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Olive leaf extract as natural corrosion inhibitor for pure copper in 0.5 M

NaCl solution: A study by voltammetry around OCP

Chahla Rahal, Mohamed Masmoudi, Ridha Abdelhedi, René Sabot, Marc

Jeannin, Mohamed Bouaziz, Philippe Refait

PII: S1572-6657(16)30106-0

DOI: doi: 10.1016/j.jelechem.2016.03.010

Reference: JEAC 2540

To appear in: Journal of Electroanalytical Chemistry

Received date: 6 October 2015

Revised date: 9 March 2016

Accepted date: 12 March 2016

Please cite this article as: Chahla Rahal, Mohamed Masmoudi, Ridha Abdelhedi,René Sabot, Marc Jeannin, Mohamed Bouaziz, Philippe Refait, Olive leaf extractas natural corrosion inhibitor for pure copper in 0.5 M NaCl solution: A study

by voltammetry around OCP, Journal of Electroanalytical Chemistry (2016), doi:10.1016/j.jelechem.2016.03.010

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http://dx.doi.org/10.1016/j.jelechem.2016.03.010http://dx.doi.org/10.1016/j.jelechem.2016.03.010http://dx.doi.org/10.1016/j.jelechem.2016.03.010http://dx.doi.org/10.1016/j.jelechem.2016.03.010


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Olive leaves extract as natural corrosion inhibitor for pure copper

in 0.5 M NaCl solution: a study by voltammetry around OCP

Chahla RAHAL1, Mohamed MASMOUDI1 , Ridha ABDELHEDI1, René SABOT2, Marc

JEANNIN2, Mohamed BOUAZIZ1, Philippe REFAIT2, *

1 Laboratory of Electrochemistry and Environment (LEE), Sfax National Engineering School

(ENIS) BPW 3038 Sfax, University of Sfax, Tunisia.2 Laboratoire des Sciences de l’Ingénieur pour l’Environnement (LaSIE),

UMR7356 CNRS - Université de La Rochelle, Bât. Marie Curie, Av. Michel Crépeau,

17042 La Rochelle cedex 01, France.

*Corresponding author.

Tel.: (33) 5 46 45 82 27 / Fax: (33) 5 46 45 72 72 / E-mail address: [email protected]

Abstract

The inhibiting action of olive leaves extract on corrosion of copper in 0.5 M NaCl solution

was investigated via potentiodynamic polarization and electrochemical impedance

spectroscopy. For the highest inhibitor concentration considered here the inhibition efficiency

reached 90% after 24 hours of immersion. Polarization curves recorded around OCP were

computer fitted with various kinetic laws to obtain detailed information on the inhibition

process. This innovative procedure led to results consistent with those deduced from

electrochemical impedance spectroscopy and confirmed that the olive leaves extract act as a

cathodic-type corrosion inhibitor mainly hindering the transport and thus the reduction of

dissolved O2. High performance liquid chromatography showed that oleuropein was the major

compound of the leaves extract and thus more likely the main inhibiting species.

Keywords: Olive leaves extract; Oleuropein; Voltammetry; Copper; Corrosion; HPLC; EIS


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1. Introduction

Corrosion inhibitors are commonly used in industry to reduce the corrosion rate of

metals and alloys. For copper protection in chloride-containing media, various substances

were considered [e.g. 1-8], and it was shown that nitrogen, sulfur and aromatic containing

organic compounds reduced efficiently the corrosion rate. The inhibiting action of these

organic compounds is usually attributed to their adsorption on the metal surface, the polar

functional groups playing the main role in the adsorption process [5]. In general, this process

depends on the chemical structure of the inhibitor, the nature and surface charge of the metal,

the adsorption mode and the electrolyte [8].

However, most of these compounds are synthetic chemicals which may be very

expensive and hazardous to living creatures and environments. Because of economic and

environmental factors, an inhibitor must not only be efficient but also cheap, nontoxic and

innocuous in the environment. Various parts of plants proved to contain several compounds

that satisfy these criteria and plants thus constitute a potential source of new corrosion

inhibitors. Numerous studies were recently achieved to determine the effects of some

naturally occurring substances on the corrosion of various metals, including copper, in

different corrosive media [e.g. 9-22].

In the Mediterranean coastal zone, olive leaves are one of the by-products of farming

of the olive grove; they exist in high amounts in the olive oil industries (10% of the total

weight of the olives). They also accumulate during the pruning of the olive trees [23]. Olive

leaves are a cheap raw material that can however be the source of high-added value products,

e.g. phenolic compounds. This has recently raised interest for their chemical composition and

specific properties. It was for instance shown that the olives leaves extract has anti-oxidative,

anti-inflammatory and antimicrobial activities against bacteria and fungi. It also exhibits anti-

viral activities against several species like haemorragic septicaemia rhabdo virus [24].


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Following the same line, the present study aimed to determine the efficiency of a

natural olive leaves extract as a corrosion inhibitor for pure copper in 0.5 M NaCl solution.

The organic species present in the extract were characterized by high performance liquid

chromatography. Electrochemistry was used to study the evolution over time of the inhibition

efficiency during 24 hour experiments, the influence of inhibitor concentration and the role of

temperature. Results obtained with electrochemical impedance spectroscopy (EIS) were

compared to those given by voltammetry. Polarization curves were recorded as usual on a

wide range of potential (from -0.55 V/SCE to + 0.6 V/SCE) at the end of the experiment.

