Experimental, adsorption, quantum chemical and molecular dynamics simulation studies on the corrosion inhibition performance of Vincamine on J55 steel in acidic medium

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Experimental, adsorption, quantum chemical and molecular dynamics simulation studies on the corrosion inhibition performance of Vincamine on J55 steel in acidic medium Nkem B. Iroha , N.A. Madueke , V. Mkpenie , B.T. Ogunyemi , Lebe A. Nnanna , Sangeeta Singh , Ekemini D. Akpan , Eno E. Ebenso PII: DOI: Reference:

S0022-2860(20)31848-2 https://doi.org/10.1016/j.molstruc.2020.129533 MOLSTR 129533

To appear in:

Journal of Molecular Structure

Received date: Revised date: Accepted date:

31 August 2020 28 September 2020 27 October 2020

Please cite this article as: Nkem B. Iroha , N.A. Madueke , V. Mkpenie , B.T. Ogunyemi , Lebe A. Nnanna , Sangeeta Singh , Ekemini D. Akpan , Eno E. Ebenso , Experimental, adsorption, quantum chemical and molecular dynamics simulation studies on the corrosion inhibition performance of Vincamine on J55 steel in acidic medium, Journal of Molecular Structure (2020), doi: https://doi.org/10.1016/j.molstruc.2020.129533

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HIGHLIGHTS 

The effect of Vincamine (VCA) as inhibitor for the corrosion of J55 steel in 1 M HCl was examined utilizing gravimetric and electrochemical methods.



Surface morphology was studied using fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques.



Polarization results indicated that VCA exhibited mixed-type corrosion inhibitor characteristics.



The adsorption of VCA on the surface of J55 steel followed the Langmuir isotherm.



Quantum chemical studies and molecular dynamics (MD) simulation were used to relate the restraint capability of VCA with its electronic structural parameters.

Experimental, adsorption, quantum chemical and molecular dynamics simulation studies on the corrosion inhibition performance of Vincamine on J55 steel in acidic medium Nkem B. Iroha1*, N. A. Madueke1, V. Mkpenie2, B. T. Ogunyemi1, Lebe A. Nnanna3, Sangeeta Singh4, Ekemini D. Akpan4, and Eno E. Ebenso4,5 ,* 1

Electrochemistry and Material Science Unit, Department of Chemistry, Federal University Otuoke, Bayelsa State, Nigeria 2 Inorganic and Computational Chemistry Group, Department of Chemistry, University of Uyo, Uyo, Nigeria 3 Department of Physics, Michael Okpara University of Agriculture Umudike, Abia State, Nigeria 4 Materials Science Innovation & Modelling Research Focus Area, Faculty of Natural and Agricultural Sciences, North West University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa 5 Nanotechnology and Water Sustainability Research Unit, College of Science, Engineering and Technology, University of South Africa, Johannesburg, South Africa *Corresponding authors: Emails: [email protected] (NBI); [email protected] (EEE)

ABSTRACT The effect of Vincamine (VCA) as an inhibitor for the corrosion of J55 steel in 1 M HCl was examined utilizing gravimetric and electrochemical methods. Surface morphology was studied using Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. The outcome showed that VCA hindered the dissolution of J55 steel in the acidic medium. From the gravimetric method, results showed that inhibition efficacy depended on the concentration of VCA and the temperature of the medium. Electrochemical impedance spectroscopy (EIS) studies revealed a rise in resistance of charge transfer, with a corresponding decline in the capacitance of double layer when there is a rise in the concentration of VCA. Polarization results indicated that VCA exhibited mixed-type corrosion inhibitor characteristics. The range of inhibition efficiencies obtained is; 79.4 to 90.2% for EIS studies, 76.0 to 91.4% for weight loss and 77.4 to 88.6% for polarization studies at 30 C. Polarization. SEM, FTIR, and AFM investigations established the adsorption of VCA on the surface of J55, thereby reducing the deterioration reaction. The adsorption of VCA on the surface of J55 steel followed the Freundlich adsorption isotherm. The adsorption characteristics were theoretically evaluated by quantum chemical studies and molecular dynamics (MD) simulation to relate the restraint capability of VCA with its electronic structural parameters. Keynotes: Vincamine, Corrosion Inhibitor, J55 steel, Charge transfer resistance, Freundlich isotherm.

2

1. Introduction The degradation of facilities by corrosion is a major problem in automobiles, construction companies, chemical industries, shipbuilding, oil and gas, and other engineering firms. Over the years, prevention and control of metallic corrosion is attracting the attention of researchers. Amongst the various methods available for preventing and minimizing metal corrosion, the use of inhibitors stands out amongst other methods, mostly in acidic systems [1]. A larger part of the corrosion inhibitors are organic molecules, which in their structures contains atoms of nitrogen, sulphur, oxygen, as well as benzene rings or conjugated double bonds, which are the most important adsorption centres [2-6]. The success of an organic molecule as an inhibitor for any metal corrosion is principally dependent upon the capability to get adsorbed onto metal surfaces. Absorbed organic compounds block active sites and create impediment between the metallic surface and the aggressive medium by shaping a deterrent layer on the metallic surface, thereby reducing the metal dissolution. In literature, several synthesized organic compounds and natural products have been explored as corrosion inhibitors of metal dissolution and reported as being successful inhibitors of metal corrosion [7–12]. The functional groups found in organic compounds bestows corrosion inhibition capabilities [13]. Vincamine (VCA) is a nootropic monoterpenoid indole alkaloid seen in lesser periwinkle (Vinca minor) leaves, comprising by weight, 25-65% of its indole alkaloids. It has been reported to have certain abilities as an antihypertensive agent, a vasodilator agent, and a metabolite, and can be synthesized from related alkaloids [14]. The choice of studying the corrosion inhibition property of VCA was based on its size, functionalities, and geometry of this molecule. Supposedly, this will be the first account on the utilization of VCA as an inhibitor of J55 steel corrosion in HCl media. It is anticipated that the geometry of the molecule, as well as the present of functional groups will aid adsorption on the surface of J55 steel, thereby inhibiting attack from corrosive environment. This present work is aim at investigating the potential of VCA in inhibiting the corrosion of J55 steel, commonly used in building well tubing, in 1 M of HCl by weight loss analysis and electrochemical techniques. Atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), and energy-dispersive X-rays spectroscopy (EDS) were employed as post-exposure analyses. Density function theory (DFT) calculations and molecular dynamics recreation were used to theoretically elucidate the impact of molecular and electronic properties of VCA over J55 steel surface and to ascertain the most active metal/inhibitor contact sites. The structure of the VCA used in this study is represented in Fig. 1.

