International Research Journal of Environment Sciences________________________________ ISSN 2319–1414Vol. 4(4), 36-43, April (2015) Int. Res. J. Environment Sci. International Science Congress Association 36 Corrosion Inhibition of Naturally Occurring Gum Exudates of Araucaria columnaris on Mild Steel in 1 M HSOBrindha T , Revathi P and Mallika J 1*Department of Chemistry, PSG College of Arts and Science, Coimbatore, Tamil Nadu, 641014, INDIA Department of Science and Humanities, SNS College of Technology, Coimbatore, Tamil Nadu, 641035, INDIAAvailable online at: www.isca.in, www.isca.me Received 19th January 2014, revised 24th February 2015, accepted 23th March 2015 AbstractThe corrosion inhibitive performance of naturally occurring gum exudates of Araucaria columnaris (AC) and its synergistic effect with halides and metal cations on mild steel in 1 M HSO has been studied using weight loss and electrochemical methods. The inhibition efficiency was found to increase with increase in concentration and decrease with temperature. The adsorption of AC gum on the mild steel obeys Langmuir adsorption isotherm. Potentiodynamic polarization studies showed that AC gum acting as mixed type of inhibitor. The formation of protective layer on mild steel surface is confirmed by SEM analysis. Keywords: Gum exudates of Araucaria columnaris (AC), Mild steel, Synergism, Electrochemical methods, SEM. Introduction Mild steel has remarkable economic and attractive materials for industries owed to its low cost and easy availability. To prevent the metal dissolution and minimize acid consumption, inhibitors are added to the corrosive environment1-3. The most widely used inhibitors are organic compounds containing oxygen, nitrogen and sulphur4,5. Lots of work related to green corrosion inhibitors have been reported and this may be due to the presence of complex organic species such as tannins, alkaloids, carbohydrates and proteins as well as their acid hydrolysis products6-8. Several researchers reported that even polysaccharides are more efficient corrosion inhibitors due to their solubility in water, abundant availability and less expensive. Some investigations have been reported using gum exudates of Pachylobus edulis andRaphia hookeri, Dacroydes edulis,Acacia seyal var seyal10-13 for the corrosion inhibition of various metals. In the present investigation of natural corrosion inhibition materials, gum exudates of Araucaria columnaris is used as safe and cheap corrosion inhibitor for mild steel in acidic medium by weight loss and electrochemical methods. Material and MethodsMild steel specimen: Mild steel specimens of the following composition have been used all over the present investigations (Carbon: 0.07 %, Sulphur: Nil, Phosphorous: 0.008%, Manganese: 0.34%, remaining ferrous).Mild steel with the dimensions 2.5 cm x 1.0 cm x 0.1 cm were used for weight loss measurements. The mild steel specimens were polished with different grades of emery papers (1/0, 2/0, 3/0 and 4/0), washed with double distilled water, dried, degreased with acetone. AR grade sulphuric acid was used for preparing the test solution. Purification of gum: The gum exudates of Araucaria columnaris (AC) were collected locally and dissolved in doubly distilled water and filtered to remove insoluble dust and impurities. It is then dried in a desiccator to obtain a glassy mass of purified gum. Electrochemical measurements: Electrochemical experiments were conducted in a three-electrode glass cell of 100 ml capacity. A platinum counter electrode and a saturated calomel electrode (SCE) as reference electrode are used. The SCE was connected via Luggin’s capillary. The electrochemical impedance measurements were carried out over the frequency range of 10 KHz to 0.01 Hz carried with AC signal amplitude of the 10 mV at the corrosion potential. The measurements were automatically controlled by Zview software and the impedance diagrams were given as Nyquist plots. From the plots the electrochemical parameters such as double layer capacitance (Cdl) and charge transfer resistance (R) were calculated. The potentiodynamic polarization