Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 3(10), 16-23, October (2013) Res. J. Chem. Sci. International Science Congress Association 16 Corrosion Behaviour of Carbon Steel in DTPMP-ST-Zn2+ System: An Eco-Friendly SystemB. Balanaga Karthik1*, P. Selvakumar and C.Thangavelu1*Department of Chemistry, KCG College of Technology, Karappakkam, Chennai, TN, INDIA Department of Chemistry, Chettinad College of Engineering and Technology, karur, TN, INDIA Department of Chemistry, Periyar E.V.R College (Autonomous), Trichy, TN, INDIA Available online at: www.isca.in, www.isca.me Received 16th July 2013, revised 4th August 2013, accepted 12th October 2013Abstract Diethylenetriamine penta (methylene phosphonic acid) (DTPMP) in the presence of Zn2+ ions are used to functions as the corrosion inhibitors in controlling the corrosion of carbon steel in 60 ppm Cl. To reduce the toxic nature of Zn2+ has to add other agents like Sodium Tungstate (ST). DTPMP is environment- friendly, non-toxic and non-polluting agent is used to control the rate of corrosion. Here, the corrosion control of carbon steel in 60 ppm Cl was investigated by Weight-loss study, electrochemical measurements and surface examination studies. The corrosion Inhibition Efficiency (IE) offered by 25 ppm of DTPMP, 25 ppm of ST, 10 ppm of Zn2+ was 92 %. The synergistic behaviour of inhibiting action was also calculated. Electrochemical Impedance spectra indicates that surface film formed on the carbon steel shows that high charge transfer resistance and low Double layer capacitance, which suggests the film, is non-porous and protective. Potentiodynamic polarization study shows that the system works as an cathodic inhibitor. The FT-IR spectra reveals that the protective film consists of Fe2+ - DTPMP complex, Fe2+ - ST complex and Zn(OH) formation. Keywords: Carbon steel, phosphonic acid, corrosion inhibition, F-Test, FT-IR spectra Introduction Large number of chemicals can inhibit the corrosion of metals in neutral aqueous solutions. The metal and inhibitor having different properties for different environment. The molybdate1,2, chromates have been used as corrosion inhibitors. Tungstate having more efficiency when compared to chromates, molybdate. Tungstate is good corrosion inhibitor for Al 3–5 and with Zinc in neutral and acidic solutions. Even though it acts as a effective corrosion inhibitor, in presence of other substance like phosphonic acids, the inhibition efficiency will be improved. DTPMP combined with different metal ions such as Zn2+etc., has been used in the cooling water systems9-14. Corrosion inhibition and scale prohibiting properties of some phosphonic acids and DTPMP have also been investigated15. The inhibiting property of Diethylenetriamine penta (methylene phosphonic acid) as corrosion inhibitor in association with a cation like Zn2+ has been discussed16. The present study evaluated the synergistic effect of the sodium tungstate-Zn2+system and influence of DTPMP. Inhibition efficiency (IE) of the ST-Zn2+ system. The type of inhibitor is evaluated by electrochemical studies and the formation of protective film is confirmed by the FT-IR data. Material and Methods Preparation of Carbon Steel specimen: Carbon steel samples with the composition (C – 0.188% , S – 0.016 %, Si – 0.346 %, Mn – 1.15%, P – 0.036 %, Cr – 0.557%, Mo – 0.225 %, Ni – 0.0847%, Al – 0.0417%, Cu- 0.0342%, Ti- 0.0149%, V – 0.0313%, Pb- 0.0006% and rest Iron 97.27%) were used for weight-loss study and electrochemical measurements, specimen of the size 1.0 cm x 0.2 cm x 4.0 cm were cut, polished to mirror finish by table grinding wheels, degreased with Trichloroethylene. The environment chosen for the study is 60 ppm Cl. The Physico-chemical parameters of water used to prepare the solutions for making medium and inhibitor are given in table 1. Table-1 Physico-chemical parameters of water used to prepare solutions S. No. Parameters Level of Content 1 Appearance Clean and Clear 2 Odour None 3 TDS 5 ppm 4 pH 7.04 5 Total Hardness as CaCO 3 3 ppm 6 Chloride as Cl 2 ppm 7 Fluoride as F Nil 8 Sulphate as SO 4 Nil 9 Phosphate as PO 4 Nil Weight – Loss Measurement: Carbon Steel specimen in Triplicate were immersed in 60 ppm Cl with and without inhibitor. After the immersion period is over specimens were taken