Acidic Corrosion Inhibition of Copper by Hydroxyethyl

British Journal of Applied Science & Technology 4(9): 1445-1460, 2014

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Acidic Corrosion Inhibition of Copper by Hydroxyethyl Cellulose I. O. Arukalam1, I. C. Madufor1, O. Ogbobe1 and E. E. Oguzie2* 1

Department of Polymer and Textile Engineering, Federal University of Technology, P.M.B. 1526 Owerri, Nigeria. 2 Electrochemistry and Materials Science Research Laboratory, Department of Chemistry, Federal University of Technology, P.M.B. 1526 Owerri, Nigeria. Authors’ contributions

This work was carried out in collaboration between all authors. Authors OO, ICM and EEO designed, oversaw the study and wrote the draft manuscript. Author IOA performed the experimental work. All the authors read and approved the final manuscript.

st

Original Research Article

Received 21 June 2013 th Accepted 24 September 2013 rd Published 3 February 2014

ABSTRACT Aims: To investigate the inhibition performance of hydroxyethyl cellulose on the acid corrosion of copper. Study Design: This study was designed to investigate the corrosion inhibition efficiency of hydroxyethyl cellulose, as well as ascertain how the additive modifies the corrosion behavior of copper in the test acid media and hence the corrosion inhibition mechanism. Place and Duration of Study: This study was carried out in the Electrochemistry and Materials Science Research Laboratory, Federal University of Technology, Owerri, Nigeria between April and September, 2012. Methodology: The corrosion inhibition performance of HEC on copper in aerated 1 M HCl and 0.5 M H2SO4 acid solutions was assessed using weight loss measurements. The influence of HEC as well as HEC + KI on the corrosion behavior of copper was investigated using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements. The electronic structure of the HEC molecule was modeled using density functional theory (DFT) -based quantum chemical computation, while molecular dynamics (MD) simulations were performed to illustrate the adsorption process of the HEC molecule on copper. Results: Weight loss measurements results revealed that HEC effectively inhibits copper corrosion in the studied acid media, with maximum inhibition efficiency ~95%. Maximum ____________________________________________________________________________________________ *Corresponding author: E-mail: [email protected];

British Journal of Applied Science & Technology, 4(9): 1445-1460, 2014

efficiency in 1 M HCl was obtained after 1 day of immersion and that in 0.5 M H2SO4 after 5 days. The impedance response revealed two capacitive time constants and this mechanism was not altered on addition of HEC, which functioned by adsorption onto the copper surface. The potentiodynamic polarization profile in 1 M HCl shows features of active-passive transition, whereas that in 0.5 M H2SO4 shows spontaneous passivation. These mechanisms were not modified by the inhibitor. The computational studies confirmed the corrosion inhibiting potential of HEC. The HEC/Cu adsorption energy estimated by means of molecular dynamics simulation (-55.09 kJ/mol) suggests a spontaneous physical adsorption process. Keywords: Adsorption; acid corrosion; corrosion inhibition; hydroxyethyl cellulose.

1. INTRODUCTION Metallic copper is a material of immense industrial and commercial importance, but suffers significant corrosion damage in the presence of strong acids, dissolving to form a solution containing Cu (II) ions together with hydrogen gas, H2 as effervescence [1,2]. Copper-based alloys are deployed for the fabrication of marine components, with wide applications in seawater services and also in power stations and sometimes heat exchangers [3]. The widespread use of these alloys is a function of a combination of high corrosion resistance, high thermal and electrical conductivity, and excellent workability [1,36]. Furthermore, copper and copper-based components may be deployed in service environments where they may get in contact with acid solutions or fumes [7-9]. To minimize the metal loss under such conditions, several approaches have been exploited [10-12]. Among the corrosion protection methods available, the use of organic inhibitors has proved to be effective and efficient [13-17]. The use of polymers (natural and synthetic) as organic inhibitors for metal corrosion in aggressive environments has received much attention recently [18-31]. Generally, polymers form elastic films when in solution. As inhibitors, they films get adsorbed on the metal surface and form protective barrier to corrosive environments, thereby reducing the corrosion susceptibility of the metal surface [18, 28]. Consequently, the service life of the metal is prolonged. Reports show that the inhibitive properties of polymers are owing to the presence of hetero-atoms such as nitrogen, N-; sulphur, S-; oxygen, O- and aromatic rings as well as π electrons in their structures. -

