corrosion inhibition of carbon steel during acid

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CORROSION INHIBITION OF CARBON STEEL DURING ACID CLEANING PROCESS BY A NEW SYNTHESIZED POLYAMIDE BASED ON THIOUREA a

a

A. M. Al-Sabagh , M. A. Migahed & M. Abd El-Raouf a

a

Egyptian Petroleum Research Institute (EPRI), Nasr City, Egypt

Available online: 08 Mar 2012

To cite this article: A. M. Al-Sabagh, M. A. Migahed & M. Abd El-Raouf (2012): CORROSION INHIBITION OF CARBON STEEL DURING ACID CLEANING PROCESS BY A NEW SYNTHESIZED POLYAMIDE BASED ON THIOUREA, Chemical Engineering Communications, 199:6, 737-750 To link to this article: http://dx.doi.org/10.1080/00986445.2011.596597

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Chem. Eng. Comm., 199:737–750, 2012 Copyright # Taylor & Francis Group, LLC ISSN: 0098-6445 print=1563-5201 online DOI: 10.1080/00986445.2011.596597

Corrosion Inhibition of Carbon Steel during Acid Cleaning Process by a New Synthesized Polyamide Based on Thiourea

Downloaded by [Enstinet], [M. A. Migahed] at 10:29 12 March 2012

A. M. AL-SABAGH, M. A. MIGAHED, AND M. ABD EL-RAOUF Egyptian Petroleum Research Institute (EPRI), Nasr City, Egypt In this work, a new polyamide (PA) based on thiourea and tartaric acid was synthesized and characterized by FT-IR spectroscopy. The efficiency of the new synthesized compound was evaluated as a corrosion inhibitor for carbon steel in 1 M HCl solution using weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy techniques (EIS). The results show that PA is a good corrosion inhibitor and its inhibition efficiency reaches 90.6% at 250 ppm. The values of the inhibition efficiency calculated from various techniques are in reasonably good agreement. The results obtained from EIS measurements show that the charge transfer resistance (Rt) increases with increasing inhibitor concentration, while the electrochemical double layer capacitance (Cdl) decreases. The inhibition process was attributed to the formation of an adsorbed film that protects the metal surface against corrosive medium. The adsorption of PA on the steel surface was found to obey Langmiur’s adsorption isotherm. The mechanism of the adsorption process was discussed in the light of the chemical structure of PA. Scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDX) were used to confirm the existence of such protective film. Keywords Carbon steel; Corrosion inhibitor; Energy dispersive X-ray analysis and impedance; Polarization measurements; Polyamide; Scanning electron microscope

Introduction Acid solutions were widely used in industrial acid cleaning, acid de-scaling, acid pickling, and oil-well acidizing. In these acid solutions corrosion inhibitors have been considered as the first line of defense against corrosion (Ali et al., 2003). Corrosion inhibitors for the oil production industry are classified according to the type of aggressive media into two types, water-soluble and hydrocarbon-soluble corrosion inhibitors. Different types of water-soluble corrosion inhibitors are used in the production, pretreatment, and transportation of water-cut oil and scale removal treatment process using acids. The efficiency of the inhibition process depends on the corrosive environment, metallic material, and the molecular structure of the inhibitor (Quraishi and Rawat, 2003; Quraishi and Jamal, 2003; Ebenso et al., Address correspondence to M. A. Migahed, Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo 11727, Egypt. E-mail: [email protected]

