doi: 10.4152/pea.201606417
-->Electrochemical, Adsorption and Anticorrosion Study on Fluvoxamine and its Blends for X80 Steel Protection in Well Acidizing Fluids
Ekemini Ituena,b,* , Abosede Jamesc and Onyewuchi Akarantab,c
a Department of Materials Physics and Chemistry, China University of Petroleum, Qingdao, China
b African Centre of Excellence for Oilfield Chemicals Research, Institute of Petroleum Studies, University of Port Harcourt, Nigeria
b Department of Pure and Industrial Chemistry, University of Port Harcourt, Nigeria
Abstract
]]> Corrosion of steel pipes, tubing, and casings in the oilfield by acidizing fluids is a major cause of materials failure. Corrosion of X80 steel in hydrochloric acid, a typical acidizing solution, was probed using gravimetric and electrochemical techniques. Fluvoxamine (FLU) was used as anticorrosive additive to the acid, and monitored between 30 °C to 90 °C. FLU offered efficient protection for the X80 steel by inhibition. The inhibitive effect increased with a higher FLU concentration, but decreased as temperature increased. The highest concentration of FLU (10×10-5 M) afforded inhibition efficiency of 93.5% and 80.4% in 1 M and 15% HCl, respectively, at 30 oC. The efficiency decreased to 88.5% and 73.3%, respectively, at 90 °C. Inhibition efficiency was improved by blending FLU with glutathione, polyethyleneglycol and potassium iodide. Adsorption and thermodynamic studies reveal that the inhibitor functions, by spontaneous exothermic adsorption on X80 surface, were best described by Langmuir adsorption model. Impedance measurement reveals increase in charge transfer resistance with decrease in double layer capacitance. Polarization studies show that FLU acts as a mixed type inhibitor. Scanned micrographs of X80 surface show a lower pitting in the presence of FLU than in the free acid, demonstrating the effectiveness of FLU as X80 steel corrosion inhibitor.Keywords: acid corrosion, charge transfer resistance, corrosion inhibitor, double layer capacitance, SEM, EIS.
Introduction
Acidizing is an important field technique in oil and gas production, comprising wellbore, matrix and fracture acidizing procedures [1]. In wellbore acidizing, acid is used to wash the wellbore in order to remove scales, corrosion products, and some acid soluble debris from drilling, completion, production treating or stimulation [2]. Matrix acidizing involves treating the formation with acid beyond the wellbore [3]. In fracturing, acid is forced under high pressure through the well bore into the reservoir rock formation to chemically react with rocks (calcite, limestone and dolomite) and dissolve them, and to open new flow channels and enlarge existing ones [4, 5]. In the process, cracks are produced in the formation, and acidic solution is introduced into the fracture to etch the flow channels and enlarge pore spaces [4]. Techniques for enhanced oil recovery, removal of scales and drilling mud damage are also associated with the introduction of acids [2].
The most commonly used acid in acidizing procedure is HCl, and at a concentration up to 15% [2]. Formic acid, acetic acid and hydrofluoric acid have also been used, and sometimes as a mixture with sulphuric, phosphoric, nitric, citric, methanesulphonic and chloroacetic acids [2, 5, 6]. Hydrochloric acid is more economical and reacts very fast with the formation, but is highly corrosive. Owing to this corrosive nature of HCl, corrosion inhibitors (CIs) are usually added to protect the surface of steel materials from corrosion and pitting. Many corrosion inhibitors have been reported to be effective in various acid solutions in different concentrations [5, 7-9]. However, it is more desirable that a corrosion inhibitor (CI) be cheap and environmentally and people friendly. It should also be easy to synthesize or processed from inexpensive sustainable sources.
Fluvoxamine is a non-toxic compound which has been used as anti-depressant [10], for treatment of obsessive-compulsive disorder (OCD) [11], and as a potent inhibitor of the metabolism of caffeine in vitro [12]. It is also one of the major compounds that can be obtained from seeds of Griffonia simplicifolia [13]. Many electron rich sites found in efficient organic corrosion inhibitors reported in literature [5, 7] are also found in the chemical structure of FLU (Fig. 1).
