A combined electrochemical and theoretical analysis of environmentally benign polymer for corrosion protection of N80 steel in sweet corrosive environment

Accepted Manuscript A combined electrochemical and theoretical analysis of environmentally benign polymer for corrosion protection of N80 steel in sweet corrosive environment Ambrish Singh, Neetesh Soni, Yu Deyuan, Ashish Kumar PII: DOI: Reference:

S2211-3797(18)33508-3 https://doi.org/10.1016/j.rinp.2019.02.052 RINP 2116

To appear in:

Results in Physics

Received Date: Revised Date: Accepted Date:

28 December 2018 15 February 2019 15 February 2019

Please cite this article as: Singh, A., Soni, N., Deyuan, Y., Kumar, A., A combined electrochemical and theoretical analysis of environmentally benign polymer for corrosion protection of N80 steel in sweet corrosive environment, Results in Physics (2019), doi: https://doi.org/10.1016/j.rinp.2019.02.052

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A combined electrochemical and theoretical analysis of environmentally benign polymer for corrosion protection of N80 steel in sweet corrosive environment Ambrish Singh1,2*, Neetesh Soni1, Yu Deyuan1, Ashish Kumar3 1

School of Materials Science and Engineering, Southwest Petroleum University, Chengdu-610500,

Sichuan, China. 2

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum

University, Chengdu, Sichuan 610500, China. 3

School of Mechanical and Civil Engineering, Lovely Faculty of Engineering and Technology, Lovely

Professional University, Phagwara, Punjab, India. *Corresponding author E-mail: [email protected]; [email protected] Ph.No.: +86-18384155035

Abstract The corrosion inhibition efficiency of aniline, formaldehyde and piperazine based polymer (ADPD) on N80 steel in 3.5% NaCl solution saturated with carbon dioxide was investigated using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, weight loss, scanning electrochemical microscopy (SECM), scanning electron microscopy (SEM) measurements, density functional theory (DFT) and molecular dynamics simulation (MD). The adsorption of polymer onto N80 steel surface followed Langmuir adsorption isotherm model. Potentiodynamic polarization study confirmed that inhibitor is mixed type with cathodic predominance. SECM study reveals the current values decreases with the increasing concentration of polymer. SEM study supports the smooth metal surface texture. DFT and MD calculations are in agreement with the experimental findings.

Keywords: Polymer; DFT; N80 steel; EIS; SECM; MD simulations 1. Introduction Carbon steel is widely used in petroleum industry due to their endearing properties such as hardness, durability, ductility and low cost. Nevertheless, they are very much prone to undergo corrosion in aggressive media in which they are present i.e. high concentration of chloride ions in presence of carbon dioxide [1, 2]. Carbon dioxide forms carbonic acid (H 2CO3) after reacting with water containing chloride ions and this weak acid causes the corrosion of carbon steel. One of the most common and suitable way to overcome from corrosion problem without modifying corrosive environment is the use of corrosion inhibitors. Polymeric compounds enclosing heteroatoms (N, O, S), π-electrons and phenyl rings provide the ability to adsorb onto the metal surface through adsorption. Polymeric compounds possess high molecular weight that helps them to shield additional surface area of the metal and thus make them excellent candidates for corrosion inhibition. Survey of literature reveals that a number of natural and synthetic polymers such water-based acrylic terpolymer, polyanthranilic acid, chitosan, carboxymethyl cellulose, Gum Arabic have been stated as corrosion mitigators [3-8]. The existence of a large number of free nitrogen atoms in the studied polymer assets their corrosion inhibition properties and ability to form complex with iron ions [9]. The current effort aims the synthesis and assessment of corrosion mitigation efficiency of polymer derived from aniline, formaldehyde and piperazine (ADPD) on N80 steel in 3.5% NaCl solution flooded with carbon dioxide by means of weight loss, and electrochemical techniques. The surface of steel samples was observed using SECM and SEM measurements. The interaction of inhibitor molecules with the metal surface was studied by MD simulation study. 2. Experimental details

