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Egyptian Journal of Petroleum xxx (2018) xxx–xxx

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Magnetite nanoparticles/polyvinyl pyrrolidone stabilized system for corrosion inhibition of carbon steel Eman A. Khamis, Amal Hamdy, Rania E. Morsi ⇑ Egyptian Petroleum Research Institute, Cairo 11727, Egypt

a r t i c l e

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Article history: Received 19 December 2017 Revised 26 January 2018 Accepted 1 February 2018 Available online xxxx Keywords: Carbon steel Magnetite nanoparticles Stabilization Potentiodynamic polarization Impedance spectroscopy

a b s t r a c t The corrosion inhibitive effects of new polyvinylpyrrolidone stabilized crystalline super-paramagnetic nanoparticles (5–20 nm) were investigated. Several characterization techniques confirmed the high stability of the prepared stabilized nanoparticles in solution. The polarization and EIS measurements showed that the inhibition efficiency increased with increasing concentration of the magnetite nanoparticles. The results obtained from EIS and electrochemical polarization curves are in reasonably good agreement. The obtained results suggest that the prepared stabilized system is an excellent inhibitor for carbon steel corrosion in 1 M HCl solution. Ó 2018 Production and hosting by Elsevier B.V. on behalf of Egyptian Petroleum Research Institute. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction Corrosion protection studies of carbon steel alloys are very vital in the industry due to their enormous input in many industrial applications especially in the petroleum industry and power plants [1]. Acid solutions are widely used in industrial applications being utilized as acid pickling of steel, chemical cleaning and processing for removal of rust and scale and oil well acidification [2,3]. Hydrochloric acid is one of the most common acids used for these purposes. However, this acid attacks the metal and initiate corrosion; this corrosion may cause serious damages to the metal and degrade its characteristics, hence limiting its applications [4], thereby corrosion causes serious operational problems leading to economic loss [5,6]. It is, therefore, essential to add corrosion inhibitors to the solution during pickling in order to reduce the degree of metal attack [7–10]. Although corrosion inhibitors are the most effective and flexible solution for corrosion control in most systems, the selection of inhibitors are actually complicated because of the different corrosive media in these systems [11]. The effective inhibitor should have many advantages such as high inhibition efficiency, low price, low toxicity and easy production. Generally, nanomaterials can provide such advantages beside the enhancement of the environmental impact [12]. In this regard, iron oxide nanoparticles can be used as additive for reducing

Peer review under responsibility of Egyptian Petroleum Research Institute. ⇑ Corresponding author. E-mail address: [email protected] (R.E. Morsi).

corrosion problems. Generally, modified magnetite nanoparticles were previously used as corrosion inhibitors [13,14]. It is well known that the effectiveness of corrosion inhibitors is related to the extent to which they adsorb and cover the metal surface. Adsorption depends on the structure of the inhibitor, on the surface charge of the metal, and on the type of electrolyte [15]. Iron metal forms self-adhering films when exposed to moisture and oxygen based on magnetite to act as protective film from corrosive environment but this film is unstable to acidic solutions and salts [16]. However, it is well known that the stabilization is a significant factor for these nanoparticles to exhibit high corrosion inhibition efficiency, so iron oxide nanoparticles should gain a good stability at all conditions, in order to have a better corrosion protection for the metals and alloys. To achieve this; iron oxide nanoparticles should be functionalized with organic compound as stabilizing agent [17–19]. Polymers can be considered as very good choice for stabilization of corrosion inhibiting nanoparticles because polymeric compounds showed great effectiveness as low-cost and stable corrosion inhibitors for metallic materials in acidic medium. Inorganic nanomaterials can be dispersed in a polymer matrix or it can be chemically bonded to the polymer matrix to form a metal complex [20,21]. The inhibitive effect of polymers is directly correlated to the structure of polymers that posses different active centers of adsorption such as cyclic rings, hetero-atoms as oxygen and nitrogen. Such polymers can form complexes with metal ions and on the metal surface, these complexes occupy a large surface area thereby isolating the surface and protecting the metals from corrosive medium [22].

