Himanshu Shekhar Shukla, Nilesh Haldar and Gopinatham Udaybhanu
Keywords: mild steel, synergistic behavior, adsorption, FTIR, micrographs,
Abstract:
The corrosion inhibition effect of Gum Acacia (GA) as green inhibitor was investigated by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods and SEM observations for mild steel in H2SO4 (pH=1) medium . The results show that GA is a good inhibitor, and the adsorption of GA on metal surface obeys Temkin adsorption isotherm. The inhibitive effect of GA in addition of halide ions is found to promot at all the concentration of GA. Polarization curves show that GA acts as a mixed-type inhibitor. The results obtained from gravimetric experiments were in good agreement with the electrochemical methods results. Comprehensive adsorption of the inhibitor molecules on the mild steel surface was suggested based on the thermodynamic parameters and on the comparative FT-IR spectral analysis of pure and metal surface product. An inhibition mechanism was proposed in terms of strongly adsorption of inhibitor molecules on mild steel surface.
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ISSN 1466-8858 Volume 14, Preprint 28 submitted 27 July 2011 Corrosion inhibition behavior of Gum Acacia as natural occurring polymer for mild steel and synergistic effect of halide ions in H2SO4 medium H. S. Shuklaa, N. Haldar and G. Udaybhanu Dept. of Appl. Chem., Indian School of Mines, Dhanbad-826004, India Abstract The corrosion inhibition effect of Gum Acacia (GA) as green inhibitor was investigated by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods and SEM observations for mild steel in H2SO4 (pH=1) medium . The results show that GA is a good inhibitor, and the adsorption of GA on metal surface obeys Temkin adsorption isotherm. The inhibitive effect of GA in addition of halide ions is found to promot at all the concentration of GA. Polarization curves show that GA acts as a mixed-type inhibitor. The results obtained from gravimetric experiments were in good agreement with the electrochemical methods results. Comprehensive adsorption of the inhibitor molecules on the mild steel surface was suggested based on the thermodynamic parameters and on the comparative FT-IR spectral analysis of pure and metal surface product. An inhibition mechanism was proposed in terms of strongly adsorption of inhibitor molecules on mild steel surface. Key words: mild steel, synergistic behavior, adsorption, FTIR, micrographs, H. S. Shuklaa ; Corresponding author, E mail; ismhs_shukla@yahoo.com 1 © 2011 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, subject 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 1. Introduction: Volume 14, Preprint 28 submitted 27 July 2011 Mild steel is one of the most important widely used engineering materials particularly for the structural and automobile applications. However, it undergoes rusting easily in the humid atmosphere and its rate of corrosion is quite high in acidic environment. Acid solutions are widely used in industries for pickling, acid cleaning, descaling and oil well acidizing. Sulfuric acid is generally the choice in the steel surface treatment over the other acids basically due to its lower cost, minimal fumes and non-corrosive nature of the SO42− ion1,2. Hence corrosion of mild steel is of fundamental academic and industrial concern that has received a considerable amount of attention. Among different corrosion protection methods, application of inhibitors is one of the most convenient methods to reduce corrosion rate of metallic materials especially in acidic media. Most of the well known acid inhibitors are organic compounds containing electron donor atoms particularly nitrogen, sulfur, oxygen in their functional groups with aromatic and heterocyclic rings3-5. Most of the corrosion inhibitors act by adsorption of their molecules on the metal surface. Their action depends on the nature and surface charge of the metal, nature of the medium and the chemical structure of the inhibitor. Though many organic and inorganic compounds show good anticorrosive activity, most of them are highly toxic to both human beings and environment. The investigation of new nontoxic or low-toxic and green corrosion inhibitors is essential to overcome this