2.1. Chemical composition and preparation of N80 steel The steel samples of size 5.0 cm × 2.5 cm were used for weight loss experiments and 2.0 cm × 1.0 cm for electrochemical experiments. The surface was cleaned carefully, followed by abrading to a mirror finish. 2.2. Preparation of test solutions The corrosive medium (3.5 wt % NaCl) was prepared using analytical grade NaCl and double distilled water. The concentration ranges of each tested inhibitor used in the course of the experiments were 50 to 400 mg/L. Before the experiments, N2 gas was bubbled for 3 h in the corrosive solution in order to remove the oxygen. Then, the solution was deoxygenated by purging CO 2 gas for 4 h. The specimens were then immersed into the solution while the CO2 gas-purging at a pressure of 6 MPa was maintained to ensure a full saturation throughout the test. The electrochemical setup was sealed during the experiment. The pH of the corrosive medium was 4. The gravimetric tests were completed for 24 h under static condition. 2.3. General procedure for inhibitor synthesis The synthesis of inhibitor was completed according to the reported literature method [10]. The synthesis scheme is shown in Fig. 1. The developed polymer was characterized by 1H NMR and

13

C

NMR. The 1H NMR and 13CNMR spectrum are included in the supplementary file. 2.4. Spectral analysis of polymer White color powder; 1H NMR (300 MHz, DMSO-d6) δ (ppm): Signal at 6.742-6.483 ppm is for aromatic protons. Peak at 2.342 represent methylene protons of piperazine. Signal at 4.897 and 3.518 is for Ar–CH2–N and Ar–NH2 (primary amine group) groups respectively,

13

C NMR, δ (ppm): Signals at 47.12, 51.62, and 55.28 ppm is for Ar–CH2–Ar, Ar–CH2–N, and N–

CH2–CH2–N group, respectively. Peak at 131.34 and 149.84 is for aromatic carbons attached to the methylene (CH2) and NH2 groups respectively. 2.5. Corrosion evaluation methods 2.5.1. Gravimetric tests In gravimetric tests samples were submerged in 3.5% NaCl solution for 24h. The subsequent equation was used to evaluate the corrosion rate (CR) and inhibition efficiency (η%).

CR 

8.76  m  At

(1)

where CR be the corrosion rate (mmy-1), ∆m be the weight loss (g), ρ be the density (g cm-3), A be the exposed area (cm2) and t be the time (h), respectively.

 CR0  CR    CR 

 %  100 

(2)

where C0R and CR are the corrosion rates evaluated from weight loss with and without inhibitor, respectively. 2.5.2. Electrochemical Autolab workstation was used to carry out all the electrochemical tests by means of a cell assembly with N80 steel as working electrode, graphite rod as counter electrode and saturated calomel as reference electrode. The data obtained was analyzed with the help of Nova and Zsimpwin softwares. The working electrode altogether with the cell assembly was immersed in the test solution for 30 minutes prior to each test to establish a stable potential. The electrochemical impedance spectroscopy (EIS) was performed in the range of frequency 100 kHz to 10 mHz at an amplitude of 10 mV. The Potentiodynamic polarization in absence and presence of inhibitor was conducted at the potential range of -250 mV to +250mV vs OCP at a constant scan rate of 1 mV/s.

2.5.3. Scanning electrochemical microscopy (SECM) CHI900C workstation was used to conduct the SECM tests with the three cell electrode assembly. All the tests were performed at room temperature with and without inhibitor. To maintain a stable potential, the tests were started after an immersion time of 30 minutes. 2.6. Surface analysis (SEM) To detect the modifications at the metal surface before and after corrosion they were exposed to ZEISS SEM machine. The surface was analyzed at various regions of the metal surface using different magnification levels. Prior to the test the N80 steel samples were submerged in the test solution at room temperature for 24 h. Then the samples were cleaned by double distilled water followed by washing through sodium bicarbonate solution. The samples were then dried and further exposed to the SEM tests. 2.7. DFT study The inhibitor molecules were optimized using the DFT/ B3LYP basis set 6-31(d, p) as implemented in Gaussian 09 software [11]. DFT values related to the highest occupied molecular orbital (EHOMO), lowest unoccupied molecular orbital (ELUMO), and energy gap (∆E= ELUMO-EHOMO) were evaluated. The corrosion occurs in aqueous phase, so, it becomes obligatory to determine the quantum chemical calculation in aqueous phase in order to mimic the experimental conditions. Therefore, self-consistent reaction field (SCRF) theory and polarized continuum model (PCM) was used to conduct all DFT calculations in aqueous phase. 2.8. MD simulations and Radial distribution function All simulations were conducted using BIOVIA Materials Studio® commercial software [12]. The Fe (110) surface with a slab size of 5 Å was selected for MD simulation owing to its high stability and its highly filled assembly. To ensure wide surface area for the steel and inhibitor to interact the tests were

conducted in a box of size 24.82×24.82×35.69 Å3 with 9Cl-, 491H2O, 9

and inhibitor molecules.

