A combined experimental and theoretical study of papain as a biological eco-friendly inhibitor for copper corrosion in H2SO4 medium

Journal Pre-proofs Full Length Article A combined experimental and theoretical study of papain as a biological ecofriendly inhibitor for copper corrosion in H2SO4 medium Lanzhou Gao, Shini Peng, Xiaomei Huang, Zhili Gong PII: DOI: Reference:

S0169-4332(20)30202-6 https://doi.org/10.1016/j.apsusc.2020.145446 APSUSC 145446

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Applied Surface Science

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1 December 2019 15 January 2020 16 January 2020

Please cite this article as: L. Gao, S. Peng, X. Huang, Z. Gong, A combined experimental and theoretical study of papain as a biological eco-friendly inhibitor for copper corrosion in H2SO4 medium, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145446

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A combined experimental and theoretical study of papain as a biological eco-friendly inhibitor for copper corrosion in H2SO4 medium Lanzhou Gao*, Shini Peng*, Xiaomei Huang, Zhili Gong School of Civil Engineering, Chongqing University, Chongqing 400044, China E-mail address: [email protected] (L Gao). [email protected] (S. Peng).

Abstract: Papain freeze dried (PFD) was used as a novel, eco-friendly and efficient inhibitor for copper corrosion in H2SO4 medium via experimental and theoretical calculation methods. These test methods include classical weight loss experiment, electrochemical measurements, surface morphology researches, and theoretical calculations. The data of weight loss experiment proves that with the increase of temperature, PFD still exhibits excellent anti-corrosion performance. The corrosion inhibition efficiency of PFD can still be maintained at about 95% at 313 K. The results of electrochemical experiments indicate that PFD is the mixed-type inhibitor that can simultaneously restrain the reaction of anode and cathode. Both the impedance experiment results and polarization experiment data show that the corrosion inhibition efficiency of PFD exceeds 95%. The atomic force microscope (AFM) and scanning electron microscope (SEM) test results intuitively show the excellent inhibition performance of PFD. The X-ray photoelectron spectroscopy (XPS) experimental data demonstrated the formation of N-Cu bond, indicating that PFD can effectively separate the corrosive medium by chemical adsorption on the copper surface. Molecular dynamics simulations (MDS) manifest that PFD can adsorption on the Cu (111) surface via the parallel pattern. Besides, the adsorption process of PFD on copper surface obey the Langmuir mono-layer adsorption. Keywords: Copper; Papain freeze dried; Corrosion inhibitor; weight loss experiment; XPS; Langmuir adsorption. 1. Introduction 1

Metal materials are damaged by the corrosive effects of surrounding corrosive media during service, which is called corrosion of metals. Metal corrosion has caused huge economic losses and waste of resources [1-4]. At the same time, it also brings huge security risks and poses potential threats to human production and life. Therefore, the corrosion protection work of metal materials has always been a hot issue for researchers. Copper is one of the most frequently used metal materials for human [5-8]. It greatly improves the process of human civilization. As the continuous improvement of metal smelting technology, the production of pure copper has been greatly improved. Copper has been widely used due to its excellent properties, such as relatively perfect electrical and thermal conductivity, excellent metal ductility and so on [9, 10]. However, Copper equipment are also subject to corrosive media under complicated work conditions, which greatly affects its service life and deteriorating the working environment. Therefore, the corrosion protection of copper is particularly significant. Among the many corrosion protection methods, inhibitor is a significant way to anticorrosion because of its simple operation and remarkable effects [11-13]. It is recognized that the organic inhibitors usually contain same heteroatoms such as nitrogen (N), oxygen (O), sulfur (S), phosphorus (P), and some polar functional groups [14-20]. Although many organic corrosion inhibitors have significant protective effects for metal materials, it also has its own defects. Many organic substances can cause irreversible damage to the ecological environment [17]. Therefore, corrosion protection researchers have begun to explore the novel, environment friendly and efficient inhibitors over the years. For example, Tan et al. [2] researched the anticorrosion performance of copper in the H2SO4 medium by three food flavor inhibitors. Liao et al. [21] researched the anti-corrosion nature of lychee fruit extract on steel in acidic medium. Zhang et al. [22] studied the anetholi trithionum as a green inhibitor for copper corrosion in H2SO4 medium. They used food flavors, fruit extracts, and drug as corrosion inhibitors, respectively. Obviously, the works of their study are the green 2