problem. Among eco-friendly organic compounds, the natural occurring polymers have many advantages such as high inhibition efficiency, low price, low toxicity and easy production6-8. It is known that polymers are adsorbed stronger than their monomer analogs; hence polymers will be better corrosion inhibitors than the corresponding monomers9. The improved performances of the polymeric materials are ascribed to their multiple adsorption sites for bonding with the metal surface. The polymer displaces many water molecules from the metal surface during adsorption, thus making the process entropically favorable and the presence of multiple bonding sites slower the desorption process10,11. Inhibition effects of natural products and different plant extracts have been examined by many researcher12-15. Synergistic inhibition is an effective method to improve the inhibitive force of individual inhibitors, to decrease the amount of usage and to diversify the application of inhibitor in acidic media16,17. The synergistic inhibition effects of organic inhibitor/metallic ion mixture, organic inhibitor/organic inhibitor mixture and inorganic inhibitor/ inorganic inhibitor mixture on 2 © 2011 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, subject 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 14, Preprint 28 submitted 27 July 2011 corrosion of steel in acidic media have been studied. Investigations on synergism between organic inhibitors and halide ions on steel corrosion in acidic solution have been researched extensively by many authors18-20. In this present investigation Gum Acica (GA) have been tested as corrosion inhibitor for mild steel in H2SO4 solution (pH=1). Corrosion experiments were carried out on the variation of corrosion rate due to the metal corrodibility as functions of time, temperature and inhibitor concentration together with analyses of the metal surface products for a better understanding of the inhibition mechanism with the help of FTIR spectroscopy. Morphological study of corroded metal surface has been carried out by SEM technique. A plausible adsorption mechanism for the inhibitor molecules on the metal surface in acid solution has also been proposed. 2. Experimental: Experiments were performed to understand the inhibition behavior of GA towards corrosion on mild Steel in pH=1 H2SO4 at different experimental conditions. The gravimetric experiments, potentiodynamic polarization and AC Impedance measurements at different exposure periods (6, 12, 18 and 24h) and at different temperatures (303, 313, 323 and 333K) in absence and in presence of inhibitor at different concentrations (10, 50, 75, 100, 150, 200, 250 and 300ppm) were carried out to determine the corrosion parameters. The synergistic effect of halides (KCl, KBr and KI) has been studied with the optimum concentration of GA at different exposure periods and at different temperatures. Mild steel sample used for the study was analyzed and the composition of the tested steel was given in Table 1. Table 1 Composition of the mild steel coupons % of C % of S % of P % of Si % of Al % of Mn % of W % of Fe 0.12 0.02 0.01 0.15 0.01 0.57 0.015 Rest The steel coupons used for the gravimetric studies were cut out of the size 4.4 cm x 2.2 cm x 0.15 cm from the mild steel sheets with a small hole ~1 mm diameter at the upper edge of the specimen. The surface of the steel coupons were cleaned by mechanical abrasion and polished with increasingly finer grades of emery papers to remove the scratches. After polishing these were degreased with acetone, washed and finally dried in a stream of warm air. In the present study, experiments were performed in pH=1 H2SO4 test solution which was prepared in 3 © 2011 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, subject 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 14, Preprint 28 submitted 27 July 2011 distilled water using Analytical grade H2SO4 supplied by Ranbaxy Fine Chemicals. For immersion test 500 ml of pH=1 H2SO4 was used and for the electrochemical experiments, test solution used was 250 ml. The potentiodynamic polarization curves were recorded in absence and in presence of the inhibitors at different concentrations with the mild steel (area 1 cm2) as working electrode using Potentiostat at ambient