https://doi.org/10.1016/j.ejpe.2018.02.001 1110-0621/Ó 2018 Production and hosting by Elsevier B.V. on behalf of Egyptian Petroleum Research Institute. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: E.A. Khamis et al., Magnetite nanoparticles/polyvinyl pyrrolidone stabilized system for corrosion inhibition of carbon steel, Egypt. J. Petrol. (2018), https://doi.org/10.1016/j.ejpe.2018.02.001

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Several research groups have evaluated the effectiveness of polymers as corrosion inhibitors for metals in aggressive media. Polymers such as polyvinyl alcohol, polyethylene glycol, polyvinyl pyridine, polyvinyl bipyridine, polyvinyl pyrrolidine, polyvinyl pyrrolidone (PVP), polyethylene imine, polyacrylic acid, polyaniline, polyacrylamide and polyvinyl imidazoles have been investigated [23–29]. Most acid inhibitors are recognized for their specific action. However, the combination of inhibitors will provide advantages over using one specific inhibitor as the interaction of two inhibitors can maximize the inhibition action required for efficient corrosion control. The present investigation is an attempt to prepare a new stabilized compound based on two materials having good inhibiting action against corrosion. The prepared nanoparticles were characterized by TEM, DLS and XRD techniques. Then the inhibition effect of the prepared stabilized compound on carbon steel corrosion in 1 M HCl solution was investigated using potentiodynamic polarization and electrochemical impedance spectroscopy.

loaded while ultrasonication. An Ultrasonic bath (ElmasonicS15H, Germany) with frequency of 47 kHz, was used for stabilization and homogeneous dispersion of the magnetite nanoparticles for 2 h. Several characterization techniques were adopted to visualize the stability of the prepared suspension. Average particles size, particles size distribution and zeta potential were measured using dynamic light scattering (Malvern-ZS) nano-series. In addition, the produced suspension was imaged by transmission electron microscopy (TEM).

2. Experimental

2.5. Potentiodynamic polarization measurements

2.1. Materials

Volta-lab80 (Tacussel-radiometer PGZ402) potentiostat was used for electrochemical polarization measurements which controlled by Tacussel corrosion analysis software model (Volta master 4). The exposure surface area of the working electrode was 0.7 cm2. This area was abraded with emery paper (grade 320, 400, 600, 800, 1000, 1200) on the test face, rinsed with distilled water, degreased with acetone, and then dried. After that the working electrode was immersed in the test solution for 2 h until the open circuit potential is established. After that the working electrode was polarized in both cathodic and anodic directions. All potentials were measured against a saturated calomel electrode (SCE) as a reference electrode, where as a platinum electrode was used as an auxiliary electrode. The concentration range of synthesized inhibitors varied from 0.05 to 0.1%. The polarization curves were recorded by change the electrode potential automatically from 800 mV to 300 mV with a scan rate 2 mVs1 at 30 °C.

Polyvinyl pyrolidone (PVP), Sigma-Aldrich (Germany), analytical grad, was used as stabilizer. FeCl36H2O, FeCl24H2O, ammonium hydroxide and all other reagents, supplied from SigmaAldrich (Germany), were high grade and used as received without further purification. Deionized water was used to prepare all solutions. 2.2. Preparation of magnetite nanoparticles (M.NPs.) Magnetite nanoparticles were prepared via chemical precipitation method using molar ratio (1:2) of Fe (II): Fe (III), respectively in basic solution as follows [30,31]:

2FeCl3 þ FeCl2 þ 8NH3 þ 4H2 O ! Fe3 O4 þ 8NH4 Cl

ð1Þ

Briefly, 200 mL of purified water was bubbled by nitrogen gas for 30 min. Then 5.2 g FeCl3 and 2.0 g FeCl2 were dissolved in water with mechanical stirrer. Under the protection of nitrogen, 1.5 mol/ L NH4OH solution was added drop-wisely to the above mixture under vigorous stirring. After an initial brown precipitate, a black precipitate was formed which was then isolated, after completing the reaction, by an external magnetic field and the supernatant was decanted. The product was washed with water several times and then vacuum dried. The prepared material was characterized by X-ray diffraction (XRD) using a Philips X’Pertpro Pan-analytical instrument. The data were taken for the 2h range of 0 to 80 degrees with a step of 0.02 degree. High Resolution Transmission Electron Microscopy (HRTEM) imaging was performed using a Jeol-JEM, Japan 2100 operating at 200 kV, the samples for TEM were prepared by sonication of the samples in ethanol and depositing onto a copper coated carbon grid and letting the solvent to evaporate. Magnetic measurement was carried out using magnetometer using VSM Unit (Lake Shore 7410) with the filed sweeping from 20000 to +20000 Oe at room temperature. 2.3. Stabilization of magnetite nanoparticles (M.NPs.) Three stabilized magnetite nanoparticles dispersed samples were prepared in concentrations of 0.05, 0.075 and 0.1 wt% using 1% PVP solution as stabilization media. After homogenization of PVP and distilled water by magnetic stirrer, nanoparticles were

2.4. Chemical composition of the carbon steel The electrochemical corrosion experiments were performed in triplicates using carbon steel of the following composition (by weight, wt%): C = 0.07, Mn = 0.19, P = 0.03, Si = 0.04, Cr = 0.07, Al = 0.02, Cu = 0.13 and balance Fe. The aggressive solutions, 1 M HCl, were prepared by dilution of analytical grade 37% HCl with distilled water.

2.6. Electrochemical impedance spectroscopy (EIS) Nyquist plots for various concentrations of the investigated inhibitor were carried out using Volta lab 80 potentiostat (Tacussel-radiometer PGZ402) controlled by Tacussel corrosion analysis software model (Volta master 4). The working electrode was immersed in the corrosive media for 2 h then impedance measurements were carried out by applying a frequency range of 100 kHz and 50 mHz using 20 steps per frequency decade while 20 mV amplitude peak to peak AC signal was used to perturb the system. 3. Results and discussion 3.1. Magnetite nanoparticles preparation and characterization Magnetic nanomaterials (M.NPs) have gained special attention in many fields based on their numerous advantages such as high separation efficiency, simple manipulation process, kind operation conditions and easy specifically functional modifications [32,33]. The X-ray diffraction pattern of the prepared M.NPs (Fig. 1a) shows the identical peaks for magnetite, which were located at 30.10, 35.50, 43.10, 53.40, 57.00, and 62.60 corresponding to their indices (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 0 0) respectively [34]. In addition, a search match on the library of the instrument ICDD-PDF data base was held and confirmed matching with the standard M.NPs card (insertion of Fig. 1a). The magnetic nanopar-

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Fig. 1. XRD patterns with the insertion of matching with the ideal magnetite in the instrument library (a) HR-TEM with insertion of the diffraction pattern (b) and magnetic hysteresis curves of the prepared MNPs (c).

ticles were examined under HR-TEM and were seen as clusters of small spherical dots. Each individual dot was about from 5 to 20 nm in diameter as shown in Fig. 1b. The electron diffraction pattern (the insertion of Fig. 1b) indicated that the nanoparticles were highly crystalline, and could be well indexed to the cubic structure of pure Fe3O4 [35]. The magnetic properties of M.NPs were investigated at room temperature. Fig. 1c shows the magnetic hysteresis curves measured at 300 oK, with the field sweeping from 20000 to 20,000 Oe. From the obtained data, it was found that the prepared magnetite nanoparticles exhibit a super-paramagnetic characteristics and the saturation magnetization of M.NPs. was 45.43 emu/g. 3.2. Magnetite nanoparticles stabilization (M.NPs/PVP) The obtained results show that PVP stabilized magnetite nanoparticles exhibits much better dispersion stability. Magnetic iron oxide nanoparticles tend to agglomerate because of strong magnetic attractions among particles, the van der Waals force, and high surface energy [36]. Consequently, the agglomerated iron oxide nanoparticles can be rapidly eliminated by a polymer forming a stabilized system. Different weight percentages (0.05–0.1%) of magnetite nanoparticles were prepared using PVP solution as stabilization agent. PVP was found to modify the nanoparticles surface by formation of thin layer surrounding the nanoparticles (as shown in Fig. 2a) providing a high stability against sedimentation and improving thus their dispersion property in water [37]. The stability duration of magnetite nanoparticles stabilized system

