Inhibition of corrosion of copper by 2,5-dimercapto-1,3,4-thiadiazole in 3.5% NaCl solution

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1 Indian Journal of Chemical Technology Vol. 17, March 2010, pp Inhibition of corrosion of copper by 2,5-dimercapto-1,3,4-thiadiazole in 3.5% NaCl solution M Yadav*& Dipti Sharma Department of Applied Chemistry, Indian School of Mines University, Dhanbad , India yadav_drmahendra@yahoo.co.in Received 24 March 2009; revised 7 December 2009 The inhibitor 2,5-dimercapto1,3,4-thiadiazole shows percent inhibition efficiency in controlling corrosion of copper in 3.5%NaCl solution at ph 7.0. The mechanistic aspects of corrosion inhibition, based on the results obtained from the weight loss method, potentiostatic polarization study, AC-impedance study, UV-visible absorption study and surface study by FTIR, ESCA and SEM have been discussed. The inhibitor 2,5-dimercapto-1,3,4-thiadiazole appears to inhibit corrosion process through formation of protective film which was found to consist of Cu(I)-inhibitor complex, cuprous chloride, CuCl or CuCl 2 complex ion or both and no oxide of copper on the surface. Keywords: Copper, Corrosion inhibition, 2,5-Dimercapto-1,3,4-thiadiazole Copper, despite being noble metal, corrodes significantly in water containing chloride ions. Extensive use of copper as a structural material in cooling systems of nuclear installations, automobiles, power plant, oil refineries, sugar factories etc. 1-3, has prompted a thorough investigation on the kinetics and mechanism of copper corrosion in water containing chloride ions with various amounts of inhibitor added to it. Compounds like derivatives of aminothiazoles 4, benzotriazoles 5-7, thioimidazoles 8, thiadiazoles 9, mercaptobenzothiazole 10 and thiosemicarbazides 11 are some examples of good inhibitors for this system. In the present work, the inhibitive effect of 2,5- dimercapto-1,3,4-thiadiazole in controlling the corrosion of copper in 3.5%NaCl solution at various ph values in the range 6.0 to 9.0, has been studied using the weight loss method. A suitable mechanism of corrosion inhibition has been proposed based on the results obtained from potentiostatic polarisation study, AC impedance study, UV-visible absorption study, FTIR, and ESCA spectra. Experimental Procedure Preparation of specimens Copper specimens used for experiments were supplied by M/s Good Fellow Metals Ltd., England. The samples were 99.99% pure with the composition: Ag = 500, Bi < 10, Pb < 50 and other metals < 300 ppm. Sodium chloride (NaCl, Merk, 99%) was used for the preparation of 3.5%NaCl solution. The samples for the weight loss and electrochemical polarisation studies were of the size cm and cm, respectively. The samples were polished successively with 1/0-4/0 grade emery papers, washed with benzene followed by hot soap solution and finally with distilled water. These were degreased by immersing in acetone for 1-2 min., dried and stored in vacuum desiccator. Weight loss method Copper specimens, in triplicate, were immersed in 300 ml 3.5%NaCl solutions both in the absence and presence of the various concentrations of the inhibitor for a period of 24 h. The weights of specimens before and after immersion were determined. The inhibition efficiencies were evaluated using the formula %IE = θ 100 (1) where θ is the fraction of surface area covered by inhibitor and is given by θ = a b a where, a is weight loss of the sample in absence of the inhibitor, b is weight loss of the sample in presence of the inhibitor. Potentiostatic polarisation study The potentiodynamic polarization studies were carried out with copper strips having an exposed area

