Experimental and theoretical investigation on the self-assembling inhibition mechanism of dithioamide derivatives on mild steel

Journal of Molecular Structure 1202 (2020) 127286

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Experimental and theoretical investigation on the self-assembling inhibition mechanism of dithioamide derivatives on mild steel Guanglong Zhang, Long Zhou, Fengcai Li, Shuwei Xia*, Liangmin Yu Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, 266100, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 June 2019 Received in revised form 22 October 2019 Accepted 23 October 2019 Available online 26 October 2019

The corrosion inhibiting performance and mechanism of two dithioamide derivatives, N,N0 -di-ethyldithiodipropinoamide (ED) and N,N0 -di-ethyoxylpropyl-dithiodipropinoamide (EPD), for mild steel in 1.0 M HCl solution were investigated by experiments, DFT calculation and MD simulations. EPD and ED are both mixed-type inhibitors with excellent inhibition efficiency (95.2% and of 92.6%, respectively) by weight loss experiments; SEM images also demonstrate the formation of compact inhibiting monolayer. On molecular level, both molecules can be tightly parallel absorbed on Fe (110) surface through SeFe and OeFe coordination bounds by the overlapping of heteroatom p orbitals and Fe 3d orbitals, with binding energies of 369.56 kJ/mol and 608.55 kJ/mol. Strong multi-intermolecular H-binding network drives ED and EPD molecules orderly self-assemble on iron surface and form protective monolayer. EPD network keeps intact up to 1.0 MPa, guarantees its potential applications in deep-sea environment. © 2019 Elsevier B.V. All rights reserved.

Keywords: Corrosion inhibition Molecular dynamics Chemisorption Self-assembled Pressure

1. Introduction Though the most widely used mild steel has remarkable economic and substantial applications, the corrosion greatly limits its service life, especially in acidic solutions [1e4]. Among various anticorrosion methods, adding inhibitors to form protective layers on metal surface is one of the most effective and economical choices [5,6], which may reduce either or both of the anodic and cathodic reaction rates [7,8]. In recent years, organic compounds containing polar functions or heteroatom (O, N, S and P) may form strong coordinate bond with Fe atoms, as well as heterocyclic group with conjugated double bonds, which may separate metal surface from corrosion media, such as H2O, Hþ, Cl and so on. Those compounds have been reported as potential inhibitor candidates in acidic medium: iron and copper could be effectively protected by a highly compacted hydrophobic inhibitor membrane [9e12]. However, most of them are toxic, thus it is imperative to explore their eco-friendly counterparts with high efficiency [13e15]. Since the mechanism of inhibition is mainly forming a compact protection film on metal surface, molecular self-assemble becomes

* Corresponding author. E-mail address: [email protected] (S. Xia). https://doi.org/10.1016/j.molstruc.2019.127286 0022-2860/© 2019 Elsevier B.V. All rights reserved.

more attractive due to its one-step procedure in film formation. The fabrication process is a combination of inhibitor molecule adsorption and extension on metal surface. Therefore, single or several atom thinness monolayer would evenly cover on metal, exhibiting excellent inhibition performance. Great progresses have been made in recent years, for example: a self-assemble membrane, came from trimethoxysilane (3-mercaptopropy) and n-dodecanethiol, was successfully synthesized on copper by F. Sinapi [16], with over 98% inhibition efficiency. Besides, several teams systemically investigated the self-assemble of alkyl hydrosulfide [17,18] and imidazoline membranes [19] on copper and iron surface: all believed single layer membrane on metal surface was the key to their excellent inhibition performance. Though the high efficiency inhibitors were developed experimentally, the underlying mechanism, such as the interaction between molecules and metal surface, and self-assemble driving force, remain vague, especially on molecular level. Density functional theory (DFT) computations and molecular dynamic (MD) simulation have long been utilized to understand the complex mechanism of interactions between organic inhibitors and metal surface [20e25]. I.B. Obot [26] compared the inhibition performance of three pyrazine compounds (MP, AP, ABP) by DFT and MD simulations: the author thought the remarkable inhibition capabilities were mainly contributed to their strong binding strength with iron surface.

2

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Inhibition behaviors in deep-sea is also of great interests, which may provide guidance for applications in marine petrol platform and pipeline. Some researchers believed the giant pressure underwater would influence corrosion behaviors significantly [27]. However, few investigations had focused on such harsh condition: Yang [28] believed the pitting initiation rate and propagation of X70 steel may be accelerated by the increase of hydrostatic pressure. However, it is extreme difficult and costly to simulate deep-sea environments experimentally, so the exploration for steel corrosion behavior under pressure by MD seems becomes acceptable. In such situation, two nontoxic and inexpensive ithioamide derivatives, (N,N0 -di-ethyl-dithiodipropinoamide (ED)), N,N0 -diethyoxylpropyl-dithiodipropinoamide (EPD)), with high electron density donor atoms (O, N, S) and hydrophobic alkyl chains, synthesized as fungicide [11] were selected to characterize their inhibitor performances. Inhibition effects of ED and EPD for mild steel in HCl solution reached 92.6% and 95.2%, respectively, by weight loss experiments. The inhibition relative quantum chemical parameters, such as molecular orbital compositions, orbital energies (EHOMO, ELUMO), Fukui functions and dipole moment exhibited same trends with experimental results. MD simulations with and without pressure were performed to reveal the filming process of ED and EPD on iron surface and simulate deep-sea surroundings to further comprehend the fabrication mechanism of protective layer on molecular level and the influence of deep-sea condition on inhibition performance. Their high inhibition efficiency mainly originated from strong coordination bonds between heteroatoms and iron atoms; also, the formation of orderly orientated molecular layer was driven by intermolecular hydrogen bonds. Such hydrogen bonds connected networks remained integrity even under 1.0 MPa, indicating outstanding inhibition performance in deep sea.

