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Green and sustainable local biomaterials for oilfield chemicals: Griffonia simplicifolia extract as steel corrosion inhibitor in hydrochloric acid
Accepted Manuscript Green and sustainable local biomaterials for oilfield chemicals: Griffonia simplicifolia extract as steel corrosion inhibitor in hydrochloric acid
Ekemini Ituen, Onyewuchi Akaranta, Abosede James, Shuangqin Sun PII: DOI: Reference:
S2214-9937(16)30035-5 doi: 10.1016/j.susmat.2016.12.001 SUSMAT 30
To appear in:
Sustainable Materials and Technologies
Received date: Revised date: Accepted date:
4 June 2016 15 December 2016 15 December 2016
Please cite this article as: Ekemini Ituen, Onyewuchi Akaranta, Abosede James, Shuangqin Sun , Green and sustainable local biomaterials for oilfield chemicals: Griffonia simplicifolia extract as steel corrosion inhibitor in hydrochloric acid. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Susmat(2016), doi: 10.1016/j.susmat.2016.12.001
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ACCEPTED MANUSCRIPT Green and sustainable local biomaterials for oilfield chemicals: Griffonia simplicifolia extract as steel corrosion inhibitor in hydrochloric acid Ekemini Ituen1,2* Onyewuchi Akaranta2,3 Abosede James3 Shuangqin Sun1 1
Department of Materials Physics and Chemistry, China University of Petroleum, Qingdao. African Centre of Excellence in Oilfield Chemicals Research, Institute of Petroleum Studies, University of Port Harcourt, Nigeria. 3 Department of Pure and Industrial Chemistry, University of Port Harcourt, Nigeria.
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Correspondence should be addressed to [email protected]
Abstract
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Mild steel (MS), X80 and J55 steel are commonly used in construction of oilfield line pipes, casings, tubing and storage facilities. Their corrosion behaviour was investigated in 1 M HCl solution in absence and presence of seed extracts of Griffonia simplicifolia (SEGS) at 303K. Corrosion rates obtained using weight loss technique at 303K followed the trend J55>X80>MS. Inhibition efficiency up to 91.73 %, 79.78 % and 75.41 % for MS, X80 and J55 respectively at 1000 ppm SEGS. SEGS was spontaneously physisorbed on the steel surfaces as best described by Temkin adsorption isotherm. Electrochemical measurements also conducted in the absence and presence of SEGS yielded comparable results. Charge transfer resistance increased as double layer capacitance decreased due to increase in thickness of SEGS protective film formed. SEGS acted as mixed type inhibitor with anodic predominance. Scanning Electron Microscopy (SEM) micrographs of the corroded surfaces reveal efficient protection of all the metal surfaces by addition of SEGS. SEGS could find application as anticorrosive oilfield chemical additive for acidizing, enhanced oil recovery and well treatment fluids.
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Graphical Abstract 60
(i) Nyquist plots for J55
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Keywords: Oilfield chemical; corrosion inhibitor; EIS; Polarization; Griffonia simplicifolia.
ACCEPTED MANUSCRIPT 1. Introduction
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The oil and gas industry employs a variety of steel materials for line pipes, tubing, casings and construction of storage and other facilities. Depending on application, one of the key factors considered in selecting the grade of steel is ability to withstand very severe conditions, among which is corrosive attack [1, 2]. These materials deteriorate when they come in contact with aggressive agents especially acid. Acids like HCl can be introduced into the pipe work through well stimulation, scale removal or enhanced oil recovery operations [3]. Other acids can be generated in situ through CO2 and H2S dissolution or wastes from some bacteria. Water itself is a cause of oilfield corrosion because it is a medium where noncorrosive agents like CO2 and H2S dissolve to form corrosive acids [4, 5].
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Materials selection of corrosion resistant grades of steel is aimed at reducing corrosion problems. Such steel grades like API 5L X70, X80, X60, X65, X52 and X42 have now been extensively preferred for line pipe applications [2, 6, 7]. Others include API-5CT J55, J55N, N80-P110, and K55 for welded casing and tubing pipes and mild steel for pipework and storage facility construction [8, 9]. The basic difference in their metallurgy is in the relative amounts of elements blended to fabricate the alloys [10]. However, these alloys still corrode or fail and may require replacement or maintenance which could warrant shutting down of plant, the down production time and other risks which no industry would like to take [11, 12].
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Corrosion inhibitors are usually employed to retard the rate of corrosion and increase materials longevity. A wide range of organic adsorption inhibitors currently used in the field are toxic and very expensive. Paradigm has shifted to ecologically friendly (green) corrosion inhibitors from cheap and sustainable sources. Plant and other biomaterials sourced locally provide the cheapest and most non-toxic source of corrosion inhibitors [13-15]. Many plants have been tested as corrosion inhibitors for different metals in various corrosive media by several researchers [1620], the present study reports SEGS as corrosion inhibitor for different grades of steel used in the oilfield. The wild plant is a shrub, widely distributed in Southern Nigeria and can propagate rapidly if cultivated by seeds. It has been classified taxonomically [21] and is also called children whistle (English), and local names are Arin, Olabahun, Mba-aba, Alukoko, and Tapara. Application of SEGS as corrosion inhibitor would contribute to local content development, reduce importation and create internal wealth.
