Experimental, quantum chemical calculations and molecular dynamics (MD) simulation studies of methionine and valine as corrosion inhibitors on carbon steel in phase change materials (PCMs) solution

Accepted Manuscript Experimental, quantum chemical calculations and molecular dynamics (MD) simulation studies of methionine and valine as corrosion inhibitors on carbon steel in phase change materials (PCMs) solution

Zhe Zhang, Wenwu Li, Weipeng Zhang, Xiaodong Huang, Le Ruan, Ling Wu PII: DOI: Reference:

S0167-7322(18)33792-9 doi:10.1016/j.molliq.2018.09.081 MOLLIQ 9687

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

23 July 2018 7 September 2018 17 September 2018

Please cite this article as: Zhe Zhang, Wenwu Li, Weipeng Zhang, Xiaodong Huang, Le Ruan, Ling Wu , Experimental, quantum chemical calculations and molecular dynamics (MD) simulation studies of methionine and valine as corrosion inhibitors on carbon steel in phase change materials (PCMs) solution. Molliq (2018), doi:10.1016/j.molliq.2018.09.081

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ACCEPTED MANUSCRIPT

Experimental, quantum chemical calculations and molecular dynamics (MD) simulation studies of methionine and valine as corrosion inhibitors on carbon steel in phase change

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materials (PCMs) solution Zhe Zhang a,b, Wenwu Li a, Weipeng Zhang a, Xiaodong Huang a, Le Ruan a,b*, Ling Wu c

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a Guangxi Key Laboratory of Electrochemical and Magneto-chemical Functional

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Materials, College of Chemistry and Bioengineering, Guilin University of Technology,

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Guilin 541004, PR China

b Guangxi Key Laboratory of Geomechanics and Geotechnical Engineering, Guilin

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University of Technology, Guilin 541004, PR China

c School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100,

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PR China

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*Corresponding author.

Tel.: +86 15296001527; Email: [email protected].

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Abstract Corrosion inhibition of methionine (Met) and valine (Val) molecules on carbon steel was investigated in phase change materials solution. Electrochemical measurements

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showed that these inhibitors can protect steel against corrosion, with a maximal protection efficiency up to 96.85% by the molar ratio of Met/Val is 1/1 and the concentration of Met

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is 0.05 mol·L-1. All inhibitors acted as anode type inhibitors. The analysis result of

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scanning electron microscope (SEM) showed that the surface morphology of the carbon

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steel sample immersed in the phase change materials solution in the presence of the

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Met/Val inhibitor was significantly better than that of the uninhibited sample. The inhibition mechanism was theoretically investigated through the quantum chemical

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calculation and molecular dynamic simulation, that showed the Methionine and valine

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molecules could adsorb on Fe (1 1 0) surface, N atoms, S atoms, and -COOH group were the main adsorption active sites. Finally, the migration rate of Cl- ions in the film of three

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corrosion inhibitors (concentration: 0.05 mol·L-1) was studied.

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Key words: Carbon steel; EIS; Polarization; Modeling study; Inhibition

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*Corresponding author.

Tel.: +86 15296001527; Email: [email protected].

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1. Introduction Phase change materials (PCMs) are high-concentration ion mixed solutions composed of inorganic salts and water and other additives, which can absorb and release a

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large amount of heat from the environment during the phase change of temperature rise and decrease[1, 2]. Because of high latent heat, relatively high thermal conductivity,

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non-toxicity, non-flammability and low cost[3], PCMs are widely used in solar energy

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storage[4-6], energy-saving buildings[2, 7, 8], storage and transportation of aquatic

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products[9-11]. However, as a kind of inorganic salt phase change cold accumulating material, PCMs have a high ion concentration liquid phase which will cause serious

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corrosion of metal containers[12-16], so the development and application of PCMs are

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hindered.

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PCMs are usually hydrated salts containing additives such as pour point depressants (KCl, NaCl, NH4Cl, etc.) and nucleating agents (borax and nano-BaCl2, etc.), which have

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a liquid pH of 5 < pH < 8[17, 18]. The corrosion reaction of metal materials such as

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carbon steel in PCMs is completely different from the corrosion reaction in hydrochloric acid because of the more complicated composition. For this reason, the corrosion product (Fe2O3 or NH4FePO4·nH2O) can be converted to soluble FeCl2 and dissolved in the PCMs, making Cl- ions a key factor in the corrosion of carbon steel in PCMs[19]. At present, there are a few methods to reducing the corrosion of carbon steel in PCMs solution. Ryo Fukahori et al[20], studied the corrosion behavior of engineering ceramics (such as Al2O3, AlN, Si3N4, SiC and SiO2) in high-temperature PCMs. Experiments showed that Al2O3, 3

ACCEPTED MANUSCRIPT AlN and Si3N4 have high corrosion resistance to high-temperature PCMs and were suitable for containers of PCMs. Moreno et al[21], studied the corrosion rate of copper, aluminium, stainless steel 316 and carbon steel in contact with PCMs, and suggested the use of different metals and metal alloys in PCMs based on the obtained corrosion rate and

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observation of the sample. Ferrer et al[7]. Studied the corrosive effects of five metals

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(aluminium, copper, carbon steel, stainless steel 304 and stainless steel 316) when in

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contact with four different PCMs (one inorganic mixture, one ester and two fatty acid eutectics). The results indicated that aluminum samples were not suitable for inorganic

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PCMs, copper could be used in fatty acid PCMs, and stainless steel was suitable for all

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PCMs.