measurements were made for a potential range of -200 mV to +200 mV with respect to open circuit potential, at scan rate of 1 mV/sec. From the plot of E Vs log I, the corrosion potential (Ecorr), corrosion current (Icorr) were obtained. Tafel slopes b and b were obtained in the absence and presence of inhibitors.Surface analysis: The surface of the mild steel immersed in the absence and presence of inhibitors were examined using scanning electron microscopy (JEOL-JSM-35-CF). Results and Discussion Weight loss measurements: The corrosion inhibition of 1 M SO in the presence of AC gumhas been studied by weight International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(4), 36-43, April (2015) Int. Res. J. Environment Sci. International Science Congress Association 37 loss measurements. The inhibition efficiency and corrosion rats of AC gum with different concentrations (50 ppm to 600 ppm) have been evaluated and the results are given in table-1. Analysis of table-1 shows that the inhibition efficiency increases with increase in concentration of the AC gum. The highest inhibition efficiency is obtained at 400 ppm and any further increase in concentration does not affect the performance of the inhibitor. The corrosion inhibition of mild steel by AC gum is due to the adsorption at the electrode/solution interface. AC gum is a polysaccharide consisting of 1,2-benzenedicarboxylic acid, bis (2-ethylhexyl) ester, diisooctyl-phthalate, phthalic acid, isobutyl and isopropyl ester14. The presence of hetero oxygen atom and carboxylate anions in the structure of AC gum forms a co-ordinate type linkage with the mild steel surface through the transfer of lone pairs of electron of oxygen atoms, giving a stable chelate with ferrous ions. The simultaneous adsorption of oxygen atoms, forces the AC gum molecule to be horizontally oriented at the metal surface, which led to increase the surface coverage, resulting in an increase in the inhibition efficiency. Effect of synergism: It has been reported that halide ions have the most adsorbable character of steel. Addition of I, Cl- and Brto an inhibitor enhances the inhibitory action. Analysis of table-2 indicates that the inhibition efficiency of AC gum is enhanced by the addition of halides. The addition of Iodide ions to the inhibiting solution is found to have highest inhibition efficiency when compared to the addition of bromide and chloride ions. The maximum inhibition efficiency caused by I can be explained as follows. In general, the presence of halide ions in acidic medium has synergistically increased the inhibition efficiency of most of the organic compounds. The halide ions increases the adsorption of the organic cations by forming intermediate bridges between the organic inhibitor and charged metal surface.15 The synergistic effect of AC gum with halides is studied using weight loss methods and electrochemical impedance spectroscopy. Analysis of the result shows that the inhibition efficiency of AC gumis enhanced by the addition of halides. The synergistic ability of halides increased in the order Cl- Br I and similar observation has been reported by several researchers16,17The highest synergistic inhibition efficiency influenced by iodide ion is attributed to its large ionic radius, higher hydrophobicity and low electro negativity compared to other halide ions. The effect of metal cations on corrosion of mild steel in 1M SO is studied with and without the addition of AC gum. The calculated values of inhibition efficiencies and corrosion rates are given in table-2. Table-1 Inhibition efficiencies of AC gum on mild steel in 1M HSO from weight loss measurement at room temperature Inhibitor Concentration (ppm) Inhibition Efficiency (%) Corrosion Rate (mmpy) Degree of Coverage ads (kJ/ mol) Blank -- 0.0561 -- -- 50 42.0 0.0325 0.420 16.85 100 46.2 0.0302 0.462 15.50 200 47.5 0.0294 0.475 13.92 400 55.1 0.0251 0.551 12.94 600 54.4 0.0255 0.544 11.85 Table-2 Synergistic effect of halides and metal cations on corrosion inhibition of AC gum on mild steel in 1M HSO4 from weight loss measurement at room temperature Inhibitor Concentration