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Corrosion inhibitors are believed to act by adsorption [22] resulting in formation of a thin protective film on the metal surface. Since corrosion is an electrochemical process, the adsorbed layer can be considered as a double layer formed at the interface between the electrolyte (corrodent) and metal electrode surface. The self-assembled films formed between SEGS and the different grades of oilfield steel were investigated and characterized using electrochemical, gravimetric and surface examination techniques. Specimens of mild steel (MS), X80 and J55 steels which are frequently used in the oilfield were used for the study. The nature of interaction of the adsorbed molecules on the respective steels surfaces is predicted using adsorption and thermodynamic models. Hydrochloric acid which is used in acidizing, formation fracture and enhanced oil recovery was used to simulate the corroding medium. 2. Material and methods 2.1. Preparation of surfaces of metal specimens
ACCEPTED MANUSCRIPT Mild steel (MS) sheets were purchased from Building and Construction Materials Market in Uyo, Akwa Ibom state. The J55 and X80 steel were purchased from Qingdao Tengxiang Instrument and Equipment Co. Ltd., China. They were cut into coupons of dimension 2 cm x 2 cm for gravimetric experiments; 1 cm x 1 cm for electrochemical studies and 2 cm x 1 cm for surface analysis. The surfaces of the metal coupons were prepared by following standard procedures reported earlier [23]. In addition, the coupons for electrochemical studies were abraded using different grades of silicon carbide paper and finished to mirror surface with CC22F P1000 grade. The chemical compositions of the steels were as shown in Table 1 below:
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Table 1
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X80
Chemical composition (wt. %) C (0.13), Si (0.18), Mn (0.39), P (0.40), S (0.04), Cu (0.025), Fe (balance) C (0.24), Si (0.22), Mn (1.1), P (0.103), S (0.004), Cu (0.5), Ni (0.28), Mo (0.019), Fe (balance) 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)
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Steel MS J55
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Chemical composition of the steel grades
2.2. Preparation of Test solutions
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The acid was prepared by diluting analytical grade 37 % HCl to a concentration of 1.0 M. Mature seeds of Griffonia simplicifolia were harvested from a local bush in Ikot Ambon Ibesikpo, Uyo, Nigeria. The pericarps were removed and the seeds were air-dried in the laboratory for one week, ground to powder and soaked in acetone for 48 hours. The filtrate (SEGS) was allowed to air-dry. The dry extract was prepared in the 1 M HCl to concentrations 100 ppm, 500 ppm and 1000 ppm using distilled water. 2.3. Weight loss measurement
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The coupons were weighed using Sartorius CPA225D analytical balance of sensitivity ± 0.01 mg and immersed in the test solutions with and without different concentrations of the inhibitor maintained at 303 K for 5 hours. Thereafter, they were retrieved, cleaned with detergent solution with soft brush to remove the corrosion products, washed in distilled water, dried in air after rinsing in acetone, and reweighed. The mass losses were recorded and used to -1 -3 calculate the corrosion rate ( (cmh )) of iron of density ( (gcm )) in the steel specimens of surface area ( (cm2)) according to Eq. 1. The corrosion inhibition efficiency ( ) and degree of surface coverage (θ) were calculated using Eq. 2 and 3 respectively. (1) (2) (3)
ACCEPTED MANUSCRIPT where and are the weights (g) of the coupons before and after immersion in the test solutions at a time interval, t (s). The obtained corrosion rate values were converted to mpy units using conversion factors described in literature [24]. 2.4. Electrochemical monitoring techniques
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The corrosion process was monitored using Gamry ZRA REF 600-18042 electrochemical workstation. The conventional three electrode set up was used consisting of saturated calomel electrode (SCE) as reference electrode, platinum as counter electrode and the different steel coupons as working electrode. Electrochemical impedance measurements (EIS) were conducted at frequency of 105 to 10-2 Hz for open circuit immersion time of 30 minutes at 303 K. The voltage was changed to -0.15 V to +0.15 V vs. EOC at scan rate of 1 mV/s for Potentiodynamic Polarization measurements (PDP). Electrochemical Frequency Modulation (EFM) measurements were conducted using two frequencies: 2 Hz and 5 Hz. The base frequency was 1 Hz, hence the waveform repeats after 1 s. A peak voltage of 10 mV was used. EChem software package was used for data fitting and analyses.
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Charge transfer resistance from EIS measurements were used to compute the inhibition efficiency according to Eq. 4. The inhibition efficiency from PDP and EFM was calculated from the corrosion current densities using Eq. 5. (4)
are charge transfer resistances in the absence and presence of inhibitor
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where and respectively. )
(5)
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where and are the corrosion current densities in the absence and presence of the inhibitor respectively; and and are the polarization resistances with and without the inhibitor respectively. The magnitude of the double layer capacitance ( ) of the adsorbed film was calculated from constant phase element (CPE) constant ( ) and charge transfer resistance ( ) using Eq. 6
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= (6) where n is a constant showing degree of roughness of the metal surface obtained from the phase angle given that is the phase angle of CPE and = 2α⁄(π ) is the CPE exponent. 2.5. Scanning Electron Microscopic study By using AMETEX S4800 TSL model operated at 5.0 kV, the micrographs of the metal surfaces were recorded in the vacuum mode after immersion in 1 M HCl in the absence and presence of 1000 ppm SEGS for 5 hours. 3. Results and discussion 3.1. Weight loss measurement
ACCEPTED MANUSCRIPT The corrosion rates of the different steel grades were higher in the free acid solution than in the inhibited solutions. The highest corrosion rate was recorded for J55 steel and the corrosion rate decreased according to the trend J55>X80>MS (Table 2). However, in the presence of the inhibitor, corrosion rate decreased greatly as can be seen by downward movement of the curves representing different concentrations of the inhibitor in Fig. 1. This observation demonstrates effective inhibition of corrosion of the different steel grades by the SEGS.
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The effectiveness of the extracts in reducing the corrosion rate of the metals is estimated in terms of the inhibition efficiency ( ). The calculated values are given in Table 1. Inhibition efficiency increased as concentration of inhibitor increased. The highest of 91.73% was obtained for MS in 1000 ppm SEGS and decreased according to the trend MS>X80>J55 at all concentrations. This implies that the inhibitor is more effective with MS than the other grades. Higher efficiency observed for MS may be attributed to larger surface area available for adsorption of inhibitor molecules than J55 and X80 and/or differences in chemical (Fe) composition of the steel grades.
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Fig. 1. Corrosion rates of MS, J55 and X80 steels in 1 M HCl containing different concentrations of SEGS at 303 K. Table 2 Calculated corrosion rate (mpy), inhibition efficiency (%) and degree of surface coverge for corrosion of MS, J55 and X80 in 1 M HCl containing different concentrations of SEGS at 303 K. System
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26.72 8.49 3.39 2.21
68.23 87.31 91.73
0.6823 0.8731 0.9197
54.87 23.35 15.87 13.49
57.45 71.08 75.41
0.5745 0.7108 0.7541
39.45 13.84 9.47 7.98
64.92 76.14 79.78
0.6492 0.7614 0.7978
3.2. Surface coverage and adsorption isotherm
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The fractional surface coverage (θ) of SEGS on the different steel grades followed the same trend as as shown in Table 2. The coverage of SEGS on metal active sites and formation of thin protective film that shields metal surface from the aggressive acid could have led to the reduction in corrosive attack. Fractional surface coverage data were fitted into different adsorption isotherm models to help explain the nature of interaction(s) in the adsorbed layer. The models tested were Langmuir, Temkin, Freundlich, Florry Hugins, Frumkin and El-Awady et al. While the coverage data of SEGS on all the metal specimens best fitted into Temkin models (R2≥ 0.998). The plots in Fig. 2 depict linear fittings obtained from Temkin adsorption isotherm. The parameters deduced from the plot are listed in Table 3.
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The Temkin isotherms can be expressed as shown in Eq. 7 below. -
(7)
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where C is the inhibitor concentration, is the adsorption-desorption equilibrium constant used to describe how strongly the inhibitor molecules are adsorbed on the metal surface, is the lateral molecular interaction parameter used to predict whether there is attraction or repulsion in the adsorbed layer [25].