The simulations were carried out at constant temperature (303 K), a time step of 1fs and 2000 PS simulation time to ensure an equilibrium state. Among the available functions COMPASS force field was chosen to perform the energy minimization and MD calculation processes [13].The relation of the inhibitor molecules adsorbed on Fe (110) surface can be established using the subsequent equations [14].

Einteraction  Etotal  ( Esurface+solution +Einhibitor )

(3)

EBinding   Einteraction

(4)

where, Etotal be the energy of the entire system, the Esurface+solution , be the entire energy of Fe (110) and

Einhibitor be the whole energy of inhibitor molecules. The radial distribution function is demarcated by Hansen and McDonald as [15, 16]:

1 N A N B (rij  r ) g AB (r )    B local N A iA jB 4 r 2 1

(5)

where  B local represents the particle density of B averaged over all shells around particle A. 3. Results and discussion 3.1. Gravimetric measurement 3.1.1. Corrosion rates and efficiency of inhibition The calculation of corrosion in the form of mass loss of the sample exposed in the corrosive media at definite time interval was investigated by gravimetric method. The effects of different concentration of inhibitor on the corrosion mitigation property of N80 steel in the temperature range of 303-353 K are shown in Fig. 2. Fig. 2a, b displays figure of corrosion rate and inhibition efficiency versus inhibitor concentration respectively at different temperature. It could be observed that at

particular temperature the corrosion rate decreases when inhibitor concentration increases and increases when temperature increases. Roughening of the steel sample reduces the adsorption ability of the polymer at elevated temperatures [17]. The increase in inhibition efficiency with inhibitor concentration is due to the more adsorption and surface coverage of metal. 3.1.2. Activation and thermodynamic studies Arrhenius process is followed by the corrosion reaction of steel in acidic medium. So, as to obtain the activation energies (Ea) Arrhenius equation was used as included below. log CR 

 Ea  log  2.303RT

(6)

where, R be the gas constant, T be the temperature, Ea signifies the activation energy and λ be the preexponential factor. The activation energy (Ea) was acquired from slope of the Arrhenius plots as presented in Fig. 3a and evaluated values are given in Table 1. The values of Ea are greater for inhibitor at different concentration. This is due to the formation of extraordinary energy shield for corrosion reaction to occur. The enthalpy of activation (H*) and entropy of activation (S*) were determined by the equation below.

 R   S *   H *  C  log  R   log      T   N a h   2.303R   2.303RT 

(7)

where, h be the Planck’s constant and N be the Avogadro number, respectively. The slope and intercept of log (CR /T) vs. 1/T graph (Fig. 3b) was used to determine the H* and S* values and the evaluated values are shown in Table 1.

The values of ∆H* are positive both with and without inhibitor suggests that the formation of activated-complex is endothermic in nature and this in literature it represents that steel dissolution

process is hindered [18]. The values of S* without inhibitor is negative and with inhibitor is positive. The variation in S* values correspond to the ordering and disordering of the inhibitors onto the electrode surface [19]. Table 1 suggests that that variation in the Ea and ∆H* values are in the similar trend but the values of ∆H* are lower than those of Ea. So it can be said that the corrosion reaction consists of hydrogen gas evolution and is connected with the reduction in overall volume of the reaction [20]. 3.1.3. Adsorption Isotherm The adsorption of the inhibitor molecule on the steel surface is the key to good protection from corrosion. Adsorption can be determined using different isotherms to get information’s about the steelinhibitor interactions. Several isotherms including Langmuir, Temkin, Frumkin were studied and the data was plotted using them. Out of all Langmuir provided the best fit for the experimental values of mass loss, impedance and polarization. The isotherm was plotted between C inh/θ and Cinh (Fig. 4) using the equation below and it gave a linear fit with correlation coefficient (R2) values close to 1 [21-23]. Cinh