2 96 INDIAN J. CHEM. TECHNOL., MARCH 2010 of 1 cm 2. A conventional three electrode cell consisting of copper as working electrode, Platinum as counter electrode and a saturated calomel electrode as reference electrode was used. Polarisation studies were carried out using VoltaLab 10 electrochemical analyser and data was analysed using Voltamaster 4.0 software. The potential sweep rate was 0.1 mvs -1. The inhibition efficiency (IE) was calculated from corrosion current determined using the Tafel extrapolation method. The samples were exposed for 24 h before polarization studies. The %IEs were calculated by using the following formula: I 0 I inh % IE = 100 (2) I 0 where, I 0 = Corrosion current in absence of inhibitor I inh = Corrosion current in presence of inhibitor AC impedance study AC impedance studies were carried out in a three electrode cell assembly using computer controlled VoltaLab 10 electrochemical analyser, using copper as the working electrode, platinum as counter electrode and saturated calomel as reference electrode. The data was analysed using Voltamaster 4.0 software. The electrochemical impedance spectra (EIS) were taken in the frequency range 10 KHz to 1 mhz at the rest potential by applying 5 mv sine wave AC voltage. The charge transfer resistance (R) and double layer capacitance (C dl ) were determined from Nyquist plots. The inhibition efficiencies (IEs) were calculated from charge transfer resistance values. % IE = R R t R t 100 (3) where R t = charge transfer resistance in presence of inhibitor R = charge transfer resistance in absence of inhibitor UV-visible spectra The UV-visible absorption spectra of various solutions before and after immersion of the metal specimen for 24 h were recorded using the Shimadzu model UV-160A spectrophotometer. FTIR spectrum The FTIR spectrum of the film formed on the surface of metal specimen was recorded using Shimadzu 8201 PC FTIR spectrophotometer. ESCA study The copper specimens were immersed in various test solutions for 24 h. After 24 h, the specimens were taken out, washed with water and dried. The nature of the film formed on the surface of the metal specimens was analysed by ESCA. The ESCA patterns of protective films formed on the surface of copper specimen were recorded using ESCA-LAB MK 11, VG Scientific Limited, UK with Mg K α radiation at a rating of 10 kv, 10 ma with a sensitivity of 0.2 ev and a detection limit of upto 2 percent. Results and Discussion Weight loss method Corrosion rate of copper in 3.5% NaCl solution at various ph values in the range of 6-9 and in the absence and presence of inhibitor at various concentrations were determined by weight loss method. The percentage inhibition efficiencies at different concentrations of the inhibitor at ph 7.0 are as shown in Table 1. The maximum inhibition efficiency in the range 94 percent is obtained with 60 ppm of the inhibitor concentration at ph 7.0. It is evident from the data in Table 1 that the inhibition efficiency values become more or less constant after an initial increase in inhibition efficiency values with the concentration of the inhibitor. The same trend is observed at other ph values in the range 6-9. The increase in %IE values with increase in concentration of the inhibitor is due to formation of protective film at the surface of copper. It is also observed from these studies that corrosion inhibition efficiencies usually increase with increase in ph from 7-9 and decrease with decreasing ph from 7 to 6. Nevertheless, the inhibition efficiency of the system ranges between to percent in the ph range 6.5 to 9. It shows that this inhibitor is very Table 1 Corrosion rates of copper in 3.5% NaCl solution (ph 7.0) in the absence and presence of various concentrations of the inhibitor (result obtained by weight loss method) 2,5-dimercapto-1,3,4-thiadiazole Inhibitor concentration (ppm) Corrrosion rate (mpy) I.E. (%)

3 YADAV & SHARMA : CORROSION INHIBITION OF COPPER BY 2,5-DIMERCAPTO-1,3,4-THIADIAZOLE 97 much effective in the ph range of 6.5 to 9 at 25 C and less effective at ph 6.0 with an efficiency of % only. Potentiostatic polarisation study The potentiostatic polarisation curves of copper immersed in 3.5% NaCl solution at various ph values in absence and presence of 2,5-dimercapto-1,3,4- thiadiazole are shown in Fig. 1 and corresponding anodic and cathodic Tafel slope values are given in Table 2. It is evident from the figure that at ph 6.5 there is a slight shift in corrosion potential, E corr (from 87 to 81 mv versus SCE) towards cathodic site. It is observed from the data that the shifts in the anodic and cathodic slopes are nearly equal indicating that this system acts as a mixed inhibitor at this ph. At ph 7.0, the corrosion potential is shifted towards anodic side (from 71 to 84 mv versus SCE). When the shifts in Tafel slopes 12 are compared, it is predominantly cathodic in nature. At ph 9.0 the corrosion potential is shifted towards cathodic side (from 94 to 62 mv versus SCE). It is also observed that the shift in the anodic slope is higher (16 mv/decade) than the shift in cathodic slope (3 mv/decade). Hence, it can be said that the same inhibitor predominantly controls the anodic reaction at ph 9.0. It is clear from the data in Table 2 that there is a increase in the anodic and cathodic Tafel slope values in presence of inhibitor indicating the formation of complex at the surface of copper which inhibit both copper dissolution and reduction of oxygen reactions, resulting in decrease in both the anodic and cathodic currents and causing inhibition of corrosion of copper. AC impedance studies The Nyquist plots of copper immersed in 3.5% NaCl solution at various values of ph (6.5 and 9) at 25 C in the absence and presence of the inhibitor after 1 h and 24 h immersion period are shown in Figs 2 and 3, respectively. The corresponding impedance parameters are given in Table 3. From the data, it is observed that the R values increased in the presence of inhibitor, whereas C dl values were found to decrease. The decrease in C dl values was due to the adsorption of the inhibitor at the surface of copper. The R values for copper in 3.5% NaCl solution without inhibitor decreases with immersion time, however, in presence of inhibitor it increased with immersion time. At the same time, C dl values in the blank solution increased with immersion Fig. 1 Potentiostatic polarization curves of copper immersed in in 3.5% NaCl solution in absence and presence of 60 ppm of 2,5-dimercapto-1,3,4-thiadiazole at (a) ph= 6.5 (b) ph= 7.0 (c) ph = 9.0 at 25 C. Table 2 The anodic and cathodic Tafel slope values obtained from Fig. 1 (a, b, c) Sl. no. Inhibitor concentration (ppm) ph Anodic Tafel slope (mv dec -1 ) Cathodic Tafel slope (mv dec -1 )