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The values of obtained using Temkin isotherm were all negative indicating that there was repulsion in the adsorbed layer [26]. The strength of repulsion followed the trend X80>J55>MS. The adsorption-desorption equilibrium constant values show considerable binding strength between the inhibitor and metals, with stronger strength on X80 surface. This demonstrates that SEGS would provide better protection for pipework, casings and tubings. Table 3
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Parameters deduced from Temkin adsorption isotherm for SEGS Steel grade MS J55 X80
4.765 6.316 7.642
The free energy of adsorption (
(ppm-1) 6.992 14.560 208.71
(kJmol-1) -15.016 -16.865 -23.572
) was calculated from
0.998 0.993 0.994 values using Eq. 8.
(9) where 55.5 represents the molar concentration of water. In literature, when values of around -20 kJmol-1, the adsorption involves electrostatic interaction (physisorption) while
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(ii) Temkin adsorption isotherm 0.9
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Fig. 2. Temkin adsorption isotherm for SEGS adsorbed on MS, J55 and X80 at 303 K 3.3. Electrochemical Impedance Spectroscopy
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Corrosion of MS, J55 and X80 steel grades were assessed in 1 M HCl in the absence and presence of different concentrations of SEGS. Nyquist and Bode diagrams obtained with the different steel grades are shown in Fig. 3. A single capacitive loop with one capacitive time constant observed demonstrates the occurrence of only one phenomenon. The capacitive loop indicates that charge transfer and double layer phenomenon occurs at the metal-electrolyte interface. The charge transfer resistance was obtained by fitting the curves into equivalent circuit shown in Fig. 4.
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The shapes of the Nyquist and Bode diagrams for all the steel grades and test solutions were similar indicating that the mechanism of corrosion remains the same with or without SEGS. Therefore, addition of inhibitor does not influence the mechanism of corrosion. Diameters of the Nyquist plot increase as concentration of SEGS in the electrolyte increase corroborating with trend of charge transfer resistance obtained. This also demonstrates that the influence of the inhibitor on the corrosion rate is concentration dependent. Charge transfer resistance can be associated with impedance offered by the thin insulating film formed by SEGS due to adsorption of its molecules on the steel surface. The thickness and protective efficiency of this film increases with increase in Rct and inhibitor concentration. A similar behaviour can also be observed with the Bode plots. The Nyquist plots produced imperfect semicircles, signifying deviations from ideal behaviour. This kind of deviation has been explained in terms of frequency dispersion of interfacial impedance and attributed to inhomogeneity of the metal surface arising from surface roughness [28]. The degree of deviation can be estimated using values of of the CPE phase shift, most of which decreased with increase in inhibitor concentration (Table 4). The CPE was introduced instead of a pure capacitor to compensate for the deviation and the values of and of the CPE were used to compute the double layer capacitance. The values of decrease on addition of
ACCEPTED MANUSCRIPT inhibitor (Table 4) which indicate a decrease in the charge and discharge rates of the capacitor [28]. Decrease in can also be due to decrease in local dielectric constant and increase in thickness of the double layer formed, suggesting strong adsorption of SEGS molecules on the metal surface [27]. A similar trend was observed for all the grades of steel studied with X80 grade showing the highest difference in from that of the blank acid.
(b) Nyquist plots for J55 400
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Fig. 3. Nyquist and Bode plots for corrosion of MS, J55 and X80 steels in 1 M HCl in the absence and presence of different concentrations of SEGS.
Fig. 4. Equivalent circuit model used for fitting EIS data.
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(a) Nyquist plot for MS 400
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The calculated inhibition efficiency increased with increase in SEGS concentration, similar to the trend of charge transfer resistance. The highest inhibition efficiency of 96.73 % was obtained with X80 grade at 1000 ppm inhibitor concentration. This implies that more SEGS molecules are assembled on X80 making the inhibitor more effective against X80 corrosion than J55 and MS. This can also be attributed to adsorption of its phyto-compounds like 5-hydroxytryptophan, clomipramine, griffonin, amitriptyline, fluvoxamine, melatonin, etc. [29] which are rich in heteroatoms (oxygen and nitrogen) and pie electrons (see structures in Fig. 5). These molecules have large sizes and many potential adsorption sites, hence could adsorb on the steel surface, block the metal actives sites, achieve large surface coverage and reduce corrosion rate of the metals. However, at 100 ppm inhibitor concentration, inhibition efficiency was the lowest (55.52%) with J55 grade.
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(i) Melatonin
(ii) Fluvoxamine
(iii) 5-Hydroxytryptophan
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Fig. 5. Molecular structures of some chemical compounds in SEGS
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Table 4
(Ωcm2) 1.035 0.999 0.873 0.869 1.131 1.058 0.532 0.983 1.063 1.097 1.038 1.028
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(μΩ-1sn) 157.7 81.62 80.50 60.20 139.6 113.2 115.3 117.8 206.4 126.2 113.8 75.9
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0.572 0.566 0.547 0.538 0.555 0.554 0.555 0.625 0.551 0.541 0.554 0.561
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(Ωcm2) 102.3 385.5 1089.0 1144.0 76.2 171.3 868.9 1003.2 30.64 124.0 372.1 936.3
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EIS parameters calculated for MS, J55 and X80 steel grades
(μFcm-2) 127.7 62.2 10.0 3.4 35.1 11.9 3.5 0.8 126.3 10.4 6.5 2.1
(%) 73.46 90.61 91.06 55.52 91.23 92.40 75.29 91.77 96.73
3.4. Potentiodynamic Polarization
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Potentiodynamic polarization measurements were conducted with the different steel grades to evaluate cathodic and anodic corrosion kinetics in the absence and presence of the inhibitor. The partial electrode reactions were approximated using Tafel plots (Fig. 6) and the compromise or free corrosion potential ( ) was determined as well as the corresponding corrosion current density ( ). Linear fitting of the Tafel slopes afforded tafel cathodic and anodic constants, and respectively. The corrosion potentials obtained for all the metals shifted to relatively more positive values suggesting that the inhibitor could be anodic type [30]. However, the differences in values of the inhibitors from those of the free acid solution were not up to 85mV, hence not sufficient to categorize the inhibitor as cathodic or anodic type. Therefore, EGS can be considered as mixed type inhibitor which inhibits the corrosion process by geometric blocking of both cathodic and anodic surface active sites of the different steel grades. Also, corrosion current density reduced in the presence of the inhibitors and with increase in SEGS concentration (Table 5) demonstrating the inhibitive effects of SEGS. The inhibition efficiency also increased with increase in inhibitor concentration, similar to EIS and weight loss results. Addition of inhibitor was also found to influence the Tafel constants in different ways for the different metals. While it seems to increase the constants for MS and X80, the constants showed no definite trend for J55. Inspection of the Tafel constants reveals the trend MS>J55>X80 for
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and X80>MS>J55 for . Also, there are larger dispersion in values than which suggests anodic predominance of the inhibitor. This demonstrates that SEGS is a mixed type inhibitor but with predominant anodic activity. The shapes of the curves are similar, supporting that the inhibitor does not change the mechanism of partial corrosion reactions at the electrodes [30].