Additionally, other polarization curves were recorded before each EIS analysis. They were

acquired around the open circuit potential (OCP) to limit modifications of the electrode

surface. All these curves were analyzed via a computer fitting procedure which provided

information about the influence of the inhibitor on the cathodic process. This innovative

approach proved a useful complementary method to EIS.

2. Experimental

2.1. Electrodes and electrolytes

The work electrodes were all made with the same pure copper (99.99%) rod of 6 mm

in diameter. The surfaces were prepared via an abrading procedure with silicon carbide (from

grade 240 to grade 1200), washed with distilled water and degreased with acetone. The 0.5 M

NaCl solution used as aggressive medium was prepared with 98% min. purity NaCl and de-

ionized water (resistivity 18.2 M cm). The solutions were not stirred during the

experiments. The influence of the olive leaves extract concentration was studied at room

temperature (RT) i.e. at 25±2°C when the experiments were performed (may-june). The effect

of temperature on the inhibition efficiency was studied in the 25 – 55°C range. A thermostat


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controlled the temperature with an accuracy of ±0.5°C. The evolution of the system was

followed with time during 24 hours.

2.2. Extraction solution and extract preparation

The olive leaves were first air-dried at room temperature (~25°C) for one month in the

absence of light. Then, 100 g of powdered leaves were mixed with 1 L of de-ionized water

under stirring for 1 h at 75°C. After cooling, the mixture was filtered through a filter paper

and the filtrate, stored in a freezer at 4°C, was used during the following days. The pH of the

filtrate was measured at 5.8 after preparation. The obtained leaves extract were added to 0.5

M NaCl solutions in various amounts (see section 3. 1.).

2.3. Characterization of the olive leaves extract by HPLC.

The quantitative analysis of olives leaves extract was carried out using an Agilent

Technologies series 1100 liquid chromatography system (HPLC, Agilent Technologies,

Karlsruhe, Germany) equipped with an automatic injector, composed of a vacuum degasser, a

quaternary pump, a column oven and a diode array detector (DAD). An Eclipse XDB-C18

column (250 x 4.6 mm, i.d., 5 µm particle size; Waters Co., Milford, MA) was used at room

temperature (25◦C) with an injection volume of 10 µL. The mobile phase was composed of

0.25% acetic acid in water (solvent A)/ methanol (solvent 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 then 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 [25]. Peaks were identified by congruent retention times compared with

available standards. Each phenolic compound was quantified in comparison with its standard


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when it was available while the other detected compounds were quantified by other equivalent

compounds.

2.4. Electrochemical measurements

Electrochemical measurements, including potentiodynamic polarization and

electrochemical impedance spectroscopy (EIS) were performed in a classical three-electrode

cell. Pure copper, platinum foil and saturated calomel electrode (SCE: XR110, Radiometer-

Analytical) were used as working, counter and reference electrodes, respectively. The open-

circuit potential (OCP) was recorded as a function of time except during the potentiodynamic

polarization and EIS experiments. The first series of potentiodynamic polarization around

OCP and EIS experiments was performed after 1 hour, a time necessary for the OCP to reach

a quasi-stationary value, and the other series after 6 and 24 hours.

These polarization curves were acquired on a limited range of potential around OCP to

induce only a minor perturbation of the metal surface. The potential was swept from the OCP

to OCP+70 mV, then down to OCP-70 mV and back to OCP, at a scan rate d E /dt =

0.5 mV s-1. Each curve was analyzed using a computer fitting procedure described in section

2.5. The EIS measurements were carried out in each case just after the acquisition of the

polarization curve. Impedance diagrams were obtained over a frequency range of 100 kHz to

10 mHz with ten points per decade using a 10 mV peak-to-peak sinusoidal voltage. The

linearity of the system was checked in varying the amplitude of the ac signal applied to the

sample. The impedance spectra were fitted using electrical equivalent circuits with EC-Lab

software (Bio-Logic).

Finally, after the last EIS experiment, i.e. approximately after 24 hours of immersion,

a polarization curve was drawn from -0.55 V/SCE to + 0.6 V/SCE at a sweep rate d E /dt = 0.5

mV s-1

.


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Each experiment was performed at least three times and the values given for the

various determined parameters are the average of the various measurements.

2.5. Voltammetry around OCP ( E = ±70 mV): methodology

The determination of corrosion rates by electrochemical methods generally implies to

modify the potential of the metal electrode. The range of this variation of potential has

however to be kept small so as to limit modifications of the electrode/electrolyte interface and

obtain information on the processes really occurring at OCP. In a recent study [26], it was for

instance observed that after cathodic polarization, the corrosion potential E corr of the metal

could be decreased of 200 mV because of O2 consumption and increase of interfacial pH,

even if a rather small scan rate of 0.2 mV/s was used (the variations of E corr should increase

with increasing scan rate). Moreover, if the potential range is really important, the method

must be considered as destructive because the modifications of the interface can be significant

and irreversible. For such reasons, the measurement of the polarization resistance R p

performed on a small potential range (typically ±15 mV around OCP) may be more adequate.