Amino acids are considered to be popular green corrosion inhibitors because of their

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non-toxicity, low cost, etc[22-24]. In this paper, the corrosion inhibition properties of methionine and Valine on carbon steel in PCMs were investigated. The molecular information of methionine (Met) and Valine (Val) is shown in Table 1. Electrochemical

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measurements were used to study corrosion inhibition efficiency. The characterization

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technique of SEM was used to verify the corrosion inhibition effect. The corrosion inhibition mechanism of the system was studied by quantum chemical calculation and molecular dynamics simulation.

2. Experiments 2.1. Preparation of electrodes A cylindrical carbon steel (composition, wt.%: C 0.45%, Si 0.17%, Mn 0.5%, S 4

ACCEPTED MANUSCRIPT 0.035%, P 0.035%, Cr 0.25%, Cu 0.25%, Ni 0.30%, and Fe 98%) having a length of 2.5 cm and an exposed geometrical area of 0.5024 cm2 was used as a working electrode to which a copper wire was connected and epoxy resin was mounted in a suitable size glass tube.

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Before the experiment, metallographic sandpaper (300#, 800#, 1200#, 1400#) was

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used to grind the exposed end face of the working electrode step by step to the light level

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of the mirror. The polished working electrode was rapidly washed with distilled water and

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placed in anhydrous ethanol for ultrasonic cleaning for 5 minutes, and then replaced with fresh anhydrous ethanol and ultrasonically cleaned again for 1 minute. The cleaned

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electrode was sealed in absolute ethanol for use.

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2.2. Preparation of phase change materials (PCMs)

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The composition (wt.%) and role of the phase change cold storage material in this experiment were as follows[19]: Na2SO4·10H2O (43.2%, main phase change material),

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Na2HPO4·12H2O (12.3%, main phase change material), NH4Cl (6.2%, pour point

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depressant), KCl (3.1%, pour point depressant), NH4H2PO4 (3.1%, buffer), borax (1.9%, nucleating agent), CMC (2.5%, flocculant ), H2O (27.8%, solvent). Each component material was placed in an Erlenmeyer flask and dissolved under magnetic stirring at a water bath temperature of 40 oC to prepare a eutectic salt phase change virtual cold material for use.

The corrosion inhibitors were methionine (> 99%, Macklin), proline (> 99%, Mackline), and the combination of two corrosion inhibitors. A certain amount of corrosion 5

ACCEPTED MANUSCRIPT inhibitor was weighed out and added to the prepared PCMs solution, and PCMs corrosion solution samples containing different concentrations of corrosion inhibitors were prepared and allowed to stand for 12 hours at 25 oC. The thermal physical properties of the prepared PCMs were tested by the step cooling curve test method. The test results are shown in Fig.

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1.

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From Fig. 1, it can be seen that the PCMS solution prepared in this experiment has a

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crystallization temperature of about 6.4 oC, a phase transition temperature of about 7.3 oC,

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and a degree of undercooling of less than 2 oC. At room temperature, it has phase change energy storage performance and good cycle performance. In addition, the PCMs had a pH

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of 6.5 as measured by a pH meter. Therefore, there is no obvious effect on the performance of PCMs after the addition of inhibitors, so methionine and valine molecules

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can be used as the research object for this experiment.

2.3. Electrochemical measurements

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The traditional three-electrode test system was used to test the corrosion inhibitor of

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carbon steel corrosion inhibition performance in PCMs solution at constant temperature 25 C. The volume of the electrolysis cell is 100 ml, the working electrode is a carbon steel

electrode, the auxiliary electrode is a platinum plate (1.0 cm × 1.8 cm), and the reference electrode is a saturated calomel electrode (SCE). The electrode potentials described herein are all relative to SCE.

The electrochemical impedance spectroscopy (EIS) measurements and the potentiodynamic polarization measurements are all based on the IM6 electrochemical 6

ACCEPTED MANUSCRIPT workstation (ZAHNER, Germany). Before testing, the working electrode was immersed in the test solution for 1h until the open circuit potential (OCP) was stable. Impedance tests were conducted at open circuit potential and a sinusoidal applied voltage of 5 mV was applied to the electrode system. The test frequency range was 100 kHz to 0.1 Hz,

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sweeping from high frequency to low frequency. The AC impedance test data was fitted

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using ZView 2 software. The potentiodynamic polarization curve test has a potential scan

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range of OCP ± 200 mV, sweeping from low potential to high potential, and scanning speed of 2 mV/s. The data of the potentiodynamic polarization curve test were all

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processed by the Tafel extrapolation method from the electrochemical workstation's own

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fitting software.

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2.4. surface morphological studies The pretreated carbon steel specimens (3 mm × 3 mm × 1 mm) were soaked in blank

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and PCMs solutions containing corrosion inhibitors for 48 h. Due to the large surface

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activity of the surface of the corroded carbon steel specimens, it is easily oxidized. Therefore, the etched specimens was quickly washed with distilled water, then placed in a certain amount of absolute ethanol, and the sample was dried by evaporation under reduced pressure. The surface morphology of specimens was observed and detected by a HITACHI (SU 5000) scanning electron microscope (SEM).