PDP parameters calculated for MS, J55 and X80 steel grades
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94.06 98.41 98.55 88.42 89.85 93.26 83.56 90.93 92.84
(iii) Tafel plot for X80
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-499 -487 -483 -482 -473 -472 -470 -461 -487 -476 -474 -469
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(iii) Tafel plots for J55
(mV/SCE)
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(μAcm-2) 969.3 48.8 15.4 14.0 614.9 71.2 62.4 41.5 693.7 114.0 63.0 49.7
log I
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(mVdec-1) 64.7 100.3 98.7 92.7 111.2 97.7 78.9 81.2 98.5 124.6 85.0 103.5
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(mVdec-1) 95.4 65.9 107.5 124.4 71.3 76.1 60.4 95.8 82.1 103.5 107.3 115.1
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Table 5
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Fig. 6. Tafel plots for MS, J55 and X80 3.5. Electrochemical Frequency Modulation
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E (V vs. SCE.)
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E (V vs. SCE.)
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EFM measurement was also carried out using small signals to obtain corrosion current values without prior knowledge of the Tafel slopes. The corrosion rate, corrosion potential and casuality factors (CF-2 and CF-3) were also obtained as displayed in Table 6. Inhibition efficiency calculated from corrosion current values varied with inhibitor concentration similar to results of other measurements, but lower in magnitudes compared to EIS, PDP and LPR results. Experimental causality factors CF-2 and CF-3 calculated from the frequency spectrum of the current responses were close to theoretical values with MS and J55 steels indicating that the measurements are of good quality [31]. However, CF-2 exceeded theoretical values with X80 steel, suggesting the influence of electrochemical noise on that measurement.
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Table 6 EFM parameters for MS, X80 and J55 steels
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(mVdec-1) 107.8 94.4 97.9 100.2 93.4 83.8 97.6 117.4 42.7 105.2 103.3 93.0
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(mVdec-1) 64.4 87.5 90.6 92.3 83.7 87.5 85.9 89.5 40.8 93.1 84.6 85.6
CF-2 1.966 1.898 1.487 1.706 2.177 2.655 2.089 2.137 0.945 1.895 1.708 1.350
CF-3 2.358 2.362 2.206 2.240 2.727 2.094 2.792 2.211 1.704 3.728 3.000 2.828
(%) 60.04 60.87 74.33 70.85 77.34 86.56 41.59 62.60 78.39
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J55
(μAcm-2) 162.6 64.9 63.6 41.7 367.4 107.1 87.3 49.4 473.9 276.8 181.0 102.4
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X80
Conc. (ppm) 0 100 500 1000 0 100 500 1000 0 100 500 1000
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Steel MS
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3.6. Surface morphology and protection
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Figure 8 shows scanned micrographs of the surfaces of different steel grades after immersion in 1M HCl in the absence and presence 1000 ppm SEGS. It can be seen from the micrographs that the surface of the metals, especially J55 steel were greatly damaged (images on the left) in the absence of the inhibitor. However, in the presence of SEGS, the pitting or corrosive attack on the metals surfaces reduced. Inspection of the images on the right hand column reveals that SEGS provided good protection for all the metals studied. J55
X80
MS
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Fig. 7. SEM micrographs of J55, X80 and MS immersed in 1M HCl without (left column) and with (right column) 1000 ppm SEGS
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Acknowledgements
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The authors are pleased to acknowledge the World Bank for support to carry out laboratory work abroad through the Robert S. McNamara Fellowship Programme. We also appreciate Materials Physics and Chemistry Department, China University of Petroleum Qingdao for providing facilities for carrying out this research and African Centre of Excellence in Oilfield Chemicals Research for their support. EI is grateful to Prof. A. P. Udoh, Prof. Hu, Dr. Li, Dr. Wang, Ubong Jerome, Min, Cheng and Xiang Xiqiang in UPC for their assistance. References
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[1]. B. D. Craig, Selection guidelines for corrosion resistant alloys in the oil and gas industry, SPE Reprint Series. (1997) 143-149.
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[2]. J. L. Kennedy, Oil and gas pipeline fundamentals, Pennwell books, 1993.
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[3]. C. Ren, D. Liu, Z. Bai, T. Li, Corrosion behavior of oil tube steel in simulant solution with hydrogen sulfide and carbon dioxide, Mat. Chem. Phys. 93 (2005) 305-309.
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[4]. M. Kermani, A. Morshed, Carbon dioxide corrosion in oil and gas production-A compendium, Corros.. 59, 8 (2003) 659-683. [5]. J. L. Crolet, Acid corrosion in wells (CO2, H2S): Metallurgical aspects, J. Petrol. Technol. 35, 8 (1983) 1553-1558.
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[6]. R. Heidersbach, Metallurgy and corrosion control in oil and gas production. John Wiley & Sons, Vol. 14. 2010. [7]. F. Siciliano, D. G. Stalheim, J. M. Gray, Modern high strength steels for oil and gas transmission pipelines. in 2008 7th International Pipeline Conference. 2008. American Society of Mechanical Engineers. [8]. M. Ueda, H. Takabe, P. I. Nice, The development and implementation of a new alloyed steel for oil and gas production wells, In Corros. 2000, NACE International. 2000. [9]. L. Smith, Control of corrosion in oil and gas production tubing. Brit. Corros. J. 34, 4 (1999) 247-253.
ACCEPTED MANUSCRIPT [10]. S. Nešić, Key issues related to modeling of internal corrosion of oil and gas pipelines–A review, Corros. Sci. 4,12 (2007) 4308-4338. [11]. M. Kermani, D. Harr, The impact of corrosion on oil and gas industry. SPE Annual Technical Conference and Exhibition, Bahrain, March 1995 (SPE 29784). [12]. C. R. Azevedo, Failure analysis of a crude oil pipeline. Eng. Failure. Anal. 14, 6 (2007) 978-994.
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[13]. P. B. Raja, M. G. Sethuraman, Natural products as corrosion inhibitor for metals in corrosive media—a review, Mat. Let. 62, 1 (2008) 113-116.
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[14]. B. Rani, B. B. J. Basu, Green inhibitors for corrosion protection of metals and alloys: An overview. Int. J. Corros. doi:10.1155/2012/380217. [15]. V. S. Sastri, Corrosion inhibitors: principles and applications, Wiley, New York, 1998.
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[16]. R. S. Nathiya, V. Raj, Evaluation of Dryopteris cochleata leaf extracts as green inhibitor for corrosion of aluminium in 1 M H2SO4. Egypt. J. Petrol. doi.org/10.1016/j.ejpe.2016.05.002.
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[17]. K. K. Alaneme, S. J. Olusegun, O. T. Adelowo, Corrosion inhibition and adsorption mechanism studies of Hunteria umbellata seed husk extracts on mild steel immersed in acidic solutions. Alex. Eng. J. 55, 1 (2016) 673-681.
