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Evaluation of inhibitors and biocide on the corrosion control of copper in neutral aqueous environment
Corrosion Science 47 (2005) 151–169 www.elsevier.com/locate/corsci
Evaluation of inhibitors and biocide on the corrosion control of copper in neutral aqueous environment S. Ramesh *, S. Rajeswari Department of Analytical Chemistry, University of Madras, Guindy Campus, Chennai 600 025, India Received 2 October 2003; accepted 10 May 2004 Available online 6 July 2004
Abstract Polarisation and impedance measurements were performed on copper in neutral aqueous solution with and without inhibitors and biocide. The inhibitors used were 3-benzylidene amino 1,2,4-triazole phosphonate (BATP), 3-cinnamalidene amino 1,2,4-triazole phosphonate (CATP), 3-salicylalidene amino 1,2,4-triazole phosphonate (SATP) and 3-paranitro benzylidene amino 1,2,4-triazole phosphonate (PBATP). Effect of inhibitors and biocide against the corrosion of copper in neutral aqueous solution has been studied. Results elucidate the minimal interference between biocide and inhibitors system. Surface evaluation techniques like FT-IR, XRD and EDXA were used to determine the nature of the protective film formed on the metal surface. 2004 Elsevier Ltd. All rights reserved. Keywords: Corrosion inhibition; Copper; Triazole phosphonate; Biocide
1. Introduction Copper is commonly used in heating and cooling systems due to its excellent thermal conductivity and mechanical workability. However, it is susceptible to different forms of corrosion such as pitting, induced by different corrosive species such as chloride, sulphate and nitrate ions [1,2]. Biological fouling decreases thermal efficiency and can cause localised corrosion attack; hence, resistance to biofouling is sometimes important.
*
Corresponding author. Tel.: +91-44-22351137x221; fax: +91-44-22352494. E-mail address: [email protected] (S. Ramesh).
0010-938X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.05.013
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The corrosion inhibition of copper and its alloys is an important topic; thus, in the presence of an organic inhibitor, the properties of an oxide protective layer can be modified. Corrosion inhibitors can effectively prevent further metal dissolution. Organic compounds cannot be designed especially as anodic or cathodic inhibitors. They are mixed inhibitors that inhibit both anodic and the cathodic reactions. The effectiveness of organic inhibitors is related to the extent to which they adsorb and cover the metal oxide surface. The adsorption depends on the structure of the compounds, the surface charge on the metal, and on the type of the electrolyte [3]. Heterocyclic organic compounds containing nitrogen, sulphur or oxygen atoms are often used to protect metals form corrosion. Thus, azoles have been intensively investigated as effective corrosion inhibitors [4–9]. In aquatic systems, aerobic and anaerobic microorganisms may degrade metallic substrates. The role of microbes in corrosion processes is mainly due to metabolisms associated with microbial growth and reproduction. In the presence of microbes, biofilm formation begins immediately after exopolymeric substance excretion and cell adhesion. The microbial colonisation markedly modifies the corrosion behaviour of the metallic substrate. Biofilms influence the surface, liquid phase and the interfacial process, which results in contamination of water systems [10] and reduction of heat transfer [11]. Most industries add biocides and inhibitors at the same point in cooling water systems. Generally, for the control of fouling and corrosion, continuous addition of inhibitors as well as the addition of biocides every week or once in a fortnight is explored. It is often not known whether interference between biocides and inhibitors will lead to an adverse effect. Hence, it is quite essential to study the possible interference between an inhibitor and a biocide for a cooling water system. In the present study synthesised triazole phosphonates were used as inhibitors along with molybdate and cetyl trimethyl ammonium bromide (CTAB) is used as biocide.
2. Experimental 2.1. Synthesis of triazole derivatives All the tested azoles were synthesised [12–14] by condensing 3-amino 1,2,4triazole, hypophosphorus acid with various aldehydes (benzaldehyde, cinnamaldehyde, salicylaldehyde and p-nitro benzaldehyde). The products were characterised by their FT-IR and NMR spectra. The structures of the compounds are given in Fig. 1. A schematic representation of the synthesis is shown below.
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Fig. 1. Structure and name of the triazole derivatives.