q¼

v0  v v0

where W0 (mg) and W (mg) are the weights of mild steel before and after exposure to testing solution, S (cm2) is the total area of test coupon, t (h) is the immersion time, n0 (mg$cm2 h1) and n (mg$cm2 h1) are the corrosion rates in the absence and presence of inhibitor. 2.3. Electrochemical experiments Electrochemical tests were conducted by electrochemical workstation, ParStat4000, controlled by NOVA 1.8 software at 25  C in standard three-electrode system, with 1 cm2 mild steel sample as working electrode (WE), saturated calomel electrode (SCE) coupled to Luggin capillary as reference electrode and a platinum counter electrode (CE). Before tests, working electrodes were immersed in 250 ml test solution (same to weight loss tests) for 30 min to reach steady state (Fig. S1). Potentiodynamic polarization was obtained from 250 mV to 250 mV with respect to open circuit potential (OCP) at a rate of 1 mV/s. Impedance tests were performed at OCP in the range from 100 kHz to 10 mHz. The inhibition efficiency (hp) was calculated as follows:

hp ¼

3.0  3.0  0.1 cm mild steel, containing 0.18 wt% C, 0.02 wt% Si, 0.45 wt% Mn, 0.02 wt% S, 0.01 wt% P and balance Fe, were used for weight loss tests. Cubical steel with 1.0 cm2 exposed area was embedded in epoxy resin and covered by Teflon holder, as working electrode in electrochemical experiments. The steel samples and electrodes were mechanically polished by SiC paper (600, 1000, 1500, 3000 grit in turn), washed by distilled water and ethanol, dried at 25  C, then stored in vacuum desiccator before tests. ED and EPD were synthesized according to Zhang and Miao’s report [29,30].

2.2. Weight loss experiments Weight loss tests were performed at 25 ± 1  C in thermostatcooling water bath. After weighing, mild steel samples were fully immersed in 250 ml 1.0 M HCl solution with and without 0.1, 0.25, 0.5, 1.0 and 2.0 mM of each inhibitor; 24 h later, all samples were removed, washed, dried and weighed. The corrosion rate (n), inhibition efficiency (hW) and surface coverage (q) were expressed as:

W0  W St

hW ¼

v0  v  100% v0

(4)

Rct  Rct0  100% Rct

(5)

where Rct and Rct0 are charge transfer resistances in the presence and absence of inhibitor.

2.1. Materials

v¼

icoor0  icoor  100% icoor0

where icoor and icoor0 are the corrosion current densities with and without inhibitor. The inhibition efficiency (hR) are defined as following equation:

hR ¼ 2. Experimental details

(3)

(1)

(2)

2.4. Scanning electron microscope (SEM) In order to observe changes on mild steel surface before and after inhibition treatment, SEM samples were prepared after immersing in 250 ml 1 M HCl in the absence and presence of 2 mM ED and EPD for 24 h at 25  C. SEM figures were obtained via TESCAN VEGA3 scanning electron microscope under the condition of N2 and 20.0 kV EHT. 2.5. Quantum chemical calculations Since both weight loss and electrochemical experiments undergo in liquid phase, it is necessary to consider the effect of solvent in all theoretical calculations. DFT/M062X [31] method with 6311 þ G (d, p) basis sets in Gaussian 09 [32] was utilized to optimize geometrical structures of two molecules; quantum chemical parameters, such as dipole moment (m), global hardness (g), global softness (s), electrophilicity index (u), fraction of electron transfer (DN), frontier molecular orbital compositions and orbital energies (EHOMO, ELUMO), were obtained in both neutral and protonated states. Theoretical calculations were carried out in aqueous phase with self-consistent reaction field (SCRF) theory, and Tomasi’s polarized continuum model (PCM) [33,34]. Frequency analysis in the same theoretical level was performed to ensure their stability. According to Koopmam’s theorem [35], the ionization potential (I) and the electron affinity (A) can be defined as: I ¼ -EHOMO and A ¼ -ELUMO. In addition, the absolute electronegativity (c), global hardness (g), global softness (s) and electrophilicity index (u) are

G. Zhang et al. / Journal of Molecular Structure 1202 (2020) 127286

the energy of Fe surface together with H2O, H3Oþ, Cl, Einhibitor is the energy of inhibitor molecule.

calculated by the following formula [36,37]:

c¼

3

IþA 2

(6) 3. Results and discussion

IA g¼ 2 1

(7) 2

s¼ ¼ g IA u¼

(8)

c2

(9)

2g

Then DN can be calculated using:

DN ¼

cFe  cinh 2ðgFe þ ginh Þ

(10)

Commonly, we choose global hardness gFe z 0 eV [38], due to its extremely small value. As for the value of cFe, 7.0 eV is quoted almost exclusively in literature [39]. The value of 7.0 eV equals to the free electron gas Fermi energy of iron in free electron gas model [40]; thus, it’s conceptually inaccurate to choose 7.0 eV here, since the electron interaction is neglected in free electron gas model. Therefore, the work function (F) of metal surface, a reasonable measure of electronegativity, is applied to replace cFe to calculate DN [41e43]:

DN ¼

F  cinh 2ðgFe þ ginh Þ

(11)

where F (4.82 eV) was obtained from Kokalj’s work [40]. As the most important local reactivity index, Fukui function, obtained by finite difference approximation [41,44], reveals the reactivity in local sense [26].