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[18]. M. Jokar, T. S. Farahani, B. Ramezanzadeh, Electrochemical and surface characterizations of morus alba pendula leaves extract (MAPLE) as a green corrosion inhibitor for steel in 1 M HCl. J. Taiwan Inst. Chem. Eng. 63 (2016) 436-452.
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[19]. M. Prabakaran, S. Kim, K. Kalaiselvi, V. Hemapriya, I. Chung, Highly efficient Ligularia fischeri green extract for the protection against corrosion of mild steel in acidic medium: Electrochemical and spectroscopic investigations. J. Taiwan Inst. Chem. Eng. 59 (1026) 553562.
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[20]. M. Sangeetha, S. Rajendran, T. Muthumegala, A. Krishnaveni, Green corrosion inhibitors– An overview. Zastita Materijala, 52, 1 (2011) 3-19.
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[21]. P. Kumar, T. Jain, N. Prakash, B. Jitendra, A review on Griffonia simplicifolia - an ideal herbal anti-depressant. Int. J. Pharm. Life Sci. 1, 3 (2010) 174-181. [22]. A. Al-Sabbagh, M. Osman, A. Omar, I. El-Gamal, Organic corrosion inhibitors for steel pipelines in oilfields. Anti-corros. Meth. Mat. 43, 6 (1996) 11-16. [23]. M. M. Solomon, S. A. Umoren, In-situ preparation, characterization and anticorrosion property of polypropylene glycol/silver nanoparticles composite for mild steel corrosion in acid solution. J. Colloid Interface Sci. 462 (2016) 29-41. [24] Z. Ahmad, Principles of Corrosion Engineering and Corrosion Control, ButterworthHeinemann. Elsevier, 2006.
ACCEPTED MANUSCRIPT [25]. S. K. Shukla, E. E. Ebenso, Corrosion inhibition, adsorption behavior and thermodynamic properties of streptomycin on mild steel in hydrochloric acid medium. Int. J. Electrochem. Sci. 6 (2011) 3277-329. [26]. M. Deyab, S. A. El-Rehim, Influence of polyethylene glycols on the corrosion inhibition of carbon steel in butyric acid solution: weight loss, EIS and theoretical studies. Int. J. Electrochem. Sci. 8 (2013) 12613-12627.
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[27]. P. Singh, A. Singh, M. Quraishi, Thiopyrimidine derivatives as new and effective corrosion inhibitors for mild steel in hydrochloric acid: Electrochemical and quantum chemical studies. J. Taiwan Inst. Chem. Eng. 60 (2016) 588-601.
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[28]. N. El Hamdani, R. Fdil, M. Tourabi, C. Jama, C. F. Fouad, Alkaloids extract of Retama monosperma (L.) Boiss. seeds used as novel eco-friendly inhibitor for carbon steel corrosion in 1M HCl solution: Electrochemical and surface studies. Appl. Surf. Sci. 357 (2015) 1294-1305.
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[29]. W. Xin-Zhi, F. Wu, W. Qu, J. Liang. A new β-carboline alkaloid from the seeds of Griffonia simplicifolia. Chin. J. Nat. Med. 11, 4 (2013) 401-405.
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[30]. C. G. Dariva, A.F. Galio, Corrosion inhibitors–principles, mechanisms and applications. Developments in Corrosion Protection, InTech, 2014.
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Ekemini Ituen, Onyewuchi Akaranta, Abosede James, Shuangqin Sun PII: DOI: Reference:
S2214-9937(16)30035-5 doi: 10.1016/j.susmat.2016.12.001 SUSMAT 30
To appear in:
Sustainable Materials and Technologies
Received date: Revised date: Accepted date:
4 June 2016 15 December 2016 15 December 2016
Please cite this article as: Ekemini Ituen, Onyewuchi Akaranta, Abosede James, Shuangqin Sun , Green and sustainable local biomaterials for oilfield chemicals: Griffonia simplicifolia extract as steel corrosion inhibitor in hydrochloric acid. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Susmat(2016), doi: 10.1016/j.susmat.2016.12.001
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ACCEPTED MANUSCRIPT Green and sustainable local biomaterials for oilfield chemicals: Griffonia simplicifolia extract as steel corrosion inhibitor in hydrochloric acid Ekemini Ituen1,2* Onyewuchi Akaranta2,3 Abosede James3 Shuangqin Sun1 1
Department of Materials Physics and Chemistry, China University of Petroleum, Qingdao. African Centre of Excellence in Oilfield Chemicals Research, Institute of Petroleum Studies, University of Port Harcourt, Nigeria. 3 Department of Pure and Industrial Chemistry, University of Port Harcourt, Nigeria.
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Correspondence should be addressed to [email protected]
Abstract
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Mild steel (MS), X80 and J55 steel are commonly used in construction of oilfield line pipes, casings, tubing and storage facilities. Their corrosion behaviour was investigated in 1 M HCl solution in absence and presence of seed extracts of Griffonia simplicifolia (SEGS) at 303K. Corrosion rates obtained using weight loss technique at 303K followed the trend J55>X80>MS. Inhibition efficiency up to 91.73 %, 79.78 % and 75.41 % for MS, X80 and J55 respectively at 1000 ppm SEGS. SEGS was spontaneously physisorbed on the steel surfaces as best described by Temkin adsorption isotherm. Electrochemical measurements also conducted in the absence and presence of SEGS yielded comparable results. Charge transfer resistance increased as double layer capacitance decreased due to increase in thickness of SEGS protective film formed. SEGS acted as mixed type inhibitor with anodic predominance. Scanning Electron Microscopy (SEM) micrographs of the corroded surfaces reveal efficient protection of all the metal surfaces by addition of SEGS. SEGS could find application as anticorrosive oilfield chemical additive for acidizing, enhanced oil recovery and well treatment fluids.
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(i) Nyquist plots for J55
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Keywords: Oilfield chemical; corrosion inhibitor; EIS; Polarization; Griffonia simplicifolia.
ACCEPTED MANUSCRIPT 1. Introduction
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The oil and gas industry employs a variety of steel materials for line pipes, tubing, casings and construction of storage and other facilities. Depending on application, one of the key factors considered in selecting the grade of steel is ability to withstand very severe conditions, among which is corrosive attack [1, 2]. These materials deteriorate when they come in contact with aggressive agents especially acid. Acids like HCl can be introduced into the pipe work through well stimulation, scale removal or enhanced oil recovery operations [3]. Other acids can be generated in situ through CO2 and H2S dissolution or wastes from some bacteria. Water itself is a cause of oilfield corrosion because it is a medium where noncorrosive agents like CO2 and H2S dissolve to form corrosive acids [4, 5].