4 98 INDIAN J. CHEM. TECHNOL., MARCH 2010 Fig. 2 AC impedance curves of copper immersed in 3.5% NaCl solution at various ph values in the absence of the inhibitor after 1 h and 24 h immersion periods. Table 3 Impedance parameters of copper in 3.5% NaCl solution at various values of ph in the absence and presence of inhibitor after 1 h and 24 h immersion periods Sample no. Inhibitor concentration (ppm) ph Charge transfer resistance R (kωcm 2 ) Capacitance C dl (µfcm -2 ) % IE Immersion period: 1 h Immersion period: 24 h Fig. 3 AC impedance curves of copper immersed in 3.5% NaCl solution at various ph values in the presence of (60 ppm), 2,5-dimercapto1,3,4-thiadiazole after 1 h and 24 h immersion periods. time. This indicated that the impedance of the inhibited substrate increased with increasing exposure time. The change in R and C dl values was due to desorption of the inhibitor molecules on the metal surface, reducing the extent of the dissolution reactions 13. The values of corrosion inhibition efficiencies calculated by impedance method are in good agreement with the results obtained by weight loss studies. It is also noticed that there is a gradual increase in the inhibition efficiency from ph 6.5 to ph 9.0 after 1 h and 24 h immersion periods. The inhibition efficiency of the inhibitor also increases with increasing immersion time. UV- visible absorption spectra The UV-visible absorption spectra of pure 2,5-dimercapto-1,3,4-thiadiazole and the surface film compound of the exposed specimen in ethanol are shown in Fig. 4. From the spectra it is clear that there

5 YADAV & SHARMA : CORROSION INHIBITION OF COPPER BY 2,5-DIMERCAPTO-1,3,4-THIADIAZOLE 99 Fig. 4 UV-visible absorption spectra of (a) pure inhibitor (b) surface film compound. is a shift in λ max (300 to 282 nm) from curve (a) to curve (b) which indicates the formation of a complex between copper and the inhibitor in solution. FTIR spectra The FTIR spectra of 2,5-dimercapto-1,3,4- thiadiazole and reflectance absorption FTIR spectra of the exposed specimen were recorded. The pure compound shows IR bands at 2520, 1450,1050 and 715 cm -1 due to the presence of S-H, C=N, N-N and C-S-C endocyclic groups, respectively. The disappearance of νs-h band and appearance of a new band at 450 cm -1 due to νcu-s in reflectance FTIR spectra of the exposed specimen indicates the formation of Cu-inhibitor complex film through the sulphur atom of the inhibitor. The presence of other bands of the inhibitor with minor shift in reflectance spectra of the exposed specimen was also reported which indicates the interaction of the inhibitor molecule with copper. ESCA spectra The ESCA patterns of the protective films formed on copper surface immersed in 3.5%NaCl solution in the absence and presence of inhibitor are shown in Figs 5 and 6, respectively. This ESCA pattern can be interpreted with the help of data obtained from the literature 14 and experimental data taken from the Regional Sophisticated Instrumentation Centre, IIT Chennai, India for various elements exhibiting peaks at characteristic binding energy values. From Fig. 6, it is observed that in the presence of inhibitor, the peak at 72 ev corresponds to 3p electrons of copper and the peaks at 934 and 950 ev are due to 2p electrons of copper. The peak at 198 ev is due to 2p electrons of chlorine. The peak at 286 ev is due to 1s electron of carbon atom. The observed peak at 162 ev is due to 2p electrons of sulphur atom. The peak at 401 ev is due to 1s electron of nitrogen atom. The presence of all the elements present in the inhibitor at the surface Fig. 5 ESCA pattern of the surface film formed on copper immersed in 3.5% NaCl solution at ph 7.0 in the absence of the inhibitor. Fig. 6 ESCA pattern of the surface film formed on copper immersed in 3.5% NaCl solution at ph 7.0 in the presence of the inhibitor.