Inhibition Efficiency (%) Corrosion Rate (mmpy) Degree of Coverage ( Blank -- 0.0931 -- 0.1 mM KI 93.3 0.0061 0.933 0.1 mM KI+400 ppm AC gum 97.5 0.0023 0.975 0.1 mM KCl 75.2 0.0231 0.752 0.1 mM KCl+ 400 ppm AC gum 80.1 0.0184 0.801 0.1 mM KBr 82.6 0.0167 0.826 0.1 mM KBr+ 400 ppm AC gum 88.4 0.0118 0.884 0.1 mM Zn 2+ 50.1 0.0259 0.501 0.1 mM Zn 2+ +400 ppm AC gum 82.6 0.0089 0.826 0.1 mM Ni 2+ 72.9 0.0139 0.729 0.1 mM Ni 2+ + 400 ppm AC gum 84.3 0.0081 0.843 International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(4), 36-43, April (2015) Int. Res. J. Environment Sci. International Science Congress Association 38 It is evident from the table that Zn2+ and Ni2+ ions enhance the inhibition efficiency of AC gum remarkably. When we compare the effect of Zn2+ ions on the corrosion of mild steel with Ni2+ ions, the Ni2+ ions have enhanced inhibition efficiency. The improvement in the corrosion inhibition efficiency can be endorsed to the formation of complex between metal cations and AC gum. Therefore, the inhibitor molecules are readily transported from the bulk to the metal surface18.Remarkably, inhibition efficiency is found to more pronounced even at higher temperatures. Effect of temperature: The effect of temperature (303 - 323 K) on the corrosion behavior of mild steel in the presence of AC gum with halides and metal cations are investigated by weight loss techniques. Data in table-3 indicate that the corrosion rate is not significantly affected with rise in temperature. This behavior reveals that the AC gum acting as an efficient corrosion inhibitor in the temperature range studied. Organic compounds containing hetero atoms are used as inhibitors as they adsorbed onto the metal/solution interface. Adsorption of the inhibitor on the mild steel specimen mainly depends on its chemical structure, chemical composition of the test solution, the nature of the electrode, temperature and electrochemical potential at the metal-solution interface. The adsorption provides information about the, adsorbed molecules as well as their interaction with the metal surface. The surface coverage ) of the inhibitors at different concentrations is used to obtain the best adsorption isotherm. The values have been calculated using the following equations (1 and 2): (1) Where: W(inh) = weight loss obtained in the presence of inhibitor and W = weight loss obtained for blank. It is well known that the adsorption isotherms are useful to understand the mechanism of corrosion inhibition. The present experimental data fit to Langmuir adsorption isotherm and it is shown in figure-1. The Langmuir adsorption isotherm is expressed as: (2) Where: K is the equilibrium constant for the adsorption process and C is the concentration. The above equation may be modified as: (3) The plot of C/ versus C gives a straight line with slope equal to unity. The present experimental data fit to Langmuir adsorption isotherm.Free energy of adsorption (ads) is calculated using the equation (4) and the values are included in table-1. (4) The values of ads obtained for AC gum point out the spontaneity of the adsorption process. In the present study, ads values of AC gum lies below -20 kJ/ mol, indicating the operation of physisorption mechanism. The activation energy (E) is calculated from the plot of log (corrosion rate) versus 1000 / T by using the formula and its representative curves are given in the figures-2(a)(b), 3. The activation energies (E) obtained for the inhibited solutions are found to be higher than the E of blank. The higher value of in the inhibiting solution supports the physisorption mechanism. The energy barrier increases with increase in concentration of inhibitors, which results in increase in activation energy19. (5) Figure-1 Langmuir adsorption isotherm for the AC gum on mild steel in 1 M HSO at room temperature Figure-2 Arrhenius plot for AC gum with Zn2+ on mild steel in 1 M SO (b) Arrhenius plot for AC gum with Ni2+ on mild steel in 1 M HSO International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(4), 36-43, April (2015) Int. Res. J. Environment Sci. International Science Congress