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2.5. Quantum chemical calculations Using the DFT/B3LYP method in Gaussian 03w software[25], the geometric configuration and single-site energy calculation of amino acid molecules were performed

occupied orbital energy (E

HOMO),

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at the 6-311G (d, p) unit level. The quantum chemical parameters such as the highest the lowest unoccupied orbital energy (ELUMO), the

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energy gap (∆E =ELUMO - EHOMO), the dipole moment (μ), and the surface charge

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distribution of each amino acid molecule were obtained. Finally, Gaussian View 5.0

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software was used to visualize the results of quantum chemistry calculations, and the

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differences in properties caused by structural changes were visually demonstrated.

2.6. Molecular dynamic (MD) simulations

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The interaction of Met, Val and Met/Val compound corrosion inhibitors and Fe

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crystals in PCMs solution was simulated and studied by Accerrys Inc.'s Material Studio 6.0 software[26]. In addition, the diffusion of Cl- ions in three corrosion inhibitor films

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was simulated and analyzed. The steps and parameters of the simulation system

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construction have been detailed in Zhang Zhe's work[19].

The first simulation system was built using an amorphous cell module. The most stable and dense Fe (1 1 0) surface was selected for supercell creation[27, 28]. The composition of the solution layer is 30 Na+, 10 SO42-, 5 PO43-, 5 H3O+, 5 NH4+, 5 Cl-, 500 H2O and a corrosion inhibitor molecule. The second simulation system consists of 50 corrosion inhibitor molecules and one Cl- ion.

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ACCEPTED MANUSCRIPT The optimized after the system was built. Finally, the molecular dynamics simulation process was performed before the temperature and energy of the entire system were stabilized.

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3. Results and discussion

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3.1. Electrochemical impedance spectroscopy (EIS) measurements

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As shown in Fig. 2, the impedance plots for the absence and presence of different concentrations (0.001 mol·L-1, 0.005 mol·L-1, 0.01 mol·L-1, 0.02 mol·L-1, 0.03 mol·L-1,

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0.04 mol·L-1 and 0.05 mol·L-1) of inhibitor samples have been tested in PCMs solution at

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25 ◦C, with an immersion time of 1 h (The working electrode (WE) was immersed in the test solution for 1 h to obtain a stable open circuit potential before each test). It can be

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clearly seen from the Nyquist plots that all curves exhibit a concave semicircular in the

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high frequency region, the diameter of which is represents the charge transfer resistance (Rct). Usually, the measured impedance loop is a concave semicircle whose center is lower

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than the real axis. This phenomenon is called dispersing effect[29-31]. In addition, it is

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found from the Bode plots that the phase angle of the corrosion inhibitor is larger than the blank sample, and the phase angle value of the corrosion inhibitor increases as the concentration of the corrosion inhibitor increases. It is shown that the corrosion inhibitor molecules form an adsorption film on the surface of carbon steel, and the coverage of the corrosion-inhibiting film layer increases with the increase of the corrosion inhibitor concentration[32].

The impedance experimental data were fitted to obtain the parameters of each circuit 9

ACCEPTED MANUSCRIPT component in the equivalent circuit by ZView 2 software. The equivalent circuit was shown in Fig. 3, with the parameters of each component were listed in Table 2. Excellent fit results can be obtained with this equivalent circuit. In the equivalent circuit diagram, Rs is the solution resistance between the working electrode and the reference electrode. It is

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considered that the PCMs solution with high-concentration salt contains a large amount of

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PO43- and SO42- anions capable of strong adsorption on the iron surface. C1 and R1 are the

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electric double layer capacitance and charge transfer resistance between the solution and an anion concentration layer such as PO43- and SO42-, respectively. Rct is the charge

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transfer resistance between the working electrode surface and the solution layer. The

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electric double layer resistor does not appear to be an ideal capacitor in consideration of the dispersion effect. Therefore, CPE is used as a substitute for capacitors and is used to

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explain the deviation caused by surface roughness[33-37]. The admittance (Y) of the CPE

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is defined as[36]:

Y CPE  Y 0  j 

(1)

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n

Where Y0 and n are modulus and deviation parameters, respectively. ω represents the

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angular frequency at the maximum imaginary part of the impedance. The inhibition efficiencies (ηR) of inhibitors is calculated according to the Eq (2):



%  Rct  Rct 100 0

Rct

(2)

where R0ct is the charge transfer resistances of Working electrode in the absence of inhibitors and Rct is the charge transfer resistances of Working electrode in the presence of 10

ACCEPTED MANUSCRIPT inhibitors.

As can be seen from Table 2, the values of C1 and R1 obtained were small and did not change, indicating that the electric double layer of sulfate and phosphate ions is not resistant to corrosive ions such as Cl- ions. In addition, as the concentration of the inhibitor

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increases, the value of Y0 decreases and the charge transfer resistance increases. This can

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be attributed to a decrease in the local dielectric constant or an increase in the thickness of

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the electric double layer. The decrease in the Y0 value may be due to the replacement of

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water molecules and other molecules adsorbed on the iron surface by the corrosion inhibitor molecules. It was confirmed that the corrosion inhibitor molecules act by

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adsorption at the metal/solution interface[38]. The value of the deviation parameter (n) can be used as an indicator to predict the dissolution mechanism in the system[39]. The

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stability of the n value indicates that the charge transfer process controls the dissolution mechanism in the system without and with different concentrations of corrosion inhibitor. The order of the maximum charge transfer resistance values of the three corrosion

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2 (g).