The method called here “voltammetry around OCP” involves a larger range, for

instance ±70 mV as in the present study, but assumed to be sufficiently small to have only a

small impact of the metal/electrolyte interface. The aim is not to determine R p (in principle,

the difference between the experimentally determined R p and the true R p value increases with

the potential range) but to obtain information on the kinetics of the anodic and cathodic

processes occurring at OCP. However, none of the two processes can be neglected with

respect to the other at the vicinity of OCP. As a consequence, no linear part could be observed

on the log|j| vs. E curve acquired around OCP even if both reactions should obey Tafel’s law.

For this reason, a fitting procedure of the experimental polarization curve is required, based

on electrochemical kinetic laws. The main advantage with respect to the so called Tafel


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method usually used on polarization curves acquired on a wider potential range is that various

kinetic laws can be used, in particular for the cathodic reaction (typically O2 reduction).

The experimental polarization curves j vs. E were computer fitted assuming that the

anodic reaction obeys Tafel’s law and that the cathodic reaction, i.e. the reduction of O2, was

under mixed activation-diffusion control. The expression of the anodic current ja is then:

where a is the anodic Tafel coefficient, in V-1.

The expression of the cathodic current jc, derived from the Koutecky-Levich equation, is:

where c is the cathodic Tafel coefficient, in V-1, and jlim the limiting current density.

The experimental function used for the mathematical modeling of the potentiodynamic

polarization curve j vs. E is then:

The corrosion potential ( E corr ) was directly read on the log| j| vs. E curve while the

corrosion current density ( jcorr ), the Tafel coefficients β a and β c and limiting cathodic current

density jlim were obtained via the computer fitting of the potentiodynamic polarization curves.

In some cases, the cathodic part of the log| j| vs. E curve was linear. The computer

fitting of the j vs. E curve was then achieved using Tafel’s law for both anodic and cathodic

currents. The experimental function used for the mathematical modeling of the curve is in this

case:

In each case, only the part of the curve corresponding to the negative-going scan from

OCP +70 mV down to OCP -70 mV was computer fitted.


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3. Results and discussion

3.1. Phenolic compounds contents of the studied olive leaves extract

The determination of phenolic compounds contents of the olive leaves extract was

carried out by high performance liquid chromatography (HPLC) using a solution obtained

with 100 g of powdered leaves mixed with 1 L of distilled water. The HPLC chromatogram

(Fig. 1) shows several peaks corresponding to phenolic compounds. Eight phenolic

compounds are identified in the olive leaves extract, namely, 1: hydroxytyrosol, 2: tyrosol: 3:

caffeic acid, 4: pcoumaric acid, 5: luteolin 7- glucoside, 6: apigenin 7- glucoside,7:

verbascoside and 8: oleuropein, in agreement with previous works [25,27-29]. The most

abundant compound was oleuropein, as it was also reported by several authors [28,29].

The quantitative study led to an estimated concentration of oleuropein of 2.42

mmol L-1 (C1). For this study the solution C1 was diluted two and five times which led to the

solutions C2 (1.21 mmol L-1) and C3 (0.48 mmol L-1).

3.2. Effect of olive leaves extract concentration

3.2.1. Potentiodynamic polarization between -0.55 and +0.6 V/SCE

Figure 2 shows the polarization curves obtained on a large potential range at the end of

the 24 hours of immersion in the various olive leaves extract concentrations C1-C3. They are

compared to the curve obtained in similar conditions in a 0.5 M NaCl solution without olive

leaves extract, solution referred to as “blank ”.

In the potential range considered here, the cathodic reaction is mainly the reduction of

dissolved oxygen:

The anodic part of the curve shows three main regions of potential. In region (I) an apparent

Tafel behavior is observed and the current density increases up to jmax when the potential


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increases. The dissolution process of copper in the presence of Cl- ions proceeds via a two-

step reaction mechanism [30,31]. The first step leads to the formation of CuCl adsorbed on

the metal surface according to the following reaction:

Copper dissolution occurs in the second step via the formation of CuCl2- complex:

In region (I), mixed charge transfer and mass transport controlling kinetics are usually

assumed and thus the Tafel behavior of the curve is only “apparent”. In region (II), the current

density decreases from jmax to jmin due to the formation of a CuCl film [30]. In region (III), the

potential increases again before to stabilize when it reaches a limiting current density. The

dissolution of the film and/or metal is then controlled by the diffusion of CuCl2- [31].

The presence of olive leaves extract results in a marked shift of the cathodic branch of

the polarization curve towards smaller current density values, and to a lesser extent, of the

anodic branch. This effect is increased when the inhibitor concentration is increased and

reaches a maximum for the largest considered concentration C1. As a result, the corrosion

potentials shifted to more negative values in presence of olive leaves extract, which confirms

that this extract has a stronger influence on oxygen cathodic reduction than on copper

oxidation reaction. The E corr values measured for the blank, C3, C2 and C1 solutions are

respectively equal to -210, -222, -278 and -295 mV/SCE. So the E corr shift reaches 85 mV for

the highest concentration (C1), a value sufficiently high to classify the olive leaves extract as

a cathodic inhibitor rather than a mixed inhibitor [32].

