Corrosion inhibition of carbon steel by adipic acid Zn 2+ system

Transcription

1 Indian Journal of Chemical Technology Vol. 12, July 2005, pp Corrosion inhibition of carbon steel by adipic acid Zn 2+ system G Ruba Helen Florence a, A Noreen Anthony a, J Wilson Sahayaraj b, A John Amalraj b & Susai Rajendran b* a Department of Chemistry, Holy Cross College, Tiruchirappalli , India b Corrosion Research Centre, Department of Chemistry, GTN Arts College, Dindigul , India Received 22 July 2004; revised received 28 March 2005; accepted 25 April 2005 The inhibition efficiency (IE) of adipic acid (AA)-Zn 2+ system in controlling corrosion of carbon steel immersed in well water has been evaluated by weight-loss method. The formulation consisting of 50 ppm of AA and 50 ppm of Zn 2+ has 95% IE. At lower ph value(ph=6) IE decreases and in alkaline medium (ph=8) IE increases. Polarization study reveals that AA- Zn 2+ system functions as a mixed inhibitor. AC impedance spectra reveal that a protective film is formed on the metal surface. FTIR spectra reveal that the protective film consists of Fe 2+ -AA complex and Zn(OH) 2. Keywords: Carbon steel, corrosion inhibition, adipic acid, well water, zinc ion IPC Code: C23F11/00 Several carboxylates such as sodium salicylate 1, sodium cinnamate 2, anthranilate 3, and adipate 4 have been used as inhibitors. These inhibitors are described by the formula R(COO - ) n where R can be alkyl or aryl and n is usually 1 or 2 but can be 3-6. A very wide range of such chemicals have been shown to be effective inhibitors of the corrosion of mild steel. The chief requirement seems to be that the R.COOH acid should have a pk a value of at least 4. Thus, in the straight chain monocarboxylic acids, the sodium salt of formic H.COOH acid with pk a =3.75 is not inhibitive whereas the higher members of the series, beginning with acetate, pk a =4.76 are inhibitive. Similarly, with the dicarboxylates (CH 2 ) n (COO - ) 2 oxalate with n=0 and pk a =1.23 and malonate with n =1 and pk a =2.54 are non-inhibitive whereas higher members of the series ie., succinate (pk a =4.17) azelate (pk a =4.54) and sebacate (pk a =4.55) are very good inhibitors 1. It has also been shown 1 that for aryl carboxylates ortho-substituted benzoates are less effective than meta- or para-substituted compounds. This effect is also presumably associated with the lower pk a value of the ortho-compounds since there is little difference in efficiency between ortho- and parasubstituted cinnamates for which the pk a values are similar. This observation on the acid strength is probably the only reliable statement that can be made *For correspondence ( srmjoany@sify.com) in predicting whether a carboxylate will be an inhibitor and even this prediction may relate only to mild steel. The inhibitive properties of carboxylates to other metals and alloys are, so far, impossible to predict. Benzoate is a good inhibitor of corrosion of mild steel but not of cast iron or zinc 1 whereas the structurally related cinnamate is effective for these other metals. A ring structure for the inhibitor molecule is not essential for the protection of cast iron since some aliphatic dicarboxylates have this property 2. Generally, the substituted cinnamates are better than substituted benzoates as inhibitors. Experience with benzoate suggests that carboxylates are safe inhibitors in the sense that they are less likely to promote localized attack than some other anodic inhibitors in the presence of excess chloride or sulphate. Reviews of carboxylates as corrosion inhibitors have appeared from time to time 1, 3-6, 7. More detailed studies of particular carboxylates have also been published. Some examples included sodium salicylate 8, sodium cinnamate 9, sodium phenyl acetate 10, anthranilate 11, thiodivaleric 12, adipate 13. Corrosion of tin in citric acid solution and effect of some inorganic anions have been studied 14. Synergistic effect of succinic acid and Zn 2+ in controlling corrosion of carbon steel has been reported 15. The present work is undertaken: (i) to evaluate the inhibition efficiency of adipic acid (AA) as its sodium salt in controlling corrosion of carbon