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inhibitors is Met/Val (17861 Ω cm2) >Met (4262 Ω cm2) >Val (2165 Ω cm2), as shown in Fig.

The synergistic parameter (S) obtained from the corrosion inhibition efficiency value (η) was used to evaluate the interaction relationship and synergistic inhibition effect of the mixed corrosion inhibitor. According to previous experience[40-44], the following equation can be used to calculate the synergistic parameter:

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ACCEPTED MANUSCRIPT S 

1   12 

(3)

1   1 2 

where η(1+2) = (η1 + η2 ) − (η1 ×η2 ), η1 and η2 are the inhibition efficiency values representing Met and Val, respectively. η(1/2) is the inhibition efficiency of the mixed

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inhibitor. Table 2 shows that the S of Met/Val compound inhibitor is less than 1 when the

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inhibitor concentration is less than 0.03 mol·L-1. Moreover, charge transfer resistance of

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the Met/Val compound inhibitor is less than the single inhibitor. This result indicates that the competitive adsorption exists between Met and Val molecules. When the inhibitor

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concentration is greater than 0.04 mol·L-1, the S of the Met/Val compound inhibitor is

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greater than 1, and charge transfer resistance of the Met/Val compound inhibitor is greater than the single inhibitor. This indicates that the interaction between Met and Val molecules

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is synergistic adsorption.

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3.2. Potentiodynamic polarization measurements

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As shown in Fig. 4, the polarization curves for the absence and presence of different concentrations of inhibitor samples have been tested in PCMs solution at 25 ◦C. The

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electrochemical parameters such as corrosion potential (Ecorr), Tafel slopes of cathodic and anodic (βc and βa), corrosion current density (icorr) and inhibition efficiencies (ηi) were received from processing polarization curves and are showed in Table 3. The ηi are calculated according to Eq. (4)[38]:

 i % 

icorr  icorr 100 0 icorr 0

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(4)

ACCEPTED MANUSCRIPT where i0corr and icorr are corrosion current density in absence and presence of inhibitors, respectively.

From Fig. 4 and Table 3, it is apparent that as the concentration of the three corrosion inhibitors increases, the anode and cathode current densities are significantly reduced and

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the anode current density is suppressed to a greater extent than the cathode current density.

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It indicates that the Met and Val inhibitor molecules can adsorb on the iron surface to form

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a corrosion inhibitor film, which effectively slows down the anode dissolution rate and

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prevents Cl- ions erosion[45, 46]. These results demonstrates that the three corrosion

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inhibitors are anode type corrosion inhibitors[47].

In addition, from the value of icorr in Table 3, it is clear that the addition of each

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corrosion inhibitor leads to a significant reduction in corrosion current density (icorr). This

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indicates that as the inhibitor concentration increases, the amount of inhibitor molecules adsorbed on the surface of the WE increases during the same immersion time, making the

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inhibitor film layer denser and more effectively reduce the contact between carbon steel and solution, reducing the corrosion rate of carbon steel. The maximum corrosion

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inhibition efficiency of the three inhibitors is Met/Val (96.85%) >Met (87.66%) >Val (63.71%), as shown in Fig. 4 (d). This result is consistent with the EIS results.

3.3. SEM characterization-inhibition effect From Fig. 5, it can be seen that the carbon steel sheet is immersed in the PCMs solution in the absence of the inhibitor for 48 h, and its surface is severely corroded to have obvious corrosion cracks and deep pores. The surface morphology of the sample 13

ACCEPTED MANUSCRIPT immersed in the PCM solution in the present of the Met/Val inhibitor is superior to the surface morphology of the uninhibited sample. In contrast, in the presence of the Met/Val inhibitor the appearance of the carbon steel sheet surface is significantly improve, with almost no pores except for the polishing lines. Therefore, the results showed that the

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Met/Val inhibitor has good inhibition performance.

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3.4. Quantum chemical calculation

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In order to study the influence of electronic properties and molecular structure on the

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corrosion inhibition efficiency of Met and Val inhibitors and to prove that the inhibitor

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molecules can adsorb on the iron surface, quantum chemical calculation technology was used. In this work, the molecular structure of Met and Val inhibitors was optimized and

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the single point energy was calculated by B3LYP/6-311G (d, p) algorithm in Gaussian

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03W software. And obtained the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) map of each inhibitor molecule (As shown in

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Table 4). In addition, the EHOMO, ELUMO, ΔE (ΔE= ELUMO-EHOMO) and μ (diple moment) directly affects the electronic interaction between the corrosion inhibitor molecule and the

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metal surface[48, 49]. The calculated quantum chemical parameters are listed in Table 5.

In general, the EHOMO represents the electron donating ability of a compound. If the EHOMO value is larger, the electron donating ability of the compound molecule is stronger. In contrast, the ELUMO represents the electron-accepting ability of a compound. The smaller the ELUMO value, the stronger the electron-accepting ability of a molecule. Moreover, the lower ΔE can infer that the inhibitor molecules have strong chemisorption 14

ACCEPTED MANUSCRIPT on the metal surface[50, 51].