3

(a)

(b)

Fig. 1 Structure of (a) VCA and (b) optimized geometric structure; C = grey, O = red, H = white and N = blue.

2. Materials and Method 2.1. Preparation of material The J55 steel with percentage composition by weight: C, 0.24; Mn, 1.1; Si, 0.22; P, 0.103; Ni, 0.28; S, 0.004; Mo, 0.019; Cu, 0.5 and the remaining being Fe was obtained from Qingdao Tengxiang Ltd. in China. Each of the steel sheets with a thickness of 5 mm, was cut into coupons of dimension 30 mm × 20 mm for electrochemical measurements, 40 mm × 40 mm for the gravimetric experiment, and 30 mm × 20 mm for surface analysis examination. The coupons surfaces were polished with a series of emery paper (220-1200 mesh). The polished coupons were cleaned in acetone, ethanol, and air-dried before used. The blank corrodent was a solution of 1 M HCl. The VCA powder from Meyers Co. Ltd. was used as provided without purification and concentrations of 0.001 M to 0.010 M were prepared. 2.2. Gravimetric measurements The previously cleaned and accurately weighed steel samples with tiny holes on the upper edges were submerged in 1 M HCl solution (100 mL) using hangers, with diverse concentrations of VCA and also without VCA, at 30 C. All the test arrangements were kept in the open-air. Exactly five hours after immersion, the J55 steel specimens were retrieved from the respective test solutions, rinsed in running water, then in ethanol, dried in the air, and the weight accurately retaken to calculate the weight loss [15]. The change in masses of the samples before and after dipping in the corrosive media was deduced as the change in weight. For good reproducibility, each test was run thrice and for different temperatures (40, 50, and 60 °C), respectively. The average value of weight losses from the triplicate tests was reported. 2.3. Electrochemical techniques Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) which are the two main electrochemical techniques utilized in corrosion inhibition study were implored to study the inhibition behavior of VCA. The electrochemical measurements were

4

conducted in a CHI 660A electrochemical work station. The conventional cell made of three electrodes was used, which consists of J55 steel as the working electrode, saturated calomel electrode (SCE) as the reference electrode, and a platinum foil as the counter electrode (CE). The samples of J55 steel were first immersed in a blank solution for 1800 s at room temperature, to set up an invariable open circuit potential (OCP). The PDP experiments were accomplished in the potential range of −0.15 V to +0.15 V with respect to corrosion potential at a scan rate of 0.2 mV/s. An AC signal (0.01 V amplitude) at the predetermined OCP and within a spectrum range of 100 kHz - 10 MHz was used for the EIS experiments. Each experiment was performed thrice to check reproducibility. 2.4. Scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS) SEM-EDS examination of J55 steel specimens immersed without, and in the presence of the maximum studied concentration of VCA powder at 30 C was studied using AMETEX S4800 EDAX TSL analyzer operated at 5 kV. The J55 steel specimens prepared as reported in Section 2.1 were dipped for five hours in 1 M HCl medium and in a solution containing 0.010 M VCA. The coupons were withdrawn from the two solutions, cleaned with deionized water and acetone, dried, and then subjected to SEM-EDS examination. 2.5 Fourier-transform infrared (FTIR) Analysis A TENSOR II FTIR spectrophotometer (Brucker International) was used for all FTIR recording. The instrument recorded the spectra of VCA powder and that of the adsorbed surface films scrapped from the J55 steel coupons surface, which was retrieved from 1 M HCl containing 0.010 M VCA. 2. 6. Atomic Force Microscopy (AFM) The Veeco diInnova model of atomic force microscope was utilized for AFM observation of the specimens’ surfaces. J55 steel samples were submerged in the various assay solutions for 24 h without and in the presence of 0.010 M VCA at 30 °C. The coupons were removed, cleaned with deionized water, air-dried, and subjected to AFM examination. 2.7 Calculation of quantum chemical parameters Gaussian 09 program package [16,17] was employed in optimizing the VCA molecule in aqueous medium, with basis set and method previously reported [18,19]. The integral equation formalism variant of the polarizable continuum model (IEF-PCM) [19,20] was employed as solvation method. IEF-PCM is prominent among other PCM methods for corrosion inhibition studies [21]. The geometry converged without any imaginary frequency showing that a minimum has been reached (RMS force = 0.2 x 10-5). The values of the energies of LUMO (ELUMO) and HOMO (EHOMO) obtained from the calculation were used in calculating some quantum chemical parameters such as electronegativity (χ), ionization potential (IE), band gap (ΔE), global hardness (η), global softness (σ), and electron affinity (EA) as shown in Equations 1-6 [17,22]:

5

(1) (2) (3) (4) (5) (6) The number of quantity of electrons transported ΔN from VCA to the surface of Fe was determined as shown in Equation 7: (7) where ϕ represents Fe(110) surface work function with a value of 4.82 [23], ηinh and χinh represent the global hardness and electronegativity of VCA, respectively. 2.8 Determination of Fukui indices and Mulliken atomic charges These parameters were determined with Dmol3 program [24] of Materials Studio version 2017, using nonlocal B3LYP functional at DFT level of theory. Geometry optimization was performed with a Conductor-like Screening Model (COSMO) [25,26] in the aqueous phase to account for the solvent effects in the calculation. To obtain higher accuracy in the estimation, the DNP (double numeric basis set with polarization) [27,28] was utilized at the level of 4.4. The following criteria were used to judge the geometry convergence: energy 1 x 10 -5 Ha, force 2 x 10-3 Ha/Å and displacement 5 x 10-3 Å. There was no observed made-up frequency in the vibrational evaluation [23]. 2.9 Determination of protonated site Forcite geometry optimization was performed using COMPASS force field [29] and smart algorithm on a protonated structure obtained by the addition of a proton to each heteroatom, respectively. The geometry convergence was based on the following criteria: energy 1.0 x 10-4 kcal/mol, force 5.0 x 10-3 kcal/mol/Å and displacement 5.0 x 10-5 Å. The possible protonation sites are: O(20), O(22), O(24), N(8), and N(13). The protonated structure obtained after geometry optimization having the minimum total electronic energy was assumed to be the protonated VCA. 2.10 Molecular dynamics (MD) simulation The experimental procedures for the MD simulation studies are as previously reported [17, 29]. An inhibitor cell having similar lattice parameters as that of Fe supercell (24.758 Å) was built. This cell contained the inhibitor (VCA) in 1 M HCl according to the composition: 1 VCA, 5 H3O+, 5 Cl- and 533 H2O. The interaction energy (Einteraction) between the Fe surface and VCA was calculated according to Equation 8 [17, 28]:

6

(8) Here, Etotal is the total energy of the system containing Fe surface, inhibitor (VCA), and acid solution, EFe+solu is the total energy of Fe surface and acid solution with no inhibitor, E Inh is the energy of inhibitor and Ei(si) is intermolecular interaction energy between the inhibitor and acid solution. Ei(si) was determined, as reported in the literature [25]. 3. Results and Discussion 3.1. Gravimetric measurements Weight loss studies were conducted using J55 steel samples submerged in 1 M HCl solutions without and in the presence of various concentrations of VCA at 30 – 60 C. The corrosion rate of J55 steel, coverage (θ), and inhibition efficacy (IEWL) were computed from the changes in weight information utilizing Equations (9, 10, and 11), respectively. (9) (10) (11) where Δw represents a difference in weight (g), A is the surface area of the steel (m2), t is the time of exposure (h), CRu, and CRi are the degrees of corrosion (g m-2 h-1) in uninhibited and inhibited solutions, respectively. The plots of corrosion rate and inhibitor efficacy against inhibitor concentration at the temperatures range of 30 – 60 C are depicted in Figs. 2a and 2b, respectively. It is evident from the graphs that CR of J55 steel in the acidic solutions decreased with increasing concentration of VCA (Fig. 2a) while inhibition efficiency increased as inhibitor concentration increases (Fig. 2b). This is a clear indication that, the VCA inhibitor retarded the deterioration of J55 steel in the acid medium in a concentration dependant manner. The diminishing rates of corrosion and the increase in protection efficacy with a rise in inhibitor concentration could be attributed to the adsorption of the molecules of VCA on the surface of J55 steel, ensuing in shielding the reactive sites of the exposed specimen in HCl solution [30]. It is noted that CR augmented with rising temperatures both in uninhibited solution and in the solution containing VCA. This indicates that the steel is susceptible to quicker deterioration with an increase in thermal agitation of the corrosive medium. Alhaffar and co-workers, and other researchers have reported similar observations [30-32]. It is likewise detected that inhibition efficacy decreased with temperature rise. The obtained diminishing inhibition efficacy as temperature rises is typical of a physisorption type of adsorption [33].

7

5

30 40 50 60

(a)

CR (g m-2 h-1)

4

C C C C

3

2

1

0 0.000

0.002

0.004

0.006

0.008

0.010

Inhibitor Concentration (M) 95

(b)

Inhibition efficacy, IEWL (%)

90

85

80

75

30 40 50 60

70

65 0.000

0.002

0.004

0.006

0.008

o

C C o C o C o

0.010

Inhibitor Concentration (M)

Fig. 2 Plot of (a) degree of corrosion, (b) inhibition efficacy (IEWL) versus inhibitor concentration for J55 steel containing and without diverse concentrations of VCA in 1 M HCl at various temperatures from weight loss measurements. 3.2. Potentiodynamic polarization (PDP) studies Potentiodynamic polarization (PDP) experiments gives insight into the kinetics of anodic dissolution, as well as the reaction involving the evolution of hydrogen at the cathode [30]. The results of PDP experiments with J55 steel in the corrosive solution containing and without studied concentrations of VCA at 30 C are represented in Fig. 3. Electrochemical factors, which includes corrosion potential (Ecorr), cathodic and anodic Tafel slopes (βc and βa), and current density (Icorr) are presented in Table 1.

8

Table 1: Electrochemical factors and inhibition efficacy deduced from polarization curves at 30 C for J55 steel in 1 M HCl with and without VCA. Test Solution Blank 0.001 M 0.005 M 0.010 M

Ecorr (V) -0.479 -0.490 -0.498 -0.514

βa 107.3 113.6 121.5 142.9

Icorr (mA) 773.8 174.7 137.6 88.1

βc 88.2 79.4 82.9 86.1

%IEPDP 77.4 82.2 88.6

-1 Blank 0.001 M 0.005 M 0.010 M

-2

-2

log /I/ (Acm )

-3 -4 -5 -6 -7 -0.7

-0.6

-0.5

-0.4

-0.3

E (V vs. SCE)

Fig. 3 Potentiodynamic polarization plots of J55 steel in 1 M HCl without and containing selected concentrations of VCA at 30 C. The data disclosed that Icorr diminished when VCA powder was added, which becomes more noticeable as the concentration of VCA increases. This shows that in the presence of VCA, there is a huge drop in the corrosion rate of the J55 steel, indicating the corrosion inhibiting ability of VCA. The corrosion potential (Ecorr) values show a slight shift towards a more positive potential upon addition of the studied inhibitor. Literature reports suggest that when the maximum displacement in Ecorr becomes larger than 85 mV; at that point, the inhibitor is anodic or cathodic and if under 85 mV, it is considered as a blended type inhibitor [34, 35]. In the presence of VCA powder, the calculated maximum displacement in Ecorr is 35 mV, which establishes that VCA behaved as a mixed-type corrosion inhibitor. Considering the data presented in Table 1, it is also observed that values of βc and βa varied in contrast with the uninhibited system, signifying that the inhibitor concurrently altered the anodic and cathodic reactions, thus strengthening the statement that VCA is a mixed-type inhibitor. Corrosion inhibition efficacy (IEPDP) was deduced from the values of Icorr using Equation 12:

9

(

)

(12)

where Icorr(u) and Icorr(i) depicts corrosion current densities without and in the existence of VCA, respectively. The values of inhibition efficacy from the PDP studies calculated utilizing Equation 11 are recorded in Table 1. The %IEPDP increases with augmenting the concentration of VCA with the optimal value of 88.6% obtained for the optimum inhibitor concentration. 3.3. Electrochemical impedance spectroscopy (EIS) studies The deterioration of J55 steel in 1 M HCl in the existence of VCA was examined by EIS studies at 30 °C. Nyquist plots without VCA and in the presence of VCA are presented in Fig. 4. It is ascertained that the plots displayed a single, capacitive-like, and depressed semi-circular loop over the entire frequency range and the distance across of the semicircle augments with increment in VCA concentration. This observations suggested that, the J55 steel corrosion was ordered by a process of charge transfer occurring between molecules of the VCA and the metal surface, which acts on the variant of the double-layer capacitance [36, 37]. The imperfect semiround circle quality of the capacitive loops, which demonstrates the frequency distribution of impedance data, could be due to inhomogeneities and rough texture of the J55 steel surface [38, 39]. The comparable circuit used to model the impedance results after fitting with Zsimpwin software is inserted in Fig. 4. The circuit contains constant phase element (CPE) used in place of the double-layer capacitance (Cdl), the resistance of charge transfer (Rct) and solution resistance (Rs). The admittance, YCPE, and impedance, ZCPE, are expressed as in Equations (13) and (14), respectively [40]: (13) (14) where, Y0 is the CPE proportional factor, j is the square root of -1, is the angular -1 frequency in rad s ( = 2πfmax), n is the phase shift, which is used to estimate the roughness or heterogeneity of the steel surface. The double-layer capacitance (Cdl) data were determined via Equation (15): (15)

( ( ⁄ ))

The inhibition efficacy (IEEIS) from EIS values was deduced by contrasting the data of charge transfer resistances without VCA and in the presence of VCA as follows: (

)

(16)

10

where ct(i) and ct(u) are the resistances of charge transfer in the presence and without VCA, respectively. The values of electrochemical parameters are recorded in Table 2. From Table 2, it is evident that the value of Rct increases as the concentration of the VCA increases while Cdl value diminishes as inhibitor concentration augments. The increase in Rct values could be as a consequence of the development of deterrent film on the metal/solution interface [41, 42]. The decline in Cdl is probably due to the electronic double layer increase [43]. These observations illustrates that in the presence of VCA, the rough texture of J55 steel surface decreases because of the adsorption of VCA on the surface of the steel. The values of the inhibition proficiency of VCA obtained from the EIS technique are similar to those obtained from potentiodynamic polarization measurements. 800 Blank 0.001 M 0.005 M 0.010 M

-Z¢¢ (W cm2)

600

400

200

0 0

200

400 600 800 2 Z¢ (W cm ) Fig. 4 Impedance plots for J55 metal in 1 M HCl in the absence and with selected concentrations of VCA at 30 °C.

Table 2 EIS parameters for J55 steel in 1 M HCl without and with selected concentrations of VCA at 30 °C. Test solution (x 10-4) ( ) Blank 0.72 0.84 66.99 704.27 0.06 0.001 M 0.77 0.85 325.78 561.30 0.11 79.4 0.005 M 0.96 0.87 550.02 332.51 0.09 87.8 0.010 M 1.17 0.89 688.85 68.23 0.32 90.2

11

3.4. SEM and EDS analysis Examination of the J55 steel specimens’ surfaces was carried out using scanning electron microscope (SEM). The coupons were submerged in 1 M HCl for five hours at 30 ºC without and in the presence of 0.010 M concentration of VCA. The specimens were then retrieved from the solution, dried, and kept in a desiccator before the surface morphological studies with SEM. The SEM images are shown in Fig. 5 whereas EDS microchemical analysis of the corroded surface film is given in Fig. 6. The SEM pictures indicate that the steel samples retrieved from the inhibitor solution were better protected, having a surface relatively free from roughness while the surface of J55 steel retrieved from acid solution without VCA appears rough with more grains. This demonstrates that VCA molecule deterred the deterioration of J55 steel by creating a deterrent film on the surface in a manner that diminishes the corrosion rate.

Fig. 5 SEM micrograph of J55 steel immersed in 1 M HCl (a) blank solution (b) containing 0.010 M of VCA. The EDS study was conducted to ascertain the existence of elements on the J55 steel surface without and in the presence of VCA inhibitor. The result of the EDS spectra confirmed the presence of heteroatoms of nitrogen and oxygen, which depicts adsorption. The elements found on J55 steel surface without VCA and 0.010 M VCA are listed according to their percentages in Table 3. In the absence of VCA, the spectrum and the corresponding elemental composition shows a high percentage of Fe and low traces of O, C, N, and Cl. This could be due to oxides formation, some J55 steel composites and the electrolyte. In the presence VCA, elemental analysis shows more traces of O, C, and N. This suggests that O, C, and N atoms contained in VCA powder get adsorbed on the steel surface, hence its capability to shield J55 steel from corroding.

