Alginate biopolymer as green corrosion inhibitor for

Journal of Molecular Structure 1157 (2018) 408e417

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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc

Alginate biopolymer as green corrosion inhibitor for copper in 1 M hydrochloric acid: Experimental and theoretical approaches A. Jmiai a, B. El Ibrahimi a, *, A. Tara b, S. El Issami a, O. Jbara b, L. Bazzi a a b

Applied Chemistry-Physic Team, Faculty of Sciences, University of IBN ZOHR, B.P.8106 Cit e Dakhla, Agadir, Morocco Laboratory of Engineering and Materials Science, University of Reims, PB 1039-F-51687 Reims, France

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 28 December 2017

The anti-corrosion behavior of sodium alginate (SA) on copper in the 1 M hydrochloric medium was carried out using weight loss and electrochemical measurements. The obtained results show that the inhibition increases with SA concentration and then reaches a maximum of 83% at a concentration of 0.1 mg L1. The effect of temperature on the reactions of copper corrosion inhibition and analyzing the thermodynamic parameters revealed that the mode of adsorption has a physical nature and obeys the Langmuir isotherm. The surface morphology was performed by scanning electron microscopy coupled with energy dispersive X-ray spectrometry and atomic force microscopy. To better understand the adsorption mechanism, describing the relationship between inhibitory ability and the molecular structure of SA, quantum calculations using density functional theory were performed. Monte Carlo simulation approache was performed to know well of the relationship between the inhibition ability and molecular structure of alginate. © 2017 Elsevier B.V. All rights reserved.

Keywords: Biopolymer Copper HCl Corrosion Alginate Inhibitor

1. Introduction Using corrosion inhibitors are a means method of protecting against corrosion of metals, especially in acidic media [1e6]. These are considered as one of the most efficient and cost-effective methods. Among the most widely known inhibitors are the derivatives of azoles considered as the most effective inhibitors against the corrosion of copper in the acid medium by virtue of the functional groups (containing N, S and O atoms). These groups are responsible for the adsorption of these molecules on the metal surface preventing the attack acidic solutions [7e10]. However, these azoles are toxic and are classified as environmental pollutants [11,12]. So, the development of novel corrosion inhibitors with a minimal or zero negative effects has been considered to be more important and desirable, so called eco-friendly or green corrosion inhibitors. Some researcher groups have stated the successful use of naturally occurring substances (e.g. plant extracts), drugs, surfactants, and ionic liquids, rare earth (e.g. CeCl3 and LaCl3) compounds as green corrosion inhibitors for metallic materials in many corrosives media [13]. Moreover, some biopolymers such as chitosan,

* Corresponding author. E-mail address: [email protected] (B. El Ibrahimi). https://doi.org/10.1016/j.molstruc.2017.12.060 0022-2860/© 2017 Elsevier B.V. All rights reserved.

cellulose, glucose and polysaccharides are widely used as corrosion inhibitors of metals, as reported in several research activities in recent times [12,14e20]. This is due to their polar grouping and their ability to complex with metal surfaces, which makes it possible to reinforce the adsorption of these compounds on the surfaces of metals in different media. Sodium alginate is a carbohydrate biopolymer of formula (C6H7Na1/2O6)n, extracted from brown algae, such as Laminaria or Fucus, widely used in the food industries and textures. This biomacromolecule consisting of carbohydrate units forming a chain [21e23] is reported as inhibitor against corrosion of various metals in some environments. Several works in the literature deals with this subject, we can cite in this case the study of the effect of sodium alginate on anticorrosion of; aluminum [24,25], magnesium [26,27], carbon steel [28] and mild steel [29], in various media. Since copper is more widely used in several industrials domains, particularly in heating and cooling systems [30,31], there is no work on the inhibition of copper with hydrochloric medium. The purpose of this study is to investigate the effect of sodium alginate (SA) on copper corrosion in 1 M HCl solution using weight loss and electrochemical methods. This method consists of determining the corrosion rate and inhibition efficiency. To complete experimental methods, the state of inhibitor adsorption is monitored by studying the concentration, immersion time and

A. Jmiai et al. / Journal of Molecular Structure 1157 (2018) 408e417

temperature effects. Characterization technics, namely: scanning electron microscopy coupled to energy dispersive X-ray spectrometry and the atomic force microscopy is performed to describe the morphology and the surface roughness of the examined copper samples when they are inhibited and uninhibited. In order to fully understand the adsorption phenomenon of SA onto copper surface, theoretical methods based on the functional theory of density and Monte Carlo simulation have been carried out [28,32].

