Research Journal of Recent Sciences _________________________________________________ ISSN 2277-2502 Vol. 3(ISC-2013), 177-184 (2014) Res. J. Recent. Sci. International Science Congress Association 177 Histidine as Synergistic additive on Poly (N-Methyl Aniline) for mild Steel Corrosion in 0.5 M HSOR. Menaka, M. Nandhini and S. Subhashini Department of chemistry, Avinashilingam Institute for Home science and Higher Education University for WomenAvailable online at: www.isca.in Received 1st December 2013, revised 17th February 2014, accepted 19th March 2014 AbstractAn inhibitor system composed of Poly(N-Methyl Aniline) [PNMANI] and histidine has been evaluated for its synergistic corrosion inhibition performance for mild steel in 0.5M HSO. PNMANI was prepared by free radical polymerisation using ammonium peroxodisulphate as initiator. 100ppm PNMANI exhibited 70% inhibition efficiency. In order to enhance the inhibitive action of PNMANI, histidine was added as a synergistic additive. The influence of histidine on PNMANI has been evaluated by weight loss and electrochemical methods. As expected, the inhibition efficiency gradually increased with increase in histidine concentration. The maximum inhibition efficiency of 89% was achieved for the inhibitor system viz 100ppm of PNMANI-130ppm of histidine. The enhanced inhibition performance of the inhibitor system has been proven thermodynamically. Keywords: PNMANI, histidine, mild steel, synergism. Introduction Steel is widely used as the constructional material in most of the major industries particularly in food, petroleum, power production, chemical and electrochemical industries, especially due to its excellent mechanical properties and low cost. Mild steel undergo considerable dissolution when they are exposed to acid solutions or aggressive environment. The use of inhibitors is one of the most practical methods to prevent the corrosion or to reduce the corrosion rate. Organic inhibitors1-3 containing hetero atoms like oxygen, nitrogen, sulphur and phosphorus etc shows better corrosion inhibition by forming protective film on the metal surface. To improve the performance of organic corrosion inhibitor, extensive synergistic studies have been carried out. Synergism has become one of the most important effects in inhibition processes and serves as the basis for all modern corrosion inhibitor formulations. The synergistic inhibition effect of combination of organic compounds with certain metal cations4-6and halide anions7-8 have also been reported. The use of polymer as effective corrosion inhibitor for mild steel have drawn considerable attention due to their inherent stability and cost effectiveness. A number of polymers have been reported to inhibit the corrosion of mild steel in acid media. Poly(diphenyl amine), polyethylene glycol, polyvinyl alcohol, polyethylene glycol methyl ether are some of the reported polymers9-13. Polymeric compounds adsorb more strongly on the metal surface with their multiple adsorption sites. Their inhibition efficiency has been upgraded by addition of small amount of cations or anions which proves the existence of synergistic action. A series of reports highlighted the synergistic effect of halide ions on corrosion inhibition of metals by polymers14-16. The studies have also reported the effect of polymer- polymer mixtures or blends on corrosion inhibition of metals17. The present study is to investigate the synergistic effect of poly (N- methyl Aniline) and Histidine on the mild steel corrosion in acid medium. It was found that the inhibition efficiency was enhanced on adding aminoacid(Histidine) to the polymer (PNMANI). The present study reports on the corrosion inhibition effect of poly(N- methyl Aniline) –Histidine mixture for mild steel in HSO using weight loss and electrochemical techniques. Material and MethodsLR grade of L-Histidine and 98% Analytical grade of HSOwere used for the corrosion inhibition studies. LR grade of N-Methyl Aniline was used for the polymer synthesis. Mild steel sheet with composition of C=0.084%, Mn=0.207%, Si= 0.021%, P= 0.019%, S=0.012%, Cr=0.026%, Mo=0.029%, Ni=0.010% and remaining Fe was cut into rectangular pieces of area 1x5cm. The polished specimens were washed with distilled water, thoroughly dried and stored in a desiccator for weight loss measurements. Synthesis of Poly(N-Methyl Aniline) PNMANI: 0.1 mole of distilled N-Methyl Aniline was dissolved in 100 ml of 1M hydrochloric acid. The solution was cooled at 0-5 and 2% ammonium persulphate was added to the mixture with constant stirring. The reaction was continued for two hours and the Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(ISC-2013), 