was investigated using dynamic light scattering technique where the average aggregate size of the stabilized system was measured over one week and the results was illustrated in Fig. 2b. From the figure, it is clear that the prepared systems are stable along one week without significant aggregation or sedimentation of the particles. It is worth to mention that the slight fluctuation in the average aggregate size is most probably due to the Brownian motion of the aggregated particles in the system. It can also be observed that the freshly prepared system has the largest aggregate size for all stabilized systems which was decreased after one day. This can be attributed to the high magnetization properties of magnetite nanoparticles which resulted in an attraction between the magnetite nanoparticles causing thus a decrease in the average aggregate size. 3.3. Polarization measurements The results of the cathodic and anodic polarization curves of steel in 1 M HCl in the absence and presence of different inhibitor concentrations are shown in Fig. 3. It is apparent that both the cathodic and anodic branches of the polarization curves show lower values of current density compared to the uninhibited solution either for PVP or PVP stabilized magnetite nanoparticles, this may be explained by the suggestion that the addition of both inhibitors to the blank solution is accompanied by formation of a protective layer on the surface. The presence of such adsorbed layer causes a decrease in reaction rate of iron dissolution and hydrogen reduction on the surface of carbon steel. The corrosion parameters

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Fig. 2. (a) TEM of magnetite nanoparticles stabilized in PVP solution and (b) Stability over time of the prepared system.

Fig. 3. Polarization curves of carbon steel corrosion in absence and presence of different concentrations of magnetite nano-particles.

such as corrosion potential (Ecorr), corrosion current density (icorr), cathodic and anodic Tafel slopes (bc and ba) were obtained from the polarization curves and are listed in Table 1. The percentage inhibition efficiency (IE%) was calculated using the following equation [38]:

IE% ¼ 1 

  i  100 io

ð2Þ

where, i and io represent the corrosion current densities of inhibited and uninhibited electrodes respectively. The data listed in the table show that Ecorr values do not change significantly in inhibited solution as compared to uninhibited one,

which suggests that the compounds act as mixed-type inhibitor [39–40]. Also, it can be noticed that, the icorr values for the stabilized magnetite nanoparticles are less than that of PVP indicating the better inhibiting action of this stabilized compound. Furthermore, it is observed that, increasing the concentration of magnetite nanoparticles resulting in a further decrease in icorr values and hence an increase in the inhibition efficiency. This can be explained on the base of average aggregate size which increases with the increase of magnetite concentration, hence the surface coverage increase and consequently enhancement in the inhibition efficiency can be observed. The polarization measurements show also that, ba and bc values are slightly changed indicating that, the

Table 1 Potentiodynamic electrochemical parameters for the corrosion of steel in 1 M HCl solution in the absence and presence different conc. of the magnetite nano-particles at 30 °C. M.NPs content

Ecorr, mV vs. SCE

icorr, mA cm2

ba, mV dec1

bc, mV dec1

gp , %

Blank P 0.05% 0.075% 0.1%

558.2 535.1 548.7 553.0 588.6

1.31 0.19 0.09 0.042 0.021

128.5 143.1 118.3 111.4 103.7

144.1 155.2 141.2 130.4 122.7

0 85.49 93.12 96.79 98.39

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Fig. 4. Nyquist plots for the carbon steel in 1 M HCl in absence and presence of different concentrations of PVP stabilized magnetite nano-particles.

