The inhibition mechanism of maize gluten meal extract as green inhibitor for steel in concrete via experimental and theoretical elucidation

Construction and Building Materials 198 (2019) 288–298

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The inhibition mechanism of maize gluten meal extract as green inhibitor for steel in concrete via experimental and theoretical elucidation Zhaocai Zhang a,b,c,⇑, Hengjing Ba a,b,c, Zhenyu Wu a,b,c,⇑, Yu Zhu d a

School of Civil Engineering, Harbin Institute of Technology, Harbin 150090, China Key Lab of Structures Dynamic Behavior and Control of the Ministry of Education, Harbin Institute of Technology, Harbin 150090, China Key Lab of Smart Prevention and Mitigation of Civil Engineering Disasters of the Ministry of Industry and Information Technology, Harbin Institute of Technology, Harbin 150090, China d School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, Henan, China b c

h i g h l i g h t s
Extract’s main basic constituent responsible for inhibition is Glu, Pro and Leu.
MGME and its main basic constituent are mixed-type inhibitors.
The antagonistic effect between Glu, Pro and Leu in extract was investigated.
Bonding of main basic inhibitors with steel was studied by theoretical calculation.

a r t i c l e

i n f o

Article history: Received 10 May 2018 Received in revised form 31 October 2018 Accepted 23 November 2018

Keywords: Green inhibitor Steel Adsorption Quantum chemical calculation Monte Carlo simulations

a b s t r a c t The inhibition mechanism of maize gluten meal extract (MGME) for steel was systematically elucidated combining experimental analysis with theoretical calculation. It is revealed that the MGME and its main basic constituent are responsible for inhibition including Glu, Pro and Leu, their inhibition efficiency are 83.15% and 79.27%, respectively. The antagonistic effect between the main basic molecules of Glu, Pro and Leu in MGME and the barrier film formed on steel surface were verified by electrochemical measurement and XPS result, respectively. Besides the quantum chemical calculation and Monte Carlo simulation were used to clarify inhibition mechanism of co-adsorption. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Since their low cost and outstanding mechanical property, steels are widely used as the reinforced building material. However, the main drawback of mild steel is easy to suffer from corrosion under severe environments [1]. Subsequently, the carbon dioxide, chloride ion ingress (seawater, de-icing salt and saline alkali land.etc) and sulphate attack on the passive film surrounding reinforcing steel, causes severe damage in the duration of its design life [2,3]. So far, the addition of inhibitors is one of the effective methods applied for retard of steel corrosion in concrete. ⇑ Corresponding authors. Tel.: +086 188 4563 8691. E-mail addresses: [email protected], [email protected] (Z. Zhang), [email protected] (Z. Wu). https://doi.org/10.1016/j.conbuildmat.2018.11.216 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.

Therefore, many organic chemicals have been developed as corrosion inhibitors in the past several decades. However, the major usage of inhibitors has been controlled owing to the high cost and toxicity. Growing environmental concerns have promoted researchers to focus on the development of environmental friendly corrosion inhibitors [4]. Therefore, studying green inhibitors for reinforcing steel in corrosive condition are significant not only in practice field but also for sciatica value. In this work, a novel green inhibitor for steel in concrete was firstly extracted from maize gluten meal, a by-product from the production of corn starch industry. And for further elucidation of its inhibitive mechanism, the compounds in MGME was separated and identified by liquid chromatography method. Then, a comparative anticorrosion study of MGME and the main contributor alone as well as their interactive effects were evaluated for steel rebar in

Z. Zhang et al. / Construction and Building Materials 198 (2019) 288–298

synthetic concrete pore solution contained 3.0 wt% NaCl; and the inhibition mechanism of major basic compounds was also discussed by quantum chemical computations and molecular dynamics simulations. 2. Experimental 2.1. Materials and samples preparation Chemicals and reagents were all purchased from Shanghai Macklin/Aladdin chemical Co., Ltd. Maize gluten meals were purchased from a local vendor of biological products, and the corn plants were grown in Heilongjiang province. The mild steel were produced by the Guangzhou Steel Group Co. Ltd. in China. 2.1.1. Extraction method Maize gluten meal from the byproducts of food processing industry was obtained according to previous procedure described