This motivated us to investigate FLU as alternative corrosion inhibitor for acidizing fluid. Different concentrations of HCl were used to simulate acidizing fluids. The X80 steel was used in this study, because such steel grades like API 5L X70, X80, X60, X65, X52 and X42 are extensively preferred for line pipes, transfer tubings, drilling pipes, casings, etc. [14, 15].The use of FLU as corrosion inhibitor for X80 steel is being reported for the first time. Electrochemical and gravimetric techniques were employed to determine the inhibition efficiency and mechanism of inhibition. The morphology of the protected metal is studied using SEM. The effect of temperature on the inhibitor performance is also examined. CI is also blended with synergistic additives to assess their performances at high temperature.
]]>Experimental methods
Preparation of steel specimens
X80 steel specimens with 2 cm × 2 cm dimensions were supplied by Qingdao Tengxiang Instrument and Equipment Co. Ltd., China. As-received surface of X80 steel was treated as provided by NACE Recommended Practice RP-0775 and ASTM G-1 & G-4, for surface finishing and cleaning of weight loss coupons. Coupons for electrochemical studies were abraded with various grades of silicon carbide paper, and 1 cm2 of the exposed surface was finished to mirror surface with CC-22F P2000 grade of silicon carbide paper. The prepared specimens were enclosed in sealed water-proof bags and stored in moisture free desiccator prior to use. The chemical compositions (wt. %) of X80 were 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).
Preparation of test solutions
Analytical grade HCl was diluted to concentrations of 1 M and 15% using double-distilled water. As received powdered fluvoxamine (AR), supplied by Meyers Co. Ltd., China, was prepared to different concentrations (1×10-5, 3×10-5 , 5×10-5 and 10×10-5 M) in HCl solutions. The additives used in blending are: industrial grade polyethyleneglycol, PEG-4000 (PEG) and analytical grade potassium iodide (KI) supplied by Richest group Ltd., Shanghai; analytical grade sodium gluconate (NaG) and industrial grade glutathione (GLU) supplied by Wuhan Yuancheng Gongchuang Technology Co. Ltd., China; and potassium chloride (analytical grade) supplied by Meyer Chemical Technology Co. Lt., Shanghai, China. All reagents were used as supplied, without further analysis or purification. 1×10-6 M of each additive was blended with 10×10-5 M FLU in the ratio 1:1 and prepared in the HCl with vigorous stirring.
Weight loss measurement
Weight loss measurements were conducted according to ASTM standard method explained in literature [5]. All weight measurements were conducted using a Sartorius CPA225D analytical balance with sensitivity ±0.00001 g. Pre-weighed X80 coupons were immersed in acid solutions with and without different concentrations of FLU for five (5) hours, and maintained at 30 °C in a water bath. The retrieved coupons were cleaned in 20% NaOH solution containing about 200 g/L of zinc dust, dried in air after rinsing in acetone, and weighed to determine the weight loss. This was repeated using the inhibitor blends. Triplicates of the experiments were conducted per test solution, and the means of the weight losses (g) were computed and reported. The corrosion rate (CR) of iron, percentage inhibition efficiency (εWL) and degree of surface coverage (θ) were calculated as follows
]]>
where ΔW
Electrochemical measurements
The electrochemical workstation used for measurement was Gamry ZRA REF 600-18042 Potentiostat/Galvanostat. The conventional three electrode set up was used, consisting of saturated calomel electrode (SCE) as reference electrode, platinum as counter electrode and X80 coupons as working electrode. The EIS were conducted at a frequency of 10 kHz to 10 mHz, for open circuit immersion time of 30 minutes, at 30 °C. The voltage was changed to -0.15 V to +0.15 V vs. Eoc at scan rate of 0.2 mV/s for PDP measurements. Linear polarization resistance measurements (LPR) were carried out at -0.20 V to +0.20 V vs. Eoc at 1 mV/s. 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. E- Chem software package was used for data fitting and analysis.
Charge transfer resistance obtained from analysis of Nyquist plot was used to compute the inhibition efficiency according to Eq. 4.
where RctB and RctI are charge transfer resistances in the absence and presence of the inhibitor, respectively.