2 FLORENCE et al.: CORROSION INHIBITION OF CARBON STEEL 473 steel in well water, in the absence and presence of Zn 2+ (ii) to study the various ph values on the IE of the AA- Zn 2+ system, (iii) to analyze the protective film by FTIR (iv) to make use of polarization study and AC impedance spectra to know the mechanistic aspects of corrosion inhibition, and (v) to propose a suitable mechanism of corrosion inhibition based on the results from the above studies. Experimental Procedure Preparation of the specimens Carbon steel specimens (S 0.026, P 0.06, Mn 0.4, C 0.1% and the rest iron) of the dimensions cm were polished to a mirror finish and degreased with trichloroethylene, and used for the weight-loss method and surface examination studies. AR grade chemicals were used in the present study. Weight-loss method The characteristics of the well water used in the present study have been shown in Table 1. Carbon steel specimens in triplicate were immersed in 100 ml of the solutions containing various concentrations of the inhibitor (sodium salt of adipic acid) in the presence and absence of Zn 2+ (as ZnSO 4.7H 2 O) for one day. The weight of the specimens before and after immersion were determined using a Shimadzu AY62 model balance. The corrosion products were cleansed with Clarke s solution 16. The corrosion inhibition efficiency (IE) was then calculated using the equation IE =100[1-(w 2 /w 1 )]% where, w 1 = corrosion rate(mdd) in absence of inhibitor w 2 = corrosion rate(mdd) in presence of inhibitor. Surface examination studies The carbon steel specimens were immersed in various test solutions for a period of one day. After one day, the specimens were taken out, washed with distilled water and then dried. The nature of the film formed on the surface of metal specimens was analyzed by the following surface analysis techniques. FTIR spectra The film was carefully removed with sharp edged glass rod, mixed thoroughly with KBr and made into pellets, and the FTIR spectra were recorded on a Perkin-Elmer 1600 spectrophotometer. Potentiostatic polarization study This study was carried out using EG and G electrochemical impedance analyzer model A three-electrode cell assembly was used. Carbon steel was used as working electrode, platinum was used as counter electrode and saturated calomel electrode(sce) was used as reference electrode. Corrosion potential, corrosion current and Tafel slopes were calculated. AC impedance measurements EG and G electrochemical impedance analyzer model 6310 has introduced a very effective approach to AC impedance measurements. The cell set-up was the same as that used for polarization measurements. A time interval of 5 to 10 min was given for the system to attain a steady state open circuit potential. Then over this steady state potential, AC potential of 10 mv was superimposed. The real part(z ) and imaginary part(z ) of the cell impedance were measured in ohms for various frequencies. The R t (charge transfer resistance) and C dl (double layer capacitance) values were calculated. Results and Discussion Weight-loss study The corrosion inhibition efficiencies of adipic acid (AA)-Zn 2+ systems are given in Table 2. It is found Table 1 Physico- chemical parameters of well water Parameters Value ph 8.38 Conductivity 3111 μmhos/cm TDS 2010 ppm Chloride 665 ppm Sulphate 14 ppm Total hardness 1100 ppm Table 2 Inhibition efficiency (IE%) of various AA-Zn 2+ systems, when carbon steel is immersed in well water for one day Inhibition efficiency (IE%) Zn 2+ (ppm) AA ppm