289

in reports [5,6] with some modifications. Maize gluten meal extracts (MGME) were extracted as follows; Firstly, maize gluten meal was ground to generate particles smaller than 85 lm. A mixtures of a-Amylase/maize flour (20 mg/g) were homogenized with 100 g/L distilled water adjusted to pH 4–7 with 2 M NaOH, which was performed at around 70 °C water bath for 2–3 h to decompose the residue starch by enzymatic reaction, then the precipitants were washed with water. Secondly, the wetting powders were dispersed in 10% (w/v) acetone for 30 min, and the extract was centrifuged to remove odor and color impurities at room temperature. Thirdly, the precipitants dispersed with 10 times weight 70% ethanol water at 60 °C for 2–4 h, followed by centrifugation. Fourthly, the precipitate was collected and treatmented with 0.1 M NaOH for 10% (w/v) suspensions. And then the alkaline extraction was performed at 60 °C for 2–4 h. Finally, the supernatant was separated, and precipitated at pH 4–6 with 1.0 M HCl, and the residuals were washed by deionized water to remove the chloride, then dried below 50 °C in oven. The dried precipitants

Step 1. The grinding and screening process of raw materials, as well as the decomposition of residual starch via heated and stirred.

Step 2. Acetone extracts pigment and separated by centrifugation

Step 3. Ethanol soluble zein and separated by centrifugation

Step 4. Alkali soluble MGME and separated by centrifugation

Step 5. Acid precipitation of MGME supernatants and separated by centrifugation Fig. 1. The main process of MGME extraction.

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of maize protein were soluble in dilute alkali solution used as inhibitors for steel in simulated concrete pore solution. The main extraction process of MGME were described in Fig. 1. 2.1.2. The main constituents in extract determined by HPLC Identification and quantification of the main compounds contained in the extracts were carried out by using a High Performance Liquid Chromatography (HPLC), which can facilitate to deeply and broadly reveal the mechanism of the anticorrosion behavior of MGME based on chemical nature of the extract, composition and structure. HPLC with Fluorescence and DiodeArray Detection (Agilent Technologies 1260 series, Germany) using an G1311B quaternary pump, a G1329B SL auto-sampler, a G1316A column oven was used to identify the type of amino acids present in MGME extraction. In detail, the MGME powder was hydrolyzed with 6 M HCl for 24 h, then the pH was adjusted to 7.0 with 6 M NaOH, and the filtrates separated by 0.45 um membrane were used as the mobile phase for HPLC test controlled by Agilent LC chemstation software. Table 1 indicates that the major compounds are Glu (glutamic acid, 1770.30 lmol/g), Pro (proline, 883.35 lmol/g) and Leu (Leucine, 504.35 lmol/g), Val (Valine, 428.20 lmol/g), Gly (Glycine, 389.80 lmol/g), Ala (Alanine, 316.25 lmol/g) and Ile (Isoleucine, 288.60 lmol/g) in 1.0 g MGME. Most of them possess the similar functional groups of amino and carboxyl groups, besides, the Pro exhibits a pyrrole ring and Glu has two amino and carboxyl groups. Therefore, the 1927.20 lmol/g Leu could be considered as the sum of Leu (504.35 lmol/g), Val (428.20 lmol/g), Gly (389.80 lmol/g), Ala (316.25 lmol/g) and Ile (288.60 lmol/g), then a comparative experiment of 1.0 g/L MGME and their main ingredients (Glu1.77 mmol/L, Pro-0.88 mmol/L, and Leu 1.93 mmol/L) alone as well as the synergism effects of them were performed. In other word, 1.77 mmol/L Glu + 0.88 mmol/L Pro + 1.93 mmol/L Leu was the major amino acid and constitutes 77.12% by mole of all the amino acid in 1.0 g/L MGME. The chemical and structural formula of all constituents in MGME can be found in reference [7]. The corrosive medium was a synthetic concrete pore solution [8]: 0.6 mol/L potassium hydroxide + 0.2 mol/L sodium hydroxide + saturated calcium hydroxide solution with 3 wt% NaCl. The testing solution was prepared using the synthetic concrete pore solution with different concentrations (MGME: 1.0 g/L MGME, Glu: 1.77 mmol/L, Pro: 0.88 mmol/L, Leu: 1.93 mmol/L) of the inhibitors and combination of them (Glu + Pro: 1.77 mmol/L Glu + 0.88 mmol/L Pro; Glu + Leu: 1.77 mmol/L Glu + 1.93 mmol/L Table 1 The Chemical characterization of extracts.