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1999; El Achouri et al., 2001). Various organic inhibitors have been studied as corrosion inhibitors in different acidic solutions (Touhami et al., 2000; Chebabe et al., 2003; Morelli et al., 2004; Negm et al., 2010; Khaled, 2003). The high inhibition efficiency of these inhibitors can be attributed to the strong adsorption ability of the inhibitor molecules on the metal surface (Vracar and Drazic, 2002; Kertit and Hammouti, 1996). It is well known that the organic compounds containing heteroatoms, such as P, S, N, and O, which have lone pairs of electrons, are useful inhibitors for metallic corrosion (Quraishi et al., 2002; Chelouani et al., 2003). The inhibition efficiency of some thiourea derivatives on the corrosion rate of carbon steel in acid environments was studied (Umoren et al., 2010). The data obtained showed that the values of the inhibition efficiency were in the range of 17.9–66.6%. The choice of the inhibitor used is based on the following factors: (i) this polyamide contains three kinds of high electron density atoms (two nitrogen, one sulfur, and four oxygen atoms) as active centers donating lone pairs of electrons, therefore, PA molecules can be easily adsorbed on the metal surface; (ii) it can be easily produced in pure state; and (iii) the polymeric compounds with relatively high molecular weight exhibit higher inhibition efficiency than the monomeric compounds (Migahed et al., 2004; Gopi et al., 2000). No data are reported on the inhibition efficiency of polyamide based on thiourea and tartaric acid. The present work is aimed at studying the inhibitive effect of a new synthesized polyamide based on thiourea on the corrosion rate of carbon steel in 1 M HCl solution during acid cleaning process using various techniques. Our attention extended to examining the surface morphology of the carbon steel samples in the absence and presence of PA using scanning electron microscopy (SEM). Also, the EDX technique was used to give an idea of the nature of the protective film formed on the carbon steel surface.

Experimental Section Chemical Composition of Carbon Steel Alloy Carbon steel coupons of size 2.0 cm 7.0 cm 0.2 cm having the chemical composition (wt.%) of 0.09 C, 0.18 Si, 1.48 Mn, 0.012 P, 0.004 S, 0.02 Cr, 0.03 Ni, 0.002 Mo, 0.04 Al, and the balance Fe were used. Preparation of Aggressive Medium Analytical grade 37% HCl (Merck) was used to prepare 1 M HCl solution by diluting the appropriate volume of the concentrated acid with bi-distilled water. The concentration of the acid was checked by titration with a standard solution of sodium carbonate. Synthesis of Inhibitor The selected inhibitor was synthesized in the laboratory from the reaction between tartaric acid and thiourea in the presence of 20 mL dimethylesulfoxide (DMSO) as a solvent. The schematic diagram of the synthesis of the inhibitor is described in Scheme 1. The chemical structure of the synthesized inhibitor was confirmed by Fourier transform-infrared (FT-IR) spectroscopic analysis, as shown in Figure 1. A typical

Corrosion Inhibition of Carbon Steel

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Scheme 1. Schematic diagram of synthesis PA inhibitor.


amide carbonyl absorption band was observed at 1666.2 cm1 together with a broad band in the 3100–3500 cm1 region for N-H stretching. The appearance of amidic carbonyl band in the range 1640–1670 cm1 confirms the postulated chemical structure of the prepared compound. Weight Loss Measurements The experiments were carried out using API XL 60 type carbon steel alloy, which is commonly used in petroleum fields. The samples were allowed to stand for 16 h in 1 M HCl solution in the absence and presence of various concentrations of the inhibitor. All measurements were carried out at 25 0.2 C. Triplicate specimens were exposed for each condition and the mean weight loss was reported. The experiments were performed according to ASTM G31-1990 method (ASTM, 1972). Potentiodynamic Polarization Measurements For potentiodynamic polarization studies, carbon steel electrodes with 1.0 cm2 exposed surface area were used. Potentiodynamic polarization studies were carried out using a VoltaLab PGZ 301 potentiostat controlled by VoltaMaster 4 software. A platinum electrode was used as auxiliary electrode and a saturated calomel electrode (SCE) was used as reference electrode. All the experiments were carried out

Figure 1. FT-IR spectrum of PA.

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at a temperature 25 0.2 C. The polarization curves were recorded by change in the electrode potential automatically from 800 to 300 mV with a scan rate 5 mVs1.

Electrochemical Impedance Spectroscopy Measurements (EIS)


Impedance spectra were obtained out in the frequency range from 100 kHz to 20 mHz at open circuit potential (OCP) after 3 h of immersion time. An AC signal with 10 mV amplitude peak to peak was used to perturb the system. EIS diagrams are given in the Nyquist plots. The impedance data were analyzed and fitted using VoltaMaster 4 graphing and analysis impedance software.