1  Cinh K ads

(8)

where, Cinh be the concentration of inhibitor, Kads be the equilibrium constant and θ be the surface coverage. The values of Kads were evaluated from the reciprocal of the intercept and are given in Table 2. The equilibrium constant (Kads) is correlated to the standard free energy of adsorption (ΔG0ads) using the equation below [24]. ο Gads   RT ln(1106 Kads )

(9)

where T be the absolute temperature, R be the molar gas constant, and 1×10–6 be the concentration of water molecules. Generally, physical adsorption is seen in compounds with

values of ΔG0ads up to -20 kJ mol-1 whereas, values greater than -40 kJ mol-1 show chemical adsorption [25, 26]. The adsorption of the polymer on the steel surface was spontaneous as is evident from the negative values of ΔG0ads (Table 2). The values of ΔG0ads are in the range of -25.59 to -28.28 kJ mol1

suggesting that the adsorption followed the mixed process by both physical and chemical reaction

[27, 28]. 3.2. Electrochemical studies 3.2.1. Electrochemical impedance study Electrochemical study is a common method used to determine the corrosion rate at the steel/ solution interface. Is gives vital information about the capacitive and resistive behaviour of the surface which further helps to determine the exact mechanism of inhibitor action. The impedance graphs of N80 steel in 3.5% NaCl solution with carbon dioxide in the absence and presence of polymer is shown in Fig. 5a. The recorded Nyquist plot without inhibitor addition comprises of only one capacitive semi-circle at the higher frequency region (HF). But, adding up of different concentration of polymeric inhibitor (ADPD) affects the shape and increase in the diameter of Nyquist plots is been observed with an inductive loop at the low frequency. The phenomenon of increase in the diameter of capacitive loop is due to the increase in the corrosion resistance property of N80 steel in presence of inhibitor, which is related to the adsorption of the inhibitor molecule on the N80 steel surface. The small inductive semicircle can be endorsed to the layer equilibrium caused due to the corrosion process at the steel surface containing polymer and its reactive products [29]. The figures obtained from the Nyquist plot were extrapolated with the help of two circuits as shown in Fig.5 b-c. Randle’s circuit was used for the 3.5% NaCl figure without inhibitor as depicted in Fig. 5b. The circuit includes Rs as the solution resistance, Rct as the charge transfer resistance and CPE as the constant phase element. In presence of inhibitor the circuit could not fit appropriately so,

inductance (L) was added in the circuit to get the best fit parameters as shown in Fig. 5c [30-33]. The CPE and impedance can be related as [34]: ZCPE  Yo 1  i 

n

(10)

where Y0 be the magnitude of CPE, ω be the angular frequency, and n be an exponent to measure heterogeneity [34]. All the obtained parameters after fitting the circuit to the figures are listed in Table 3. The equation below was used to evaluate the double layer capacitance (Cdl) [31]: Cdl  Yo Rct1n 

1/ n

(11)

As can be observed from the Table 3 the values of Cdl reduces and Rct rises with the adding up of inhibitor. This phenomenon is related to the adsorption of inhibitor molecules on the steel surface that form a shield and retards the corrosion reaction [35,36]. In absence of inhibitor the value of n is 0.755 which represents a surface with roughness while, the value of n increases in presence of inhibitor from 0.793 to 0.901 suggesting a better and smooth surface. This can be ascribed to the adsorption of inhibitor on the steel surface that further increases its homogeneity [37, 38]. Fig. 5d shows the Bode and phase angle figures for N80 steel in 3.5% NaCl solution with and without polymer at 303 K. At subordinate frequencies the impedance value is enhanced in presence of polymer while it is seen to decrease in the absence of polymer. This occurrence can be endorsed to the formation of a protective barrier on N80 steel due to adsorption of the polymer that protects the steel from the corrosive media [39]. The values of the phase angle increases towards the more negative side from -32.10o (Blank) to -65.40o (at 400 mg/L) as can be seen in Fig. 5e. This characteristic is seen for all potential inhibitors and it suggests the better inhibitive action of the studies polymer on the N80 steel surface. 3.2.2. Potentiodynamic polarization