The inhibition efficiency from PDP was calculated from the corrosion current densities obtained from analysis of Tafel plot using Eq. 5.
]]>
where ibcorr and iicorr are the corrosion current densities in the absence and presence of the inhibitor, respectively.
The magnitude of the double layer capacitance (Cdl) of the adsorbed film was calculated from constant phase element (CPE), constant (Yo) and charge transfer resistance (Rct) using Eq. 6.
where n is a constant showing a degree of roughness of the metal surface obtained from the phase angle, given that (j2=-1)α is the phase angle of CPE and n=2α/(π) is the CPE exponent.
SEM study
To study the morphology of the metal in both uninhibited and inhibited solutions, the steel coupons of size 1 cm × 2 cm were abraded to mirror finish, as described above. The SEM images were recorded in the vacuum mode before and after immersion in HCl using AMETEX S4800 SEM/EDAX TSL. This was repeated with a coupon immersed in HCl containing 10×10-5 M FLU solution. The instrument was operated at 5 kV.
]]> Results and discussion
Weight loss experiment: effect of inhibitor and acid concentration
The corrosion rate (CR), inhibition efficiency (%I) and fractional surface coverage obtained from weight loss measurements for the corrosion of X80 in both 1 M HCl and 15% HCl containing the different concentrations of FLU are presented in Table 1.
Inhibition efficiency obtained increased from 76.25% to 94.30%, when the concentration of FLU was increased from 1×10-5 M to 10×10-5 M, at constant temperature. The efficiency of FLU was also tested for both 1 M HCl and 15% HCl, and results were compared. The effectiveness of FLU declined when the acid concentration was increased from 1 M to 15%. For instance, the inhibition efficiency decreased from 94.3% in 1 M HCl to 80.4% in 15% HCl at 30 oC. This represents about a 14.7% decrease in inhibition efficiency on about a 340% increase in acid concentration, which is reasonable and may be improved using some synergistic intensifiers.
Effect of temperature
Nowadays, many industries have ventured into the production and recovery of hydrocarbons from deep pay zones. Since the temperature at the surface is usually different from down-hole temperature, understanding the effect of temperature on a corrosion inhibitor is essential. As one goes down the well, the difference in temperature per unit well length has been described as geothermal gradient. It is believed that the universal average geothermal gradient is about 25 °C per km of depth (1°F per 70 feet of depth) [17, 18]. From literature, the average geothermal gradient in Nigeria Niger delta fields is 28 °C/km [19].
Therefore, minimally deep wells in the Nigerian Niger-delta region have been reported to be about 2-4 km deep, corresponding to average downhole temperatures of 65-125 °C [20].
On this basis, the performance of FLU was examined at higher temperatures up to 90 °C. Results obtained are shown in Table 2.
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Inhibition efficiency initially increases as temperature increases, and later decreases with higher temperatures. With 10×10-5 M FLU in 1 M HCl solution, the inhibition efficiency increases from 94.3% at 30 °C to 95.1% at 60 °C, before decreasing to 88.5% at 90 °C. Similar trend was obtained in 15% HCl.
This behaviour is graphically illustrated in Fig. 2.
This implies that if FLU is used as CI in acidizing fluid, it will perform more efficiently at surface conditions than at down hole conditions. In literature, similar trend of inhibition efficiency is associated with both physical and chemical adsorption mechanism [21].
Effect of additives
It has been reported that certain substances (called intensifiers) can increase the inhibition efficiency of some corrosion inhibitors [5]. Intensifiers are desirable, because corrosion inhibitors often may not provide adequate protection to steels, at high temperatures and long exposure time.
Therefore, 10×10-5 M FLU in both 1 M and 15% HCl solutions were blended with potassium iodide (KI), polyethylene glycol (PEG), sodium gluconate (NaG), and glutathione (GLU). The results obtained (Tables 3-4) show that the additives improved the inhibition efficiency of FLU.
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For instance, at 90 °C, the blends containing PEG, KI and GLU afforded %I of 93.5%, 94.5% and 95.6 %, respectively, in 1 M HCl and 79.6%, 80.8%, and 84.5% in 15% HCl.