It can be seen from Table 4 that the highest occupied molecular orbital (HOMO) of the Met and Val molecules mainly covers on the N atoms and S atoms, while the lowest unoccupied molecular orbital (LUMO) covers on the -COOH group. As seen from Table 5,

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the EHOMO of Met is higher than that of Val molecule, which indicates that the electron

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donation capability of Met molecule is stronger than that of Val molecule. Furthermore,

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the ELUMO of Met is lower than that of Val molecule, which suggests that the electron

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accepting capability of Met molecule is stronger than that of Val molecule. According to the ΔE values in Table 5, the ΔE of Met is lower than Val, so that the chemisorption

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capacity of Met and Val follow the order of Met > Val. This is consistent with the experimental results and also satisfies the number of transferred electrons (ΔN) from the

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inhibitor to the metal surface calculated by Eq. (5)[52, 53]:

 Fe   inh 2 Fe   inh

(5)

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N 

Where, χFe (7 eV) and γFe (0 eV) are the electronegativity and chemical hardness value of

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the iron atom, respectively. χinh and γinh are the electronegativity and global chemical hardness value of the inhibitor molecules, respectively[54, 55].

The ionization potential (I) and electron affinity (A) were calculated according to the Koopmans’ theorem and the the Hartree-Fock theorem[31, 56, 57]. Their operation and approximation were defined as Eq. (6) and Eq. (7):

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 

 

IA

(6)

2 IA 2

(7)

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The relationship between the energies of the HOMO and the LUMO and ionization

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potential and affinity was established, respectively. The frontier orbital energy is given as

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Eq. (8) and Eq. (9):

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I   E HOMO

(9)

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A   E LUMO

(8)

As shown in Table 5, the ΔN value represents the ability of the compound to supply

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electrons to the metal surface, and the inhibitory effect of the inhibitor molecule can be

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directly evaluated. If ΔN < 3.6[28, 58], the ηi value increases with increasing of the ΔN value. From Table 5, it is shown that the ΔN of Met is higher than that of Val, which is

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consistent with the inhibition efficiency of the inhibitor molecules.

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3.5. Molecular Dynamic (MD) simulation Molecular dynamics (MD) simulation has become the primary tool for studying the interaction between inhibitors and metal surfaces. Molecular dynamics (MD) simulations can reasonably predict the optimal molecular structure of inhibitor molecules on metal surfaces[31, 56, 59, 60]. In addition, it can provide a basis for judging the inhibition performance of the inhibitor by obtaining the values of adsorption energy (Eadsorption) and binding energy (Ebinding) between the inhibitor molecule and the metal surface. In this 16

ACCEPTED MANUSCRIPT experiment, the interaction between the Met and Val molecules and Fe (1 1 0) surface was studied by molecular dynamics simulation, and the adsorption configuration of each inhibitor molecule on the iron surface was obtained, as shown in Fig. 6. Obviously, the inhibitor molecules are absorbed almost flat on the Fe (1 1 0) surface. This parallel or flat

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position on the iron surface can be attributed to the nearly equal distribution of the density

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of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular

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orbital (LUMO) over the entire molecule. In addition, the entire inhibitor molecule can provide electrons to the unoccupied orbital of the iron surface to form stable coordination

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bonds, and can also accept electrons from the d orbitals of iron with π bonds[61].

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Generally, if the bond distance is less than 3.5 Å, it indicates that a strong chemical bond has formed between the atoms. Conversely, a longer bond length indicates that the

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interaction force between atoms is Van der Waals force[62]. The values of the shortest

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bond distance between the heteroatoms of the three inhibitors (Met, Val and Met/Val) and the Fe (1 1 0) surface were calculated as follows:

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Met-Fe adsorption model: (N-Fe = 4.07 Å, S-Fe = 3.37 Å, O-Fe = 3.13 Å);

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Val-Fe adsorption model: (N-Fe = 3.13 Å, O-Fe = 3.05 Å);

Met/Val-Fe adsorption model: (N-Fe = 3.11 Å, S-Fe = 3.27 Å, O-Fe = 3.11 Å).

These values confirm the presence of chemical bonds between the three corrosion inhibitors and the Fe (1 1 0) surface. Longer bond distances indicate van der Waals forces in the adsorption of inhibitors and Fe. In addition, the S atoms and -COOH group of the corrosion inhibitor molecules in the frontier molecular orbital map were also verified as 17

ACCEPTED MANUSCRIPT the main adsorption sites.

To study the degree of interaction between the corrosion inhibitors and the Fe crystal. The binding energy (Ebinding) and adsorption energy (Eadsorption) values of the three corrosion inhibitors can be calculated according to Eq. (10) and Eq. (11)[27]:

(10)

(11)

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Ebinding   Eadsorption

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Eadsorption  Etotal  Esurfacesolution  Einhibitorsolution  Esolution

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Herein, Etotal is the total energy of the entire simulation system. It includes iron crystals, adsorbed corrosion inhibitor molecules and solutions. The Esurface+solution and the

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Einhibitor+solution represent the total energy of the system without corrosion inhibitor and the total energy of the system without iron crystals, respectively. The Esolution is the energy of

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the PCMs solution.