The potentiodynamic polarization method was employed to investigate the kinetics of cathodic and anodic region in 3.5% NaCl with carbon dioxide with and without polymer at 303 K. Fig. 6 includes the obtained polarization plots with prominent anodic and cathodic regions. A shift towards the lower current region is observed from the figure for inhibited surface. This could be due to the adsorption of polymer on the N80 steel surface that mitigates the corrosion reaction [40]. The mechanism of corrosion process is not modified as is evident by the similar shape of the polarization plots with and without polymer indicating that the mitigation was achieved by blocking the active centers on the steel surface. All the extracted kinetic parameters from the polarization plots are shown in Table 4. The corrosion current density (icorr) is quite enhanced for 3.5% NaCl solution (102.7 A/cm2), while it is reduced (7.2 A/cm2 at 400 mg/L) in presence of polymer as is indicated in the table. This phenomenon also supports the formation of a protective shield by adsorption of polymer on the steel surface that reduces the corrosion current density. Using the icorr values, inhibition efficiency was calculated:



 %  1  

icorr(i)   100 icorr 

(12)

where, icorr be the current density for 3.5% NaCl and icorr(i) be the corrosion current density for polymer, respectively. As can be noticed from the Table 4 that both the anodic and cathodic Tafel constants (βa, βc) show a slight variation signifying that the mechanism is intact with adding up of inhibitor. Although this phenomenon refers to the mixed mode of inhibitor action but, since the Ecorr shift is more towards the negative direction with the addition of inhibitor it can be categorized as mixed type with predominantly cathodic nature. 3.2.3. Scanning electrochemical microscopy (SECM)

Fig. 7 displays the 3D pictures of AC-SECM with and without different concentration of inhibitor [41, 42]. The platinum probe of the machine recorded higher current for the N80 steel surface without inhibitor (surface acting as a conductor) as shown in Fig. 7a (x and y axis) [43]. This may be due to the direct contact of the probe with the metal surface. While, when the probe was brought close to the N80 steel surface with adsorbed polymer a lower current was observed (surface acting as insulator) as shown in Fig. 7b, 7c, 7d, and 7e. This may be due to the protective film of polymer on the steel surface which blocks the direct contact of the probe with the metal [43]. 3.3. Surface morphology analysis (SEM) The N80 steel surface was exposed to SEM tests and the results are displayed in Fig. 8a-e. The steel surface without polymer was very rough, damaged and with visible cracks (Fig. 8a). This took place due to the attack of aggressive media on the steel directly. The surface of the steel with polymer covering was smooth and less damaged (Fig. 8b-e). This can be ascribed to the good inhibitive effect of the polymer forming a protective shield on the steel surface and not allowing the aggressive solution to break through and attack the steel. 3.4. Density Functional Theory (DFT) Analyses 3.4.1. Global reactivity The optimized neutral and protonated forms of the studied inhibitors along with the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) are represented in Fig. 9. EHOMO signifies the electron donating ability of the inhibitor as per the frontier orbital theory. So, higher the EHOMO values, better is the electron donation to the suitable acceptor sites. On the other hand, ELUMO signifies the electron accepting ability of the inhibitor molecules. This implies that lower the value of ELUMO, more available tendency to accept electrons [44,45]. Table 5 and Table 6 shows the computed parameters for the neutral as well as protonated form of the inhibitor in the aqueous

phase. As can be observed from the table that the EHOMO in case of neutral inhibitor is more as compared to protonated one so, the protonated form has less electron donation tendency than the neutral form. The ELUMO values for protonated inhibitor are more positive than the neutral inhibitor therefore, neutral form has lower ability to accept electron than the protonated form. Energy gap (∆E) is one of the important parameter to define the adsorption tendency of the inhibitor molecules on the steel surface. Now, as can be seen that the value of ∆E is lower in case of neutral form of inhibitor that suggests the greater ability to adsorb on the steel surface in the neutral form. The ability of the inhibitor to adsorb increases with decrease in the energy gap. According to Fig. 9, the distribution of HOMO and LUMO regions in neutral and protonated inhibitor forms are opposite to each other in other words the neutral HOMO regions are almost similar to protonated LUMO and neutral LUMO regions are almost similar to protonated HOMO. In both neutral and protonated inhibitor forms HOMO and LUMO are dispersed on the NH 2 and phenyl groups. 3.5. MD simulations and Radial distribution function Despite many studies on CO2 corrosion have been conducted, a slight exploration has been done to the computational simulation of the corrosion inhibition process. The inhibitor-steel interactions are believed to be affected at the molecular levels. So, the molecular dynamic simulations have been computed on neutral and protonated forms of the inhibitor molecule in presence of a simulated electrolyte aiming to mimic the experimental conditions as much as possible. Herein, an important difference that sets our model more suitable is the inclusion of carbonate into the simulated solution which makes the estimation of the interactions between tested inhibitor and steel surface more informative. All the computations were run till the structures reached an equilibrium, then, the interaction energies were estimated by calculating the single point energies of all system constituents