177-184 (2014) Res. J. Recent. Sci. International Science Congress Association 178 mixture is refrigerated for 24 hours. The polymer solution was neutralized with ammonium hydroxide. The polymer was precipitated using ethanol and then dried. Weight loss measurement: Weight loss measurements were performed with the clean and dry rectangular strips. The mild steel strips were immersed in triplicates in 100 ml of 0.5 M SO4 in the absence and presence of 100ppm poly(N- methyl Aniline) and different concentrations (40-130ppm) of Histidine for different immersion periods at room temperature. At elevated temperatures, a constant immersion period of half an hour was selected and the studies were conducted for the same concentration of inhibitor mixture. The samples were washed, dried and reweighed. From the obtained weight loss, corrosion rate, inhibition efficiency (IE) and surface coverage( were calculated using following formulae, Where, W and W are the weight loss of the coupon in the absence and presence of inhibitor, A is the area of the specimen in cm,D is the density of the material in g/cm, t is the time of exposure in hours. Electrochemical measurements: Frequency Response analyzer (Solartron 1280B) supported with corrware and z- plot corrosion software was used for data acquisition and analysis. The conventional three- electrode system consisting of saturated calomel electrode (SCE) as reference electrode, platinum foil as counter electrode and MS strips having exposed area of 1 cm2 as working electrode was used. The electrodes were immersed in 0.5 M HSO solution for 30 minutes until a steady – state potential was reached. The polarisation studies were carried out over a potential range of 200mV to 1500mV with respect to reference electrode and its current response was measured at a scan rate of 1mV sec-1. Anodic and cathodic tafel segments were extrapolated to obtain corrosion potential (Ecorr) and corrosion current density (Icorr). The IE was evaluated using the following formula: Where, Icorr and Icorr are corrosion currents in the presence and absence of the inhibitor. Impedance measurements were carried out at each corrosion potential. An AC sine wave of 10 mV amplitude is applied to the electrode and the frequency was varied from 10 kHz to 10 MHz. The real Z and the imaginary Z’ were plotted in the Nyquist plot. From Nyquist plot, polarization resistance R, charge transfer resistance Rctand double layer capacitance values Cdlare obtained,and IE were calculated using the following formulae. Results and DiscussionEffect of Histidine on PNMANI: Weight loss measurements were carried out in 0.5 M HSO in absence and presence of different concentrations of PNMANI to determine its inhibitive effect against corrosion. The inhibition behavior of PNMANI was found to be 70% at 100 ppm. In order to enhance the inhibition efficiency of PNMANI, various concentrations (40-130ppm) of histidine was added. The corrosion rate of mild steel samples in 0.5 M HSO in presence of PNMANI with histidine were determined by weight loss measurements at different immersion time and concentration. The obtained results are shown in Figure figure 1 and 2. The inhibition efficiency was found to be affected by both time and concentration, and maximum efficiency of 89% was obtained for 130ppm Histidine+100ppm PNMANI after 3 hours of immersion. This may be due to adsorption of inhibitor molecules on the metal surface. A decrease in inhibition efficiency was observed for prolonged immersion i.e. after 3 hours till 24 hours which could be attributed to desorption of inhibitor from the metal surface. Figure-1 Effect of PNMANI-Histidine with immersion time Figure 2Variation of IE with concentration of PNMANI- Histidine Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(ISC-2013), 177-184 (2014) Res. J. Recent. Sci. International Science Congress Association 179 Synergism considerations: The mild steel corrosion in 0.5 M SO4 in absence and presence of different concentration of Histidine along with 100 ppm of PNMANI was studied. Addition of histidine concentration further enhanced the inhibition efficiency values (table 1). The synergistic inhibition effect was analyzed by estimating the synergism parameter (S) from inhibition efficiency obtained from weight loss method. According to Aramaki and Hackerman18 (S) is calculated using the equation: =1-I1+2 /1-I’1+2. Where I is the inhibition efficiency of histidine, I is the inhibition efficiency of PNMANI, I’1+2 is the inhibition efficiency of PNMANI in combination with histidine. The values of S for PNMANI-histidine are found to be more than unity suggesting that enhanced