Surface protection of steel in oil well acidizing fluids using L-theanine-based corrosion inhibitor formulations: Experimental and theoretical evaluation

Accepted Manuscript

Surface protection of steel in oil well acidizing fluids using L-theanine-based corrosion inhibitor formulations: Experimental and theoretical evaluation Ekemini Ituen , Victor Mkpenie , Emmanuel Dan PII: DOI: Reference:

S2468-0230(18)30491-7 https://doi.org/10.1016/j.surfin.2019.04.006 SURFIN 319

To appear in:

Surfaces and Interfaces

Received date: Revised date: Accepted date:

11 October 2018 28 March 2019 9 April 2019

Please cite this article as: Ekemini Ituen , Victor Mkpenie , Emmanuel Dan , Surface protection of steel in oil well acidizing fluids using L-theanine-based corrosion inhibitor formulations: Experimental and theoretical evaluation, Surfaces and Interfaces (2019), doi: https://doi.org/10.1016/j.surfin.2019.04.006

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Highlights

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Natural LTN powder extracts from green tea is used to formulate corrosion inhibitors for X80 steel in simulated acidizing fluid Electroanalytical, thermogravimetric ans surface screening techniques are employed to evaluate steel protection Theoretical parameters and adsorption behaviour of LTN is also explained using quantum chemical models and dynamic molecular simulations Rigid adsorption energy, interaction energy and Fukiu electrophilic and nucleophilic centers are predicted at different temperatures.

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ACCEPTED MANUSCRIPT Surface protection of steel in oil well acidizing fluids using L-theanine-based corrosion inhibitor formulations: Experimental and theoretical evaluation Ekemini Ituen1(*), Victor Mkpenie2, Emmanuel Dan1 1

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Corresponding author’s email: [email protected]

Abstract

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Materials and Oilfield Chemistry Group, Department of Chemistry, Faculty of Science, University of Uyo, Uyo, Nigeria. 2 Inorganic and Computational Chemistry Group, Department of Chemistry, Faculty of Science, University of Uyo, Uyo, Nigeria.

Corrosion of X80 steel in 1 M and 15 % HCl solutions and its inhibition using L-theanine (LTN), a natural white

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powder of green tea leaves extracts, was investigated using electrochemical frequency modulation (EFM), potentiodynamic polarization (PDP), linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS) and weight loss (WL) techniques. Increase in concentration of LTN improves its adsorption on X80 steel surface, forms thin protective film that increases charge transfer resistance and inhibition efficiency, but reduces corrosion current density and double layer capacitance. LTN acts as mixed type inhibitor with anodic

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predominance. Adsorption of LTN is spontaneous, exothermic and best approximated by Temkin adsorption isotherm. Composites of LTN and some synergistic compounds afford higher efficiencies (up to 89.2%).at 90 oC

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than pure LTN. Quantum chemical computations were also carried out by Conductor-like Screening Model (COSMO-GGA/PW91 of Dmol3) to support experimental findings. The active adsorption sites were predicted as C(5), O(12) and N(11) based on Mulliken charge distribution and Fukui electrophilic and nucleophilic functions.

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The energies of interaction were compared with rigid adsorption energies at studied temperatures (30 – 90 oC).

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FTIR analyses support that O and N sites were actively involved in adsorption. Keywords: Oil well stimulation, adsorption energy, protective film, EIS, SEM, molecular simulations INTRODUCTION

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1.

Recovery of petroleum from reservoir is not always continuous. A time is reached when the well depletes and there

is

need

to

enhance

the

recovery

of

hydrocarbons

using

suitable

techniques.

One

of the techniques popularly used is well stimulation. Well stimulation is an intervention procedure executed on depleted wells to improve flow of hydrocarbons and increase production [1]. During stimulation, new channels are opened up in the formation rock and this allows trapped hydrocarbons to flow through these channels. Stimulation treatments may be carried out in any of three ways: use of explosives to break up the rock; injection of acid into the formation (to partially dissolve it and create new flow channels) and hydraulic fracture (using acid or propants to split up formation rock) [2].

ACCEPTED MANUSCRIPT Matrix stimulation by use of hydrochloric acid (5-10%) is commonly employed in carbonate formation because HCl reacts fast with the rock [3]. Apart from enlarging pore spaces, the acid also dissolves clays and mud solids near the wellbore which had choked the pores and leaves sand and remaining fines in water wet condition. Some fracture procedures involve the use of hydrochloric acid up to 10-15% concentration [3]. The amount of the acid used depends on the type of carbonate formation and the expected reaction is depicted in Eqs. 1-3. While well stimulation improves production, contact of the acid with steel used for well construction leads to corrosion because HCl is highly corrosive.

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(1) (2) (3)

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The stimulation acid is usually mixed with some substances to achieve desired effect. Some of the substances include surfactants added to provide retardation, achieve deep penetration and high reservoir temperature compatibility [4]. Others include iron controller, mutual solvent, friction reducer, clay stabilizers and corrosion inhibitors. The function of corrosion inhibitor is to retard the rate of corrosion of the steel. Otherwise, corrosion of steel structural materials could result in untimely failure of materials, spills and perhaps disasters which would be

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expensive to manage. Chemical corrosion inhibitors used in oilfields are limited by toxicity and high cost. Toxic corrosion inhibitors can cause environmental pollution especially those that are non-biodegradable. This is why

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research into environmentally benign corrosion inhibitors from cheap sources is very active. L-theanine is being investigated in this study as an alternative corrosion inhibitor for acidizing.

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L-theanine (L-γ-glutamylethylamide or N⁵-ethyl-L-glutamine) is an amino acid and is abundant in tea and other plant and fungal species. Majority of the studies conducted on L-theanine has been on its effect on mental state,

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cognitive function, especially in synergy with caffeine [5-9]. The combination of both L-theanine and caffeine has been shown to improve intellectual quotient (IQ) scores [8]. L-theanine has been used to prepare supplements beneficial for stress reduction, decreasing generalized anxiety, and reducing the severity of insomnia [10]. The

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molecular structure of L-theanine (Figure 1) has some electron-rich centers like oxygen and nitrogen. Adsorption of LTN could occur through these centers as reported for other efficient organic corrosion inhibitors [11, 12]. Better still, LTN is not toxic and is commercially available at very cheap cost. These motivated its choice for application as green corrosion inhibitor for oilfield acidizing. In this study, the inhibitive effect of natural LTN is investigated by thermo-gravimetric, electro-analytical and quantum chemical methods. The oilfield stimulation liquor is simulated using both 1 M HCl and 15 % HCl to mimic lowly and highly acidic real field conditions respectively. The tests were conducted at different temperatures between 30 oC to 90 oC without exclusion of oxygen. This is to demonstrate the effect of

ACCEPTED MANUSCRIPT temperature on the inhibitor as it is pumped downhole. Some compounds, namely, glutathione (GLU), N-acetyl cysteine (NAC), 5-hydroxytryptophan (5-HTP), polyethylene glycol (PEG) and potassium iodide (KI) were blended with LTN to obtain different formulations and tested under similar conditions. Some of these compounds

(b)

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(a)

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have been reported to inhibit corrosion in some media, but L-theanine has not been reported.

Fig. 1. Structure of L-theanine (a): Molecular structure (b): Optimized structure at DFT/GGA-PW91. O - red; N blue; C - gray; H - white. Experimental procedures

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2.1 Preparation of LTN formulations

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Analytical grade HCl (Sigma Aldrich) was diluted to 1 M and 15% concentrations using double distilled water. Natural white powder of green tea leaves extracts (solvent extraction, 99 % L-theanine) was obtained from

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Chengdu Health-life Biotechnology Co. Ltd., China. The additives GLT and NAC were supplied by Wuhan Yuancheng Gongchuang Technology Co. Ltd.; PEG was supplied by Richest group Ltd. Shangai; 5-HTP supplied

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by Shaanxi Kanglai Ecology Agriculture Co. Ltd.; and KI was supplied by Meyer Chemical Technology Co. Ltd. Shanghai, all in China. These compounds were used as supplied without further characterization or purification.

