Research Journal of Chemical Sciences ___ ______________________________ ______ ____ ___ ISS N 22 31 - 606X Vol. 3 ( 2 ), 54 - 58 , February (201 3 ) Res. J. Chem. Sci. International Science Congress Association 54 Investigation of Corrosion Inhibition of Stainless steel by Sodium tungstate Patric Raymond P . 1* , Peter Pascal Regis A . 1 , Rajendran S . 2, 3 and Manivannan M . 4 1 PG and Research Departent of Cheistry, St. Joseph’s College (Autonoous), Tiruchirappalli, Ta il Nadu, INDIA 2 Corrosion Research Centre, PG and Research Department of Chemistry, GTN Arts College, Dindigul, Tamil Nadu, INDIA 3 Department of Chemistry, RVS School of Engineering and Technology, Dindigul, Tamil Nadu, INDIA 4 Department of Chemistry, Chet tinad College of Engineering and Technology, Karur, Tamil Nadu, INDIA Available online at: www.isca.in Received 15 th December 201 2 , revised 18th January 201 3 , accepted 28 th January 201 3 Abstract The corrosion inhibiti on of sodium tungstate (ST) in controlling corrosion of stainless steel 410 in sea water in the absence and presence of Zn 2+ has been evaluated by weight loss method. The formulation consists of 250 ppm of ST and 50 ppm of Zn 2+ has 98% IE. AC impedance spe ctra reveals that the formation of protective film on the metal surface. The nature of the protective film has been characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Keywords: sodium tungstate, sea water, SEM, AFM I ntroduction Corrosion can be defined as the destruction of metals and alloys by electrochemical reaction with its surroundings. The corrosion occurs because of the normal trend of metals to come back to their thermodynamically stable native state. It canno t be avoided, but it can be controlled and prevented by using suitable protective procedures like cathodic protection, anodic protection, coating, alloying and using inhibitors, etc. Of these the application of inhibitors reduces the aggressiveness of the corrosive and unsafe aqueous surroundings and preventing the metal and alloy from corrosion by forming a protective layer over the metal surface. Especially inhibitors find applications in cooling water system, shipboard condensers, nuclear power plant con densers and heat exchangers, etc 1,2 . The corrosion behaviour of stainless steel and other metals in sea water has been studied by many researchers 3 - 5 . The corrosion inhibition of various metals in different aqueous surrounding using sodium tungstate has be en studied 6 - 8 . The objective of the present investigation is to study corrosion inhibition of stainless steel 410 in sea water in the presence and absence of ST and Zn 2+ combination. The physicochemical parameters of sea water are given in t able 1 and whic h is collected from Bay of Bengal at Cuddalore Port, Cuddalore, Tamil Nadu, India. The corrosion inhibition was calculated using AC impedance spectra. The protective film formed on the metal surface has been characterized using surface morphological studie s such as scanning electron microscopy (SEM) and atomic force microscopy (AFM). Material and Methods Preparation of specimens : Stainless steel 410 specimens (0.095 % C, 0.001 % P, 6.03.4 % Mn, 0.340 % Si, 0.001 % P, 13 % Cr, 0.710 % Ni and the rest Fe) o f dimensions 1.0 cm x 4.0 cm x 0.2 cm were polished to a mirror finish and degreased with trichloroethylene. Table - 1 Physicochemical Parameters of Sea water Parameter Value Total Dissolved Solid 36441 ppm Electrical conductivity 53590 micro - mho/cm pH 6 .91 Total Hardness (CaCO 3 equivalent) 84 ppm Mg 11 ppm Na 8900 ppm Chlorides 16900 ppm Sulphate 4239 ppm Weight - loss method : Stain s tainless steel specimens in triplicate were immersed in 100 mL of the solutions containing various concentrations of the inhibitor in the presence and absence of Zn 2+ for seven days at 25 + 0.1 o C. The weight of the specimens before and after immersion was determined using a Shimadzu balance, model AY62. The corrosion products were cleansed with Clarke’s solution 9 . The IE was then calculated using the equation (1). IE = 100 [1 - (W 2 / W 1 )] % (1) where W 1 and W 2 are corrosion rate in the absence and presence of the inhibitor respectively. The corrosion rate (CR) was calculated using the equation (2). CR = Weight loss in mg / (0.11 dm 3 x