6 100 INDIAN J. CHEM. TECHNOL., MARCH 2010 of the metal suggests the adsorption of the inhibitor and formation of Cu(I)-inhibitor protective layer at the surface of the metal. SEM studies Figure 7 (a, b, c) shows the SEM microphotographs for copper in 3.5% NaCl solution in the absence and presence of 60 ppm of 2,5-dimercapto-1,3,4- thiadiazole inhibitor, at 500x magnification. On comparing these micrographs, it appears that in the presence of inhibitor, the surface of the test material has improved remarkably with respect to its smoothness. The smoothening of the surface would have been caused by the formation of the Cu-inhibitor protective film at the surface of the copper. Mechanism of inhibition of corrosion The results of weight loss method reveal that this system has maximum inhibition efficiency in the Fig. 7 SEM microphotographs of copper exposed to 3.5% NaCl solution: (a) Polished specimen (b) In presence of 3.5% NaCl solution (c) In presence of 60 ppm of the inhibitor. range of 82 to 94 percent at 60 ppm of the inhibitor concentration in the ph range of at 25 C. The polarization studies show that this inhibitor acts as mixed inhibitor, though there is variation depending upon the ph, whether it is predominantly controlling the cathodic or anodic reaction. The FTIR, UV-visible and ESCA patterns clearly reveal the formation of Cu-inhibitor complex film and presence of chloride and all the elements of the inhibitor on the copper surface but not oxygen of any oxide film. In other words, the protective film consists of cuprous chloride, CuCl or CuCl 2 complex ion or both and also the Cu (I)-inhibitor complex. There is no cuprous oxide or any oxide of copper in the protective film. The anodic dissolution mechanism of copper in the presence of the inhibitor in neutral and alkaline aqueous environment containing chlorides in the presence of oxygen is as follows: Cu Cu + + e - Cu + + Cl - CuCl CuCl + Cl - CuCl 2 In the inhibition process, the diffusion of the inhibitor molecules at the surface of copper takes place and the inhibitor molecules are chemisorbed at the surface of copper. Now at the anodic sites, surface reaction takes place between copper ion and the chemisorbed inhibitor molecules, resulting in the formation of protective film of Cu- inhibitor insoluble complex. On the cathodic sites the inhibitor molecules are simply chemisorbed. Once the protective film is formed, the anodic dissolution of copper is prevented and diffusion of oxygen to the surface is also prevented by chemisorbed inhibitor molecules. Thus, controlling both anodic and cathodic reactions and the inhibitor acts as mixed inhibitor. The protective film consists of cuprous chloride CuCl or CuCl 2 complex ion or both and also the Cu(I)-inhibitor complex. Conclusion A maximum corrosion inhibition efficiency of percent is obtained by 2,5-dimercapto-1,3,4- thiadiazole at 60 ppm of inhibitor concentration at ph 7.0. The inhibitor is more effective in the ph range and less effective at ph 6.0 with an efficiency of only 62.48%. At ph 7.0, it acts as a mixed inhibitor. The protective film consist of copper (I)-2,5-dimercapto-1,3,4-thiadiazole complex and cuprous chloride and does not consist of any oxide of copper.

7 YADAV & SHARMA : CORROSION INHIBITION OF COPPER BY 2,5-DIMERCAPTO-1,3,4-THIADIAZOLE 101 Acknowledgement Financial assistance from Department of Science and Technology, New Delhi, India, under the Fast Track Young Scientist Scheme to M. Yadav is gratefully acknowledged. References 1 Stupnisek E & Cafuk I, Corrosion, 54 (1998) Gasparac R, Martin C R & Lisac E S, J Electrochem Soc, 147 (2000) Zaky A M, Br Corros J, 36 (2001) Sherif E M & Park Su-Moon, Corros Sci, 48 (2006) Zhon G D M Z & Notoxa T, Bull Electrochem Soc, 7(2) (1991) Suoto R M, Fox V, Laz M M, Perez M & Gangatez R S, J Electroanal Chem, 411(1-2) (1996) John Berchmans L, Sivan V & Venkata Krishna Iyer S, Mater Chem Phys, 98 (2005) Mamoud S S & Malidy E G A, Egypt J Chem, 39(4) (1996) Bastidas J M & Otero E, Werk Korros, 47(6) (1996) Cicileo G P, Rosales B M & Vilche J R, Corros Sci, 40 (1998) Singh M M, Rastogi R B, Upadhyay B N & Yadav M, Mater Chem Phys, 80 (2003) Tafel J & Fur Z, Physiki Chemie, 24 (1905) Bentiss F, Traisnel M & Lagrenee M, Br Corros J, 35 (2000) Wagner C D, Handbook of X-ray photoelectron spectroscopy (Perkin-Elmer-Corp. Eden Prairie MN), 1998.