environmentally friendly corrosion inhibitors, and the impact on the environment is negligible. Therefore, in view of their works, we began to explore the anti-corrosion performance of PFD for copper in H2SO4 medium. PFD can be extracted from immature papaya fruit. Its water solubility is very good. PFD is widely applied in life and production. For example, it can be used for the tenderization of meat and the clarifying agent of beer in the catering industry. In the pharmaceutical industry, drugs containing PFD can fight cancer, tumor, lymphocytic leukemia, protobacteria and parasites, tubercle bacilli, etc., which can reduce inflammation, gallbladder, relieve pain and help digestion. Because it naturally exists in papaya fruit, it is a green corrosion inhibitor. In this work, we used weight loss experiment, electrochemical measurements, surface morphology research and quantum chemistry calculations to explain the adsorption mechanism of PFD on copper surface from various angles. 2. Experimental 2.1 Preparation of materials PFD purchased from Adamas Co., Ltd. Its molecular formula is shown in Fig.1 and its purity is 99.5%, no purification before use. The purity of the copper sample used in this work is greater than 99.5%. Copper was cut into samples of different sizes before various tests. Copper was cut into 2 × 2 × 1 cm3 squares for weight loss test. The copper was cut into 0.1 × 1 × 1 cm3 specimens for AFM and XPS measurements. Copper samples were cut into 0.5 × 0.5 × 0.5 cm3 squares for SEM test. Before conducting the electrochemical experiment, the copper electrode was carefully isolated with epoxy resin, leaving a 1 × 1 cm2 working surface in the corrosive medium. Prior to each measurement, the copper electrode was sequentially polished on 400-2000 # emery papers until the entire copper surface was smooth and flat. The 0.5 M H2SO4 corrosion solution was formulated with concentrated H2SO4 and deionized water. The PFD was diluted sequentially into 0.2, 0.5, 1, 2, and 5 mM gradients of the test solution. 3

Fig. 1. Molecular formula of papain freeze dried.

2.2 Weight loss test Firstly, the copper samples of the weight loss test were sanded on 400 to 2000 mesh emery papers until the 6 faces were smooth. Then, the copper blocks were douche d with ultrapure water and absolute ethanol. Finally, degreased with acetone. The dried copper samples were weighed and recorded. Then use the fishing lines to suspend the copper blocks and completely immerse them in the corrosive solution. Three copper sample blocks were immersed at the same concentration for testing to obtain their average value. The copper samples were soaked in the corrosion solution for 24 h at temperatures ranging from 298 K to 313 K. After the samples were immersed for 24 h, the copper samples were taken out and then immersed in 0.1 M HCl to remove surface oxidation products. The copper samples were washed sequentially with deionized water and absolute ethanol. After drying at room temperature, the weight of the copper sample was weighed using a high-precision analytical balance. 2.3 Surface topography tests The copper samples (0.5 × 0.5 × 0.5 cm3) were sequentially polished on emery papers of 400 to 7000 # to obtain the bright test surface. The polished copper samples were sequentially douched with ultrapure water, absolute ethanol, and acetone. The copper samples were soaked in the 0.5 mol/L H2SO4 with and without 5 mM PFD for 4

30 hours at 303 K. After the surface of copper sample was cleaned, the SEM test was carried out under vacuum environment, and the model of the SEM is JEOL-JSM-7800F. Similarly, in the AFM and XPS tests, the copper pieces were immersed for 10 hours, the AFM model is MFP-3D-BIO. The copper sample surface must be absolutely dry for the XPS test, and the XPS was carried out with Al Kα X-ray source (1486.6 eV) via a PHI 5700 spectrometer. 2.4 Electrochemical experiment tests The electrochemical tests were executed in a traditional three-electrode system, and the model of the electrochemical workstation was CHI760e. In this system, the copper electrode was used as the working electrode, the platinum (Pt) electrode (2 × 2 cm2) was the counter electrode, and the saturated calomel electrode (SCE) was a reference electrode using a Luggin capillary connected to the test solution. A 250 mL beaker was used as the tested system bottle. Keep the entire test system connected to the atmosphere at each test. The test time of open circuit potential (OCP) was 1800 seconds to insure that the copper electrode surface reaches a steady state. As shown in Fig. 2 (a), it can be found that in the last 300 seconds, the fluctuation scope of the open circuit potential is significantly less than 3 mV, so it can be judged that the copper electrode surface has reached a transient state. Next, the electrochemical impedance spectroscopy (EIS) experiment was carried out. The initial voltage was the stable EOCP value. The frequency scope was from high frequency 100000 Hz to low frequency 0.01 Hz and with 5 mV as AC disturbance signal. EIS data in the paper was fitted using ZSimDemo software. Finally, the potentiodynamic polarization curves were tested. The scope of polarization voltage was EOCP  250 mV. The polarization scan rate was 0.167 mV/s. Experiments with the identical conditions were carried out 3 times to guarantee well-pleasing reproducible data. 2.5 Theoretical calculation details The molecular structure of PFD was fully optimized with Gaussian 09 via density 5

functional theory (DFT). The calculation parameters are selected as following, the work type is OPT+Frep. The calculation method is DFT, and the Basis Set is 6-311G++d p. We calculate the frontier molecular orbitals, dipole moment, and energy gaps of PFD, respectively. The adsorption configuration of PFD on ideal surface of Cu (111) was calculated via the Materials Studio (MS) 8.0. We create a 3D model of Cu (111) with 8×8×8. The size of the entire model is 30 Å × 30 Å × 50 Å. A 20 Å vacuum layer was established and then filled with 1000 H2O molecules and a PFD molecule. The Dynamics as the calculated task, the Ensemble was NVT, the temperature was set to 298 K, 1 fs as the time step, and the 1000 ps as the total simulation time. Forcefield was COMPASS, Electrostatic and van der Waals all select Ewald. 3. Results and discussion 3.1 Analysis of weight loss experiment The weight loss results of copper immersed in the 0.5 M H2SO4 medium with and without PFD for 24 h at different temperatures are presented in Table 1. In order to quantitatively analyze the anti-corrosion nature of PFD, corrosion rate (v), anticorrosion efficiency (η) and the surface coverage (), are obtained by below formulas [23-27]:  