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LTN, herein referred to as the base component, was prepared into four different concentrations (0.00001 M, 0.0001 M, 0.001 M and 0.01 M). The concentration of each of the additives prepared was 10-5 M. Table 1: Formulations of LTN and the different compositions Formulation Code LTN-P LTN-G LTN-H LTN-N LTN-K LTN-PK

Composition LTN + PEG LTN + GLU LTN + 5-HTP LTN + NAC LTN + KI LTN + PEG + KI

ACCEPTED MANUSCRIPT LTN-GK LTN-HK LTN-NK

LTN + GLU + KI LTN + 5-HTP + KI LTN + NAC + KI

2.2 Metal surface preparation The X80 steel coupons (dimension 2 cm x 2 cm) were supplied by supplied by Qingdao Tengxiang Instrument and Equipment Co. Ltd., China and has composition C (0.065), Si (0.24), Mn (1.58), P (0.011), S (0.003), Cu (0.01), Cr (0.022), Nb (0.057), V (0.005), Ti (0.024), B (0.0006), Fe (balance). NACE Recommended Practice

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RP-0775 and ASTM G-1 & G-4 procedures for surface finishing and cleaning of weight-loss coupons [13] were followed in preparing the X80 steel surface. Coupons for electrochemical studies were abraded to mirror surface using CC-22F P2000 grade and 1 cm2 of the surface area was exposed. Coupons for SEM test were of dimensions 2 cm x 1 cm. Prepared specimens were enclosed in sealed water-proof bags and stored in moisture free desiccator prior to use.

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2.3 Electrochemical measurements

Gamry Potentiostat/Galvanostat (ZRA REF 600-18042) electrochemical workstation was used for this measurement. This consists of the conventional three electrode set up, with saturated calomel electrode (SCE) as reference electrode, platinium as counter electrode and X80 steel coupon as working electrode under de-aerated

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condition. The corrosion behavior of X80 steel in the test solutions was probed by electrochemical frequency modulation (EFM), potentiodynamic polarization (PDP), linear polarization resistance (LPR), and electrochemical

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impedance spectroscopy (EIS). Initially, the working electrode was immersed in the test solution for 30 mins to obtain steady open circuit potential (OCP) [14]. The EIS was conducted at a frequency of 100 kHz to 10 mHz at 30 oC. A voltage range of −0.15 V to + 0.15 V vs. OCP and scan rate of 0.2 mV/s was used for PDP

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measurements. LPR measurements were taken at Ecorr of ±10 mV at a scan rate of 0.25 mV/s. EFM was conducted with a base frequency of 0.01 Hz and amplitude of 10 mV. Data analyses and curves fittings was

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achieved using Gamry E-Chem software.

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2.4 Weight loss technique

The ASTM procedures for weight loss measurement were followed [13] at temperatures between 30 to 90 oC by means of a water bath. The coupons were immersed in the test solutions for five hours, then retrieved, washed in 20 % NaOH solution containing about 200 g/L of zinc dust until clean, dried in air after rinsing in acetone, reweighed for weight loss (

). Corrosion rate (CR) of iron, percentage inhibition efficiency (

of surface coverage (θ), were calculated using Eq. 4, 5 and 6 respectively. (4)

) and degree

ACCEPTED MANUSCRIPT (5) (6) where

are the corrosion rates (cmh-1) in the absence and presence of the inhibitor respectively,

and

the density of iron,

2

is the average surface area (cm ) of the metal specimens and

is

is the immersion time (h).

The CR values obtained were converted to mmpy units as described in literature [15]. This procedure was

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repeated at 50, 70 and 90oC.

2.5 Quantum chemical calculation

The geometry of LTN was optimized using Dmol3 module [16] of Materials Studio developed by Dassault Systemes BIOVIA, with the generalized gradient approximation (GGA) functional obtainable within the density functional theory (DFT). The exchange-correlation functional of Perdew and Wang (PW91) [17] was employed.

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The basis file of the double numerical basis set with polarization function (DNP) was set to 4.4 ensuring improved calculation accuracy [18]. The convergence tolerance of energy, force and displacement were set to 1x10-5 Ha, 2x10-3 Ha/Å and 5x10-3 Å, respectively. The optimization was performed in aqueous phase (water as solvent) using the Conductor-like Screening Model (COSMO) which allow for the treatment of solvation effects [19, 20]. COSMO is a continuum solvation model (CSM) in which the solute molecule forms a cavity within the dielectric

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continuum of permittivity that represents the solvent. The geometry of the optimized structure converged with energy and force under 0.59x10-5 Ha and 0.86x10-3 Ha/Å, respectively. The absence of imaginary frequency in

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the vibrational analysis attest to the attainment of a minimum. The molecular properties obtained by DFT/GGAPW91 calculation includes the energy of the highest occupied molecular orbital (EHOMO), energy of the lowest

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unoccupied molecular orbital (ELUMO), energy gap (ΔEgap), ionization potential (IE), electron affinity (EA), electronegativity (χ), global hardness (η) and softness (σ), Fukui electrophilic (f(-)) and nucleophilic (f(+)) indices

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and Mulliken atomic charges. These properties were used to predict the probable active adsorption sites of LTN. Values of the energies of HOMO and LUMO orbitals of LTN are related to the ionization energy (IE) and

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electron affinity (EA) according to Eqn. 7 and Eqn. 8, respectively.

The energy gap (

(7) (8)

) between LUMO and HOMO molecular orbitals is calculated using Eqn. 9. Electronegativity

(χ) and the global hardness (η) can be calculated using Eqn. 10 and Eqn. 11 [21]. Global softness also known as electron polarizability, which is electron receiving capacity of a molecule is represented as a reciprocal of global hardness as shown in Eqn. 12. (9) χ = (IP + EA)/2

(10)

η = (IP - EA)/2

(11)

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(12)

The number of electrons transported between Fe surface and LTN, ΔN can be calculated using Equ. (13): ΔN = Φ – χLTN / 2 ηLTN

(13)

where Φ is the work function given as 4.82 [21], χLTN is the electronegativity of LTN, and ηLTN is the global hardness of LTN.

2.6 Surface energy calculation

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The Fe (body centered 3D cubic) crystal was obtained from BIOVIA Materials Studio database with lattice parameters a = b = c = 2.8664 Å and α = β = γ = 90o belonging to m-3m crystal class. The cell and atomic positions were optimized using Forcite geometry optimization module utilizing Smart algorithm with convergence tolerance of energy, force and displacement, set to 1x10-4 kcal/mol, 5x10-3 kcal/mol/Å and 5x10-5 Å, respectively. The lattice parameters obtained after optimization (a = b = c = 2.859 Å; α = β = γ = 90 o) are in

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agreement with the experimental lattice parameters of Fe having bcc structure (a = b = c = 2.856 Å; α = β = γ = 90o)[22]. Forcite is a computational package that allows fast geometry optimization and energy calculation on both periodic systems and molecules. The forcefield chosen for the optimization was the Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS). COMPASS is a technology break-through in forcefield methods that enables accurate calculation of gas-phase and condensed-phase

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properties for a broad range of inorganic and organic systems. Fe (110) surface was cleaved from optimized Fe crystal with a fractional thickness of 6, a 6x6 supercell was created from the cleaved surface and the surface was

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optimized with the same Forcite parameters used previously. The 6x6 supercell exposes a large surface area for the LTN molecule. The energy parameters obtained from the single point energy calculation of the Fe(110)

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surface were used to compute surface energy.

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2.7 Calculation of rigid adsorption energy The rigid adsorption energy of LTN on Fe(110) surface was calculated using Adsorption Locator module of Materials Studio BIOVIA. A slab was built from the optimized 6x6 supercell with a vacuum thickness of 30 Å

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oriented along the C-axis (crystal orientation standard: C long Z, B in YZ plane) and LTN was loaded on the slab. The rigid adsorption energy was calculated by simulated annealing using COMPASS forcefield of Forcite engine and the same convergence setting used previously. Adsorption Locator implements a Monte Carlo search of the configurational space of the substrate-adsorbate system as the temperature is slowly decreased. The maximum simulated annealing temperature was set to 100,000 K and this monotonically decreased to the temperature, 303 K, 323 K, 343 K and 363 K in the respective calculations corresponding to 30 – 90 oC temperature range. The high annealing temperature at the beginning of the simulation is to allow the system to explore the entire conformational space within the periodic boundary for low energy configuration.