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Materials selection of corrosion resistant grades of steel is aimed at reducing corrosion problems. Such steel grades like API 5L X70, X80, X60, X65, X52 and X42 have now been extensively preferred for line pipe applications [2, 6, 7]. Others include API-5CT J55, J55N, N80-P110, and K55 for welded casing and tubing pipes and mild steel for pipework and storage facility construction [8, 9]. The basic difference in their metallurgy is in the relative amounts of elements blended to fabricate the alloys [10]. However, these alloys still corrode or fail and may require replacement or maintenance which could warrant shutting down of plant, the down production time and other risks which no industry would like to take [11, 12].
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Corrosion inhibitors are usually employed to retard the rate of corrosion and increase materials longevity. A wide range of organic adsorption inhibitors currently used in the field are toxic and very expensive. Paradigm has shifted to ecologically friendly (green) corrosion inhibitors from cheap and sustainable sources. Plant and other biomaterials sourced locally provide the cheapest and most non-toxic source of corrosion inhibitors [13-15]. Many plants have been tested as corrosion inhibitors for different metals in various corrosive media by several researchers [1620], the present study reports SEGS as corrosion inhibitor for different grades of steel used in the oilfield. The wild plant is a shrub, widely distributed in Southern Nigeria and can propagate rapidly if cultivated by seeds. It has been classified taxonomically [21] and is also called children whistle (English), and local names are Arin, Olabahun, Mba-aba, Alukoko, and Tapara. Application of SEGS as corrosion inhibitor would contribute to local content development, reduce importation and create internal wealth.
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Corrosion inhibitors are believed to act by adsorption [22] resulting in formation of a thin protective film on the metal surface. Since corrosion is an electrochemical process, the adsorbed layer can be considered as a double layer formed at the interface between the electrolyte (corrodent) and metal electrode surface. The self-assembled films formed between SEGS and the different grades of oilfield steel were investigated and characterized using electrochemical, gravimetric and surface examination techniques. Specimens of mild steel (MS), X80 and J55 steels which are frequently used in the oilfield were used for the study. The nature of interaction of the adsorbed molecules on the respective steels surfaces is predicted using adsorption and thermodynamic models. Hydrochloric acid which is used in acidizing, formation fracture and enhanced oil recovery was used to simulate the corroding medium. 2. Material and methods 2.1. Preparation of surfaces of metal specimens
ACCEPTED MANUSCRIPT Mild steel (MS) sheets were purchased from Building and Construction Materials Market in Uyo, Akwa Ibom state. The J55 and X80 steel were purchased from Qingdao Tengxiang Instrument and Equipment Co. Ltd., China. They were cut into coupons of dimension 2 cm x 2 cm for gravimetric experiments; 1 cm x 1 cm for electrochemical studies and 2 cm x 1 cm for surface analysis. The surfaces of the metal coupons were prepared by following standard procedures reported earlier [23]. In addition, the coupons for electrochemical studies were abraded using different grades of silicon carbide paper and finished to mirror surface with CC22F P1000 grade. The chemical compositions of the steels were as shown in Table 1 below:
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Table 1
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X80
Chemical composition (wt. %) C (0.13), Si (0.18), Mn (0.39), P (0.40), S (0.04), Cu (0.025), Fe (balance) C (0.24), Si (0.22), Mn (1.1), P (0.103), S (0.004), Cu (0.5), Ni (0.28), Mo (0.019), Fe (balance) 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)
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Steel MS J55
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Chemical composition of the steel grades
2.2. Preparation of Test solutions
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The acid was prepared by diluting analytical grade 37 % HCl to a concentration of 1.0 M. Mature seeds of Griffonia simplicifolia were harvested from a local bush in Ikot Ambon Ibesikpo, Uyo, Nigeria. The pericarps were removed and the seeds were air-dried in the laboratory for one week, ground to powder and soaked in acetone for 48 hours. The filtrate (SEGS) was allowed to air-dry. The dry extract was prepared in the 1 M HCl to concentrations 100 ppm, 500 ppm and 1000 ppm using distilled water. 2.3. Weight loss measurement
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The coupons were weighed using Sartorius CPA225D analytical balance of sensitivity ± 0.01 mg and immersed in the test solutions with and without different concentrations of the inhibitor maintained at 303 K for 5 hours. Thereafter, they were retrieved, cleaned with detergent solution with soft brush to remove the corrosion products, washed in distilled water, dried in air after rinsing in acetone, and reweighed. The mass losses were recorded and used to -1 -3 calculate the corrosion rate ( (cmh )) of iron of density ( (gcm )) in the steel specimens of surface area ( (cm2)) according to Eq. 1. The corrosion inhibition efficiency ( ) and degree of surface coverage (θ) were calculated using Eq. 2 and 3 respectively. (1) (2) (3)
ACCEPTED MANUSCRIPT where and are the weights (g) of the coupons before and after immersion in the test solutions at a time interval, t (s). The obtained corrosion rate values were converted to mpy units using conversion factors described in literature [24]. 2.4. Electrochemical monitoring techniques
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The corrosion process was monitored using Gamry ZRA REF 600-18042 electrochemical workstation. The conventional three electrode set up was used consisting of saturated calomel electrode (SCE) as reference electrode, platinum as counter electrode and the different steel coupons as working electrode. Electrochemical impedance measurements (EIS) were conducted at frequency of 105 to 10-2 Hz for open circuit immersion time of 30 minutes at 303 K. The voltage was changed to -0.15 V to +0.15 V vs. EOC at scan rate of 1 mV/s for Potentiodynamic Polarization measurements (PDP). Electrochemical Frequency Modulation (EFM) measurements were conducted using two frequencies: 2 Hz and 5 Hz. The base frequency was 1 Hz, hence the waveform repeats after 1 s. A peak voltage of 10 mV was used. EChem software package was used for data fitting and analyses.
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Charge transfer resistance from EIS measurements were used to compute the inhibition efficiency according to Eq. 4. The inhibition efficiency from PDP and EFM was calculated from the corrosion current densities using Eq. 5. (4)
are charge transfer resistances in the absence and presence of inhibitor
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where and respectively. )
(5)
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where and are the corrosion current densities in the absence and presence of the inhibitor respectively; and and are the polarization resistances with and without the inhibitor respectively. The magnitude of the double layer capacitance ( ) of the adsorbed film was calculated from constant phase element (CPE) constant ( ) and charge transfer resistance ( ) using Eq. 6
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= (6) where n is a constant showing degree of roughness of the metal surface obtained from the phase angle given that is the phase angle of CPE and = 2α⁄(π ) is the CPE exponent. 2.5. Scanning Electron Microscopic study By using AMETEX S4800 TSL model operated at 5.0 kV, the micrographs of the metal surfaces were recorded in the vacuum mode after immersion in 1 M HCl in the absence and presence of 1000 ppm SEGS for 5 hours. 3. Results and discussion 3.1. Weight loss measurement
ACCEPTED MANUSCRIPT The corrosion rates of the different steel grades were higher in the free acid solution than in the inhibited solutions. The highest corrosion rate was recorded for J55 steel and the corrosion rate decreased according to the trend J55>X80>MS (Table 2). However, in the presence of the inhibitor, corrosion rate decreased greatly as can be seen by downward movement of the curves representing different concentrations of the inhibitor in Fig. 1. This observation demonstrates effective inhibition of corrosion of the different steel grades by the SEGS.