Leu; Pro + Leu: 0.88 mmol/L Pro + 1.93 mmol/L Leu; Glu + Pro + Leu: 1.77 mmol/L Glu + 0.88 mmol/L Pro + 1.93 mmol/L Leu). The solution with absence of inhibitors was seemed as blank for comparison. The steel, U6mm in diameter and 100 mm long, were washed by 12 wt% HCl and polished successively by emery paper of different grades (400, 600 and 800) to a near mirror finished surface, then carefully welded lead and coated with epoxy resin on both ends to expose area 15.08 cm2, then washed with distilled water, degreased with ethanol, finally dried at room temperature for electrochemical test. 2.2. Electrochemical measurements The electrochemical experiments were carried out in a container cell with three electrodes connected to RST5200 electrochemical work station. The pretreated steel was the working electrode; platinum wire was used as the counter electrode and saturated calomel electrode (SCE) as the reference electrode. All potentials were measured versus the SCE reference electrode. Prior to electrochemical test, the working electrode was put in the corrosive solution until a steady state of free corrosion potentials were achieved. Subsequently, EIS measurements were carried out under potentiostatic conditions in a frequency range from 10 kHz to 10 mHz with an amplitude of 7 mV, and the impedance data were analyzed and fitted using impedance analyzing software (ZSimpWin version 3.50). Finally, Tafel polarization curves (TP) were obtained by changing the electrode potential automatically in the range of 300 mV more or less versus the free corrosion potential (Ecorr ) at a scan rate of 1 mV/s. The temperature of all those test solutions was at room temperature. 2.3. XPS analysis The surface chemical composition of the adsorbed film of inhibitors on steel was detected by X-ray Photoelectron Spectroscopy (XPS) (ESCALAB 250Xi Spectrometer, Thermo Fisher Scientific) using a standard Al Ka (1486.6 eV) as the excitation source and the binding energy was calibrated by measuring C 1 s peak (285.1 eV). The film formulation were prepared by immersion of the steel substrate in 3 wt% NaCl simulated concrete pore solution with Glu + Pro + Leu (1.77 mmol/L Glu + 0.88 mmol/L Pro + 1.93 mmol/L Leu) at room temperature. The steel specimen was removed from the immersion solution and vacuumed dry for XPS test. 2.4. Calculation method

Amino acid in extracts

lmol/g MGME

Asp (Asparagine) Glu (Glutamic acid) Ser (Serine) His (Histidine) Gly (Glycine) Thr (Threonine) Arg (Arginine) Ala (Alanine) Tyr (Tyrosine) Cys (Cysteine) Val (Valine) Met (Methionine) Phe (Phenylalanine) Ile (Isoleucine) Leu (Leucine) Lys (Lysine) Pro (Proline)

195.10 1770.30 317.15 86.75 389.80 129.70 217.50 316.25 176.10 211.60 428.20 102.70 280.50 288.60 504.35 83.10 883.35

Bold values of Glu, Pro and Leu are the main constituents in MGME.

2.4.1. Quantum chemical method The generalized gradient approximation density functional of BLYP (GGA/BLYP) method was employed to geometrical optimizations and frequency calculations via DMol3 module in Materials Studio software 8.0 from Accelrys Inc [9]. The ‘‘DNP” doublenumeric basis set, fine convergence accuracy and global orbital cutoffs, were performed in all computations. In order to obtain more reasonable data, the water as solvent was accomplished by using conductor-like screening model (COSMO). Otherwise, the quantum chemical parameters computation for the optimized geometry situations were conducted with no symmetry constraints, and the spin unrestricted was chose to assure the minimum value on potential energy surface during the optimization process. 2.4.2. Monte Carlo simulations The adsorption process of the main contributor (Glu, Leu and Pro molecule) on Fe surface was studied using Monte Carlo simulations by Materials Studio software 8.0 from Accelrys Inc [9].

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The interaction simulations between the studied molecules and Fe (1 1 0) surface were carried out in a simulation box (29.788Å  29.788Å  30.134Å), and a representative part of an interface devoid of any arbitrary boundary effects was simulated by periodic boundary conditions. The box included of a vacuum layer, a water slab filled with the investigated elements and a Fe slab. All atoms in the Fe (1 1 0) systems including both the amino and water were freely interacted with the iron surface in the simulating process. The Condensed phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS) was used for the structure optimizations of all constituents in the simulation system.

3. Results and discussion The chemical composition of green inhibitor shows that the Glu, Pro and Leu can represent the major amino acid and constitutes 77.12% by mole of all the amino acid in 1.0 g/L MGME. Evaluation of the inhibition characteristics of Glu, Pro and Leu alone as well as the synergism effects were carried out by Tafel polarization and EIS measurements.