ACCEPTED MANUSCRIPT 2.8 Molecular dynamics (MD) simulation A simulation box 17.15 x 24.26 x 40.11 (Å3) built from 6x6 supercell with a 30 Å vacuum thickness having the first three layers of Fe atoms unconstrained was used for molecular dynamics simulation. The LTN molecule was placed and oriented so that it lies flat, hovering on the Fe slab surface. With the help of close contact monitor, the least distance between LTN and Fe surface atoms was indicated as 2.40 Å and the system was optimized using Forcite. The structure returned from this system optimization provides an energy minimum which is only a guess at the minimum energy adsorption site. To obtain the global energy minimum, Quench molecular dynamics

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approach was employed to sample many different configurations. All the surface atoms were selected and constrained and Quench dynamics calculation was performed at different temperatures (30, 50, 70 and 90 ºC) using constant volume/constant temperature (NVT) thermodynamic ensemble with a time step of 1.0 fs and a total simulation time of 100 ps. The Nosé-Hoover-Langevin (NHL) thermostat [22, 23] was used and the geometry optimization option utilized Smart algorithm with the same convergence tolerance setting used previously. The

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lowest energy configuration corresponding to a global energy minimum was obtained and after removing the surface atom constrain, this configuration was used to perform single point energy calculations for the LTN and Fe(110) surface to obtain the interaction (adsorption) energy according to Eqn. (14). Binding energy of LTN on Fe(110) surface is simply the negative of interaction energy (Ebinding = - Einteraction). (14)

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Interaction Energy (Einteraction) = EFe/LTN - (ELTN + EFe)

energy of the Fe(110) surface.

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Where EFe/LTN is the total energy of Fe(110) surface and adsorbed LTN, ELTN is the energy of LTN and EFe is the

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In order to ascertain the accuracy of the calculation, important geometrical parameters of LTN (especially around the hetero atoms) obtained were compared with the ones obtained from experiment [23], and good agreements

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were observed between the COMPASS prediction and experiment. For example, Nitrogen atom bond lengths, C(2)-N(11), C(5)-N(6) and N(6)-C(7), calculated as 1.490, 1.341 and 1.456 Å were very accurate compared to the

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experimental values given as 1.490, 1.351 and 1.450 Å, respectively. Relative error in the measurement of nitrogen bond length lies within 0 - 0.74%. For oxygen atom bonds, the calculated versus experimental values are as follows: C(1)-O(9), 1.370 vs 1.252 Å; C(1)-O(10), 1.559 vs 1.524 Å; C(5)-O(12), 1.216 vs 1.237 Å with error ranging from 1.70 – 9.42%. The 9.42% error shown in the measurement of C(1)-O(9) is due to the fact that, in the experiment, the bond was treated as partial double bond due to delocalization of pi-electrons. Other bond lengths are: C(2)-C(3), 1.534 vs 1.534 Å; C(3)-C(4), 1.534 vs 1.511 Å; C(7)-C(8), 1.528 vs 1.479 Å. Bond angles were also compared and the error in the measurement versus experimental values is given in parenthesis: C(5)-N(6)C(7), 120.65 vs 119.1º (1.3%); C(4)-C(5)-N(6). 113.98 vs 115.20º (1.06%); C(4)-C(5)-O(12), 123.77 vs 125.20º

ACCEPTED MANUSCRIPT (1.14%); N(6)-C(7)-O(12), 122.26 vs 119.60º (2.22%). It can be concluded that geometry optimization using COMPASS forcefield agrees well with experimental method in the determination of crystal structure.

2.9 Proton affinity calculation For the theoretical calculation of gas-phase proton affinity (PA), the geometries of the neutral and protonated LTN molecules were fully optimized using Gaussian 09 program [24] Since there are multiple protonation site, a proton (H+) was added to each site labeled N(6), N(11), O(12), O(10) and O(9), respectively (Fig. 1b) and the

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structures were optimized with DFT method using hybrid functional of Becke, Lee, Yang and Parr (B3LYP) with triplet split-valence basis set with polarization function 6-311G(d,p) [25]. The B3LYP/6-311G(d,p) calculation afforded quantum parameters: electronic energy (Eel) and zero point energy (ZPE) that were used to calculate PA according to Eq. 15 [26]. PA = – ΔEel – ΔZPE + 5/2RT

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(15)

where ΔEel and ΔZPE are the change in electronic energy and change in zero point energy, respectively between the neutral and protonated LTN, R is the gas constant given as 1.9872 calmol-1K-1 and T is temperature (298 K).

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The accuracy of the calculation was tested by determining the PA of water at the B3LYP/6-311G(d,p) level and the result indicated a value of 164.40 kcal/mol against experimental value of 165 kcal/mol. This confirms that the

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theoretical method is accurate. 2.10 FTIR study

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The spectra of the LTN powder and that of the surface film formed by 0.01 M LTN on the steel were obtained by

subtracted.

RESULTS AND DISCUSSION

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3.

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FTIR scan at 600-4500 cm-1. Both materials were mixed with KBr and the peak corresponding to KBr was

3.1 EFM measurements The essence of conducting EFM measurement was to obtain corrosion current values using small signals. These corrosion current values were used to compute the inhibition efficiency (Eq. 16) and displayed in Table 2. The obtained inhibition efficiency increased with increase in LTN concentration. Experimental causality factors CF-2 and CF-3 were also deduced from analyses of the frequency spectrum of the current responses. The obtained values of CF-2 and CF-were close to theoretical values of 2 and 3 respectively which indicates that the measurements are of good quality [27, 28]. Tafel anodic (

) and cathodic ( ) constants were also deduced and

ACCEPTED MANUSCRIPT results show more spread in

values than

values, indicating that the inhibitor has more influence on the

anodic than cathodic reaction. ) where

and

(16)

are the corrosion current densities in the absence and presence of the inhibitor respectively.

LTN. 1 M HCl 112.58 66.90 477.1 1.884 2.187 -

0.00001 M 92.10 71.42 94.4 1.623 2.423 80.2

0.0001 M 89.24 80.20 41.5 1.751 2.594 91.3

0.001 M 91.62 78.96 15.3 1.893 2.718 96.8

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Parameters (mV/decade) (mV/decade) (μA) CF-2 CF-3 (%)

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Table 2: EFM parameters for corrosion of X80 steel in 1 M HCl without and with different concentrations of

3.2 Potentiodynamic Polarization

0.01 M 74.60 82.30 12.9 1.944 2.776 97.3

Since X80 steel is an alloy of many metals, various oxidation reactions could take place at the anode. Typical

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anodic oxidation reactions may be represented in Eq. 17, and for iron, which is the major composition in X80 steel, the oxidation reaction is represented by Eq. 18. The reaction at the cathode could involve evolution of

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hydrogen following reduction of water (Eq. 19 -20).

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Anodic reaction:

(17) (18)

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Cathodic reaction:

(19) (20)

Potentiodynamic polarization measurements can be used to control the potential (being the driving force) in order to measure the current as a function of net change in reaction rate. The sum of currents from anodic and cathodic processes was rationalized to obtain a compromise current called the corrosion current density ( corresponding potential (

) and a

) from appropriate Tafel plots (Fig. 2). The Tafel cathodic and anodic constants

ACCEPTED MANUSCRIPT (

and

) were also obtained alongside with some other PDP parameters (Table 3). Inhibition efficiency (

)

can be calculated using Eq. 21. ) where

and

(21)

are the measured corrosion current densities without and with inhibitor, respectively.

From the results obtained,

values decreases when LTN concentration increases. Current density value was

protective film of LTN on X80 steel surface. The inhibited solutions implying displacement of

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highest in the uninhibited solution than solutions containing LTN, attributed due to formation of adsorbed value measured in 1.0 M HCl is more negative than in the values to more positive regions. Inhibitors that exhibit this

behavior are usually identified as anodic inhibitors, otherwise, cathodic inhibitors [29]. Thus, LTN has dominant influence on the partial anodic reaction by retarding the rate of metallic oxidation. However, the maximum displacement of

of the inhibited media from that of the free acid is less than -85 mV, hence not sufficient

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to classify the inhibitor as either cathodic or anodic type. Therefore, LTN can be regarded as mixed type inhibitor with anodic predominance [30]. In other words, LTN inhibits both the iron dissolution and hydrogen evolution processes but more actively inhibiting iron oxidation. The mechanism could be activation or diffusion controlled. The values of

and

obtained follow a similar trend as that of EFM measurement with the highest range of . This indicates that LTN exhibits more influence on anodic reaction.