-

2-

2-

The presence of anions such as Cl , I , SO4 , and S in an aggressive solution may either inhibit or stimulate corrosion processes [32,33]. Halide ions in acidic media may synergistically increase the corrosion inhibition performance of some organic compounds [8,34-36]. Corrosion inhibition synergism results from increased surface coverage arising from ion-pair interactions between the protonated organic molecule and the halide ions [37,38]. In recent times, the complex processes associated with metal-inhibitor interactions have been investigated at the molecular level by means of computer simulations of suitable models and the density functional theory (DFT) has been used widely in this regard [39-43]. The idea is to use a combination of DFT-based quantum chemical calculations and molecular dynamics simulations to theoretically evaluate inhibiting potential based on structure-activity type relationships that estimate basic molecular descriptors such as frontier molecular orbitals, charge densities, ionization potential etc, which correlate with the corrosion inhibiting performance. Molecular dynamics (MD) simulations have also been used 1446


to qualitatively and quantitatively illustrate the adsorption of the molecules on metal surfaces at a molecular level and to compute the corresponding interaction energies, which is related to the adsorption strength [44-46]. Hydroxyethyl cellulose (HEC) is a non-ionic, water soluble polymer used as a thickening agent for aqueous cosmetic and personal care formulations [47]. It is a derivative of cellulose with a wide array of practical uses [48]. However, solutions of HEC form films which are tough and flexible, and these properties are retained over a wide range of temperatures. The present study investigates the corrosion inhibition performance of HEC on copper in hydrochloric and sulphuric acid solutions, as well as ascertains the extent of modification of the electrochemical corrosion behaviour of copper induced by HEC as well as HEC + KI. DFT-based quantum chemical computations and molecular dynamics simulation were employed to assess the electronic structure of HEC and discuss the adsorption mode on the copper surface.

2. EXPERIMENTAL DETAILS 2.1. Materials Preparation Copper sheets of puratronic type, 99.999% were used. The copper specimens used in the weight loss measurements were cut into rectangular coupons of width, 2 cm; length, 4 cm; and thickness, 0.05 cm. The same types of coupons were used for the electrochemical studies. However, in electrochemical studies the specimens were cut to 1 cm width; length, 2 2 cm and thickness, 0.05 cm such that only 1 cm area was exposed to the electrolyte. Before both measurements, the samples were polished using different grade emery papers followed by washing in ethanol, acetone and finally with distilled water. The aggressive media for the study were prepared from reagent grade HCl (Pro Analysis) and H2SO4 (BDH Chemicals). The potassium iodide (KI), used was from M & B Laboratory Chemicals. The test inhibitor, hydroxyethyl cellulose (HEC) was obtained from Sigma Aldrich. Inhibitor solutions were prepared by dissolving desired amounts of HEC respectively in 1 M HCl and 0.5 M H2SO4 solutions to yield four different concentrations of HEC (500, 1000, 1500 and 2000 mg/L) in each corrodent. To study the effect of halide salt addition, 500 mg/L of KI was added to 2000 mg/L HEC o solutions. All the tests were performed in aerated medium at room temperature (29±1 C) and normal atmospheric pressure. The optimized structure of hydroxyethyl cellulose is given in Fig. 1.

Fig. 1. Optimized structure of one molecule of HEC 1447


2.2 Weight Loss Measurements Corrosion testing using weight loss method was conducted on the copper coupons whose dimensions have been stated above. These were used as cut without further polishing but were degreased in absolute ethanol, dried in acetone and weighed. Experiments were carried out under total immersion conditions in 300 ml of test solutions maintained at room temperature. The coupons were retrieved at 24-h intervals progressively for 120 h, immersed in 20% NaOH solution containing 200 g/l of zinc dust, scrubbed with bristle brush, washed, dried and weighed [49]. The weight loss results were calculated as the difference between the final weight at a given time and the initial weight. The measurements were repeated 3 times and the results presented are average of the 3 measurements. From the values of weight loss obtained, the inhibition efficiencies ( IE % ) of HEC were computed using the relation [17]:

 W IE %  1  inh  Wblank where

  x 100 

(1)

Winh and Wblank are weight loss values in the presence and absence of the additive

respectively.