W St

(1)



 0  0

(2)



v0  v  100 v0

(3)

where w is the lost quality of copper in the weight loss experiment. S represents the total area of the six sides of copper specimen. t is the soaking time of copper sample in test solution. The v and v0 indicate the rate of corrosion with and without PFD. 6

The corrosion rate of copper in H2SO4 medium increases sharply with increasing temperature without PFD in Table 1. The copper corrosion rate is 214.4 mg m2h1at 298 K. When the temperature rises to 313 K, the corrosion rate rises sharply to 339.2 mg m2h1. It shows that with the increase of temperature, the corrosion of copper obviously accelerated by H2SO4 corrosive medium. This is attribute to the increase in temperature, which exacerbates the thermal motion of the corrosive medium, thereby increasing the frequency of contact with the copper sample. Therefore, the corrosion rate of the copper is augment. After adding PFD to the H2SO4 solution, the corrosion rate of copper observably reduced. At 298 K, the corrosion rate of copper after soaking for 24 hours with 5 mM PFD has dropped to 3.2 mg m2h1. The inhibition efficiency reach 98.5 % relative to the blank solution. It is also worth mentioning that with the temperature augment, the η value of PFD decrease, which manifests that the temperature increase is not conducive to PFD adsorption on the copper surface. Fortunately, even at very low concentration (0.2 mM) of PFD, it still exhibits excellent anti-corrosion performance. Table 1. Weight loss experimental data of copper samples containing disparate concentrations of PFD at different temperatures. T

C

ν

η

(K)

mM

(mg m-2h-1)

(%)

0

214.4

－

－

0.2

17.3

90.2

0.902

2.1

0.5

13.9

93.5

0.935

3.0

1

10.5

95.1

0.951

1.9

2

7.9

96.3

0.963

2.3

5

3.2

98.5

0.985

2.8

0

247.4

－

－

0.2

26.2

89.4

0.894

1.8

0.5

17.8

92.8

0.928

2.5

1

13.1

94.7

0.947

2.8

2

11.6

95.3

0.953

2.7

5

5.4

97.8

0.978

2.2

0

280.1

－

－

0.2

32.5

88.4

0.884

298

303

7

θ

SD

2.3

308

313

0.5

22.1

92.1

0.921

2.6

1

17.1

93.9

0.939

2.7

2

16.5

94.1

0.941

1.7

5

9.8

96.5

0.965

2.1

0

339.2

－

－

0.2

48.2

85.8

0.858

1.6

0.5

29.8

91.2

0.912

2.8

1

26.8

92.1

0.921

2.5

2

23.1

93.2

0.932

2.3

5

15.9

95.3

0.953

2.6

SD represents the standard deviation of three parallel experiments

3.2 Open circuit potential (OCP) and potentiodynamic polarization curves analysis Fig. 2(a) and (b) is the diagram of OCP and polarization curves of copper electrode at 303 K in 0.5 M H2SO4 with and without PFD, respectively. Fig. 2(a) reveals that the entire OCP curves tend to be stable in the last 300 seconds after 1800 seconds. This can indicate that the copper electrode surface has reached a steady state. In addition, it can be found that after the addition of PFD, the open circuit potential is clearly shifted to the negative direction. This may be because the adsorption of PFD on the copper electrode surface changes the state of copper/solution interface. As shown in Fig. 2(b), the icorr is observably descend with the PFD concentration increase. In addition, it can be found that the downward trend of the cathodic branch is significantly faster than the anodic branch. Therefore, it can be shown that the inhibitory effect of PFD on the cathodic reaction is significantly greater than that of the anodic reaction. The anti-corrosion mechanisms of the cathode and anode of the copper in the H2SO4 medium are as follows [7, 8, 28]:   Anodic reactions: Cu  Cuads  e  2 Cuads  Cusol  e

Cathodic reaction:

1 O2  2 H   2e   H 2O 2

fast

(4)

slow

(5) (6)

Therefore, it can be found that copper is oxidized to cuprous in H2SO4. The adsorption 8

mechanism of cuprous and PFD on the copper electrode surface is as follow [28]:  Cuads  nInh  [Cu  Inhn ]ads

(7)

The PFD adsorb on the copper surface by complex formation with Cu+, thereby effectually isolating the corrosive medium. In the subsequent XPS experimental analysis section, we will discuss in depth the chemical bonds formed between PFD and copper. In addition, by observing Fig. 2(b), the corrosion voltage significantly moves to the cathodic direction. Some researchers believe that this is attributed to the inhibition of copper cathodic reaction by PFD is greater than the anodic reaction. In the anodic branch, as the PFD concentration and the polarized potential increase, the anodic branch slope increases sharply, indicating that the PFD desorption behavior occurs on the copper electrode surface.

9

Fig.2. The (a) OCP diagram and (b) potentiodynamic polarization curves of PFD at different concentrations at 303 K.

