Substituted imidazoles as corrosion inhibitors for N80 steel in hydrochloric acid

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1 Indian Journal of Chemical Technology Vol., November 13, pp Substituted imidazoles as corrosion inhibitors for N80 steel in hydrochloric acid M Yadav 1, *, P N Yadav 2 & Usha Sharma 1 1 Department of Applied Chemistry, Indian School of Mines, Dhanbad , India 2 Department of Physics, Post Graduate College, Ghazipur , India Received 23 June 12; accepted 2 March 13 Three synthesized imidazole derivatives, namely 1-[hydrazinyl(4-methoxyphenyl)methyl]-1H-imidazole [HMPMI], 1-[hydrazinyl(phenyl)methyl]-1H imidazol [HPMI] and 1-[hydrazinyl(chlorophenyl)methyl]-1H-imidazol [HCPMI] have been used as corrosion inhibitors for N80 steel in 15% HCl using weight loss, electrochemical polarization, AC impedance and SEM techniques. The results show that the inhibition efficiency of all inhibitors increases with the increase in inhibitors concentration. All studied inhibitors act as mixed inhibitors and obey the Langmuir adsorption isotherm. Corrosion inhibition takes place through adsorption phenomenon. Keywords: Corrosion inhibition, Electrochemical impedance spectroscopy, Electrochemical polarization, Imidazoles, N80 steel N80 steel is widely used as a construction material for pipe work in the oil and gas production, such as down hole tubular, flow lines and transmission pipelines in petroleum industry. Mineral acids particularly hydrochloric acid are frequently used in industrial processes involving acid cleaning, acid pickling, acid descaling, and oil well acidizing 1-3. Acidization of a petroleum oil well is one of the important stimulation techniques for enhancing oil production. It is commonly brought about by forcing a solution of 15 28% hydrochloric acid into the well to open up near bore channels in the formation and hence to increase the flow of oil. To reduce the aggressive attack of the acid on tubing and casing materials (N80 steel), inhibitors are added to the acid solution during the acidifying process. Most of the well-known acid inhibitors are organic compounds containing nitrogen, oxygen and/or sulphur atoms, heterocyclic compounds and pi-electrons 4-7. The polar function is usually regarded as the reaction centre for the establishment of the adsorption process 8. It is generally accepted that organic molecules inhibit corrosion via adsorption at the metal solution interface 9,10, resulting in formation of adsorption layer as a barrier thus isolating the metal from the * Corresponding author. yadav_drmahendra@yahoo.co.in corrosion 11. The effective acidizing inhibitors that are usually found in commercial formulations are acetylenic alcohols, alkenyl phenones, aromatic aldehydes, nitrogen-containing heterocyclics, quaternary salts and condensation products of carbonyls and amines However, these inhibitors suffer from drawbacks, such as they are effective only at high concentrations and are harmful to the environment due to their toxicity. Hence, it is important to search for new nontoxic and effective organic corrosion inhibitors for N80 steel 15% hydrochloric acid system. Imidazole derivatives, because of their good solubility, high stability, and lower toxicity, have been widely used The encouraging results obtained with imidazole derivatives have incited us to synthesized some imidazole derivatives and extend their use in the corrosion inhibiting action on N80 steel in 15% HCl solution. Thus, the present study was aimed at preparing three imidazole compounds namely 1-[hydrazinyl (4-methoxyphenyl)methyl]-1H-imidazole [HMPMI], 1-[hydrazinyl(phenyl)methyl]-1H imidazol [HPMI] and 1-[hydrazinyl (chlorophenyl) methyl] -1Himidazol [HCPMI] to assess their inhibitive properties for oil-well tubular steel (N80) in 15% hydrochloric acid solution.