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3.1. EIS measurement The Nyquist plots for steel in corrosive medium without and with the different concentrations of MGME and Glu, Pro, Leu as well as their combination were displayed respectively in Fig. 2. As shown in Fig. 2 all Nyquist plots exhibit one single capacitive loop indicating that a charge-transfer process mainly controls the corrosion of steel, which is similar to the previous EIS with different MGME concentration. Besides, all the impedance spectra show a similar shape, suggesting the corrosion characteristic is quite not changed. Compared with the blank medium, the diameter of the semicircles raises slightly with the presence of lone Glu, Pro and Leu. However, the diameter of profiles enhances dramatically with

Fig. 3. Equivalent circuit diagram used to fit impedance data.

Fig. 2. The Nyquist plots for steel in corrosive solution without and with different inhibitors.

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Table 2 Impedance parameters and inhibition efficiency for steel in simulating concrete pore solutions with different inhibitors. Inhibitor

RS /(X)

Y0  103 /(X1  sn  cm2 )

n

Rct /(X)

IEEIS /(%)

S

Blank Glu Pro Leu Glu + Pro Glu + Leu Pro + Leu Glu + Pro + Leu MGME

26.49 24.05 18.65 20.69 28.07 23.26 26.16 32.56 40.66

0.66 0.66 0.78 0.71 0.59 0.66 0.62 0.55 0.21

0.78 0.74 0.73 0.73 0.72 0.73 0.75 0.71 0.86

1141.38 3744.42 2332.85 2999.09 4609.02 3292.34 4264.36 5532.27 8164.96

– 69.52 51.07 61.09 75.24 65.33 73.20 79.37 86.02

– – – – 0.60 0.34 0.71 – –

combination of any both molecules, showing a high corrosion inhibition performance with combination effect. Similarly, the fitting impedance parameters derived using the equivalent circuit in Fig. 3 were listed in Table 2. The impedance of CPE is defined as follows [10]

Z CPE ¼

1 n Y 0 ðjxÞ

where Y0 is the modulus of CPE constant, n represents the CPE exponent, corresponding to the deviation parameter with regard to a phase shift, j2 = 1 is an imaginary number, and x is the angular frequency. Depending on n, CPE can describe resistance (ZCPE = R, n = 0), inductance (ZCPE = L, n = 1), Warburg impedance (ZCPE = W, n = 0.5), or capacitance (ZCPE = C, n = 1). The inhibition efficiency from the impedance data is calculated from the following equation.

IEEIS ¼

Rct  Rct;0  100% Rct

where Rct and Rct,0 are the charge transfer resistances for mild steel in 3 wt% NaCl simulated concrete pore solution with presence and absence of inhibitors, respectively.

As indicated in Table 2, it is clear that the Rct enhance with an inhibition order of Pro < Leu < Glu. Especially, the strengthening trends of Rct values for any both combination of them are also apparent. Also shown the green MGME and Glu + Pro + Leu exhibit an inhibition efficiency of 86.02% and 79.37%, respectively. These results show that the Glu, Leu, and Pro molecules are well the component responsible for the corrosion inhibition of MGME. In order to further explain the interactive of Glu, Pro and Leu, the synergism parameter (S) are calculated as follows [1] and also presented in Table 2.

S¼

1  IE1  IE2 þ IE1  IE2 1  IE1þ2

where IE1, IE2, and IE1+2 are the surface coverages (defined by inhibition efficiency) of Glu, Pro and Leu as well as both of them coexistence in the synthetic concrete pore solution with 3.0 wt% NaCl. Normally, S < 1 indicates an antagonistic effect, S = 1 means no interaction, whereas S > 1 represents a synergistic interaction between the inhibitors towards adsorption on steel surface. However, all the value of S were less than 1, as shown in Table 2, which mentions that an antagonistic effect occurs between their major constitutes.

Fig. 4. The Tafel curve for steel in corrosive solution without and with different inhibitors.