[46]. The obtained equilibrium configurations of neutral and protonated forms of inhibitor molecule on the Fe (110) surface in solvent are shown in Fig. 10. As can be seen from the results in Fig. 10 that both forms of the polymer adsorbed on the surface of Fe (110) with a parallel mode. Interestingly, the whole inhibitor molecule adsorbed on the steel surface, indicating that there are more adsorption centers. It is believed that this parallel disposition can enhance the inhibitive effects of the said compound, this is because parallel adsorption can ensure more surface coverage, thus prevent steel surface from corrosion. The binding (interaction) energies of inhibitor achieved under equilibrium state for neutral and protonated forms are 675.760 KJ/mol (-675.760 KJ/mol) and 601.088KJ/mol (-601.088 Kj/mol) respectively. The results show that the collaboration between inhibitor molecule and steel is robust and steady in neutral form than that in protonated form [47]. This can be endorsed to the strong solvation of polymer in the water solvent which opposes adsorption [48]. The adsorption takes place through more than one centers as is evident from the magnitude of the binding energies for the tested inhibitor molecule [48, 49]. To allow for further insights into the extent of adsorption in both inhibitor forms, we computed a RDF as a significant means for structural depiction. Here, the total radial dissemination function was evaluated for both forms using MD computations. Whether the interactions of an inhibitor with iron atoms are meaningful can be judged by comparison of the first prominent peaks in the RDF curves. The peak of 1 Å ~ 3.5 Å indicates a smaller bond length that is correlated to chemisorption, whereas, peaks longer than 3.5 Å indicates the physical interactions [50]. The RDFs results are graphically represented in the Fig. 11, from which we can easily observe that the first prominent peak for both forms are observed, situated at distances less than 3.5 Å. These results are important and an interesting point that emerges from this data is that both inhibitor forms seem to be effective in inhibition of steel corrosion.

4. Mechanism of polymer interaction The mechanism of adsorption of polymer onto the steel surface consists of three adsorption modes in presence of carbonic acid. First mode of adsorption includes chemical bond formation in case of neutral form of inhibitor. This adsorption consists of sharing of extra pair of electrons present on the O, N, to the empty d- orbital of iron and also donor-acceptor interactions between π- electrons of benzene ring and vacant d-orbitals of iron. The second mode suggests that, in acidic medium steel surface bears positive charge [51,52], thus, it becomes difficult for protonated inhibitor forms to adsorb on the steel surface because of the electrostatic repulsion. However, the degree of hydration of chloride ions are small, so they can easily convey additional negative charges towards the interface and favor more adsorption of the positively charged molecules through electrostatic exchanges between the positively charged molecules and negatively charged steel surface. The third mode includes back-donation of electrons from occupied iron orbitals to the vacant anti-bonding orbital’s present in the inhibitor molecules. All the three adsorption modes are shown in Fig. 12.

5. Conclusions (1) The developed polymer derivative considerably enhanced the corrosion mitigation effectiveness of N80 steel in 3.5% NaCl saturated with carbon dioxide. (2) Potentiodynamic polarization measurement established that ADPD molecules successfully reduce corrosion reaction and characterized into mixed type category with dominantly cathodic. (3) The adsorption of inhibitor followed Langmuir isotherm prototype. (4) SEM and SECM pictures suggests the adsorption of polymer onto the steel surface. (5) DFT computation reveals that neutral inhibitor has more electron donation ability than the protonated form.