CR ¼

DW

2.1. Materials and experimental setup For weight loss measurement, precision balance, class I type ALD, was used with high screen to measure the mass loss of each copper sample. The samples examined are copper plates of the same dimensional parameters (2.4 cm 1.6 cm 0.02 cm) with an area of 7.7 cm2. Electrochemical measurements were performed using a threeelectrodes electrochemical cell. The working electrode (WE) was a copper cylinder rod (99.999%) sealed with Araldite® epoxy glue resin. Before each manipulation, the surface of the electrode was rubbed with grade 600 and 1200 SiC abrasive papers. The circular cross-section of a surface (1 cm2) was exposed in the solution. The saturated calomel electrode (SCE) and platinum foil, were employed as a reference and counter electrodes, respectively. The electrolyte used for electrochemical corrosion measurements was a commercial hydrochloric acid solution with a concentration of 37%, provided by Sigma Aldrich. The 1 M concentration test solutions were prepared by diluting the analytical HCl stock with distilled water. Sodium alginate (molar mass ¼ 198 g/mol) was purchased from Sigma-Aldrich Co. Ltd, and its chemical structure is shown in Fig. 1. The concentrations of inhibitor ranging from 5 103 to 1 101 (mg L1) were prepared by dissolving sodium alginate in 1 M HCl solution.

2.2. Gravimetric method In this study, gravimetric method used to evaluate the corrosion rate of copper in acid medium with and without SA inhibitor. This method is based on the measuring of the samples mass losses before and after the addition of different SA concentrations at several immersion times. Before experiments, the copper plates are first washed with distilled water and there after weighed. The prepared samples are then immersed in 100 mL of a 1 M HCl solution in absence and in presence of SA. For an immersion time, different inhibitor concentrations are used. Several tests are envisaged at different temperatures (298, 308, 318 and 328 K). After immersion, the samples were washed by distilled water, dried at ambient temperature and the final mass loss is measured. The average of weight losses were taken to determine corrosion rate (noted: CR):

Fig. 1. Molecular structure of Alginate.

(1)

St

where DW is the mass loss (mg cm2), S is the area of the plate (cm2) and t is the period time of immersion (h). The inhibition efficiency, IEG (%), was calculated by the following equation:

IEG ð%Þ ¼ 2. Methodology

409

CR CRðinhÞ CR

100

(2)

where CR(inh) and CR represent the corrosion rates of copper without and with addition of SA at different concentrations. 2.3. Electrochemical methods Electrochemical measurements were performed using potentiostat/galvanostat VoltaLab PGZ301 model monitored by VoltaMaster4 software. The potentiodynamic (current-potential) curves were obtained by applying different potential values to working electrode from 500 to þ250 mV/SCE with a scanning rate of 1 mV/ s. The electrochemical impedance spectroscopy measurements were carried out at open circuit potentials over a frequency range from 105 Hz to 25 103 Hz with a perturbation wave amplitude of 10 mV. The impedance data were analyzed and fitted to the appropriate equivalent circuits by using ZSim software. 2.4. Surface characterization The surface state of copper sample immersed for one day in 1 M HCl solution without and with SA compound at 101 mg L1 were conducted by scanning electron microscopy (Jeol JSM-6460LAV model) coupled to energy dispersive X-ray spectrometry and the atomic force microscopy (Intergated Dynamics Engineering model D-65479 Raunheim nanoscope model). These analyses give the surface morphology and the elemental composition of the formed species on the metal surface. 2.5. Chemical quantum calculations Quantum chemical calculations were carried out using the density functional theory (DFT) with three Becke and Lee-Yang-Parr (B3LYP) functional. The 6-31G þþ (d, p) was employed as basis set for the calculations of a fragment of SA with one unites. All these calculations are made by a Gaussian09 software in gas and aqueous phase [33,34]. 2.6. Monte Carlo simulation The Monte Carlo simulation was carried out by using Materials Studio 6.0 software to simulate the adsorption configuration (energy and structure) of different SA fragment long (with different unite). The simulation box (84.35 Å 84.35 Å 74.61 Å) with periodic boundary conditions (PBC) was modeled by two regions. The first was modeled by eight-layer of Cu (111) plane, which each ones containing 64 copper atoms (8 8). The second was a large vacuum region with a thickness of 60 Å, which was constructed above of the Cu plane (111). The simulation was performed with a single SA fragment in gas phase. For the energy calculation, COMPASS force field type has been used. In other hand, the calculation of electrostatic and van der Waals potentials were performed by the using Ewald and atoms-based summation technique, respectively.