fþ k ¼ qk ðN þ 1Þ  qk ðNÞ

(12)

f k ¼ qk ðNÞ  qk ðN  1Þ

(13)

where qk (N þ 1), qk (N-1) are the net charge of atom k in a molecule  with Nþ1 and N-1 electrons;f þ k and f k represent the nucleophilic attacking index and the electrophilic attacking index. 2.6. Molecular dynamics (MD) simulations MD simulations were performed by Discover module in Materials Studio 7.0 (from Accelrys Inc): the interaction between inhibitor and iron surface were performed in a 39.29 Å  24.56 Å  33.96 Å box, containing a six atomic-layer Fe (110) surface and inhibitors; periodic boundary conditions were applied to avoid any arbitrary boundary effects; H2O, H3Oþ, Cl were added to the system to simulate our experimental condition; optimized inhibitor molecules were placed on proper position (parallel or vertical to Fe (110) surface). COMPASS (Condensed Phase Optimized Molecular Potentials for Atomistic Simulation Studies) force field and NVT ensemble were selected to perform at 298 K for 200 ps with a time step of 1.0 fs The interaction energy (Einteraction) between inhibitor molecules and iron surface was calculated as:



Einteraction ¼ Etotal  Einhibitor þ EsurfaceþH2 OþH3 Oþ þCl



(14)

where Etotal is the total energy of the system, EsurfaceþH2OþH3OþþCl- is

3.1. wt loss experiments Corrosion rate (n), inhibition efficiency (hW) and surface coverage (q) of two inhibitors with different concentrations are summarized in Table 1. The inhibition efficiencies of ED and EPD increase with the rising of inhibitor concentrations, which may be induced by the rising coverage effectively isolate mild steel surface from medium. ED and EPD reach their top efficiencies (92.6% and 95.2%, respectively) at 2.0 mM, indicating both dithioamide derivatives are excellent corrosion inhibitors for mild steel in 1.0 M HCl solution, better than former reports [9,45]. Moreover, higher hW of EPD at same concentration reveals its better inhibitive performance compared with ED.

3.2. Potentiodynamic polarization Polarization of mild steel in both presence and absence of different ED and EPD concentrations are shown in Fig. 1(A) and (B), respectively. Corrosion potential (Ecoor), cathodic Tafel slope (bc), anodic Tafel slope (ba), corrosion current densities (icoor) and inhibition efficiency (hp) are listed in Table 2. The addition of either inhibitor causes the reduce of corrosion rates (Fig. 1): both anodic and cathodic curves shift to lower current densities; and inhibition effect enhances with the increase of concentration, which may be due to the adsorption and formation of barrier film upon electrode [46]. Tafel slopes of bc and ba changed upon addition of two inhibitors, indicating both molecules controlled the anodic dissolution of metal and cathodic hydrogen evolution reaction [47]. According to reports [48], if the difference of Ecoor between the inhibited and uninhibited system is greater than 85 mV, the inhibitor could be regarded as either anodic or cathodic type, whereas it may be recognized as mixed one. The largest displacement of Ecoor here was approximately 47 mV (Table 2) for both inhibitors, which could be concluded that ED and EPD were mixed-type inhibitors. In Table 2, icoor decreased obviously in the presence of ED or EPD comparing with and without inhibitor, and reduced with the increasing of concentration. Accordingly, hp increased with inhibitor concentration, may be due to the rising of inhibitor coverage on WE surface. hp of 2.0 mM inhibitor reached maximum of 90.7% for ED and 92.5% for EPD, which also confirmed that two dithioamide derivatives were good inhibitors in 1.0 M HCl. hp followed the order of EPD > ED in same concentration. Table 1 Corrosion parameters after weight loss experiments for mild steel in 1.0 M HCl with different concentrations of ED and EPD at 25  C. Inhibitor

Concentration (mM)

n (mg$cm2$h1)

hW (%)

q

Blank ED

/ 0.10 0.25 0.50 1.00 2.00 0.10 0.25 0.50 1.00 2.00

0.8508 0.1413 0.1059 0.0921 0.0756 0.0630 0.0947 0.0797 0.0598 0.0505 0.0409

/ 83.4 87.6 89.2 91.1 92.6 88.9 90.6 93.0 94.1 95.2

/ 0.834 0.876 0.892 0.911 0.926 0.889 0.906 0.930 0.941 0.952

EPD

4

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Fig. 1. Potentiodynamic polarization curves for mild steel in 1.0 M HCl with different concentrations of ED and EPD at 25  C: (A) ED and (B) EPD.

Table 2 Polarization parameters for mild steel in 1.0 M HCl with different concentrations of ED and EPD at 25  C. Inhibitor

Concentration (mM)

ba (mV$dec1)

-b c (mV$dec1)

Ecoor (mV/SCE)

icoor (mA$cm2)

hp(%)