2.3 Electrochemical Measurements Electrochemical experiments were conducted in a conventional three-electrode glass cell of capacity 400 mL using a VERSASTAT 3 Complete DC Voltammetry and Corrosion System, with V3 Studio software. A graphite rod was used as counter electrode and a saturated calomel electrode (SCE) as reference electrode. The latter was connected via a Luggin’s capillary. The copper working electrode was wet-polished using SiC abrasive paper (up to 1200 grit) then dried in acetone and warm air. Measurements were performed in naturally aerated and unstirred solutions at the end of 30 minutes of immersion at 30 ºC. Impedance measurements were made at corrosion potentials (Ecorr) over a frequency range of 100 kHz – 10 mHz, with a signal amplitude perturbation of 5 mV. The potentiodynamic polarization experiments were undertaken in the potential range of −250 mV to +1700 mV with a sweep rate of 1 mV/s.

2.4 Computational Studies In order to gain insight into the inhibitive capability of HEC, DFT method was used to analyze the chemical reactivity of HEC by means of molecular orbital theory. This was achieved 3 using the DFT electronic structure program DMol using a Mulliken population analysis. Electronic parameters for the simulation include restricted spin polarization using the DND basis set and the Perdew Wang (PW) local correlation density functional. This basis set helps in providing accurate geometry and electronic properties of HEC. The molecular orbitals, HOMO and LUMO energy were analyzed and this helps to find out from molecular structure of HEC the possible sites of nucleophilic and electrophilic reactions with the copper surface. The local reactivity of HEC was also analyzed by means of Fukui indices [50].

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Molecular dynamics (MD) simulation of the non-covalent interaction between the inhibitor molecules and the copper surface was performed using Forcite quench molecular dynamics to sample many different low energy configurations and identify the low energy minima [2,15]. Calculations were carried out, using the COMPASS force field and the Smart algorithm, in a simulation box 30 Å x 25 Å x29 Å with periodic boundary conditions to model a representative part of the interface, devoid of arbitrary boundary effects. The box comprised of the Cu slab, cleaved along the (1 1 0) plane and a vacuum layer of 20 Å height. The geometry of the bottom layer of the slab was constrained to the bulk positions whereas other degrees of freedom were relaxed before optimizing the Cu (1 1 0) surface, which was subsequently enlarged into a 10 x 8 supercell. The inhibitor molecule was made to adsorb on one side of the slab. Temperature was fixed at 303 K, with NVE (microcanonical) ensemble, with a time step of 1 fs and simulation time 5 ps. The system was quenched every 250 steps.

3. RESULTS AND DISCUSSION 3.1 Weight Loss Results The weight loss data of the copper specimens in 1 M HCl and 0.5 M H2SO4 without and with the test inhibitor, hydroxyethyl cellulose (HEC) is presented in Table 1 and shows that HEC actually retarded mild steel corrosion at all concentrations and the weight loss, hence corrosion rate in uninhibited and inhibited systems increased with exposure time. Table 1. Weight loss values (g) of copper corrosion in (a) 1 M HCl and (b) 0.5 M H 2SO4 in the absence and presence of HEC from 1 – 5 days Conc. (mg/L) Blank 500 1000 1500 2000