Scanning Electron Microscopy (SEM) and Energy Dispersive Analysis of X-Rays (EDX) The surface examination was carried out using a scanning electron microscope (JEOL 5400, Japan). The energy of the acceleration beam employed was 30 kV. All micrographs were taken at a magnification power of 500. An EDX system attached to the scanning electron microscope was used for elemental analysis or chemical characterization of the film formed on the steel surface. As a type of spectroscopy, it relies on the investigation of the sample through interaction between electromagnetic radiation and the matter. A detector was used to convert X-ray energy into voltage signals. This information is sent to a pulse processor, which measured the signals and passed them into an analyzer for data display and analysis.

Results and Discussion Weight Loss Test The corrosion rate (CR) was calculated from the following equation: CR ¼ W =St

ð1Þ

where W is the average weight loss of three parallel carbon steel samples, S is the total surface area of the specimen, and t is the immersion time. Dependence of the corrosion rate on the inhibitor concentration is shown in Figure 2. It is apparent that the corrosion rate of carbon steel in 1 M HCl solution decreases by increasing the concentration of PA. The values of inhibition efficiency (Ew%) obtained by the weight loss technique were determined by the following equation: Ew % ¼ ðCRuninh CRinh Þ=CRuninh 100

ð2Þ

where CRuninh and CRinh are the values of the corrosion rate of carbon steel in the absence and presence of the inhibitor, respectively. The values of Ew% were found to increase with increasing the inhibitor concentration, as listed in Table I, and attain the maximum value of 90.6% at 250 ppm.



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Figure 2. Relationship between rate of corrosion and inhibitor concentration for carbon steel dissolution in 1 M HCl in the absence and presence of various concentrations of PA at 25 0.2 C. (Figure provided in color online.)

Potentiodynamic Polarization Measurements The values of inhibition efficiency (EI%) obtained from the potentiodynamic polarization technique were calculated using the following relationship (Qu et al., 2007; Elkadi et al., 2000): EI % ¼ ½IcorrðuninhÞ IcorrðinhÞ Þ=IcorrðuninhÞ 100

ð3Þ

where Icorr(uninh) and Icorr(inh) are the values of the corrosion current density in the absence and presence of the inhibitor, respectively. The values of polarization resistance were calculated according to the following equation (Stern and Geary, 1957): Rp ¼ ba bc =2:303Icorr ðba þ bc Þ

ð4Þ

Figure 3 shows the potentiodynamic polarization curves of carbon steel in 1 M HCl with different concentrations of PA at 25 2 C. As can be seen from Figure 3,

Table I. Weight loss data obtained for carbon steel immersed in 1 M HCl solution in the absence and presence of various concentrations of PA Inhibitor Blank PA

Concentration (ppm) 0 50 100 150 200 250

Corrosion rate 103 (mg cm2 h1)

Ew (%)

32.00 14.5 8.8 6.5 4.9 2.9

— 54.6 72.4 77.5 84.6 90.6


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Figure 3. Potentiodynamic polarization curves for carbon steel in 1 M HCl in the absence and presence of various concentrations of PA at 25 0.2 C. (Figure provided in color online.)

the additive exhibits a significant effect on the current-potential relations. The following points could also be drawn: . The Tafel lines are shifted to more negative and more positive potentials for the cathodic and anodic processes, respectively, relative to the blank specimen. This means that the additive affects both anodic dissolution of the metal and cathodic evolution of hydrogen (i.e., the additive acts as a mixed-type inhibitor). . The slopes of the cathodic and anodic Tafel lines are different and dependent on the inhibitor concentration (Bartos and Hackerman, 1992). . By increasing the concentration of the additive, the corrosion current densities (Icorr) were decreased. The potentiodynamic polarization parameters including corrosion current density (Icorr), corrosion potential (Ecorr), anodic Tafel slope (ba), cathodic Tafel slope (bc), polarization resistance (Rp), and inhibition efficiency (EI%) were calculated and are listed in Table II. The results indicated that the values of (EI%) increase with increasing PA concentration and reach a maximum value of 90.7 at 250 ppm. Electrochemical Impedance Spectroscopy (EIS) The corrosion behavior of carbon steel in 1 M hydrochloric acid solution, in the absence and presence of various concentrations of PA, was also investigated using the EIS technique at 25 0.2 C after 3 h of immersion time. Typical Nyquist plots for carbon steel in 1 M HCl at various concentrations of (PA) are presented in Figure 4. The values of charge transfer resistance (Rt) were calculated from the difference in real impedance (Zre) at lower and higher frequencies as follows (Yadav et al.): Rt ¼ Zreðat lower frequencyÞ Zreðat higher frequencyÞ