The beneficial effect of the inhibitor on the substrate is also illustrated by the shape of

the curves in region (III). For the two largest concentrations C1 and C2, the current density

remains stable at j = jmin when E increases. The adsorption of the inhibitor more likely hinders


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the formation of the CuCl2- adsorbed species (reaction 3) preventing the dissolution of the

CuCl film and/or favoring its formation.

3.2.2. Voltammetry around OCP ( E = ±70 mV)

The polarization curves obtained after 24 hours of immersion in the various solutions

on a limited potential range ( E = ±70 mV) were computer fitted as described in section 2.5

to determine the corrosion current density ( jcorr ), the corrosion rate (CR) and the inhibition

efficiency ( ). Figure 3 shows as an example the experimental polarization curve of copper

obtained after 24 h in 0.5 M NaCl solution, the computed curve and the corresponding anodic

ja and cathodic jc components of the current density.

The inhibition efficiency ( ) is then calculated according to the following equation:

where j0

corr and jcorr are the corrosion current densities in the absence and presence of

inhibitor, respectively.

The values obtained for the various determined parameters are given in Table 1. First

of all, it appears clearly that the corrosion rate decreases strongly with the increasing olive

leaves extract concentration, which definitively confirms that this extract contains at least one

species acting as a corrosion inhibitor, i.e. more likely oleuropein (OLE). The inhibition

efficiency reaches 86±5% for the highest concentration considered here.

The anodic Tafel coefficient a is approximately constant around an average value of

59±15 V-1. This value corresponds to a Tafel slope ba equal to 0.041±0.009 V decade-1, in

agreement with the values found for copper static electrodes in aerated neutral 3.0-3.5% g L-1

NaCl solutions that range between 0.045 and 0.060 V decade-1 [2]. The cathodic Tafel

coefficient c was determined at -26±5 V-1 in the blank solution but it decreases from -18±5


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V-1 to -33±2 V-1 with the increase of the olive leaves extract concentration, which may be

related to the fact that the inhibitor mainly influences the cathodic reaction. It is also

interesting to note that the curves obtained in the blank solution and with the smallest

inhibitor concentration C3 could be fitted in most cases with activation controlled cathodic

reaction (i.e. using equation 4). In contrast, O2 reduction was under mixed activation-diffusion

control for the larger C2 and C1 concentrations and equation 3 had to be used. Moreover, it

appears that the limiting cathodic current jlim decreases (in absolute value) when the inhibitor

concentration increases. This clearly shows that the inhibitor hinders oxygen reduction, i.e.

decreases the cathodic reaction rate.

Looking back to the polarization curves acquired on a large range of potential (figure

2), it can be seen that O2 reduction is completely controlled by diffusion in any solution (even

in blank) for the more cathodic potentials (around -0.5 V/SCE). In contrast, the computer

fitting of the polarization curves acquired around OCP showed that the kinetic of oxygen

reduction was whether partially controlled by diffusion or mainly controlled by charge

transfer but was never totally controlled by diffusion (in this last case jcorr = jlim). However,

one must recall that the results obtained by voltammetry around OCP relate to the kinetics at

OCP, which are those of main interest. The kinetic of the cathodic reaction may be partially

controlled by charge transfer at OCP and totally controlled by diffusion at more cathodic

potentials. Actually, in the case of a mixed activation – diffusion control, the limiting effects of

diffusion are increasing when the potential decreases, until jlim is finally reached for strongly

cathodic potentials. So the observation of a plateau ( j = jlim) at -0.5 V/SCE does not

necessarily means that the kinetic is mainly controlled by diffusion at OCP (i.e. -0.2 V/SCE);

it may be partially controlled by charge transfer. This gives another example of the relevance

of voltammetry around OCP.


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Some computer fittings of the polarization curves obtained in the blank required the

use of a mixed activation-diffusion controlled kinetic for oxygen reduction while others were

achieved using Tafel’s law for the cathodic process. It is probable that the impact of diffusion

was too small to be detected by voltammetry around OCP while EIS revealed the diffusional

process. The method may not be sufficiently accurate in some cases.

3.2.3. Electrochemical impedance spectroscopy (EIS)

Figure 4 shows the Nyquist diagrams obtained after 24 h of immersion in 0.5 M NaCl

solution without and with various concentrations of olive leaves extract at 25°C. The Nyquist

diagrams obtained without inhibitor (blank) and with the small inhibitor concentration C3

shows a depressed semicircle in the high frequency region. This high frequency semicircle is

mainly due to the charge transfer and double-layer capacitance [34]. For concentrations C2

and C1, and more particularly for C1, the shape of the diagram in the high frequency region

differs and appears to involve two overlapping semi-circles. The second semi-circle may

correspond to the film of corrosion products. For concentration C3, a slight inductive behavior

is observed at low frequency. It could be linked to a non-stationarity of the system. However,

the evolution of OCP was followed with time (results not shown) and it was observed that in

any case, the OCP was stable after less than 15 minutes. So even after 1 hour it seems that the

system has reached stationary conditions. Consequently, such a behavior may rather be due to

adsorption phenomena and thus may point out the adsorption of the inhibitor on the copper

surface. This inductive behavior was neglected, i.e. the data corresponding to the highest

frequencies of this peculiar impedance diagram were excluded for the modelling. Actually it

is masked by a stronger effect at the higher concentrations C1 and C2. For these two

concentrations and for the blank as well, the curves deviate from their semicircular shape at

low frequency, which may be related to a diffusion phenomenon, more likely that of dissolved


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O2. From the analysis of the potentiodynamic polarization curves obtained with E = ±70

mV, it was deduced that O2 reduction was under mixed activation-diffusion control for the

larger C1 and C2 concentrations and mainly under activation control at concentration C3. The

EIS results are therefore clearly consistent with those given by voltammetry. In the case of the

blank solution, both situations were met and the Nyquist plot shown here corresponds to a

case where the cathodic reaction was partially controlled by diffusion.