410


Table 1 Corrosion parameters obtained for copper in 1 M HCl solution with and without SA at different concentrations after immersion for one day (T ¼ 25 C). Concentration (mg L1)

CR (mg cm2 h1)

IEG (%)

Q

Blank 5 103 1 102 5 102 1 101

0.0616 0.0269 0.0199 0.0171 0.0134

e 56.40 67.67 72.19 78.20

e 0.5640 0.6767 0.7219 0.7820

3. Results and discussion 3.1. Gravimetric method 3.1.1. Concentration effect Table 1 shows the corrosion parameters obtained from mass loss measurements such as the rate of corrosion, the surface coverage and the inhibition efficiency in hydrochloric medium after one day of immersion at 25 C. It is seen that the corrosion rate decreases by adding SA in low concentration (between 5 and 100 mg L1) to corrosive solution and similarly the inhibition efficiency increases with increasing SA concentration. Inhibition efficiency towards the attack of chloride ions reaches a maximum value IEG of 78% at a concentration of 101 mg L1. This is probably due to the increasing of the adsorption surface coverage (Q) of SA which results in blocking the formation of corrosion products on the copper surface [17,19,26]. 3.1.2. Immersion time effect Fig. 2 (a) shows the inhibition efficiency of SA at different concentration for different immersion times. For the tested SA concentrations, it is seen that the inhibition effectiveness increases by increasing the immersion time, the maximum IEG values were obtained after 24 h for all SA concentrations. The highest value of the inhibition efficiency was observed for the concentration of 101 mg L1. From (Fig. 2 (b)), we observe during the first hours (0 he24 h) a drop in corrosion rate as a function of time, whereas the inhibition efficiency increases gradually and stabilizes. This behavior can be attributed to the high stability of adsorbed inhibitor on the copper surface [35,36]. Indeed, a slow rearrangement of the SA molecule reflects a high stability of the adsorption layer on the copper surface [36,37]. During the time interval 24 h up to 72 h a reverse behavior is observed (i.e., decreased of IEG and increased of CR followed by stabilization for both). This could be due to the saturation and depletion of SA molecules in the solution which adsorbs on the copper surface [36,38]. 3.1.3. Temperature effect and corrosion activation parameters To determine the adsorption nature of the inhibitor, the effect of temperature variation on the reactions of copper corrosion inhibition was studied in the absence and presence of 101 mg L1 SA. The temperature was varied in the range 298 Ke328 K and the immersion time was kept at one day. The results obtained are summarized in Table 2. These results show that the inhibition efficiency decreases and the corrosion rate increases with temperature both in absence and in presence of the inhibitor. This decreases of IEG is due to the displacement of adsorption equilibrium towards the acceleration of desorption of SA molecules on the copper surface, indicating the dissolution of the metal [18,39]. In order to understand the corrosion phenomenon occurring on the metal surface, we determine its activation energy using Arrhenius plot. Fig. 2 (c) shows the natural logarithm of the corrosion rate (Log (CR)) versus 1000/T for copper corrosion in 1 M