Blank ED

/ 0.10 0.25 0.50 1.00 2.00 0.10 0.25 0.50 1.00 2.00

111.9 110.2 73.9 108.3 89.2 110.9 123.0 115.6 105.4 111.9 113.6

68.5 70.6 74.1 76.6 71.6 57.4 63.7 63.5 59.7 61.9 56.8

464.9 481.5 512.3 464.9 480.8 476.5 492.8 478.7 472.0 474.3 466.0

429.6 73.0 63.4 54.2 48.6 40.0 67.1 56.6 50.9 40.6 32.4

/ 83.0 85.2 87.4 88.7 90.7 84.4 86.8 88.2 90.5 92.5

EPD

3.3. Electrochemical impedance spectroscopy Nyquist plots obtained for mild steel in 1.0 M HCl in various concentrations of ED and EPD are shown in Fig. 2. Single capacitive loops could be observed in Nyquist diagrams, indicating the corrosion mainly controlled by charge transfer process, and related to double layer behavior [49]. The imperfect semicircle capacitive loops were attributed to the frequency dispersion effect, caused by electrode surface roughness and inhomogeneousness [50]. From Fig. 2(A) and (B), the diameters of the loop with either inhibitor were larger than blank, and increased with inhibitor concentration. Electrochemical equivalent circuit was accepted to obtain Rs and Rct based on impedance datum (Fig. 3). Rs represents solution resistance, whereas Rct is charge transfer resistance. To acquire a more accurate fitting, a double layer capacitance (Cdl) was substituted by constant phase element (CPE). The CPE impedance is given by the equation:

n ZCPE ¼ Y 1 0 ðjuÞ

(15)

where Y0 is a proportional factor, n represents phase shift. For n ¼ 0, CPE is a resistance, n ¼ 0.5 a Warburg element, n ¼ 1 a capacitance and n ¼ 1 an inductance [36]. The double layer capacitance (Cdl) and inhibition efficiency (hR) are defined as following equations:

1=n  Cdl ¼ Y0 ,R1n ct

(16)

hR ¼

Rct  Rct0  100% Rct

(17)

where Rct and Rct0 are charge transfer resistances in the presence and absence of inhibitor, respectively. The electrochemical parameters of Rs, Rct, Y0, n, Cdl and hR are listed in Table 3. The corresponding Bode and phase angle plots for mild steel with and without inhibitors (open potential circuit) are given in Figs. 4 and 5. In Fig. 4, the absolute value of impedance at low frequencies increases with the concentration of ED or EPD, demonstrating the enhanced corrosion protection in the presence of ED or EPD, and the better protection with the increasing of concentration [51]. The phase angle shifts to more negative values with the increasing concentration of both ED and EPD in 1.0 M HCl, implying their better inhibitive activity (Fig. 5): more adsorbed inhibitor molecules increases the coverage on mild steel surface. In addition, only one peak at median frequency appears in Fig. 5, representing one time constant for both corrosion inhibitors referring to the electrical double layer [52]. The low chi squared (c2) values in Table 3 indicate the good fitness of EIS plots. Smaller Cdl value, comparing with blank solution, was caused by the reducing of local dielectric constant and/or the increase of electrical double layer thickness, indicating such inhibitor molecules were activated after by their adsorption on the metal/solution interface [47]. Rct values increased together with concentration, consequently; same trend was also observed on hR values. Furthermore, in same concentration, Rct and hR values followed the order of EPD > ED, implying the better performance of EPD. The hR value at 2.0 mM was 93.4% and 94.4% for ED and EPD,

G. Zhang et al. / Journal of Molecular Structure 1202 (2020) 127286

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respectively, exhibiting excellent inhibition efficiencies. Furthermore, it can be seen from Fig. S2 that same trend in electrochemical measurements were obtained as those in weight loss experiments. 3.4. Adsorption isotherm Since the chemisorption of inhibitor on metal surface can significantly reduce its corrosion, their interaction mechanism can be revealed by adsorption isotherm [53]. The C/q against C of both ED and EPD yield straight lines with liner correlation coefficient (r2) and slope almost equal to 1 (Fig. 6), suggesting their adsorption follow Langmuir adsorption isotherm, represented as [54]:

C

q

¼

1 þC Kads

(18)

where C represents the inhibitor concentration; q is surface coverage calculated by weight loss experiment (Table 1) and Kads is the adsorptive equilibrium constant. The standard adsorption free energy (DG0ads ) was also calculated [55]:

DG0ads ¼  RT lnð55:5Kads Þ

(19)

where R ¼ 8:314 J,mol1 ,K 1 (gas constant); T is temperature (K); and 55.5 (mol$L1) represent the molar concentration of water in acid solution [56]. Calculated DG0ads values are also presented in Table 4. Negative D G0ads

indicated the adsorption was a spontaneous process; the lower

the DG0ads , the stronger the interaction and the more highly adsorption efficiency [57]. Since EPD exhibits a lower DG0ads value, further demonstrates its better inhibitive performance than ED. 3.5. Morphological analysis Fig. 2. Nyquist plots for mild steel in 1.0 M HCl with different concentrations of ED and EPD at 25  C: (A) ED and (B) EPD.

To better evaluate effects of both ED and EPD against mild steel corrosion, SEM images of metal surface morphologies in the absence and presence of inhibitors were taken (Fig. 7). The rough surface (Fig. 7(a)) shows serious damage under acid conditions; with respect to blank case, the surface of the sample with 2 mM ED (Fig. 7(b)) or 2 mM EPD (Fig. 7(c)) is much smoother, implying effective protection from both inhibitors. 3.6. Quantum chemistry calculations

Fig. 3. Equivalent circuit model used to fit the data of EIS experiment.