Finally, it is clearly visible that the diameter of the curve grows with increasing olive

leaves extract concentration. This indicates that the corrosion rate decreases due to the

inhibitor in agreement with voltammetry experiments.

In accordance with this preliminary description of the EIS results, the analysis of

impedance data were performed with the various equivalent circuits presented in Figure 5.

These models were used in various studies to describe the behavior of copper or copper alloys

in chloride containing solutions, with or without adsorption inhibitors [30,34-37]. That of fig.

5a proved suitable for the Nyquist plot of copper in the blank solution. In this model, Rs is the

solution resistance, Rct is the charge transfer resistance, Qdl corresponds to the capacitance of

the double layer and W is the Warburg impedance related to the diffusion processes in the low

frequency region. This shows that the contribution of the film of corrosion products formed

on the metal surface is negligible. In contrast, the equivalent circuits used for copper in

solutions containing olive leaves extract include a contribution of the film of corrosion

products via the resistance Rf and the element Qf (Figs. 5b and c). Actually, Qdl and Qf

represent constant phase elements (CPE) used in place of capacitors to compensate for

deviations from ideal dielectric behavior. These deviations arise from the inhomogeneous

nature of the electrode due to surface roughness, inhibitor adsorption, porous layer formation,

etc. that leads to a distribution of time constants on the electrode surface. The equivalent

circuit of fig. 5b does not include Warburg impedance and was used for concentration C3.


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The equivalent circuit of fig. 5c includes Warburg impedance and was used for concentrations

C2 and C1. A typical example of the results given by the computer fittings is presented in

figure 6. The model shows a good agreement with the experimental results for both Nyquist

and Bode plots.

The values obtained after the computer fitting of the Nyquist diagrams are listed in

Table 2. The parameters n1 and n2, associated with Qf and Qdl respectively, are fractional

parameters describing the departure of the electrode from an ideal surface: for n = 1 the CPE

describes an ideal capacitor. The inhibition efficiency was computed according to:

where R p0 and R p are total polarization resistance of copper in the solution in the absence and

presence of inhibitor, respectively. In each case, R p is the sum of Rf and Rct.

As it was already visible from the Nyquist plots, Rct increases with the inhibitor

concentration from 2500 to 22000 Ω cm2. The resistance Rf of the film of corrosion products

also increases with the inhibitor concentration up to 3000 Ω cm2, which explains why the

shape of the curves changes with the inhibitor concentration. However, the increase of R p is

mainly due to the increase of Rct with the inhibitor concentration. The inhibitor, by adsorbing

on the metal surface, blocks the cathodic sites so that only the small proportion of the surface

not covered by the inhibitor remains active. Schematically, if only 10% of the surface is not

blocked, then I corr is divided by 10. The current density is still expressed using the overall

surface S , and therefore jcorr (and also jlim) is then also divided by 10. In contrast Rct, expressed

in cm2, is multiplied by 10: Rct = Rct

mes × S = 10 ( Rctmes × S /10), where Rct

mes is the measured

charge-transfer resistance in , and Rctmes × S /10 is in first approximate equal to the Rct value

measured without inhibitor, i.e. describes the behavior of the metal surface not covered by the

inhibitor. As shown by the increase of Rf , the inhibitor also adsorbs on the film of corrosion


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products, favoring its stability and increasing its protective ability. This result is consistent

with what was observed on the polarization curves acquired in a large potential range (figure

2): the adsorption of the inhibitor on the CuCl film limits its dissolution at strongly anodic

potentials. So, even if the 90% efficiency of olive leaves extract is mainly due to the

adsorption of organic molecules directly on the metal surface there is also a slight

contribution (i.e. Rf /( Rf + Rct) = 12% of the inhibiting effect) coming from the adsorption of the

molecules on corrosion products that have formed on parts of the metal not protected by the

inhibitor.

Finally, it can be seen that the values of determined from EIS spectra (Table 2) are

very similar to those determined from the mathematical modelling of the potentiodynamic

polarization curves recorded around OCP (Table 1).