The values of binding energy (Ebinding) and adsorption energy (Eadsorption) are listed in

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Table 6. The order of the values of the binding energies of the corrosion inhibitors is:

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Met/Val (6.108 eV) > Met (3.732 eV) > Val (2.438 eV). According to theoretical analysis, the higher value of Ebinding indicates that the adsorption system is more stable, and the corrosion inhibitors should have higher corrosion inhibition efficiency, such as: Met/Val > Met > Val. This is consistent with the experimental results. In addition, the Ebinding value of Met/Val is less than the Ebinding value of a single Met and Val, namely: Ebinding (Met) + Ebinding (Val)

=3.732 + 2.438 eV = 6.170 eV > Ebinding (Met/Val) =6.108 eV, this may be caused by the

competitive adsorption of Met molecules and Val molecules in the system[63]. 18

ACCEPTED MANUSCRIPT In order to verify the experimental results more accurately and truthfully, we studied and calculated the velocity of Cl- ions in the corrosion inhibitor film formed by three corrosion inhibitors with the concentration of 0.05 mol·L-1 (the best corrosion inhibition efficiency) by molecular dynamics simulation. Because pitting of Cl- ions is a major factor

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in the corrosion of carbon steel in PCMs solution.

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Generally, the value of the diffusion coefficient (D) can be used to assess the rate of

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migration of corrosive materials in the inhibitor film. A small value of the diffusion

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coefficient means that the inhibitor film has a good inhibitory effect, which results in high corrosion inhibition efficiency. The formula for calculating the diffusion coefficient (D) is

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as shown in Eq. (12) and Eq. (13)[64, 65]:

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D

2 1 N MSDt     Ri t   Ri 0   N i1 





(13)

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2 1 d n D  lim  Rit  Ri0 6 t  dx i

(12)

Where, Ri(t), Ri(0) represent the positions of corrosive species at time t and 0,

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respectively. |Ri(t) − Ri(0)|2 stands for mean-square displacement (MSD).

It can be seen from Fig. 7 that the corrosion inhibitor film layer of Met/Val is more compact than the corrosion inhibitor film layers of Met and Val, and can better reduce the migration rate of Cl- ions and achieve good inhibition efficiency. Further, as shown in Fig. 8 and Table 7, the order of the migration rate values of Cl- ions in each of the corrosion inhibitor film layers is Met/Val
ACCEPTED MANUSCRIPT inhibitors is confirmed to be Met/ Val>Met>Val. At the same time, from the value of the diffusion coefficient of the corrosion inhibitor, the value of Met/Val (0.000018×10-9 m2 s-1) is much smaller than the value of Met (0.004089×10-9 m2 s-1) and the Val (0.004885×10-9 m2 s-1). This may be due to the similar structure of Met and Val, both of which are chain

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molecules. Besides, the corrosion inhibitor molecules act synergistically under high

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concentration conditions, stacking on each other to supplement the adsorption on the iron

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surface, making the corrosion inhibitor film layer tighter and greatly reducing the migration rate of Cl- ions in the corrosion inhibitor film, achieving good Corrosion

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inhibition efficiency. This confirms that in the electrochemical test results, when the

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Met/Val corrosion inhibitor reaches 0.05 mol·L-1, the corrosion inhibition performance is better than that of the single corrosion inhibitor.

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4. Conclusion

The corrosion inhibition performance of carbon steel in PCMs solution was evaluated

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by electrochemical method using Met, Val and Met/Val corrosion inhibitors. The inhibition mechanism of Met, Val and Met/Val was studied by SEM, quantum chemical calculation

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and molecular dynamics simulation. In addition, the results of this experiment are compared with the results of research on the corrosion inhibition performance of carbon steel in PCMs solution by methionine (Met) and proline (Pro)[19].

Electrochemical test results show that both Met and Val inhibitors can reduce the corrosion of carbon steel in PCMs solution, and the inhibition efficiency is positively correlated with the concentration. The highest corrosion inhibition efficiencies of Met and 20

ACCEPTED MANUSCRIPT Val inhibitors are 87.66% and 63.71%, respectively. However, Met/Val has the highest corrosion inhibition efficiency, which is 96.85%, which is higher than the inhibition efficiency (94.3%) of Met/Pro. In addition, the results of compounding effect analysis showed that there was competitive adsorption between Met and Val inhibitors at a

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concentration of less than 0.03 mol·L-1, and when the concentration was greater than 0.04

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mol·L-1, synergistic adsorption occurred between Met and Val. The polarization curve test

SC

shows that Met, Val and Met/Val are both anode type inhibitors, and the corrosion

NU

inhibition efficiency is consistent with the EIS test results.

Quantum chemical calculations and molecular dynamics simulations show that both

MA

Met and Val and Met/Val inhibitors can adsorb on Fe (1 1 0) surface, and N atoms, S atoms, and −COOH group are the main adsorption active sites. Met/Val has a stronger

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adsorption capacity than Met and Val single inhibitors. However, the adsorption capacity of Met/Val is less than the sum of the adsorption energies of the single inhibitors, indicating that there is a competitive adsorption between the Met and Val inhibitors when

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the concentration of the inhibitor is low. In addition, the migration rate of Cl- ions in the

AC

film of three corrosion inhibitors (concentration: 0.05 mol·L-1) was studied. It was found that the migration rate of Met/Val was the smallest, and it was also much smaller than the migration rate of Met/Pro(inner salt). It shows that the corrosion inhibition performance of Met/Val is stronger than that of Met/Pro. This is attributed to the fact that the chain inhibitor molecule (Val) is more suitable for compounding with Met than the heterocyclic inhibitor molecule (Pro).

