Research Journal of Chemical Sciences ______ ______________________________ ______ ____ ISSN 2231 - 606X Vol. 2 ( 3 ), 52 - 57 , March (201 2 ) Res.J.Chem.Sci. International Science Congress Association 52 Corrosion Inhibition of Carbon Steel by Polyacrylamide Manimaran N. 1 , Rajendran. S 2,3 , Manivannan. M 4 and John Mary.S 5 1 Department of Chemistry, Yadava College, Madurai - 625014, Tamilnadu, INDIA 2 PG and Research Department of Chemistry, GTN Arts College, Dindigul - 624005, Tamilnadu, INDIA 3 Department of Chemistry, RVS School of Engineering and Technology, Dindigul - 624005, Tamil Nadu, INDIA 4 Department of Chemistry, Chettinad College of Engine ering and Technology, Karur 639114, Tamil Nadu, INDIA 5 Department of Chemistry, Loyola College, Chennai 600 034, Tamil Nadu, INDIA Available online at: www.isca.in (Received 27 th January 2012 , revised 2 nd Februry 2012 , accepted 28 th Februry 2012 ) Abstract The inhibition efficiency (IE) of Polyacrylamide (PAA) in controlling corrosion of carbon steel in ground water in the absence and presence of Zn 2+ has been evaluated by weight loss method. The formulation consisting of 250 ppm PAA and 50 ppm Zn 2+ has 98% IE. It is found that the inhibition efficiency (IE) of PAA increases by the addition of Zn 2+ ion. A synergistic effect exists between PAA and Zn 2+ . The mechanistic aspects of corrosion inhibition have been studied using polarization study. Also FTIR spectra reveal that the protective film consists of Fe 2+ – PAA complex and Zn (OH) 2 . The scanning electron microscopy (SEM) study confirms the protection of carbon steel surface by strong adsorption of PAA. A suitable mechanism for corrosion inhibition is proposed based on the results from the above studies. Keywords: Corrosion inhibition, polyacrylamide, carbon steel, synergistic effect, ground water Introduction Corrosion is the destruction of metals and alloys by chemical and electrochemical reactions with its environment. It is a natural phenomenon which cannot be avoided, but it can be controlled and prevented using appropriate pre ventive techniques like metallic coating, anodic protection, cathodic protection and using inhibitors, etc. Inhibitors imparts very good role in the process of corrosion inhibition. The organic inhibitors containing hetero atoms like oxygen, nitrogen, sulp hur and phosphorus, etc shows better corrosion inhibition by forming protective film on the metal surface. The order of corrosion inhibition efficiency of the compounds containing heteroatoms follows, O N S P 1 - 4 . Application of polymers as corrosion inhibitors have been attracted several researchers 5 - 7 . Corrosion inhibition by conducting polymer has been studied 8 . The studies on corrosion inhibition of Polyacrylamide grafted with fenugreek mucilage 9 and polyvinylpyrrolidone have been reported 10 . The c orrosion inhibition studies of mild steel 11 , aluminium 12 and zinc 13 , etc in various aqueous environment have been studied. The aim of the present study was to investigate synergistic corrosion inhibition for the Polyacrylamide (PAA) and Zn 2+ combination to carbon steel in ground water collected from Yadava College at Madurai, Tamil Nadu, India. The physico - chemical parameters 14 of the ground water taken in the present study have been given in table 1. The corrosion inhibition efficiency was c alculated using weight loss and polarization studies. The protective film formed on the metal surface characterized using surface morphological studies such as Fourier Transform Infrared spectra (FTIR) and scanning electron microscopy (SEM). Table - 1 Physic o - Chemical Parameter of Ground Water Parameters Value pH 7.3 Total Hardness as CaCO 3 460 ppm Calcium 32 ppm Magnesium 91 ppm Nitrate 8 ppm Chloride 270 ppm Fluoride 0.8 ppm Sulphate 100 ppm Phosphate 0.46 ppm