Volume 11 Preprint 2


Study of Corrosion of Mild Steel in Propyl Triphenyl Phosphonium Bromide/Sulfuric Acid Solution by Conventional Weight Loss and Electrochemical Methods

Kalpana Bhrara and Gurmeet Singh

Keywords: Propyl triphenyl phosphonium bromide, weight loss studies, Langmuir’s isotherm,

Abstract:
Propyl Triphenyl Phosphonium Bromide (PTPPB) has been evaluated as a corrosion inhibitor for mild steel in 0.5M H2SO4 solutions using weight loss measurements. The data showed that the presence of PTPPB in aerated 0.5 M H2SO4 solutions decreases corrosion rate to a great extent and the corrosion rate decreases with increasing inhibitor concentration at a constant temperature. At 298K, inhibition efficiency was found to be 93.5% for 10−7 M PTPPB which increased to about 99% for the PTPPB concentration of 10−2 M. The effect of temperature on the corrosion behavior of mild steel was studied at five different temperatures ranging from 298 to 338K. Adsorption on the mild steel surface follows the Langmuir isotherm. The values of free energy of adsorption indicate strong adsorption of PTPPB on mild steel surface. The weight loss results were found to be in agreement with the electrochemical results. The polarization curves clearly indicate that PTPPB acts as a mixed type inhibitor.

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ISSN 1466-8858 Volume 11, Preprint 2 submitted 20 May 2008 Study of corrosion of mild steel in Propyl Triphenyl Phosphonium Bromide/ Sulfuric Acid solution by conventional weight Loss and Electrochemical methods Kalpana Bhraraa and Gurmeet Singhb* a,b Department of Chemistry, University of Delhi, Delhi-110007, India. Email: gurmeet123@yahoo.com, kbhrara@yahoo.com, Tel.:91-011-27667725 Ext.1628 ABSTRACT Propyl Triphenyl Phosphonium Bromide (PTPPB) has been evaluated as a corrosion inhibitor for mild steel in 0.5M H2SO4 solutions using weight loss measurements. The data showed that the presence of PTPPB in aerated 0.5 M H2SO4 solutions decreases corrosion rate to a great extent and the corrosion rate decreases with increasing inhibitor concentration at a constant temperature. At 298K, inhibition efficiency was found to be 93.5% for 10−7 M PTPPB which increased to about 99% for the PTPPB concentration of 10−2 M. The effect of temperature on the corrosion behavior of mild steel was studied at five different temperatures ranging from 298 to 338K. Adsorption on the mild steel surface follows the Langmuir isotherm. The values of free energy of adsorption indicate strong adsorption of PTPPB on mild steel surface. The weight loss results were found to be in agreement with the electrochemical results. The polarization curves clearly indicate that PTPPB acts as a mixed type inhibitor. Key Words Propyl triphenyl phosphonium bromide, weight loss studies, Langmuir’s isotherm, potentiodynamic polarization. * Author for Correspondence © 2008 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject1 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Introduction Volume 11, Preprint 2 submitted 20 May 2008 A considerable amount of interest has been generated in the study of organic compounds as corrosion inhibitors owing to their usefulness in several industries: during the pickling of metals, cleaning of boilers, acidification of oil wells, etc [1]. Most effective inhibitors are organic compounds which are rich in hetero atoms such as phosphorous, nitrogen, sulfur and oxygen. Such compounds and their derivatives are excellent inhibitors for iron and mild steel in sulfuric acid [2-26]. Phosphonium compounds are known to act as corrosion inhibitors of various metals in acidic solutions [27-39]. In the present study, Propyl Triphenyl Phosphonium Bromide (PTPPB) has been evaluated as a corrosion inhibitor for mild steel in 0.5M H2SO4 solutions using weight loss studies. The study was also complemented