Association 39 Figure-3 Arrhenius plot for AC gum with KI on mild steel in 1 M SO4 Potentiodynamic polarization studies: The potentiodynamic polarization curves for mild steel in 1 M HSO4 in the absence and presence of AC gum with halides and metal cations are given in figures-4 to 6. The addition of inhibitors causes a remarkable decrease in the corrosion rate, i.e., shifts both anodic and cathodic curves to lower current densities. Electrochemical parameters like corrosion potential (Ecorr), and corrosion current density (Icorr), Tafel constant ba and bc are listed in tables-5 and 6. The addition of inhibitors causes a remarkable decrease in the corrosion rate, i.e., shifts both anodic and cathodic curves to lower current densities. In other words, both the anode and cathodic reactions of mild steel is drastically inhibited by the inhibitors. Icorr values significantly decreases as additives are added to the corrosive medium and this indicates the inhibiting nature of the gum. It is clear from the tables-5 and 6 that the Icorrdecreased considerably in the presence of inhibitors while no definite trend was observed in the shift of Ecorr. The AC gum adsorbed onto the mild steel surface suppresses both the anodic and cathodic reaction and thereby hindering the active sites, and these results suggested that the addition of inhibitors reduces the anodic dissolution and also retards the cathodic hydrogen evolution reaction, indicating that these inhibitors act as mixed type of inhibitor. Table-3 Inhibition efficiencies of AC gum with halides at different temperatures for 1 M HSOInhibitor concentration 303 K 308 K 313 K 318 K 323 K 400 ppm AC gum 55.1 52.1 48.3 44.9 43.1 0.1 mM KCl 75.2 70.9 69.1 66.3 64.2 0.1 mM KCl+ 400 ppm AC gum 80.1 78.4 75.8 73.5 70.9 0.1 mM KBr 82.6 80.2 79.2 76.1 73.8 0.1 mM KBr+ 400 ppm AC gum 88.4 85.6 81.4 76.1 74.0 0.1 mM KI 93.3 90.3 89.1 88.3 87.4 0.1 mM KI+ 400 ppm AC gum 97.5 97.1 96.4 95.3 94.3 0.1 mM Zn 2+ 51.2 50.9 49.4 45.1 43.4 0.1 mM Zn 2+ + 400 ppm AC gum 70.91 70.1 68.9 66.3 64.2 0.1 mM Ni 2+ 46.94 43.1 41.2 39.3 38.7 0.1 mM Ni 2+ + 400 ppm AC gum 71.45 70.1 68.2 67.2 64 Table-4 Energy of activation parameters in the presence of AC gum with halides and metal cations Composition a (kJ/mol) Blank With 400ppm AC gum H 2 SO 4 0.36 0.51 H 2 SO 4 + halides 0.44 1.01 H 2 SO 4 + Zn 2+ 0.48 0.97 H 2 SO 4 + Ni 2+ 0.49 1.20 Table-5 Potentiodynamic polarization parameters for the corrosion inhibition of mild steel in 1 M HSO with and without inhibitors Inhibitor Concentration corr (mA cm-2) × 10-4corr (mV/SCE) a (mV/dec) c ( mv/dec) Corrosion rate (mmpy) Inhibition efficiency (%) Blank 0.93 -0.51 0.086 0.129 14.1 -- 400 ppm AC gum 0.57 -0.49 0.075 0.135 8.7 38.7 0.1 mM KCl+ 400 ppm AC gum 0.63 -0.49 0.076 0.119 9.4 33.3 0.1 mM KBr+400 ppm AC gum 0.56 -0.50 0.076 0.122 8.6 39.7 0.1 mM KI+400 ppm AC gum 0.15 -0.48 0.066 0.128 2.4 83.8 International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(4), 36-43, April (2015) Int. Res. J. Environment Sci. International Science Congress Association 40 Table-6 Potentiodynamic polarization parameters for metal cations on corrosion inhibition of mild steel in 1M HSO with and without inhibitorsInhibitor Concentration corr (mA cm-2) x 10-4corr (mV/SCE) a (mV/dec) c mV/dec)Corrosion rate (mmpy) Inhibition efficiency (%) Blank 0.93 -0.508 0.086 0.129 14.1 -- 400 ppm AC gum 0.57 -0.495 0.075 0.135 8.7 38.7 0.1 mM Zn 2+ 0.74 -0.506 0.081 0.112 10.4 20.4 0.1 mM Zn + +400 ppm AC gum 0.29 -0.492 0.064 0.137 4.3 68.8 0.1 mM Ni 2+ 0.68 -0.522 0.063 0.105 10.3 26.8 0.1 mM Ni 2+ +400 ppm AC gum 0.25 -0.4989 0.061 0.108 3.82 63.2 Figure 4 Potentiodynamic polarization curves for AC gumon mild steel in presence of halide ions in 1 M HSO4 Figure-5 Potentiodynamic polarization for AC gum on mild steel in presence of Ni2+ ion in 1 M HSO Figure-6 Potentiodynamic polarization curves for the AC gum on mild steel in presence of Zn2+ ion in 1 M HSOElectrochemical impedance spectroscopic studies: The corrosion inhibition of mild steel in 1 M HSO in absence and presence of AC gum with halides and metal cationsare investigated by