in Fig. 5. For these two curves, in contrast with the three previous ones, the cathodic branch was characterized by a Tafel linear behavior while the anodic branch did not. The fit was consequently performed from the most cathodic potential, i.e. −0.55 V vs SCE, to a potential close to Ecorr + 50 mV, i.e. −0.4 V vs SCE. It was assumed that in this potential range, the anodic reaction rate did obey Tafel law, i.e. that the discrepancy with Tafel law appeared only at high overvoltage. The curves were then fitted using Eq. (1) presented in Section 2.4. This assumption is questionable and the obtained βa value (13 V−1 for both SA and SAH solutions) must be considered cautiously. However, the estimated value of jcorr relies also on the fitting of the cathodic part of the curve, which should limit the influence of the kinetic model considered for the anodic reaction. The inhibition efficiency is determined at 81.2% in SA solution and 97.4% in SAH solution. The significant difference between the IE values obtained in these two solutions is difficult to explain because both solutions contain relatively high amounts of hydroxytyrosol and elenolic acid. In any case, the various attempts to fit one voltammogram converged on one unique mathematical solution. The problems encountered while fitting voltammograms acquired on a short potential range (ΔE = OCP ± 30 mV; i.e. ΔE = 60 mV, [28]) were then solved by increasing the analyzed potential range. In the present work, potential ranges from ΔE = 150 mV to ΔE = 400 mV were considered, which proved sufficient to determine a reliable (i.e. a unique) value of the corrosion current density (and any other electrochemical parameter). As for any other kind of fitting procedure, the experimental data must contain a sufficient amount of information with respect to the number of adjustable parameters.

the diagram increases strongly, with confirms the inhibiting effect already clearly observed via voltammetry. The strongest inhibiting effect is observed with SAH solution. More accurate information was obtained via the analysis of the impedance data with electrical equivalent circuits (EEC). The two considered EEC are displayed in Fig. 7. In these circuits, Rs is the solution resistance, Rct is the charge transfer resistance, Qdl corresponds to the capacitance of the double layer, Rf and Qf are associated with the film of corrosion products formed on the metal surface and W is the Warburg impedance related to the diffusion processes in the low frequency region. Actually, Qdl and Qf are constant phase elements (CPE) used in place of capacitors to compensate for deviations from ideal dielectric behavior that are due to the heterogeneous nature of the electrode surface. The results obtained after the computer fitting of the experimental impedance data are listed in Table 3. The parameter n describes the departure of the electrode from an ideal surface, which corresponds to n = 1 (and the CPE to an ideal capacitor). In table3, n1 and n2 are associated with Qf and Qdl respectively. The choice of these two EEC was made using an approach based on the four points described hereafter: (1) If the presence of two time constants was not evidenced by the EIS plots, then only Rs, Rct and Qdl were used. This first trial revealed that Rf and Qf were required in any case to obtain a satisfactory fitting. The physical meaning of Rf and Qf was then attributed to the presence of a very thin film of corrosion products, that was seen visually at the end of experiments performed in the “blank” solution. (2) The aim is to study the evolution of the same corrosion system, i.e. copper in 0.5 M NaCl solution, as inhibiting species are added. For that reason, the modelling was made with a number of different EEC as low as possible, and a number of differences between the various EEC as low as possible too. (3) CPEs were not systematically used. A first modelling was performed with capacitances but did not lead in most cases to an acceptable result. Thus, a CPE was used in any case according to point (2). As it can be seen in Table 3, the CPE exponents are all far from 1 (between 0.65 and 0.9) except in one case (n2 for the blank). The coefficient n2 = 0.99 for the blank actually indicates that Qf is here a pure capacitance. (4) Similarly, the Warburg element was omitted a priori, i.e. the EEC of Fig. 7a was first used except when the EIS plots clearly revealed the influence of a diffusion process (case of the “blank”). The Warburg element was then added only if necessary, i.e. if a satisfactory fitting could not be obtained (case of “S” extract). The values of n1, obtained for SA and SAH, i.e. 0.66 and 0.65, respectively, are however rather low. In principle, if n = 0.5, the CPE corresponds to a Warburg

3.4. Electrochemical impedance spectroscopy (EIS) Fig. 6 shows the Nyquist diagrams obtained for pure copper electrodes after 24 h of immersion in 0.5 M NaCl solutions without (blank) and with olive leaf extract (S, SH, SA and SAH). For the blank, the diagram mainly shows a capacitive loop at high frequency and an almost linear curve, which makes an angle of 45° with the Re(Z) axis, at low frequency (see also Fig. 8). This shape clearly shows that diffusion influences significantly the corrosion process. In the presence of olive leaf extract, the diameter of

Fig. 5. Experimental polarization curve of copper obtained after 24 h at RT in solution SAH, computed curve and corresponding anodic ja and cathodic jc components of the current density.