Immersion period) (2) AC Impedance Spectra : AC Impedance Spectra were carried out in an H and CH electrochemical workstation impedance analyzer, model CHI 660A. A three - electrode cell assembly was used. The working electro de was stainless steel. A saturated calomel electrode (SCE) was used as the reference electrode and a rectangular Pt foil was used as the counter electrode. The real Research Journal of Chemical Sciences ___ _ _ _______________________________ ______________ _ ________ ISSN 22 31 - 606X Vol. 3 ( 2 ), 54 - 58 , February (201 3 ) Res. J. Chem. Sci. International Science Congress Association 55 part (Z’) and iaginary part (Z”) of the cell ipedance were measured in ohms for various frequencies. The corrosion parameters such as charge transfer resistance (R t ), double layer capacitance (C dl ) and impedance log (Z/ohm) values were calculated. Surface examination study : The stainless steel specimens were immersed in various test solution s for a period of seven days. After seven days, the specimens were taken out and dried. The nature of the film formed on the surface of the metal specimens was characterized by surface analytical techniques such as SEM and AFM. Scanning Electron Micros copy (SEM) : The stainless steel specimens immersed in various test solutions were taken out, rinsed with double distilled water, dried and subjected to the surface examination. The surface morphology measurements of the stainless steel were carried out by SEM using HITACHI S – 3000 H SEM. Atomic Force Microscopy (AFM) : Atomic Force Microscope (AFM) is an exciting new technique that allows surface to be imaged at higher resolutions and accuracies than ever before. The microscope used for the present study w as PicoSPM I Molecular Imaging, USA make. Polished specimens prior to the initiation of all corrosion experiments were examined through an optical microscope to find out any surface defects such as pits or noticeable irregularities like cracks, etc. Only those specimens with a smooth pit - free surface were subjected for AFM examination. The protective films formed on the stainless steel specimens after immersion in the inhibitor systems for different time durations were examined for a scanned area of 30 x 30  m 2 and 15 x 15  m 2 . The two - dimensional topography of surface films gives average roughness parameters of the film. Results and Discussion Analysis of results of weight loss study : The corrosion rates (CR) of stainless steel 410 immersed in sea water in the absence and presence of inhibitor is given in t able 2. It is observed that as the concentration of ST increases, the IE increases. This is due to the fact that as the concentration of ST increases the protective film formed on the surface controls the anodic reaction and protects the metal. Thus a synergistic effect exists between ST and Zn 2 + . The combination of 250 ppm of ST and 50 ppm of Zn 2+ has a high IE, namely 98 percent. In the presence of Zn 2+ more amount of ST is transported towards the met al surface. On the metal surface Fe 2+ - ST complex is formed on the anodic sites of the metal surface. Thus the anodic reaction is controlled. The cathodic reaction is the generation of OH - , which is controlled by the formation of Zn(OH) 2 on the cathodi c sites of the metal surface. Thus the anodic reaction and cathodic reaction are controlled effectively. This accounts for the synergistic effect existing between Zn 2+ and ST 10,11 . Table - 2 Inhibition efficiencies (IE) and Corrosion rates (CR) obtained from ST – Zn 2+ systems, when stainless steel immersed in sea water Na 2 WO 4 ppm Zn 2+ ppm IE% CR (mdd) 0 0 - 6.75 0 50 25 5.06 50 50 59 2.77 100 50 64 2.43 150 50 73 1.82 200 50 81 1.28 250 50 98 0.14 Analysis of AC impedance spectra : AC impedanc e spectra have been used to confirm the formation of protective film on the metal surface. If a protective film is formed on the metal surface, charge transfer resistance (R t ) increases; double layer capacitance value (C dl ) decreases and the impedance log( Z/ohm) value increases. The AC impedance spectra of stainless steel immersed in sea water in the absence and presence of inhibitors are shown in figure 1 to 3. The AC impedance parameters such as R t , C dl and log(Z/ohm) are given in t able 3. It is observed