2 364 INDIAN J. CHEM. TECHNOL., NOVEMBER 13 Experimental Procedure Materials The working electrode and specimens for weight loss experiments were prepared from oil-well N80 steel sheets having the %wt composition: C 0.31, Mn 0.92, Si 0.19, P 0.01, S 0.008, Cr 0., Fe remainder (in %wt). Weight measurements The specimens for the weight loss experiments were of the size 3 cm 3 cm 0.1 cm and for electrochemical studies the size of the electrodes was 1 cm 1 cm 0.1 cm with a 4 cm long tag for electrochemical contact. Both sides of the specimens were exposed for both the techniques. The specimens were mechanically polished successively with 1/0, 2/0, 3/0 and 4/0 grade emery papers. After polishing with the paper of each grade, the surface was thoroughly washed with soap, running tap water, distilled water and finally degreased with acetone. The samples were dried and stored in a vacuum dessicator before immersing in the test solution. For weight loss experiments 300 ml of 15% hydrochloric acid solution was taken in 500 ml glass beakers with lids. The inhibition efficiencies (%IE) were evaluated after a pre-optimized time interval of 6 h using, 50,, 150, and 250 ppm of inhibitors. The specimens were removed from the electrolyte, washed thoroughly with distilled water, dried and weighed. The inhibition efficiencies were evaluated using the following formula: W W % IE = i (1) W where W is the weight loss in absence of inhibitor; and W i, the weight loss in presence of inhibitor. Electrochemical procedure The electrochemical experiments were carried out in a three necked glass assembly containing 150 ml of the electrolyte with different concentrations of inhibitors ( - ppm by weight) dissolved in it. The potentiodynamic polarization studies were carried out with N80 steel strips having an exposed area of 1 cm 2. A conventional three electrode cell consisting of N80 steel as working electrode, platinum as counter electrode and a saturated calomel electrode as reference electrode were used. Polarisation studies were carried out using VoltaLab 10 electrochemical analyser and data was analysed using Voltamaster 4.0 software. The potential sweep rate was 0.1 mvs -1. All experiments were performed at 25 ± 0.2 C in an electronically controlled air thermostat. For calculating % IE by electrochemical polarization method, the following formula was used: I I IE = inh (2) I 0 % 0 where I 0 is the corrosion current in absence of inhibitor ; and I inh, the corrosion current in presence of inhibitor. AC-impedance studies were carried out in a three electrode cell assembly using computer controlled VoltaLab 10 electrochemical analyzer, as well as N80 steel as the working electrode, platinum as counter electrode and saturated calomel as reference electrode.the data were analysed using Voltamaster 4.0 software. The electrochemical impedance spectra (EIS) were aquared in the frequency range from 10 khz to 1mHz at the rest potential by applying 5mV sine wave AC voltage. The charge transfer resistance (R ct ) and double layer capacitance (C dl ) were determined from Nyquist plots. The inhibition efficiencies were calculated from charge transfer resistance values by using the following formula: % IE = R ct ( inh) R R ct ( inh) ct (3) where R ct is the charge transfer resistance in absence of inhibitor ; and R ct(inh), the charge transfer resistance in presence of inhibitor. Synthesis of inhibitors The imidazole derivatives 1-[hydrazinyl(phenyl) methyl]-1h-imidazole (HPMI), 1-[hydrazinyl(4-methoxyphenyl)methyl]-1H-imidazole (HMPMI), and 1-[hydrazinyl(4-chlorophenyl)methyl]-1H-imidazole (HCPMI) were synthesized by the reported method as shown in Scheme 1. A mixture of imidazole (0.1 mol), hydrazine hydrate (0.1 mol) and 4-subsituted benzaldehyde (0.1 mol) in ethanol was refluxed for 5 h. It was cooled and poured in to icecold water. The precipitate was obtained in a few Scheme 1 Synthetic route of inhibitors: HMPMI, HPMI and HCPMI