12

Fig. 6 EDS spectra of corroded J55 metal surface immersed in (a) corrosive solution and (b) 0.010 M VCA powder in 1 M HCl. Table 3 Elemental composition of J55 steel surface obtained from EDS spectra in blank and 0.010 M VCA at 30 ºC. 1 M HCl Blank solution

0.010 M VCA powder

Element

Weight %

Atomic %

Element

Weight %

Atomic %

C

2.52

10.68

C

2.90

11.83

N

0.04

0.14

N

0.12

0.27

O

0.16

0.49

O

1.35

4.13

Cl

0.11

0.16

Cl

0.00

0.00

Fe

97.17

88.52

Fe

95.63

83.76

Total:

100.00

100.00

Total:

100.00

100.00

3.5. FTIR Analysis This study was undertaken to identify the various functional groups existing in the VCA molecule and also, to establish the type of bond formation that takes place between the VCA molecule and the J55 steel surface. The FTIR band of VCA molecule and that of the film adsorbed on J55 steel specimen are presented in Fig. 7, to make some comparison. In the spectra of pure VCA powder, the peak appeared at 3301 cm-1, which corresponds to the hydroxyl group, H-bonded OH stretch; 3473 cm-1 corresponds to heterocyclic amine, NH stretch; 3602 cm-1 matches the non-bonded hydroxyl group, OH stretch, and 1712 cm-1 depicts the presence of C=O group. The peaks found in the spectrum of the adsorbed film are similar to those in VCA powder spectrum, which indicates that the adsorbed film contains VCA. Inspection of the peaks displays the absence or deviations of some of the peaks. This explains the possible adsorption of VCA on the surface of J55 steel. 13

Transmittance (a.u.)

60

40

20

Vincamine powder Adsorbed film 0

1000

2000

3000

4000 -1

Wavenumber (cm )

Fig. 7 FTIR spectra of VCA molecule and adsorbed film on J55 steel specimen submerged in 1 M HCl solution having 0.010 M VCA. 3.6. Atomic Force Microscope (AFM) Analysis Atomic force microscope is a vital tool in characterizing the surface of the metal specimen. The analysis was undertaken to look at the surface morphology resulting from the influence of VCA on the corrosion progress on the J55 steel. The three-dimensional (3D) AFM images of J55 steel without and in the presence of 0.010 M VCA in 1 M HCl are depicted in Figs. 8a and b. Close examination of the image in Fig. 8a reveals a rough surface because of the interaction of the acid with the steel surface without the inhibitor, and the average roughness value is around 9.9 μm. In the existence of VCA (Fig. 8b), the average roughness value changed to 6.2 μm with an evener surface. This inhibitive effect is assumed to be the result of the adsorption of VCA molecules onto the steel surface. The result obtained agrees with SEM results

14

Fig. 8 The AFM images in 3D for J55 steel surface in (a) blank (b) presence 0.010 M VCA. 3.7. Adsorption isotherm Adsorption isotherms are utilized to obtain evidence on the contact between inhibitors and metal surfaces [44]. VCA retards the deterioration of J55 steel by adsorption on its surface. The surface coverage for different inhibitor concentrations was fitted with different adsorption isotherms, including Temkin, Langmuir, Frumkin, and Freundlich. The Freundlich isotherm given in Equation 17 was found to fit the obtained data properly. (17) where θ stands for the degree of coverage on the surface, Kads represents the adsorption equilibrium constant, C is VCA concentration, and n the interaction parameter. The plots obtained with values of ln θ against ln C are linear with good correlation coefficient (R2), which suggests that adsorption of VCA follows Freundlich adsorption isotherm (Fig. 9). The Kads data were evaluated from the intercept of the plots and are connected to the by Equation 18: (18) where 55.5 is the concentration of H2O in mol L-1, T represents temperature, and R is the universal gas constant. The and Kads deducted data are presented in Table 4. It is observed

15

that Kads values diminishes with increment in temperature, demonstrating that the interplay between the adsorbed VCA molecule and the steel surface are lessened. Such data explains the diminishing inhibition efficacy with temperature rise. The negative values of suggests that -1 VCA adsorption was spontaneous. Usually, values of up to -20 kJ mol or less negative suggest physisorption interaction between charged inhibitors while values around -40 kJ mol-1 and less positive is suggestive of chemisorption, which concerns transfer or sharing of charge between the steel surface and the inhibitor molecule to produce a kind of chemical bond [30,45]. In this study, the evaluated values of , ranges between -18.44 and -18.93 kJ mol-1, therefore suggesting that VCA adsorbed through physisorption mechanism (Table 4). -0.05 -0.10 -0.15

ln θ

-0.20 -0.25 -0.30

30 40 50 60

-0.35 -0.40 -7.0

-6.5

-6.0

-5.5

-5.0

o

C C o C o C o

-4.5

ln C (M)

Fig. 9 Freundlich isotherm for VCA on J55 steel in 1 M HCl solution at diverse temperatures. Table 4: Calculated functions from Freundlich isotherm for J55 steel corrosion in 1 M HCl at various temperatures. Temp. (C) 30 40 50 60

R2 0.9951 0.9909 0.9888 0.9712

Kads 27.24 25.86 20.72 16.11

(kJ mol-1) -18.44 -18.91 -18.93 -18.81

3.8. Temperature effect The impact of temperature on the inhibitive behavior of VCA molecule was evaluated by gravimetric analyses at 30 – 60 C temperature range. From the results obtained (Fig. 2a), it was discovered that the rate of J55 steel corrosion upsurges with temperature rise while the inhibition 16

efficacy of VCA decreases with temperature rise (Fig. 2b). This result reveals that at all temperatures considered, VCA was adsorbed on the surface of the metal via physisorption mechanism. The activation energy (Ea) for the deterioration reaction without and in the presence of VCA was determined from Equation 19 [27]: (

)

(19)

where CR represents corrosion rate and all other quantities are the standard quantities of Arrhenius equation. The Ea values were calculated from the slope of the Arrhenius plot and are presented in Table 5. The entropy of activation (ΔS*) as well as enthalpy of activation (ΔH*) for the corrosion process were deduced from Equation 20 [30]: ( )

*(

(

)

(

))+

(20)

where N represents Avogadro's number, and h is the Planck's constant. The plot of log (CR/T) against 1/T (Fig. 10) is linear, and values of ΔS* and ΔH* were obtained accordingly and depicted in Table 5. 0.8

-1.8

(b)

(a)

0.6

-2.0 Log (CR/T) (mm yr-1 K-1)

Log CR (mm yr-1)

0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0

Blank 0.0010 M 0.0025 M 0.0050 M 0.0075 M 0.0100 M 3.00

3.05

-2.2 -2.4 -2.6 -2.8 -3.0 -3.2 -3.4 -3.6

3.10

3.15

3.20

3.25

3.30

Blank 0.0010 M 0.0025 M 0.0050 M 0.0075 M 0.0100 M 3.00

-1

3.05

3.10

3.15

3.20

3.25

3.30

-1

1/T X 1000 (K )

1/T X 1000 (K )

Fig. 10 Arrhenius plots (a) of log CR and Eyring’s plot (b) of log CR/T versus 1/T for J55 steel in 1 M HCl in the absence and containing various concentrations of VCA.