(a) 1 M HCl 1d 0.107 0.034 0.011 0.032 0.014

2d 0.225 0.131 0.074 0.067 0.017

3d 0.285 0.194 0.187 0.151 0.033

(b) 0.5 M H2SO4 4d 0.373 0.313 0.301 0.256 0.021

5d 0.579 0.440 0.433 0.382 0.013

1d 0.011 0.006 0.006 0.009 0.007

2d 0.040 0.020 0.011 0.008 0.016

3d 0.050 0.020 0.007 0.009 0.018

4d 0.080 0.040 0.006 0.011 0.022

5d 0.090 0.050 0.002 0.018 0.020

The efficiency of inhibition was quantified by comparing corrosion rates in the absence and presence of HEC according to as described in Eq. 1. Fig. 2 illustrates the variation of inhibition efficiency with HEC concentration and exposure time. Efficiency in 1 M HCl increased steadily with HEC concentration and generally decreased with exposure time, but at 2000 mg/L HEC concentration efficiency increased with exposure time, reaching maximum value of about 95%. Inhibition efficiency in 0.5 M H2SO4 on the other hand peaked at 1000 mg/L HEC concentration, attaining the maximum value (96%) at prolonged exposure time. Thus the main difference in the corrosion inhibition performance of HEC for copper in 1 M HCl and 0.5 M H2SO4 has to do with the influence of exposure time on inhibition performance; with efficiency decreasing and increasing with exposure time in 1 M HCl and 0.5 M H2SO4 respectively. Both media also have one vital feature in common – high maximum inhibition efficiencies (~95%).

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Fig. 2. Variation of inhibition efficiency with HEC concentration and exposure time for copper in (a) 1 M HCl and (b) 0.5 M H2SO4 3.2 Electrochemical Impedance Spectroscopy The weight loss experiments have demonstrated the corrosion inhibiting efficacy of HEC for copper in 1 M HCl and 0.5 M H2SO4. Impedance studies were subsequently undertaken to provide insight into the kinetics of electrode processes at the Cu/1 M HCl and Cu/0.5 M H2SO4 interfaces in absence and presence of HEC, hence show if and how the inhibitor modifies the corrosion behavior of the copper specimen. The 2000 mg/L HEC concentration was chosen for the impedance experiments. Experiments were also carried out in 2000 mg/L HEC containing 500 mg/L KI, in order to assess possible improvements in the inhibition performance arising from synergistic interactions with iodide ions. The impedance responses of these systems are given in Fig. 3-5 for 1 M HCl and 0.5 M H2SO4 in the Nyquist and Bode formats. The Nyquist plot in 1 M HCl generally comprise of two capacitive semicircular arcs at low and high frequency region (inset Fig. 3a), corresponding to two time constants as reflected by the two phase angle peaks in the Bode plot (Fig. 3b). Similar observation has been reported in the literature [53-55], where the high and low frequency loops are attributed to charge transfer resistance for electrical double layer relaxation and diffusion mass transport of Cu ions from the surface respectively. The depressed nature of the capacitive semicircle is typical for solid metal electrodes that show frequency dispersion of the impedance data. Several reports have described various techniques to compensate for the non ideal frequency response and non-ideal dielectric behaviour [56-58]. Introduction of HEC as well as HEC + KI did not modify the shape of the impedance response of Cu in 1 M HCl, but resulted in an increase in corrosion resistance as reflected by the increases in the diameter of the Nyquist semicircle and phase angle peak. This means that HEC inhibits Cu corrosion without modifying the mechanism of the corrosion process. The capacitance plots in Fig. 4 reflect the decrease in double layer capacitance in the presence of the additives. Such decrease in Cdl values, which normally results from a decrease in the dielectric constant and/or an increase in the double-layer thickness, often results from substitution of preadsorbed water molecules on the metal/electrolyte interface by adsorbed organics (with lower dielectric constant), is evidence that HEC is adsorbed on and modifies the metal/electrolyte interface.

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Fig. 3. Impedance plot of copper in 1 M HCl in the absence and presence of HEC and HEC + KI The impedance response of copper in 0.5 M H2SO4 follows the same trend as above. The corrosion inhibiting effect of HEC is however stifled in this system, and this can be attributed to the short exposure period for the electrochemical measurements (30 minutes) which is insufficient for significant adsorption of HEC on the copper surface. This point of view is supported by the trend of increasing inhibition efficiency with exposure time as determined from weight loss measurements.

