Conversion of imidazole to N-(3-Aminopropyl) imidazole towards

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Materials and Interfaces

Conversion of imidazole to N-(3-Aminopropyl)imidazole towards enhanced corrosion protection of steel in combination with carboxymethyl chitosan grafted poly(2-methyl-1-vinylimidazole) Ubong Eduok, Enyinnaya Ohaeri, and Jerzy Szpunar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00378 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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Industrial & Engineering Chemistry Research

Conversion of imidazole to N-(3-Aminopropyl)imidazole towards enhanced corrosion protection of steel in combination with carboxymethyl chitosan grafted poly(2-methyl-1-vinylimidazole) Ubong Eduok*, Enyinnaya Ohaeri, Jerzy Szpunar Department of Mechanical Engineering, College of Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, S7N 5A9, Saskatchewan, Canada. _____________________________________________________________________________________ Abstract In this study, the synthesis route for preparing N-(3-Aminopropyl)imidazole (APM) from imidazole has been reported via hydrogenation using Raney®-Nickel catalyst. The inhibition performance of the resultant imidazoline product against pipeline steel corrosion has been investigated in CO2 saturated acidic oilfield formation water. The contributions of carboxymethyl chitosan grafted poly(2-methyl-1-vinylimidazole) copolymer (polyCMCh-graft-polyMVI) newly synthesized from chitosan revealed a synergy of protective capacity in combination with APM. Corrosion inhibition by APM/graft copolymer conjugate is attributed to molecular adsorption and the formation of more stable protective inhibitor films on the steel surface compared to individual compounds. In-depth electrochemical corrosion investigation is reported by mean of electrochemical impedance spectroscopy and potentiodynamic polarization as well as surface analytical techniques. Adsorbed ferrous carbonate phases are observed at acid pH with apparently amorphous morphologies. Modelled mechanisms of corrosion and corrosion inhibition has been illustrated from experimental evidences in line with the barrier performance on adsorbed films. Keywords: Sweet corrosion; Pipeline steel; Imidazole-chitosan conjugate; Corrosion inhibitors; Carbonate scales; Raney®-Nickel catalyst *Corresponding author. Tel.: +1 (306) 966 7752. Fax: +1 (306) 966 5427 ([email protected]; [email protected]) 1.

INTRODUCTION

High-strength carbon steel is a suitability appropriate pipeline material utilized in oilfield applications. Their high-demand for conveying oil and gas products in the petroleum industries has contributed to enormous economic advantages worldwide.1 Over the years, the strengths of most high-grade pipeline steel materials have been improved to reduce construction cost as well as foster transportation efficiency.2 Modification of wall thicknesses has also added to their versatility when used to convey contents with largeflow and high pressure.3 To improve their performances, most pipeline materials are also subjected to some processing techniques. Thermo-mechanically controlled processing and micro-alloying are examples of the most recent techniques utilized towards enhancing the mechanical strengths, deformability and corrosion resistance of pipeline materials.4 However, pipeline failure continues to be a problem in modern times due to corrosion. Carbon dioxide is also another causative agent and contributing factor towards corrosion of pipeline steel since it is a byproduct of oil and gas production. This gas is even more aggressive when dispersed in aqueous brine rich in chloride/sulphates ions and acetic acid.5 The rate of pipeline steel corrosion may even worsen depending on the concentrations of these ions within oilfield formation water systems. In most extreme cases, erosion of the internal structures intensifies material loss due to factors related with high-pressure dynamic conditions (e.g. turbulence). Prolong and untreated corrosion cases may 1 ACS Paragon Plus Environment

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lead to huge damages and the eventual leakage of the pipelines. This directly affects production and gross profits due to economic loss with adverse environmental implications.6,7 With acetic acid (in oilfield formation water) causing significant corrosion problems due to unrestricted cathodic and anodic corrosion processes in pipelines, there is a need for corrosion mitigation.6 The inherent presence of organic acids during oil production makes CO2 corrosion a complex problem. The design of corrosion inhibitor formulations using aromatic cyclic organic compounds with heteroatoms (nitrogen, oxygen, sulfur, phosphors, etc.) has assisted in protecting pipelines against CO2 saturated brines.8-11 Some of these compounds are derivatives of simple amide and imidazole, and they are mostly utilized due to their unique film-forming and adsorptive abilities on metal surfaces.5 The use of imidazoles as corrosion inhibitors have been widely reported within the literature,12-15 to mention but a few. The structures of imidazoles make them a unique class of compounds with anticorrosive potentials. They can simultaneously bond on metal surfaces via their aromatic rings and lone sp2 electron pair on their hetero N atoms.16 Imidazoles are soluble in most media and readily form highly stable films on metal surfaces. Chitosan and its derivatives are another class of effective corrosion inhibitors with potentials for industrial applications due to their unique adsorption patterns on surfaces and impedance against corrosive species. They are also benign and green with no adverse impacts on the environments. More compounds could also be derived from chitosan by introducing more reactive functional groups via its amine group,17 and a few them have been recently reported with protective properties for steel.18-20 However, without the synergistic corrosion inhibitive effects of most imidazoles (as well as other organic compounds) with other additives capable of forming stable protective films, their usage as single molecules towards corrosion inhibition would eventually fail. The combination of principal inhibitor molecules and these additives results in composites with combined inhibitive properties when compared with each of them. These new hybrid protective films then chemisorb on surfaces, forming strong bonds even for deprotonated molecules.21 Most additives readily enhance surface adsorption of organic inhibitor molecules by forming intermediate bridges with charged metal surfaces, hence, improving corrosion inhibition.22 In this work, the synergistic inhibiting performance between N-(3-Aminopropyl)imidazole (APM) and poly (2-methyl-1vinylimidazole) grafted carboxymethyl chitosan is investigated. The synthesis route for preparing APM from imidazole via hydrogenation using Raney®-Nickel catalyst has been reported. To the best of our knowledge, this is the first featured report using the resultant imidazoline derivative from this synthesis route as an inhibitor against pipeline steel corrosion in CO2 saturated acidic oilfield formation water. The contributions of carboxymethyl chitosan grafted poly(2-methyl-1-vinylimidazole) copolymer (polyCMChgraft-polyMVI) newly synthesized from chitosan towards improved corrosion inhibition is also investigated within the corrosive aqueous system. Inhibition performance is probed by means of 2 ACS Paragon Plus Environment

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electrochemical analyses and in-depth surface characterization. It was pertinent to study the effect of corrosion of pipeline steel in oilfield produced-water since its formation during petroleum exploration processes adversely affects industrial processing. Using efficient corrosion inhibitor formulations is a step towards solving corrosion menace. This report, mimicking actual conditions during oil and gas transportation, is our contribution towards that noble research goal.23 2.

EXPERIMENTAL PROCEDURES

2.1.

Materials

The steel substrate studied within this work was donated by EVRAZ North America. It belongs to X70 grade in line with API (American Petroleum Institute) 5L specification for line pipes. It was cut into smallsized coupons (3 cm × 3 cm ×1 cm) with one masked surface at exposure area of 1 cm2. The steel substrate has the elemental composition: C (0.047%), Mn (1.65%), S (0.0018%), P (0.009%), Si (0.18%), Cu (0.29%), Ni (0.07%), Cr (0.06%), V (0.001%), Nb (0.073%), Mo (0.247%), Sn (0.01%), Al (0.044%), Ca (0.0014%), B (0.0001%), Ti (0.022%), with Fe making up the balance. The steel coupons were abraded with different grades of silicon carbide abrasive paper (400, 600, 800, 1000, 1200, 2400, 4000) before deploying the final cleaning procedure using 3 and 1 μm diamond-paste suspensions. These abraded coupons were repeatedly washed with anhydrous ethanol then degreased and sonicated in ethanol/acetone (30:70) mixture before drying with warm air. Each coupon was used for each corrosion test without replacement. 2.2.