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3.2. Polarization measurement The polarization experiments of steel in aggressive solution with the same species and concentrations of inhibitors were performed to verify the results of EIS investigations. Fig. 4 shows the anodic and cathodic polarization plots for steel in corrosive medium without and with the different concentrations of MGME and Glu, Pro, Leu as well as their combination. It can be seen that the presence of inhibitors lead to a obvious shift in both cathodic and anodic branches of polarization curves, towards a lower current density with respect to the blank. And the shapes of all the polarization plots are similar. That means the green inhibitors behave as a mixed-type inhibitor and decrease the corrosion rate but not alter the chemical reaction responsible for corrosion. The respective corrosion kinetic parameters derived from theses curves are given in Table 3. It is apparent that the inhibition efficiency increases with the increase in its mole of inhibitor, and no significant change in the corrosion potential Ecorr is observed. A maximum inhibition efficiency of 79.17% is recorded for Glu + Pro + Leu, is close to the 80.27% for MGME, This reveals that the 1.77 mmol/L Glu + 0.88 mmol/L Pro + 1.93 mmol/L Leu (Glu + Pro + Leu) are the main anticorrosion contributor for the MGME at concentration of 1.0 g/L. Otherwise, All the S values are also less than 1 and consistent with the EIS results, which displays that an antagonistic effect exists between the major contributors. Table 4 summarized the average of inhibition efficiency for steel in synthetic concrete pore solution without and with different inhibitors. As shown in Table 4, three major ingredients in MGME, namely Glu, Pro, and Leu, have a comparative efficiency from 53.76% to 69.40% with an inhibition rank of Glu > Leu > Pro. With regard to any two combinations of them, it can be seen that the inhibition efficiency strengthened in the order of Pro + Leu < Glu + Leu < Glu + Pro. Besides, the inhibition efficiency obtained from Glu + Pro + Leu and MGME are very close for the two test method, and the high performance of MGME could also be due to the larger size of hydrolysate than that of any amino acid(Glu/Pro/Leu) which cover wide areas on the steel surface and thus inhibiting corrosion. Consequently, inhibition performance enhance with increasing the concentration of inhibitors and varied from 53.76% to 69.40% for single Glu, Pro or Leu, 70.22%-79.27% for both of them and 79.27% for coexistence of Glu, Pro and Leu, respectively. The Glu + Pro + Leu exhibited an higher inhibition efficiency of 79.37% than

that of any two inhibitor combination, because the cumulative effect of the inhibitor concentration overcome the antagonistic action between them in inhibition perspective. The result of polarization measurements are in perfect agreement with the results got from EIS studies. Both results confirm that the Glu, Leu, and Pro molecule can be regarded as being the main ingredient responsible for inhibition property of MGME. The MGME and its main constituents (Glu + Pro + Leu) with similar inhibition efficiency are 83.15% and 79.27%, respectively. 3.3. Surface analysis Although there are negative synergistic effect of the Glu, Pro and Leu, any of their concentration increases lead to the enhance of anticorrosion behavior, which is attributed to the surface film on steel consisting of Glu, Pro or Leu. As shown in Fig. 5, the surface chemical elements of the adsorbing film of green inhibitors on steel rebar were analyzed by X-ray Photoelectron Spectroscopy (XPS). XPS measurement was carried out to give insight into the chemistry characteristics of the green inhibitors/steel interface and to study the adsorbing nature of the Glu + Pro + Leu. The XPS spectrum curves (C1s, O1s, N1s and Cl2p) of steel surface in 3 wt% NaCl simulated concrete pore solution with 1.77 mmol/L Glu + 0.88 mmol/L Pro + 1.93 mmol/L Leu, were displayed in Fig. 5. All spectra indicate complex type which has been attributed to the relating species. As shown in Fig. 5(a), The O1s spectrum of around 530.0 eV can be deconvoluted into three component peaks, ascribed to oxide, hydroxide and adsorbed water. Indeed, the peak for O2 at 530.1 eV peak is assigned to ferric oxides such as Fe2O3 and/or Fe3O4, while the peak at about 531.0 eV belongs to OH of hydrous iron oxides (i.e., FeOOH) [11]. From Fig. 5(b), the centre of the C1s peak is mainly located at 283.7 eV, and the weak and wide peak at 288.1 eV related to C@O [12]. Otherwise, around 286.0 eV is associated with C–N between carbon and pyrrole nitrogen, indicates that Pro molecule have adsorbed on mild steel surface. If bonding between carbon and oxygen has to be taken into account, the centers of the CAO and C@O peaks are located at 286.5 eV and 288.6 eV [13]. Then in Fig. 5(c), the chloride concentration is obviously, as evidenced by a considerable peak whose centre was located at approximately 198.9 eV for Cl2p3/2 and 201.5 eV for Cl2p1/2 [14]. The peak of N1s spectra is less clearly

Table 3 The polarization parameters and relating inhibition efficiency for steel in corrosive solution without and with different inhibitors. Inhibitor

Ecorr /(V)

bc /(mV/Dec)

bc /(mV/Dec)

icorr /(lA)