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Table 5: Calculated activation parameters for J55 steel in 1 M HCl containing and without varying concentrations of VCA. Conc. (M) Blank 0.0010 0.0025 0.0050 0.0075 0.0100

∆H* (kJ/mol) 19.54 27.65 31.76 33.71 34.51 36.96

Ea (kJ/mol) 27.67 34.88 37.42 39.54 43.32 45.12

∆S* (J/mol/K) -185.32 -181.81 -178.93 -172.98 -168.72 -166.30

It is apparent from the data in Table 5 that Ea values augmented with increment in the concentrations of VCA. This was ascribed to a reduction in the adsorption strength of VCA on the J55 steel surface with rise in temperature. Findings from different researchers [46 – 48] have it that larger Ea values in the presence of inhibitors comparative to the blank solution are credited to physical adsorption method of the inhibitors while the opposite is referred to chemical adsorption mechanism. However, Ea values for physical adsorption are usually between 20 - 40 kJ mol–1. In this study, the value ranges between 27.67 and 45.12 kJ mol-1, suggesting that the adsorption of VCA also involves chemisorption. Assessment of Table 5 also shows that the values of ΔS* in both the inhibited and uninhibited solutions were negative. This entails that the activation complex is the rate deciding step representing association instead of dissociation, connoting that a drop-off in the level of disorderliness existed when moving from reactant to the activated complex [30,49]. It is also clear that the positive values of ΔH* in both inhibited and uninhibited solutions show that the adsorption process of VCA on the J55 steel surface is endothermic. 3.9 Density functional theory calculations Quantum chemical calculations offers a viable computational method for studying inhibition of metal corrosion [50-52]. The electronic energies obtained for the protonation at various heteroatom sites are as follows: N(8) 155.10; N(13) 46.27; O(20) 132.18; O(22) 126.47; O(24) 125.95 kcal/mol. This shows that VCA-H+ has maximum stability as a result of protonation at N(13) compared to other heteroatom sites. The calculated quantum parameters for the two species of Vincamine: neutral (VCA) and protonated (VCA-H+) are given in Table 6. VCA exhibits higher EHOMO and wider ΔEgap whereas VCA-H+ shows lower ELUMO and lower ΔEgap. Since Lower ELUMO is linked to a higher electron accepting ability and higher EHOMO related with higher electron donating capability, it can be inferred that VCA-H+ has a higher leaning to accept electrons donated from the metal orbital, whereas VCA has a higher propensity to donate an electron to the metal surface. The ΔEgap, which controls the overall reactivity of the inhibitor, favors VCA-H+ indicated with a lower value.

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Table 6. Calculated quantum parameters for VCA and VCA-H+. Inhibitor VCA VCA-H+

EHOMO (eV) -0.194 -0.229

ELUMO (eV) -0.009 -0.060

ΔEgap (eV) 0.185 0.169

IP (eV) 0.194 0.229

EA (eV) 0.009 0.060

χ (eV) 0.102 0.145

η (eV) 0.093 0.085

σ ΔN (eV-1) 10.753 25.366 11.765 27.500

The values of electronegativity obtained for VCA-H+ and VCA are inferior compared to the Fe(110) surface work function. As a consequence, a flow of electron from the lower electronegative value of the inhibitor to higher electronegative value of the Fe surface is expected. This flow is an attempt to balance chemical potentials setup when the Fe surface is in contact with the inhibitor solution. The fraction of electron transported (ΔN) to the metal surface is greater for VCA-H+, indicating that VCA-H+ has a higher propensity to contribute electrons compared to VCA. The value of ΔN does not actually signify the number of electrons donated, but a way of qualifying the electron-donating capability of the molecule. The positive values of ΔN for VCA and VCA-H+ show that electron is expected to be donated by the inhibitors to the steel surface. Also, the ΔN values greater than 3.6 suggests a decrease in the inhibition efficacy as the values of the electron-donating capability increases. In such a scenario, it is suggested that the inhibitor may have been attached to an electron acceptor prior to its adsorption on the metal surface, hence the reason for the decreased efficacy [53]. The value of global hardness shows that VCA in comparison to VCA-H+ is a hard compound [17]. Information on the resistance that atom exhibits regarding the deformation of its electron cloud could be deducted from global hardness and softness [25,28]. It is demonstrated that hard molecules are characterized by wider ΔEgap, which portrays lower reactivity, as shown by VCA. Therefore, in Fe-surface-inhibitor interaction, VCA-H+ is expected to show more reactivity based on lower global hardness and higher global softness. 3.10 Location of active sites on VCA and VCA-H+ Three parameters were considered in order to ascertain the active adsorption sites of VCA and VCA-H+ and these are the Fukui functions, frontier orbital plots and distribution of Mulliken atomic charge [26]. Fukui functions segregate the molecule into regions based on reactivity towards electrophilic or nucleophilic attack. Those regions with higher values of the Fukui electrophilic (fk-) or Fukui nucleophilic (fk+) parameters could be said to be vulnerable to the respective reactions. As indicated in Table 7, fk- is assigned to C(11) and fk+ assigned to C(21) for VCA, whereas for VCA-H+, they are assigned to N(8) and C(12), respectively. The Fe2+ ions on the surface of J55 steel interact with fk- reactive regions of the VCA, and in so doing, promote chemisorption and surface complex formation on the metal surface [28,29]. Considering the Mulliken atomic charge distribution, C(21) and O(22) are associated with the maximum positive and negative charge for both VCA and VCA-H+. This implies that protonation induced a negligible effect on the inhibitor’s charge distribution. The charged centers within the inhibitor undergo electrostatic interactions with the steel surface having positive 19