2.2. Sample preparation Copper specimens of size 1.0 cm · 1.0 cm · 0.3 cm having the following compositions were used: P––0.021%, Fe––0.07%, Ni––0.004%, Zn––1.59%, Cr––0.006%, As––0.003%, S––0.06% and Cu––balance. Before each experiment the specimens were mechanically abraded with silicon carbide papers (from grades 120 to 1000), washed with distilled water, degreased with acetone and dried at room temperature, then placed in a test solution. Lake water was used as the test solution and a typical analysis of this electrolyte is given in Table 1. A copper specimen was immersed in a test solution for a period of three days. Each test was done in triplicate. After immersion period the efficiencies were assessed for the following systems.
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System
Material used
Medium
Parameters of interest
System I System II System III
Inhibitors with Mo CTAB Inhibitors with molybdate and CTAB (biocide)
Lake water Lake water Lake water
System IV
CTAB (biocide) with Lake water inhibitors and molybdate
Inhibition efficiency Biocide efficiency Interference in the inhibitors and biocide (inhibitors and biocide are added at same time) Biocide are added first, after 24 h inhibitors were added
Table 1 Analysis of lake water (electrolyte) Parameter
Value
pH Conductivity (lS/cm) Alkalinity Total hardness (ppm) Ca (ppm) Mg (ppm) Chloride (ppm) Sulphate (ppm)
7.99 1002 201 236 59 21 170 30
Total dissolved solids (ppm)
738
2.3. Potentiodynamic polarisation measurement Potentiodynamic polarisation studies were carried out using a Vibrant Potentiostat/Galvanostat model No. VSM/CS/30 at a scan rate of 1 mV/s under static condition. A platinum electrode and a saturated calomel electrode (SCE) were used as auxiliary and reference electrodes respectively. The working electrode (WE) was a copper specimen of area 1 cm2 . All the experiments were carried out at 30 ± 2 C with natural lake water as an electrolyte. Polarisation studies were carried out in lake water containing various concentrations of additives (systems I–IV).
2.4. Electrochemical impedance measurement Ac impedance investigations were carried out using EG&G PARC Potentiostat Model 6310 analyser with M 398 software at 30 ± 2 C. Electrochemical impedance spectroscopy was employed in the region of 10 kHz to 100 mHz with the perturbation amplitude of 10 mV.
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2.5. Accelerated-leaching study (ICP-AES) In this study the concentration of metal ions present in the test solution was determined after ageing the WE for 1 h at the impressed steady state potential. The analysis was carried out using inductively coupled argon plasma-atomic emission spectroscopy (ICP-AES)––Thermo Jarrel Ash-Atom Scan, USA. 2.6. Surface evaluation study Copper specimens were immersed in various test solutions for a period of 30 days. After this period, the specimens were removed and dried. The nature of the film formed on the copper surface was ascertained by the following surface analytical techniques. 2.6.1. FT-IR Fourier Transform Infrared (FT-IR) spectra were recorded with a Perkin-Elmer 1600 FT-IR spectrophotometer. 2.6.2. X-ray diffraction (XRD) XRD patterns of the film formed on the metal surface were recorded using a computer controlled X-ray powder diffractometer, JEOL JDX 8030, with CuKa at a rating of 40 kV, 20 mA. (Ni-filtered) radiation (k ¼ 1:5418 A) 2.6.3. Energy dispersive X-ray analysis (EDXA) The surface film formed on the metal specimen was examined by energy dispersive X-ray analysis (EDXA). This was carried out with a Philips 501 SEM in conjugation with an energy dispersive spectrometer. The spectra were recorded on samples immersed for a period of 30 days in natural lake water with and without the inhibitors and biocide.
3. Results and discussion 3.1. Potentiodynamic polarisation measurements Cathodic and anodic polarisation of copper in lake water in the presence and absence of various concentrations of triazole derivatives after three days were carried out. Each inhibitor was studied at different concentration levels (2, 3, 4, 5, 6, 8 and 10 ppm). The inhibition efficiency was found to increase appreciably with increase in concentration of inhibitor after which it decreased. The optimum concentrations of inhibitors were evaluated based on inhibition efficiency. The potentiodynamic polarisation parameters of copper immersed in lake water for all the systems are given in Tables 2–4 and corresponding polarisation curves are also shown in Figs. 2–6. The corrosion current density was decreased considerably in the presence of inhibitor. In the presence of inhibitors, the corrosion potential
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Table 2 Potentiodynamic polarisation parameters of copper in lake water with and without the optimum concentration inhibitors (system I) Inhibitors
Ecorr (mV)
icorr (lA/cm2 )
Corrosion rate (mpy)
I.E. (%)
Blank BATP CATP SATP PBATP BATP + Mo CATP + Mo SATP + Mo PBATP + Mo
)156 )230 )220 )218 )185 )254 )200 )180 )253
1.6 0.46 0.43 0.40 0.60 0.44 0.41 0.38 0.55
0.735 0.211 0.197 0.183 0.275 0.202 0.188 0.174 0.252
– 71.25 73.12 75.00 62.50 72.50 74.37 76.25 65.62
I.E.––inhibition efficiency.