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Tafel constants obtained with -0.3

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1 M HCl 0.00001 M LTN 0.0001 M LTN 0.001 M LTN 0.01 M LTN

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E (V vs SCE)

-0.4

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-0.5

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-0.6

-0.7 -7

-6

-5

-4 log I (Acm-2)

-3

-2

-1

Fig. 2. Tafel plots for X80 steel corrosion in 1 M HCl without and with different concentrations of LTN. Table 3: PDP parameters for X80 steel corrosion in 1M HCl without and with different concentrations of LTN. Parameters

(mV/decade) (mV/decade) (μA)

1 M HCl

0.00001 M LTN

0.0001 M LTN

0.001 M LTN

0.01 M LTN

105.2 73.1 957.1

77.9 80.7 130.2

88.5 84.8 26.8

89.4 79.7 13.4

91.8 82.6 1.91

ACCEPTED MANUSCRIPT -0.525 -

(V) (%)

-0.511 86.4

-0.504 97.2

-0.503 98.6

-0.501 99.8

3.3 Linear Polarization Resistance LPR experiment was carried out to complement the inhibition efficiency values obtained from PDP measurements. The Stern-Geary equation (Eq. 22) is used to deduce polarization resistance, which is applied in

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calculating the inhibition efficiency according to Eq. 23. (22)

( and

(22)

are respectively the polarization resistances of 1 M HCl solution with and without different

concentrations of LTN. The

values obtained (Table 4) increases as LTN concentration increases, similar to

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where

)

PDP trend. Also, inhibition efficiency values calculated are comparable with PDP results. Table 4: LPR parameters for corrosion of X80 steel in 1 M HCl containing different concentrations of LTN.

2

0.00001 M LTN

0.0001 M LTN

0.001 M LTN

0.01 M LTN

26.38 -

189.8 86.1

1099.2 97.6

1758.7 98.5

2198.3 98.8

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( cm ) (%)

1 M HCl

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Parameters

3.4 Electrochemical Impedance Spectroscopy

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EIS experiments were also conducted using the X80 steel in 1 M HCl without and with the different concentrations of LTN. The resulting data afforded Nyquist plots (Fig. 3) which is used to deduce EIS parameters

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for description of the corrosion process. Nyquist plots yield imperfect semicircles having similar shapes but different diameters. The diameter size is influenced by addition of a proportional concentration of LTN. This supports that inhibiting potential increases as LTN concentration increases. The observed imperfection in shape of

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the semicircles may be ascribed to surface roughness or inhomogeneity of the X80 steel [31]. The single capacitive loop signifies charge transfer controlled mechanism of corrosion [31]. Similarity in the shape of the plots also signifies similar corrosion mechanism with or without the inhibitor. The equivalent circuit (Fig. 4) corresponding to

model was used to fit the experimental data and a

good fitness of the order of 10-4 was obtained. The constant phase element (CPE) is used to compensate surface roughness or inhomogeneity of the steel. The CPE is can be estimated using

and , related to impedance by Eq.

24. =

(24)

ACCEPTED MANUSCRIPT where

is the impedance of the CPE,

complex number,

is the CPE constant,

is the phase angle of CPE and

is the angular frequency,

is an imaginary

= 2α⁄(π ). Inhibition efficiency was calculated

using charge transfer resistance according to Eq. 25. The double layer capacitance relates to the local insulation effect caused by the thin adsorbed film of LTN and was computed using Eq. 26 (

)

(25)

where

and

(26)

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=

are measured charge transfer resistances without and with inhibitor respectively,

phase element (CPE) constant and n is a constant obtained from the phase angle given that 2α⁄(π ).

and

=

increase on addition of more

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Some EIS parameters were calculated and presented in Table 5. The values of

constant

LTN molecules, showing increases in surface roughness of the steel due to more LTN adsorption [32]. It also denotes that the CPE is not just a single resistance, capacitance or inductive element but there is relative and/or integrated influence of all these on the CPE [33, 34]. Thus, as LTN is added to the HCl solution, values of n increase due to insulation of the metal/solution interface by the surface film formed [32]. This also causes charge

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transfer resistance to increase in the concentration dependent pattern. This demonstrates improved ‘blanketing’ property of the film as LTN concentration increases. The trend of inhibition efficiency obtained is consistent with

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other measurements and comparable to some extents.

1 M HCl 0.00001 M HCl 0.0001 M HCl 0.001 M HCl 0.01 M HCl

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1600

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Z(cm2)

1200

800

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400

0 0

400

800

1200

Z(cm2)

1600

2000

Fig. 3. Nyquist plot for the inhibition of X80 steel corrosion in 1 M HCl without and with different concentrations of LTN

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Fig. 4. Equivalent circuit used for EIS data analyses.

n x10-9 (F) (%)

1 M HCl 34.8 1.026 162.8 0.884 102.6 -

0.00001 M LTN 347.5 1.211 131.6 0.888 69.5 89.9

0.0001 M LTN 1044.2 1.248 155.7 0.891 23.6 96.7

0. 001 M LTN 1377.3 1.273 159.2 0.893 8.4 97.5

0.01 M LTN 2014.8 1.368 163.5 0.896 1.7 98.3

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EIS Parameters ( cm2) ( cm2)

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Table 5: EIS parameters for X80 steel corrosion in 1 M HCl without and with different concentrations of LTN.

3.5 Weight loss measurement

Weight loss values obtained were used to compute the corrosion rate of X80 steel in both 1 M and 15 % HCl and the results are displayed in Table 6 and Table 7 respectively. It can be inferred from these tables that X80 steel

M

corrodes more in 15 % HCl than 1 M HCl, hence the effect of acid concentration on the corrosion rate. Another vital observation is that inhibition efficiency declines as temperature increases but increases as LTN concentration increases. These trends are similar to those observed in literature [35-37]. However, when concentration of LTN

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was increased from 0.001 M to 0.01 M, the inhibition efficiency did not increase significantly, except at 90 oC. It can be assumed that an optimum concentration of LTN has been reached whereby increase in LTN concentration

PT

does not increase further adsorption of LTN molecules. Nevertheless, in 15% HCl, inhibition efficiency of 0.01 M is higher than that of 0.001 M HCl which also portrays the influence of concentration on the inhibition efficiency.

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At 90 oC, the steel was completely dissolved in 15 % HCl hence the inhibition efficiencies could not be estimated.

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Table 6: Corrosion rate and inhibition efficiency for the corrosion of X80 steel in 1 M HCl without and with

Test Solution

1 M HCl 0.00001 M LTN 0.0001 M LTN 0.001 M LTN 0.01 M LTN

different concentrations of LTN. 30 oC

(mmpy) 39.44 4.61 1.36 0.24 0.16

50 oC (%) 88.3 96.5 99.4 99.6

(mmpy) 47.82 9.95 6.03 0.72 0.57

70 oC (%) 79.2 87.4 98.5 98.8

(mmpy) 69.48 26.89 18.97 2.57 2.29

90 oC (%) 61.3 72.7 96.3 96.7

(mmpy) 88.41 40.14 34.57 9.90 8.58

(%) 54.6 60.9 88.8 90.3

ACCEPTED MANUSCRIPT Table 7: Corrosion rate and inhibition efficiency for the corrosion of X80 steel in 15 % HCl without and with different concentrations of LTN. Concentration (M) 0 0.001 0.01

Corrosion rate (mmpy) 30 C 50 oC 70 oC 90 oC 53.9 101.5 208.5 SL 13.7 50.4 154.1 211.6 9.6 38.9 130.5 109.4 o

Inhibition efficiency (%) 30 oC 50 oC 70 oC 74.6 50.3 26.1 82.1 61.6 37.4

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3.6 Comparative study of inhibition efficiency obtained For clarity, the values of inhibition efficiency obtained using the various methods were compared. The mean and highest deviation from the mean (H.D) of the values were also evaluated as shown in Table 8. It is clear from the table that the values obtained using the different methods were comparable and fairly stable, with maximum deviation obtained with EFM measurements. In all the measurements, the inhibition efficiency for 0.001 M and

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0.01 M LTN are almost similar which further supports that is 0.001 M could be the optimum concentration of LTN at 30 oC. Above this concentration, there is no significant increase in the inhibition efficiency. Table 8: Summary of inhibition efficiencies using various methods. PDP 86.4 97.2 98.6 99.8