This demonstrates that the inhibitor blends obtained could be suitable alternatives for various oilfield acidizing procedures associated with high temperature operations. However, a study is ongoing to formulate the FLU using surfactants, other intensifiers, solvents, etc., to optimize effectiveness at high temperatures.
Adsorption study
Adsorption of CIs can be either by physical or chemical adsorption mechanism. To predict this mechanism, surface coverage (θ) data were fitted into adsorption models namely, Langmuir, Temkin, Freundlich, Flory Huggins and El-Awady et al. The best fit was obtained with Langmuir isotherm (Fig. 3), with adjusted R2 ≥ 0.99255.
The expression for the model is given in Eq. 7 [22].
]]>
where Cinh is the concentration of inhibitor (M) in the acid solution and is Kads the adsorption-desorption equilibrium constant, which relates to the free energy change of adsorption (ΔGads), according to Eq. 8.
where 55.5 represents the concentration of water molecules displaced by the inhibitor molecules, R is the universal gas constant and T is the absolute temperature. The values of Kads and ΔGads obtained are given in Table 5.
The constant, Kads, relates to the strength of the inhibitor-metal surface interaction. It can be observed that Kads decreases as temperature increases, which indicates that the strength of adsorptive binding of FLU to the steel surface decreases as temperature increases, perhaps due to desorption of its molecules from the surface. The Kads values are also higher in 1 M HCl than in 15% HCl, demonstrating that the inhibitor is more strongly adsorbed on X80 steel in the former than in the later.
The ΔGads values obtained are higher than -40 kJ mol-1, suggesting that the mechanism is of chemical adsorption [23]. This indicates that the adsorption of FLU on X80 surface is facilitated by chemical interactions between FLU electrons and empty d-orbitals of iron in the steel. The Langmuir adsorption isotherm assumes equivalent X80 sites with the monolayer of FLU molecules, as shown in the almost unity values of the slope obtained. Therefore, the results obtained are in agreement, since chemisorption mechanism is consistent with monolayer adsorption. In addition, adsorption of FLU is more spontaneous as temperature increases.
]]> Kinetic and thermodynamic studiesThe corrosion rate data were fitted into Arrhenius kinetic model (Eq. 9), and activation energy was calculated from the slope of linear plots of Log(CR), against reciprocal of temperature (Fig. 4).
where Ea is the activation energy, A is the Arrhenius pre-exponential factor or frequency factor, R is the universal gas constant and T is the absolute temperature.
The obtained activation energy increased on addition of the inhibitor, depending on the inhibitor's concentration. From the concept of activation and collision theory, it can be considered that, before the acid solution corrodes the steel, the molecules of the acid must collide with the metal molecules on the surface. The acid molecules should possess energy up to a minimum threshold called activation energy.
In the presence of the inhibitors, the activation energy values were higher than in the uninhibited solutions (Table 6).
]]> Therefore, the acid molecules must acquire extra (higher) energy in the presence of the inhibitor, for corrosion to occur, hence, corrosion inhibition [24]. The energy also increases as the inhibitor concentration increases. The activation energy was lower in 15% HCl than in 1 M HCl, implying that molecules of 15% HCl solution require a lower energy barrier to cross the activated complex and form corrosion products with mild steel than that of 1 M HCl. The increase in activation energy in the presence of the inhibitor in both acids is consistent with trends reported in literature, and it is associated with physical adsorption mechanism [25].
The other activation parameters given in Table 6 were derived from the transition state equation (Eq. 10)
where ΔH* and ΔS* are the enthalpy and entropy changes of activation, respectively.
Linear plots of Log(CR)/T against reciprocal of temperature are given in Fig. 5)
The values of ΔS* are all negative, which implies that a decrease in disorderliness of the system takes place on moving from reactants to the activated complex. It also indicates that the activated complex in the rate determining step involves an association of the inhibitor on metal surface, instead of dissolution of the metal [26]. The negative sign of ΔH* reflects the exothermic nature of X80 steel corrosion in HCl. Thermodynamically, it is consistent for an exothermic process with decreasing entropy to be spontaneous at temperatures that are not too high [27].