temperature (25 to 300C). The details about the experimental procedures for the above study are explained elsewhere21, 22 The organic compound Gum Acacia selected from natural polymers and has been explored as corrosion inhibitor for mild steel in pH=1 H2SO4. Gum Acacia has very complex structure (Fig. 1) and is slightly acidic; pH- 4.5-5.0, MW range is from 250,000 to 1,000.000023,24. Fig. 1 Structure of Gum Acacia (GA) 3. Results and discussion: The inhibition efficiency increases with the inhibitor concentration up to 250 ppm (70.02%) and then tends to a constant value for 6h exposure at room temperature has been shown in Fig 2. The inhibition efficiency of GA was 39% at 10 ppm. In the present study degree of surface coverage (θ) was tested graphically for fitting a suitable adsorption isotherm. The best fit 4 © 2011 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, subject 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 14, Preprint 28 75 submitted 27 July 2011 70 65 IE% 60 55 50 45 40 35 0 50 100 150 200 250 300 350 Concentration (ppm) Fig. 2 Variation of inhibition efficiency with the inhibitor concentration was obtained with Temkin adsorption isotherm in which a straight line was obtained from the plot of Log C against θ. The corrosion parameters for individual and combination of GA with halides have been given in Table 2. Further it is found that, in addition of halide ions, the Table 2 Corrosion parameters in presence of GA at different inhibitor concentration Concentration (ppm) CR (mpy) IE% Surface Coverage (θ) s - - - 133.02 63.00 0.63 - 0 113.45 68.46 0.68 1.84 700 0 92.41 74.31 0.74 1.57 0 0 700 65.61 81.76 0.82 1.40 250 0 0 0 107.59 70.09 0.70 - 250 700 0 0 104.06 71.07 0.71 1.79 250 0 700 0 84.93 76.39 0.76 1.16 250 0 0 700 46.87 86.97 0.87 1.38 Gum Acacia Chloride Bromide Iodide 0 0 0 0 359.70 100 0 0 0 100 700 0 100 0 100 5 © 2011 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, subject 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 14, Preprint 28 submitted 27 July 2011 inhibitive effect of GA is promoted at all the concentration of GA (Table 3). The highest efficiency (~ 87%) was obtained in case of 250 ppm GA with 700 ppm iodide. The combined inhibition behavior of GA with halides can be explained in more details with the synergism parameter (s). The highest value of synergism parameter (1.84) was obtained for 700 ppm chloride with 100 ppm GA. The synergistic behavior of halides with GA might be due to the preferential adsorption of halide ions (Cl-, Br-, I-) on the metal surface and makes the negatively charged surface which facilitate further adsorption of inhibitor molecules25,26. Therefore halide ions enhance the adsorption of GA and improve the inhibitive effect to a considerable extent. Table 3 Corrosion parameters in presence of GA and GA + Halides at different exposure period Exposure Period (h) 6 12 18 24 Concentration (ppm) CR (mpy) PI Gum Acacia Chloride Bromide Iodide 0 250 250 250 250 0 0 700 0 0 0 0 0 700 0 0 0 0 0 700 359.70 107.59 104.06 84.93 46.87 70.09 71.07 76.39 86.97 0 250 250 250 250 0 0 700 0 0 0 0 0 700 0 0 0 0 0 700 370.61 102.46 96.03 77.98 46.18 72.34 74.09 78.96 87.54 0 250 250 250 250 0 0 700 0 0 0 0 0 700 0 0 0 0 0 700 392.42 97.46 90.37 79.31 39.56 75.43 76.97 79.79 89.92 0 250 250 250 250 0 0 700 0 0 0 0 0 700 0 0 0 0 0 700 421.27 91.82 82.91 69.68 39.77 78.24 80.32 83.46 90.56 Inhibition efficiency obtained increases in the order GA + Iodide > GA + Bromide > GA + Chloride > GA 6 © 2011 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, subject 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 14, Preprint 28 submitted 27 July 2011 It might be due to the radii and the electronegativity of the halide ions play a significant role in the adsorption process20. The corrosion rate and percentage inhibition in presence of optimum inhibitors concentration for the tested exposure periods have been listed in Table 3. The corrosion rate increases with the exposure period up to 24h in case of the free acid whereas corrosion rate decreases with the exposure period up to 24h in case of the inhibited acid. The inhibition efficiency for all the inhibitors increases (Fig 3) with time and the combination of GA and Iodide anion offered maximum inhibition (~ 91%) at 24h exposure. 