3.3. Effect of immersion time

3.3.1. Voltammetry around OCP ( E = ±70 mV)

Fig. 7 shows the potentiodynamic polarization curves of copper in 0.5 M NaCl

solution without and with olive leaves extract obtained after (a) 1 hour, (b) 6 hours and (c) 24

hours of immersion. Only the larger inhibitor concentration C1 was considered here. The

kinetic parameters were obtained from these curves as explained in section 3.2.2 by computer

fitting of the curves using equation (6) or equation (7). They are gathered in Table 3. It can be

seen that in any case the corrosion current density of copper in the presence of olive leaves

extract is lower than that of the copper electrode in NaCl solution. More precisely, the values

of are all higher than 70 %. The value of (86±5%) reached after 24 h of immersion is

higher than the others. The anodic coefficient a is not changed by the inhibitor, which

indicates that the mechanism of the anodic process is not modified. Actually, it is also clear

from the curves of figure 7 that only the cathodic reaction is inhibited. This once again


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confirms that the inhibitor behaves as a barrier hindering the access of O2 to the cathodic sites

of the copper surface.

The curves obtained in the blank solution for this series of experiment were fitted in

any case with O2 reduction under mixed activation-diffusion control. It can however be seen

in table 3 that the values computed for jlim are significantly higher (in absolute value) than

those determined with inhibitor concentration C1, e.g. -19 µA cm-2 vs -5.1 µA cm-2 after 1

hour. Actually, the inhibition efficiency could also be computed using | jlim| instead of jcorr in

equation (8). The values of are then 73±10%, 62±10% and 90±4 %, i.e. values similar to

those computed from jcorr . This shows once again that the inhibitor influences the rate of O2

reduction.

Note finally that all the experiments were performed once again, but with voltammetry

around OCP applied only at the end of the 24 hours of immersion. The results obtained

proved consistent with those of the main series of experiments, where voltammetry around

OCP was applied after 1 h, 6 h and 24 h. This shows that the method did not lead to

significant changes of the electrode surface; it can then be used as a non-destructive technique

to follow the evolution of a system over time.

3.3.2. Electrochemical impedance spectroscopy

Figure 8 shows the Nyquist plots obtained for copper in 0.5 M NaCl solution without

(a) and with (b) inhibitor concentration C1 at different immersion times (1, 6 and 24 h). In

both solutions, the shape of the Nyquist diagram does not change with increasing time. Only

its diameter varies. Actually, it clearly increases with time in the presence of the inhibitor. In

contrast, there is no clear trend in the blank solution. As already noted in section 3.2, the

impedance diagrams of copper in the blank solution show only one capacitive flattened loop

at high frequency. This loop relates to the charge-transfer process of copper dissolution


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occurring at the metal/electrolyte interface. The growing film of corrosion products has not a

significant influence on the corrosion process for such small immersion times. In contrast, the

Nyquist diagrams obtained with inhibitor concentration C1 clearly reveal the presence of two

overlapping capacitive loops, one corresponding to charge transfer, the other to the

contribution of the film formed on the surface. Another time constant appears in the low-

frequency (LF) region related to mass transport, i.e. more likely O 2 diffusion. SIE indicates

that the process is partially controlled by diffusion in both solutions, in agreement with the

results given by voltammetry for this series of experiments.

The impedance data could be modelled using equivalent circuits 5a for the blank

solution and 5c for inhibitor concentration C1 (see fig. 5). The determined values of the

various parameters are given in table 4. It can be noted first that the Rct value obtained for the

blank solution does not vary significantly over time (taking into account the dispersion of the

measurements). This is consistent with the assumption that the film of corrosion products

does not influence the corrosion process, i.e. that it has a poor protective ability. In agreement,

the corrosion rate determined by voltammetry around OCP (table 3) remains approximately

constant. In contrast, Rct clearly increases over time for the inhibitor concentration C1. Note

that Qdl decreases while Rct increases. Moreover, Rf also increases, which shows that both

beneficial effects of the inhibitor increase over time. Accordingly, the highest inhibition

efficiency is observed after 24 hours.

In conclusion, the inhibition efficiency increases with time and is only around 60-70%

at the beginning of the corrosion process (at least during 6 hours). It reaches 90% only after a

longer immersion time (but before 24 hours). This is in fact consistent with the observed

effect of the inhibitor on the corrosion products. The film of corrosion product was mainly

formed on some parts of the steel surface while the inhibition efficiency was still moderate,

which explains why the organic molecules finally adsorb on this film.


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3.4. Effect of temperature

The effect of temperature on the inhibition efficiency was investigated by voltammetry

around OCP ( E = ±70 mV) in the temperature range 298 – 328 K. The results obtained in 0.5

M NaCl solution with inhibitor concentration C1 are shown in Fig. 9. The kinetic parameters

determined by computer fitting are listed in table 5.

It can be noted first that, once again, the cathodic reaction could only be modelled by

the kinetic law corresponding to a mixed activation-diffusion control. Secondly, it can be seen

from Fig. 9 that increasing the temperature increased the current density of the

potentiodynamic polarization curves. This is more clearly illustrated in Table 5 by the

variations of jcorr with temperature. Similarly, the limiting current jlim increases (in absolute

value) significantly with temperature, from -2.9 µA cm-2 at 298 K to -127 µA cm-2 at 328 K.

This is due to the fact that diffusion is a thermally activated process with a higher effect on

the reaction rate than the decrease of O2 concentration linked to the increase of temperature.