3 YADAV et al.: SUBSTITUTED IMIDAZOLES AS CORROSION INHIBITORS FOR N80 STEEL IN HCl 365 mints, collected by filtration. The precipitate was dried and recrystallised by absolute ethanol. Results and Discussion Weight loss measurement The percentage inhibition efficiencies (% IEs) in presence of, 50,, 150, and 250 ppm concentration of HMPMI, HPMI and HCPMI have been evaluated by weight loss technique at 25 0 C and the results are summarized in Table 1. It is evident from these values that all the three inhibitors are significantly effective even at low concentrations like ppm and there is a linear increase in %IE in the whole range of concentrations studied. The %IE of all the three studied inhibitors increases on increasing the concentration of inhibitors and becomes almost constant above ppm concentration.the structure of the inhibitors are given in Scheme 2. Scheme 2 Structures of three inhibitors used. Table 1 Corrosion paramters in absence and presence of HMPMI, HPMI and HCPMI at different concentrations Conc. ppm HMPMI HPMI HCPMI CR IE% CR IE% CR IE% mmpy mmpy mmpy It is observed that HMPMI is most efficient among all the three tested inhibitors. The protective ability of the inhibitors for all the tested concentrations is found to decrease in the order HMPMI >HPMI> HCPMI. The extent of %IE of different inhibitor at fixed concentration depends upon the surface area of the inhibitor molecules, the number of active centers such as N, S and O atoms and the intensities of lone pair of electrons on these sites along with the intensities of π-electron on aromatic rings. The percentage inhibition efficiency exhibited by these inhibitors is high which is supposed to be due to strong adsorption of the inhibitor molecules on the metal surface, thereby preventing corrosion of N80 steel in hydrochloric acid solution. The inhibitors are expected to get adsorbed through the lone pairs of electrons on N atoms of amino group and imidazole ring as well as π-electron density on the phenyl and imidazole ring by their coordination with metal surface. The participation of phenyl ring in addition to that of N atom during the adsorption process may be confirmed by changing the π-electron density on phenyl ring by substituting electron donating (-OCH 3 ) and electron withdrawing (-Cl) groups. Generally, electron donating groups increase the inhibition efficiency and presence of electron withdrawing groups decrease the inhibition efficiency of the inhibitors. The inhibitors HMPMI, HPMI and HCPMI have nearly same size and number of active centers but HMPMI shows higher inhibition efficiency (IE%) than HPMI and HCPMI due to higher delocalized π-electron density at benzene ring. The delocalized π-electron density at benzene ring in case of HMPMI is more than in HPMI due to electron donating nature of methoxy ( OCH 3 ) group. The delocalized π-electron density at benzene ring in case of HCPMI is less than in HPMI due to electron withdrawing nature of chloro ( Cl) group. It may be noted that there does not exist any direct correlation between magnitude in increase in IE values and the number of expected sites of adsorption and size. This may be due to the fact that the number of active centers actually involved in adsorption may be different than the number of active centers present in the molecules owing to their geometry. Electrochemical polarization Electrochemical polarization curves of HMPMI, HPMI and HCPMI for N80 steel in 15% hydrochloric acid at 25 o C are shown in Fig.1 and various parameters obtained are given in Table 2. The shift in