Fig. 6. Nyquist plots for copper electrodes immersed in 0.5 M NaCl solutions at RT for 24 h without (blank) and with various olive leaf extracts (solutions S, SH, SA and SAH). 6

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Journal of Electroanalytical Chemistry 859 (2020) 113834

Fig. 7. Equivalent electrical circuits used to fit the EIS experimental data.

Fig. 8. Experimental Nyquist diagram and its mathematical fitting: Example of copper in 0.5 M NaCl solution (blank). Squares: experimental curve, line: computed curve.

Table 3 Electrochemical impedance parameters for copper electrodes after 24 h in 0.5 M NaCl solutions without (blank) or with olive leaf extract, i.e. solutions S, SH, SA and SAH, at RT. Solution

Rs (Ω cm2) Rct (Ω cm2) Qdl (10−3 Ω−1 cm−2 sn1) n1 Ceff,dl (μF cm−2) Rf (Ω cm2) Qf (10−6 Ω−1 cm−2 sn2) n2 W (Ω−1 cm−2 s0.5) IE %

Blank

S

SH

SA

SAH

8.5 ± 3 767 ± 17 0.15 ± 0.02 0.79 ± 0.03 84 ± 14 382 ± 30 100 ± 9 0.99 ± 0.01 257 ± 18 –

8±3 6250 ± 200 0.089 ± 0.028 0.72 ± 0.03 71 ± 30 1156 ± 20 23 ± 3 0.9 ± 0.01 528 ± 50 79.3

7.4 ± 3.6 9900 ± 400 0.079 ± 0.01 0.7 ± 0.02 71 ± 13 2176 ± 120 22.6 ± 3.3 0.9 ± 0.03 – 86.9

11.34 ± 0,1 12,367 ± 500 0.055 ± 0.02 0.66 ± 0.06 45 ± 3 3216.5 ± 150 20.16 ± 1.7 0.89 ± 0.011 – 89.5

8.51 ± 1 17,588 ± 800 0.013 ± 0.008 0.65 ± 0.005 6±5 3380 ± 250 21.8 ± 0.9 0.89 ± 0.02 – 92.6

element. These low values then indicate that diffusion may play a role in these two cases. For the blank solution, as explained above (point 1), it was first considered that the modelling could be done with only one capacitive loop and a Warburg impedance, i.e. omitting Rf and Qf in circuit (b) of Fig. 7. However, it proved impossible to obtain a satisfactory result. The possible influence of the thin film of corrosion products observed visually after the experiment was then considered and circuit (b) of Fig. 7 was used. The Nyquist diagram of Fig. 8 and the Bode plots of Fig. 9 illustrate the accuracy of the computer fitting. For the solutions containing olive leaf extracts two cases can be distinguished. For solution S, the EEC (b) of Fig. 7 was used again. This is consistent with the analysis of the polarization curves (Section 3.3) that showed a mixed activation/diffusion control of the cathodic reaction for copper in solution S (and in blank as well). For other solutions, the Warburg impedance proved unnecessary and the impedance data could be modelled with circuit (a) of Fig. 7. For solutions SA and SAH, this result is fully consistent with the analysis of the polarization curves, which revealed that the cathodic reaction was controlled by charge transfer as it predominantly corresponded to water reduction (see Section 3.2). The CPE coefficient n1 is however rather low, equal to 0.65 or 0.66, which suggests that the reduction of dissolved O2, controlled by diffusion, is involved as a minor cathodic process. However, the polarization curve of copper in solution SH was fitted using a mixed activation/diffusion controlled cathodic reaction.

Fig. 9. Experimental Bode plots and their mathematical fitting: Example of copper in 0.5 M NaCl solution (blank). Squares: experimental curves, lines: computed curves. 7