by potentiodynamic polarization studies. Experimental Mild Steel (C=0.15%, Si=0.08%, S=0.025%, P=0.025% and Mn=1.02%) specimens of dimensions 2 cm x 0.8 cm x 0.8 cm were abraded successively by emery papers of different grades, i.e. 150, 320, 400 and 600 and finely polished with a 4/0 polishing paper to obtain mirror like finish. The specimens were degreased in an ultrasonic cleaner, dried in dessicator for 24 hours and weighed. They were dipped in 0.5M sulfuric acid solution and in 0.5M sulfuric acid solution containing 10-2M, 10-3M, 10-5M and 10-7M inhibitor. After six hours samples were taken out of the medium, washed with water to remove corrosion products, dried in dessicator for 24 hours and weighed again to calculate weight loss. The detailed procedure for potentiodynamic polarization studies is described in [36-38]. 0.5M sulfuric acid solution containing 10-2, 10-3, 10-5 and 10-7M PTPPB were used for corrosion studies. The cathodic and anodic polarization studies were conducted at 298, 308, 318, 328 and 338K. Typical polarisation curves for the aerated solutions of 0.5M sulfuric acid with various concentrations of PTPPB at different temperatures were plotted. The corrosion current for different sets of solution were found from the extrapolation of the polarization curves back to the OCP. © 2008 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject2 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 11, Preprint 2 Results submitted 20 May 2008 Weight Loss Studies: Inhibitor efficiency was calculated using mass loss data from the equation: I% = ((w0 - wI) /w0) x 100 Where w0 and wI are weight losses in the absence and presence of inhibitor. The various data, i.e. corrosion rate, I%, surface coverage (θ) etc. are reported in table 1 for the adsorption of PTPPB on mild steel in 0.5M sulfuric acid. Table 1. Mass Loss data for corrosion of mild steel in 1N sulfuric acid in the absence and the presence of PTPPB Concentration mol l-1 Weight Loss g Corrosion Rate g cm-2 day-1 Efficiency % θ 298K 10-2 10-3 10-5 10-7 H2SO4 0.00120 0.00156 0.03113 0.00708 0.11975 0.00062 0.00081 0.01621 0.00369 0.06237 99.0 98.7 74.0 93.5 - 0.99 0.99 0.74 0.93 - 10-2 10-3 10-5 10-7 H2SO4 0.00136 0.00136 0.11896 0.09767 0.13565 0.00071 0.00071 0.06196 0.05087 0.07065 99.0 99.0 12.3 28.0 - 0.99 0.99 0.12 0.28 10-2 10-3 10-5 10-7 H2SO4 0.00157 0.00188 0.00721 0.11158 0.15671 0.00082 0.00098 0.00375 0.05811 0.08162 99.0 98.8 95.4 28.8 - 0.99 0.99 0.95 0.29 - 10-2 10-3 10-5 10-7 H2SO4 0.00180 0.00180 0.01361 0.01755 0.16402 0.00094 0.00094 0.00709 0.00914 0.08543 98.9 98.9 91.7 89.3 - 0.99 0.99 0.92 0.89 - 10-2 10-3 10-5 10-7 H2SO4 0.00188 0.00342 0.11978 0.11123 0.17112 0.00098 0.00178 0.06238 0.05793 0.08912 98.9 98.0 30.0 35.0 - 0.99 0.98 0.30 0.35 - 308K 318K 328K 338K © 2008 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject3 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 11, Preprint 2 submitted 20 May 2008 Various adsorption isotherms were studied but Langmuir’s isotherm was found to be the best fit for the concentrations and temperatures studied with average R 2 = 1.0000 for the adsorption of PTPPB. Langmuir’s isotherm is given by C / θ = 1/ K + C Where C is inhibitor concentration in mol l-1 and K is an equilibrium constant of adsorption, K which is related to standard free energy of adsorption G0ads by the equation K= (1/55.55) exp (- G0ads / RT) Fig.1 shows the dependence of C/θ as a function of C. From the intercepts, the values of K and G0ads are calculated and are given in Table 2. 