out, rinsed in running tap water and kept in a desiccators. The corrosion products were cleaned with Clark’s solution17. Then Weight – loss determined in order to calculate the Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(10), 16-23, October (2013) Res. J. Chem. Sci.International Science Congress Association 17 Inhibition Efficiency and Corrosion Rate (CR) using the following formulae. – WI. E = X 100 Where, W = Weight – loss in absence of an inhibitor, W = Weight – loss in presence of an inhibitor. Loss in Weight (mg) CR = SA (dm) x IP (days) SA – Surface Area of the specimen in dm, IP – Immersion Period in days Electrochemical Impedance Spectra: The electrochemical measurements presented in this study were performed using the Electrochemical Workstation (Model No. CHI760, CH Instruments, USA). Prior to the electrochemical measurements, the metal specimens were prepared according to the above described procedure. The experiment was carried out at room temperature with 60 ppm Cl as an electrolyte. The platinum electrode and saturated calomel electrode (SCE) were used as auxiliary and reference electrodes respectively. Working electrode constitute Carbon steel specimen of 1 cm area. In order to minimize the ohmic potential drop the tip of the reference electrode is positioned very close proximity to the working electrode. Potentiodynamic polarization study was carried out at a scan rate of 0.1 mV/s. The open circuit potential (OCP) was established first and then the polarization measurement was the polarization for carbon steel specimens in the test solution with and without inhibitors. The real part (Z’) and the imaginary part (Z”) of the cell impedance were measured. The Charge transfer resistance (Rct) and Double layer Capacitance (Cdl) value were calculated using the following relation. 1 dl = 2 Rct maxSurface Examination studies: The carbon steel specimens were immersed in blank as well as inhibitor solutions, for a period of 3 days. After the immersion period is over, the specimens were taken out and dried. The nature of the thin film formed on the surface of the metal specimens was analyzed by various surface analysis techniques. FT-IR spectra: Shimadzu IR affinity – 1 KBr dei set method spectrophotometer the film formed on the Carbon steel specimen were taken out, dried and the film was carefully removed, mixed thoroughly with KBr and made in to pellets and the FT-IR spectra were recorded. Synergism Parameter: The synergism parameter can be calculated by using the equation indicated the synergistic effect existing between the inhibitors18 . SI value is found to be greater than one suggesting that the synergistic effect between the inhibitors. SI = 1- I1+2 / 1- I’1+2 – Inhibition efficiency of the inhibitor 1, I – Inhibition efficiency of the inhibitor 2, I ‘1+2 – Combined Inhibition efficiency of inhibitor 1 and 2. Analysis of Variance (ANOVA): An F- test was carried out to investigate whether the synergistic effect existing between the inhibitor systems is statistically significant19 is greater than 5.32 for 1, 8 degrees of freedom, the synergistic effect proves to be statistically significant. If it is less than 5.32 for 1,8 degrees of freedom, it was statistically insignificant at a 0.05 level of significance. Results and Discussion Weight – loss method: Evaluation of improvement of IE of Sodium Tungstate with Zn2+: The inhibition efficiency (IE) of Sodium Tungstate (ST) in controlling corrosion of carbon steel immersed in 60 ppm Cl for a period of three days in the absence and the presence of Zn2+ is also given in Table-2. It can be seen from the data that ST alone shows some IE, Whereas Zn2+ alone has some IE. In the absence of ST, the rate of transport of Zn2+ from the bulk of the solution towards the metal surface is slower than the rate of the corrosion process on the metal surface. Hence the lower corrosion inhibition takes place in the absence of ST, Similar observation has already been reported20. When ST is combined with Zn2+ ions, it is found that the IE increases. For example, 25 ppm ST has only 11 % IE and 10 ppm Zn2+ has only 24 % interestingly, their combination shows 31 % IE. This suggests a synergistic effect between the binary inhibitor formulation ST and Zn2+ ions; ST is able to transport Zn2+ towards the metal