3 474 INDIAN J. CHEM. TECHNOL., JULY 2005 that the IE increases as the concentration of AA increases. As the concentration of Zn 2+ increases, IE also increases. A synergistic effect exists between AA and Zn 2+. For example, 50 ppm of AA has 62% IE; 50ppm of Zn 2+ has 22% IE. However, the formulation consisting of 50 ppm of AA and 50 ppm of Zn 2+ has 95% IE. That is mixture of inhibitors shows better inhibition efficiency than the individual inhibitors. Influence of ph on the IE of AA- Zn 2+ system The influence of ph (addition of H 2 SO 4 or addition of NaOH) on the IE of AA and AA-Zn 2+ systems is given in Table 3. It is found that when ph=6(acidic), the IE decreases. This is due to the fact that in acidic medium the protective film is broken by the acid; moreover in acid medium adipic acid exists in unionized form. So, it cannot easily coordinate with Fe 2+ to form Fe 2+ -AA complex on the metal surface. When ph is increased, IE slightly increases for AA system. This is due to the fact that ionization of AA is increased by the presence of OH - in solution, and hence coordination of AA with Fe 2+ is enhanced. When ph is increased IE slightly decreases for the AA- Zn 2+ system. This is due to the fact that when NaOH is added to the AA- Zn 2+ system Zn(OH) 2 is precipitated in the bulk of the solution. Zn 2+ is not free to transport AA towards the metal surface. Hence, a decrease in the IE is noticed. However, when more NaOH is added (at ph 11) sodium zincate is formed which is in solubilized form. Now, Zn 2+ is free transport AA towards metal surface to form Fe 2+ -AA complex on the metal surface and hence an increase in IE is noticed. Surface analysis The protective film formed on the surface of the metal in the presence of AA-Zn 2+ system has been analyzed by FTIR spectroscopy. Analysis of FTIR spectra FTIR spectra of pure adipic acid (AA) and film formed on the surface of metal after immersion in well water containing 50 ppm of AA and 50 ppm of Zn 2+ are shown in Fig. 1a and Fig. 1b respectively. The C=O stretching frequency of carboxyl group shifts from 1719 cm -1 (Fig. 1a) to 1700 cm -1 (Fig. 1b). This indicates that the oxygen atom of carboxyl group has coordinated with Fe 2+ -AA complex on the anodic sites of the metal surface. The peak at 1350 cm -1 is due to Zn(OH) 2 formed on the cathodic sites 17,18. Table 3 Influence of ph on the inhibition efficiency of the adipic acid (AA)(50 ppm) and AA (50 ppm)-zn 2+ (5 ppm) systems Immersion period: one day Sl. No. ph Inhibitor system AA AA-Zn 2+ IE (%) Table 4 Corrosion parameters of carbon steel immersed in well water in the presence and absence of inhibitor obtained by polarization method AA Zn 2+ E corr I corr b a b c ppm ppm mv vs SCE A/cm 2 mv mv Fig. 1 FTIR Spectra: (a) Pure adipic acid (KBr), (b) Film formed on carbon steel after immersion in well water containing 50 ppm of adipic acid and 50 ppm of Zn 2+ Analysis of potentiostatic polarization curves The potentiostatic polarization curves of carbon steel immersed in various test solutions are shown in Fig. 2. The corrosion parameters are given in Table 4. When carbon steel is immersed in well water, the corrosion potential is 511 mv versus saturated calomel electrode (SCE). The formulation consisting of 50 ppm of AA and 50 ppm of Zn 2+ shifts the corrosion potential to 477 mv versus SCE. This suggests that the anodic reaction is controlled predominantly, since more AA is transported to the anodic sites in the presence of Zn 2+. The Tafel slopes(b a and b c ) are equal (53 mv). These results suggest that the AA-Zn 2+ formulation functions as a mixed inhibitor. The corrosion current for well water is A/cm 2. The corrosion current for formulation consisting of AA(50 ppm)-zn(50 ppm) has decreased to A/cm 2.