4 366 INDIAN J. CHEM. TECHNOL., NOVEMBER 13 Table 2 Electrochemical corrosion parameters in absence and presence of HMPMI, HPMI and HCPMI Inhibitors Conc. Tafel slope I corr E corr IE% ppm Anodic β a mv dec -1 Cathodic β c µa/cm 2 mv mv dec -1 Blank HMPMI HPMI HCPMI Fig. 1 Potentiodynamic polarization curves in absence and in presence of inhibitors at different concentrations the cathodic and anodic partial curves in presence of the inhibitors may be due to adsorbed inhibitor species on the surface of the steel that affects both the anodic and cathodic areas. The minor shift of E corr in negative direction indicates the interference of these inhibitors with the cathodic partial processes. The variation in the values of β a and β c in presence of the inhibitors may indicate that both the anodic and cathodic processes are controlled. All these inhibitors are mixed type and predominantly control the cathodic reaction. The significant reduction in I corr at higher concentration level ( ppm) indicates better inhibition performance at higher concentration level. It is realized from these observations that the inhibitors molecules retard the corrosion process without changing its mechanism in the medium of investigation. The magnitude of the shift in current density is directly proportional to the concentration of the inhibitors, indicating that the inhibitive property of the inhibitor is concentration dependent. It is clear from the polarization curves of the inhibitors that the shift in current density towards lower current density for anodic as well as cathodic curve increases on increasing the concentration of the inhibitor. The negative shift in the E corr in presence of inhibitors on increasing the concentration of the inhibitors is due to the decrease in the rate of cathodic reaction. Moreover, the increase in the cathodic and anodic Tafel slopes (β c and β a ) is related to the decrease in both the cathodic and anodic currents. Both the inhibitors affect both the anodic as well as cathodic sites, so these are mixed inhibitors. AC impedance study The impedance data of N80 steel, recorded in presence of, and ppm of the inhibitors HMPMI, HPMI and HCPMI in 15% HCl solution at 25 o C as Nyquist plots, are shown in Fig. 2. The impedance data of the N80 steel electrode in presence of, and ppm of these inhibitors were analyzed using the equivalent circuit as shown in Fig. 3. The impedance parameters derived from this investigation are given in Table 3. The values of charge transfer resistance (R ct ) are obtained by

5 YADAV et al.: SUBSTITUTED IMIDAZOLES AS CORROSION INHIBITORS FOR N80 STEEL IN HCl 367 Table 3 Electrochemical impedance corrosion parameters in absence and presence of HMPMI, HPMI and HCPMI Inhibitors Conc. ppm R ct Ωcm 2 C dl IE% µf cm -2 Blank HEPMI HPMI HCPMI The double layer capacitance C dl is expressed in the Helmotz model by: Cdl εε δ 0 = S (6) Fig. 2 Nyquist plots of the corrosion of N80 steel in 15% HCl solution without and with different concentrations of inhibitors Fig. 3 Equivalent circuit model used in the fitting of the impedance data of N80 steel in 15% HCl solution at 25 o C subtracting the high frequency impedance from the low frequency, as shown below 21 : R ct = Z r (at low frequency) Z r (at high frequency) (4) The values of electrochemical double layer capacitance (C dl ) were calculated at the frequency (f max ) at which the imaginary component of the impedance is maximal ( Z i ) using the following equation 22 : C dl =1/2πf max R ct (5) where d is the thickness of the deposite; S, the surface of the working electrode, ε 0, the permittivity of the air; and ε, the medium dielectric constant. The decrease in C dl values may be interpreted either by a decrease of local dielectric constant (ε) or by increase in the thickness of the adsorbate layer of inhibitor at the metal surface 23,24. Table 3 shows that by increasing the concentration of inhibitors, R ct values increase and C dl values decrease, indicating a decrease in the local dielectric constant and/or an increase in the thickness of the electrical double layer, suggesting that the inhibitor molecule functions by formation of the protective layer at the metal surface. The C dl values tend to decrease due to displacement of the water molecules by the inhibitor molecules at the electrical double layer, which suggests that the inhibitors molecules function by adsorption at the metal solution interface 23. It can be seen from Table 3 that the inhibition efficiency has increased with increase in inhibitors concentration implying that the large charge transfer resistance is associated with a slower corroding system. In contrast, better protection provided by inhibitors can be associated with a decrease in capacitance of the metal. The depression in Nyquist semicircle is a feature for solid electrodes, often referred as frequency dispersion and attributed to the roughness and other inhomogenities of the solid