Reagents and chemicals

All reagents used in this study are analytical grade purchased from Sigma Aldrich; they were used aspurchased without further purification. Imidazole (purity ≥99.5%) was purchased from the same outlet to synthesize N-(3-Aminopropyl)imidazole. Other chemicals utilized in this synthesis included Raney®Nickel catalyst (Al/Ni), methanol (purity ≥99.9%), ammonium hydroxide (purity ≥99.99%), acrylonitrile (purity ≥99%), tetrahydrofuran (purity ≥99.9%). 2-Methyl-1-vinylimidazole (99.9% purity) monomer and chitosan (Ch; degree of deacetylation 75%) were purchased to synthesize polyCMCh-graft-polyMVI. It was purchased alongside chloroacetic acid (99% purity) and potassium persulfate (99.99% trace metals basis). 2.3.

Microstructural characterization of test substrate

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The starting microstructure on the RD-TD plane of pipeline steel was investigated by standard metallographic analysis using a piece of 10 mm (RD) x10 mm (TD) x 12.8 mm (ND) specimen. First, the piece of sectioned sample was hot mounted in Bakelite and abraded with up to 4000 grit silicon carbide abrasive. Further polishing was done using 3 μm and 1 μm diamond suspension medium. The abraded samples were then etched in 5% Nital solution for 50 sec before examining the steel under an Olympus BXKMA-LED Optical Microscope (OM) and a Field Emission Hitachi SU6600 Scanning Electron Microscope (SEM). In-depth microstructural and crystallographic texture analyses were conducted with Electron Backscatter Diffraction (EBSD) and Energy Dispersive X-ray Spectroscopy (EDS) detectors coupled to the SEM. EBSD scans were recorded with an accelerating voltage of 30 kV. Electron diffraction patterns were acquired on Oxford Instrument’s AZTEC 2.0 software suite using a step size of 0.14 μm, and post-processed with the Channel 5 software. Also, grain boundary distribution with misorientation angle (θ) in the range of 5° < θ < 15° were categorised as low angle grain boundary (LAGB), whereas 15° < θ < 62.5° were regarded as high angle grain boundaries (HAGB). Sub-grains were considered to be separated by misorientation angle (θ) ranging from 1° < θ < 7.5°, while grains are separated by θ > 7.5°. The kernel average misorientation (KAM) measurements covered a range of θ < 5°. 2.4.

Synthesis of N-(3-Aminopropyl)imidazole

APM was synthesized by catalytic hydrogenation of imidazole using Raney®-Nickel catalyst in the presence of methanol and ammonium hydroxide after reacting imidazole with acrylonitrile. The reaction scheme is presented in Figure 1 as reported by Wright, jr. et al.24 with some modifications. In this work, a 50:50 mixture of imidazole and acrylonitrile was stirred at 50 oC for 5 h in a Parr high pressure reactor system (Parr Instrument) equipped with magnetic drive and turbine agitator. The pressure of the system was later reduced to concentrate the resultant product suspension, in turn removing excess acrylonitrile. A 500 ml ammonium hydroxide was then added with 500 ml methanol alongside 10g catalyst. Following a 6-h continuous hydrogenation, this reaction was allowed within the reactor until hydrogen uptake ceased; the agitation speed and temperature were monitored by a controller (4842). A 400 ml tetrahydrofuran was introduced before filtering the reaction suspension after 6 h. Toluene was utilized in concentrating the aminopropyl oil filtrate and stored appropriately. Prior to the reaction, the reactor was charged with the reactants before purging it with nitrogen to ensure atmospheric inertness and heated to the desired temperature. No further purification procedure was carried out before NMR and FTIR characterization. The as-synthesized product has the boiling point of 300oC, slightly denser than the pure compound (1.85 g/ml at 30oC) and soluble in water. 2.5.

Synthesis of polyCMCh-graft-polyMVI 4 ACS Paragon Plus Environment

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To synthesize polyCMCh-graft-polyMVI, 4.5 g chitosan was reacted with chloroacetic acid in 120 ml 20% NaOH for 45 mins. The temperature of the solution was slightly raised to 40 oC and allowed to stir for six more hours. The pH of the reaction mixture was then slightly reduced (pH 4) by neutralization using 50 ml acetic acid (10%). It was sparged with nitrogen gas before adding 3 g 2-methyl-1-vinylimidazole monomer at 60 oC and kept stirring for 45 min. The stirring speed was increased during the addition of 1 g potassium persulfate initiator; it was dissolved within the aqueous suspension to initiate graft copolymerization before concentrating the polymeric product using acetone. The precipitated polyCMCh-graft-polyMVI was aged for 10 h and then pass through Wheaton soxhlet apparatus for extraction, re-washed in acetone and vacuumdried. Earlier studies on the synthesis of imidazole-carboxymethyl chitosan graft copolymers have been reported elsewhere.25,26 The proposed formation scheme for the graft copolymer from chitosan is presented in Figure 2. 2.6.

NMR and FTIR analyses

Before characterization of the reaction products from the two synthesis routes, their solubilities in polar solvents were tested. The degree of product conversions from precursors was monitored using Nuclear magnetic resonance (NMR) spectroscopy and this technique was also used to confirm their proposed molecular structures. This work presents comparative spectra for principal reactants and their products, recorded using an Avance III HD 600 MHz NMR Spectrometer (Burker, US). Spectroscopy analysis also includes functional group studies using Fourier Transform Infra-red Spectroscopy (FTIR) with the aid of a Bio-RAD FTS-40 spectrophotometer (Bio-Rad Lab) equipped with a DTGS KBr detector. FTIR spectra were recorded in absorbance mode at 64 scans. 2.7.

Electrochemical corrosion tests

EIS measurements were conducted in-situ during the immersion tests with the three-electrode system with the steel substrate in within the corrosive media. The corrosion medium was prepared to depict formation water in oil production. It consisted of NaCl (82.4 g), CaCl2 (1.3 g), MgCl2.6H2O (12.6 g), NaHCO3 (1.1 g), Na2SO4 (2.6 g) per litre water containing 3 mM acetic acid. Appropriate concentrations of APM and APM/ polyCMCh-graft-polyMVI were then dispersed within the media and utilized as corrosion inhibitors. Before the corrosion tests, this medium was purged and saturated with 99.99% CO2 for 4 h; this was maintained throughout the durations of all corrosion tests at 60 oC. Accompanying EIS spectra were recorded by applying a 10-mV ac potential of a rms amplitude around the Eoc on steel samples between 100 kHz and 1 mHz. Potentiodynamic polarization experiment was also conducted by applying electrical potential range between ‒0.25 and +0.25 V vs open circuit potential (Eoc) at a scan rate of 0.5 mV/s after 5 ACS Paragon Plus Environment


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the Eoc reached a steady-state state. Before fitting appropriate equivalent circuit models to the experimental impedance data, their linearity was by Kramers-Kronig transformation. The three-electrode system utilized in these electrochemical measurements consist of steel coupons as working electrode; Ag/AgCl (sat. KCl) and

platinum

rod

as

reference

and

counter

electrodes,

respectively,

connected

to

a

Potentiostat/Galvanostat/ZRA (Interface 1000, Gamry Instruments) electrochemical workstation. The measured values of potentials within this study were collected relative to the reference electrode. Analyses of electrochemical results was conducted using the EChem Analyst software. 2.8.