inhibition efficiency caused by the synergistic action of histidine. Effect of temperature: Temperature can modify the interaction between mild steel and inhibitor molecule in acid medium. Performance of PNMANI- Histidine system was studied for ½ hour of immersion at various temperatures in order to investigate the effectiveness and mechanism of inhibition. From the table 2, it can be seen that the corrosion rate increased with increasing temperature and the inhibition efficiency decreased with rise in temperature. A decrease in inhibition efficiency at elevated temperature may be attributed to physical adsorption of the inhibitor system19. At higher temperature, a decrease in inhibition efficiency may also due to desorption of inhibitor molecules. Kinetic and Thermodynamic parameters of dissolution: Figure 3 shows the Arrhenius plot of log corrosion rate (CR) versus the reciprocal of absolute temperature (1/T). The apparent activation energy (Ea) was calculated by the following equation, = -2.303 *R*slope of the Arrhenius plot Table-1 Synergism Parameters for 100ppm PNMANI with various concentrations of Histidine IE of 100ppm PNMANIConcentration of histidine (ppm) IE of histidine (%) IE of PNMANI-histidine (%) Synergism parameter,S I 63.2 40 30.51 66.26 1.420 50 34.18 67.22 1.455 60 36.11 68.03 1.466 70 40.28 69.38 1.498 80 41.04 74.36 1.407 90 42.09 80.11 1.318 100 47.43 84.87 1.307 110 53 85.79 1.358 120 54.9 87.57 1.352 130 56.56 89.3 1.344 Table 2 Performance of PNMANI - Histidine at various temperatures Concentration (ppm) Temperature(K) 303 313 323 333 343 353 CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE ( %) CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE ( %) Blank 1107.68 - 4849.37 - 6642.08 - 16257.6 - 28224.0 - 29898.6 PNMANI Histidine 100 100 100 100 100 100 100 100 100 100 100 - 40 50 60 70 80 90 100 110 120 130 505.87 470.98 462.26 453.53 418.65 401.20 348.87 305.26 279.10 261.65 252.93 54.33 57.48 58.26 59.05 62.20 63.77 68.50 72.44 74.80 76.37 77.16 1535.05 2407.24 2232.80 2215.36 2110.69 2075.81 1997.31 1613.55 1395.50 1378.06 1151.29 68.34 50.35 53.95 54.31 56.47 57.19 58.81 66.72 71.22 71.58 76.25 4299.89 3959.74 3863.80 3732.97 3663.19 3628.31 3410.54 3296.87 3078.83 3056.83 2952.72 35.3 40.38 41.82 43.79 44.88 45.40 48.65 50.36 53.67 53.97 55.54 9873.19 9515.59 9332.43 9201.60 9114.38 8599.79 8530.01 7954.37 7692.71 7682.61 7542.12 39.27 41.46 42.96 43.40 43.93 47.10 47.53 51.07 52.68 52.74 53.60 18333.4 18760.8 18734.6 18682.3 18664.8 17958.3 17740.3 16641.3 16414.6 16214.0 15708.1 35.04 33.52 33.62 33.80 33.86 36.37 37.14 41.03 41.84 42.55 44.34 18534.0 23418.3 22860.0 21787.3 22903.7 21516.9 22310.6 18595.0 18534.0 18411.9 16353.5 38.01 21.67 23.54 26.42 27.12 28.03 34.01 37.80 38.01 38.41 45.30 Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(ISC-2013), 177-184 (2014) Res. J. Recent. Sci. International Science Congress Association 180 Figure-3 Arrhenius plot of PNMANI - Histidine The values of Ea obtained from the slope of Arrhenius plot are listed in table 4. It can be seen from the table the Ea value (67.658 kJ/mole) in presence of PNMANI is higher than that in the acid solution (58.344 kJ/mole), and the Evalues are still higher in the presence of PNMANI along with different concentration of histidine. Hence the synergistic action of histidine on PNMANI is proved by increasing Evalues. The decrease in inhibition efficiency with increase in temperature and the increase of E values in presence of inhibitor system indicates the physical adsorption mechanism20. Table-3 Thermodynamic parameters of adsorption of PNMANI – Histidine Concentration (ppm) - DDG (kJ / mole) Change in enthalpy ---DDDH kJ / mole) Change in entropy (DDS kJ / mole) 313 K 323 K 333 K 343 K 353 K Blank - - - - - - - PNMANI HISTIDINE 100 - 9.983447 13.99639 13.95924 14.89556 14.95364 18.3655 0.096111 100 40 12.82543 14.31937 14.63898 16.04771 18.27751 25.5861 0.12262 100 50 12.62934 14.34489 14.65974 16.2317 18.16614 25.6888 0.122942 100 60 12.75958 14.30208 14.78879 16.39281 17.90408 24.2987 0.118803 100 70 12.68981 14.34628 14.897 16.55808 17.97721 26.1815 0.124615 100 80 12.7622 14.4434 14.70119 16.40679 18.01123 25.586 0.122714 100 90 12.72981 14.23809 14.80314 16.46653 17.34767 25.0712 0.120567 100 100 11.97986 14.1921 14.55283 16.14652 17.01448 26.5007 0.123885 100 110 11.55885 13.96683 14.50934 16.19047 17.1315 29.5047 0.132681 100 120 11.63405 14.05935 14.63149 16.24013 17.21833 30.2681 0.135176 100 130 11.11803 14.00852 14.65894 16.1591 16.51626 28.4932 0.129093 Table-4 Activation parameters of PNMANI- Histidine in 0.5 M HSO Concentration (ppm) E a kJ/mol H a kJ/mol S a J/mol