The dependence of the corrosion rate on temperature can be expressed by the

Arrhenius equation:

where jcorr is the corrosion current density, A is the frequency factor, E a is the

activation energy of the copper corrosion reaction, T is the absolute temperature and R the

universal gas constant (8.314 J mol-1 K -1). The obtained Arrhenius plot Ln jcorr vs 1/T is given

in Fig. 10. It is compared to that of copper in 0.5 M NaCl solution without inhibitor, drawn

using the E a value of 17.9 kJ mol-1 given in previous work [38] and the corrosion current

density obtained at 25°C in our experimental conditions (i.e. 4.3 µA cm-2, see table 1). The

experimental curve obtained with the olive leaves extract corresponds approximately to a

straight line and the E a value can be determined from the slope of this line. The fitting of this


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curve with a straight line gave a value of 47±6 kJ mol -1 for the activation energy E a of the

corrosion process with inhibitor.

This value is significantly higher than that of 17.9 kJ mol-1 obtained previously for

copper without inhibitor (and used for drawing the corresponding line in fig. 9) [38], which

explains why the variations of jcorr are more pronounced in the presence of the inhibitor. This

effect can be quantified by computing the inhibition efficiency at each temperature. The

results, given in table 5, show that decreases when the temperature increases, from 86% at

298 K to 63% at 328 K. An increase of the corrosion activation energy in the presence of

inhibitor, associated with a decrease in inhibition efficiency with increasing temperature, is

frequently interpreted as due to the formation of an adsorption film of physical nature, i.e.

involving electrostatic interactions with the metal surface [37,39]. Conversely, a

chemisorption mechanism corresponds to an increase in inhibition efficiency with

temperature and a lower activation energy in the presence of the inhibitor [40-41]. Our results

suggest a predominant physisorption of the inhibiting species. Actually, it has been proposed

that physisorbed molecules are bound to the metal at cathodic sites and mainly inhibit the

corrosion process by hindering the cathodic reaction. This is consistent with the voltammetry

analysis that shows that the decrease of the corrosion current density was mainly due to a

decrease of the cathodic reaction rate (see figures 2 and 7).

4. Conclusions

The olive leaves extract was found to be an effective green inhibitor of copper in 0.5

M NaCl. The inhibition efficiency was found to increase with time during the 24 hour

experiments performed in this study, and reached 90% for the largest considered inhibitor

concentration. Experiments over longer times are to be performed to specify if the efficiency

can further increase. Two electrochemical methods were used, EIS and voltammetry around


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OCP ( E = ±70 mV). In this last case, the polarization curves were computer fitted using

various kinetic laws for the cathodic reaction, which provided information on the inhibition

process. Both methods led to consistent results, for instance the values of = 90±4 % (EIS)

and 86±5 % (voltammetry) after 24 h for the largest inhibitor concentration considered in this

study. From an applied point of view, this means that could be increased using a larger

inhibitor concentration.

The inhibiting species present in the olive leaves extract act as a cathodic inhibitor

adsorbed on the metal surface. The decrease of the inhibition efficiency with the temperature

and the value obtained for the apparent activation energy of the corrosion process indicate a

predominant physisorption mechanism. Such type of inhibitors are efficient at ambient

temperature, but are characterized by a loss in inhibition efficiency at elevated temperatures.

The inhibiting effect is due to the adsorption of the various phenolic compounds,

present in the olive leaves extract onto the copper surface. HPLC analysis showed that

oleuropein was the most abundant of these compounds in the leaves extract and thus more

likely the main inhibiting species. Additional work is however required to detail the

mechanisms of the inhibition process and the role of various substances present in the olive

leaves extract.

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Tables

Table 1. Electrochemical kinetic parameters and inhibition efficiency obtained from

potentiodynamic polarization curves ( E = ±70 mV, see figure 3) at RT (~25°C).

j corr β a β c j lim CR

Solution (µA cm-2

) (V-1

) (V-1

) (µA cm-2

) (mm year-1

) (%)

Blank 4.3±1.3 38±13 -26±5 - (*) 0.05±1.3 -

C3 1.5±0.3 70±2 -18±5 - 0.017±0.3 59±6

C2 1.8±0.3 65±14 -24±7 -6.1 0.02±0.3 60±8

C1 0.6±0.2 47±2 -33±2 -2.9±0.7 0.007±0.2 86±5

(*) Four experiments were performed. In only 1 case the curve had to be fitted with cathodic

reaction under mixed control. The obtained jlim value was -28 µA cm-2.

Table 2. Electrochemical impedance parameters for copper electrodes after 24 h in 0.5 M

NaCl solutions with or without olive leaves extract at RT (~25°C).

Solution

Blank C3 C2 C1

R s (Ω cm2) 8.5±3 7.607±0.011 8±3 6±1

R ct (Ω cm2) 2500±800 4465±100 6250±1000 22000±5000

Qdl 10-3

( Ω-1

cm-2

sn1

) 0.15±0.12 0.09± 0.01 0.09±0.03 0.07±0.01

n1 0.74±0.07 0.73±0.02 0.73±0.03 0.55±0.01

R f (Ω cm2) - 275±125 1150±200 3000±500

Qf 10-6

(Ω-1

cm-2

sn2

) - 39±5 23±3 13.5±2.5

n2 - 0.89±0.01 0.87±0.01 0.90±0.02

W (Ω-1

cm-2

s0.5

) 216±18 - 378±100 334±100

% - 47±2 65±4 90±4


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Table 3. Electrochemical kinetic parameters and inhibition efficiency at RT (~25°C) obtained

from potentiodynamic polarization curves ( E = ±70 mV) for concentration C1 compared to

those obtained without inhibitor (blank) at different times.