Material and Methods Preparation of the specimens: Carbon steel specimen (0.026% S, 0.06% P, 0.4% Mn and 0.1% C and rest Fe) of the dimensions 1.0 X 4.0 X 0.2 cm were polished to a mirror finish and degreased with trichloroethylene and used for the weight - loss method and surfa ce examination studies. Weight – Loss Method: Carbon steel specimens in triplicate were immersed in 100 mL of the ground water containing various concentrations of the inhibitor in the presence and absence of Zn 2+ for 3 days. The corrosion product cleaned with Clark’s solution 15 . The parameter of the marine media is given in Table 1. The weights of the specimens before and after immersion were determined using a balance, Shimadzu AY 210 model. Then the inhibition efficiency was calculated using the equation (1) IE = 100 [1 – (W 2 / W 1 )] % ... (1) Research Journal of Chemical Sciences ______ _ _ _______________________________ ______________ _ ____ ISSN 2231 - 606X Vol. 2 ( 3 ), 52 - 57 , March (201 2 ) Res.J.Chem.Sci International Science Congress Association 53 Where, W 1 and W 2 are Corrosion rate in the absence and presence of inhibitor respectively. The corrosion rate (CR) was calculated using the formula (2) CR = [(Weight los s in mg) / (Area of the specimen in dm 2 x Immersion period in days)] mdd …….(2) Potentiodynamic Polarization Study: Polarization study was carried out in Electrochemical Impedance Analyzer model CHI 660A using a three electrode cell assembly. The working electrode was used as a rectangular specimen of carbon steel with one face of the electrode of constant 1 cm 2 area exposed. A saturated calomel electrode (SCE) was used as reference electrode. A rectangular platinum foil was used as the counter electrodes . Polarization curves were recorded after doing iR compensation. The corrosion parameters such as Tafel slopes (anodic slope b a and cathodic slope b c ), corrosion current (I Corr ) and corrosion potential (E Corr ) values were calculated. During the polarizatio n study, the scan rate (V/s) was 0.005; Hold time at Ef (s) was zero and quiet time (s) was 2. Surface Examination Study: The carbon steel specimens were immersed in various test solutions for a period of one day. After one day the specimens were taken ou t and dried. The nature of the film formed on the surface of metal specimens was analyzed by surface analysis technique, FTIR spectra and SEM. FTIR spectra: The carbon steel specimens immersed in various test solutions for one day were taken out and dried. The film formed on the metal surface was carefully removed and thoroughly mixed with potassium bromide (KBr), so as to make it uniform throughout. The FTIR spectra were recorded in a Perkin – Elmer – 1600 spectrophotometer. Scanning electron microsco py (SEM): The carbon steel specimens immersed in various test solutions for one day were taken out, rinsed with double distilled water, dried and subjected to the surface examination. The surface morphology measurements of the carbon steel surface were car ried out by scanning electron microscopy (SEM) using HITACHI S - 3000H SEM. Results and Discussion Analysis of results of weight loss study: The calculated inhibition efficiencies (IE) and corrosion rates (CR) of PAA in controlling corrosion of carbon steel immersed in ground water both in the absence and presence of Zn 2+ ion are given in table 2 and also shown in figure 1. The IE’s of the PAA - Zn 2+ systems as a function of concentrations of PAA are shown in Fig.1. The calculated value indicates the ability o f PAA to be a good corrosion inhibitor. The IE is found to be enhanced in the presence of Zn 2+ ion. PAA alone shows some IE. But the combination of 250 ppm PAA and 50 ppm Zn 2+ shows 98% IE. This suggests a synergistic effect exists between PAA and Zn 2+ ion 16 . Analysis of Polarization curves: The potentiodynamic polarization curves of carbon steel immersed in ground water in the absence and presence of inhibitors are shown in figure 2. The corrosion parameters such as corrosion potential (E Corr ), Tafel slopes (anodic slope b a and cathodic slope b c ), linear polarization resistance and corrosion current (I Corr ) values were calculated and are given in table 3. When carbon steel was immersed in ground water the corrosion potential was - 680 mV vs SCE (Satura ted calomel electrode). When PAA (250 ppm) and Zn 2+ (50 ppm) were added to the above system the corrosion potential shifted to the cathodic side - 698 mV vs SCE. This indicates that the PAA - Zn 2+ system control the cathodic reaction predominantly. Further, the LPR value increases from 18820.2 ohm cm 2 to 29682.6 ohm cm 2 ; the corrosion current decreases from 2.016 x 10 - 6 A/cm 2 to 1.471 x 10 - 6 A/cm 2 . Thus, polarization study confirms the formation of a protective film on the metal surface. However the shift is not very much. Therefore it is concluded, that the system functions as a mixed type inhibitor. The anodic reaction is controlled by the formation of Fe 2+ - PAA confirms on the anodic sites. The cathodic reaction (generation of OH - ) is controlled by formatio n of Zn(OH) 2 on the cathodic sites on the metal surface. Thus anodic reaction and cathodic reaction are controlled. This accounts for synergistic effect 17, 18 . Table - 2 Inhibition efficiencies (IE %) and Corrosion rates (CR) obtained from PAA - Zn 2+ systems, when carbon steel immersed in ground water PAA ppm Zn 2+ ppm 0 5 10 25 50 CR mdd IE % CR mdd IE % CR mdd IE % CR mdd IE % CR mdd IE % 0 15.15 --- 14.09 7 13.64 10 13.33 12 12.88 15 25 14.39 5 6.82 55 9.06 40 7.58 50 3.03 80 50 12.45 18 5.30 65 6.06 60 5.30 65 2.73 82 75 11.36 25 4.55 70 3.64 76 3.64 76 2.42 84 100 9.85 35 3.79 75 3.03 80 1.82 88 1.52 90 125 8.75 42 2.27 85 2.73 82 1.52 90 0.91 94 250 7.85 48 2.12 86 1.82 88 0.91 94 0.30 98 Research Journal of Chemical Sciences ______ _ _ _______________________________ ______________ _ ____ ISSN 2231 - 606X Vol. 2 ( 3 ), 52 - 57 , March (201 2 ) Res.J.Chem.Sci International Science Congress Association 54 Figure - 1 Corrosion rates (CR) of carbon steel immersed in various test solutions Table - 3 Corrosion parameters of carbon steel immersed in ground water in the absence and presence of inhibitor system obtained from potentiodynamic polarization study System E corr mV vs SCE b c mV/de cade b a mV/decade I corr A/cm 2 LPR ohm cm 2 Ground Water - 680 172 177 2.016 x 10 - 6 18820.2 Ground Water + PAA (250 ppm) + Zn 2+ (50 ppm) - 698 177 212 1.471 x 10 - 6 29682.6 Figure - 2 Polarization curves of carbon steel immersed in various test solutions (a) Ground water (blank) (b) Ground water + PAA (250 ppm) + Zn 2+ (50 ppm) Research Journal of Chemical Sciences ______ _ _ _______________________________ ______________ _ ____ ISSN 2231 - 606X Vol. 2 ( 3 ), 52 - 57 , March (201 2 ) Res.J.Chem.Sci International Science Congress Association 55 Analysis of FTIR spectra: FTIR spectra have been used to analyze the protective film formed on the metal surface 19 . The structure of pure PAA is shown in figure 3. The FTIR spectrum (KBr) of pure PAA is shown in figure 4(a). The N - H stretching and deformation frequencies appear at 3442 cm - 1 and 1641 cm - 1 respectively. The C=O stretching frequency appears at 1650 cm - 1 . The C - N stretching frequency appears at 1021 cm - 1 . The FTIR spectrum of the film formed on the metal surface after immersion in the solution containing ground water, 250 ppm of PAA and 50 ppm Zn 2+ is shown in figure 4(b). The N - H stretching frequency has shifted from 3442 cm - 1 to 3445 cm - 1 . The N - H deformation frequency has shifted from 1641 cm - 1 to 1645 cm - 1 . The C=O stretching frequency has shifted