HCl solution without and with addition of SA at 101 mg L1. In both case, a draw straight lines are obtained with a correlation coefficients of 0.89 and 0.91, respectively. The obtained values of the slopes lead to calculate the corrosion activation energy, (Ea), by the following the relationship:

Ea þA Log ðCRÞ ¼ 2:303 RT

(3)

where R is the molar gas constant, T is the absolute temperature and A is the Arrhenius constant. The calculated activation energy (Ea) for the blank solution (i.e. 1 M HCl) is found to be 32.864 (kJ mol1) and in the presence of inhibitor is 43.843 (kJ mol1). These results suggest a physical adsorption of SA molecules on the copper surface, that can be attributed to the formation of an energy barrier on the surface responsible of the change in the corrosion process [20,24,40,41]. This explains the high inhibition efficiency of tested bio-polymer. 3.2. Adsorption isotherm To understand more the adsorption process of an inhibitor into metal surface, it is important to evaluate it through the adsorption isotherm study. According to the results of the weight loss method, the plotting of the C/q graph as a function of C (Fig. 2 (d)) gave a straight line with a slope close to the unit. And the Langmuir adsorption isotherm provides the best description of the SA adsorption behavior on the copper surface [12]. DGads is the thermodynamic Gibbs adsorption energy which can determine the influence of the adsorption molecules with the metal surface. The latter was computed from the equilibrium constant K (given by adsorption isotherm) using equation [36]:

DGads ¼ RT ln (55.5 K)

(4)

where R is the gas constant, T is the absolute temperature (K) and K is the adsorption/desorption equilibrium constant, while 55.5 value is the concentration of water in solution (mol L1). The DGads value calculated from mass-loss data at 298 K is 24.2 kJ mol1. This negative value of the adsorption energy (DGads) is attributed to the stability and the spontaneity of the formed film by adsorbed SA molecules onto the metal surface [29]. According to the literature [39,42], if DGads takes a value about 20 kJ mol1, an electrostatic interaction surface (i.e. physisorption) between the compound molecules and the substratum can be occur, while a chemical interaction (i.e. chemisorption) between the compound molecules and the surface of the material occurs if DGads is greater or around 40 kJ mol1. In our case, the value obtained for DGads (i.e. 24.2 kJ mol1) indicates that the adsorption process between the SA molecules and the copper surface in 1 M HCl solution is spontaneous physical adsorption, which is in a good agreement with the previous conclusion based on the calculation of activation energy (Ea). These results indicate that the long SA biopolymer chain can be interacted with the surface through several sites of interaction, especially the hetero atoms, which increases the adsorption energy [19,36,43]. 3.3. Electrochemical methods 3.3.1. Polarization curve and electrochemical impedance measurements The electrochemical method permits us to understand the corrosion phenomena. Fig. 3(a) shows the potentiodynamic polarization curves obtained for copper with and without addition of SA into 1 M HCl solution at 25 C. Table 3 lists the obtained


411

electrochemical parameters, such as the corrosion current density (Icorr), the corrosion potential (Ecorr), the cathode and anode slopes (bc, ba) determined using the EC-Lab software, also, the corrosion rate (milli-inch per year, mpy) according to the following equation:

(a)

CRðmpyÞ ¼

Icorr kEW dA

(5)

where k is a constant of corrosion rate (k ¼ 128.800 milli-inches (amp cm year)), A the area surface of copper electrode (cm2), d the density of copper (8.92 g cm3) and EW the equivalent weight in grams/equivalent of copper (EW ¼ 31.75) [12]. The corrosion inhibiting efficiency (IEp) was obtained as follows:

IEpð%Þ ¼

(b)

(c)

(d)