E a -H a kJ/mol RT kJ/mol Blank 58.344 55.446 17.74 2.898 2.934 PNMANI Histidine 100 - 67.658 64.731 287.17 2.927 100 40 68.329 65.400 400.24 2.929 100 50 68.831 65.901 468.19 2.930 100 60 68.565 65.635 415.84 2.929 100 70 70.607 67.670 863.08 2.936 100 80 70.084 67.149 685.97 2.934 100 90 72.684 69.741 1713.53 2.946 100 100 73.203 70.258 1845.44 2.944 100 110 75.383 72.432 3868.35 2.951 100 120 76.170 73.216 5071.79 2.953 100 130 76.295 73.341 4953.64 2.954 Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(ISC-2013), 177-184 (2014) Res. J. Recent. Sci. International Science Congress Association 181 From the transition state theory, the graph of log CR/T vs. 1/T is plotted. From the plot the slope of /2.303R and an intercept of logR/Nh+/2-303R were obtained, from which the values of -and -were calculated respectively and are tabulated (table 4). These values increase with increase in histidine concentration. The positive sign of the -a reflect the endothermic nature of metal dissolution process21. The increase in the values of Eand -with increase in the concentration of histidine suggests the increase in energy barrier of metal dissolution reaction22. The higher values of Ealso indicate the higher protecting efficiency23. We state that Eand -values vary in the same manner with the inhibitor concentration. This can be used to verify the known thermodynamic relation between E and -a.=RT The calculated values are very close to RT which is equal to 2.934 kJ /mole at 353 K24. The positive values of entropy in the absence and presence of inhibited system infers that the rate determining step for the activated complex is dissociation step rather than association. The increase in entropy is considered to be a driving force for the adsorption of inhibitor onto the metal surface. Thermodynamic parameters of adsorption: The free energy of adsorption (ads) was calculated using the equation, Log C = Log ( /1-)-Log B Log B = -1.74 – (ads/2.303 RT) The (ads) values are listed in Table 3 for the studied inhibitor system at different temperatures. The negative values of (ads) indicate the spontaneous adsorption of the inhibitor system and its stability on the mild steel surface25. It can also be seen that the values of ads slightly increase with inhibitor concentration and temperature. It is universally accepted that the values of ads around -20kJmol-1 regarded as physisorption physical adsorption, which is due to electrostatic interaction between the charged molecules and the charged metal, and those values around -40 kJmol-1 regarded as chemisorptions chemical adsorption, which is due to charge sharing or a transfer from the inhibitor molecule to metal surface forming a covalent bond26-27. In the present study, the free energy of adsorption (ads) are found to be in the range of (9-16 kJmol-1) indicating the spontaneous physical interaction between mild steel surface and the inhibitor system. The thermodynamic functions of adsorption reaction adsand ads were calculated from the plot of adsvs T. The intercept and slope of the plot gives enthalpy of adsorption (ads) and entropy of adsorption (ads) respectively. The obtained adsvalue is negative which indicates that the adsorption of inhibitor is an exothermic process28. For an exothermic process, physisorption is differentiated from chemisorption by the absolute values. When the ads values are less than 40kJmol-1it corresponds to physisorption, and if ads is around 100kJmol-1 it corresponds to chemisorption29. In this case the enthalpyof adsorption (ads) values are less than 40kJmol-1confirming the physical nature of adsorption30. The adsorption of inhibitor molecules is accompanied by positive values of ads. This can also be attributed to the increase in solvent entropy which is due to desorption of water molecules and thereby causing the adsorption of the inhibitor on the metal surface31. Adsorption isotherms: Inspection of tables suggests that PNMANI- Histidineis an effective corrosion inhibitor for mild steel corrosion. The adsorption of the inhibitor is an essential step of the inhibition mechanism and it can provide information about the nature of metal-inhibitor interaction. The surface coverage () is an important parameter in discussing the adsorption characteristics and it is calculated by, By using values of different inhibitor concentrations, several adsorption isotherms like Langmuir,Temkin and Freundlich etc., were assessed. The plot of log /(1- ) Vs. log C gave straight lines with slope values close to unity suggesting theadsorption of PNMANI in presence of Histidine obeys Langmuir isotherm figure 4. Figure-4 Langmuir adsorption isotherm for PNMANI-Histidine