Metal surface analyses after corrosion test

Scanning electron microscopy was utilized in studying the extent of surface corrosion of steel coupons. These surface analyses were examined using an SEM Hitachi SU6600 scanning electron microscope (Hitachi High-Tech.) on steel substrates prior to and after exposed to CO2 saturated acidic oilfield formation water for 12 h. Evidence of adsorption of protective passive film on the surface of X70 substrate within the corrodent was also probed by SEM. Coupled with Energy Dispersive X-ray analyzer, SEM also qualitatively mapped, the elemental composition of the adsorbed films on steel. The SEM analysis was corroborated with Atomic force microscopy (AFM) using a PicoSPM instrument (Molecular Imaging) with the aid of a gold-coated Si cantilever at intermittent contact. Magnitudes of resonance frequency and spring constant stood at 26 kHz and 1.6 N/m, respectively, for all topological images of representative steel surfaces. This work also presents maximum peak-to-valley height analyses of topological AFM micrographs of impacted steel surfaces. Analyses of the composition of the adhering corrosion product aggregates on steel substrates after the corrosion test were conducted using X-ray photoelectron spectroscopy (XPS, Kratos AXIS Supra X-ray Photoelectron Spectrometer). XPS spectra were collected using spectrometer coupled with an Al K𝛼 X-ray source (0–1200 eV at an emission angle of 90°). Widescan and high-resolution XPS spectra within this work were deconvoluted and appropriately analyzed by means of Gaussian–Lorentzian combination with the aid of a CasaXPS software. A Data Physics contactangle measurement equipment was utilized in recording the aqueous contact-angle values (degree unit) of impacted and unimpacted metallic surfaces using pendant drop method. The results presented within this work are mean values of five trials of 5 μl droplets collected at different spots on respective metallic surfaces. 3.

RESULTS AND DISCUSSION

3.1.

Microstructural evaluation


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Low-alloyed thermomechanically processed pipeline steel display complicated microstructures, which often contain constituents such as polygonal ferrite, acicular ferrite, martensite/retained austenite, quasipolygonal ferrite and pearlite. The initial microstructure of as-received X70 pipeline steel is revealed in Figure 3. The SEM (Figure 3 a-b) and optical (Figure 3 c) micrographs indicate large presence of ferrite, with grain sizes smaller than 5 μm. Some whitish carbide precipitates are seen around the grain boundaries in the higher resolution SEM image (Figure 3 b), with sparse appearances of pearlitic grains. In Figure 3 c, the dark features indicate martensite/retained austenite microconstituents. Additionally, precipitate particle found within the steel structure was analysed by EDS. The maps corresponding to Figure 3 d are evidence that the particle contain Ti-N-O. Meanwhile, there are proof of several kinds of inclusion particles in pipeline steel, mainly sulphide, and oxides of elements such as Ti, Ca, Mg, Al.27,28 These multi-component fragments often found in X70 pipeline steel are present due to the complexities associated with microalloying. Interestingly, the location of some inclusion components has been reported as preferred site for different forms of corrosion; most especially localised/crack related corrosive degradation.29-32 A recent work33 established that particles of TiN as well as 𝛿-ferrite as quite unfavourable for corrosion resistance in 321 stainless steel. The reason being that incoherence between these inclusions and the steel matrix could result in porosity or voids, which will eventually become preferred sites where corrosion could begin. Moreover, the galvanic couple created between these particles and neighbouring steel structure can help in exacerbating corrosion. The EBSD results for the steel considered in this study are shown in Figure 4. The IPF color map in Figure 4a represents different orientation of grains. Various colors featuring on the map suggest that grains are randomly oriented, with mainly HAGBs and some LAGBs (Figure 4b). Although the steel investigated in this study is not textured with any specific crystallographic orientation, it is worth mentioning that {111} planes oriented parallel to the surface have shown great potentials in improving corrosion resistance [3436]. Atoms are densely packed in a {111} textured structure, which might be the reason for its reduced corrosion behaviour compared to other planes such as {001} and {011} with loosely packed atoms. Therefore, removal of atoms during he process of corrosion is more difficult when they are tightly packed within the lattice and vice versa. The {001} and {011} orientation are usually attributed to high surface energy, due to their reduced atomic density.37 Arafin and Szpunar35 observed that the resistance of {111} grains to stress corrosion cracking relates to the type of boundaries present around the region. They noticed that LAGB and coincident site lattice (CSL) boundaries offered better resistance to failure than HAGB. In the current study, pipeline steel featured primarily non-equiaxed grains; and Figure 4c confirms that the structure is highly recovered. However, few recrystallized (blueish) and deformed (reddish) regions are also seen. The highly deformed grains in Figure 4c corresponds to those with relatively high degree of 7 ACS Paragon Plus Environment


misorientation in Figure 4d. Such higher KAM regions can be regarded as an indirect measure of dislocation density or stored strain due to deformation, which might help facilitate crack related degradation in pipeline steel.38 3.2.

Structural elucidation: NMR and FTIR analyses

The percentage yields for APM and polyCMCh-graft-polyMVI products were 80 and 85%, respectively. The molecular structures of these compounds were appropriately characterized; their NMR spectra are presented in Figure 5. The chemical shifts of few protons on these products and their precursors are marked in these spectra. It could be observed that the proton peak around 11 ppm (assigned to imidazoline N-H) has disappeared on the 1H NMR spectrum of the product (Figure 5 b). This could be ascribed to the addition reaction between C=C (on acrylonitrile) and N-H (on imidazole).39 It is also important to note that peaks corresponding to protons b and c are labelled as H1 and H2 on the NMR spectrum of the as-synthesized polyCMCh-graft-polyMVI copolymer (Figure 6a). The broadened peaks between 1 and 3 ppm could be due to inherent interactions between protons within the polymer chain while other peaks common with the chitosan molecule (in Figure 6 a) can also be observed in Figure 6b. The FTIR spectra of as-synthesized products and their precursors are represented in Figure 7. IR absorption peaks characteristic of imidazole rings are common amongst in Figure 7 (a). The doublet at 3400 cm─1 denotes N-H stretching vibration; this peak also overlaps with the O-H between 3000 and 3400 cm─1 in Figure 7 (b). The stretching vibrations of C=N absorbs at 1491 and 1320 cm−1 while peaks between 1200 and 1500 cm─1 could be assigned to C=C and C-N bending vibrations of the imidazoline ring on both IR spectra. The presence of peaks at 750 and 1374 cm─1 are due to N-H wag and C=C ring stretching vibration and ring deformation, respectively.40 Other IR vibration signals corresponding to imidazole are bending and out of plane bending between 850 and 1444 cm─1, C-H stretching vibrations between 2800–3100 cm─1, C-H in-plane bending between 1100 and 1150 cm─1.41 The IR spectrum presented in Figure 7(b) reveal absorption peaks common with chitosanic moiety for polyCMCh-graft-polyMVI product and its chitosan precursor. There are peaks corresponding to the glycosidic bond (C-O-C) between 900 and 1100 cm−1 and the glucosamine’s carbonyl group at 1640 cm−1 assigned to amide Ι and II bands.42 There are also C─N axial stretching, N-H deformation, C-H stretching vibration peaks at 1420, 1380, 2900 cm─1, respectively. Figure 8 depicts the SEM micrographs of the as-prepared graft copolymer with an amorphous powdery morphology compared to its flaky and physically sub-fibrous chitosan precursor (see inset). The solubility of this graft copolymer could, in part, be ascribed to this unique surface structures with densely packed morphology after grafting 2-methyl-1-vinylimidazole to the carboxymethyl chitosan chain. One gram of this powder was completely soluble in 100 ml water and also in deionized water containing 10-20% acetic 8 ACS Paragon Plus Environment

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acid at room temperature. As expected, the graft copolymer product from this catalyzed hydrogenation reaction was soluble in water and other polar solvents, including ethanol. 3.3.