In order to quantitatively analyze the electrochemical results, the Tafel extrapolation way was employed to fit the polarization curves. The fitted parameters were shown in Table 2. The E is the corrosion potential, i is the corrosion current density, and βc and βa are the slopes of the cathodic and anodic branches, respectively. The η values are calculated using the following formula [29-33]:

 (%) 

icorr,0  icoor icorr, 0

100

(8)

The corrosion potential is 47 mV without PFD. When the concentration of PFD is 5 mM, the corrosion potential is 101 mV. The maximum change value is 54 mV, significantly less than 85 mV, so the PFD is the mixed-type inhibitor [34]. In addition, the values of βc and βa significantly changed to the blank solution. Therefore, it can also be judged that the PFD is a mixed-type inhibitor. Table 2. The electrochemical parameters of potentiodynamic polarization curves for copper in 0.5 M H2SO4 with and without PFD at 303 K.

10

C

Ecorr

βc

icorr 2

βa 1

1

η(%)

SD

(mM)

(mV/SCE)

(μA cm )

(mV dec )

(mV dec )

Blank

47

18.52

281.2

81.3

－

0.2

74

1.96

175.5

49.6

89.4

1.8

0.5

83

1.42

186.8

55.5

92.3

1.7

1

86

1.05

189.2

44.9

94.3

2.3

2

92

0.77

179.4

79.3

95.8

2.8

5

101

0.72

165.2

67.4

96.1

2.2

PFD

SD represents the standard deviation of three parallel experiments.

3.3 Electrochemical impedance spectroscopy analysis EIS is widely used as an effective electrochemical way to research corrosion inhibitors [35]. Fig. 3 shows the plots of the Nyquist and Bode for the copper electrode in 0.5 M H2SO4 at 303 K with and without PFD, respectively. As shown in Fig. 3(a), the Nyquist diagram shows depressed semicircle with its center below the real X-axis and a straight line with a certain angle at the low frequency area. The linear part of the low frequency region is called the Warburg impedance. It is usually due to diffusion of the corrosion products towards or away from the copper surface or the diffusion of dissolved oxygen to the copper electrode surface [33, 36-38]. We can see that the presence of Warburg impedance is still observed when the PFD concentrations are very low (0.2-1 mM). Which manifests that the adsorption protection film by PFD onto the surface of copper is not dense and orderly enough. When the concentration of PFD reaches 2 mM, it can be found that the Warburg impedance has disappeared. This indicates that PFD adsorb on the copper surface to form a dense and orderly barrier film. It is also worth mentioning that all the capacitive loop show an imperfect semicircle or flattened semicircle, which is due to the uneven adsorption film on the electrode surface [21]. Fig. 3(b) is the Bode diagram, which clearly shows that the impedance modulus augments significantly by an order of magnitude in the low frequency area. In addition, the slope of the impedance modulus map in the intermediate frequency area is 1, 11

indicating that the copper electrode surface has a capacitance property [39]. Observing the phase angle maps can clearly reveal two peaks, which manifests that the adsorption of PFD on the copper surface produces two relaxation effects. One is the adsorption process of PFD at the copper electrode, and the other is ascribe to the electric double layer capacitance [8].

12

Fig. 3. Impedance spectra of PFD at 303 K at different concentrations: (a) Niqust plot, (b) Bode plot.

The electrochemical impedance spectroscopy parameters in Table 3 are obtained by fitting using the corresponding equivalent circuit diagrams. The chi-square values of the fits were all less than 102, which indicates that the error between the fitted and experimental value is small. The equivalent circuit diagrams of Fig. 4 apply to fitting the data of EIS without and with PFD, respectively. In Table 3, Rf stands for the film resistance (molecular film formed by PFD or oxide film on copper surface), Rct stands for the charge transfer resistance, and Rp (Rct + Rf) is the polarization resistance. W stands for the Warburg impedance, and CPEdl and CPEf are the constant phase angle elements, reflecting the Cf (film capacitance) and the Cdl (double layer capacitance). The calculating CPE impedance formula is as following [40-42]:

Z CPE 

1 Y0 ( j ) n

(9)

13

Y0 stands for CPE constant, j stands for the imaginary root,  stands for the angular frequency, and n stands for the deviation index, reflecting the electrode surface heterogeneity. Besides, the values of capacitances (Cdl and Cf) are calculated via the formula (10) [43, 44]:

C  Y0 () n1  Y0 (2f ZimMax ) n1

(10)

where Y0 can be obtained by fitting. It can be clearly seen in Table 3 that as the concentration of PFD augments, the values of Rct and Rf augment distinctly, indicating that the adsorption of PFD markedly slows the corrosion of copper electrode. The values of η can be calculated by the equation (11) [38]:

 (%) 

RP  RP , 0 Rp

100

(11)

where Rp and Rp, 0 indicate the polarization resistance with and without PFD. As shown in Table 3, the expressions of Cf and Cdl are as following [45, 46]:

Cf  Cdl 

F 2S 4 RT

 0 d

(12)

S

where F is the Faraday’s constant.

(13)

 0 and  are air dielectric constant and double layer

local dielectric constant. S stands for the uncovered area of the copper electrode in the H2SO4 corrosive medium. The d represents the thickness of the Cdl. The more obvious the decrease in Cf and Cdl values, the more H2O molecules on the Cu surface are replaced by PFD.