surface. Evaluation of synergistic effect of DTPMP with ST-Zn2+system: In order to examine the role of DTPMP in the ternary inhibitor formulation, experiments were conducted with DTPMP alone, ST and with Zn2+, DTPMP in a wide concentration range. The highest inhibition efficiency with these systems is shown in the table-2. The synergistic effect in the ST- Zn2+- DTPMP system is evident from the data in table 2. The Zn2+ ions acts as one synergist and DTPMP acts as the other. From the date in table 2, it is seen that at relatively higher concentrations of both ST and Zn2+, 92 % IE was obtained. However, such efficiency is not obtained with concentrations of ST and DTPMP, even at relatively high concentrations. Thus, it may be concluded that Zn2+ is the primary synergist and DTPMP is the secondary synergist and both play a significant synergistic role in inhibiting corrosion. Hence, the highest IE is obtained at such low concentrations of each of the components in the ternary inhibition formulation. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(10), 16-23, October (2013) Res. J. Chem. Sci.International Science Congress Association 18 Table-2 Inhibition Efficiencies (IE) of carbon steel in 60 ppm Cl, the absence and the presence of inhibitors and Inhibition efficiency (IE) obtained by weight – loss method ST ppm Zn 2+ ppm DTPMP ppm IE % 25 0 0 11 0 10 0 24 0 0 25 10 25 10 0 31 0 10 25 36 25 10 5 45 25 10 10 58 25 10 25 92 25 10 50 82 25 10 75 76 Influence of immersion period on the ST-Zn2+ - DTPMP system: The influence of immersion period on the IE of ST (25ppm) – Zn2+ (10ppm)-DTPMP (25ppm) is shown in table 3. It is found that as the immersion period increases, the inhibition efficiency decreases21. This is due to the fact that as the immersion period increases the protective film is ruptured by the continuous attack of Cl- present in the solution. There is a competition between the two processes, namely the formation of Fe-ST and Fe-DTPMP complex and iron chloride. It appears that the formation of iron chloride is more favored than the formation of iron complexes. Moreover, the iron complexes of ST and DTPMP film formed on the metal surface is converted into iron chloride, which goes into the solution, and hence, the IE decreases as the immersion period increases. Synergism Parameter: The values of synergism parameters are shown in table 4. The values of SI are greater than one, suggesting a synergistic effect. SI approaches 1 when no interaction exists between the inhibitor compounds. When SI � 1, this points to the synergistic effect. In the case of SI 1, the negative interaction of inhibitors prevails i.e. Increase in corrosion rate taking place22. From table 4 the most values are greater than unity, suggesting that the synergistic behaviour is existing between the inhibitors. Surface Analysis: The protective film formed on the surface of the metal in the presence of Zn2+-WO2 has been analyzed by FTIR spectroscopy. Analysis of FTIR Spectra: The FT-IR spectrum of Pure Sodium Tungstate (ST) is shown in figure 2a. The WO2-stretching frequency of the sodium tungstate appears at 1687 cm-1. The FT-IR spectrum of Pure DTPMP is shown in figure 2b. The C – N stretching frequency appears at 1116 cm-1 and P – O stretching frequency appears at 1059 cm-1. The FT-IR spectrum of pure DTPMP is shown in figure 2(b). The P – O stretching frequency appears at 1059 cm-1. The C – N stretching frequency appears at 1116 cm-1. P- OH group causes absorption at 3332 cm-1 and P(O) OH group at 3402 cm-1. The absorption band at 1059 cm-1 represents P – O stretching frequency. The absorption band at 1346 cm-1 represent P = O stretching. Table-3 Influence of Immersion Period on the IE of ST– Zn2+– DTPMP system System Immersion Period (Days) 1 3 5 7 9 60 ppm Cl - CR (mdd) 0.0051 0.0075 0.0155 0.0295 0.0376 60 ppm Cl - + ST (25ppm) + Zn 2+ (10 ppm) + DTPMP (25 ppm)CR (mdd) 0.0003 0.0006 0.0018 0.0040 0.0054 IE % 95 92 88 86 85 Figure-1 Influence of immersion period variation Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(10), 16-23, October (2013) Res. J. Chem. Sci.International Science Congress Association 19 Table-4 Synergism parameter of Carbon steel immersed in 60 ppm Cl– in the presence and absence of inhibitor ST +Zn 2+ (I 2 ) IE %DTPMP (I 1 ) IE %ST-Zn 