Fig. 5. Equivalent circuit used to fit the metal/acid interface containing different concentrations of the tested inhibitors.

inhibitors blocked the cathodic and anodic sites without changing the corrosion mechanism. 3.4. Impedance measurements EIS measurements provide a better understanding of the corrosion mechanism taking place at the electrode surface including the kinetics of the electrode processes and simultaneously about the surface properties of the investigated systems. The shape of impedance gives mechanistic information [41]. The corrosion of carbon steel in acidic solution has been investigated by the EIS method for further investigation of the corrosion inhibition performance of PVP stabilized M.NPs. Nyquist diagrams for the substrate in the presence and absence of investigated inhibitors are shown in Fig. 4. The Nyquist curve only displays a capacitive loop slightly deviating from an ideal semicircle, which is caused by surface roughness and is known as the ‘‘dispersing effect’’ [42]. The diameter of the capacitive loop is approximately equal to the value of the charge-transfer resistance of the process of corrosion reaction and is associated with the corrosion resistance ability of the tested compounds in HCl solution. It is clear from the demonstrated plots

that the impedance response of carbon steel in uninhibited HCl solution has been significantly changed after the addition of inhibiting compounds in the corrosive solution. The results indicate that the impedance of inhibited substrate increases with increasing inhibitor concentration and consequently the inhibition efficiency increases. All EIS spectra can be analyzed and fitted with the equivalent circuit shown in Fig. 5. It represents a single charge transfer reaction and fitted well with our experimental results. The intersection of the capacitive loop with the real axis represented the ohmic resistances of the corrosion product films and the solution enclosed between the working electrode and the reference electrode, Rs [43]. Rct represents the charge transfer resistance and its value was a measure of electron transfer across the surface and was inversely proportional to the corrosion rate [44]. The values of charge transfer resistance (Rct) and double layer capacitance (Cdl) are obtained from impedance measurements. These impedance parameters and values of inhibition efficiency are given in Table 2. The inhibition efficiency is calculated from charge transfer resistance as follows:



RctðIÞ  RCt  100 RctðIÞ

ð3Þ

where Rct and Rct(I) are the charge transfer resistance values with and without inhibitor, respectively for carbon steel in 1 MHCl. It was found from the listed data that, as the concentration of inhibiting compound increased the obtained Rct values increased, but the Cdl values tend to decrease. This increase in the magnitude of Rct and the corresponding decrease in Cdl values, with the addition of inhibitor are directly correlated to an increase in inhibition efficiency. Generally, the high Rct values are associated with slower corroding systems [45]. This noticed behavior in the EIS

Table 2 EIS parameters of carbon steel electrode in aqueous 1 M HCl solution in the absence and presence of various concentrations of the prepared inhibitor at 298 K. M.NPs content

Coefficient

Rs (ohm cm2)

Rct (ohm cm2)

Cdl (mF cm2)

gI (%)

Blank P 0.05% 0.075% 0.1%

0.975 0.975 0.973 0.980 0.988

12.17 10.92 12.73 11.43 10.24

35 247 502 650 982

110.34 100.43 95.35 87.13 77.20

– 84.61 92.43 94.15 96.13

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measurements can be attributed to a decrease in dielectric constant and/or an increase in the thickness of the electrical double layer, suggesting that the inhibitor molecules act by adsorption mechanism at carbon steel/hydrochloric acid solution interface [46]. So, it can be suggested that the decrease in Cdl values is a direct result to the gradual replacement of water molecules in the double layer by the adsorbed inhibitor molecules which form an adherent film on the metal surface and lead to a decrease metal dissolution [47]. The thickness of the protective layer, dorg is related to Cdl by the following equation [48]:

dorg ¼ o s =Cdl

ð4Þ

where eo is the dielectric constant and es is the relative dielectric constant. This decrease in the Cdl, which can result from a decrease in local dielectric constant and/or increase in thickness of the electrical double layer, suggested that the compounds act via adsorption

at the metal/solution interface [49]. It could be assumed that the decrease of Cdl values is caused by the gradual replacement of water molecules by adsorption of organic molecules on the electrode surface, which decreases the extent of the metal dissolution [50]. The adsorption can occur either directly on the basis of donoracceptor interaction between the unshared electron pairs and/or p-electrons of inhibitor molecule and the vacant d-orbitals of the metal surface or by interaction of the inhibitors with already adsorbed chloride ions [51–53]. The inhibition efficiencies, got from electrochemical impedance measurements, show good agreements with those obtained from polarization measurements. 3.5. Anti-corrosive action It is well known that, the variation in inhibitive efficiency mainly depends on the type and the nature of the substituent present in the inhibitor molecule. The obtained results show that PVP

Fig. 6. Effect of M. NPs concentration in the stabilized solution on the aggregate size as measured by DLS.