Icorr IcorrðinhÞ Icorr

100

where Icorr and Icorr(inh) are the corrosion current densities in the absence and presence of the inhibitor. According to Fig. 3(a) and Table 3, it can be seen that the addition of SA biopolymer displaces the value of Ecorr to more negative potentials, this might be due to the decrease in the cathodic reaction rate. Thus, it surpress hydrogen evolution [26]. In the presence of inhibitor, the values of the anodic Tafel lines (ba) show minor modifications with the addition of SA and the Icorr density values show a remarkable decrease with respect to the blank (Icorr(blank) ¼ 26.22 mA cm2 and Icorr(Inh) ¼ 4.44 mA cm2) resulting in a decrease in CR and simultaneous increase in inhibition efficiency (its maximum value, 83.06%, at 101 mg L1). This reduction in Icorr can be related to the adsorption of SA onto the copper surface leading to form an adsorbed film, which protects the metal against dissolution [19,20,36]. Fig. 3(b) show the Nyquist plots of the copper in the 1 M HCl solution uninhibited and inhibited and Fig. 3(c) shows the equivalent electrochemical circuits obtained after the analysis of the impedance spectra. The electrochemical parameters obtained are listed in Table 4. The Rs represents the resistance of solution, Rf is the surface film resistance, Cf is the capacitance of corrosion product film on copper surface and Cdl represents the double-layer capacitance at the interface between metal and the solution and Rct the charge-transfer resistance. The impedance diagrams obtained show depressive semicircles (loop capacitive), generally associated with a double layer capacitance corresponding to the transfer of charge [27]. This semicircular depression is due to the inhomogeneity of the metal surface [12,19]. From the data of Table 4, it can be seen that the values Rct in the inhibited medium are greater than those of the uninhibited medium. Such behavior is probably due to the effect of SA molecules on the copper/solution interface, which leads to higher coverage of the electrode surface [35]. The strong decrease in the double layer constant (Cdl ¼ 153 mF cm2) at higher SA concentrations is due to limited access of charged particles to the surface [28]. This can be explained by the formation of a protective film which prevents attack of the acid solution on the metal surface [26,36]. The inhibition efficiency (IEIm %) is calculated as follows:

Rt 100 IEIm % ¼ 1 R tðInhÞ Fig. 2. (a) Variation of the inhibition efficiency of SA as a function of concentration for different immersion times, (b) variation of the corrosion rate (CR) and inhibition efficiency (IEG %) with time of copper in 1 M HCl solution (T ¼ 25 C), (c) Arrhenius plot for copper corrosion in 1 M HCl solution with and without addition of SA at 101 mg L1 and (d) Langmuir isotherm plot of SA molecules/copper surface/1 M HCl solution system.

(6)

(7)

where Rt(Inh) and Rt are the total resistances in the absence and presence of inhibitor. Table 4 clearly shows that the inhibition performance of SA increases by concentration increasing, which value of 82.12% at 101 mg L1. These results are consistent reach a with polarization results under the same conditions

412


Table 2 Corrosion parameters obtained for copper in 1 M HCl solution without and with 101 mg L1 of SA after one day of immersion at various temperatures. Temperature (K)

Conc. (mg L1)

Q

CR (mg cm2 h1)

IEG (%)

298

Blank 1 101 Blank 1 101 Blank 1 101 Blank 1 101

e 0.7820 e 0.6363 e 0.5482 e 0.4254

0.0616 0.0134 0.2902 0.1055 0.8013 0.3620 0.9837 0.5652

e 78.20 e 63.63 e 54.82 e 42.54

308 318 328

Fig. 3. (a) Potentiodynamic polarization curves and (b) Nyquist plots of copper in 1 M HCl solution with and without addition of SA at different concentrations (T ¼ 25 C). (c) Used equivalent circuit model to fit the obtained EIS data.

3.4. Surface characterization 3.4.1. SEM/EDS characterization The surface Morphology of copper specimens immersed in different mediums were examined by SEM (Fig. 4(aec)). Fig. 4 (a)