95 Inhibition Efficiency 90 85 80 75 GA GA + Chloride GA + Bromide GA + Iodide 70 65 0 5 10 15 20 25 30 Time (h) Fig. 3 Variation of inhibition efficiency in presence of the inhibitors at different exposure period Temperature dependence of the corrosion parameters of the corrosion process both in the absence and in the presence of inhibitors lead to some conclusions concerning the mechanism of the inhibiting action. The corrosion experiments were carried out at high temperatures with the optimum inhibitor concentration to understand the effect of temperature. The corrosion rate in absence of the inhibitors has been found to increase steeply from 303 K to 333 K whereas in presence of the inhibitor the corrosion rate increased slowly. The corrosion parameters in absence and in presence of the inhibitor in the temperature range 303 K to 333 K have been summarized in Table 4. The inhibition efficiency for all the inhibitors decreases (Fig 4) with 7 © 2011 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, subject 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 14, Preprint 28 submitted 27 July 2011 temperature and the combination of GA and Iodide anion offered maximum inhibition (~ 71%) at 333K. The lower inhibition efficiency at higher temperature in case of the inhibitor might be Table 4 Corrosion parameters in presence of GA and GA + Halides at different exposure period Exposure Period (h) Concentration (ppm) CR (mpy) PI Gum Acacia Chloride Bromide Iodide 303 0 250 250 250 250 0 0 700 0 0 0 0 0 700 0 0 0 0 0 700 603.80 190.55 186.63 140.93 111.34 68.45 69.09 76.66 81.56 313 0 250 250 250 250 0 0 700 0 0 0 0 0 700 0 0 0 0 0 700 1215.89 433.31 384.71 346.89 305.31 64.37 68.36 71.47 74.89 323 0 250 250 250 250 0 0 700 0 0 0 0 0 700 0 0 0 0 0 700 2111.95 801.05 761.15 654.45 552.91 62.06 63.96 69.01 73.82 333 0 250 250 250 250 0 0 700 0 0 0 0 0 700 0 0 0 0 0 700 2900.68 1115.02 1087.6 928.21 844.39 61.56 62.52 68.00 70.89 due to decrease in hydrogen evolution overpotential and also the higher desorption rate than the adsorption at high temperature27,28. Thermodynamic parameters (Table 5) in presence of the inhibitor have been calculated from the well known equations. 8 © 2011 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, subject 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 14, Preprint 28 submitted 27 July 2011 85 GA GA + Chloride GA + Bromide GA + Iodide Inhibition efficiency 80 75 70 65 60 300 305 310 315 320 325 330 335 Temperature (K) Fig. 4 Variation of percentage inhibition with temperature in presence of GA Table 5 Thermodynamic parameters in absence and in presence GA Thermodynamic Blank GA parameters (kJ/mol) GA + Chloride GA+ Bromide GA + Iodide (250 + 700)ppm (250 + 700) ppm (250 + 700) ppm Heat of adsorption - -55.891 -54.589 -59.969 -62.764 Activation energy 44.306 49.821 50.223 52.998 56.11 Average Free energy - 8.750 -8.443 -12.158 -16.696 Entropy of adsorption 0.154 0.137 0.165 0.173 The Ea value for dissolution of mild steel in sulphuric acid (pH=1) in absence of inhibitor has been reported29 as ~44 kJ/mol. Higher values of Ea were obtained in presence of the studied inhibitor combinations indicated the formation of an adsorptive film of an electrostatic character. The lower negative values of ΔGads suggested the spontaneous adsorption of the inhibitors on the metal surface. The negative values of ΔHads revealed the exothermic and physical adsorption of the inhibitors on the metal surface causes the dissolution of steel is difficult. Positive values of 9 © 2011 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, subject 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 14, Preprint 28 submitted 27 July 2011 ΔSads were observed for the inhibitors may be due to involvement of less ordered transition state in the adsorption process. The adsorption of the inhibitor molecules occur after desorption of water molecules initially adsorbed on metal surface due to stronger attraction between the metal surface and inhibitors molecules compared to water 30. The electrochemical parameters for corrosion of the experimental steel in the acid containing GA are summarized in Table 6. The E0 value in presence of the inhibitors was found to shift slightly