Usually, it’s blind to speculate inhibition mechanisms purely based on experimental results; thus, quantum chemical calculations are commonly utilized to comprehend the underlying

Table 3 Impedance parameters for mild steel in 1.0 M HCl with different concentrations of ED and EPD at 25  C. Inhibitor ED

EPD

Concentration (mM)

R s (U$cm2)

Rct (U$cm2)

Y0  106 (mF$ cm2)

n

Cdl (mF$cm2)

hR (%)

c2 10-3

Blank 0.10 0.25 0.50 1.00 2.00 0.10 0.25 0.50 1.00 2.00

1.3 0.9 0.9 0.7 0.4 0.3 1.2 0.9 1.0 0.8 0.7

28.5 124 224 312 388 430 172 252 400 441 507

193.1 106.0 67.3 62.6 46.7 38.6 104.6 59.5 43.9 37.2 40.1

0.91 0.89 0.87 0.90 0.89 0.90 0.91 0.87 0.86 0.88 0.92

452.4 342.3 283.4 187.6 156.9 113.6 275.7 250.3 215.5 139.7 95.0

e 77.0 87.3 90.9 92.7 93.4 83.4 88.7 92.9 93.5 94.4

0.90 0.58 1.01 0.63 0.83 0.56 0.66 2.18 3.78 1.51 0.64

6

G. Zhang et al. / Journal of Molecular Structure 1202 (2020) 127286

Fig. 4. Bode plots of mild steel in 1.0 M HCl in the absence and presence of different concentrations of ED and EPD at 25  C.

Fig. 5. Phase angle plots of mild steel in 1.0 M HCl solution in the absence and presence of different concentrations of ED and EPD at 25  C.

principles in molecular level. Due to the existence of heteroatoms in ED and EPD, both inhibitors are favorable to be protonated in acidic medium [58]. From the structures of investigated inhibitors, there are many active centers on both inhibitors for protonation. Optimization of possible protonated structures with different active center were carried out: only N atoms protonated forms were kept intact without imaginary frequency. Fig. 8 is optimized structures of neutral and protonated ED and EPD: both molecular contain heteroatom, such as N, O, S atoms, and hydrophobic groups (-CH2CH3), coordination bonds between free electron pairs of these heteroatoms and the empty d-orbital of Fe atom can be expected. Quantum chemical properties of neutral and protonated of ED and EPD are listed in Table 5. Protonation leads to remarkable torsions of molecular structure over the whole skeleton. Bond length (Å) and bond angles ( ) of neutral and protonated optimization structures were listed in Table S1. After protonation, bond lengths change drastically ED: N3eC4, C12eN13 and C12 ¼ O15 enlarged up to 17.6%, whereas C4¼O8 shortened from 1.237 to 1.186 Å. The biggest changes upon protonation for EPD were: N7eC8, C15eN16 and C8¼O24

elongated from 1.8% to 22%, whereas C15 ¼ O17 reduced about 4.1%. After protonation, the bond angle of C11eC12 ¼ O15, C11eC12eN13 and N13eC12 ¼ O15 of ED became smaller than neutral; similarly trends could be observed in N7eC8¼O24, N7eC8eC9 and C9eC8¼O24 of EPD. Thus, the ends of both inhibitors molecular chain tend to approach the other after protonation, such trend of EPD is greater than ED; therefore, more EPD could be excepted to be adsorbed on iron surface, leading to a more compacted protective layer. Since active sites of molecules mainly depend on their molecular orbitals, frontier molecule orbital density distributions of neutral and protonated ED and EPD are performed and shown in Fig. 9. HOMOs and LUMOs of both neutral molecules are mainly distributed around S atoms: the contributions of S atoms are 86.32%/60.89% and 87.10%/57.90% for ED and EPD, respectively (Table S2), which make S atoms as favorable active sites for electron donating and accepting. However, distributions of HOMO/LUMO for protonated ED and EPD become different: both HOMO and LUMO concentrate around O atoms, 25.92%/26.74% for ED and 28.89%/ 29.78% for EPD. Such changes illustrate that active sites in

G. Zhang et al. / Journal of Molecular Structure 1202 (2020) 127286

Fig. 6. Langmuir isotherm plots for mild steel in 1.0 M HCl with different concentrations of ED and EPD at 25  C.

7

protonated molecules move from S atoms to O atoms. Such shift will make it easier for both molecules to be planar adsorbed on iron surface; thus increases the inhibition efficiency by the rising of surface coverage. EHOMO is often used to represent electron donating capacity of a molecule; therefore, inhibitors with high EHOMO values are more likely to donate electrons to the empty molecular orbital of acceptors. Conversely, ELUMO indicates electron accepting capacity: the lower the value, the higher the electron accepting capability. From Table 5, EHOMO follows the order of ED < EPD, which is in conformity to inhibition efficiency results discussed above. However, ELUMO of EPD is bigger than that of ED; such conflict result suggests the complex nature of corrosion inhibition process [59]. The higher EHOMO and lower ELUMO of protonated ED and EPD indicate their better electron donating and accepting abilities than neutral. A feedback electron donation from iron to inhibitor leads to the formation of Fe2þ species, which connects inhibitors and Fe surface by O atoms through electrostatic interactions, further strengthens the binding. Thus, the adsorption of ED and EPD on Fe in HCl is a mixture of chemisorption and physisorption. The energy gap DE (DE ¼ ELUMO-EHOMO) is a factor to reveal the

Table 4 Parameters of the straight lines of C/q e C and adsorption free energy (DG0ads ) in 1.0 M HCl at 25  C. Inhibitor

Linear correlation coefficient (r2)

Slope

Kads (L mol1)

DG0ads (kJ mol1)

ED EPD

0.999 0.999

1.07 1.05

5.32  104 7.58  104

26.97 27.85

Fig. 7. SEM micrographs of mild steel samples after 24 h of immersion in 1.0 M HCl at 25  C: (a) blank, (b) 2 mM ED, (c) 2 mM EPD.

8

G. Zhang et al. / Journal of Molecular Structure 1202 (2020) 127286

Fig. 8. Optimized molecular structures of ED and EPD in the neutral and protonated form.