Fig. 7. Effect of M. NPs concentration on the Zeta potential values as measured by DLS.

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stabilized magnetite nanoparticles exhibits much better anticorrosive properties than PVP. This difference in protection action can be attributed to the superior properties of magnetite nanoparticles which deposited on the surface of carbon steel electrode, acting as protective layer and reduce thus the interaction between HCl and carbon steel surface. Magnetite is classified as ferromagnetic materials a long time ago, however, the research in nanosized magnetic materials has found that the magnetism of materials is highly size dependent. The general rule is that as the size of ferromagnetic substances is sufficiently small, they will be like a single magnetic spin, which has a larger response to the applied magnetic field. Below such a size, the substances display the super paramagnetic property. Nano-sized Fe3O4 with particle size less than 20 nm are often considered in the range of a single domain and exhibit a super paramagnetic property. Magnetic iron oxide nanoparticles tend to agglomerate but the agglomerated iron oxide nanoparticles can be rapidly eliminated by a polymer forming a stabilized system. PVP was found to modify the nanoparticles surface by formation of thin layer surrounding the nanoparticles and this composite form can have a double action for metal surface protection. The effect weight percentage of magnetite nanoparticles in solution on the average aggregate size measured by dynamic light scatterings can be seen in Fig. 6. It was found that as the concentration of the magnetite nanoparticles increased, the average aggregate size increased leading to enhancement of adsorption on carbon steel surface and consequently increasing the inhibition efficiency. Zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to the particle. The zeta potential indicates the degree of repulsion between adjacent, similarly charged particles in dispersion [54]. Generally, the polymer/magnetite nanoparticles possess positive or negative charges based on polymer types and pH of aqueous solution. The results obtained from the Zeta potential measurements (Fig. 7) show that as concentration of magnetite increased, the zeta potential increases and the system becomes less negative which facilitate the adsorption of the inhibitor on the metal surface. Thus, PVP/magnetite nanoparticles system form adherent film on the carbon steel through interaction between the metal surface and the inhibitor molecules and consequently increases the covered surface by the inhibitor and consequently decreases the attack of the metal surface by the aggressive medium.

4. Conclusions From this study, some concluding points can be reported as follows: 1. Super-paramagnetic highly crystalline magnetite nanoparticles of 5 to 20 nm could be prepared by chemical precipitation. 2. The limited dispersion and stability of magnetite nanoparticles in solutions generally limit their solutions related applications. 3. Magnetite nanoparticles could be perfectly stabilized in PVP aqueous solution by utilization of ultra-sonication. 4. The polarization and EIS measurements showed that, the stabilized magnetite nanoparticles are excellent mixed type inhibitors for carbon steel in 1 M HCl solution. The results obtained from both techniques are in reasonably good agreement. 5. The protection efficiency increased with increasing concentration of the magnetite nanoparticles in the stabilized system. 6. The decrease in the capacitance double layer (Cdl) with concentration indicated that, the studied compounds are adsorbed on the metal surface creating a physical barrier to charge and mass transfer for metal dissolution.

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7. The results obtained from the DLS measurements show that as concentration of magnetite increases, the average aggregate size increased providing higher surface coverage and, in addition, the zeta potential increases and the system becomes less negative which facilitate the adsorption of the inhibitor on the metal surface.

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Please cite this article in press as: E.A. Khamis et al., Magnetite nanoparticles/polyvinyl pyrrolidone stabilized system for corrosion inhibition of carbon steel, Egypt. J. Petrol. (2018), https://doi.org/10.1016/j.ejpe.2018.02.001