from 1650 cm - 1 to 1623 cm - 1 . This observation suggest that PAA has coordinated with Fe 2+ through the nitrogen atom of N - H group resulting in th e formation of Fe 2+ - PAA complex on the anodic sites of the metal surface. The peak at 1384 cm - 1 is due to Zn - O stretching. The OH stretching frequency appears at 3442 cm - 1 . This confirms that Zn(OH) 2 is formed on the cathodic sites of metal surface 20 - 22 . Thus the FTIR spectral study leads to the conclusion that the protective film consist of Fe 2+ - PAA complex and Zn(OH) 2 . Figure - 3 Structure of polyacryl amide Figure - 4 FTIR spectra (a) Pure PAA (b) Film formed on the metal surface Scanning Electron Microscopy (SEM): SEM provides a pictorial representation of the surface. To understand the nature of the surface film in the absence and presence of inhibitors and the extent of corrosion of carbon steel, the SEM micrographs of the surfa ce are examined. The SEM micrograph (X 1000) of a polished carbon steel surface (control) in figure 5(a) shows the smooth surface of the metal. This shows the absence of any corrosion products or inhibitor complex formed on the metal surface. The SEM micro graph (X 1000) of carbon steel specimen immersed in the ground water for one day is shown in figure 5(b) and figure 5(c) respectively. Figure - 5 SEM micrographs of carbon steel surface ( Magnification – X 1000) (i) Polished Carbon ste el (control) (ii) Carbon steel immersed in ground water (iii) Carbon steel immersed in ground water containing PVA (250 ppm) and Zn 2+ (50 ppm) Research Journal of Chemical Sciences ______ _ _ _______________________________ ______________ _ ____ ISSN 2231 - 606X Vol. 2 ( 3 ), 52 - 57 , March (201 2 ) Res.J.Chem.Sci International Science Congress Association 56 The SEM micrograph of carbon steel surface immersed in ground water is shown in figure 5(b). Thi s shows the roughness of the metal surface which indicates the corrosion of carbon steel in ground water. The figure 5(c) indicates that in the presence of 250 ppm PAA and 50 ppm Zn 2+ mixture in ground water, the surface coverage increases which in turn results in the formation of insoluble complex on the metal surface. In the presence of PAA and Zn 2+ , the surface is covered by a thin layer of inhibitors which effectively control the d issolution of carbon steel 23 - 29 . Mechanism of corrosion inhibition: With these discussions, a mechanism is proposed for the corrosion inhibition of carbon steel immersed in ground water by 250 ppm PAA and 50 ppm Zn 2+ system : When the formulation consisti ng of 250 ppm of PAA and 50 ppm of Zn 2+ in ground water there is a formation of PAA – Zn 2+ complex in solution. When carbon steel is immersed in this solution PAA – Zn 2+ complex diffuses from the bulk of the solution towards the metal surface. PAA – Zn 2+ c omplex is converted into PAA – Fe 2+ complex on the anodic sites of the metal surface with the release of Zn 2+ ion, Zn 2+ - PAA + Fe 2+ ---------- � Fe 2+ - PAA + Zn 2+ The released Zn 2+ combines with OH – to form Zn (OH) 2 on the cathodic sites of the metal s urface, Zn 2+ + 2OH - --------------- � Zn(OH) 2  Thus the protective film consists of Fe 2+ – PAA complex and Zn (OH) 2 . In near neutral aqueous solution the anodic reaction is the formation of Fe 2+ . This anodic reaction is controlled by the formation of PAA – Fe 2+ complex on the anodic site of the metal surface. The cathodic reaction is the generation of OH – . It is controlled by the formation of Zn(OH) 2 on the cathodic sites of the metal surface. Fe → Fe 2+ + 2e - (Anodic reaction) Fe 2+ + Zn 2 + - PAA complex →Fe 2+ - PAA complex + Zn 2+ O 2 + 2OH - + 4e - → 4OH - (Cathodic reaction) Zn 2+ + 2OH - → Zn(OH) 2  This accounts for the synergistic effect of PAA – Zn 2+ system. 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