Effect of APM and polyCMCh-graft-polyMVI concentrations on X70 steel corrosion

The gradual accumulation of protective layers on metal surfaces due to molecular adsorption could be analyzed by electrochemical impedance spectroscopy. In this study, the corrosion resistance of the adsorbed APM and polyCMCh-graft-polyMVI films on X70 steel surface was probed in CO2 saturated oilfield formation water. Representative impedance curves are presented in Figure 9 for different concentrations of APM and the graft copolymer. The curves reveal single-time constant capacitive loop in high frequency regions; this is related with the charge transfer and diffuse layer resistances.43,44 Impedance curves with wider diameters are observed for steel coupons within corrosive medium with higher APM and graft copolymer concentrations; this is an indication of corrosion inhibition as protective inhibitor films form on the metal surfaces.45,46 Steel corrosion is influenced by the charge transfer process as more inhibitor molecules are transported to adsorb at the metal/solution interface.47,48 To further analyze the trend of electrochemical parameters, the impedance curves were fitted into appropriate electrical equivalent circuit model (Rsoln(Qdl(Rct))) and the parameters were obtained are presented in Table 1. Rsoln and Rct denote solution and charge transfer resistances, respectively, while Yo and 𝛼 are associated with the properties of electroactive species and phase shift (─1 ≤ 𝛼 ≤ 1), respectively. The magnitude of charge transfer resistance (Rct) is observed to increase with APM and polyCMCh-graft-polyMVI concentrations within the acid corrodent as mass transports initiate the formation of passive film on the metal surface.47 Values of Rct up to 135.6, 200.4, 225.6 Ω cm2 were recorded for steel exposed to the corrodent containing 100, 250 and 500 ppm APM, respectively, relative to the substrate within the bare acid medium (89.9 Ω cm2). Values of Rct are higher for polyCMCh-graft-polyMVI: 260.2, 300.5, 310.4 Ω cm2 were recorded at 50, 100, 200 ppm concentrations. Corrosion inhibition is due to the buildup of resistance at the metal/solution interface.45,46 Changes in the magnitude of CPE (𝑌𝑜) with both inhibitor concentrations was also monitored. A steady reduction in this parameter is observed at higher ppm concentrations and the events leading to this could be related with the displacement of adsorbed molecular water by accumulating APM and the graft copolymer species on the steel surfaces. Adsorption of inhibitor species led to the isolation of the metal surfaces from further corrosion. The electrochemical impedance curves were recorded for respective systems at steady open circuit potential (Eoc). Figure S1 represents the trend of Eoc values for a 30-min duration. Most positive Eoc values are observed for more inhibitive systems, and this trend may denote reduction towards localized corrosion attack. The corresponding values of corrosion protection efficiency (𝜂%) were computed from 𝑅𝑐𝑡 using the relation: (𝜂% =

𝑅𝑐𝑡 ― 𝑅𝑜𝑐𝑡 𝑅𝑐𝑡

× 100); these values are presented in Table



1. It can be observed that 𝜂% increases with APM and polyCMCh-graft-polyMVI concentrations up to 66 and 75% for 500 and 200 ppm, respectively. In this equation, 𝑅𝑜𝑐𝑡 and 𝑅𝑐𝑡 denotes charge transfer resistance (𝑅𝑐𝑡) in the absence (blank) and presence of the inhibitor molecules. Unlike electrochemical impedance spectroscopy, potentiodynamic polarization technique assesses the kinetics of electron transfer during anodic steel dissolution and accompanying cathodic reactions within the metal substrates exposed to corrosive solutions.49 The Tafel polarization curves recorded for X70 metal substrate exposed to CO2 saturated oilfield formation water containing different APM and polyCMChgraft-polyMVI concentrations are presented in Figure 9 (b). From these polarization curves, values of corrosion current density (jcorr) and potential (Ecorr) as well as Tafel slopes (anodic (βa) and cathodic (βc)) were extrapolated via Tafel fitting and presented in Table 1. When compared with the values of jcorr for the bare steel within the corrosive electrolyte, those for both inhibitor compounds showed steady reduction with increasing concentrations. Values of jcorr for steel exposed to corrodent containing the graft copolymer are less than that of APM. This trend is suggestive to increased corrosion reduction as more polyCMChgraft-polyMVI molecules adsorbed on the metal substrate, in turn, blocking approaching corrosive species. Values of jcorr up to 200.0, 189.0, 109.0 µA cm−2 were recorded for steel exposed to the corrodent containing 50, 100 and 200 ppm polyCMCh-graft-polyMVI, respectively, relative to the substrate within the bare acid medium (909.0 µA cm−2). Higher jcorr values are recorded for 50, 100, 200 ppm APM concentrations: 390.0, 300.0, 269.0 µA cm−2. The corresponding values of 𝜂% were also computed for this technique from jcorr using the relation: (𝜂% =

𝑗𝑜𝑐𝑜𝑟𝑟 ― 𝑗𝑐𝑜𝑟𝑟 𝑗𝑜𝑐𝑜𝑟𝑟

× 100); these values are presented in Table 1. It can be observed that

𝜂% increases with APM and polyCMCh-graft-polyMVI concentrations up to 70 and 88 % for 500 and 200 ppm, respectively. In this equation, 𝑗𝑜𝑐𝑜𝑟𝑟 and 𝑗𝑐𝑜𝑟𝑟 are the values of corrosion current density in the absence (blank) and presence of the inhibitor molecules. Magnitudes of Ecorr are more positive with increasing concentrations of both inhibitor compounds and this is suggestive of alteration in anodic dissolution. Adsorption of APM and the copolymer on the steel predominantly influences anodic dissolution.50,51 Generally, metal surface molecular adsorption is dependent on factors associated the pH of solution, type of metal and corrosive medium, their natures, inhibitor concentrations and the functional groups within them.49 The trend of results from electrochemical impedance spectroscopy and potentiodynamic polarization techniques suggests that APM and the graft copolymer acted as corrosion inhibitors, however, the graft copolymer inhibited steel corrosion more than APM within the range of concentration under study. This could be attributed to enhanced surface coverage by the chitosan-based polymer after adsorption compared to the simple APM imidazole derivative. The adsorption of most polymers normally will restrict the extent of anodic dissolution and/or deters associated 10 ACS Paragon Plus Environment

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cathodic reactions by active site blocking due to the formation of protective polymer layers on the metal surfaces. Corrosion inhibitor is fostered by physical coverage and molecular adsorption while the mechanisms of protection are influenced by factors surrounding macromolecular weights, cyclic rings, chemical composition and their unique molecular and electronic structures of these adsorbed species. The corrosion inhibiting ability of APM and the graft copolymer could be linked to their unique molecular structures. Metal surface adsorption could be achieved via their electron-rich heteroatoms (O and N), capable of electron transfer and covalent bond coordination with the empty or partially occupied Fe orbitals on ferrous substrates.52 3.4.