6 368 INDIAN J. CHEM. TECHNOL., NOVEMBER 13 electrode 24. The inhibition efficiencies calculated from impedance data are in good agreement with those obtained from electrochemical polarization and weight loss measurement. Adsorption isotherms The adsorption of inhibitor molecules on the surface of the corroding metal has been considered as the root cause of corrosion inhibition. Assuming that the percentage area covered by the inhibitors is directly proportional to retardation in the corrosion rate, the compounds should obey Langmuir adsorption isotherm 25,26, as shown below: θ Q log = log A + log C 1 θ 2.3 RT (7) where θ is the surface coverage; C, the concentration of inhibitors; A, the temperature independent constant; and Q, the heat of adsorption. The validity of Langmuir isotherm is confirmed from the linearity θ of the log vs log C plot having the slope value 1 θ θ to be unity. The plots of log vs log C for the 1 θ investigated inhibitors at 25 C are shown in Fig. 4. It is observed that although these plots are linear, the gradient are not unity, contrary to what is expected for the ideal Langmuir adsorption isotherm equation. The Fig. 4 Langmuir adsorption isotherm in presence of HMPMI, HPMI and HCPMI deviation in the values of the slopes of Langmuir plots from unity may be advocated to be due to the mutual interaction between adsorbed molecules in a close vicinity 27. Organic molecules and metal complexes having polar atoms or groups which are adsorbed on the metal surface may interact by mutual repulsion or attraction and hence may affect the heat of adsorption. θ The plots of log vs log C yield a straight line 1 θ with a correlation coefficient (R 2 ) values , , for HMPMI, HPMI and HCPMI respectively at 303 K. All the inhibitors follow the Langmuir adsorption isotherm, indicating that the adsorption of inhibitors at the surface of N80 is the root cause of corrosion inhibition. The adsorption of tested compounds at N80 steel/hydrochloric acid interface can be attributed to the presence of hetero atom, imidazole ring and aromatic ring, thus the possible reaction centers are unshaired electron pair on nitrogen atoms and π- electrons on imidazole ring and aromatic ring. It is also known that the adsorption of the inhibitors can be influenced by the nature of anions in acidic solution. The presence of Cl - in the solution should characterized with strong adsorbability on the metal surface which brings about a negative charge favoring the adsorption of cation type inhibitors 28,29. In aqueous acidic solution, HMPMI, HPMI and HCPMI exists either as neutral molecule or in the form of cations thus the adsorption of the inhibitors as neutral molecule on the metal surface can occur directly involving the displacement of water molecule from the metal surface and sharing of electrons between the nitrogen atom and the metal surface. 30 The protonated and unprotonated inhibitor molecules may be adsorbed on the metal surface through charge transfer or charge sharing mechanism. These heterocyclic nitrogen compounds may also be adsorbed through electrostatic interaction between the positively charged nitrogen atom and the negatively charged metal surface. 31 In addition π electron interaction between the aromatic nucleus and the positively charged metal surface may also play role. Among these three inhibitors, HMPMI shows maximum inhibition (89.21%) at ppm towards corrosion of N80 steel in 15% HCl due to presence of methoxy group which increases electron density due to +I effect on aromatic ring.