charges. The steel surface in the presence of the acid is known to possess excess positive charges [54,55]. Such electrostatic interaction between the negatively charged centers of the inhibitor, O(22), and the metal surface is a leading factor in physisorption. The anion, Cl- from the acid electrostatically interacts with the positively charged atoms, C(21) of the inhibitor and may also align on the Fe surface. The molecular orbital plots are useful plots used to predict adsorption of the inhibitor. As shown in Fig. 11, the major HOMO for VCA is concentrated within the indole fragment because of the electron-rich nature of the ring. This infers that the indole fragment is the most active site for interaction. A minor contribution to HOMO is received from N(8). The metal surface receives electrons via the HOMO centers of the inhibitor. The LUMO involves primarily the atoms of carbon within the indole fragment, and donation from the metal surface to the inhibitor is sent via the LUMO [26]. VCA-H+ shows significant changes in the HOMO compared to VCA, having its LUMO ultimately outside the indole fragment and centered on N(8) and few carbon atoms within the region. The LUMO of VCA and that of VCA-H+ are very similar and based on this, and in agreement with quantum chemical parameters (EHOMO and ELUMO) calculated, it is suggested that the neutral (VCA) and protonated (VCA-H+) species may exist in equilibrium. In such a situation, the HOMO of VCA donates electrons to the surface of Fe while the LUMO of VCA-H+ accepts feedback donations from the Fe surface. The HOMO centers coordinate the donor abilities while the LUMO centers coordinate the acceptor abilities of the inhibitor [56]. The donor-acceptor interaction is deemed necessary in the corrosion inhibition process, and this has been used to describe the surface-complex formation amid the vacant metal d-orbital and the VCA [28,57,58].

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Table 7 Mulliken atomic charges and Fukui indices for VCA and VCA-H+.

Atom

C(1) C(2) C(3) C(4) C(5) C(6) C(7) N(8) C(9) C(10) C(11) C(12) N(13) C(14) C(15) C(16) C(17) C(18) C(19) O(20) C(21) O(22) C(23) O(24)

Mulliken atomic charges 0.086 0.212 -0.174 -0.150 -0.109 -0.117 -0.035 -0.380 -0.215 0.014 0.031 0.276 -0.338 0.276 -0.138 -0.079 -0.021 -0.128 -0.133 -0.561 0.664 -0.532 0.069 -0.493

VCA Fukui electrophilic fk0.019 0.043 0.078 0.046 0.025 0.076 -0.012 -0.000 -0.012 -0.016 0.113 0.095 0.030 -0.004 -0.006 -0.005 -0.009 -0.003 -0.005 0.018 0.009 -0.004 -0.000 0.015

VCA-H+ Fukui Mulliken Fukui nucleophilic atomic electrophilic + fk charges fk0.032 0.140 0.003 0.017 0.227 0.009 0.050 -0.156 0.013 0.031 -0.098 0.005 0.011 -0.064 0.006 0.055 -0.084 0.015 -0.005 -0.034 -0.028 -0.001 -0.378 0.200 -0.004 -0.227 -0.014 -0.009 0.015 -0.031 0.031 0.126 0.026 0.047 0.273 0.000 -0.006 -0.432 -0.005 0.005 0.297 0.000 -0.014 -0.144 -0.008 -0.003 -0.076 -0.018 -0.005 -0.022 -0.035 -0.002 -0.128 -0.017 -0.004 -0.137 -0.007 0.034 -0.507 0.007 0.003 0.179 0.668 0.035 -0.000 -0.523 -0.011 0.066 -0.000 0.132 -0.457 0.004

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Fukui nucleophilic fk+ 0.055 0.060 0.035 0.013 0.051 0.072 -0.007 -0.002 -0.008 -0.013 0.066 0.074 -0.014 0.019 -0.008 -0.008 -0.007 -0.002 -0.005 0.029 0.040 0.006 -0.003 0.050

VCA HOMO

VCA LUMO

VCA-H+ HOMO

VCA-H+ LUMO

Fig. 11 Frontier molecular orbital plots of VCA and VCA-H+.

3.11 Molecular dynamics simulation The effectiveness of corrosion inhibition of organic compounds hangs on the adsorption on the surface of the metal. This is induced primarily by numerous factors, including the electronic structure, availability of functional groups, steric, electronic density, and mass of the molecule [59]. These features together determine the structural alignment of the inhibitor on the metal surface, thereby representing the binding mode. In order to deduce the nature of binding (inhibitor configuration), Quench MD simulation (QMD) was employed. QMD has the advantage over other MD simulation methods in its ability to precisely pinpoint the global minimum amongst different local minima possible within the inhibitor-metal surface configurational space. The arrangement of VCA and VCA-H+ on the surface of Fe is presented in Fig. 12.

22

VCA:

Front view

Side view

VCA-H+: Front view

Side view

Fig. 12: The configuration of VCA and VCA-H+ on J55 steel surfaces. The configurational patterns of VCA and VCA-H+ are quite different on the Fe surface. It is observed that VCA-H+ displays planar configuration, whereas VCA does not. Planarity allows the inhibitor to align the HOMO and LUMO centers for maximum donor-acceptor interaction. The oxygen atoms are pushed above the ring axis, and this is more pronounced in VCA-H+ than VCA. Due to polarization of C(21) by the electron-withdrawing effect of O(22) and O(24) and by extension, O(20), C(21) has a resident high positive charge, which is the reason it is indicated as the highest positive charged center according to Mulliken atomic charge distribution. This somewhat explains the non-participation of the oxygen atoms as HOMO or LUMO centre. The ethyl hydrocarbon and methyl carboxylate fragments align above the Fe surface in both VCA and VCAH+ indicating their non-involvement in the corrosion inhibition process. The minimum distance between Fe surface and VCA or VCA-H+ is less than 3 Å, and this suggests that the inhibitor is chemisorbed on J55 surface [60]. The protection of Fe surface by VCA and VCA-H+ from corrosion hangs on its adsorption, and this could be determined by evaluating the energy of