(6) MD outcomes suggests that neutral form of inhibitor has more binding energy than protonated form. Declaration of interest None. Acknowledgements Dr. Ambrish Singh is grateful for the Sichuan 1000 Talent Fund and financial assistance provided by the open fund of southwest petroleum university (No. X151518KCL30). Authors are also thankful to Dr. K. R. Ansari (KFUPM, Saudi Arabia) for manuscript rechecking and, Dr. Hassane Lgaz (Konkuk University, South Korea) for MD simulations. Author Contributions The concept and design was prepared by Dr. Ambrish Singh. The experiments were carried out by Dr. Ashish Kumar, and Neetesh Soni. The characterizations were done by Yu Deyuan. The rewriting of DFT computations was done by Dr. Ashish Kumar and Dr. Ambrish Singh. Further, Dr. Ambrish Singh interpreted and prepared the final draft of the paper. References [1] B. Wang, M. Du, J. Zhang, C.J. Gao, Electrochemical and surface analysis studies on corrosion inhibition of Q235 steel by imidazolines derivative against CO2 corrosion, Corros. Sci. 53 (2011) 353361. [2] A. Singh, K. R. Ansari, X. Xu, Z. Sun, A. Kumar, Y. Lin, An impending inhibitor useful for the oil and gas production industry: Weight loss, electrochemical, surface and quantum chemical calculation, Sci Rep. 2017 Nov 2;7(1):14904. doi: 10.1038/s41598-017-13877-0

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Figure captions Figure 1:

Synthetic scheme of inhibitor

Figure 2a:

Variation of weight loss with inhibitor concentration in different temperature

2b:

Variation of corrosion rate with inhibitor concentration at different temperature

2c:

Variation of inhibition efficiency (η %) with inhibitor concentration at different temperature inhibitor

2d:

Variation of inhibition efficiency (η %) with solution temperature at different concentration of inhibitor

Figure 3a:

Arrhenius plot for N80 steel corrosion in the absence and presence of different concentrations of inhibitor

Figure 3b:

Transition state plot for N80 steel corrosion in the absence and presence of different concentrations of inhibitor

Figure 4:

Langmuir’s isotherm plot for adsorption of inhibitor

Figure 5a:

Nyquist plots for N80 steel in absence and presence of different concentration of inhibitor at 303 K

5b,c: Equivalent circuit model used to fit the EIS data 5d:

Bode (log f vs. log |Z|) and phase angle (log f vs. α◦) plots of impedance plots for N80 steel in absence and presence of different concentration of inhibitor at 303 K

Figure 6:

Potentiodynamic polarization curves for N80 steel absence and presence of different concentration of inhibitor at 303 K

Figure 7a-d: SECM micrographs of N80 steel surfaces (a) blank 3.5% NaCl (b) at 50 mg/L (c) at 100 mg/L (d) at 200 mg/L (e) at 400 mg/L

Figure 8a-d: SEM micrographs of N80 steel surfaces (a) blank 3.5% NaCl (b) at 50 mg/L (c) at 100 mg/L (d) at 200 mg/L (e) at 400 mg/L Figure 9a, b: Optimized geometries (a) Neutral (b) Protonated Figure 9c, d: Frontier molecular orbital’s of neutral inhibitor (c) HOMO (d) LUMO Figure 9e, f: Frontier molecular orbital’s of protonated (e) HOMO (f) LUMO Figure 10a:

Side and top views of the adsorption of the neutral inhibitors on the Fe (110) surface in solution

Figure 10b:

Side and top views of the adsorption of the protonated inhibitors on the Fe (110) surface in solution

Figure 11:

RDFs of neutral and protonated forms of tested corrosion inhibitors adsorbed on the Fe (110) surface in solution

Figure 12:

Mechanism of corrosion inhibition

Graphical Abstract A combined electrochemical and theoretical analysis of environmentally benign polymer for corrosion protection of N80 steel in sweet corrosive environment Ambrish Singh1,2*, Neetesh Soni1, Yu Deyuan1, Ashish Kumar3 1

School of Materials Science and Engineering, Southwest Petroleum University, Chengdu-610500, Sichuan, China. 2 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, China. 3 School of Mechanical and Civil Engineering, Lovely Faculty of Engineering and Technology, Lovely Professional University, Phagwara, Punjab, India. *Corresponding author E-mail: [email protected]; [email protected] Ph.No.: +86-18384155035

Abstract The corrosion inhibition efficiency of aniline, formaldehyde and piperazine based polymer (ADPD) on N80 steel in 3.5% NaCl solution saturated with carbon dioxide was investigated using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, weight loss, scanning electrochemical microscopy (SECM), scanning electron microscopy (SEM) measurements, density functional theory (DFT) and molecular dynamics simulation (MD). The adsorption of polymer onto N80 steel surface followed Langmuir adsorption isotherm model. Potentiodynamic polarization study confirmed that inhibitor is mixed type with cathodic predominance. SECM study reveals the current values decreases with the increasing concentration of polymer. SEM study supports the smooth metal surface texture. DFT and MD calculations are in agreement with the experimental findings.