14

Table 3. Impedance spectrum parameters. C

Rf

Rct

Rp

Cf

n1

Cdl

n2

W

ƞ (%)

SD

(mM)

(kΩ cm2)

(kΩ cm2)

(kΩ cm2)

(μF cm-2)

Blank

0.01

0.14

0.15

32.62

0.87

87.98

0.59

1.41

－

0.2

0.02

1.63

1.65

16.37

0.91

40.23

0.61

0.25

90.9

1.2

0.5

0.04

3.06

3.10

15.52

0.93

30.28

0.73

0.19

95.2

1.1

1

0.06

3.79

3.85

11.19

0.93

23.13

0.75

0.03

96.1

1.9

2

0.15

5.04

5.19

8.54

0.94

19.23

0.65

－

97.1

2.5

5

0.18

6.15

6.33

4.09

0.96

16.34

0.67

－

97.6

2.0

(μF cm2)

(×102 Ω cm2 s1/2)

PFD

SD represents the standard deviation of three parallel experiments.

Fig. 4. The equivalent circuit diagrams for fitting the impedance spectrum parameters: (a) with Warburg impedance (b) without Warburg impedance.

3.4 Adsorption isotherm model study The basic information of PFD adsorption on copper/solution interface was researched via isotherm models. We used a variety of adsorption isotherm models to fit the weight loss experimental data at different temperatures. When using Langmuir adsorption isotherm simulation, its linear regression coefficient R2 is very close to 1. Hence, the adsorption of PFD on the copper surface is obey the Langmuir adsorption. Langmuir adsorption isotherm is shown below [47]: 15

Langmuir isotherm equation:

C 1  C  Ka d s

(14)

where C stands for the PFD concentration,  is the surface coverage, and Kads is the adsorption equilibrium constant. The fitted data present in Fig. 5. In order to explore the adsorption type (physical or chemical adsorption) of PFD onto the copper surface, the value of G0ads can be calculated by the formula (15) [27]:

K ads 

G 0 1 exp(  ads ) 55.5 RT

(15)

It is well known that large Kads value and small G0ads value indicate that corrosion inhibitors exhibit excellent anti-corrosion performance. It can be judged by the G0ads and Kads values in Fig. 5 that PFD can exhibit distinguished anti-corrosion nature for copper in H2SO4 corrosion medium. It is generally believed that when the absolute value of G0ads is less than 20 kJ/mol, it can be judged as physical adsorption, that is, the molecules of corrosion adsorb onto the metal surface via electrostatic force. The absolute value of G0ads is more than 40 kJ/mol, it can be judged as chemisorption, that is, the inhibitor molecules are adsorbed onto the metal surface by charge transfer or sharing. If the absolute value of G0ads is between 2040 kJ/mol, which indicates that physical and chemical adsorption coexist. The G0ads values are 35.4, 35.9, 36.5 and 37.0 kJ/mol at temperatures of 298 K to 313 K, respectively. Hence, the physical and chemical adsorption coexist when PFD is adsorbed on the copper surface [48]. Besides, we can find that the absolute G 0ads of G is closer to 40 kJ/mol with increasing temperature. This indicates that there are more PFD molecules on the copper surface by chemisorption.

16

17

Fig. 5. Langmuir adsorption isotherm plots of PFD on copper surface at 298 K to 313 K.

The results of weight loss experiment show that temperature is one of the significant factors affecting the anti-corrosion nature of PFD. In order to further understand thoroughly the anti-corrosion mechanism of PFD for copper in sulfuric acid 18

solution. We separately calculated the relevant thermodynamic parameters using the following formulas [41, 49]: 0 H ads ln K ads    cons tan t RT

(16)

0 0 0 Gads  H ads  TS ads

(17)

Among them, the calculated G0ads (standard free energy), H0ads (enthalpy) and S0ads (entropy) values are listed in Table 4. The value H 0ads can be obtained by using equation 16, lnKads is the Y axis, and 1/T is the X axis. In Fig. 6, the calculated H0ads value is 50.27 kJ/mol, and the value of H0ads is negative. It manifests that the PFD adsorption on the copper surface is an exothermic process. Therefore, the increase of temperature is a disadvantage condition to the PFD adsorption onto the copper surface. The S0ads of PFD are negative values, indicating PFD lose its translation freedom when they are adsorption onto the copper surface. Table 4. Thermodynamic parameters of PFD adsorption on the copper samples at different temperatures Temperature

K ads

0 Gads

0 H ads

0 S ads

(K)

(103 L/mol)

(kJ/mol)

(kJ/mol)

(J K1mol1)

298

28.9

35.4

303

28.3

35.9

308

27.4

36.5

44.71

313

26.3

37.0

42.39

19

49.89 50.27

47.42

Fig. 6. The relationship between ln Kads and 1/T for copper in 0.5 mol/L H2SO4 including diverse concentrations of PFD.