298 K 0.012 303 K 0.01 C/ θ 0.008 313 K 0.006 323 K 0.004 333 K 0.002 0 0 0.005 0.01 0.015 C/ mol/ l Fig. 1 Langmuir isotherm for the adsorption of PTPPB Table 2 Various parameters calculated from the Langmuir isotherm for PTPPB Temperature K R2 Slope K -∆Gads kJ mol-1 298 308 318 328 338 1.0000 1.0000 1.0000 1.0000 1.0000 1.010 1.007 1.010 1.010 1.009 1.00 x 106 3.33 x 104 5.00 x 106 3.33 x 106 1.00 x 105 44.19 36.96 51.41 51.92 43.65 © 2008 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject4 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 11, Preprint 2 submitted 20 May 2008 Potentiodynamic Polarization Studies: Polarization studies on mild steel in 0.5M sulfuric acid solution containing 10 -2M, 10-3 M, 10-5M and 10-7M PTPPB were performed. Fig. 2 shows the representative cathodic and anodic polarization curves for mild steel in 0.5M sulfuric acid in presence of 10 -2 M PTPPB at 298K. The corrosion currents for different sets of solution were found from the extrapolation of the polarization curves back to the OCP. The inhibitor efficiency was calculated using: I% = (i0 – icorr)/i0 x 100 in which i0 and icorr are the corrosion currents ( A cm-2) in uninhibited and inhibited solutions respectively . From the polarisation curves at various temperatures, corrosion parameters e.g. OCP, corrosion current and inhibition efficiencies etc. of acid corrosion of mild steel in the presence of PTPPB were calculated and are given in Table3. -200 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 -300 -400 C-10-2M A-10-2M C-10-3M A-10-3M C-10-5M A-10-5M C-10-7M A-10-7M -500 -600 Pot. vs. SCE / mV ----> -700 -800 -900 -1000 -1100 -1200 2 Log(i/A) μA/cm ----> Figure 2 Galvanostatic Polarisation curve of Mild Steel in 0.5M sulfuric acid in presence of PTPPB at 298K © 2008 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject5 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 11, Preprint 2 submitted 20 May 2008 TABLE 3 Corrosion Parameters of Mild Steel 1N H2SO4 in the presence of PTPPB Conc. (mol l-1) 298 K 10-2 10-3 10-5 10-7 H2SO4 308K 10-2 10-3 10-5 10-7 H2SO4 318K 10-2 10-3 10-5 10-7 H2SO4 328K 10-2 10-3 10-5 10-7 H2SO4 338K 10-2 10-3 10-5 10-7 H2SO4 -OCP (mV) i corr mA/cm2) I (%) 530 480 479 487 481 0.060 0.072 1.820 0.389 7.080 99.1 99.0 74.3 94.5 - 491 474 490 480 474 0.072 0.072 7.080 6.026 7.244 99.0 99.0 2.3 16.8 - 510 506 473 483 501 0.145 0.204 0.589 11.749 12.883 98.9 98.4 95.4 8.8 - 507 527 477 478 483 0.229 0.263 1.259 1.622 15.136 98.5 98.3 91.7 89.3 - 509 498 477 480 477 0.282 0.417 13.183 13.183 17.378 98.4 97.6 24.1 24.1 - ( Discussion: The corrosion rate increases and then decreases with the increase in concentration of the inhibitor at lower temperatures. At higher temperatures corrosion rate decreases with the © 2008 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject6 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 11, Preprint 2 submitted 20 May 2008 increase in concentration of the inhibitor. For 10-2 and 10-3M PTPPB, corrosion rate almost remains constant with increase in temperature whereas for 10 -5 and 10-7M PTPPB it increases and then decreases, passes through a minimum and then increases again. For the lower concentrations studied, the minimum in corrosion rate (maximum in I %) is observed at 328K. It can be clearly seen (Table 1) that PTPPB is a very efficient inhibitor at most of the temperatures studied especially at higher concentrations. It performs best at 318 and 328K where inhibitor efficiencies are in the range of 89.3-99.0%. Its performance decreases considerably at 338K at lower concentrations. The fact that inhibition efficiencies are higher at higher temperatures than those at 298K also indicates that the adsorption of PTPPB is not merely a physical or a chemical adsorption but a comprehensive adsorption [40]. Table 2 shows that all the linear correlation coefficients (R2) are almost equal to 1 and all the slopes are very close to 1, which indicates the adsorption of inhibitor onto steel surface accords with the Langmuir adsorption isotherm. The result also indicates that there were no interactions among the adsorbed species [41, 42]. The negative values of ∆G0ads