IETP/(%)

S

Blank Glu Pro Leu Glu + Pro Glu + Leu Pro + Leu Glu + Pro + Leu MGME

0.56 0.51 0.54 0.53 0.50 0.52 0.49 0.50 0.55

26.71 193.82 225.64 213.95 184.48 180.12 174.72 177.84 59.36

4.08 215.54 202.99 209.60 252.27 216.09 264.38 250.58 123.01

21.03 6.45 9.15 7.73 5.47 4.99 6.89 4.37 4.14

– 69.27 56.45 63.21 73.99 76.33 67.23 79.17 80.27

– – – – 0.51 0.48 0.49 – –

Table 4 The average of inhibition efficiency for steel in corrosive solution without and with different inhibitors. IE

Glu

Pro

Leu

Glu + Pro

Glu + Leu

Pro + Leu

Glu + Pro + Leu

MGME

IETP/% IEEIS/% IEavg/%

69.27 69.52 69.40

56.45 51.07 53.76

63.21 61.09 62.15

73.99 75.24 74.62

76.33 65.33 70.83

67.23 73.20 70.22

79.17 79.37 79.27

80.27 86.02 83.15

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(a)

(b)

(c)

(d)

Fig. 5. The surface chemical composition of the adsorbed layer on steel was detected by XPS.

confirmed and is present like a shoulder as shown in Fig. 5(d). The two peaks centered at around values of 400.9 eV and 399.6 eV are related to the existence of C-NH-C (pyrrole ring) and C-N-steel connection, respectively [15]. The appearance of the nitrogen species bonding in different types with the iron surface reveals that amino acid inhibitors can adsorbed by physically or chemically [16]. 3.4. Theoretical study: interaction between inhibitor and steel surface 3.4.1. Quantum chemical computations Quantum chemical computations have been extensively used to study the relations of molecular/electronic structure of an organic inhibitor and its inhibition property. The electrostatic potential (ESP) map, frontier molecular orbital density distribution including the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) and optimized structure of Glu, Pro, and Leu molecule are shown in Fig. 6. The HOMO spread is located on the ANH2 groups and pyrrole ring. The LUMO comprises a large lobe that is centered on the ACOOH group near the a carbon atom, indicating they can receive electrons from the metal by this carboxylic group. These observations suggest that the molecule is liable to interact with ferrous orbital using mainly the centers with nitrogen or oxygen atom resulting from their existence of negative charge. The HOMO and LUMO surfaces and contours for Glu, Pro and Leu suggest a inhomogeneous distribution of electron density around the molecule, indicating that the amino acid molecule is nonsymmetric. On the one hand, all these molecules exhibit the yellow (negative) field associated with nucleophilic reactivity, which indicates the molecules are easily sharing or transfer free electron pairs to iron; On the other hand, they also have the blue (positive) field with electrophilic reactivity. Hence, it is clearly that both competitive and

cooperative adsorbing process may exist simultaneously. That may be also the reason of antagonistic effect occurs between the Glu, Pro, and Leu. Table 5 revealed the frontier molecular orbital energies including the EHOMO, ELUMO, and DE = ELUMO  EHOMO. As shown in Table 5, the only small difference in the frontier molecular orbital energy can be found between Glu, Pro and Leu, which implied that the electron donating and accepting capacity of these amino acids is similar. Furthermore, a higher difference of dipole moment (l) value implied the more inhibitors accumulation on the steel surface thereby benefit the synergistic inhibition effect. However, the similar size of l value implying the antagonistic effect among inhibitors is confirmed again in theoretical perspective. But the amino acid with higher value of dipole moment and the polarizability have a greater tendency to interact with other species through electrostatic interactions [7]. Donating or accepting electron behavior can be further established by computing the fraction of electrons (DN) transferred from adsorbate to steel. According to Koopman’s theorem [17], the DN value can be computed as follows

DN ¼

/  vln 2ðcFe þ cln Þ

where c ¼ ðEHOMO þ ELUMO Þ is global hardness, v ¼ ðEHOMO þ ELUMO Þ=2 is absolute electro-negativity, a global hardness of cFe  0 is accepted due to the extremely small number [18],the work function / values calculated from DFT are 3.91 eV, 3.88 eV and 4.82 eV for Fe (1 0 0), (1 1 1) and (1 1 0) interfaces, respectively [19]. The defined DN measures the electron transfer from metal to inhibitors if DN < 0 and vice versa if DN > 0 [20]. As shown in Table 5, all the values of DN for three ferrous planes are positive, which exhibiting that the amino acid inhibitors can donate electrons to Fe surface

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Optimized

HOMO

LUMO

ESP

Glu

Leu

Pro Fig. 6. The HOMO orbital, LUMO orbital and ESP map for Glu, Pro and Leu inhibitor.