in 0.5M HSOPotentiodynamic polarization measurements: The influence of histidine on PNMANI on the cathodic and anodic potentiodynamic polarization curves of mild steel in 0.5M SO is shown in figure 5. Electrochemical parameters such as Tafel slope (b and b), corrosion current (Icorr), corrosion Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(ISC-2013), 177-184 (2014) Res. J. Recent. Sci. International Science Congress Association 182 potential (Ecorr) and inhibition efficiency (IE) are depicted in Table 5.From the table, the values of cathodic and anodic Tafel slopes show a marked change with inhibitor concentration indicating the mixed behavior of the inhibitor system32. The marginal shift in the Ecorrvalue indicates that the mixed type inhibitor controlled both the cathodic and anodic reactions33. The Icorr values decreases with the increase in the inhibitor concentration thus by reducing the metal dissolution. These results are consistent with weight loss measurements. 10-510-410-310-210-110 -1.25-1.00-0.75-0.50-0.25 0 I (Amps/cmE (Volts)blank H2SO4.cor PANI-100.cor 100ppm pani+80ppm histidine.cor 100ppm pani+100ppm histidine.cor Figure-5 Polarization curves for mild Steel in presence of PNMANI- Histidine Impedance measurements: Figure 6 is the impedance spectra of mild steel with and without the addition of various concentrations of histidine on PNMANI in 0.5MHSO. The slightly depressed nature of the semicircle has been attributed to the roughness and inhomogeneities of the solid electrode34-35. From Nyquist plot, polarization resistance R, charge transfer resistance Rct and are obtained and depicted in Table 6. The R and Rct value increases with increasing inhibitor concentration which indicates that the adsorption takes place at the metal surface forming a protective layer. The change in Rctvalue clearly suggests that charge transfer process controls the dissolution of mild steel. 05101520 -15-10-5Z'Z''blankH2SO4.z PANI100.z 100ppm pani+80 histidine.z 100pani+120ppm histidine.z Figure-6 Nyquist plots of mild steel in the presence of PNMANI-Histidine Table-5 Polarization Parameters PNMANI- Histidine Conc. (Ppm) b a (mV/dec) b c (mV/dec) I corr (µA/cm 2 ) E corr (V/dec) IE (%) Blank 268.6 163.48 5.566 -0.48045 - 100ppm PNMANI 288.6 151.36 2.31 -0.46323 58.67 100ppm PNMANI + 40ppm histidine 214.08 123.63 3.3521 -0.47006 39.77 100ppm PNMANI +80ppm histidine 191.24 115.03 3.0446 -0.45537 45.30 100ppmPNMANI +100ppm histidine 119.92 84.207 1.0672 -0.46291 80.82 Table-6 Impedance parameters in the presence of PNMANI-Histidine Conc. (Ppm) R p (Ohm /cm 2 ) IE (%) R ct (Ohm/cm 2 ) IE (%) Blank 7.743 - 6.5895 - 100ppm PNMANI 19.29 59.86 9.945 33.71 100ppm PNMANI+ 40ppm histidine 19.99 61.27 10.84 39.25 100ppm PNMANI +80ppm histidine 28.15 72.49 14.77 55.40 100ppm PNMANI 100ppm histidine 31.21 75.19 17.23 61.75 Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(ISC-2013), 177-184 (2014) Res. J. Recent. Sci. International Science Congress Association 183 Mechanism of inhibition: The inhibition of PNMANI-histidine on mild steel corrosion in 0.5M HSO can be explained by adsorption of inhibitor molecules on metal surface. Initially sulphate ions are adsorbed on the metal surface in SOmedium. PNMANI which exist as protonated species in acidic solution may interact with already adsorbed sulphate ions on the iron surface. The adsorption may also due to the interaction between unshared electron pairs of nitrogen atom on PNMANI and vacant d-orbitals of the iron atom on the metal surface. The enhanced inhibition efficiency is obtained by synergistic effect of histidine. Due to smaller size and zwitterionic nature of the histidine, they can easily get adsorbed on the mild steel surface. Thus histidine inhibits metal from corrosion and enhances the corrosion protecting efficiency of PNMANI. Conclusion PNMANI-histidine showed effective inhibition against mild steel corrosion in 0.5M HSO. The presence of histidine enhanced the inhibition efficiency of PNMANI and found to be synergistic in nature. The inhibitor system obeyed the adsorption model, Langmuir isotherm. The calculated values of ads, ads, ads indicated the spontenity of the adsorption and its physical nature. Electrochemical measurements showed that PNMANI-histidine acted as a mixed type inhibitor. Also, the higher values of E indicated high inhibition efficiency. Acknowledgements The authors wish to acknowledge the Avinashilingam Institute for Home science and Higher Education University for Women, Coimbatore for providing lab facilities. One of the author thanks UGC for the financial support. 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