along with high K indicate a spontaneous adsorption process and a good chemical stability of inhibitor, which may be derived from the chemical bond between metal and inhibitor molecules. Magnitudes of K and ∆G 0ads values confirm that PTPPB is adsorbed on the metal surface at all the temperatures but is most efficient at 328K. The inhibitor efficiencies remain almost constant (98-99%) with temperature at higher concentrations. The π- electron system of this inhibitor molecule possibly overlaps with the vacant d-orbitals of the metal surface resulting in a strong dπ-pπ interaction [27] which is further assisted by the synergistic effect of Br- ions [29]. This electrostatic interaction probably leads to a stronger adsorption of the inhibitor and formation of a barrier between the metal surface and reactive sites. At 10-5M PTPPB, efficiency decreases and then increases with temperature and shows maxima at 318K. This is clear indicative of the fact that the PTPPB undergoes change in orientation of phenyl rings as the temperature is increased. At 318K, they may be in the same plane, therefore enhancing the adsorption on the metal surface. These changes in © 2008 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject7 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 11, Preprint 2 submitted 20 May 2008 orientation of phenyl rings are not possible beyond a certain temperature because of the shorter time lag between adsorption-desorption processes. At 10-7M PTPPB, efficiency is maximum at 298K, decreases and then shows maxima at 328K. The adsorption of PTPPB is assisted by other anions present in the solution and therefore maximum adsorption at room temperature whereas the maxima at 328K can be explained due to maximum adsorption through π-electron density because of planar orientation of four phenyl rings. Table 3 shows that the electrochemical results supplement the weight loss results. OCP remains more or less constant with a slight shift towards cathodic direction for all the temperatures. Therefore, PTPPB is a mixed type of inhibitor. It acts in both ways affecting cathodic and anodic partial processes by blocking the active sites of the metal surface. Conclusions From the overall data of adsorption of PTPPB on mild steel surface in acid solution studied, it may be concluded that (i) PTPPB retards corrosion at ordinary temperatures and shows better inhibition efficiency at higher temperatures especially at 318K and 328K. (ii) The Langmuir adsorption isotherm was found to be closest to the description of the adsorption behavior of the studied inhibitor. (iii) The negative values of ∆G0ads along with high K indicate a spontaneous adsorption process at all the temperatures but is most efficient at 318 and 328K. (iv) The electrochemical results are in good agreement with the weight loss data. (v) PTPPB is a mixed type of inhibitors. OCP values in the presence of PTPPB shows a slight predominance in cathodic direction. References 1. B.G. Clubley, ‘Chemical inhibitors for corrosion control’, 1990, Cambridge, Royal Society of Chemistry. 2. J. Rawat and M. A. Quraishi: Brit. Corros. J., 1999, 34(3), 220-224. 3. B. Ramesh Babu and R. Holze: Brit. Corros. J., 2000, 35(3), 204-209 4. M. Özcan, . Dehri and M. Erbil: Appl. Surf. Sci., 2004, 236(1-4), 155-164 5. M. Bouklah, A. Attayibat, S. Kertit, A. Ramdani and B. Hammouti: Appl. Surf. Sci., © 2008 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject8 to the reviewers’ comments, be published online at http://www.jcse.org in due course. 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Hornez: Corrosion, 1999, 55(10), 968. © 2008 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of 10to the reviewers’ comments, be published online at Corrosion Science and Engineering. It will be reviewed and, subject http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work.