Table 5 Quantum chemical properties for ED and EPD calculated with the DFT/M062X method with 6-311 þ G (d, p) basis in the neutral and protonated form.

Neutral form Protonated form

Inhibitor

EHOMO (eV)

ELUMO (eV)

DE (eV)

g (eV)

s (eV)

u (eV)

DN

m (Debye)

ED EPD ED EPD

7.9158 7.8997 4.6823 3.8463

0.1388 0.1399 0.4123 0.0893

8.0546 8.0396 4.2700 3.7570

4.0273 4.0198 2.1350 1.8785

0.2483 0.2488 0.4684 0.5323

1.8772 1.8724 1.5090 1.0307

0.1156 0.1169 0.5322 0.7592

4.3324 5.4363 40.7542 28.6580

interaction between organic molecule and steel surface [26,60]: low DE value indicates the higher activity of molecules, which facilitating their adsorption on metal surface [61e63]. DE of EPD is smaller than ED (Table 5), implying stronger coordination ability to metal, consistent with experimental results. The DE values of protonated ED and EPD are smaller than neutral, indicating the more possibility for protonated ED and EPD to be adsorbed on Fe surface. To further confirm our results, fraction of electron transfer (DN) computed from the inhibitor to metallic surface was performed. If DN>0, electron will transfer from molecule to metal surface and vice versa [41]. If DN<3.6, the increase of inhibition efficiency may be mainly caused by the increasing electron-donating ability of molecule to metal surface [64]. In Table 5, both inhibitors have positive DN values, indicating they could donate electrons to iron surface to form coordination bonds. The DN of EPD is higher than ED in both neutral and protonated forms, indicating the better inhibitive performance of EPD, also in good correlation with experimental inhibition efficiencies. However, little difference of DN in neutral become significant in protonated forms, where DN are 0.7592 and 0.5322 for EPD and ED, respectively, implying the better electron-donating tendency for EPD. Thus, better interaction between iron surface and EPD molecule can be expected, which is also in accordance with experimental results. Global hardness and softness are another two important factors to assess the reactivity and stability of inhibitors. Generally,

inhibitor with small global hardness and elevated softness is expected to be with outstanding inhibition efficiency [63,65]: s is higher and g, u are lower for protonated ED and EPD than neutral. EPD has a low value of g and an elevated value of s (Table 5) in both neutral and protonated forms, which proves its high inhibition efficiency. On the other hand, the capability of inhibitor to accept electrons can be clarified by the electrophilicity index [66]: inhibition efficiency increases with the decreases of u. The u of EPD is less than that of ED in both neutral and protonated forms, further demonstrating its better inhibition efficiency, in accordance with experimental results. The dipole moment (m) is utilized to evaluate the inhibition efficiency of molecules [67]: higher dipole moment can facilitate inhibitor adsorption by electronic force. From Table 5, EPD also exhibits higher value of m than ED in neutral form, indicates stronger dipole-dipole interaction with metallic surface [68], in accordance with experimental results. However, m of protonated ED and EPD follows the order of EPD < ED. Compared to neutral ones, the higher m of protonated ED and EPD further facilitated their performance as inhibitor. Comparison of quantum chemical calculations for protonated and neutral inhibitors indicate that there is a better correlation between most of the quantum chemical parameters and inhibiting efficiency in the protonated form than that in the neutral form. These results illustrate that in acid medium, protonated inhibitors

G. Zhang et al. / Journal of Molecular Structure 1202 (2020) 127286

9

Fig. 9. Frontier molecule orbital density distributions: ED and EPD in the neutral and protonated form.

make a higher contribution to anti-corrosion effect on mild steel. In both neutral and protonated states, more positive EHOMO, DN and s of EPD demonstrate its better bonding character with iron surface; thus, it is much easier for EPD to form a protective film onto mild steel than ED, further prove its better inhibition efficiency. Inhibitor molecules can be bound to metal surface through donating electrons to metal surface, as well as accepting electrons from metal surface [69]. Therefore, it is essential to evaluate the active sites for donating or accepting electrons in molecules: Fukui functions are utilized to analyze local reactivity of inhibitors. It provides information on which atom has a higher tendency to either donate or accept electrons. The nucleophilic and electro philic attacks are controlled by the maximum value of f þ k and f k :  high f þ k and f k value indicates preferable site of nucleophilic and electrophilic attack, respectively. Calculated Fukui indices for ED and EPD are listed in Tables S3 and S4. It is clear that for ED, S7, O8, S9 and O15 sites are more susceptible to nucleophilic attack, since they possess higher f þ k values. Whereas, N3, S7, O8, S9, N13 and O15 are more favorable for

electrophilic attacks, due to higher values of f  k . However, for EPD,

large f þ k values are mostly located on S11, S12, O17 and O24 atoms; large f  k are mostly located on N7, S11, S12, O17 and O24 atoms. S atoms strongly participates in bond formation, since the maximum þ  value of f þ k located on S atoms. While the value of f k /f k for pro-

 tonated ED and EPD become different: both f þ k and f k concentrated on O atoms. Based on the above discussion, we can speculate that OeFe bonds may be formed for both protonated inhibitor molecules and steel, leading to parallel adsorbed configurations.