Table 5 Quantum chemical parameters for the Glu, Pro and Leu molecule. Name

EHOMO(eV)

ELUMO(eV)

DE(eV)

l(Debye)

DN100

DN110

DN111

Glu Leu Pro

5.76 5.68 5.45

1.35 1.17 1.08

4.41 4.52 4.37

3.80 3.77 4.09

0.08 0.11 0.15

0.28 0.31 0.36

0.07 0.10 0.14

favoring the formation of adsorption bonds, and the electron donating capacity is mainly attributed to the amino group or pyrrole (as shown in HOMO spread of Fig. 6). Indeed, the amino bonding to metal is predetermined by XPS spectra with the peaks for C-NH-C (pyrrole ring) and C-N-steel connection. Otherwise, the donating electron from metal surface benefited the superoxide anion radicals to form stable substances resulting in the scavenging superoxide radical and the inhibitive ability of inhibitors increased. That is also consistent with the experimental results as mention before. 3.4.2. Monte Carlo simulation The molecule dynamics simulations were performed to investigate the interaction between the inhibitor molecules and Fe (1 1 0) surface by Monte Carlo simulation. According to the

chemical composition of MGME, the Glu, Pro and Leu is 1.77 mmol/L, 0.88 mmol/L and 1.93 mmol/L, respectively. In order to obtain more accurate simulation analysis of the adsorption process of Glu, Pro and Leu in MGME, the nearly mole ratio of Glu:Pro:Leu is set during simulation. During energy optimization process, the related parameters for inhibitor/water/Fe (110) system is noted in Table 6. The adsorption energy, is the most important parameter of Monte Carlo simulation, represents the energy of adsorbing the adsorbates onto the Fe surface. Table 6 exhibits a large value in several inhibitors coexistence meanwhile than only single inhibitor, which demonstrates a less stable adsorption configuration except the combination of Glu and Leu. Furthermore, the dEad/dNi is the differential adsorption energy, indicating the

Table 6 Parameters computed by the Mont Carlo simulation.

*

Glu:Pro:Leu:Water*

Adsorption energy

Glu: dEad/dNi

Pro: dEad/dNi

Leu: dEad/dNi

Water: dEad/dNi

1:0:0:300 0:1:0:300 0:0:1:300 1:0:1:300 0:1:2:300 2:1:0:300 2:1:2:300

1752.73 1721.84 2101.74 1818.08 1720.15 1634.49 1631.76

60.65 – – 72.28 – 46.93 17.19

– 21.52 – – 26.35 26.06 14.04

– – 17.53 54.31 58.59 – 11.99

7.87 5.29 0.11 5.42 0.85 16.15 3.99

Glu:Pro:Leu:Water = x:y:z:n represents the adsorbed system including x Glu, y Pro, z Leu, and n H2O.

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capacity of releasing a adsorbate. The low absolute value of dEad/ dNi in Table 6 indicates that adsorbing inhibitor can be easily removed gradually by aggressive species [1]. Therefore, the antagonistic effect of studied amino acids results in high adsorption energy as compared to single Glu, Pro or Leu. The side and top view of the most stable conformations for the molecules adsorbing onto Fe(1 1 0) surface with seven situations

are show in Fig. 7. It can be seen that investigated inhibitors adsorbed on a certain preferential groove of Fe(1 1 0) surface. For the single molecule circumstances, the end of the amino acid molecule in which the frontier orbital energy (as shown in Fig. 6) can be concentrated is preferentially adsorbed. In other cases, a antagonistic adsorption may occurred when combination of several kinds of inhibitors.

(a) Glu:Pro:Leu:Water= 1:0:0:300

(b) Glu:Pro:Leu:Water= 0:1:0:300

(c) Glu:Pro:Leu:Water=0:0:1:300

(d) Glu:Pro:Leu:Water=1:0:1:300 Fig. 7. Side and top image of the stable structures of inhibitors on Fe(1 1 0) surface.