3.5 Corrosion dynamics analysis By analyzing the thermodynamic parameters, we have already known that temperature is a disadvantage condition. In order to profoundly explore the adsorption mechanism of PFD, we used the Arrhenius equation and the transition state equations to analyze the corrosion rate of copper. The formulas are as follows [50, 51]: Ea  ln A RT v R S a H a ln  ln   T Nh R RT

ln v  

(18) (19)

where v is the corrosion rate of copper in test solution, Ea stands for the apparent activation energy, R stands for the gas constant, T stands for the thermodynamic temperature,  stands for the pre-factor, h is the Plank constant, N stands for the Avogadro constant, and Sa stands for the apparent activation entropy, Ha stands for apparent activation enthalpy. Fig 7 (a) and (b) show the linear fit of the corrosion of copper samples to the Arrhenius equation and the transition state equation, respectively. The calculated 20

activation kinetic parameters are listed in Table 5. The PFD was added to the H2SO4 corrosion medium, the value of the Ea increased significantly, which was attributed to the first physical adsorption of PFD onto the copper surface [52]. However, with the increase of PFD concentrations, the Ea value showed a decreasing trend, which indicated that the PFD and copper surface had chemical adsorption. Therefore, the PFD adsorption on the copper surface has the coexistence of physical adsorption and chemisorption, which is accordant with the results of the kinetic analysis. The value of Ha is positive, indicating that the dissolution process of copper has endothermic characteristics [53]. Therefore, raising the temperature is beneficial factor to the dissolution of copper. Fortunately, the addition of PFD increased the value of Ha, indicating that the dissolution of copper is difficult due to the adsorption of PFD onto the copper/solution interface. It is worth mentioning that the Sa value is larger with low PFD concentration than that in the blank solution. This is due to the competitive adsorption of PFD molecules and water molecules, resulting in increased chaos on the copper surface. However, with the concentration of PFD augments, the Sa value decreases, which indicates that the chaos of the surface of the copper electrode is reduced [51]. This is because the PFD forms a stable and orderly obstacle film onto the copper surface.

21

22

Fig.7. (a) Arrhenius plots for copper in 0.5 mol/L H2SO4 with and without various concentrations of PFD. (b) Transition state plots to copper in 0.5 M H2SO4 without and with different concentrations of PFD. Table 5. Activation parameters of copper samples in 0.5 mol/L H2SO4 containing different concentrations of PFD. PFD

H a

Ea

Sa

(mM)

(kJ/mol)

(kJ/mol)

(J K mol-1)

0

22.36

46.52

23.76

0.2

49.07

62.30

6.42

0.5

47.38

73.22

17.16

1

43.45

64.53

24.12

2

35.84

75.19

39.81

5

32.97

72.83

50.83

3.6. SEM analysis 23

-1

Fig. 8 displays the surface of copper under disparate conditions. Fig. 8(a) displays the copper surface form after it has been ground. It can clearly see the fine scratches left by the entire copper surface. The entire copper surface is very flat. Fig. 8(b) displays the surface morphology of the copper after soaking in the sulfuric acid containing 5 mM PFD for 30 hours at 303 K. We can see that the entire copper surface is still very flat and bright. The polishing scratches are faintly seen. Fig. 8(c) shows the copper surface morphology soaked in 0.5 mol/L H2SO4 medium for 30 s at 303 K. It can be observed that the copper surface has been badly damaged. The entire copper surface exhibits a uniform broken structure. By comparing the different states of copper, it can be clearly found that PFD can exhibit excellent anti-corrosion performance in the H2SO4 corrosive medium. Which is highly compatible with the experimental results.

Fig. 8. SEM image of the copper samples under different conditions: (a) copper surface just after grinding, (b) 5 mM PFD, (c) sulfuric acid solution without PFD.

3.7 AFM analysis Fig. 9 shows the 3D morphology and corresponding contour plots of the copper under disparate experimental conditions. Fig. 9 (a) and (b) are the surface morphology of the newly burnished copper and the corresponding contour map. The roughness value of the entire copper surface is only about 3.7 nm. Fig. 9 (c) and (d) show the surface 24

morphology and corresponding contour plot of the copper sample after soak in the sulfuric acid solution containing 5 mM PFD for 10 hours at 303 K. At this time, the average roughness of the copper surface is about 13 nm. Fig. 9 (e) and (f) show the surface morphology and corresponding contour plot of the copper sample after soak in the sulfuric acid solution without PFD for 10 hours at 303 K. It can be found that the peaks and troughs of corrosion can exceed 100 nm. Therefore, it can be shown that the copper sample is severely corroded in the sulfuric acid solution without PFD. At this time, the average roughness value (Ra) of the copper surface is close to 60 nm. Therefore, it can be clearly judged that PFD exhibits distinguished anti-corrosion nature for copper in sulfuric acid corrosion medium.

25

Fig. 9. AFM topography of copper under disparate conditions: (a) the surface of the copper after the sanding, (c) 5 mM PFD, (e) the sulfuric acid solution without PFD.