LPR 86.1 97.6 98.5 98.8

M

EFM 80.2 91.3 96.8 97.3

EIS 89.9 96.7 97.5 98.3

WL 88.3 96.5 99.4 99.6

MEAN 86.2 95.9 98.7 98.8

H.D 6.0 4.6 1.9 1.5

ED

conc. (M) 0.00001 0.0001 0.001 0.01

PT

3.7 Optimization of LTN for high temperature operations The inhibition efficiency of LTN was evaluated at 90 oC in both 1 M HCl and 15 % HCl. It was observed that the

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efficiency of LTN declined at high temperature due to thermal instability. To optimize LTN for field application, LTN (at optimum concentration) was unified with some synergistic additives and the efficiency obtained is shown in Table 9. Some of the blends afforded higher efficiencies at especially in 1 M HCl due to synergistic effect. The

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performance of some of the blends in 15% HCl can be considered as good and hence can be used for field application. However, study is ongoing to improve this efficiency. Table 9: Inhibition efficiencies of the different formulations of LTN and that of a commercial inhibitor at 90 oC. Formulation Code LTN alone LTN-P LTN-G LTN-H LTN-N

Inhibition efficiency (%) 1 M HCl 15% HCl 90.3 26.1 96.7 55.7 94.8 48.5 94.4 41.7 94.7 40.8

ACCEPTED MANUSCRIPT LTN-K LTN-PK LTN-GK LTN-HK LTN-NK

93.9 99.3 98.2 97.5 97.1

43.4 89.2 87.6 83.5 82.8

3.8 Adsorption and mechanistic studies Corrosion inhibitors function by adsorption on the surface they are aimed to protect [38]. The adsorption

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mechanism of LTN was investigated by fitting the surface coverage data obtained from TGA into adsorption isotherms. The Temkin adsorption isotherm model (Fig. 5) was the best fit judged from Pearson’s and Adjusted R2 values, and standard errors, and is used to explain the adsorption of LTN molecules on X80 steel. All statistical calculations are performed using Origin 8.5 Pro package. According to the Temkin model (Eq. 27),

where

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(27)

denotes the configuration factor, which depends on the physical model and assumptions underlying

the derivation of the particular model,

is the adsorption-desorption equilibrium constant, and a is the molecular

interaction parameter also useful for predicting interactions in the adsorbed layer [38]. Linear plots of surface coverage against the logarithm of inhibitor concentration afforded data in Table 10. The constant,

M

used to describe the strength of adsorbate-adsorbent interaction. From the results,

has been

decreases as temperature

increases indicating that the strength of adsorptive binding of LTN decreases as temperature increases, perhaps are all negative, hence repulsion takes place in the adsorbed layer [38]. The

ED

due to desorption. The values of

PT

value of K is related to free energy change of adsorption (

) as shown in Eq. 28 [38]. (28)

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where 55.5 represents the concentration of water in the solution, absolute temperature. The values of

and

is the universal gas constant and

obtained are given in Table 10. The

is the

values are all

negative showing that adsorption is spontaneous. Also, the values fall within the range which can be attributed to

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chemical adsorption mechanism [39]. Thus, the adsorptive film is formed from actual coordinate covalent bonds between LTN and X80 steel surface

ACCEPTED MANUSCRIPT

1.0

0.9



0.8

0.7

30 50 70 90

0.5 -5.0

-4.5

-4.0

-3.5

-3.0

log C

C C o C o C o

CR IP T

0.6

o

-2.5

-2.0

Figure 5. Temkin adsorption isotherm.

different concentrations of LTN. Slope with error 0.04 ± 0.01 0.09 ± 0.02 0.13 ± 0.03 0.14 ± 0.03

Intercept with error 1.09 ± 0.05 1.20 ± 0.08 1.27 ± 0.10 1.29 ± 0.12

Pearson’s R2 0.8976 0.9387 0.9490 0.9396

Adjusted R2 0.7884 0.8218 0.8515 0.8244

M

T ( C) 30 50 70 90 o

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Table 10: Adsorption and statistical parameters for inhibition of X80 steel corrosion in 1 M HCl containing

F-Value (ANOVA) 8.2889 14.8368 18.1341 15.0669

-12.5 -5.6 -3.8 -3.6

K (mol-1) 6.8x1011 6.2x105 1.7x104 1.0x104

(kJmol-1) -78.73 -46.59 -40.22 -39.91

ED

3.9 Kinetic and thermodynamic considerations

As already observed, the corrosion rate increases as temperature increases. The corrosion rate data were fitted into

PT

Arrhenius kinetic model (Eq. 29) and classical thermodynamic or transition state model (Eq. 30) to further characterize the adsorption process as shown in Figure 6 and Figure 7 respectively. Based on the perception of

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activation and collision theory, before the acid solution corrodes the steel surface, molecules of the acid must collide with the steel surface molecules. The collision should generate an amount of energy up to a minimum

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threshold called the activation energy (Ea). (29)

where CR is the corrosion rate, Z the Arrhenius pre-exponential or frequency factor, R is the universal gas constant and T is absolute temperature. Linear plots of log CR against inverse of temperature (1000/T) affords Fig. 6 from which the activation energy is deduced (Table 11).

ACCEPTED MANUSCRIPT 2.0

log CR

1.5 1.0 0.5

-0.5 -1.0 2.7

2.8

2.9

CR IP T

1.0 M HCl 0.00001 M LTN 0.0001 M LTN 0.001 M LTN 0.01 M LTN

0.0

3.0 3.1 1000/T (K-1)

3.2

3.3

Fig. 6. Arrhenius plot for the corrosion of X80 steel in blank 1.0 M HCl and that inhibited by different

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concentrations of LTN at different temperatures. -0.5

-1.0

M

log (CR/T)

-1.5

-2.0

ED

-2.5

-3.5 2.8

2.9

3.0

1000/T (K-1)

3.1

3.2

3.3

CE

2.7

PT

-3.0

1.0 M HCl 0.00001 M LTN 0.0001 M LTN 0.001 M LTN 0.01 M LTN

Fig. 7. Transition state plot for the corrosion of X80 steel in blank 1.0 M HCl and that inhibited by different

AC

concentrations of LTN at different temperatures. Table 11: Parameters deduced from Arrhenius plot.

Solution Slope Concentration (with error) 1.0 M HCl -0.66 ± 0.07 0.00001 M LTN -1.78 ± 0.17 0.0001 M LTN -2.59 ± 0.27 0.001 M LTN -2.94 ± 0.44 0.01 M LTN -3.16 ± 0.11

Intercept (with error) 3.76 ± 0.16 6.54 ± 0.51 8.74 ± 0.81 9.03 ± 0.52 9.59 ± 0.33

Pearson’s

Adjusted

Z -1

0.9905 0.9908 0.9894 0.9965 0.9986

0.9715 0.9727 0.9683 0.9897 0.9964

(kJmol ) 12.64 34.09 49.60 56.30 60.51

5.75x103 3.48 x106 5.50 x108 1.07 x109 3.89 x109

ACCEPTED MANUSCRIPT The obtained activation energies in the presence of LTN are higher than in 1 M HCl. This indicates the existence of higher energy barrier that molecular collision must overcome in order to initiate corrosion process due to the inhibitive effect of LTN. The trend in our results is also in agreement with some literature reports [14]. The transition state model can be represented by Eq. 30 *( and

)

(

))+

(

)

(30)

is the enthalpy and entropy change of activation respectively. Linear plots of

against

CR IP T

where

(

inverse of temperature (1000/T) were constructed as shown in Fig. 7. The

and

values were computed

from slope and intercept (Table 12). It can be inferred from the results that the adsorption process is exothermic and results in increase in entropy of the bulk solution. Exothermicity is consistent with the nature of most adsorption processes and the range of values support physical adsorption mechanism. In addition, the positive entropy changes indicate that there could have been dissociation of molecules at the activated complex. These

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inferences are consistent (from basic thermodynamic principles) with spontaneous adsorption of LTN on steel surface earlier predicted.

Table 12: Activation parameters deduced from Arrhenius and transition state plots.