23

interaction (Einteraction) of the inhibitors on the J55 steel surface. The Einteraction of VCA and VCAH+ is presented in Table 8. Table 8. Energies of interaction and binding of VCA and VCA-H+ on J55 steel surface. Energy (kcal/mol)

Equilibration temperature (°C) 30

Interaction energy

VCA-H+

VCA 40

50

60

30

40

50

60

-106.80

-154.66 -154.95

-115.35 -150.54

-151.24

-151.98

-149.58

106.80

154.66 154.95

115.35

151.24

151.98

149.58

(Einteraction) Binding energy

150.54

(-Einteraction)

The binding energy of VCA and VCA-H+ on J55 steel surface is temperature dependent [28]. For VCA, the binding energy increases with temperature, up to a maximum at 50 °C, after which there is a decline. The variation pattern shows a sharp increase from 30 - 40 °C (47.86 kcal/mol) and a sharp decrease from 50 - 60 °C (39.60 kcal/mol). Between 40 - 50 °C, the increase is more or less constant (0.25 kcal/mol), portraying saturation or approaching peak point. For VCA-H+, the variation in binding energy with temperature occurs within a narrow range: 0.70 (30 - 40 °C); 0.74 (40 - 50 °C); 2.40 kcal/mol (50 - 60 °C). Structurally, such narrow variation depicts the stability of the inhibitor due to a lack of temperature-dependent variants. A better inhibitor is evaluated based on the more negative interaction energy or higher binding energy. VCA appears to perform better at 40 - 50 °C by exhibiting higher binding (or more negative interaction) energies, whereas VCA-H+ indicates better anticorrosion behavior at the initial (30 °C) and final (60 °C) temperatures considered in this work. By overlooking the fluctuations in the mid values (40 - 50 °C) and laying emphasis on initial and final values, the overall trend can clearly be mapped out. For VCA, the overall trend shows chemisorption having the final value of binding energy higher than the initial value, whereas physisorption is indicated for VCA-H+. These findings match with the experimental data, which had already established physisorption as the mechanism for the corrosion inhibition process. In physisorption, the inhibition efficacy reduces as the temperature increases while in chemisorption, the inhibition efficacy rises with temperature [61]. For both VCA and VCA-H+, the trend within the mid temperature values shows chemisorption, and this is governed by the donation of electrons from the HOMO of VCA to the surface of J55 steel and return a donation from Fe d-orbital to the LUMO of the inhibitor. This mixed-type behavior consisting of chemisorption and physisorption had been pointed out previously [19]. For VCA-H+, chemisorption is the intermediate while physisorption, originating from a strong electrostatic interaction of charged centers within the

24

inhibitor and Fe surface, is the overall (dominating) adsorption mechanism in the mixed-type behavior. The binding energy of VCA-H+ is seen to be higher than that of VCA at the initial and final values of temperatures, which shows that VCA-H+ exhibits higher adsorptive potential and better corrosion inhibition properties on Fe surface. Therefore, the addition of a proton to VCA in acid most likely occurs since such a pathway leads to better anticorrosion behaviour. Since experimental data had already established physisorption as the adsorption mechanism, theoretical studies have confirmed that the active species exhibiting this physisorption is the protonated Vincamine. Therefore, VCA-H+ is a better corrosion inhibitor centred on the results of these findings. 3.12. Mechanism of inhibition The adsorption of VCA molecules at the steel-solution interface accounts to its ability to inhibit J55 steel corrosion in 1 M HCl solution. The morphology of the steel, type of corrosive medium and the inhibitor chemical structure influences inhibitor adsorption [62]. Generally, the adsorption process of organic inhibitors involves the substitution of pre-adsorbed water molecules ( ) with inhibitor molecule in solution ( ) as expressed below [63]: (21) where x represents the number of substituted water molecules. Most importantly, the electron density at the active center of the inhibitor molecule determines the efficacy of the inhibitor. VCA get protonated in HCl solution due to the presence of nitrogen atoms in its molecule, which converts it into quaternary compounds. Cathodic reactions are controlled by the adsorption of the protonated species on the J55 steel cathodic sites, decreasing the hydrogen evolution. Anodic sites adsorption proceeds through π- electrons of indole rings and electron lone pairs of heteroatoms interactions with the J55 steel [64]. 4. Conclusion Based on the obtained results, Vincamine (VCA) has shown to be a potential inhibitor against deterioration of J55 steel in acidic medium with the corrosion inhibition efficacy rising with increment in the concentrations of the VCA molecules, but diminishes with temperature rise. Polarization curves showed Vincamine to be is a mixed-type inhibitor. Impedance analyses affirm that as VCA concentration augments, the resistance to charge transfer also augments. This indicates that the inhibitive behavior of VCA molecules depends on its adsorption on the surface of J55 steel. The adsorption of VCA on the surface of J55 metal obeys Freundlich adsorption isotherm. The negative values of show that adsorption of the VCA molecules is a spontaneous process. The surface analysis affirms the development of a deterrent layer on the J55 steel surface. Theoretical studies confirmed that the active species exhibiting this physisorption is the protonated Vincamine.

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Acknowledgements NBI, NAM, and BTO are thankful to Federal University Otuoke, Nigeria, for supporting this work financially. EDA thanks the North-West University, South Africa, for the postdoctoral research fellowship. Conflict of interest Authors have no conflict to declare.

Contribution of authors NBI designed the study and wrote the first draft of the manuscript, NAM and BTO performed the weight loss and electrochemical experiments, while VM performed the quantum chemical studies. LAN managed the literature searches and took part in the manuscript writing; EEE and EDA managed the analyses, wrote, edited, and finalized the manuscript. All the authors read and approved the final manuscript.

CREDIT AUTHOR STATEMENT

NBI designed the study and wrote the first draft of the manuscript, NAM and BTO performed the weight loss and electrochemical experiments, while VM performed the quantum chemical studies. LAN managed the literature searches and took part in the manuscript writing; EEE and EDA managed the analyses, wrote, edited, and finalized the manuscript. All the authors read and approved the final manuscript. DECLARATION OF INTEREST: NONE References [1] [2] [3] [4] [5] [6] [7]

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