shows the SEM image of the abraded copper sample prior to immersion in the acid solution. Seen rectal lines are formed due to mechanical polishing. After immersion of the metal in the corrosive solutions without inhibitor (Fig. 4 (b)) a remarkable change of the copper surface is observed. The surface is rough with crystalline aggregates of a triangular shape indicating that the surface is strongly damaged by exposing to HCl solution in the absence of SA due to metal dissolution in an aggressive solution [17,19]. In sharp contrast, in the presence of SA (Fig. 4 (c)), a homogeneous and uninterrupted surface without formation of corrosion products aggregates is clearly observed. These observations shown that the corrosion was tangibly suppressed in the presence of tested biopolymer [26,36]. The acquired EDS spectra shown in Fig. 4 (a ', b', c ') were used to determine the elementary composition of copper during the corrosion process with and without SA in 1 M HCl solution. Fig. 4 (a ') showed characteristic peaks of the components of the abraded copper specimens prior to immersion in the solution, a mass composition of 99.05% is obtained for Cu while for residual elements such as O and C we obtain 0.83% and 0.11%, respectively. In free SA biopolymer solution, the EDS spectrum of copper given in Fig. 4 (b ') shows the appearance of large characteristic peak of chlorine and an increase in the intensity of the oxygen peak with respect to virgin copper. In the presence of SA inhibitor (Fig. 4 (c ')), the spectrum shows a decrease in the Cl intensity peak and an increase in intensities of O and C peaks. The mass percentages of the various elements composing the sample in the presence and absence of the inhibitor are listed in Table 5. When we compare the percentages corresponding to the polished copper and the copper attacked by the hydrochloric solution an increase in mass percentage of chlorine and oxygen is observed while a decrease in copper is observed. This explains that the corrosion of copper in the presence of HCl is due to the formation of products such as CuCl and/or CuCl 2 [12]. These results are consistent with our gravimetric and electrochemical studies. In the presence of SA the percentage of Cu is great than that of copper without addition of SA biopolymer, while a decrease in the percentage of Cl and an increase in the percentage of C is observed. This confirms that the SA molecules have an effect on the corrosion of copper by the formation of a molecular barrier at the electrode/ electrolyte interface which prevents the formation of the CuCl complex [20,36]. All these observations are in a good agreement with SEM images and the experimental results.

3.4.2. AFM observation The Atomic Force Microscopy (AFM) is another powerful technique for assessing the influence of inhibitors on the corrosion process at the metal/solution interface. It describes the morphology of the surface at the Nano-micro scale interval [44]. Fig. 5 shows the AFM images (two and three-dimensional) of sample surface of copper in the presence and absence of the SA biopolymer immersed in 1 M HCl solution at 298 K. The first images (a and a') obtained after one day of immersion in the uninhibited 1 M HCl solution shows a rough surface and a highly damaged copper surface with cracks due to the acid attack which increases the surface roughness at 1.7 mm [12]. In the presence of Biopolymer in the medium, the resulting image (b, b ') shows a smooth and

Table 3 Potentiodynamic polarization parameters of copper in 1 M HCl solution in absence and in presence of SA with different concentrations (T ¼ 25 C). C (mg L1)

Ecorr/(SCE) (mV)

Icorr (mA cm2)

ba (mV dec1)

-bc (mV dec1)

CR (mpy)

IEp (%)

0.0 5 103 1 102 5 102 1 101

256.64 302.67 314.10 317.70 317.00

26.22 12.28 7.30 5.60 4.44

60.90 71.30 61.70 61.20 60.70

250.10 361.50 263.80 233.70 167.80

12.02 6.54 3.35 2.50 2.04

e 53.20 71.16 78.65 83.06


413

Table 4 Obtained parameters by fitting the Nyquist plots of copper electrode in 1 M HCl solution in absence and presence of SA (T ¼ 25 C). C (mg L1)

Rs (U cm2)

Rf (U cm2)

Cf (mF cm2)

af

Rct (U cm2)

Cdl (mF cm2)

adl

Rt (U cm2)

IEI (%)

0.0 5 103 1 102 5 102 1 101

1.94 5.91 3.25 1.52 1.40

72.2 133.0 162.7 145.4 121.5

74 41 34 17 18

0.86 0.86 0.88 0.94 0.95

799 2052 2800 3800 4749

267.7 306 225 174 153

0.62 0.621 0.64 0.62 0.57

871.2 2185 2962.7 3945.4 4870.5

e 60.12 70.60 77.92 82.12

Fig. 4. SEM and corresponding EDS spectra of copper surface before and after one day of immersion in 1 M HCl solution (at T ¼ 298 K) with and without addition of SA biopolymer at 101 mg L1: (a, a’) abrade copper surface, (b, b’) without SA, and (c, c’) with SA.