7 YADAV et al.: SUBSTITUTED IMIDAZOLES AS CORROSION INHIBITORS FOR N80 STEEL IN HCl 369 Fig. 5 SEM images of (A) polished sample (B) sample in presence of 15% hydrochloric acid solution (C) sample in presence of ppm of HMPMI (D) sample in presence of ppm of HPMI, and (E) sample in presence of ppm of HCPMI Microscopic Study SEM microphotographs (Fig. 5) in absence and presence of ppm of the inhibitors at X 0 magnification were studied to analyse the change in the morphology of metal surface after corrosion tests in presence and absence of the inhibitors. Steel surface appears to be very rough in absence of inhibitors (Fig. 5B). This is due to formation of uniform flake type corrosion products on the metal surface. Fig. 5 (C), (D) and (E) show that surface of the samples become smooth due to adsorption of the inhibitors at the surface of the sample. Conclusion All the studied inhibitors (HMPMI, HPMI and HCPMI) act as efficient corrosion inhibitor for N80 steel in 15% HCl solution. HMPMI shows appreciably higher efficiency than the HPMI and HCPMI due to presence of electron donating methoxy ( OCH 3 ) group. HCPMI shows least inhibition efficiency due to presence of electron withdrawing chloro ( Cl) group. All the three imidazole derivatives inhibit corrosion by adsorption on the metal surface and follow Langmuir adsorption isotherm. The results of potentiodynamic polarization

8 370 INDIAN J. CHEM. TECHNOL., NOVEMBER 13 studies reveal that all the three inhibitors are mixed type inhibitors and predominantly act on cathodic area. In AC Impedance studies, R ct values increase while C dl values decrease as the concentration of inhibitors increases, indicating the adsorption of inhibitors at the surface of N80 steel. It is suggested from the results obtained from SEM and Langmuir adsorption isotherm that the mechanism of corrosion inhibition is occurring through adsorption process. Acknowledgement Financial assistance from Indian School of Mines, Dhanbad under FRS to one of the authors (M Y) is gratefully acknowledged. References 1 Vishwanatham S & Haldar N, Corros Sci, 50 (8) Abd El-Maksoud S A & Fouda A S, Mater Chem Phys, 93 (5) Migahed M A & Nassar I F, Electrochim Acta, 53 (8) Muralidharan S, Quraishi M A & Iyer S V K, Corros Sci, 37 (1995) Bentiss F, Lebrini M, Lagrenee M, Elfarouk A & Vezin H, Electrochim Acta, 52 (7) Cruz J, Martinez R, Genesca J & Garcia-Ochoa E, J Electroanal Chem, 566 (4) Khaled KF, Babic-Samradzija K & Hackerman N, Electrochim Acta, 50 (5) Roberge P R, Handbook of Corrosion Engineering (McGraw-Hill, New York), Olivares-Xometl O, Likhanova N V, Domínguez-Aguilar M A, Arce E, Dorantes H & Arellanes-Lozada P, Mate Chem Phys, 110 (8) Quartarone G, Battilana M, Bonaldo L & Tortato T, Corros Sci, 50 (8) Ebenso E E, Bull Electrochem, 19 (3) Poling G W, J Electrochem Soc, 114 (1967) Neemla K D, Saxena R C & Jayaraman A, Corros Prev Cont, 6 (1992) Frenier W, Growcock F, Dixon B & Lopp V R, Corrosion, 44 (1988) Stupnicek-Lisac E, Gazivoda A & Madzarac M, Electrochim Acta, 47 (2) Sivaraju M, Kannan K & Elavarasan, S, Orient J Chem, 25 (8) Khaled, K F & Amin M A, J Appl Electrochem, 39 (9) Zhang D Q, Gao L X & Zhou G D, Corros Sci, 46 (4) Christov M & Popova A, Corros Sci, 46 (4) Radha Krishnan Surendra Kumar, Akbar Idhayadhulla & Abdul Jamal Abdul Nasser, Orbital Ele J Chem, 3(1) (11) Yadav A P, Nishikata A & Tsuru T, Corros Sci, 46 (4) Galal A, Atta N F & Al-Hassan M H S, Mater Chem Phys, 89 (5) McCafferty E & Hackerman N, J Electrochem Soc, 119 (1972) Bastidas J M, Polo J L & Cano E, J Appl Electrochem, 30 (0) Popova A & Christov M, Corros Sci, 48 (6) Singh M M, Rastogi R B & Upadhyay B N, Corrosion, 50 (1994) Singh M M, Rastogi R B & Upadhyay B N, Bull Electrochem, 12 (1996) Antropov L I, Makushin E M & Panasenko V F, Inhibitors of Metal Corrosion (Russia), (Technika, Kiev), Rozenfield I I, Corrosion Inhibitors (Russia) (Khimia, Moscow), Hackerman N & Makrides A C, J Phys Chem, 59 (1955) Man C A, Trans Electrohem Soc, 69 (1936) 105.