3.7. Molecular dynamics simulations Recently, large amount of corrosion inhibition studies utilized molecular dynamics simulations to reveal the interaction between inhibitors and metal surface [26,70,71]. To better understand adsorption behaviors of ED and EPD on steel surface, MD simulations were conducted in aqueous solution. The most favored adsorption configuration of both inhibitors on metal surface after equilibration could be reasonably predicted by MD simulations;

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Table 6 Interaction and binding energies between the molecules and Fe (110) surface. Systems

Einteraction (kJ/mol)

Ebinding (kJ/mol)

ED þ Fe (110) þ H2O, H3Oþ, Cl EPD þ Fe (110) þ H2O, H3Oþ, Cl

369.56 608.55

369.56 608.55

* Ebinding

¼ -

Einteraction.

Einteraction between inhibitor molecule and surface were also calculated and shown in Table 6. The equilibrium adsorption configurations of ED and EPD are depicted in Fig. 10: both inhibitors could almost parallel adsorb on Fe surface, revealing strong binding interactions. Such flat orientation could be ascribed to the vast existence of active sites. As mentioned above, these active sites (S atoms, O atoms) could bind to iron surface by either coordinate or back-donating bonds. The flat adsorption orientation will definitely lead to large blocking area, thus high inhibition efficiency is expected, also observed experimentally. However, compared to EPD, the configuration of adsorbed ED is slight tilted, suggesting EPD provide higher coverage and leading better inhibition efficiency than ED at the same concentration, also in good accordance with experimental results. As shown in Table 6, interaction energies between ED or EPD with Fe (110) surface are negative, confirming the adsorptions are spontaneous; the binding energy of EPD is bigger than that of ED, therefore, EPD adsorbs more strongly on iron surface and possesses more inhibition efficiency. Moreover, the interaction energy of EPD is similar to a-APD [66] (Einteraction ¼ 510.85 kJ/mol, h ¼ 92.81%) and CBAT [72] (Einteraction ¼ 487.45 kJ/mol, h ¼ 94.2%), indicating their similar bonding ability to iron surface. However, all those inhibitors contain strong interaction aromatic parts; the comparable bonding character guarantees the excellent inhibition efficiency of EPD. 3.8. Projected density of states (PDOS) Since HOMO and LUMO of both molecules were mainly distributed around S atoms, also Fukui indices of S atoms were

dominant; DFT calculations were preformed to investigate the PDOS of S atoms of both molecules. The s and p orbitals of S11 atom in both isolated and absorbed EPD molecule were shown in Fig. 11. Sharp peaks could be obviously identified in isolated molecule with convolution of smaller peaks. However, the locations of these peaks changed clearly after adsorption. The 3s orbital at 8.70 eV and 15.05 eV moved to 10.80 eV and 17.34 eV, respectively. The board peak ranged from 14.22 to 9.9 eV moved to the range of 16.25 to 11.60 eV. Similar changes can also be observed for 3p orbitals, peaks at 3.92 eV and 0 eV moved to 0.25 eV and 1.49 eV, respectively; the board peak also moved to deeper energy level. The 3p orbital of S11 atom and 3d orbital of Fe atom could be effectively overlapped around the Fermi level. The s and p orbitals of S, O and N atoms of ED show the same downshift trend (Fig. S4). Similar changes of other heteroatoms (Fig. S3 and Fig. S4) could also be observed. Such orbital overlapping indicates that coordination bonds are formed between heteroatom and Fe atoms. In addition, in Table 7, the average coordination bond length between heteroatom and Fe atoms in EPD (3.033 Å) is less than that of ED (3.139 Å); so compared with ED, EPD exhibits better inhibitive performance, consistent with MD simulation results. 3.9. Molecular self-assemble Based on SEM images, ED and EPD could form compact protection film on steel surface, their self-assemble filming mechanisms were further considered. Among the driving forces of selfassemble (hydrogen bonding, coordination bond, hydrophobic effect, electrostatic interaction, van der Waals’ force and so on), the formation of H-bonds between adjacent molecules seems to be proper explanation for their network structure: both carbonyl and amine groups can generate multi C]O/HeN bonding between adjacent molecules, forming a two-dimensional net. Cross-linked H-bindings can be observed after MD simulation for two EPD and ED molecules on Fe (110) surface (Fig. S5), mainly composite by stable ‘V’ shaped C]O/HeN H-binding and other weaker hydrogen-bindings, such as C]O/HeC. Both ED and EPD molecules can be stably and planar distributed by hydrogen bonding on

Fig. 10. Equilibrium adsorption configurations of ED (a) and EPD (b) on Fe (110) surfaces obtained by molecular dynamic simulations. Top: top view, bottom: side view.

G. Zhang et al. / Journal of Molecular Structure 1202 (2020) 127286

11

Fig. 11. PDOS of S11 atom in EPD (A) and corresponding Fe atom (B) on the surface. Fermi level is represented by dashed line, a and b represent after and before adsorption, respectively.

Table 7 The distances between these heteroatoms of ED and EPD to nearest Fe atom. Inhibitor

Atom - Fe

Distance (Å)

ED

O8 S7 S9 O15 O3 S11 S12 O17 O21 O24

3.036 3.303 3.112 3.103 2.898 3.109 3.138 3.133 2.973 2.944

EPD

Fe (110) surface. The slab with more inhibitor molecules to simulate the film formation process was further considered. As shown in Fig. 12, six ED molecules are orderly spread on iron surface after MD simulations, hydrogen-bonding types are similar to low coverage, with only a little change in bond length (2.14 Å avg.). A more widespread hydrogen bond network is favored to form in high ED coverage, which makes ED filming along [1 -1 1] on iron surface. From side view of Fig. 12, hydrophobic-CH3 group is bend against iron surface, which may effectively separate surface atoms from corroding environments. Similar monolayer of EPD can also be obtained (Fig. 13). To clearly observe the self-assemble network of ED and EPD molecules on Fe (110) surface, periodic magnifications are presented in Fig. S6 and Fig. S7, while the schematic diagram of molecular self-