M

Intercept (with error) -2.33 ± 0.17 -6.25 ± 0.69 -8.87 ± 0.81 -10.83 ± 0.11 -11.58 ± 0.21

ED

Slope (with error) 0.52 ± 0.06 1.62 ± 0.02 2.41 ± 0.40 2.80 ± 0.04 3.02 ± 0.07

Pearson’s

0.9889 0.9898 0.9741 0.9998 0.9994

Adjusted

0.9670 0.9684 0.9233 0.9995 0.9984

(kJmol-1) -10.15 -31.66 -47.03 -53.78 -57.87

(kJmol-1) 0.66 0.87 0.99 1.10 1.43

PT

Solution Concentration 1.0 M HCl 0.00001 M LTN 0.0001 M LTN 0.001 M LTN 0.01 M LTN

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3.10 Quantum chemical assessment

The optimized structure of LTN is shown in Fig. 1(b). In order to understand the mechanism of inhibition offered by LTN on the X80 steel represented by Fe(110) surface, some quantum chemical parameters were calculated

AC

(Table 13). Since corrosion was conducted in aqueous medium under the corrosive influence of 1.0 M HCl, it is pertinent to first understand how solvent molecules affects the electronic properties of LTN. By comparing the quantum parameters of LTN obtained in gaseous state and in water, it is seen that water molecules have more influence on electronic parameters of LTN. Water molecules permeate the LTN molecule lowering the energies of HOMO and LUMO. The LUMO energies were reduced from -0.891 eV in the gaseous state to -1.140 eV in aqueous medium while the HOMO energies were reduced from a value of -5.80 eV to -5.936 eV. This causes corresponding reduction in energy gap (ΔEgap) from 4.909 eV in gaseous phase to 4.796 eV in aqueous phase. LTN therefore gains an energy of approximately 0.11 eV (2.6 kcal/mol) as it interacts with the water molecules. The energy gap is an important electronic parameter associated with the overall reactivity of the molecule. The

ACCEPTED MANUSCRIPT lower value of energy gap observed in aqueous medium implies an increase in reactivity of the LTN in water [40]. Thus, solvation by water molecules lowers the ΔEgap of LTN, generating facilitated adsorptive interaction in the aqueous medium. In other words, adsorption of LTN on X80 steel is more enhanced in the aqueous medium studied. Another significant feature of the interaction of LTN with solvent molecules is the reduction in global hardness and of course, increase in global softness of LTN. Global hardness and softness are terms used to portray resistance of atoms to the deformation of its electron cloud. Hard molecules are less reactive than soft molecules since they cannot easily give electrons to an acceptor molecule. Thus, a large energy gap exist in hard molecules

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while in soft molecules, the energy gap is low. The LTN molecule is softened as water molecules infiltrate its molecular structure and this is in agreement with the low energy gap observed in the aqueous phase. To determine whether the adsorption on Fe surface occurs via the neutral LTN (LTN) or protonated LTN (PLTN) in the acid, PA calculation was performed. For the reaction A + H+ → AH+, the energy difference between the protonated species (AH+) and parent molecule (A) is considered as PA. As shown in Fig. 1b, the different

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atoms, N(6), N(11), O(12), O(10) and O(9) have different affinity for proton. Gas phase PA calculated using B3LYP/6-311G(d,p) shows the following order: O(10) > O(12) > O(9) > N(11) > N(6). This order is derived from the PA values for each site as follows: O(10) 231.9285 kcal/mol; O(12) 217.8225 kcal/mol; O(9) 217.7542 kcal/mol; N(11) 217.7534 kcal/mol and N(6) 207.3148 kcal/mol. PA values at oxygen sites are higher than PA values at nitrogen sites. It is obvious that O(10) has the highest PA and is likely to be protonated in an acid

M

solution. But, analysis of the final optimized structure of the protonated species at the oxygen sites shows that when protonation occurs at O(9), the proton is transferred to N(11) site. When protonated at O(10), the proton is

ED

transferred to O(12) site, and when O(12) is the site of the protonation, the proton is accommodated there. Therefore, having a higher PA is not enough to decide the site of protonation, but the ability to accommodate the

PT

charge (proton) also play a vital role. Based on this, O(12) has the highest propensity to stabilize the proton and is

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the effective center for protonation.

Table 13 also includes quantum chemical parameters for P-LTN at O(12) site. The EHOMO and ELUMO of P-LTN in aqueous phase are higher than that of its gaseous phase. This is the reverse of the trend shown by LTN and may

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results from the interaction of charged species with water molecule or increased hydrogen bonding interaction. Although the HOMO and LUMO energies increased from gaseous phase to aqueous medium, the ΔEgap in the aqueous medium is lower than that of the gaseous phase in agreement with trend shown by the ΔEgap of LTN. This lower ΔEgap in aqueous medium will support facilitated adsorptive interaction in aqueous medium which is better for corrosion inhibition process.

The performance of any inhibitor can be compared using their EHOMO, ELUMO and ΔEgap values. Inhibitors with higher EHOMO values are better electron donor whereas those with lower ELUMO values are better electron acceptor.

ACCEPTED MANUSCRIPT The EHOMO of P-LTN is higher than that of LTN. Also, the ELUMO of P-LTN has a lower value compared to that of LTN. This implies that, the protonated form, P-LTN has a higher tendency to donate and accept electron during adsorptive interaction. Comparing the ΔEgap of LTN and P-LTN, it is obvious that at a first glance, P-LTN will be chosen based on the lower ΔEgap shown. But, since corrosion itself is a complicated process, other factors may come to bare and it is therefore necessary to study both species (LTN and P-LTN) side by side and compare their adsorption energies in order to determine the form that will exhibit better adsorptive/inhibitive properties on Fe(110) surface in the acid solution. For an organic inhibitor, its inhibitive interaction with a metal surface

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depends on a number of factors including presence of heteroatoms and distribution of charges. Since such interaction in acidic media more often leads to the formation of metal-inhibitor complex from freshly generated Fe2+ ions and adsorbed inhibitor (Inhads) [41], such complex could act as a protective layer for the anodic cell to limit the formation of Fe2+ sites [42]. The electrons lost by the Fe atoms are accepted into the LUMO since LUMO represents the electron acceptor ability of the inhibitor. The inhibitor can also donate electrons to the

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metal via the HOMO and such electrons enter the empty d-orbitals of the metal. The positions of LUMO and HOMO, the distribution of electrophilic and nucleophilic centers as well as Mulliken charges could help locate the adsorption sites on the inhibitor.

Charge partitioning by Mulliken method indicates C(1) as the highest positive center whereas O(12) is indicated as the highest negative center (Table 14) for both LTN and P-LTN. Fukui electrophilic and nucleophilic

M

assignments are in agreement with Mulliken charges separation for LTN which identify C(1) and O(12) with highest Fukui nucleophilic and Fukui electrophilic susceptibility, respectively. The electrophilic and nucleophilic

ED

attacks are measured by the highest values of Fukui functions. It therefore appears that these centers coordinate the LUMO (electron accepting) and HOMO (electron donating) activities in the inhibitor. To confirm this, the

PT

frontier molecular orbitals plots were examined. The HOMO and LUMO plots in Fig. 8(a-b) show that the LUMO is located primarily at C(1) while the largest HOMO is located at O(12), in agreement with Muliken atomic

CE

charge segregation and Fukui electrophilic and nucleophilic assignments for LTN. On the other hand, P-LTN shows a slightly different electrophilic and nucleophilic tendencies. C(5) is identified with the highest value of electrophilic and nucleophilic functions. This is possible since the presence of proton on O(12) causes a

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redistribution of electron densities which is concentrated around the C(4), C(5), N(6) and O(12) domain, with C(5) as the center. This is confirmed by the frontier molecular orbital plot of P-LTN as shown in Fig. 8(c-d). This domain of multi center atoms coordinate the LUMO and HOMO activities with LUMO largely influenced by C(5) whereas the HOMO is significantly influenced by O(12). The lower ΔEgap of P-LTN is satisfactorily confirmed by the location of HOMO and LUMO centers within the same domain, compared to LTN that has its LUMO and HOMO centers located at a distance. This implies that the closer the LUMO and HOMO centers are, the lower the ΔEgap.

ACCEPTED MANUSCRIPT

(b)

CR IP T

(a)

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(c) (d) Fig. 8. Frontier molecular orbitals plot of LTN at DFT/GGA-PW91 (a) LTN LUMO (b) LTN HOMO (c) P-LTN LUMO (d) P-LTN HOMO. Table 13: Quantum chemical parameters for LTN and P-LTN at geometry optimization, in gaseous and aqueous phases.