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Nevertheless, it must be recalled that EIS is performed at OCP whereas the polarization curves explore a large cathodic range of potentials. For solution SH, EIS shows that the cathodic reaction is mainly controlled by charge transfer at OCP, even though the low value of n1, equal in this case to 0.7, also suggests that the diffusion of dissolved O2 slightly influences the kinetic of the corrosion process. Voltammetry actually shows that the influence of diffusion increases when the potential decreases and finally becomes important as the cathodic current density tends towards a diffusion plateau at the most cathodic potentials (Fig. 2). The Nyquist diagram and Bode plots of copper in solution S are given as examples in Figs. 10 and 11, respectively, to illustrate the accuracy of the computer fitting. The overall data, listed in Table 3, show that various parameters vary in the presence of the olive leaf extracts. First, the charge transfer resistance, Rct, and the resistance associated with the film formed on the metal surface, Rf, increase. They both follow the sequence: SAH > SA > SH > S > blank. The increase of Rf and Rct is accompanied with a decrease of Qdl, the CPE used as double layer capacitance. The values are found in a sequence opposite to that of Rf and Rct: Blank > S > SH > SA > SAH. To investigate more precisely this particular point, the effective double layer capacitance Ceff,dl was calculated from the CPE parameters using the approach developed by Huang et al. [38] based on the formula proposed by Brug et al. [39]. The values obtained (Table 3) show clearly that Ceff,dl decreases only slightly with S and SH leaf extracts where oleuropein predominates, and strongly with SA and SAH where elenolic acid predominates. According to the Helmoltz model [29,40], the double layer capacitance decreases if the relative permittivity of the electrolyte solution decreases or if the thickness of the double layer increases. This decrease of Ceff,dl then clearly shows the influence of the adsorbed organic species. The difference observed via EIS between oleuropein and elenolic acid is fully consistent with that observed via voltammetry, the adsorption of elenolic acid inducing a strong decrease of Ecorr (250 mV) while that of oleuropein only induces a moderate decrease of Ecorr (70 mV). Similarly, the capacitive element Qf associated with the film formed on the metal is affected by the presence of the organic molecules. Qf is about 100 × 10−6 Ω−1 cm−2 sn2 (with n2 = 0.99) in the blank solution and approximately constant at about 20–23× 10−6 Ω−1 cm−2 sn2 (with n2 = 0.89–0.9) in other solutions. This finding suggests that the organic molecules also adsorb on the film of corrosion products thus preventing its dissolution and increasing its protective ability. The inhibition efficiency can also be computed from the results of the EIS analysis. To take into account the beneficial effects of the inhibiting species on both Rf and Rct, the polarization resistance Rp, that is the sum of Rf

and Rct, is considered and IE is then given by: IEð%Þ ¼

Rp −R0p x 100 Rp

ð8Þ

where R0p and Rp are the total polarization resistance of copper in the solution in the absence and presence of inhibitor, respectively. The lowest IE value is found for solution S and is equal to 79.3% while the highest IE is for solution SAH and is equal to 92.6%. This result is fully consistent with that deduced from voltammetry, i.e. IE = 82.2% for solution S and IE = 97.4% for solution SAH. Similarly, both methods gave an intermediate value for the IE of solution SH, that is IE = 86.9% (EIS) and 90.1% (voltammetry). The discrepancy is however significant in the case of solution SA. EIS leads to an IE of 89.5%, better than that of S and SH, while voltammetry leads to an IE of 81.2%, lower than that of SH and even slightly lower than that of S. This discrepancy more likely comes from the simplifying assumption made on the kinetic of the anodic reaction (see Section 3.3) used to determine jcorr from the polarization curve. 4. Conclusions - The aqueous extracts of olive leaf are greatly rich in phenolic compounds, mainly oleuropein at pH = 5.8 and 25 °C. After acid hydrolysis (pH = 2) and high temperature (100 °C) of extraction, hydroxytyrosol and elenolic acid (main compound) are predominant. - In 0.5 M NaCl solutions, the inhibition efficiency was found between 81% and 95% after 24 h of immersion. The best result was obtained after acid hydrolysis and high temperature of extraction, i.e. when the hydroxytyrosol and elenolic acid concentrations were the highest. It was determined at an average of 95% (97.4% with voltammetry, 92.6% with EIS). The lowest inhibition efficiency was observed after hydrolysis at 25 °C and pH = 5.8, at an average of 81% (82.2% with voltammetry, 79.3% with EIS). - A mathematical modelling of the voltammograms was achieved to determine all relevant electrochemical parameters. The fitting procedure led to a unique solution in any case because the analyzed potential range was sufficiently large, i.e. contained a sufficient amount of information with respect to the number of adjustable parameters. - Oleuropein acts as a mixed- or slightly cathodic-type inhibitor, a result in agreement with previous study [20]. Its inhibiting effect is due to the physical adsorption of the organic molecule on the metal surface [20]. Elenolic acid, the main extracted phenolic compound in acidic conditions is clearly a cathodic-type inhibitor while hydroxytyrosol may rather be a mixed-type inhibitor. - This study confirms that olive leaf, a by-product of the olive oil industry considered as industrial waste, may provide possible efficient corrosion inhibitors. Data availability The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request. CRediT authorship contribution statement Philippe Refait: Supervision, Conceptualization, Investigation, Validation, Visualization, Formal analysis, Writing - original draft, Writing - review & editing. Chahla Rahal: Investigation, Validation, Visualization, Formal analysis.Mohamed Masmoudi: Supervision, Conceptualization, Investigation, Validation, Formal analysis, Writing - review & editing. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 10. Experimental Nyquist diagram and its mathematical fitting: Example of copper in solution S. Squares: experimental curve, line: computed curve. 8

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Fig. 11. Experimental Bode plots and their mathematical fitting: Example of copper in solution S. Squares: experimental curves, lines: computed curves.

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