Combined corrosion inhibitive performance between APM and polyCMCh-graft-polyMVI

compounds As observed from the previous section, the adsorption of APM and polyCMCh-graft-polyMVI have significantly reduced X70 steel corrosion; better protection was achieved at 200 ppm polyCMCh-graftpolyMVI concentration. Corrosion resistant is attributed to molecular adsorption at the metal surface against its spontaneous dissolution within the CO2 saturated oilfield formation water. To enhance corrosion protection, the inhibiting performances of both compounds combined were investigated relative to APM and graft copolymer studied individually. Respective EIS parameters derived from Nyquist curves (Figure 10) are presented in Table 1 for X70 steel substrates exposure to the corrosive solutions containing 500 ppm APM in combination with different concentrations of the graft copolymer. Wider Nyquist curves diameter are observed for inhibitive systems with superior protection. Just like the impedance curves for the singular inhibitor systems (Figure 9), these ones also show some depressions due to lack of surface homogeneity from surface polishing and distortions on the double-layer. To account for these surface defects, constant phase element (CPE) was incorporated into the equivalent circuit model to appropriately fit the data.53 CPE also accounts for the deviation from ideal dielectric behavior arising from the roughness of the electrode surface. The CPE impedance is expressed as 𝑍𝐶𝑃𝐸 = 𝑌𝑜―1(𝑗𝜔)

―𝛼

; j is equivalent to 1

while 𝜔 is the angular frequency (measured in rad/s). Higher values of Rct were derived from impedance spectrum for APM/ polyCMCh-graft-polyMVI hybrid systems due to their enhanced protective capacities and at higher copolymer concentrations against steel corrosion. This denotes synergism, as greater total inhibition effect is attained compared to one of inhibitors investigated independently. The observed superior protective performance could be attributed to the formation of more stable organic-inorganic APM/polyCMCh-graft-polyMVI hybrid film clusters. This confirms the synergistic inhibition effect between APM and polyCMCh-graft-polyMVI.46 The magnitudes of Rct for the adsorbed hybrid films with 50, 100 and 200 ppm polyCMCh-graft-polyMVI additive (in combination with 500 APM) are 365.1, 450.8, 11 ACS Paragon Plus Environment


590.4 Ω cm2, respectively, as against 225.6 and 310.4 Ω cm2 observed for 500 ppm APM and 200 ppm polyCMCh-graft-polyMVI alone. The presence of polyCMCh-graft-polyMVI within the hybrid conjugate has enhanced the inhibitive performance for X70 steel in CO2 saturated oilfield formation water. Some inhibitor additives enhance surface adsorption of organic inhibitor molecules by forming intermediate bridges with charged metal surfaces, hence, improved corrosion inhibition.54 Co-adsorption of inhibitor species could foster enhanced corrosion protection on metal surfaces. As one charged inhibitor molecule adsorbs via electrostatic attraction, this enables the formation of ion-pairs with the other.54 Corrosion inhibition in the presence of APM on X70 steel could have been enhanced as the presence of the graft copolymer synergistically aided corrosion reduction via formation of more stable protective film on steel. While Rct increased with increasing polyCMCh-graft-polyMVI concentration within the hybrid APM/polyCMCh-graft-polyMVI conjugate, Qdl decreased in that order. The magnitudes of Qdl for the adsorbed hybrid films with 50, 100, 200 ppm graft copolymer additive (in combination with 500 APM) are 129.7, 118.8, 100.9 µF cm−2 s−(1−αc), respectively, as against 198.2 and 159.1 µF cm−2 s−(1−αc) observed for 500 ppm APM and 200 ppm polyCMCh-graft-polyMVI alone. Reduction in CPE denotes that the presence of polyCMCh-graftpolyMVI increased the thickness of the adsorbed protective layer compared to 500 ppm APM alone. This decrease is due to inhibitor adsorption, and it is also in line with modification of the electrical double-layer thickness as well as decrease in the local dielectric constant.54 This trend is consistent with the Helmholtz model given as 𝑄 = 𝜀𝜀𝑜𝐴/𝛿; 𝜀 and 𝜀𝑜 denote the medium’s dielectric constant and vacuum permittivity, respectively; A and 𝛿 are the area of the electrode and the thickness of the protective layer, respectively. Tafel curves of X70 steel substrates exposure to CO2 saturated oilfield formation water modified with 500 ppm APM in combination with different concentrations of polyCMCh-graft-polyMVI additive at room temperature are also presented in Figure 10 (b). From the parameters presented in Table 1, the magnitude of jcorr for X70 substrate significantly reduced with increasing polyCMCh-graft-polyMVI concentration within the corrosive medium relative to the metal substrate in 500 ppm APM. The observed decrease could be associated with the blocking of metal surface by the adsorbed APM/polyCMCh-graft-polyMVI conjugate. Values of jcorr up to 85.0, 59.0, 30.0 µA cm−2 were recorded for steel exposed to CO2 saturated oilfield formation water containing 50, 100 and 200 ppm polyCMCh-graft-polyMVI additive (in combination with 500 APM), respectively, relative to 269.0 and 109.0 µA cm−2 observed for 500 ppm APM and 200 ppm graft copolymer alone. Magnitudes of Ecorr are more positive with increasing polyCMChgraft-polyMVI concentrations within the APM/polyCMCh-graft-polyMVI conjugate and this is suggestive alteration in anodic dissolution during the corrosion of steel substrate. Adsorption of the hybrid inhibitor 12 ACS Paragon Plus Environment

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conjugate on the steel mostly altered anodic dissolution. The corresponding values of 𝜂% were also computed from results obtained from the two electrochemical techniques for APM in combination with polyCMCh-graft-polyMVI copolymer. It can be observed that 𝜂% increases with polyCMCh-graftpolyMVI concentration blended with APM (see Table 1), denoting improved corrosion protection between both compounds. To determine the nature of interaction between APM and polyCMCh-graft-polyMVI as corrosion inhibitors for API 5L X60 steel in CO2-saturated corrodent, the synergistic parameter (𝑆1) was calculated using the relation: (𝑆1 =

1 ― (𝜂1 + 𝜂2) 1 ― 𝜂𝜄1 + 2

× 100) and presented in Table S1. These values are found to

be greater than unity, denoting that the observed superior protective performance could have been due to the synergistic inhibiting performance between APM and polyCMCh-graft-polyMVI after forming stable organic-inorganic hybrid film clusters on steel surface. In all, greater total inhibition effect was attained between both compounds compared to each of them investigated independently, and this could also be attributed to the strong interaction between these two inhibitor molecules. 𝜂1 and 𝜂1 are values of inhibition efficiency of APM and polyCMCh-graft-polyMVI, respectively, while 𝜂𝜄1 + 2 denotes the measured inhibition efficiency for both compounds combined. 3.5.