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The effectiveness of the extracts in reducing the corrosion rate of the metals is estimated in terms of the inhibition efficiency ( ). The calculated values are given in Table 1. Inhibition efficiency increased as concentration of inhibitor increased. The highest of 91.73% was obtained for MS in 1000 ppm SEGS and decreased according to the trend MS>X80>J55 at all concentrations. This implies that the inhibitor is more effective with MS than the other grades. Higher efficiency observed for MS may be attributed to larger surface area available for adsorption of inhibitor molecules than J55 and X80 and/or differences in chemical (Fe) composition of the steel grades.
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Fig. 1. Corrosion rates of MS, J55 and X80 steels in 1 M HCl containing different concentrations of SEGS at 303 K. Table 2 Calculated corrosion rate (mpy), inhibition efficiency (%) and degree of surface coverge for corrosion of MS, J55 and X80 in 1 M HCl containing different concentrations of SEGS at 303 K. System
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X80 θ
θ
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26.72 8.49 3.39 2.21
68.23 87.31 91.73
0.6823 0.8731 0.9197
54.87 23.35 15.87 13.49
57.45 71.08 75.41
0.5745 0.7108 0.7541
39.45 13.84 9.47 7.98
64.92 76.14 79.78
0.6492 0.7614 0.7978
3.2. Surface coverage and adsorption isotherm
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The fractional surface coverage (θ) of SEGS on the different steel grades followed the same trend as as shown in Table 2. The coverage of SEGS on metal active sites and formation of thin protective film that shields metal surface from the aggressive acid could have led to the reduction in corrosive attack. Fractional surface coverage data were fitted into different adsorption isotherm models to help explain the nature of interaction(s) in the adsorbed layer. The models tested were Langmuir, Temkin, Freundlich, Florry Hugins, Frumkin and El-Awady et al. While the coverage data of SEGS on all the metal specimens best fitted into Temkin models (R2≥ 0.998). The plots in Fig. 2 depict linear fittings obtained from Temkin adsorption isotherm. The parameters deduced from the plot are listed in Table 3.
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The Temkin isotherms can be expressed as shown in Eq. 7 below. -
(7)
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where C is the inhibitor concentration, is the adsorption-desorption equilibrium constant used to describe how strongly the inhibitor molecules are adsorbed on the metal surface, is the lateral molecular interaction parameter used to predict whether there is attraction or repulsion in the adsorbed layer [25].
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The values of obtained using Temkin isotherm were all negative indicating that there was repulsion in the adsorbed layer [26]. The strength of repulsion followed the trend X80>J55>MS. The adsorption-desorption equilibrium constant values show considerable binding strength between the inhibitor and metals, with stronger strength on X80 surface. This demonstrates that SEGS would provide better protection for pipework, casings and tubings. Table 3
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Parameters deduced from Temkin adsorption isotherm for SEGS Steel grade MS J55 X80
4.765 6.316 7.642
The free energy of adsorption (
(ppm-1) 6.992 14.560 208.71
(kJmol-1) -15.016 -16.865 -23.572
) was calculated from
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(9) where 55.5 represents the molar concentration of water. In literature, when values of around -20 kJmol-1, the adsorption involves electrostatic interaction (physisorption) while
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Fig. 2. Temkin adsorption isotherm for SEGS adsorbed on MS, J55 and X80 at 303 K 3.3. Electrochemical Impedance Spectroscopy
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Corrosion of MS, J55 and X80 steel grades were assessed in 1 M HCl in the absence and presence of different concentrations of SEGS. Nyquist and Bode diagrams obtained with the different steel grades are shown in Fig. 3. A single capacitive loop with one capacitive time constant observed demonstrates the occurrence of only one phenomenon. The capacitive loop indicates that charge transfer and double layer phenomenon occurs at the metal-electrolyte interface. The charge transfer resistance was obtained by fitting the curves into equivalent circuit shown in Fig. 4.
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The shapes of the Nyquist and Bode diagrams for all the steel grades and test solutions were similar indicating that the mechanism of corrosion remains the same with or without SEGS. Therefore, addition of inhibitor does not influence the mechanism of corrosion. Diameters of the Nyquist plot increase as concentration of SEGS in the electrolyte increase corroborating with trend of charge transfer resistance obtained. This also demonstrates that the influence of the inhibitor on the corrosion rate is concentration dependent. Charge transfer resistance can be associated with impedance offered by the thin insulating film formed by SEGS due to adsorption of its molecules on the steel surface. The thickness and protective efficiency of this film increases with increase in Rct and inhibitor concentration. A similar behaviour can also be observed with the Bode plots. The Nyquist plots produced imperfect semicircles, signifying deviations from ideal behaviour. This kind of deviation has been explained in terms of frequency dispersion of interfacial impedance and attributed to inhomogeneity of the metal surface arising from surface roughness [28]. The degree of deviation can be estimated using values of of the CPE phase shift, most of which decreased with increase in inhibitor concentration (Table 4). The CPE was introduced instead of a pure capacitor to compensate for the deviation and the values of and of the CPE were used to compute the double layer capacitance. The values of decrease on addition of
ACCEPTED MANUSCRIPT inhibitor (Table 4) which indicate a decrease in the charge and discharge rates of the capacitor [28]. Decrease in can also be due to decrease in local dielectric constant and increase in thickness of the double layer formed, suggesting strong adsorption of SEGS molecules on the metal surface [27]. A similar trend was observed for all the grades of steel studied with X80 grade showing the highest difference in from that of the blank acid.
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Fig. 4. Equivalent circuit model used for fitting EIS data.
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The calculated inhibition efficiency increased with increase in SEGS concentration, similar to the trend of charge transfer resistance. The highest inhibition efficiency of 96.73 % was obtained with X80 grade at 1000 ppm inhibitor concentration. This implies that more SEGS molecules are assembled on X80 making the inhibitor more effective against X80 corrosion than J55 and MS. This can also be attributed to adsorption of its phyto-compounds like 5-hydroxytryptophan, clomipramine, griffonin, amitriptyline, fluvoxamine, melatonin, etc. [29] which are rich in heteroatoms (oxygen and nitrogen) and pie electrons (see structures in Fig. 5). These molecules have large sizes and many potential adsorption sites, hence could adsorb on the steel surface, block the metal actives sites, achieve large surface coverage and reduce corrosion rate of the metals. However, at 100 ppm inhibitor concentration, inhibition efficiency was the lowest (55.52%) with J55 grade.