ED PT

AC

CE

EHOMO (eV) ELUMO (eV) ΔEgap (eV) IE (eV) EA (eV) χ (eV) η σ ΔN

Geometry optimization Gaseous phase Aqueous phase LTN P-LTN LTN P-LTN -5.800 -3.266 -5.936 -3.160 -0.891 -1.725 -1.140 -1.649 4.909 1.541 4.796 1.511 5.800 3.266 5.936 3.160 0.891 1.725 1.140 1.649 3.346 2.496 3.538 2.404 2.454 0.771 2.398 0.756 0.407 1.297 0.417 1.323 0.300 1.507 0.274 1.598

M

Quantum parameters

Table 14: Mulliken atomic charges, Fukui electrophilic and Fukui nucleophilic indices for LTN and P-LTN at geometry optimization.

Atom

C(1) C(2) C(3)

Mulliken atomic charges 0.557 -0.122 -0.124

LTN Fukui Fukui Atom electrophilic nucleophilic 0.019 -0.016 -0.033

0.320 -0.030 -0.025

C(1) C(2) C(3)

P-LTN Mulliken Fukui atomic electrophilic charges 0.006 0.526 -0.098 -0.005 -0.149 -0.025

Fukui nucleophilic 0.032 -0.007 -0.034

ACCEPTED MANUSCRIPT C(4) C(5) N(6) C(7) C(8) O(9) O(10) N(11) O(12)

-0.265 0.428 -0.267 -0.046 -0.264 -0.438 -0.455 -0.454 -0.555

-0.002 0.060 0.034 -0.016 -0.004 0.017 0.045 0.182 0.246

-0.008 0.005 0.002 -0.001 -0.000 0.123 0.241 0.010 0.006

C(4) C(5) N(6) C(7) C(8) O(9) O(10) N(11) O(12)

-0.183 0.258 -0.333 -0.053 -0.254 -0.476 -0.493 -0.464 -0.525

-0.029 0.246 0.121 -0.031 -0.009 0.006 0.008 0.001 0.099

-0.047 0.313 0.030 -0.034 -0.005 0.017 0.028 0.001 0.043

Parameters for H atoms not included since none was indicated with highest value of the calculated function

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Energy of Fe(110) surface

Surface energy calculation on the Fe(110) surface was performed in order to determine the stability of the surface. It provides a measure of the thermodynamic stability of the cleaved plane. The surface energy was calculated according to Eqn. 31 USE = (Usurf – Ubulk)/A

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(31)

where Usurf is the energy of the cleaved (created) surface, A is the area of the cleaved surface and Ubulk is the energy of the bulk system (Fe crystal) scaled by the ratio of the number of atoms in the surface to the number of atoms in the bulk. The results reveal that the Fe(110) surface has a surface energy of 18.29 kcal/mol. The positive value indicates endothermicity which implies that the Fe(110) surface is stable. On the other hand, materials with

M

negative value of surface energy are regarded as unstable. Rigid adsorption energy of LTN on Fe(110) surface

ED

The rigid adsorption energies of LTN and P-LTN on Fe(110) surface calculated from Adsorption Locator module is given in Table 15. The rigid adsorption energy is the energy required when the unrelaxed adsorbate components are adsorbed on the surface. It can be considered as the energy that a molecule uses to attach to the surface

PT

without being relaxed on that surface. For the same surface, rigid adsorption energy changes with temperature. Even though the rigid adsorption energy calculated showed no clear pattern on its variation with temperature, it

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can be regarded as a threshold energy used to identify whether the interaction of an inhibitor with the surface is favourable or not. If the adsorption energy or energy of interaction between the inhibitor and the metal surface is

AC

more negative compared to the rigid adsorption energy, the inhibitor can be considered a good candidate to inhibit corrosion or depletion of that surface as obtained in this study.

. Table 15: Interaction energy, binding energy and rigid adsorption energy (kcal/mol) of LTN and P-LTN on Fe(110) surface obtained from MD simulations at different temperatures LTN Temp (ºC) 30 50

Rigid Adsorption Energy -107.31 -109.39

Interaction Energy

Binding Energy

Temp (ºC)

-110.06 -110.06

110.06 110.06

30 50

P-LTN Rigid Interaction Adsorption Energy Energy -105.77 -106.06 -106.48 -107.41

Binding Energy 106.06 107.41

ACCEPTED MANUSCRIPT 70 90

-109.32 -107.70

-110.06 -110.06

110.06 110.06

70 90

-106.07 -106.09

-106.22 -107.37

106.22 107.37

Quench Molecular dynamics Having located the possible adsorption sites, the interaction of LTN and P-LTN with the Fe(110) surface can now be investigated. This interaction was modeled by Quench molecular dynamics simulation. Molecular dynamics is very useful in sampling many different low energy configurations but Quench molecular dynamics can locate the global energy minimum out of many local energy minima. The LTN and P-LTN are oriented on Fe(110) surface

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as shown in Fig. 9. This orientation represents the binding modes of LTN and P-LTN on the surface of Fe atoms and were observed at all the temperatures studied. The negative centers including the heteroatoms are all oriented favourably to donate or accept electrons from Fe atoms. This transport of electron density between Fe atoms and LTN is facilitated by the low ΔEgap already observed. The number of electrons transferred ΔN was theoretically obtained as 0.247. Since this value is less than 3.6, it is inferred that the major mechanism for the adsorption

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process is principally the donation of electrons from the inhibitor to the iron surface [16], which supports chemisorption mechanism proposed from experimental findings. The positive values of ΔN shows the tendency to donate electron to the metal surface. It is also obvious that ΔN value for P-LTN is higher than that of LTN indicating a greater tendency for P-LTN to transfer electrons. The presence of a strong feedback donation from Fe atoms to the inhibitor is very obvious and is indicated by the orientation of the negative centers of both LTN and

M

P-LTN towards Fe surface. Therefore, LTN and P-LTN may be regarded as an excellent inhibitor since they are characterized by electron donating ability (to the unoccupied orbital of the metal), and electron accepting ability

ED

(into the LUMO of the inhibitor) [43]. The interaction (adsorption) energy and binding energy of LTN and P-LTN

AC

LTN

CE

PT

on Fe(110) surface are shown in Table 15.

ACCEPTED MANUSCRIPT

CR IP T

P-LTN

Fig. 9. Side and top views of the final adsorption of the neutral and protonated forms of LTN on the Fe (110) surface

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The adsorptive interaction energy of LTN on Fe(110) surface is constant and non-temperature dependent. This was observed after a 100 ps of total simulation time that allows for full equilibration of the Fe(110)-LTN system. This constant interaction energy implies that at higher temperature, LTN can still retain its inhibitive properties without any loss. This property is very rare for an organic inhibitor as most of them deteriorate at higher temperature. For P-LTN, the interaction energy varies between 0.16-1.35 kcal/mol at 30 - 90 ºC, although no

M

trend could be discovered with respect to temperature. By comparing the adsorption energies of LTN and P-LTN to their rigid adsorption energies, it can be seen that the adsorption energies are more negative than the rigid

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adsorption energies for all the temperatures investigated. This further supports the fact that LTN and L-PTN are better corrosion inhibitor. The more negative the value of the interaction energy becomes, the higher is the adsorption ability of the inhibitor on the Fe surface. The higher adsorption ability also means better inhibition

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efficiency. Table 15 has also shown that a better inhibition efficiency proceeds via LTN since large negative values of the interaction energy were obtained at all temperatures compared to P-LTN. It is notable that P-LTN

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which showed promising quantum chemical parameters such as low energy gap, higher EHOMO and lower ELUMO became worse inhibitor due its lower binding energy on Fe surface. The binding energies of LTN at the different

AC

temperatures are higher than those of P-LTN and the high magnitude of these values indicates more stable inhibitor-surface interaction [44].

Effect of solvent on the interaction energy In order to evaluate the effect of solvent on the interaction energy of LTN on Fe surface, Quench molecular dynamics calculation was performed on an optimized system (simulation box) containing the inhibitor (LTN), water and acid according to the following composition: 171 H2O, 5 H3O+, 5Cl- and 1 LTN. This composition ensured that LTN molecule is dissolved in 15% HCl. The Fe surface was in contact with the inhibitor solution (0.00 Å thickness between Fe layer and inhibitor solution layer) and a 15 Å vacuum was added above the

ACCEPTED MANUSCRIPT inhibitor solution to avoid interaction of Fe surface on both sides. The binding mode of LTN on Fe surface in

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solution is shown in Fig, 10.

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Fig. 10. Side and top views of the final adsorption of the LTN on the Fe (110) surface in solution

In order to calculate the interaction energy, different existing models proposed by researchers were explored as

Einteraction = Etotal – (Esurface+solution + Einhibitor)

(31) (32)

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Einteraction = Etotal – (Esurface + Einhibitor + Esolution)

M

shown in Eq. 31 [45-47] and Eq. 32 [48].