Surface morphology by SEM and AFM

The results from electrochemical tests have revealed enhanced corrosion protection in the presence of APM in combination with the copolymer; the relationship between both compounds is synergistic in nature. Indepth surface analyses of metal surface have also been conducted using SEM. Figure 11 shows the SEM micrographs of abraded and treated X70 substrate surfaces in corrosive CO2 saturated oilfield formation water containing APM, polyCMCh-graft-polyMVI copolymer and in combination with each other. The abraded metallic surface (a) corroded after 12 h within the corrodent, and this could be attributed to the unhindered chloride-induced and CO2 corrosion within the oilfield formation water.5-7 The presence of the corrosion products in Figure 11 (b) is evidence of the steel dissolution within corrosive media. Unlike steel in the blank corrodent, the presence of APM and polyCMCh-graft-polyMVI within the test medium has significantly reduced corrosion by formation of protective films on the metal surface. The SEM of X70 surfaces containing both APM and polyCMCh-graft-polyMVI show evidence of reduced corrosion attack due to molecular adsorption and the formation of more stable protective passive films on steel.55 The inset reveals scale coverings on the metallic surface immersed within the corrodent. This has also been corroborated with Energy Dispersive maps from X-ray elemental microanalysis of defined surface areas of the X70 substrate immersed in the APM and polyCMCh-graft-polyMVI copolymer. The presence of elements common within both compounds confirms that corrosion inhibition was due to molecular coadsorption on the metal surface. This is a direct consequence of mass transport processes as film formation 13 ACS Paragon Plus Environment


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initiates the receding on ionic currents of corrosive species at the metal/solution interface.52 Molecular adsorption could be a physical or simple electrostatic interaction between charged inhibitor species and the charged steel surface.56 Within this CO2 saturated environment, the metal substrates are characterized with less crystalline carbonate scales due to localized corrosion resistance by molecular adsorption within the temperature under study (60oC). Though there are scales on metal surfaces with APM and polyCMChgraft-polyMVI, they are also sparsely contributed (see SEM insets). These corrosion scales are dense and more evenly distributed on metal substrates in the APM/ polyCMCh-graft-polyMVI conjugates with more dispersed coverage that also impedes the free passages for corrosive media. The scales, in the presence of these inhibitor molecules, could alter the kinetics of CO2 corrosion, hence the gross corrosion rate within the media.55 Iron carbonate scales are readily formed by the combination of ferrous with carbonate ions as displayed in Equation 1.7 Depending on the thicknesses, most of the adsorbed scales could fall off as they are loosely attached to the metal surface. The few observed cracks could be attributed dehydration effect during sample preparation for SEM imaging.55 There are conspicuous surface pits on steel substrate in the black corrodent from visual inspection. Fe2 + + CO23 ― →FeCO3

(1)

A similar trend in surface morphology has also been revealed by AFM when comparing treated metal substrates within inhibited and uninhibited corrosive solutions after removing adhering scales. Figure 11 also shows AFM micrographs of selected spots on the same surface analyzed using the SEM technique. The observed micrographs show polished marks on untreated steel surface (a1) while the impacted ones (b1-d1) show significant protrusions due to surface corrosion. The average surface roughness (∅) values for steel substrate in the bare corrodent is 501 nm while those in exposed to APM, polyCMCh-graftpolyMVI and APM/polyCMCh-graft-polyMVI conjugate are 356, 304 and 234 nm, respectively. The differences in the magnitude of ∅ between untreated (72 nm) and treated steel substrates is due to inherent changes during corrosion attack while the presence of protective inhibitor films lowers the surface roughness.56 Normally, these nanoscale AFM characterizations of surface roughness accounts for interfacial phenomenon associated with corrosion protection via molecular adsorption on metal surfaces. The surface profiles showing the variation in few surface parameters (e.g. texture, waviness and roughness) are also mapped beside each AFM micrograph. By far, the surface of X70 substrate within the blank corrodent (b1) is rougher and displays uneven texture compared to those with adhering inhibitor films (c1-e1). 3.6.

XPS analyses of adsorbed inhibitor film on metal substrates


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XPS technique was utilized in analyzing the adhering corrosion products on the surface of steel within the CO2 saturated oilfield formation water. Figure 12 (a) represents the corresponding XPS wide-scan spectra for respective surfaces analyzed within the whole range of binding energies (0 to 1200 eV), between the blank corrodent, APM, polyCMCh-graft-polyMVI and APM/polyCMCh-graft-polyMVI conjugate. The spectra reveal elemental compositions consistent with the corrosive medium and the adhering inhibitor film/compounds within the adsorbed corrosion scales. These are elements seen with core level peaks like C 1s, O 1s, N 1s, and Fe 2p, to mention but a few. The peaks at 400 and 1100 eV indicate that N KLL and N 1s could be linked with adhering of inhibiting films (from their imidazole and glucosamine moieties) on steel. The charged nitrogen atoms (as well as ring-bond 𝜋-electrons) could have fostered electrostatic attraction between APM/ polyCMCh-graft-polyMVI and the iron orbitals, especially on the lone sp2 electron pairs.57 The high resolution XPS spectrograms of the outer scales on steel within the APM/ polyCMCh-graft-polyMVI conjugate were theoretical deconvoluted and presented Figure 12 (b-e). N 1s spectrum (Figure 12b) reveals an absorption peak at 397 eV corresponding to C-N and =N- bonds from the imidazoline species while the peaks between 399 and 401 eV could be linked with N atoms from amine groups.58 Peaks related to C 1s (Figure 12c) are located at 284.79, 286.41, 288.74 eV and they could be assigned to C=O, C-N, and O-C=O bonds, respectively, from the glucosamine and imidazole rings. These bonds could promote chemical coordination on the steel substrate while the peak at 284.79 eV could be related with low-carbon content corrosion scales.59 Lu et al.55 have reported that the XPS peaks between 287.3 and 289.2 eV may corresponds to adsorbed FeCO3 scales. The peaks at 529.6 eV on the O 1s spectrum (Figure 12d) is assigned to Fe-O bonds on ferrous and hydrated ferrous oxides (e.g. FeOOH) and carbonates (within the dense corrosion scales) while O-C and O=C bonds could be observed at 532.1 eV. Carbonates are also linked with the peak at 529.8 eV (this is assigned to O2− within the adsorbed carbonate).55 The Fe 2p spectrum (Figure 12e) reveal two distinct peaks at 710.26 and 722.75 eV and are assigned to Fe-O/FeOH and metallic iron, respectively. The peak at 711.6 eV confirms the presence of FeCO3. Actually, the presence of the Fe peaks is indicative of steel oxidization and formation of passivating protective oxide scales on steel.59 The XPS evidence reported within this study is a proof of inhibitor-metal surface interaction on the metallic substrate by the adsorbed inhibitor compounds during molecular adsorption. 3.7.

Measuring surface wetness

Figure 12 (f) represents the aqueous contact-angle values of impacted (Corro.) and unimpacted (Noncorro.) metallic surfaces measured after the corrosion test. The observed increase in the magnitude of contact angle for steel substrates within solutions of these inhibitor compounds confirms the formation of protective inhibitor films. Addition of APM, polyCMCh-graft-polyMVI and APM/polyCMCh-graft-



Page 16 of 30

polyMVI conjugate reveals values of contact angle up to 56, 59, 75o, respectively. This trend of results confirms that surface wettability was reduced in the presence of adsorbed protective films due to molecular adsorption. Though these inhibitor compounds have contributed to reduced surface energy, these metallic surfaces are still hydrophilic (contact angle < 90o) due to the nature of the adsorbed amorphous scales on steel and the gross impact of chloride-induced dissolution and CO2 corrosion. The unhindered corrosion of steel within the blank CO2 saturated oilfield formation water led to the formation of more corrosion products (See Figure 11b) thereby completely reducing the contact angle to zero degree (0o).60 The absence of inhibitor molecules within this medium increased the corrosion rate of steel with time. 3.8.