EIS experiment
]]> Nyquist and Bode plots shown in Fig. 6 were obtained from analysis of the EIS results for the corrosion of X80 steel in 1 M HCl, with and without different concentrations of FLU.
The Nyquist plot yields semicircles with larger diameters in the presence of FLU than in of HCl solution, depending on FLU's concentration.
The difference in diameter size from that of the HCl solution is due to the influence of FLU on the corrosion rate of X80 steel through the inhibition process. This is further supported by an increase in diameter, as the inhibitor concentration increases, as it also happens with the obtained inhibition efficiency. The semicircles are imperfect in shape, and this has been attributed to the inhomogeneity of the steel surface, due to adsorption of the inhibitor [28].
The shapes of the plots were similar in both the inhibited and free acid solution, indicating that the mechanism of steel corrosion is not influenced by the introduction of FLU. The single capacitive loop obtained also indicates that the mechanism of corrosion is mainly controlled by charge transfer process [26]. The RS(RCT . φCPE) equivalent circuit model (Fig. 7) provides a good fit (Χ2 ≤ 10-4) for the obtained data.
With this model, a constant phase element (CPE) compensates the surface inhomogeneity of the steel. The CPE is a non-integer element dependent on frequency, and its impedance can be estimated using Y0 and n in Eq. 11.
]]>
where ZCPE is the impedance of the CPE, Y0 is the CPE constant, w is the angular frequency, j is an imaginary complex number, (j2 = -1)α is the phase angle of CPE and n = 2 α/(π).
Some of the EIS parameters calculated are presented in Table 7.
The value of n decreases on addition of FLU, indicating that adsorption of FLU molecules increases the surface roughness of the steel [28]. It also shows that there is relative and/or integrated influence on the CPE, rather than just a single resistance, capacitance or inductive element. Decrease in n on addition inhibitors also signifies insulation of the metal/solution interface by formation of a surface film. Film formation increases the charge transfer resistance in the presence of the inhibitor. The charge transfer resistance increases with an increase in the inhibitor's concentration, showing that the 'blanketing' property of the film is better at higher inhibitor's concentrations.
Increase in peak heights of the Bode plots suggests a higher capacitive response of the interface, caused by the presence of the adsorbed inhibitor layer [28], due to the formation of an electrochemical double layer with a magnitude capacitance (Cdl) estimated using Eq. 6. The calculated Cdl values decrease in the presence of inhibitors, similar to what is reported in literature [28, 29]. This is attributed to a decrease in the local dielectric, or to an increase in the thickness of the double layer or both, caused by the adsorbed protective film of the inhibitors.
PDP experiment
The chemical composition of X80 steel shown above shows that it is a mixture of many metals, hence, a number of oxidation reactions may occur at the anode (Eq. 11). The oxidation of iron in the steel sample is represented by Eq. 13.
]]>
The reaction at the cathode could involve reduction of water in aqueous environments with sufficiently negative potential (Eq. 14), and/or evolution of hydrogen (Eq. 15).
The driving force (i.e. potential) was controlled so as to measure the current as a function of the net change in reaction rate. The sum of currents resulting from the electrode processes can be used to obtain the compromise current or free corrosion current density (icorr) and the corresponding potential (Ecorr) using Tafel plots (Fig. 8).
Also, the cathodic and anodic constants (βc and βa) and some other PDP parameters were obtained and are shown in Table 8.
]]> The values decreased with an increase in the inhibitor's concentration, compared to the free acid solution. This is due to the formation of an adsorbed protective film of FLU on the X80's steel surface. A displacement of Ecorr to less negative values in the inhibited solutions compared to the free acid solution was also observed. Corrosion inhibitors that displace Ecorr to less negative values are usually identified as anodic inhibitors, whereas cathodic inhibitors displace the potential to more negative values [30]. Therefore, the Ecorr values obtained indicate that the inhibitor has dominant influence on the partial anodic reaction. However, the highest difference from that of the free acid (δ Ecorr) not up to -85 mV, hence, not sufficient to categorize the inhibitor as cathodic or anodic type. Such corrosion inhibitors are usually regarded as mixed type inhibitors with anodic predominance [31]. The obtained values of βc and βa differ on addition of inhibitor from those of the free acid solution. The highest difference was obtained with βa, confirming that FLU exhibits more influence on the anodic reaction.