assemble film is shown in Fig. 14. It is clear that iron surface can be fully covered by compact inhibitor network; such protection monolayer would effectively reduce the diffusion of corrosion medium to iron surface to prevent its corrosion. MD simulation results revealed that ED and EPD film could be automatically formed on Fe (110) surface by intermolecular Hbonding network. Intermolecular energy was calculated to further investigate their stabilities. In low coverage, intermolecular energy for ED and EPD are 41.51 kJ/mol and 33.09 kJ/mol, respectively: such high values make them orderly distribute on Fe surface. Whereas in high coverage, the average interaction energies are a little bigger than low coverage, since hydrogen bindings are formed on both sides of ED or EPD molecule, which leads to the rising of intramolecular interactions (Fig. S8).Such trend can also be observed in multi EPD molecule system. 3.10. Inhibition under pressure The corrosion behavior of metal in deep sea is of great interest, and the understanding of this kind of corrosion have a fundamental meaning to design long-lived marine petrol platform, pipeline under the ocean and so on. Till now, small amount of researches on such topic have been reported [73e76], but mainly focus on the stress corrosion under pressure, few aim at the performance of inhibitors in deep sea. Thus we choose EPD to investigate the inhibition performance on Fe surface under 0.1e1.0 MPa, since it has better performance than ED. Fig. S9 is the adsorption energy of EPD on Fe (110) surface via

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Fig. 12. Equilibrium adsorption configurations of six ED molecules on Fe (110) surface obtained by molecular dynamic simulations.

Fig. 13. Equilibrium adsorption configurations of three EPD molecules on Fe (110) surface obtained by molecular dynamic simulations.

pressure: the adsorption energy increases with the rising of pressure. The approaching process of EPD to the surface in pressure can be observed with narrowing gap between the centroid of EPD and surface (Table 8): EPD becomes closer to Fe (110) surface in this range of pressure, which means that inhibition effect of EPD becomes better under pressure. From equilibrium configurations under pressure after MD simulations, little changes have been observed between with and without pressure system, revealing good stability of EPD. Besides, the distances between Fe surface and heteroatoms are no longer than 3.15 Å, coordination bonding could be found among them to further stabilize EPD adsorption and formation of protection film. Adsorption of EPD on Fe (110) surface under 1.0 MPa was also performed using the same method to further investigate the filming properties under pressure. A planar of EPD monolayer built by H-bonding on Fe (110) surface (Fig. S10) was obtained, without big changes compare to that with no pressure (Fig. S11). Shorter Hbonding length reveals that such big pressure facilities the filming process, rather than damages the network. It can be speculated that good inhibition of EPD to iron is kept even under 1.0 MPa.

4. Conclusion Two dithioamide derivatives were investigated as corrosion inhibitors for mild steel by experiments and theoretical calculations. ED and EPD acted well as inhibitors in 1.0 M HCl solution. The best inhibition efficiency for ED is over 92.6% and EPD over 95.2% at the concentration of 2.0 mM, both of them are mixed-type inhibitors; charge transfer resistance increases and double layer capacitance reduces with the rising of inhibitor concentration; SEM micrographs also demonstrate the formation of inhibitor monolayer on steel surface. The adsorption of ED or EPD on the mild surface follows Langmuir adsorption isotherm. The value of

DG0ads , 26.97 kJ/mol and 27.85 kJ/mol for ED and EPD, indicates the spontaneous interaction between studied inhibitors and mild steel surface. High contributions of heteroatoms to HOMO/LUMO make them as active sites for electrophilic and nucleophilic reactions upon steel. EPD demonstrates its better electron donating ability under neutral and protonated state, further prove its better bonding ability with iron surface. Molecular dynamics simulations reveal that ED and EPD molecules can be tightly parallel absorbed

G. Zhang et al. / Journal of Molecular Structure 1202 (2020) 127286

13

Fig. 14. The schematic diagram of self-assembling film.

Table 8 Distance from the centroid and heteroatoms of EPD to Fe (110) surface with and without pressure. Distance(Å)

centroid-Fe（110） O3 N7 S11 S12 N16 O17 O21 O24

by Qingdao National Laboratory for Marine Science and Technology (2017ASTCP-OS02). Appendix A. Supplementary data

Pressure (MPa) 0

0.1

0.2

0.5

0.7

1.0

2.983 2.744 2.958 2.910 3.056 3.076 2.704 2.779 3.010

2.982 2.874 3.089 3.049 2.914 2.840 2.663 2.861 2.866

2.964 2.998 2.982 2.969 3.052 3.104 2.888 2.644 2.645

2.969 2.661 2.906 3.022 3.034 3.058 2.629 2.902 3.121

2.951 2.858 2.892 3.014 2.942 2.950 2.856 2.861 2.669

2.935 2.857 2.973 2.921 2.947 3.032 2.786 2.765 2.683

to Fe (110) surface. Binding energies of EPD (608.55 kJ/mol) upon Fe (110) surface is bigger than that of ED (369.56 kJ/mol). Monolayer of ED or EPD could be easily self-assembled by cross-link hydrogen bonding network between eC]O and -N-H from adjacent molecules. EPD film maintains good stability under pressure up to 1.0 MPa. Both experimental and theoretical results propose ED and EPD are nontoxic effective anticorrosion inhibitors for mild steel in acidic environment. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled. Acknowledgements The authors would like to acknowledge financial supports from Natural Science Foundation (50673085). The authors also acknowledge the Ao Shan Talents Cultivation Program Supported

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