that in the presence of ST (250 ppm) - Zn 2+ (50 ppm) formulation, R t value increases from 12.315 ohm cm 2 to 39.363 ohm cm 2 and C dl value decreases from 4.141x10 - 7 F /cm 2 to 1.296 x10 - 7 F /cm 2 . Moreover, the impedance value [log (Z/ohm)] increases from 1.16 30 to 1.223. These observations suggest that the formation of protective film on the metal surface 10,11 . Table – 3 Corrosion parameters of stainless steel immersed in sea water in the absence and presence of inhibitor system obtained from AC impedance spe ctra System R t Ohm cm 2 C dl F/cm 2 log (Z/ohm) Sea water 12.315 4.141 x 10 - 7 1.1630 Sea water + ST (250 ppm) + Zn 2+ (50 ppm) 39.363 1.296 x10 - 7 1.223 Figure – 1 AC Impedance spectra (Nquist plot) (a) Sea water (b) Sea water + ST (250 ppm) + Zn 2+ (50 ppm) Research Journal of Chemical Sciences ___ _ _ _______________________________ ______________ _ ________ ISSN 22 31 - 606X Vol. 3 ( 2 ), 54 - 58 , February (201 3 ) Res. J. Chem. Sci. International Science Congress Association 56 Figure – 2 AC Impedance spectra (Bode plot) (a) Sea water (b) Sea water + ST (250 ppm) + Zn 2+ (50 ppm) Figure - 3 AC Impedance spectra (Phase Bode plot) (a) Sea water (b) Sea water + ST (250 ppm) + Zn 2+ (50 ppm) SEM analysis of metal surface : SE M provides pictorial representation of the metal surface. To understand the nature of the surface film in the absence and presence of inhibitors and extent of corrosion of stainless steel, the SEM micrographs of the surface are examined. The SEM images of different magnification (x 3000) of stainless steel specimen immersed in sea water for 7 days in the absence and presence of inhibitor system are shown in figure 4. The SEM micrographs of polished carbon steel surface (control) in figure 4(a) shows the smo oth surface of the metal. This shows the absence of any corrosion products or inhibitor complex formed on the metal surface. The SEM micrographs of carbon steel surface immersed in sea water figure 4(b) shows the roughness of the metal surface which indica tes the corroded area of stainless steel in sea water. However figure 4(c) indicates the rate of corrosion is suppressed, as can be seen from the decrease of corroded areas. The metal surface is almost free from corrosion due to the formation of insoluble complex on the surface of the metal. In the presence of ST and Zn 2+ , the surface is covered by thin layer of inhibitors, which effectively controls the dissolution of stainless steel 12,13 . Figure – 4 SEM micrographs of stainless steel (Magnificatio n – X 3000) (a) Polished stainless steel (Control) , (b) Stainless steel immersed in sea water , (c) Stainless steel immersed in sea water consists of ST (250 ppm)+Zn 2+ (50 ppm) Research Journal of Chemical Sciences ___ _ _ _______________________________ ______________ _ ________ ISSN 22 31 - 606X Vol. 3 ( 2 ), 54 - 58 , February (201 3 ) Res. J. Chem. Sci. International Science Congress Association 57 Atomic Force Microscopy (AFM) analysis of metal surface : AFM is a powerful techn ique for the gathering of roughness statistics from a variety of surfaces 14 . AFM is becoming an accepted method of roughness investigation 15 - 17 . All atomic force microscopy images were obtained in a VECCO lab incorporation AFM instrument operating in typi ng mode in air. The scan size of all the AFM images are 5 m x 5 m areas at a scan rate of 6.685 m/sec. The two dimensional (2D) AFM morphologies for polished stainless steel surface (reference sample), stainless steel immersed in sea water (blank), and stainless steel surface immersed in sea water consists of ST 250 ppm and 50 ppm of Zn 2+ are shown in the figure 5 (a), (b) and (c) respectively. AFM image analysis was performed to obtain average roughness, R a (the average deviation of all points roughness profile from a mean line over the evaluation length) 18 . Table 4 is a summary of the average roughness (R a ) value for stainless steel surface immersed in different environments. The values of R a for the polished stainless steel surface (control) are 3.3 nm . This shows that the surface is more homogenous, with some places where the height is lower than the average depth 19 . Figure 5(a) displays the non - corroded metal surface. The slight roughness observed on the polished stainless