Table 3 Potentiodynamic polarisation parameters of copper in lake water with and without the biocide (system II) Biocide
Conc. (ppm)
Ecorr (mV)
icorr (lA/cm2 )
Corrosion rate (mpy)
I.E. (%)
Blank
–
)160 )138 )130 )127 )161
1.6 0.6 0.5 0.36 0.75
0.73 0.27 0.22 0.16 0.34
– 62.50 68.75 77.50 53.12
CTAB
5 10 15 20
Table 4 Potentiodynamic polarisation parameters of copper in lake water mixing of biocide + inhibitor (systems III and IV) Inhibitors
Ecorr (mV)
icorr (lA/cm2 )
Corrosion rate (mpy)
I.E. (%)
Blank BATP CATP SATP PBATP
)156 )188 )173 )190 )180
1.6 0.43 0.39 0.35 0.46
0.73 0.197 0.179 0.160 0.211
– 73.12 75.62 78.12 71.25
Inhibitors addition after killing bacterial by addition of biocide (system IV) BATP )256 0.30 0.137 CATP )218 0.28 0.128 SATP )258 0.26 0.119 PBATP )254 0.38 0.174
81.25 82.50 83.75 76.25
slightly shifts towards the active direction in comparison to the result obtained in the absence of the inhibitor. Both the anodic and cathodic current densities were decreased indicating that all the tested inhibitors suppressed both the anodic and cathodic reactions, although mainly the cathodic one. Optimum concentrations were determined for all investigated compounds by Tafel extrapolation method. For SATP and CATP, the corrosion inhibition efficiency of 75% and 73.12% respectively was obtained at 4 ppm concentration. The
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Fig. 2. Potentiodynamic polarisation curves of copper in lake water with and without optimum concentration of molybdate and biocide.
inhibiting effect decreases at lower and higher concentrations and the compounds act as activators at the highest investigated concentration. The higher inhibition efficiency shown by SATP compared to PBATP may be attributed to the increased electron density leading to electron transfer mechanism from functional group to metal surface, producing greater coordinate bonding with a greater adsorption and inhibition efficiency. Similar explanation has been sought to explain the discrepancy in the order of inhibition efficiency by triazoline derivative [15]. SATP is the most efficient inhibitor due to the presence of electron releasing character of OH group compared to the other substituents. PBATP has the lowest protection efficiency due to the coplanarity of p-NO2 group with the phenyl ring, which imparts maximum electron withdrawal [16]. Most investigators have evaluated the inhibitive action of molybdate. Molybdate have been shown to be corrosion inhibitors in neutral or basic solution similar to cooling tower water. The inhibition efficiency with the addition of molybdate at various concentration levels to the lake water was determined. An optimum concentration of 5 ppm gave a maximum inhibition efficiency of 50.40%.
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Fig. 3. Potentiodynamic polarisation curves of copper in lake water with and without optimum concentration of BATP and biocide.
Addition of molybdate controls both cathodic as well as anodic reaction. But along with inhibitor they also predominantly control the cathodic reaction site. Influence of molybdate with inhibitors on corrosion is presented in Table 2. In presence of molybdate, the efficacy of BATP and CATP are increased to 72.50% and 74.37% respectively. Similarly SATP showed a higher efficiency of 76.25% in presence of molybdate than SATP alone (75%). The synergism is an enhanced inhibition, which may be related either to interaction between the inhibitor compounds or to interaction between the inhibitor compound and one of the ions present in the aqueous media. In system II, the inhibitive effect of CTAB on copper corrosion in neutral aqueous solution was investigated. The reason for selecting CTAB as biocide is that CTAB is not only a typical cationic surfactant, which is commercially available, but is also a quaternary ammonium salt with a long hydrocarbon chain, whose homologues have been used extensively as biocides and inhibitors [17,18]. In particular it is reported that the halides are the most effective derivatives since they increased the inhibiting tendency of the positive quaternary ammonium ions through a synergistic effect [17]. By comparing polarisation curves in the absence and presence of CTAB, it was observed that an increase in CTAB concentration shifted the open circuit potential in the positive direction with a lowering of both the anodic and cathodic current
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Fig. 4. Potentiodynamic polarisation curves of copper in lake water with optimum concentration of CATP and biocide.