The expression of Eq. 31 indicates that the surface is dissolved in the solution and this is wrong in perspective.

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For Eq. 32, the expression represents the calculation for intermolecular energy between all the molecules (species) in the system, and not interaction energy of the inhibitor on metal surface. A more satisfactory model (in terms of

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perspective) is expressed as Eq. 33 which is a variant of Eq. 31. It gives the correct perception showing that the inhibitor is dissolved in the solution to mimic the actual corrosion setup. It is notable that Eq. 31, Eq 32 and Eq. 33 do not represent accurate models for calculating interaction energy between inhibitor and metal surface in

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solution (or acid solution). It is also important to observe that Einteraction obtained from Eq. 31 and Eq. 33 do not agree in values basically due to different input of intermolecular energy (Ei) operating in each scenario. In order to obtain an accurate interaction energy (Einteraction), Eq. 33 has been modified by introducing Ei(msi) to account for intermolecular energy between all the molecules in the system and this revision is shown in Eq. 34. Ei(msi) is a correction factor for intermolecular energy involving water-water, water-acid, water-inhibitor, acid-acid and inhibitor-acid interactions. The indices ‘i’ stands for intermolecular and ‘msi’ stands for metal-solution-inhibitor system. Ei(msi) can be determined as shown in Eq. 35. Einteraction = Etotal – (Esurface + Einhibitor+solution)

(33)

ACCEPTED MANUSCRIPT Einteraction = -Etotal + Esurface + Einhibitor+solution + Ei(msi) Ei(msi)

(34)

= Etotal – (Esurface + Einhibitor + Esolution)

(35)

Therefore, Eq. 34 represents the correct expression for determining the interaction energy (Einteraction) of any inhibitor on any metal surface in solution (acid solution). Its significance is shown in the fact that only the energy of interaction between the inhibitor and the metal surface is captured, having accounted for any other interaction

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energies due to all species in solution.

Table 16 shows the Einteraction obtained from Eq. 34 at 30 ºC. These values include solvent effect on the interaction of LTN on Fe surface. The Einteraction (or binding energy) obtained without solvent inclusion (Table 15) is expected to be the maximum interaction energy possible. As solvent is introduced into the system, the inhibitor has to contend with the solvent molecules and displace them as well in order to be adsorbed on the metal surface. These

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are all energy consuming processes which cause the inhibitor to lose part of its energy. Because of this, the inhibitor binds to the metal surface with energy lower than that obtained without the solvent. It is therefore obvious that only our model agrees with these facts. The binding energy of LTN on Fe surface shown by our model is lower in the solvent inclusion system than the value obtained without solvent. The failure of other models lies in the fact that the intermolecular energy between all the species in solution was not accounted for.

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Therefore, their models still integrate these intermolecular energy which spikes the binding energy erroneously.

Table 16. Comparing existing models with our model on interaction of inhibitor on metal surface in solution at 30 ºC

Models for the calculation of interaction energy E(33)

Interaction energy

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Energy

-148.22

-592.27

-592.76

-44.47

Binding energy

148.22

592,27

592.76

44.47

E(31)

E(32)

E(34) (This studies)

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(kcal/mol)

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E(31), E(32), E(33) and E(34) are Einteraction calculated from Eq (31), (32), (33) and (34), respectively.

Adsorption mechanism The mechanism of adsorption of LTN on the X80 steel can be determined by considering the variation of the interaction energy or binding energy with temperature. As shown in Fig. 11, the variation of the binding energy with temperature indicates an interplay between two types of adsorption mechanisms. At the initial temperature of 30 °C, the adsorption mechanism is controlled basically by chemisorption. This is due to the increase in binding energy as the temperature increases, which is the behavior expected in chemisorption processes [49]. The dominance of chemisorption is seen to reach a climax at 58 °C (binding energy = 48.80 kcal/mol) and is then

ACCEPTED MANUSCRIPT taken over by physisorption at any temperature above 58 °C. In physisorption, the binding energy (inhibitive potential) is expected to decrease with temperature. Hence, the adsorption mechanism of the interaction of LTN on X80 steel is indicative of a mixed-type behaviour comprising of chemisorption and physisorption.

49.5

50, 48.5515

48.5 48

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70, 47.8956

47.5 47 46.5 46 45.5 45 44.5

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Binding energy (kcal/mol)

49

30, 44.4678

90, 44.3167

44 0

10

20

30

40

50

60

70

80

90

100

Temperaure (℃)

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Fig. 11. Variation of binding energy of LTN on Fe(110) surface with temperature

3.11. FTIR spectroscopy

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When a protective film is formed by a corrosion inhibitor on the surface it its substrate, the interaction between the film and the surface is usually facilitated by some active groups in the inhibitor. Usually, the spectrum of the

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pure inhibitor will be different from that of its surface film. The peaks corresponding to some active groups could disappear, broaden, sharpen or shift to higher or lower wave numbers [50, 51]. The obtained spectra for pure LTN and its surface film are shown in Fig. 12. The peaks around 1100 cm-1 can be assigned to C-O or C-N stretch. This

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peak shifted to 650 cm-1 indicating that there could have been slight modification due to its involvement in the adsorption process. Similar observations apply to peaks at 1360 1630 and 2930 cm-1 which can be assigned to CO stretch, C=O or N-H stretch, and -OH or C-H stretch respectively which also signifies their possible involvement in the adsorption. The peak at 3460 cm-1 which can be assigned to -N-H amine or -O-H (intermolecular bond) is found to become very broad after adsorption, a clear evidence that the sites could have been modified after taking part in the adsorption.

ACCEPTED MANUSCRIPT 50 LTN surface film pure LTN

40 35 30

620

1270

25 20 15

2930

1360 1630 1100

650

3460

780

1610

2970

1055

broad

1000

2000

3000

-1

4000

Wave number (cm )

4.

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Fig. 12. FTIR spectra off pure LTN and LTN surface film

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Transmittance (%)

45

Conclusion

L-theanine (LTN), a natural extract of green tea, was investigated as ecofriendly corrosion inhibitor for X80 steel

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in 1.0 M HCl and 15% HCl at 30-90 oC using thermogravimetric and electroanalytical techniques., complemented

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by theoretical studies. Based on results, the following conclusions are drawn: 1. LTN inhibits steel corrosion at the studied temperatures and the inhibition efficiency was higher with increased LTN concentration.

predominance.

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2. The optimum concentration of LTN is 0.001 M. The inhibitor acts as mixed type inhibitor with anodic

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3. Increase in concentration of LTN also increases charge transfer resistance and decreases both corrosion current and double layer capacitance. Temkin adsorption isotherm best describes the adsorption behavior

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of LTN as spontaneous, exothermic and resulting in increase in entropy. 4. Frontier molecular orbitals (HOMO and LUMO) energies and other theoretical parameters calculated support good adsorption properties of LTN.

5. Mullikan charge distribution and Fukui electrophilic and nucleophilic functions calculated indicate that C(5), O(12) and N(11) are the major adsorption sites for interaction with the steel surface. 6. Inhibition efficiency decrease with increase in temperature from weight loss experiment. Adsorption energies as calculated from quench molecular dynamics simulations reveals non-temperature independent.

ACCEPTED MANUSCRIPT 7. Addition of synergistic compounds such as polyethylene glycol, 5-hydroxytryptophan, N-acetyl cysteine, glutathione and potassium iodide optimized the inhibition efficiency at high temperatures close to a commercial inhibitor under similar conditions. 8. The LTN formulations reported in this study can be deployed as alternative cheap, sustainable and ecofriendly corrosion inhibitors for oilfield acidizing processes. 9. Corrosion inhibition of X80 steel proceeds via the neutral form of LTN. Protonation of LTN leads to poor adsorption and inhibition efficiency.

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10. The adsorption mechanism of LTN on X80 steel indicates a mixed-type behavior which comprises of chemisorption, dominating at lower temperature and physisorption, dominating at higher temperature. Acknowledgements

The authors are grateful to the World Bank for supporting this work through the 2015 Robbert S. MacNamara

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PhD Fellowship. We are also grateful to Materials Chemistry and Physics Laboratory, China University of Petroleum, Qingdao for hosting and providing facilities for the work. Conflict of interest

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There is no conflict of interest

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