Table 5 Mass percentage contents of elements (deduced from EDS spectra) on copper specimens in different tested mediums. Element

Abraded Cu

Cu þ 1 M HCl

Cu þ 1 M HCl þ SA

Cu Cl O C

99.05 e 0.83 0.11

68.22 17.69 11.62 0.47

93.02 3.13 2.62 1.22

homogeneous surface covered with spherical or bread-shaped particles [45], and the surface roughness was reduced at 1.4 mm [46]. These results were explained by the binding and attachment of the biopolymer molecules to the metal surface. The SA adsorption film on the copper surface confirms the rough layer of precipitate responsible for slowing the corrosion product by the formation of a barrier against aggressive ions [44,47]. Therefore, it can be concluded that the SA is an ecological inhibitor against copper corrosion in 1 M hydrochloric media.

414


Fig. 5. 2D (left) and 3D (right) AFM images of copper surface before and after one day of immersion in 1 M HCl solution (a, a’) without and (b, b’) with addition of SA biopolymer at 101 mg L1 (at T ¼ 298 K).

3.5. DFT calculations Quantum chemical study is a computational method which can give another information about the adsorption mechanism of inhibitors on metallic surface [33,48]. The geometric optimization of the studied biopolymer fragment was evaluated using the DFT with the functional B3LYP and 6-31Gþþ(d,p) as basis set in vacuum and aqueous phases. Fig. 6 (a) shows the geometry and the evolution of optimization energy curve of the SA fragment. From this figure, we see that after step 4 the total energy of the SA molecule becomes stable. This stabilization of the molecule is reached at an energy value of 761.261 Hartree. The density distributions of the frontier molecules orbitals (i.e. HOMO and LUMO) for the molecule SA are plotted in Fig. 6 (b, c). According to obtained results in aqueous and gas phase (Table 6), the presence of solvent does not change much the electronic properties of inhibitor molecule [49]. The energy (EHOMO) of SA is electron donating effect to the acceptor metal. The energy of (ELUMO) has an opposite effect to (EHOMO) which means a higher tendency of the inhibitor to receive electrons from the donor metal [35]. When the gap energy, EHOMO-ELUMO, is smaller, the molecule is more reactive [12,33]. The distribution of the charges can provide more information on the reactive sites on studied molecule [50]. The presence of the highest negative partial charge in some atoms gives them more tendencies to give electrons to metal atoms than other with less negative charge [33]. Mulliken charges calculated on SA oxygen atoms are summarized in Table 7. The highest negative partial

charge is found to locate on the oxygen atoms, this may confirm that the alginate molecules can be adsorbed through oxygen atoms to the copper surface [20]. The molecular electrostatic potential (MEP) is a very useful descriptor in understanding sites for electrophilic reactions. Fig. 6 (e) shows the contour representation of MEP of SA fragment. Regions of positive (negative) potential are colored in green (red). This molecule has only one possible site of electrophilic attack (red) found in the area of (O21, O18 and O12). Whereas the rest of the molecule is characterized by positive potential (green region). The red region rich in electrons also indicates the most probable molecular sites for binding with metallic surfaces [12,49]. Fig. 6 (d) presents the contour representation of MEP. The negative (red) region of the MEP was linked to high electron density (electrophilic reactivity) and the positive (green color) ones to the low electron density. The electron density decreases in the following order: red > orange > yellow > green > blue. The MEP plot of the inhibitor indicates that the high electron density (red color) is delocalized along a single electron-rich region with oxygen atoms (O18, O21 and O11). The rest of the inhibitor is characterized by the low electron density (green color) delocalized on the other atoms of the SA molecule. The high electron densities on the oxygen atoms can be attributed to the lone pairs of electrons on these atoms. As a result, these atoms with high-electron densities will be attracted to the Cu surface as a low-electron density region to form the coordination bonds type with copper atoms on the surface [51].