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(i) Melatonin
(ii) Fluvoxamine
(iii) 5-Hydroxytryptophan
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Fig. 5. Molecular structures of some chemical compounds in SEGS
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Table 4
(Ωcm2) 1.035 0.999 0.873 0.869 1.131 1.058 0.532 0.983 1.063 1.097 1.038 1.028
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(μΩ-1sn) 157.7 81.62 80.50 60.20 139.6 113.2 115.3 117.8 206.4 126.2 113.8 75.9
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0.572 0.566 0.547 0.538 0.555 0.554 0.555 0.625 0.551 0.541 0.554 0.561
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(Ωcm2) 102.3 385.5 1089.0 1144.0 76.2 171.3 868.9 1003.2 30.64 124.0 372.1 936.3
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EIS parameters calculated for MS, J55 and X80 steel grades
(μFcm-2) 127.7 62.2 10.0 3.4 35.1 11.9 3.5 0.8 126.3 10.4 6.5 2.1
(%) 73.46 90.61 91.06 55.52 91.23 92.40 75.29 91.77 96.73
3.4. Potentiodynamic Polarization
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Potentiodynamic polarization measurements were conducted with the different steel grades to evaluate cathodic and anodic corrosion kinetics in the absence and presence of the inhibitor. The partial electrode reactions were approximated using Tafel plots (Fig. 6) and the compromise or free corrosion potential ( ) was determined as well as the corresponding corrosion current density ( ). Linear fitting of the Tafel slopes afforded tafel cathodic and anodic constants, and respectively. The corrosion potentials obtained for all the metals shifted to relatively more positive values suggesting that the inhibitor could be anodic type [30]. However, the differences in values of the inhibitors from those of the free acid solution were not up to 85mV, hence not sufficient to categorize the inhibitor as cathodic or anodic type. Therefore, EGS can be considered as mixed type inhibitor which inhibits the corrosion process by geometric blocking of both cathodic and anodic surface active sites of the different steel grades. Also, corrosion current density reduced in the presence of the inhibitors and with increase in SEGS concentration (Table 5) demonstrating the inhibitive effects of SEGS. The inhibition efficiency also increased with increase in inhibitor concentration, similar to EIS and weight loss results. Addition of inhibitor was also found to influence the Tafel constants in different ways for the different metals. While it seems to increase the constants for MS and X80, the constants showed no definite trend for J55. Inspection of the Tafel constants reveals the trend MS>J55>X80 for
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and X80>MS>J55 for . Also, there are larger dispersion in values than which suggests anodic predominance of the inhibitor. This demonstrates that SEGS is a mixed type inhibitor but with predominant anodic activity. The shapes of the curves are similar, supporting that the inhibitor does not change the mechanism of partial corrosion reactions at the electrodes [30].
PDP parameters calculated for MS, J55 and X80 steel grades
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94.06 98.41 98.55 88.42 89.85 93.26 83.56 90.93 92.84
(iii) Tafel plot for X80
-0.4
-2
-4
1000ppm 500ppm 0ppm 100ppm
-4
log I
-4
-1
-499 -487 -483 -482 -473 -472 -470 -461 -487 -476 -474 -469
(%)
-3
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log I
-3
(iii) Tafel plots for J55
(mV/SCE)
-2
CE
-2
(μAcm-2) 969.3 48.8 15.4 14.0 614.9 71.2 62.4 41.5 693.7 114.0 63.0 49.7
log I
-1
(mVdec-1) 64.7 100.3 98.7 92.7 111.2 97.7 78.9 81.2 98.5 124.6 85.0 103.5
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X80
(mVdec-1) 95.4 65.9 107.5 124.4 71.3 76.1 60.4 95.8 82.1 103.5 107.3 115.1
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J55
Conc. (ppm) 0 100 500 1000 0 100 500 1000 0 100 500 1000
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Steel MS
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Table 5
100ppm 0ppm 500ppm 1000ppm
-6
-5 -6 -7
-0.3
-0.2
E (V vs. SCE)
-0.6
Fig. 6. Tafel plots for MS, J55 and X80 3.5. Electrochemical Frequency Modulation
-0.4
E (V vs. SCE.)
-0.7
-0.6
-0.5
-0.4
E (V vs. SCE.)
-0.3
-0.2
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EFM measurement was also carried out using small signals to obtain corrosion current values without prior knowledge of the Tafel slopes. The corrosion rate, corrosion potential and casuality factors (CF-2 and CF-3) were also obtained as displayed in Table 6. Inhibition efficiency calculated from corrosion current values varied with inhibitor concentration similar to results of other measurements, but lower in magnitudes compared to EIS, PDP and LPR results. Experimental causality factors CF-2 and CF-3 calculated from the frequency spectrum of the current responses were close to theoretical values with MS and J55 steels indicating that the measurements are of good quality [31]. However, CF-2 exceeded theoretical values with X80 steel, suggesting the influence of electrochemical noise on that measurement.
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Table 6 EFM parameters for MS, X80 and J55 steels
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(mVdec-1) 107.8 94.4 97.9 100.2 93.4 83.8 97.6 117.4 42.7 105.2 103.3 93.0
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(mVdec-1) 64.4 87.5 90.6 92.3 83.7 87.5 85.9 89.5 40.8 93.1 84.6 85.6
CF-2 1.966 1.898 1.487 1.706 2.177 2.655 2.089 2.137 0.945 1.895 1.708 1.350
CF-3 2.358 2.362 2.206 2.240 2.727 2.094 2.792 2.211 1.704 3.728 3.000 2.828
(%) 60.04 60.87 74.33 70.85 77.34 86.56 41.59 62.60 78.39
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J55
(μAcm-2) 162.6 64.9 63.6 41.7 367.4 107.1 87.3 49.4 473.9 276.8 181.0 102.4
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X80
Conc. (ppm) 0 100 500 1000 0 100 500 1000 0 100 500 1000
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Steel MS
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3.6. Surface morphology and protection
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Figure 8 shows scanned micrographs of the surfaces of different steel grades after immersion in 1M HCl in the absence and presence 1000 ppm SEGS. It can be seen from the micrographs that the surface of the metals, especially J55 steel were greatly damaged (images on the left) in the absence of the inhibitor. However, in the presence of SEGS, the pitting or corrosive attack on the metals surfaces reduced. Inspection of the images on the right hand column reveals that SEGS provided good protection for all the metals studied. J55
X80
MS
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Fig. 7. SEM micrographs of J55, X80 and MS immersed in 1M HCl without (left column) and with (right column) 1000 ppm SEGS
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Acknowledgements
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The authors are pleased to acknowledge the World Bank for support to carry out laboratory work abroad through the Robert S. McNamara Fellowship Programme. We also appreciate Materials Physics and Chemistry Department, China University of Petroleum Qingdao for providing facilities for carrying out this research and African Centre of Excellence in Oilfield Chemicals Research for their support. EI is grateful to Prof. A. P. Udoh, Prof. Hu, Dr. Li, Dr. Wang, Ubong Jerome, Min, Cheng and Xiang Xiqiang in UPC for their assistance. References
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