ð5Þ


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Table II. Electrochemical polarization parameters obtained for carbon steel immersed in 1 M HCl solution in the absence and presence of various concentrations of PA bc Concentration Ecorr mV ba Rp Icorr (vs. SCE) (mV dec1) (mV dec1) (X cm2) (mA cm2) (ppm)

EI (%)

0 50 100 150 200 250

— 61.6 76.9 83.0 86.1 90.7

532 526 518 528 524 527

115 104 102 101 100 98

79 75 74 72 71 71

29 75 103 121 163 178

0.65 0.25 0.15 0.11 0.09 0.06

The values of double-layer capacitance (Cdl) were calculated at the frequency (fmax) at which the imaginary component of the impedance is maximal (Zmax) using the following equation (Migahed, 2005): Cdl ¼ 1=2pfmax Rt

ð6Þ

The values of inhibition efficiency obtained from the EIS technique were calculated according to the following equation (Migahed and Nassar, 2008): ERt % ¼ ½RtðinhÞ RtðuninhÞ =RtðinhÞ 100

ð7Þ

where Rt(inh) and Rt(uninh) are the values of the charge transfer resistance in the presence and absence of the inhibitor, respectively.

Figure 4. Nyquist plots for carbon steel in 1 M HCl in the absence and presence of various concentrations of PA at 25 0.2 C. (Figure provided in color online.)

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Table III. Electrochemical impedance parameters obtained for carbon steel immersed in 1 M HCl solution in the absence and presence of various concentrations of PA Inhibitor

Concentration (ppm)

Blank PA

0 50 100 150 200 250

Rt (X cm2)

Cdl (mFcm2)

ERt (%)

42.9 101.3 185.4 258.2 338.3 447.2

893.5 98.5 31.2 19.4 18.7 15.9

— 57.6 76.8 83.3 87.3 90.4

The values of the charge transfer resistance (Rt), electrochemical double-layer capacitance (Cdl), and the percentage inhibition efficiency (ERt%) at various concentrations of PA were calculated and are listed in Table III. For analysis of the impedance spectra containing one capacitive loop, the equivalent circuit (EC) given in Figure 5 was used, where Rs represents the solution resistance, Rt represents the charge transfer resistance, and Cdl represents the electrochemical double-layer capacitance. In fact, the presence of PA is accompanied by an increase of the value of Rt in acid solution, indicating that the corrosion process is mainly controlled by the charge transfer process. The values of Cdl are brought down to the minimum extent by increasing the inhibitor concentration, and this decrease in the values of Cdl follows an order similar to that obtained for Icorr in polarization technique. The decrease in Cdl can be attributed to the adsorption of the inhibitor molecules on the metal surface leading to the formation of a good protective film that isolates the metal surface from acid solution (Kardas¸, 2005; Solmaz et al., 2008; Krim et al., 2008). For comparison, the values of inhibition efficiency calculated from weight loss, polarization, and electrochemical impedance techniques were plotted against the inhibitor concentrations, as shown in Figure 6. The obtained results were in good agreement. Adsorption Isotherm The transition of metal=solution interface from a state of active dissolution to the passive state is attributed to the adsorption of the inhibitor molecules on the metal surface, forming a protective film. The rate of adsorption is usually rapid; hence, several adsorption isotherms were assessed. The Langmuir adsorption isotherm

Figure 5. Equivalent circuit model for electrochemical impedance measurements. (Figure provided in color online.)