Fig. 4. Capacitance plot of copper in 1 M HCl in the absence and presence of HEC and HEC + KI Using the ZSimpwin software, the impedance spectra were appropriately analyzed by fitting to the equivalent circuit model Rs(QdlRct(QfRf)), which has been previously used to model the Cu/acid interface [53].The time constant in the high frequency region (RctQdl) represents the

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charge-transfer process of copper dissolution, where Rct describes the charge-transfer resistance and Qdl the constant phase element (CPE) relating to the double layer capacitor. The other time constant in the low frequency region (RfQf), Qf represents the CPE of the adsorbed surface layer, Rf is the resistance of the ion conducting paths that developed in the surface layer. Excellent fit was obtained with this model for experimental data without and with HEC. As an example, the Bode plots for Cu corrosion without and with HEC + KI in 1 M HCl and 0.5 M H2SO4 respectively are presented in Figs. 6 and 7 and show good fit between the experimental and simulated data.

Fig. 5. Impedance plot of copper in 0.5 M H2SO4 in the absence and presence of HEC and HEC + KI

Fig. 6. Experimental (solid) and simulated (hollow) impedance responses for copper in 1 M HCl without (a) and with HEC + KI (b)

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Fig. 7. Experimental (solid) and simulated (hollow) impedance responses for copper in 0.5 M H2SO4 without (a) and with HEC + KI (b)

3.3 Potentiodynamic Polarization Potentiodynamic polarization experiments were carried out to ascertain the influence of HEC on the anodic and cathodic polarization behavior of copper in the studied acid media. Figs. 8a and 8b depict typical potentiodynamic polarization curves for the copper specimen in 1 M HCl and 0.5 M H2SO4 without and with HEC and HEC + KI.

Fig. 8. Polarization curves of Copper corrosion in (a) 1 M HCl and (b) 0.5 M H 2SO4 using HEC, in the absence and presence of KI The polarization curves in 1 M HCl display features of active-passive transformation at about 0.0 mV vs. SCE, as has been reported elsewhere, whereas in 0.5 M H2SO4, the anodic polarization behavior shows feature of spontaneous passivation at potentials more positive than 250 mV vs. SCE. Interestingly, the additives did not visibly alter the polarization behavior of copper in the active, passive or transpassive region in either medium. The corrosion inhibition performance of the additives is as well not clearly obvious, due possibly to the short exposure time adapted in the electrochemical experiments. Subsequent studies will unravel whether the electrochemical corrosion inhibition mechanism of HEC would become altered at longer exposure periods.

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3.4 Theoretical Studies 3.4.1 Quantum chemical calculation The electronic structure of hydroxyethyl cellulose (HEC) has been fully optimized using 3 density functional theory (DFT) electronic structure program Dmol . This is to enable determination of some quantum chemical parameters of the HEC molecule, such as highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and Fukui functions. The corrosion inhibition performance of some organic compounds has been related to quantum chemical parameters such as energy of highest occupied molecular orbital (EHOMO), lowest unoccupied molecular orbital (ELUMO) and energy of gap (ΔE = ELUMO EHOMO) [62,63]. High values (more negative) of EHOMO indicate a high tendency of the molecule to donate electrons to appropriate acceptor molecules with low energy, empty molecular orbitals. On the other hand, low values of ELUMO correspond to a tendency to accept electrons into available vacant orbitals. Low values of the energy gap (ΔE) will encourage good inhibition efficiency, because the energy to dislodge electrons from the last occupied orbital will be low [64]. The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and Fukui functions of HEC are presented in Fig. 9, while Table 2 provides + calculated Mulliken atomic charges and Fukui indices for nucleophilic, f and electrophilic f attack. The HOMO orbital for HEC is localized around the glucosidic ring, while the LUMO orbital is saturated around some hydroxyl functions.