415

Fig. 6. (a) The optimization energy step of SA fragment according to B3LYP/6-31Gþþ(d,p) method. And Frontier molecule orbital distributions on the Alginat fragment: (b) HOMO, (c) LUMO using the B3LYP/6-31Gþþ(d,p), and (d,e) its electrostatic potential map: regions of positive (negative) potential are colored green (red).

Table 6 Calculated quantum chemical parameters of investigated inhibitor in gas and aqueous phases.

Gas Aqueous

DH (Hartree)

m (Debye)

ELUMO (eV)

EHOMO (eV)

DE (eV)

761.261 761.359

7.10 11.08

1.741 0.431

4.539 6.493

6.280 6.062

Table 7 Calculated Mulliken partial charges on oxygen atoms of investigated inhibitor in gas and aqueous phases (for atomic numbering see Fig. 6(a)). Atom

Gas Aqueous

O11

O12

O13

O14

O18

O20

O21

0.462 0.561

0.393 0.404

0.498 0.550

0.445 0.528

0.370 0.518

0.478 0.507

0.420 0.726

3.6. Monte Carlo simulations In order to understand the interaction of SA molecules with the copper surface, the calculations Monte Carlo simulations have been introduced. According to this study, it is possible to determine the

interaction energies (ie the binding energy Eads) which takes place between the SA biopolymer and the Cu (111) surface, Eads is calculated according to the following relation: Eads ¼ ECu-inh ‒ (Einh ‒ ECu)

(8)

where Einh and ECu are the total energy of the SA and of Cu surface, respectively, while, ECu-inh represents the total energy of new formed system between the SA and the copper surface. Fig. 7 shows the adsorption configuration of SA fragments on the Cu (111) surface. The determined adsorption (Eads) and binding energies (-Eads) of the biopolymer with the surface of the metal are summarized in Table 8. According to these images, it is seen that SA biopolymer fragment adsorbs on the surface in a parallel manner [28] and the adsorption energies obtained are negative (43.88, 72.32, … and 274.32 kJ mol1), which elucidates the spontaneity of the adsorption and the strong binding which takes place on the copper surface [37]. This energy increases simultaneously by increasing the number of alginate monomer, this due to the increasing in the numbers of the actives site of adsorption (i.e. functional groups) in the SA molecular structure [12,50].

416


(a)

(a’)

(b’)

(b)

Fig. 7. Adsorption configuration of SA inhibitor on Cu (111) surface, top view (a, a’) and side view (b, b’).

Table 8 The adsorption (Eads) and binding (-Eads) energies as function of monomer number (chain length) in SA biopolymer fragment on Cu (111) surface. Number of monomer unit

Eads (kJ mol1)

Ebind (kJ mol1)

1 2 4 6 8 10

43.88 72.32 97.74 182.12 256.75 274.32

43.88 72.32 97.74 182.12 256.75 274.32

4. Conclusion Sodium alginate (SA) as nontoxic biopolymer has showed a good inhibitory performance (IE) against the corrosion of copper molar hydrochloric solution. Mass loss method shows an increase of IE by increasing SA concentration. Stationary electrochemical studies have shown that SA is a cathodic inhibitor. Transient measurements reveal an increase in Rct with an increase in inhibitor concentration. The inhibition efficiency is found to increase with the increasing in concentration of SA, which the inhibitory efficiency reaches a value of 83% at 101 mg L1. In other hand, the adsorption of the investigated biopolymer was found to follow the Langmuir isotherm model. Further, the temperature effect and the thermodynamic parameters revealed that the mode of adsorption is of physical nature. SEM, EDS and AFM surface analysis techniques indicate that the copper dissolution is greatly diminished by the adsorption of SA molecules on the copper surface. The partial charge and the MEP representation indicate that the high electron density is localized on the oxygen atoms. Furthermore, Monte Carlo simulations explained the spontaneity of adsorption of alginate molecules on the surface of copper. The theoretical calculations are in a good agreement with the experimental results obtained.

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