Solution E corr j corr a c j lim

(%)(h) (mV) (µA cm-2

) (V-1

) (V-1

) (µA cm-2

)

Immersion time = 1 h

Blank -245±35 3.4±0.6 50±10 -28±8 -19 -

C1 -238±6 0.85±0.15 65±2 -48±8 -5.1±1.3 75±4


Blank -221±25 3.35±0.55 75±5 -35±5 -9.3±1.7 -

C1 -233±15 0.9±0.3 64±3 -52±18 -3.5±0.3 73±9


Blank -263±50 4.3±1.3 38±13 -26±5 -28 -

C1 -258±8 0.6±0.2 47±2 -33±2 -2.9±0.7 86±5


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Table 4. Electrochemical impedance parameters at RT (~25°C) obtained for concentration C1 after various times

compared to those obtained without inhibitor.

Time R s R ct Qdl 10-3

n1 R f Qf 10-6

n2 W

(h) (Ω cm2) (Ω cm

2) ( Ω

-1cm

-2s

n1) (Ω cm

2) (Ω

-1cm

-2s

n2) (Ω

-1cm

-2s

0.5) (%)

Blank

1 6.9±0.3 2600±600 0.12±0.05 0.61±0.05 - - - 478±47 -

6 9.5±3.5 3300±300 0.27±0.13 0.60±0.05 - - - 230±80 -

24 8.5±3 2500±800 0.15±0.12 0.74±0.07 - - - 216±18 -

With inhibitor concentration C1

1 8.4±0.2 8940±30 0.14±0.02 0.6±1 1000±500 16±4 0.88±0.02 434±50 73±2

6 7.8±1.2 13300±2400 0.11±0.02 0.53±0.03 650±500 9.7±3 0.94±0.03 110±50 72±8

24 6±1 22000±5000 0.07±0.01 0.55±0.01 3000±500 13.5±2.5 0.90±0.02 334±100 90±4


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Table 5. Electrochemical kinetic parameters and inhibition efficiency obtained from

potentiodynamic polarization curves ( E = ±70 mV) for concentration C1 at different

temperatures.

T E corr j corr β a β c j lim

(K) (mV) (µA cm-2

) (V-1

) (V-1

) (µA cm-2

) (%)

298 -258 0.6 47 -33 -2.9 86

308 -234 1 75 -42 -6 82

318 -286 2.4 34 -41 -14 65

328 -344 3.1 20 -21 -127 63


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Figures captions

Figure 1. HPLC chromatogram at 254 nm of the obtained olives leaves extract. 1:

hydroxytyrosol; 2: tyrosol; 3: caffeic acid; 4: pcoumaric acid; 5: luteolin 7- glucoside; 6:

apigenin 7- glucoside; 7: verbascoside; 8: oleuropein.

Figure 2. Polarization curves of copper obtained after 24 h in 0.5 M NaCl solution in the

absence (blank) and presence of different concentrations (C1-C3) of olive leaves extract at RT

(~25°C).

Figure 3. Experimental polarization curve around OCP of copper obtained after 24 h in 0.5 M

NaCl solution (blank), computed curve and corresponding anodic ja and cathodic jc

components of the current density.

Figure 4. Nyquist plots for copper electrode immersion in 0.5 M HCl solutions without

(blank) and with various concentrations (C1-C3) of olive leaves extract at RT (~25°C) for

24 h.

Figure 5. Equivalent circuits used to fit the EIS experimental data.

Figure 6. Experimental Bode and Nyquist plots and their mathematical fitting: Example of

copper in 0.5 M NaCl solution with inhibitor concentration C2 after 23 h at room temperature.

Circles and squares (phase): experimental curves, lines: computed curves.

Figure 7. Polarization curves around OCP of copper in 0.5 M NaCl solution without inhibitor

(blank) or with the largest inhibitor concentration in C1 at different times: (a) 1 h; (b) 6 h and

(c) 24 h.

Figure 8. Nyquist plots for copper at different times (a) in 0.5 M NaCl solution without

inhibitor (blank) at different times and (b) in 0.5 M NaCl solution with inhibitor concentration

C1, at RT (~25°C).

Figure 9. Effect of temperature on the polarization curves around OCP of copper in 0.5 M

NaCl solutions with inhibitor concentration C1, after 24 h.


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Figure 10. Ln jcorr vs. 1/T plot used to calculate the Arrhenius slopes for copper in 0.5 M

NaCl solution with inhibitor concentration C1, compared with that computed from [33] for

copper in 0.5 M NaCl solution without inhibitor (blank).


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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Figure 7


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Figure 8


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Figure 9


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Figure 10


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Highlights

- Voltammetry around OCP is applied to the study of a corrosion inhibitor

- Voltammetry around OCP is a useful complementary method to EIS

- The olives leaves extract acts as a cathodic-type corrosion inhibitor

- The inhibition efficiency increases with time during the 24 hour experiments

- The major phenolic compound of the olive leaves extract is oleuropein

inhibitori naturali de coroziune

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