3.8 XPS analysis In recent years, XPS has been widely applied to the chemical bond formation of heteroatoms in corrosion inhibitors and metals. Fig. 10 is the spectrum plots of Cu 2p, N 1s, and O 1s. The copper sample was soaked in the H2SO4 containing 5 mM PFD for 10 hours at 303 K. Table 6 lists the corresponding peak fitting parameters. The peaks at Cu 2p 3/2, and Cu 2p 1/2 in Fig. 10 (a) are attributed to Cu+ and copper substrate. The Cu (II) is not found in the spectrum of Fig. 10 (a) at the 933.5 eV, indicating that the PFD adsorption onto the copper surface can effectually restrain the oxidation of Cu+ into Cu2+ [54]. In Fig. 10 (b), the binding energy values are 400.4, 399.6 and 399.0 eV, which corresponding to NCu, NC/NC and N, respectively. Therefore, PFD form the coordination bond via nitrogen atom and copper base, it is chemically adsorbed onto the copper surface. Therefore, PFD can exhibit remarkable anti-corrosion performance. Fig. 9(c) is the O1s peak position, which is due to the Cu2O peak at the 531.3 eV. When the binding energy is 532.1 eV, this is due to the peak of C=O in the PFD [55]. In summary, by analyzing the spectrograms of Cu 2p, N 1s, and O 1s, it can be clearly judged that PFD molecules adsorb onto the copper surface by chemisorption of N-Cu bonds, thereby the Cu+ is inhibited from being further oxidized to Cu2+.

26

27

Fig. 10. XPS spectra from copper surface: (a) Cu 2p (b) N 1s (c) O 1s. 28

Table 6. XPS parameters of peak fitting. Level

Chemical States

Cu 2p

Cu(0)/Cu(1)

932.1

1.00

O 1s

Cu2O

531.3

1.07

C=O

532.1

1.15

NCu

400.4

1.09

NC/NC

399.6

1.02

N

399.0

1.20

N 1s

Binding Energies (eV)

FWHMs

3.9 Quantum Chemical Analysis In order to deeply explore anti-corrosion nature of PFD molecular. We use DFT to calculate its quantum chemical parameters. These parameters include dipole moment, frontier molecular orbital energy, and energy gap. In addition, the frontier molecular orbital map and electrostatic potential (ESP) of the PFD molecule were calculated to obtain the active center of the reaction. Fig. 11 reveals the molecular structure and the orbital distribution of HOMO and LUMO of PFD and its protonation. The electron cloud density in the HOMO orbitals of PFD mainly distributes the five-membered ring containing nitrogen. The electron cloud density on the LUMO orbitals can be evenly distributed throughout the PFD molecule. When the PFD molecule is protonated, the electron cloud on the HOMO orbitals is transferred to the nitrogen-containing chain, and the electron cloud density of the LUMO orbitals is transferred to the nitrogencontaining heterocycle of the five-membered ring.

29

Fig. 11. The molecular structures and the orbital distribution of HOMO and LUMO of PFD and its protonation.

Fig. 12 reveals the electrostatic potential map of the PFD molecule and its protonation, respectively. It is well known that in the ESP diagram, the reddish-brown region has nucleophilic properties, while the blue region has electrophilic properties [56, 57]. All reddish-brown area are due to in electronegative groups with heteroatoms, such as heterocycles of O, N. They can form covalent bonds with copper atoms.

Fig. 12. The ESP distribution of PFD and its protonation. 30

Fig. 13 shows the quantum chemical parameters of PFD molecule and its protonation, including energy of HOMO and LUMO orbitals, and dipoles. As well known, the HOMO orbitals correspond to the donating electron nature of molecules [53]. The large energy value of HOMO orbitals indicates that the electron donating nature of the inhibitor is stronger. Therefore, this is an advantageous factor for the corrosion inhibitor adsorb onto the metal surface. The LUMO orbitals are correspond to the electron accepting nature of the molecule [58, 59]. The low ELUMO value is beneficial for molecules to accept electrons. Generally, the reactivity of the corrosion inhibitor is judged according to the energy gap value (E = ELUMOEHOMO) [60]. The small E value facilitates corrosion inhibitor adsorption on metal surface [61, 62]. According to Fig. 13(a), we can calculate that the PFD and PFDH+ energy gaps are 5.91 eV and 3.27 eV, respectively. Therefore, it is reasonably judge that the PFD after protonation facilitates adsorption onto the copper surface. The dipole moment is one of the significant parameters for judging the anticorrosion performance of corrosion inhibitors. Most corrosion researchers believe that a large dipole moment value can change the electric double layer capacitance of the metal surface, which is beneficial to the adsorption of corrosion inhibitor onto the surface of metal [37]. The dipole moment values of PFD and PFDH+ are 4.79 Debye and 6.33 Debye in Fig. 13(b), respectively. Therefore, it can be found that when the PFD molecule is protonated, the value of the dipole moment is significantly increased, which indicates that protonation facilitates the adsorption of PFD onto the copper surface.

31

Fig. 13. The energy gap and dipole moment of PFD and its protonated.