Proposed mechanism of corrosion inhibition by APM/polyCMCh-graft-polyMVI conjugate

According to Nazari et al.61, impure water within the oilfield is rich in dissolved CO2 gas. CO2 dissolution within produced water gradually reduces the pH of the media due to the formation of weak carbonic acid. This acid is responsible for most sweet corrosion cases within production facilities. The chemical equations accompanying these changes are presented in Equations 2 and 3. The anodic reaction at acid pH (e.g. in the presence of acetic acid) involves iron ions (Fe+2) going into the solution (Equation 4) and there are three principal cathodic reactions (Equations 5-7) at 4 < pH < 6. However, above pH 6, the dominant cathodic reaction involves reduction of HCO3― ion. Apart from pH, Usman and Ali 62 have highlighted some of the parameters influencing the rate of corrosion in CO2 environment. CO2 (g)↔CO2 (aq)

(2)

CO2 (aq) + H2O(l)↔H2CO3 (aq)

(3)

Fe2 + + 2e ― →Fe

(4)

2H2CO3 + 2e ― →H2 + 2HCO3―

(5)

2HCO3― + 2e ― →H2 + 2CO23 ―

(6)

2H + + 2e ― →H2

(7)

Figure 13 (a) presents a schematic representation involving CO2 corrosion episodes. As observed within this schematic, this study has revealed intense surface pits (Figure 11a) initiated by chloride-induced dissolution and CO2 corrosion. As localized corrosion damages the metallic substrates, more steel-based elements are released into the corrodent as ions. With this, more secondary surface interactions are imminent. For example, the presence of Fe+2 ion leads to the subsequent formation of ferrous carbonate scales according to Equation 1.7 Scale formation is intensified by the presence of bicarbonate/carbonate 16 ACS Paragon Plus Environment

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ions, dissolved oxygen, and other inorganic salts within the oilfield produced water at elevated temperature (60oC is reported within this work). The presence of these scales alters corrosion kinetics to some extent.55 XPS and SEM evidences have revealed some of these adsorbed carbonate scales within the CO2 saturated medium. To further inhibit pipeline steel corrosion, the dissolved inhibitor compounds (APM and polyCMCh-graft-polyMVI) form protective films on the metal surface due to molecular adsorption. Enhanced corrosion protection is attained in the presence of both compounds combined, due to coadsorption on their protonated forms and formation of more stable protective films (Figure 11b). These inhibitor additives most have enhanced the surface adsorption of each other by forming intermediate bridges with charged metal surfaces, hence, improved corrosion inhibition. The co-adsorption of these inhibitor species was facilitated by electrostatic attraction towards the negative charged steel surface to replace adhering water molecules.54 Inhibitor film formation is a consequence of mass/charge transfer action via strong donor-acceptor or weak interfacial interactions at the metal/solution, and this is in line with improved corrosion protection between both compounds. This could only be linked with synergistic inhibition between APM and polyCMCh-graft-polyMVI as previously evaluated from synergism assumptions proposed by Aramaki and Hackermann in 1964 63. 4.

CONCLUSIONS

The following conclusions were drawn from experimental results obtained within this study: 1. N-(3-Aminopropyl)imidazole and graft vinylimidazole-chitosan copolymer were synthesized from imidazole (via hydrogenation using Raney®-Nickel catalyst) and chitosan (via copolymerization), respectively. 2. Both reaction products have inhibited steel corrosion, especially at higher concentrations, in CO2 saturated acidic oilfield formation water. However, the combination of both imidaoline-based products offered significantly superior protection against corrosion. 3. Corrosion inhibition by the APM/ polyCMCh-graft-polyMVI conjugate is attributed to molecular adsorption as well as the formation of more stable protective inhibitor films on steel. 4. Adsorption of protective films in the presence of these imidazoline-based compounds is accompanied by the formation of ferrous carbonate phases with apparently amorphous morphologies due to CO2 saturation within the medium. 5. Treated steel surfaces possessed reduced wetness due to adsorbed inhibitor films. However, metallic surfaces are still hydrophilic (contact angle < 90o) due to the nature of the adsorbed amorphous scales on steel and the gross impact of chloride-induced dissolution and CO2 corrosion.



AUTHOR CONTRIBUTIONS All of the authors approved the final version of the manuscript. NOTES The authors declare no competing financial interest. ACKNOWLEDGEMENTS Authors wish to acknowledge Saskatchewan Structural Sciences Centre and University of Saskatchewan for providing the facilities for this study. SUPPORTING INFORMATION Figure S1. Open-circuit potential curves for X70 substrates exposure to oilfield produced water containing APM, polyCMCh-graft-polyMVI copolymer and in combination with each other. Table S1. Synergism parameter (𝑆1) for different concentrations of polyCMCh-graft-polyMVI collected at room temperature from electrochemical impedance spectroscopy and potentiodynamic polarization techniques. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8)

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M.A. Amin, K.F. Khaled, Copper corrosion inhibition in O2 -saturated H2SO4 solutions. Corros. Sci. 2010, 52, 1194. Solmaz, R.; Sahin, E.A.; Döner, A.; Kardas, G. The investigation of synergistic inhibition effect of rhodanine and iodide ion on the corrosion of copper in sulphuric acid solution. Corros. Sci. 2011, 53, 3231. Sayed, G.H.; Azab, M.E.; Anwer, K.E.; Raouf, M.A.; Negm, N.A. Pyrazole, pyrazolone and enaminonitrile pyrazole derivatives: Synthesis, characterization and potential in corrosion inhibition and antimicrobial applications. J. Mol. Liq. 2018, 252, 329. Cabello, G.; Funkhouser, G.P.; Cassidy, J.; Kiser, C.E.; Lane, J.; Cuesta, A. CO and transcinnamaldehyde as corrosion inhibitors of I825, L80-13Cr and N80 alloys in concentrated HCl solutions at high pressure and temperature. Electrochim. Acta. 2013, 97, 1. Pojtanabuntoeng, T.; Salasi, M. An electrochemical study of carbon steel CO2 corrosion in the presence of monoethylene glycol: The effects of pH and hydrodynamic conditions. Electrochim. Acta. 2017, 258, 442. Umoren, S.A.; Eduok, U.M. Application of carbohydrate polymers as corrosion inhibitors for metal substrates in different media: A review. Carbohydrate Polymers. 2016, 140, 314. Zhang, D.Q.; Gao, L.X.; Zhou, G.D. Synergistic effect of 2-mercapto benzimidazole and KI on copper corrosion inhibition in aerated sulfuric acid solution. J. Appl. Electrochem. 2003, 33, 361. Umoren, S.A.; Solomon, M.M.; Udosoro, I.I.; Udoh, A.P. Synergistic and antagonistic effects between halide ions and carboxymethyl cellulose for the corrosion inhibition of mild steel in sulphuric acid solution. Cellulose 2010, 17, 635. Lu, Y.; Jing, H.; Han, Y.; Feng, Z.; Xu, L. Recommend design of filler metal to minimize carbon steel weld metal preferential corrosion in CO2-saturated oilfield produced water. Appl. Surf. Sci. 2016, 389, 609. Verma, C.B.; Ebenso, E.E.; Bahadur, I.; Obot, I.B.; Quraishi, M.A. 5-(Phenylthio)-3H-pyrrole-4carbonitriles as effective corrosion inhibitors for mild steel in 1 M HCl: experimental and heoretical investigation. J. Mol. Liq. 2015, 212, 209. Bentiss, F.; Mernari, B.; Traisnel, M.; Vezin, H.; Lagrenee, M. On the relationship between corrosion inhibiting effect and molecular structure of 2,5-bis(npyridyl)-1,3,4-thiadiazole derivatives in acidic media: AC impedance and DFT studies. Corros. Sci. 2011, 53, 487. Jiang, S.B.; Jiang, L.H.; Wang, Z.Y.; Jin, M.; Bai, S.; Song, S.; Yan, X. Deoxyribonucleic acid as an inhibitor for chloride-induced corrosion of reinforcing steel in simulated concrete pore solutions, Construct. Build. Mater. 2017, 150, 238. Ralkhal, S.; Shahrabi, T.; Ramezanzadeh, B.; Bahlakeh, G. A combined electrochemical, molecular dynamics, quantum mechanics and XPS analysis of the mild steel surface protected by a complex film composed of neodymium (III) and benzimidazole. Appl. Surf. Sci. 2019, 464,178. Singh, A.; Ansari, K.R.; Lin, Y.; Quraishi, M.A.; Lgaz, H.; Chung, I.M. Corrosion inhibition performance of imidazolidine derivatives for J55 pipeline steel in acidic oilfield formation water: Electrochemical, surface and theoretical studies. J Taiwan Inst Chem E. 2018, 000, 1. Nazari, M.H.; Allahkaram, S.R.; Kermani, M.B. The effects of temperature and pH on the characteristics of corrosion product in CO2 corrosion of grade X70 steel. Mater. Des. 2010, 31, 3559. Usman, B.J.; Ali, S.A. Carbon Dioxide Corrosion Inhibitors: A review. Arab J. Sci. Eng. 2018, 43,1. Umoren, S.A.; Eduok, U.M.; Oguzie, E.E. Corrosion Inhibition of Mild Steel in 1 M H2SO4 by Polyvinyl Pyrrolidone and Synergistic Iodide Additives, Portugaliae Electrochim. Acta 2008, 26, 533.