2+ - DTPMP (I’ 1+2 ) IE %SI 31 15 45 9.54 31 25 58 12.63 31 10 92 2.96 31 22 82 7.77 31 27 76 10.40 The absorption band due to the bending of O-P-O appears at 493,545, and 633 cm-1. The bending vibrations (-CHScissoring) of aliphatic tertiary amine grout CH-N appears at 1448 cm-1. The weak C-C stretching vibrations appears at 956 748,725 cm-1. Thus DTPMP with molecular formula 2815 is characterized by FT-IR spectrum. The FT-IR spectrum (KBr Pellet) of the film formed on the carbon steel surface after immersion in the solution containing 60 ppm Cl, 25 ppm DTPMP, 25 ppm ST and 10 ppm Zn2+ is shown in figure 2c. The tungstate stretching frequency of ST shifted from 1687 cm-1to 1600 cm-1. This suggests that WO2- of ST is coordinated with Fe2+ on the anodic sites of the metal surface also resulting in the formation of Fe2+- WO2- complex. The C – N stretching frequency shifted from 1116 cm-1 to 1049 cm-1 and P – O stretching frequency shifted from 1059 cm-1 to 904 cm-1, which suggests that oxygen and nitrogen atom of DTPMP is coordinated with Fe2+ on the anodic sites of the metal surface, resulting the formation of Fe2+ - DTPMP complex 23-25 . The band at 1317 cm-1 may be due to the in plane vibration of the OH group in Zn(OH) formed on the cathodic sites of the metal surface26,27. Analysis of Electrochemical Impedance Spectra (EIS): The EIS parameters of carbon steel immersed in various test solutions are given in table 5. 4000350030002500200015001000500102030405060 40003500300025002000150010005001020304050607080 40003500300025002000150010005005060708090100 C - NP - O% TWavenumber cm-1 c) Film formed on the metal surface after immersion of 60 ppm Cl- + 25 ppm DTPMP + 25 ppm ST + 10 ppm Zn2+ b) Pure DTPMP a) Pure ST WO2-% TWavenumber cm-1Zn(OH)WO2-C - NP - O% TWavenumber cm-1Figure-2 FT-IR spectrum Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(10), 16-23, October (2013) Res. J. Chem. Sci.International Science Congress Association 20 Table-5 EIS parameters of Carbon steel immersed in 60 ppm Cl environment S.No Cl - ppm DTPMP ppm ST ppm Zn 2+ ppm cmdl΅F / cmIE % 1 60 0 0 0 215 6.22 x 10-6 --- 2 60 25 25 10 3671 2.25 x 10-8 94.14 Inhibitor System: DTPMP – ST – Zn2+ Immersion Period: 3 Days pH = 7 Figure-3 EIS curves of carbon steel immersed in various test solutions, a) 60 ppm Cl b) 60 ppm Cl + 25 ppm DTPMP + 25 ppm ST + 10 ppm Zn2+ The Nyquist representations of the impedance behaviour of carbon steel in 60 ppm Cl with and without inhibitors are given in table 5. The existence of single semi circle showed the single charge transfer process during dissolution, which is unaffected by the presence of inhibitor. Here, for uninhibited solution the charge transfer resistance is 215 ohm cm, for inhibited solution containing 25 ppm DTPMP, 25 ppm ST and 10 ppm Zn2+ the charge transfer resistance 3671 ohm cm. At the same time the double layer capacitance (Cdl) value getting decreases from 6.22 x 10-6΅F / cm to 2.25 x 10-8 ΅F / cm 28 . It is clear that the addition of inhibitor increases the value of charge transfer resistance (Rct) and decreases the value of Cdl. The decreases in dl attributed to increases in thickness of electronic double layer29. In addition, the formation of protective film is confirmed by increase in Rct values30,31. The percentage of efficiency of the inhibitor can be calculated by using following relation. R’ct - RctI.E R’ctR’ct – Charge transfer resistance for inhibited solution, Rct - Charge transfer resistance for uninhibited solution. By using the above relation the percentage efficiency of the inhibitor was calculated. This inhibitor formulation gives 94.14 % inhibition efficiency. Thus the EIS spectral data reveal that a protective film is formed on the metal surface. Potentiodynamic Polarization study: The corrosion parameters of carbon steel in various test solutions are given table 6. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(10), 16-23, October (2013) Res. J. Chem. Sci.International Science Congress Association 21 Table-6 Corrosion parameters of carbon steel immersed in 60 ppm Cl environment obtained by the polarization study S.No Cl - ppm DTPMP ppm ST ppm Zn 2+ ppm corr.mV vs SCE mV mV I corrA/cmI E % 1 60 0 0 0 -598 5.27 5.18 1.26 x 10 - 5 - 2 60 25 25 10 -442 6.18 5.33 2.79 x 10 - 6 78 Inhibitor System: DTPMP – ST – Zn2+ Immersion Period: 3 Days pH = 7 Figure-4 Potentiodynamic polarization curves of carbon steel immersed in various test solutions, a) 60 ppm Cl b) 60 ppm Cl- + 25 ppm DTPMP + 25 ppm ST + 10 ppm Zn2+From the polarization curves the current density (icorr) was determined by Tafel extrapolation method and the corrosion rates were determined from icorr values. The anodic and cathodic potentiodynamic polarization curves for carbon steel in 60 ppm Cl– in the absence and presence of an inhibitor is represented in figure 4. Addition of inhibitors to the test solution showed a shift in corrosion potential towards cathodic direction corresponding to the predominant control of cathodic reaction. The corrosion current was decreased considerable in the presence of inhibitor formulation. The value of Corrosion rate of carbon steel in the presence of inhibitor was much smaller than that in the absence of an inhibitor. In presence of inhibitor, the I.E was about 78 %. When Carbon steel is immersed in 60 ppm Cl- environment the corrosion current Icorr is 1.26 x 10 -5 A/cm. When 25 ppm DTPMP, 25ppm ST, 10 ppm Zn2+ are added the corrosion current decreases to 2.79 x 10-6 A / cm. The significant reduction in corrosion current indicates a decrease in corrosion rate in the presence of inhibitor. For the Blank system the corrosion potential is - 598 mV vs SCE, for the inhibitor system the potential of corrosion is -0.442 mV vs SCE. This suggests this formulation controls the cathodic reaction predominantly. From the Polarization curves it can be inferred that Zn(OH) formed32 on the metal surface retards the oxygen reduction reaction and thus controls the cathodic reaction of the metal. The DTPMP- ST – Zn2+ system shifts the corrosion potential to cathodic side. Thus the ternary inhibitor formulation acts as cathodic inhibitor. This indicates the protective film formed on the metal surface. Mechanism of Corrosion Inhibition: In order to explain the experimental results, the following mechanism was proposed for corrosion inhibition. The formulation consisting of 25 ppm of DTPMP, 25 ppm of ST and 10 ppm of Zn2+ has 92 % Inhibition Efficiency. Polarization study reveals that the formulation functions as cathodic inhibitor. Electrochemical Impedance spectra reveal that the protective film is formed on the metal surface. FT-IR spectra show that the protective film consists of Fe2+ - DTPMP complex. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 3(10), 16-23, October (2013) Res. J. Chem. Sci.International Science Congress Association 22 When formulation consisting of 60 ppm Cl, 25 ppm DTPMP, 25 ppm ST and 10 ppm Zn2+ is prepared, there will be the formation of DTPMP-Zn2+ and ST- Fe2+ complexes in the solution. When carbon steel is immersed the complexes formed on the solution may diffuse to the metal. On the Carbon steel the Zn2+ complexes are converted into iron complexes33 on the anodic sites. Zn2+ - DTPMP + Fe2+ Fe2+ - DTPMP + Zn2+ Zn2+ - ST + Fe Fe2+ - ST + Zn2+The released Zn2+ ions reacts with OH ions to form Zn (OH)Zn2+ + 2OH Zn (OH)Thus the protective film consists of Fe2+ - DTPMP, Fe2+ - ST and Zn (OH)2 complexes. Thus Zn(OH) precipitate may take place at cathodic sites thus decreasing the rate of further oxygen reduction. Conclusion A formulation consisting of DTPMP, ST and Zn2+ can be used as a potent inhibitor to prevent the corrosion of Carbon steel in 60 ppm Cl- medium. DTPMP plays an excellent synergistinc role in the ST- Zn2+ - DTPMP system. The ternary system ST (25ppm) – Zn2+ (10 ppm) – DTPMP (25ppm) is effective and it gives 92 % of Inhibition Efficiency. Significant synergism was obtained by the combined application of ST- Zn2+- DTPMP. The concentration of Zn2+ can be reduced by the addition of ST. The DTPMP is an eco-friendly and is required very low concentrations. Thus this new inhibitor formulation is more environment friendly. The inhibitor formulation acts as an cathodic inhibitor. In presence of the inhibitor, the charge transfer resistance is significantly increased. Also, the double layer capacitance of surface film reduced markedly. Both ST and DTPMP from stable complexes with metal ions in the metal surface. The protective film consists of Fe2+ - ST, Fe2+ - DTPMP and Zn(OH). 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