A mixed type inhibitor acts by blocking some active anodic and cathodic sites of the metal, without changing its corrosion mechanism. In other words, FLU inhibits both the iron dissolution and hydrogen evolution processes, but more actively inhibiting iron oxidation (anodic reaction). The mechanism may be activation or diffusion controlled. The calculated inhibition efficiency also increased with an increase in concentration of the inhibitor, similar to EIS results.
LPR measurement
The polarization resistances in the absence and presence of different concentrations of FLU were obtained using insights from the Stern-Geary equation (Eq. 16).
Inhibition efficiency was calculated from the polarization resistances using Eq. 17.
where and are the polarization resistances with and without the inhibitor, respectively. Obtained values of (Table 9) increased with an increase in FLU's concentration.
]]>
The obtained inhibition efficiency followed similar trend as PDP, with respect to concentration. However, the obtained inhibition efficiency was relatively lower than that from PDP measurements, but comparable to EIS results.
EFM measurement
EFM measurement was also carried out using small signals to obtain the causality factors (CF-2 and CF-3) and Tafel constants displayed in Table 10.
The obtained causality factors CF-2 and CF-3 are close to theoretical values, indicating that the measurements are of good quality [32]. The slight differences can be attributed to the influence of electrochemical noise on the measurement. Variation in the values of the Tafel constants demonstrates that the inhibitor has more influence on βa, supporting PDP results.
SEM studies
Micrographs of an abraded X80 steel coupon prior to immersion, and then immersed in 1 M HCl, with and without FLU, for 24 hours, were recorded by SEM. Results reveal that the surface of the coupon immersed in the free acid solution (Fig. 9, left) experienced damage, due to a corrosive attack leading to pitting and undulation.
]]>
However, the surface of the coupon immersed in the solution containing FLU (Fig. 9, right) was relatively smoother compared to the free acid solution.
This demonstrates that the addition of inhibitor reduces the corrosive pitting which occurs in the free acid solution. The extent of protection can be considered to be high, since pitting decreases significantly.
The protective layer formed by the inhibitor was not evenly distributed over the metal surface, which is why some portions on the surface are smoother than others. Thus, the active sites on the steel surface might not be equivalent or possess similar affinity for the active molecules of the inhibitors. Also, some cracks, which may arise during cleaning of the coupon after retrieval, are visible on the protected surface.
Conclusions
Fluvoxamine has been assessed as an alternative anticorrosion additive for X80 steel surface protection in hydrochloric acid. The efficiency of FLU on inhibition of X80 steel corrosion decreases as temperature increases, and increases as FLU concentration increases. Blending of FLU with potassium iodide, polyethyleneglygol and glutathione further improves the efficiency. FLU behaves as a mixed type inhibitor with anodic predominance. Adsorption of FLU on X80 steel surface is spontaneous, exothermic and best described by Langmuir adsorption model. SEM micrographs of X80 steel surface are protected from the aggressive HCl solution by addition of FLU. FLU and its blends could be useful as effective alternative ecofriendly corrosion inhibitors for X80 steel materials in acidic well treatment fluids.
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Acknowledgements
The authors acknowledge support from World Bank, through the Robert S. McNamara Fellowship programme, to conduct laboratory work abroad. We also are grateful to Dr. Shuangqing Sun, of Materials Physics and Chemistry Department, China University of Petroleum Qingdao, for providing their facilities and softwares to carry out this research, and to African Centre of Excellence in Oilfield Chemicals Research, for their support. EI is grateful to Prof. A. P. Udoh, Dr. B. S. Antia, Prof. Ogbona Joel, Prof. Hu, Dr. Li, Dr. Wang, Ubong Jerome, Chen, Chao, Xiang and Min in UPC, for their assistance.
*Corresponding author. E-mail address: ekeminiituen@uniuyo.edu.ng
Received September 29, 2016; accepted October 29, 2016
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