steel surface is due to atm ospheric corrosion. Figure 5(b) displays the corroded metal surface in sea water. The average roughness value for the stainless steel surface immersed in sea water is 24 nm. This value suggest that stainless steel surface immersed in sea water has a greate r surface roughness than the polished metal surface, indicating that the unprotected stainless steel surface is rougher and were due to the corrosion of stainless steel in sea water environment. Figure 5(c) displays corroded metal surface with few pits. Th e formulation consists of 250 ppm ST and 50 ppm of Zn 2+ in sea water shows R a value of 10.9 nm reduced from 24 nm confirms that the surface appears smoother. The smoothness of the surface is due to the formation of a protective film of Fe 2+ - ST complex and Zn(OH) 2 on the metal surface thereby inhibiting the corrosion of stainless steel. The above parameters are also somewhat greater than the AFM data of polished metal surface, which confirms the formation of film on the metal surface, which is protective in nature. Table – 4 AFM data for stainless steel surface Samples Average Roughness (nm) Polished stainless steel (Control) 3.3 Stainless steel immersed in sea water (blank) 24 Stainless steel immersed in sea water consists of 250 ppm ST and 50 ppm Zn 2+ . 10.9 Mechanism of corrosion inhibition : With these discussions, a mechanism is proposed for the inhibition of corrosion of stainless steel immersed in sea water consists of 250 ppm ST and 50 ppm Zn 2+ formulation. When the formulation consists of 250 ppm ST and 50 ppm Zn 2+ in sea water there is a formation of ST – Zn 2+ complex in solution. When stainless steel is immersed in this solution ST – Zn 2+ complex diffuses from the bulk of the solution towards the metal surface. ST – Zn 2+ complex is converted int o ST – Fe 2+ complex on the anodic sites of the metal surface with release of Zn 2+ ion. Figure – 5 2D AFM images of stainless steel surface (a) Polished stainless steel (control) , (b) Stainless steel immersed in sea water (blank) , (c) Stainless steel immersed in sea water consists of ST (250 ppm) + Zn 2+ (50 ppm) ST – Zn 2+ + Fe 2+ → ST – Fe 2+ complex + Zn 2+ The released Zn 2+ combines with hydroxide to form Zn(OH) 2 on the cathodic sites of the metal surface. Zn 2+ + 2 OH - → Zn(OH) 2 Research Journal of Chemical Sciences ___ _ _ _______________________________ ______________ _ ________ ISSN 22 31 - 606X Vol. 3 ( 2 ), 54 - 58 , February (201 3 ) Res. J. Chem. Sci. International Science Congress Association 58 Thus the protective film consists of ST – Fe 2+ 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 ST – Fe 2+ complex on the anodic site of the metal surface. The cathodic reaction is the generation of hydroxide. It is controlled by the formation of Zn(OH ) 2 on the cathodic sites of the metal surface. Anodic reaction: Fe → Fe 2+ + 2 e - Cathodic reaction: H 2 O + O 2 + 2 e - → 2 OH – Fe 2+ + ST → ST – Fe 2+ complex , Zn 2+ + 2 OH - → Zn(OH) 2 This accounts for the synergistic effect of ST – Zn 2+ system. Conclusion The corrosion inhibition of ST in controlling corrosion of stainless steel in sea water has been evaluated by weight loss method. The combination of ST (250 ppm) – Zn 2+ (50 ppm) shows IE of 98%. AC impedance spectra reveal that the system controls the cathodic reaction predominantly due to the format ion of a protective film on the metal surface. The SEM micrographs and AFM images confirm the formation of protective film on the metal surface. Acknowledgement The authors are thankful to their respective managements and University Grant Commission (UGC ), New Delhi for the encouragement and support. References 1. Mary Anbarasi C., Rajendran S., Vijaya N., Manivannan M., and Shanthi T., Corrosion Inhibition by an Ion Pair Reagent – Zn(II) system, The Open Corr. J., 4 , 40 - 46 (2011) 2. Nawafleh E., Irshedat M., Bataineh T., Muhaidat R., Al - Qudah M. and Alomary A., The Effects of Inula viscosa Extract on Corrosion of Copper in NaOH Solution, Res. J. Chem. Sci., 2(9) , 37 - 41 (2012) 3. Dubey R.S. and Potdar Y., Corrosion inhibition of 301 stainless steel in sodium chlo ride by ciprofloxacin and norfloxacin, Indian J. Chem. Tech., 16 , 334 - 338 (2009) 4. 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