Z. Zhang et al. / Construction and Building Materials 198 (2019) 288–298

297

(e) Glu:Pro:Leu:Water=0:1:2:300

(f) Glu:Pro:Leu:Water=2:1:0:300

(g) Glu:Pro:Leu:Water=2:1:2:300 Fig. 7 (continued)

As we know, the inhibition mechanism of steel in corrosive solution is clarified by the adsorption of inhibitors. The inhibitive effectiveness of inhibitor is affected by several factors including steel species, aggressive media, number of adsorptive sites, the electronic properties and chemical characteristics of the inhibitor, interaction types between inhibitor and steel surface [1]. According to the previous results gave by the experimental and theoretical investigation, antagonistic effect between the main basic compounds made a certain against inhibition capacity, whereas more inhibitor species with satisfied inhibition ability. This might be attributed to the co-sorption behavior of inhibitors, which is either competitive or cooperative. With regard to the cooperative adsorption, the nucleophilic fraction of species are attracted on the iron surface and the electrophilic or neutral fraction of species are adsorbed subsequently, the Glu and Leu are nearly cooperative adsorption as shown in Fig. 7(d); besides, the nucleophilic/electrophilic fraction of Glu, Pro and Leu are shown in Fig. 6. While for the competitive adsorption, different amino acid may be adsorbed at different active centers on the steel surface, which is evidenced by Glu and Pro, Pro and Leu displayed in (e) and (f) of Fig. 7. However, both competitive and cooperative adsorption process might exist simultaneously, as shown in Fig. 7(g). In addition to, all the S values are also less than 1 from EIS and Tafel result,

which displays that an antagonistic effect exists between the inhibitors, but the Glu and Leu are nearly cooperative adsorption as shown in Fig. 7(d) in theoretical aspect, which is ascribed to the inhomogeneity of real steel surface for EIS and Tafel result, which is different from the theoretical Fe(1 1 0). In this paper, although the Monte Carlo calculation were conducted simulating the corroding surface as single-crystals and the quantum chemistry was not considering the effect of the chloride and hydroxyl ion and other electrolyte salt in concrete pore solution. However, the computational results were also beneficial to understand the relationship between the electronic structures, adsorption behaviors and inhibitive properties. Review literatures on this issue were published by several researchers [21,22]. Besides, the long-term inhibitive performance and stability of MGME in the real concrete circumstance will be studied in our next work.

4. Conclusion In this work, the inhibition mechanism of maize gluten meal extract (MGME) for steel in synthetic concrete pore solution with 3% NaCl, was systematically elucidated combining experimental

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analysis with theoretical calculation, the following conclusions could be obtained: (1) It is revealed that the MGME and its main basic constituent are responsible for inhibition including Glu, Pro and Leu, their inhibition efficiency are 83.15% and 79.27%, respectively. So its main constituents responsible for inhibition included Glu, Pro and Leu. (2) The MGME and its main basic constituent responsible for inhibition, are mixed-type inhibitors retarding both anodic and cathodic processes on the corrosion of steel. (3) The EIS and Tafel results indicate that an antagonistic effect occurs exists between three major constitutes. (4) The XPS study confirms the formation of protective layer over the steel surface by the main basic constituent of green inhibitor. (5) The experimental analysis and theoretical studies, are in good agreement, indicate that the inhibition of green inhibitors is dominated via co-adsorption barrier layers against the aggressive medium. Conflict of interest None. Acknowledgements Thanks for helps from Ph. D Candidates Zhenyun Yu and Qinglin Yu in electrochemical experiments, And the Natural Science Foundation of Henan Province (182300410249) are also gratefully acknowledged. References [1] Y.J. Qiang, L. Guo, S.T. Zhang, W.P. Li, S.S. Yu, J.H. Tan, Synergistic effect of tartaric acid with 2,6-diaminopyridine on the corrosion inhibition of mild steel in 0.5 M HCl, Sci. Rep. 6 (2016) 33305, https://doi.org/10.1038/srep33305. [2] Han-Seung Lee, Hyun-Min Yang, Jitendra Kumar Singh, Shailesh Kumar Prasad, Bongyoung Yoo, Corrosion mitigation of steel rebars in chloride contaminated concrete pore solution using inhibitor: an electrochemical investigation, Constr. Build. Mater. 173 (2018) 443–451. [3] Chan Basha Nusrath Unnisa, Gowraraju Nirmala Devi, Venkatesan Hemapriya, Subramanian Chitra, Ill-Min Chung, Seung-Hyun Kim, Mayakrishnan Prabakaran, Linear polyesters as effective corrosion inhibitors for steel rebars in chloride induced alkaline medium – an electrochemical approach, Constr. Build. Mater. 165 (2018) 866–876.

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