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Figure 6. Dependence of the percentage inhibition efficiency on the concentrations of PA as calculated from weight loss, potentiodynamic polarization, and EIS techniques. (Figure provided in color online.)

was found to be the best description of the adsorption behavior of PA molecules on carbon steel surface. Figure 7 shows the variation of the ratio of (Ci=h) as a function of PA concentrations (Ci). The obtained plot gives a linear relationship with a slope of (0.957), which is very close to unity (Migahed et al., 2011). The small deviation from unity is generally attributed to the interaction between the adsorbed inhibitor molecules and the heterogeneous nature of the carbon steel surface. Scanning Electron Microscopy Figure 8(A) shows the SEM image of a polished carbon steel surface. The micrograph shows a characteristic inclusion, which was probably an oxide inclusion (ASTM, 1980). Figure 8(B) shows an SEM of the surface of a carbon steel specimen after immersion in 1 M HCl solution for 16 h in the absence of inhibitor, while Figure 8(C) shows an SEM of the surface of another carbon steel specimen after

Figure 7. Langmuir adsorption isotherm (Ci=h vs. Ci) for PA on carbon steel surface immersed in 1 M HCl at 25 0.2 C. (Figure provided in color online.)


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Figure 8. SEM and EDX of carbon steel: (A) polished sample, (B) sample immersed in 1 M HCl solution without inhibitor, (C) sample immersed in 1 M HCl solution with 250 ppm of PA.

immersion in 1 M HCl solution for the same time interval in the presence of 250 ppm of PA. The micrographs reveal that the surface was highly damaged in the absence of the inhibitor, but damage on the carbon steel surface was decreased in the presence of 250 ppm of PA. It is clear from Figure 8(C) that the steel surface appears more uniform, which indicates that PA exhibits appreciable resistance to corrosion at this concentration.

Energy Dispersive Analysis of X-Rays (EDX) The protective film formed on the carbon steel surface was analyzed using EDX, as shown in Figure 8. The EDX spectrum of a polished carbon steel sample in Figure 8(A) shows good surface properties, while the spectrum in the case of carbon steel immersed in 1 M HCl solution without inhibitor molecules failed to do so



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Scheme 2. Adsorption of inhibitor. (Figure provided in color online.)

because it was severely weakened by external corrosion, as shown in Figure 8(B). By addition 250 ppm of PA, a decrease of the intensity of the iron band and the appearance of a new band for sulfur in the spectrum were observed, in addition to the appearance of a new peak in the EDX spectrum (Migahed et al., 2010). This behavior can be attributed to the formation of a strong protective layer of inhibitor molecules on the carbon steel surface (Ishibashi et al., 1996), as indicated in Figure 8(C). Corrosion Inhibition Mechanism Generally, it is assumed that the first stage in the action mechanism of an inhibitor in an aggressive medium is the adsorption of the inhibitor molecules onto the metal surface. The adsorption of the organic substance at the metal=solution interface may be written according to the following displacement reaction (Bockris and Swinkels, 1964): OrgðsolÞ þ nH2 OðadsÞ ¼ OrgðadsÞ þ nH2 OðsolÞ

ð8Þ

where n is the number of water molecules removed from the metal surface for each molecule of inhibitor adsorbed. Therefore, the relatively high adsorption ability of PA molecules on the metal surface can occur directly on the basis of donor=acceptor interaction between the p-electrons of the inhibitor molecules and the vacant d-orbital of the steel surface. The adsorption behavior of PA is determined by the presence of a double bond with the S-atom, which is an adsorption center, and that of p-electrons of the metal surface. The S-atom processes a vacant d-orbital, which is compatible with those of the metal atoms. The p-electrons present can overlap with the d-orbital and form dp-dp bond (Donnelly et al., 1974). The adsorption of the inhibitor is schematically presented in the Scheme 2. The scheme shows that the stability of the adsorption bond depends on the p-electron density in its center.

Conclusions The main conclusions of the present study could be summarized in the following points: . Newly synthesized polyamide acts as a good inhibitor for carbon steel corrosion in 1 M HCl solution during acid cleaning process.

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. The percentage inhibition efficiency increased by increasing the inhibitor concentration due to the adsorption of the inhibitor molecules on the carbon steel surface and blocking its active sites. . The values of inhibition efficiency determined by different techniques are in good agreement. . The adsorption of the inhibitor molecules on the steel surface obeys the Langmuir isotherm model. . The high inhibition efficiency exhibited by the new synthesized compound is attributed to the strong adsorption ability on the metal surface forming a good protective layer. . The existence of the protective layer was confirmed by both SEM and EDX techniques.

Acknowledgments This work has been financially supported by Egyptian Petroleum Research Institute (EPRI) fund. The authors are deeply thankful to the EPRI fund and support.

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