Fig. 1

(b) η (%)

CR (mmy-1)

(a)

C. (mg/L)

Fig. 2

(a)

C. (mg/L)

log CR (mmy-1)

log CR/T (mmy-1)

(b)

[(1/T). 103] / K-1

[(1/T). 103] / K-1

[(C/θ)] (mg/L)

Fig. 3

C. (mg/L) Fig. 4

-Z im (Ω cm2)

(a)

Z re (Ω cm2)

(b)

(c)

-log |Z| (Ω cm2)

(d)

log f (Hz)

-α°

(e)

log f (Hz)

log i (A/cm2)

Fig. 5

E (mV vs. SCE)

Fig. 6

(a)

(b)

(c)

(d)

(e)

Fig. 7

(a)

(c)

(b)

(d)

(e)

Fig. 8

(a)

(b)

(c)

(e)

Fig. 9

(d)

(f)

Fig. 10

2.0 Fe-Neutral

g(r)

1.5

Fe-Protonated

2.9

3.3

1.0

0.5

0.0

0

2

4

6 X

Fig. 11

(Angstrom)

8

10

12

Fig. 12

Table 1 Activation parameters obtained from weight loss measurements for APDP on N80 steel in the 303-353 temperature range

Inhibitor

H*

Concentration Ea

(mg/L)

S*

(kJ mol-1)

(kJ mol-1)

(kJ mol-1)

Blank

0

53.94

49.01

-52.08

APDP

50

89.25

84.76

51.90

100

97.73

93.25

76.60

200

109.08

104.60

109.54

400

121.55

117.09

145.92

Table 2 Thermodynamic parameters obtained from weight loss measurements for APDP on N80 steel in the 303-353 temperature range

Inhibitor

Kads

G ads

(L/g)

(kJ mol-1)

303

55.34

-27.51

313

52.49

-28.28

Temperature

(K)

APDP

333

16.45

-26.88

353

6.11

-25.59

Table 3 Electrochemical impedance parameters for N80 steel in absence and presence different concentration of APDP.

n

-

%

-

32.1

-

47.5

67

43.3

75

0.659

44.5

23

58.8

88

101

0.734

39.5

79

61.5

94

54

0.788

33.1

52

65.4

96

Solution

Rs

Rct

Y°

(mg/L)

( cm2)

( cm2)

Blank

9.6

128

0.755

298

0.496

50.2

APDP 50

8.5

507

0.793

238

0.622

APDP 100

9.2

1038

0.798

163

APDP 200

9.2

2167

0.844

APDP 400

7.9

2954

0.901

(Ω−1sn/cm2)

-S

Cdl

(F cm2)

L (H cm2)

Table 4 Polarization parameters for N80 steel in the absence and presence of different concentrations APDP.

Conc.(mg/L)

Tafel data Ecorr (V vs. SCE)

Icorr (μA cm-2)

ba -bc  -1 -1 (V d ) (V d ) (%)

Blank

-0.684

102.7

147

104

-

APDP 50

-0.710

36.3

114

55

65

100

-0.723

17.1

99

52

83

200

-0.741

12.8

102

62

88

400

-0.721

7.2

95

42

93

Table 5 Calculated quantum chemical parameters of ADPD Inhibitors

ADPD

ADPD+ -4.875

EHOMO

ELUMO

ΔE

(eV)

(eV)

(eV)

-4.787

0.0527

4.839

0.779

5.654

52

Research Highlights     

Aniline based polymer as effective corrosion inhibitor for N80 steel. Electrochemical measurements using potentiodynamic polarization and EIS. SECM was used to study the localized electrochemical behavior. SEM was used to observe the surface morphologies. DFT and molecular simulations supported the experimental results.

53