electrochemical impedance spectroscopic method at 303 K. The recorded Nyquist plots are given in figures-7 to 9. The electrochemical parameters such as Rt,, Cdland inhibition efficiency are calculated and presented in table-7. The semicircular nature of Nquist plots indicates that the corrosion of mild steel is controlled by a charge transfer process, and the presence of the inhibitor does not affect the mild steel dissolution20. The R values were calculated from the difference in impedance at low and high frequencies. The double layer capacitance values are obtained from the frequency at which imaginary Zcomponent of the impedance (-Z”max) is maximized. The relationship used is: (6) It is evident from the table 7 that R value increases with the International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(4), 36-43, April (2015) Int. Res. J. Environment Sci. International Science Congress Association 41 addition of ACgum (400 ppm) with halide ions and metal cations. The data also indicate that Cdl value decreases with the addition of ACgum with halide ions and metal cation. The decrease is due to the adsorption of gum exudates of AC on the metal surface. The inhibition efficiency of the gum is calculated directly by using the following relationship (7) Where, Rt (inh) -1 is the charge transfer resistance of the inhibited solution and R-1 is the charge transfer of the uninhibited solution. The inhibition efficiency calculated by electrochemical measurements follows the same trend as a weight loss method. Figure-7 Nyquist plots for AC gum on mild steel in presence of halide ions in 1 M HSO Figure-8 Nyquist plots for AC gum on mild steel in presence of Ni2+ in 1 M HSO Figure 9 Nyquist plots for the AC gum on mild steel in presence of Zn2+ in 1 M HSO4 SEM Micrographs: The SEM micrographs of corroded mild steel in the presence of 1M HSO are shown in figure-8. The faceting seen in the figures in a result of pits formed due to the exposure of mild steel to the acid. figure-9 shows the SEM micrograph of the mild steel in 1M HSO in the presence of AC gum. The faceting observed in figure-8 disappeared and the surface is free from pits and it is smooth. It can be concluded from figure-9, the corrosion does not occur in the presence of inhibitor and hence corrosion was inhibited strongly in the presence of inhibitors. Table-7 AC Impedance parameters for the corrosion inhibition of mild steel in 1 M HSO with and without inhibitorsInhibitor concentration (cm) dl (F cm-2) Inhibition efficiency (%) Blank 15.08 0.249 -- 400 ppm AC gum 20.62 0.232 26.8 0.1 mM KCl +400 ppm AC gum 19.04 0.250 20.7 0.1 mM KBr +400 ppm AC gum 21.4 0.243 27.6 0.1 mM KI +400 ppm AC gum 62.9 0.145 76.0 0.1 mM Zn 2+ + 400 ppm AC gum 49.54 0.204 69.5 0.1 mM Ni 2+ + 400 ppm AC gum 23.64 0.246 36.2 International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(4), 36-43, April (2015) Int. Res. J. Environment Sci. International Science Congress Association 42 Figure-10 SEM photograph of mild steel in 1 M HSO Figure-11 SEM photograph for mild steel in 1 M HSO in presence of AC gum Conclusion The AC gumact as an effective corrosion inhibitor for the mild steel in 1 M HSO acid and maximum inhibition efficiency is achieved at very lower concentration (400 ppm). It has been found that the rate of corrosion is not affected, appreciably up to 323 K, indicating the efficient inhibiting nature in temperature range studied due to the formation of an adherent, protective film on the mild steel surface. The synergistic effect of halides with the AC gum in acidic medium obeys the following order: - � Br- � Cl. The adsorption of the inhibitor onto the mild steel surface obeys Langmuir adsorption isotherm. From the potentiodynamic polarization studies, it can be concluded that the AC gumacting as a mixed type of inhibitor. The protective layer formed by the AC gum on the mild steel surface is further confirmed by SEM analysis. References 1.Abdel Hameed R.S., Ranitidine Drugs as Non-Toxic Corrosion Inhibitors for Mild Steel in Hydrochloric Acid Medium, Port. Electrochimi. 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