21

ACCEPTED MANUSCRIPT Acknowledgements This work was funded by the Nature Science Foundation of Guangxi Province of China (No. 2016GXNSFAA380061), Guangxi Key Laboratory of Electrochemical and

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CE

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Magnetochemical Functional Materials (EMFM20161104, EMFM20161203).

22

ACCEPTED MANUSCRIPT

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ACCEPTED MANUSCRIPT [64] Y. Yan, X. Wang, Y. Zhang, P. Wang, X. Cao, J. Zhang, Molecular dynamics simulation of corrosive species diffusion in imidazoline inhibitor films with different alkyl chain length, Corrosion Science, 73 (2013) 123-129. [65] A. Guo, G. Duan, K. He, B. Sun, C. Fan, S. Hu, Synergistic effect between 2-oleyl-1-oleylamidoethyl

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Theoretical Chemistry, 1015 (2013) 21-26.

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Figure Fig. 1 Cooling curves of phase change materials, (a) bare, (b) Met/Val.

Fig. 2 Nyquist plots (a), (c), (e) and the Bode and phase angle plots (b), (d), (f) of carbon

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steel in PCMs solutions with and without inhibitors at 25 °C, and (g) concentration-Rct/η

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diagram of Met, Val and Met/Val inhibitors.

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Fig. 3 Equivalent circuit used to calculate the impedance measurement of carbon steel in a

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PCMs solution with and without inhibitors at 25 °C.

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Fig. 4 Potentiodynamic polarization curves of carbon steel in PCMs solution, with and without inhibitors at 25 °C, (a) Met inhibitors, (b) Val inhibitors, (c) Met/Val compound

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inhibitors and (d) concentration-icorr/η diagram of Met, Val and Met/Val inhibitors.

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Fig. 5 The SEM micrographs of corrosion surfaces formed by the CS sheets immersed in various PCMs solution for 48 h, (a) blank PCMs solution, (c) PCMs solution in presence

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of Met/Val inhibitor, and the (b),(d) ere the detail view of (a), (c) respectively.

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Fig. 6 Molecular simulation of the most favorable adsorption mode obtained by Inhibitors on Fe (1 1 0) surface in PCMs solution, (a and b) is Met, (c and d) is Val and (e and f) is Met/Val. Fig. 7 Simulation model of Cl- ion diffusion in three inhibitor films: (a and b) is Met, (c and d) is Val and (e and f) is Met/Val. Fig. 8 The MSD curves of Cl- ion corrosive species in the three kinds of dynamic simulation systems. 32

ACCEPTED MANUSCRIPT Table Table 1 Chemical names, structural formulas, symbols and molecular weight of the two inhibitor molecules.

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Table 2 EIS parameters for corrosion of carbon steel in PCMs solution, with and without

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inhibitors at 25 ° C.

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Table 3 Potentiodynamic polarization parameters of Met, Val and Met/Val inhibitor

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molecules at 25 °C.

Table 4 Optimized geometry and and frontier molecular orbital density distribution of Met

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and Val inhibitor molecules.

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Table 5 Quantum chemical parameters of Met and Val molecules.

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Table 6 Adsorption and binding energies of the Met, Val and Met/Val inhibitor molecules

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to the Fe (1 1 0) surface are in the simulated system. Table 7 The diffusion coefficient of Cl- ion in the three kinds of inhibitor film dynamic

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simulation systems.

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Table 1

Chemical names, structural formulas, symbols and molecular weight of the two inhibitor molecules.

Structure

Symbol

L-Methionine

SC

Met

RI

Inhibitor

L-Valine

AC

CE

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MA

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Val

34

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Molecular weight (g·mol-1)

149.21

117.15

ACCEPTED MANUSCRIPT

Table 2

EIS parameters for corrosion of carbon steel in PCMs solution, with and without inhibitors at 25 ° C.

Rs

C1

CPE

R1

Inhibitors (Ω cm )

(μF cm )

(Ω cm )

Y0(μΩ S cm )

0

6.292

3.265

5.880

53.410

0.001

5.353

6.959

5.180

0.005

6.367

7.549

6.236

0.01

6.046

4.847

0.02

6.652

6.634

0.03

6.676

0.04

n

(Ω cm )

(%)

0.659

877

--

--

31.792

0.765

1848

52.56

--

23.630

0.782

2351

62.71

--

17.354

0.792

2823

68.94

--

6.603

17.223

0.801

3067

71.42

--

4.561

6.668

15.706

0.806

3244

72.97

--

6.002

4.387

5.952

16.477

0.802

3723

76.45

--

6.744

3.842

6.986

13.703

0.823

4262

79.43

--

n

SC

NU 5.851

-1

-2

0.001

6.356

6.326

5.830

33.177

0.721

1502

41.63

--

0.005

6.411

6.426

6.043

29.995

0.738

1563

43.91

--

0.01

6.376

6.493

6.053

28.130

0.749

1646

46.74

--

0.02

6.943

6.678

6.464

33.337

0.728

1728

49.27

--

0.03

6.337

6.141

5.912

29.993

0.736

1830

52.09

--

0.04

7.288

6.869

6.643

24.266

0.722

1895

53.74

--

0.05

6.179

6.925

5.871

26.123

0.751

2165

59.51

--

0.001

6.508

8.134

6.430

26.653

0.771

1741

49.64

0.55

AC

CE

Met/Val

S 2

RI

(mol L )