4 FLORENCE et al.: CORROSION INHIBITION OF CARBON STEEL 475 Fig. 2 Polarization curves of carbon steel immersed in various test solution: (a) Well water, (b) Well water containing 50 ppm of adipic acid and 50 ppm of Zn 2+ Analysis of AC impedance spectra The AC impedance spectra of carbon steel immersed in various test solutions are shown in Fig. 3. The AC impedance parameters, namely, charge transfer resistance(r t ) and double layer capacitance (C dl ) are given in Table 5. Well water has R t value of Ωcm 2 and C dl value of μfcm -2. When AA and Zn 2+ are added to well water, R t value increases tremendously from to Ωcm 2. The C dl decreases from to μfcm -2. This suggests that a protective film is formed on the surface of the metal. This accounts for the very high IE of AA-Zn 2+ system. Mechanism of corrosion inhibition Weight loss study reveals that the formulation consisting of 50 ppm of adipic acid(aa) and 50 ppm of Zn 2+ has 95% IE. Polarization study reveals that this formulation functions as a mixed inhibitor. AC impedance spectra reveal that a protective film is formed on the metal surface. FTIR spectra reveal that the protective film consists of Fe 2+ -AA complex and Zn(OH) 2. In order to explain the above facts, the following mechanism of corrosion inhibition is proposed. When the formulation consisting of 50 ppm of AA and 50 ppm of Zn 2+ is prepared, there is formation of Zn 2+ -AA complex in solution. When carbon steel is immersed in this solution, Zn 2+ -AA complex diffuses towards the metal surface. On the metal surface, Zn 2+ -AA complex is converted into Fe 2+ -AA complex on the anodic sites. Fig. 3 AC impedance spectra of carbon steel immersed in various test solutions: (a) Well water, (b) Well water containing 50 ppm of adipic acid and 50 ppm of Zn 2+ Table 5 Impedance parameters of carbon steel in well water in the presence and absence of inhibitor obtained by AC impedance method AA Zn 2+ R t C dl ppm ppm Ωcm 2 μfcm Zn 2+ -AA + Fe 2+ Fe 2+ -AA + Zn 2+ The released Zn 2+ combines with OH - to form Zn(OH) 2 on the cathodic sites. Zn 2+ + OH - Zn(OH) 2 Thus, the protective film consists of Fe 2+ -AA complex and Zn(OH) 2. Conclusion The present study leads to the following conclusions: The inhibition efficiency(ie) of adipic acid (AA)-Zn 2+ system in controlling corrosion of

5 476 INDIAN J. CHEM. TECHNOL., JULY 2005 carbon steel immersed in well water has been evaluated by weight-loss method. The formulation consisting of 50 ppm of AA and 50 ppm of Zn 2+ has 95% IE. At lower ph value (ph=6) IE decreases and in alkaline medium (ph=8) IE increases. AC Impedance spectra reveal that a protective film is formed on the metal surface. FTIR spectra reveal that the protective film consists of Fe 2+ -AA complex and Zn(OH) 2. Polarization study reveals that AA-Zn 2+ system functions as a mixed inhibitor. Acknowledgement The authors are thankful to their management, Prof. E. Kalman(Hungary), Prof. F. Zucchi (Italy) and UGC, India, for their support and encouragement. References 1 Mercer A D, 5 th European Symp Corros Inhibitors (5 SEIC) Ann Univ Ferrara, Sez v. Supp, n7 (1980) Mercer A D, ASTM Special Technical Publication, ASTM Philadelphia, USA 705 (1980) Chemistry Research for the Years 1958, 1951 and published by HMSO, London. 4 Funke W & Hamann K, Werkst Korros, 9 (1958) Hersch P, J Appl Chem, 11 (1961) Mayne J E O & Ramshaw E M, J Appl Chem, 10 (1980) Shmeleva N K & Barannik V P, Zh Prikl Khim, 36 (1963) Bogatyreva E V & Balezin S A, Zh Prikl Khim, 32 (1959) Bogatyreva E V & Nagaev V V, Zh Prikl Khim, 35 (1962) Bogatyreva E V & Balezin S A, Khim Khim Tekhnol, n1 (1959). 11 Kuznetsov I Yu, Oleynik S V, Adreev N N & Vesely S S, 6 th European Symp Corros Inhibitors, (6 SEIC), Ann Univ Ferrara, Sez V. Supp n8 (1985) Bogatyreva E V & Karepina M A, Zh Prikl Khim, 36 (1963) Klyuchnikov N G & Novoshinskaya N S, Zh Prikl Khim, 36 (1963) Rehim S S A, Sayyah S M & Deeb M M E, Mater Chem Phys, 80 (2003) Felicia Rajammal Selvarani, Santhamadharasi S, Wilson Sahayaraj J, John Amalraj A & Susai Rajendran, Bull Electrochem, 20 (2004) Wranglen G, Introduction to Corrosion and Protection of Metals (Chapman & Hall, London), 236, Silverstein R M, Bassler G C & Morrill T, Spectromatric Identification of Organic Compounds (John Wiley & Sons, New York), 1981, Sekine I & Hirakwa Y, Corrosion, 42 (1986) 276.