3.10 Molecular Dynamics Simulation (MDS) In order to vividly comprehend the stable adsorption configuration of PFDH+ on the copper surface, we used the Forcite module in MS software to calculate the PFDH+ 32

on the Cu(111) surface. The optimized configuration is presented in Fig. 14. It can be clearly observed that the PFD adsorb onto the Cu(111) surface in parallel pattern. This adsorption configuration provides maximum coverage, effectively separating the copper surface from the corrosive media. In addition, we calculated the binding energy of PFD on the copper surface by the following formulas [28, 63, 64]:

Ebinding   Eint eract

(20)

Einteract  Etot  ( Esubs  Einh )

(21)

where Etot represents the total energy of the entire simulation system. Esubs is the energy of copper substrate and water molecules. Einh is the energy of a single inhibitor molecule. The binding energy calculated by equations (20 and 21) is 172.15 kJ/mol. This indicates that the PFD molecule has a distinguished adsorption onto the copper surface.

Fig. 14. Stable adsorption configuration on Cu(111) surface of PFDH+: (a) side view, (b) top view.

3.11 Adsorption mechanism research Fig. 15 is a schematic diagram of the adsorption mechanism of PFDH+ onto the 33

copper surface. It can be proved by isothermal adsorption model that PFDH+ adsorption copper surface conforms to Langmuir monolayer adsorption. In addition, it is worth mentioning that the G 0ads can be used to judge physical and chemical adsorption coexistence of PFDH+ onto the copper/solution interface. The XPS experiment shows that cuprous ions are detected onto the copper surface after the addition of PFD. Therefore, it can be confirmed that copper has been oxidized to cuprous copper when the PFD has not been adsorbed on the copper/solution interface. Therefore, a layer of cuprous ions has been coated on the surface of the copper substrate, and these cuprous ions are capable of adsorbing sulfate ions in the solution. Sulfate ions adsorbed at the cuprous/solution interface will be physically adsorbed by protonated PFDH+. In addition, the heteroatomic nitrogen and oxygen in the PFD can provide a lone pair of electrons to form a coordinating bond with the empty orbital of copper at the metal/solution interface. Therefore, the PFD molecule can form a molecular barrier film onto the copper/solution interface, thereby effectively suppressing corrosion of copper in the H2SO4 corrosion solution.

Fig. 15. The schematic diagram of the adsorption mechanism of PFDH+ on the copper surface. 34

4. Conclusion 1. Electrochemical test data indicate that PFD exhibit distinguished anti-corrosion ability for copper in H2SO4. The PFD can simultaneously restrain the reaction of the cathode and anode of the copper electrode, and is the mixed-type corrosion inhibitor. 2. The morphology of SEM and AFM convincingly demonstrate the data of electrochemical results. XPS analysis manifests that PFD adsorb on the copper surface by N-Cu bond, thus forming a dense and orderly obstacle film, which effectively inhibited the oxidation of Cu+ into Cu2+. 3. The weight loss experiment showed that PFD showed excellent anti-corrosion nature with increasing temperature within a certain temperature range. 4. The PFD adsorption on the copper surface is spontaneous process and obeys to the Langmuir mono-layer adsorption. As the adsorption process occurs, the degree of chaos decreases, indicating the formation of a stable and disordered barrier film on the copper surface. 5. Theoretical calculations show that PFD has small energy gap value and large dipole moment value. This indicates that it has good corrosion inhibition performance. Molecular dynamics simulations showed that PFD could adsorb onto the Cu (111) surface in parallel manner. Declarations of interest: none. Acknowledgments Graduate Research and Innovation Foundation of Chongqing, China (No. CYB19090). References [1] S.A. Umoren, M.M. Solomon, S.A. Ali, H.D.M. Dafalla, Synthesis, characterization, and utilization of a diallylmethylamine-based cyclopolymer for corrosion mitigation in simulated acidizing environment, Mater. Sci. Eng. C Mater. Biol. Appl. 100 (2019) 897-914. [2] B. Tan, S. Zhang, Y. Qiang, L. Guo, L. Feng, C. Liao, Y. Xu, S. Chen, A combined experimental and theoretical study of the inhibition effect of three disulfide-based flavouring agents for copper corrosion in 0.5 M sulfuric acid, J. Colloid Interf. Sci. 526 (2018) 268-280. [3] A. Dehghani, G. Bahlakeh, B. Ramezanzadeh, M. Ramezanzadeh, Detailed macro-/micro-scale exploration of the excellent active corrosion inhibition of a novel environmentally friendly green inhibitor for carbon steel in acidic environments, J. Taiwan Inst. Chem. E. 100 (2019) 239-261. [4] S.K. Saha, M. Murmu, N.C. Murmu, P. Banerjee, Evaluating electronic structure of quinazolinone 35

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Highlights • Electrochemical test data indicate PFD can exhibit distinguished anti-corrosion ability for Cu in H2SO4 • XPS provided strong evidence for the existence of inhibition film on the Cu surface via Cu-N bond. • PFD adsorb on the copper surface in accordance with the Langmuir model. • Theoretical calculations show that PFD has small energy gap value and large dipole moment value. Molecular dynamics simulations showed that PFD could adsorb on the Cu (111) surface in parallel manner.

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Graphical Abstract

41

Author contributions Lanzhou Gao: Experiment, Methodology, Theoretical calculation, Investigation, Writing - Original Draft. Shini Peng: Experiment design, Formal analysis, Visualization,Software. Xiaomei Huang: Validation, Formal analysis, Visualization. Zhili Gong: Writing - Review & Editing, Supervision, Data Curation. All authors agree to submit to Applied Surface Science.

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