0.05

Val

2

MA

Met

-2

PT E

bare

2

D

-1

ηR

Rct

PT

Conc.inh

35

ACCEPTED MANUSCRIPT 6.531

7.593

6.373

26.882

0.759

2132

58.88

0.51

0.01

6.741

7.678

6.674

23.259

0.782

2294

61.78

0.43

0.02

6.210

5.736

6.047

21.605

0.769

2799

68.68

0.46

0.03

6.879

5.232

6.820

13.306

0.796

5317

83.51

0.79

0.04

6.498

4.444

6.288

10.431

0.797

12246

92.84

1.52

0.05

6.711

3.794

6.484

10.122

17861

95.09

1.70

AC

CE

PT E

D

MA

NU

SC

RI

PT

0.005

36

0.780

ACCEPTED MANUSCRIPT Table 3 Potentiodynamic polarization parameters of Met, Val and Met/Val inhibitor molecules at 25 °C. Conc.inh

Ecorr

βa

-βc

η

icorr

Inhibitors

S (%)

37.20

--

--

17.90

51.88

--

0.069

11.20

69.89

--

0.062

7.88

78.82

--

0.210

0.058

6.68

82.04

--

0.208

0.053

5.23

85.94

--

0.222

0.060

4.87

86.91

--

-0.673

0.206

0.059

4.59

87.66

--

-0.676

0.345

0.139

23.40

37.10

--

-0.678

0.328

0.134

23.00

38.17

--

-0.678

0.309

0.133

22.10

40.59

--

0.02

-0.675

0.296

0.110

20.10

45.97

--

0.03

-0.678

0.259

0.097

17.30

53.49

--

0.04

-0.674

0.247

0.097

17.10

54.03

--

0.05

-0.675

0.246

0.083

13.50

63.71

--

0.001

-0.667

0.297

0.100

16.90

54.57

0.66

0.005

-0.679

0.241

0.080

13.90

62.63

0.50

(V dec )

0

-0.675

0.303

0.296

0.001

-0.675

0.293

0.107

0.005

-0.675

0.269

0.01

-0.675

0.236

0.02

-0.678

0.03

-0.676

0.04

-0.677

0.05

CE

0.005

AC

0.01

NU

SC

RI

(V dec )

PT

(μA cm )

(V vs.SCE)

0.001

Val

-2

MA

Met

-1

(mol L )

D

bare

-1

PT E

-1

Met/Val

37

ACCEPTED MANUSCRIPT -0.669

0.240

0.079

11.30

69.62

0.41

0.02

-0.678

0.234

0.065

8.22

77.90

0.44

0.03

-0.674

0.204

0.056

3.73

89.97

0.65

0.04

-0.678

0.198

0.061

1.96

94.73

1.14

0.05

-0.675

0.155

0.058

1.17

96.85

1.42

AC

CE

PT E

D

MA

NU

SC

RI

PT

0.01

38

ACCEPTED MANUSCRIPT

Table 4

Optimized geometry and and frontier molecular orbital density distribution of Met and Val inhibitor molecules. Structure

HOMO

LUMO

PT

Inhibitors

SC

RI

Met

AC

CE

PT E

D

MA

NU

Val

39

ACCEPTED MANUSCRIPT

Table 5

Quantum chemical parameters of Met and Val molecules. EHOMO

ELUMO

△E

μ

(eV)

(Debye)

Met

-6.035

-0.181

5.854

2.427

Val

-6.855

-0.075

6.780

1.318

NU MA D PT E CE AC

40

△N

2.927

0.665

3.465

3.390

0.521

3.108

RI

(eV)

SC

(eV)

γ=(I-A)/2

PT

χ=(I+A)/2

Assembly molecules

ACCEPTED MANUSCRIPT

Table 6

Adsorption and binding energies of the Met, Val and Met/Val inhibitor molecules to the Fe (1 1 0) surface are in the simulated system. Eadsorption

Ebinding

ηmax

(eV)

(eV)

(%)

Met

-3.732

3.732

87.66

Val

-2.438

2.438

63.71

Met/Val

-6.108

6.108

96.85

AC

CE

PT E

D

MA

NU

SC

RI

PT

Inhibitors

41

ACCEPTED MANUSCRIPT

Table 7 The diffusion coefficient of Cl- ion in the three kinds of inhibitor film dynamic simulation systems. Diffusion coefficient /(10-9 m2s-1)

PT

Inhibitors

0.004885

Met/Val

0.000018

SC

Val

AC

CE

PT E

D

MA

NU

0.004089

RI

ClMet

42

ηmax (%) 87.66 63.71 96.85

ACCEPTED MANUSCRIPT

Highlights 1. The methionine and valine were used as corrosion inhibitors for carbon steel.

2. The inhibition performance of inhibitors in phase change materials solution.

RI

PT

3. EIS, Polarization and SEM were used to investigate the inhibition performance.

SC

4. The inhibition mechanism was studied by computer modeling studies.

AC

CE

PT E

D

MA

NU

5. Simulated the migration rate of Cl- ion in the corrosion inhibitor films.

43

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8