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Tables Table 1. Impedance and polarization parameters for steel substrates in CO2 saturated oilfield formation water modified with different concentrations of APM and polyCMCh-graft-polyMVI. Electrochemical impedance spectroscopy Rsoln CPE, Yo System under study Rct χ2 × −2 𝜂% (Ω (µF cm (Ω cm2) 10─6 2 −(1−αc) cm ) s ) Blank corrodent 2.5 75.5 490.2 102.7 100 ppm APM 3.8 135.6 236.6 123.2 44.3 250 ppm APM 3.7 200.4 201.6 169.7 62.3 500 ppm APM 2.8 225.6 198.2 157.2 66.5 50 ppm polyCMCh-graft-polyMVI 3.6 260.2 185.8 230.0 70.9 100 ppm polyCMCh-graft-polyMVI 2.6 300.5 181.8 269.4 74.8 200 ppm polyCMCh-graft-polyMVI 2.5 310.4 159.1 441.1 75.6 500 ppm APM+50 ppm polyCMCh-graft-polyMVI 2.2 392.2 129.7 265.8 80.8 500 ppm APM+100 ppm polyCMCh-graft-polyMVI 3.8 550.8 111.8 895.0 86.3 500 ppm APM+200 ppm polyCMCh-graft-polyMVI 1.9 799.5 100.9 235.7 91.0 χ2 represents Goodness of Fit; values of 𝛼 within this work are close to unity


Potentiodynamic polarization -Ecorr jcorr βa βc (mV vs (µA/c (mV/ (mV/ SCE) m2) dec) dec) 490.0 909.0 56.7 99.9 479.0 390.0 48.7 91.8 473.0 300.0 89.4 113.3 469.0 269.0 69.9 98.0 469.0 200.0 99.2 104.1 450.0 189.0 89.8 152.9 445.0 109.0 69.8 109.5 450.0 85.0 73.4 120.6 445.0 70.0 80.4 100.9 430.0 59.0 56.7 85.2

𝜂% 57.1 66.9 70.4 77.9 79.2 88.0 90.7 92.2 93.5

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Figures

N

+

N

N

CH3OH; NH4+OH─ Raney nickel catalyst, H+ N H

N

H2N

Figure 1. Proposed hydrogenation formation scheme for APM from imidazole precursor using a Raney®Nickel catalyst.

OCH2COOH

OH * HO

O

NH2

(1) Stir at 40oC for 6 h with

*20% NaOH and chloroacetic acid

O (2) Stir at 55oC/30 min with K2S2O8

O

* HO

NH

N

n N

* O

H C

*

n

CH m *

N N

Figure 2. Proposed formation scheme for polyCMCh-graft-polyMVI copolymer from chitosan.



Figure 3. Microstructural images obtained from (a, b) SEM, (c) optical microscope, and (d) EDS particle analysis.

Figure 4. EBSD results showing maps for (a) IPF (b) grain boundary distribution (c) area fraction of recrystallized, recovered and deformed regions (d) KAM.


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Figure 5. 1H NMR spectra of (a) imidazole precursor and (b) the synthesized APM product utilized within this study (solvent: CDCl3); chemical shifts are measured in ppm.

Figure 6. 1H NMR spectra of (a) chitosan precursor and (b) the synthesized polyCMCh-graft-polyMVI product utilized within this study (solvent: D2O).



Figure 7. FTIR spectra of as-synthesized (a) APM and (b) polyCMCh-graft-polyMVI products and their imidazole (IMI) and chitosan (Ch) precursors, respectively.

Figure 8. SEM micrographs of as-synthesized polyCMCh-graft-polyMVI copolymer; inset: surface morphology of chitosan flakes.


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Figure 9. Nyquist (a and b) and Tafel (c and d) curves of X70 steel substrates exposure to CO2 saturated oilfield formation water modified with different concentrations of APM and polyCMCh-graft-polyMVI.



Figure 10. (a) Nyquist and (a) Tafel curves of X70 steel substrates exposure to CO2 saturated oilfield formation water modified with 500 ppm APM in combination with different concentrations of polyCMChgraft-polyMVI additive.

Figure 11. SEM micrographs (and corresponding AFM micrographs) of abraded/polished (a) and X70 substrate surfaces showing different adsorbed corrosion products/inhibitor aggregates after a 12-h exposure to CO2 saturated oilfield formation water modified with (b) no inhibitor, (c) 500 ppm APM and (d) 500 ppm APM/200 ppm polyCMCh-graft-polyMVI conjugate. Besides each AFM micrograph (RHS) is a 28 ACS Paragon Plus Environment

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surface profile showing the variation of surface parameters mapped between penetration depth (y axis) vs distance (x axis).

Figure 12. XPS wide-scan spectra (a) of the adsorbed corrosion products/inhibitor aggregates films on the X70 metallic substrate exposed to CO2 saturated oilfield formation water containing APM/ polyCMChgraft-polyMVI conjugate (b-e, high-resolution spectra were collected for selected elements from the adsorbed APM/ polyCMCh-graft-polyMVI conjugate film on X70 substrate); (f) Mean aqueous contactangle values of impacted and unimpacted metallic surfaces.

(a) The formation of protective inhibitor films is due to molecular adsorption. This is mixed with adsorbed carbonate scales within the CO2 saturated medium.

CO2 saturated oilfield produced water at acid pH and 60oC

(b) Negatively charged metal surface

e‒

Surface pits are formed due to chlorideinduced dissolution and CO2 corrosion Dissolved ions from steel (e.g. Fe+2 and Mn+2) Corrosive ions within the corrodent (e.g. Cl─)

Synergistic inhibition is due to adsorption of both APM and polyCMCh-graft-polyMVI protonated molecules on steel

Figure 13. The proposed mechanism of corrosion (a) and corrosion inhibition (b) via adsorption of APM/ polyCMCh-graft-polyMVI conjugate on X70 surface after exposure to oilfield produced water. There may be multiple cathodic reactions (see Equations 5-7) at 4 < pH < 6, but above pH 6, the dominant cathodic reaction involves reduction of HCO3― ion. 29 ACS Paragon Plus Environment


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Conversion of imidazole to N-(3-Aminopropyl) imidazole towards

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