Fig. 9. (COLOUR ONLINE) Electronic properties of HEC (a) HOMO orbital; (b) LUMO + orbital; (c) Fukui (f ) (d) Fukui (f )function function. [C, grey; H, white; O, red] The blue and yellow is surfaces depict the electron density difference; the blue regions show electron accumulation, while the yellow regions show electron loss The computed values of EHOMO, ELUMO and ΔE are -5.589, -0.139 and 5.450 eV respectively. The f function correspond with the HOMO locations (O5, O8, O12, O13, O16 and O17), indicating the sites through which the molecules could be adsorbed on the metal surface, + whereas f correspond with the LUMO locations (O12, O13 and O17), showing sites through which the molecules could interact with the non bonding electrons in the metal.

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British Journal of Applied Science & Technology, 4(9): 1445-1460, 2014 +

Table 2. Calculated Mulliken atomic charges and Fukui indices for f and f attacks by HEC molecule +

Atom C1 C2 C3 C4 O5 C6 C7 O8 C9 C10 O11 O12 O13 C14 C15 O16 O17

f -0.009 -0.009 -0.004 -0.028 -0.007 -0.023 -0.003 -0.001 -0.012 -0.012 -0.182 0.015 0.008 -0.014 -0.003 0.000 0.007

-

-

Atom C1 C2 C3 C4 O5 C6 C7 O8 C9 C10 O11 O12 O13 C14 C15 O16 O17

f -0.001 -0.037 -0.019 -0.012 0.080 -0.018 -0.009 0.040 -0.020 -0.011 0.001 0.165 0.055 -0.036 -0.015 0.032 0.060

3.5.2 Molecular dynamics (MD) simulations Molecular dynamics (MD) simulation of the interaction between a single molecule of HEC and the Cu surface was performed using Forcite quench molecular dynamics to sample many different low energy configurations and identify the low energy minima. Geometry optimized structure of HEC, which was adsorbed on one side of the Cu (110) slab. The lowest energy adsorption models for HEC on the Cu (110) surface from our simulation are shown in Fig. 10. The HEC molecule can be seen to maintain a flat-lying adsorption orientation on the Cu surface. This orientation maximizes contact; hence augments the degree of surface coverage. Quantitative appraisal of the interaction between HEC molecule and the Cu surface was achieved by calculating the adsorption energy (Eads) using the following equation [66]:

Eads  Ecomplex  E HEC  ECu  where

ECu

is the total energy of the copper surface and

(5)

E HEC is

the total energy of HEC.

When the adsorption occurs between the inhibitor and the metal, the energy of the new system is expressed as E complex [45,66]. The obtained value of

Eads

was (-55.09 kJ/mol).

The negative value suggests that there was propensity towards adsorption, which occurs spontaneously, while the magnitude falls within the values expected for non covalent adsorption.

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Fig. 10. Adsorption configurations of HEC on Cu (1 1 0) surface showing (a) the side view and (b) the on-top view

4. CONCLUSION Hydroxyethyl cellulose has been examined and found to possess corrosion inhibition potential. The effectiveness of HEC inhibition of copper corrosion in 1 M HCl and 0.5 M H2SO4 solutions was found to be concentration-dependent by weight loss measurements. Weight loss measurements indicate that corrosion inhibition of copper in HCl medium decreased with time whereas the reverse trend was the case in H2SO4 solution. Impedance measurements at short immersion times indicate that HEC was adsorbed on the copper surface but did not alter the corrosion mechanism of the copper specimen in both media and this was corroborated by the potentiodynamic polarization studies. The inhibition susceptibility is a contribution from anodic reactions. The restricted immersion time for the electrochemical experiment was just enough to allow assessment of the influence of HEC on the electrochemical corrosion behavior of copper, but insufficient for significant inhibitor adsorption, particularly in H2SO4 solution. Addition of KI did not alter inhibition mechanism of HEC and improved the adsorption on the copper surface immersed in HCl solution. Molecular dynamics simulation was used to model the adsorption of a single HEC molecule on the copper surface.

ACKNOWLEDGMENT The authors gratefully acknowledge funding from the Nigeria Tertiary Education Trust Fund (TET Fund).

COMPETING INTERESTS Authors hereby declare that there were no competing interests from any quarters.

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Peer-review history: The peer review history for this paper can be accessed here: http://www.sciencedomain.org/review-history.php?iid=416&id=5&aid=3532

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