towards less negative side compared to that of the free acid (-0.5518V) for the higher concentration of GA and halides. The significant reduction in corrosion current (Icorr) at Table 6 Electrochemical corrosion parameters in absence and in presence of GA Concentration of E0 (V) I0 (µ amp/ cm2) Inhibitor Blank Tafel Slopes (mV) Anodic (βa ) Cathodic (βc) PI -0.5518 423.0 124.16 191.93 - 50 ppm -0.5451 184.1 99.16 148.13 56.50 100 ppm -0.5485 153.8 93.01 163.21 63.60 250 ppm -0.5478 126.1 86.28 168.83 70.21 GA GA + Chloride 100 + 700 ppm -0.5818 141.1 111.96 130.96 66.67 250 + 700 ppm -0.5417 124.9 86.76 164.85 70.47 GA + Bromide 100 + 700 ppm -0.5718 104.2 98.49 125.58 75.37 250 + 700 ppm -0.5618 94.73 93.55 121.95 77.60 GA + Iodide 100 + 700 ppm -0.5748 81.74 99.03 120.89 80.67 250 + 700 ppm -0.5358 50.88 55.58 167.20 87.97 higher concentration level indicated more adsorption of the inhibitors and better inhibition performance. Variations in values of both the Tafel slopes were observed in the presence of these inhibitors. Significant decrease in the values of anodic Tafel slopes (βa) (38 mV). The potentiodynamic polarization curves in absence and presence GA along with halides are given in 10 © 2011 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, subject 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 14, Preprint 28 submitted 27 July 2011 Fig. 5 Potentiodynamic polarization curves in presence of the GA Fig. 6 Potentiodynamic polarization curves in presence of the GA and halides Fig 5 and Fig 6. It is clear from the Tafel slopes that the anode is more polarized (βa > βc) when external current is applied for the inhibitor indicated anodic control in presence of the inhibitor31,32. The inhibition efficiencies calculated from potentiodynamic polarization study 11 © 2011 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, subject 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 14, Preprint 28 submitted 27 July 2011 were slightly different from that of the weight loss study. This may be explain as the results obtained from the weight loss measurement were average values, while the results obtained from potentiodynamic measurement were instantaneous values. Electrochemical impedance spectra for mild steel/H2SO4 interface in absence and presence of GA alone (Fig. 7) and GA with halides (Fig. 8) were recorded as Nyquist plot and the impedance data obtained were summarized in Table 7. Table 7 Electrochemical impedance parameters in absence and in presence of the inhibitors Inhibitor Concentration Rt (Ώ cm2) Cdl (μFcm−2) %IE Blank 58 121.32 - 50 ppm GA 118 29.39 51.67 100 ppm GA 140 21.11 58.57 250 ppm GA 160 16.13 63.75 100 ppm GA + 700 ppm Chloride 154 17.45 62.34 100 ppm GA + 700 ppm Bromide 213 8.74 72.77 100 ppm GA + 700 ppm Iodide 330 4.24 82.42 250 ppm GA + 700 ppm Chloride 202 10.01 71.28 250 ppm GA + 700 ppm Bromide 223 8.22 73.99 250 ppm GA + 700 ppm Iodide 378 2.89 84.49 The Rt values increases with inhibitor concentration and this in turn leads to an increase in IE% and may be attributed to the formation of protective film on the metal–solution interface. The addition of inhibitor lowers the Cdl value, suggesting that the inhibition can be attributed to surface adsorption15, 33. Similar trend have observed with the addition of halides. The maximum Rt (378 Ώ cm2) and minimum Cdl (2.89 μFcm−2) have been obtained for the combination of 250 ppm GA with 700 ppm Iodide which is in accordance with the gravimetric and potentioostatic corrosion experiments. 12 © 2011 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, subject 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 14, Preprint 28 submitted 27 July 2011 Fig. 7 Electrochemical impedance plots in absence and in presence of GA Fig. 8 Electrochemical impedance plots in absence and in presence of GA + Halide ions 13 © 2011 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, subject 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 14, Preprint 28 submitted 27 July 2011 GA showed typical bands of the -OH bond at 3425 cm-1 and it was obtained at 3422 cm-1 in case of the metal surface product (Fig 9). Two strong bands at 1616 and 1423 cm-1 are due to asymmetric and symmetric stretching vibration of the carboxylic acid for pure GA and these were at 1561 and 1401 cm-1 respectively in the metal surface product34-36. The band obtained at 1042 cm-1 due to the stretching of the CO bond and it was at 1023 cm-1 for metal surface product. The peaks arise due to ring breathing vibrations were obtained at 641 cm-1 for GA and at 1673 cm-1 for metal surface product. FT-IR spectra revealed the presence of GA in the metal surface product after corrosion experiments in the inhibited acid solution. The slight difference in the peak or band position might be due to adsorption of the inhibitor molecules on the metal surface. Fig. 9 FT-IR spectra of GA both as pure form and in metal surface product A uniform flake type corrosion product was seen in case of the free acid (Fig. 10 B). SEM study revealed that the metal surface was partially covered with the quasi globular inhibitor products along with flake type deposition may be of metal oxides and metal hydroxides 37 in presence of all the inhibitors. A more compact layer has been observed in case of the mixture of GA with bromide and iodide (Fig. 10 E and 10 F). 14 © 2011 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, subject 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 14, Preprint 28 A submitted 27 July 2011 B C D E F Fig. 10 SEM Micrographs of the metal surface in absence and in presence of the inhibitors (A) Before exposure, (B) Blank, (C) 250 ppm GA, (D) 250 ppm GA + 700 ppm chloride, (E) 250 ppm GA + 700 ppm bromide (F) 250 ppm GA + 700 ppm iodide The strong chemisorption of halide ions on the metal surface is responsible for the synergistic effect in combination with inhibitor in their cationic form38,39. The inhibitors are then adsorbed by coulombic attraction on the metal surface where halide ions are already adsorbed by chemisorption. Stabilization of adsorbed halide ions with inhibitors in forms cations leads to greater surface coverage and thereby greater inhibition. 15 © 2011 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, subject 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 5. 1. Conclusion Volume 14, Preprint 28 submitted 27 July 2011 (1) Gum Acacia acts as inhibitors for the corrosion of mild steel in H2SO4 (pH = 1) medium. The maximum inhibition efficiency for the inhibitors was obtained for the combination of 250 ppm GA and 700 ppm Iodide (~87%) for 6h exposure period. (2) The adsorption of GA on mild steel surface in acidic medium follows Temkin adsorption isotherm. (3) The spontaneous physisorption of the inhibitors on the metal surface revealed from the negative values of ΔGads and lower negative ΔHads. (4) Synergistic effect was observed (s>1) between GA and the halides; inhibition efficiency of the combinations follow the trend Iodide > Bromide > Chloride might be due to the size and the electronegativity of the halide ions. (5) The potentiodynamic study suggested the mixed type inhibition and preferentially anodic control for all the inhibitors. In strong acid the protonated polymer molecules were responsible for the adsorption on the metal surface preferentially on anodic sites or on the negatively charged surface resulted from preadsorption of halide ions. (6) The comparative FT-IR spectral analysis revealed the presence of GA in the metal surface product and the slight difference in the peak or band position might be due to interaction of adsorbed inhibitor molecules with the metal surface. 16 © 2011 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, subject 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 References Volume 14, Preprint 28 submitted 27 July 2011 1. J. Aljourani, M.A. Golozar, K. Raeissi; Mater. Chem. Phys. 121, (2010) 320. 2. R. Hasanov, S. Bilge, S. Bilgic, G. Gece, Z. Kılıc; Corros. Sci. 52, ( 2010) 984. 3. L. Wang; Corros. Sci. 43 (2001) 1637–1644. 4. M.J. Bahrami, S.M.A. Hosseini, P. Pilvar; Corrosion Science 52 (2010) 2793–2803. 5. I.B. Obot, N.O. Obi-Egbedi; Surf. Rev. Lett. 15 (6) (2008) 903. 6. P. C. Okafor, E. E. Ebenso, U. J.Ekpe; Int. J. Electrochem. Sci. 5 (2010) 973 – 998. 7. E. E Oguzie, C. K.Enenebeaku, C. O Akalezi, S. C. Okoro, Ayuk, A.A., Ejike; J. Coll. Interf. Sci. 349 (2010) 283 – 292. 8. P. B. Raja, M. G. Sethuraman; Materials Letters 62 (2008) 113–116. 9. 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