densities. The corrosion potential and corrosion current density for copper in CTAB were obtained and the results are listed in Table 3. The results revealed that the I.E. increases with the CTAB concentration up to a maximum (77.5%) in a concentration of 15 ppm and then it decreases. In system III (Table 4) the inhibition efficiency was about 78.12% and 75.62% respectively for SATP and CATP. In system IV, the corrosion inhibition percentage was higher compared to system III are shown in Table 4. The inhibition efficiencies of SATP and PBATP (system IV) were about 83.75% and 76.25% respectively. From the observation of results, SATP found to act as better inhibitor. Besides in system IV, the inhibitor recorded higher efficiencies than system III and elucidates the low interference between biocide and inhibitors system. In system III, the biocide (CTAB) adsorption may disturb the absorption of triazole phosphonate, which may interfere with the inhibitor action. But in system IV the interference between biocide and inhibitor was least. The I.E. of all inhibitors with biocide decreased in the following order SATP > CATP > BATP > PBTAB. The polarisation curve for the blended mixture (system IV) indicates a shift in the corrosion potential towards the negative values of potential compared with the control sample. The cathodic region shows a decrease in the current density as a
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Fig. 5. Potentiodynamic polarisation curves of copper in lake water with and without optimum concentration of SATP and biocide.
result of cathodic inhibition. However, according to corrosion potential the inhibitor trended towards cathodic inhibition. 3.2. Electrochemical impedance spectroscopy (EIS) The admittance plots of copper immersed in natural aqueous environment (lake water) in the absence and presence of inhibitors (4 ppm of BATP, CATP, SATP and 5 ppm of PBATP, 5 ppm of molybdate and CTAB-15 ppm) are shown in Fig. 7. As the admittance plots were not semicircles, these plots were not used for calculating the impedance parameters. The electrochemical parameters were obtained by using the semicircle fitting method [19]. This can be achieved by selecting the best fitting for the semicircle on the complex plane (Nyquist) plot. The data was plotted and analysed using the software Z view version 1.5b, 1996, Scribner Associates Inc. As seen from the impedance results the increase in resistance in the presence of SATP (compared to inhibitor-free solution) is related to the corrosion protection effect of the molecule. The value of Cdl decreases in the presence of SATP with biocide mixture, indicating that the surface oxide layer thickness decreases and changes the influence of the oxide layer on the kinetics of the electrode process.
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161
Fig. 6. Potentiodynamic polarisation curves of copper in lake water with optimum concentration of PBATP and biocide.
The EIS parameters were calculated and presented in Table 5. Copper with CTAB + SATP + Mo mixture exhibited higher resistance and impedance but lower capacitance than for the blank. The inhibition efficiencies calculated from ac measurement show the same trend as those observed from dc polarisation results. Maximum efficiency was noticed for the compounds with the methoxy phenyl substituents, since the basicity of the coordinating atoms were increased by electron donating groups. The electron withdrawing groups such as p-nitro group retards this electron transfer process, which results in decreased inhibition efficiency. The results are followed the same order, were observed in dc polarisation measurement. 3.3. Accelerated-leaching studies (ICP-AES) To determine the quantity of leached copper, accelerated-leaching studies were carried out on the WE. In the leaching studies, the WE was immersed in the test solution for 1 h at the impressed potential of )200 mV. At the end of each experiment, the test solution was analysed for metal ion (Cu), by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The results of the accelerated-leaching study are presented in Fig. 8, which show the concentration of copper leached from the metal. Among the options studied, the
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Fig. 7. (a) Nyquist and (b) Bode plot of copper in lake water with optimum concentration of various inhibitors with biocide mixture (system IV).
most effective treatment for reducing copper dissolution was the application of 4 ppm of SATP + 5 ppm of molybdate with 15 ppm of CTAB to lake water. The less effective treatment in this study was the application of only PBATP with biocide to water, although copper dissolution was somewhat reduced.
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Table 5 Electrochemical impedance parameters of copper immersed in lake water with optimum concentration of inhibitors and biocide (system IV) Compounds
Rct 105 (X)
Cdl 106 (F)
Total impedance · 104
Blank SATP CATP BATP PBATP
0.2055 0.4371 0.4192 0.3666 0.2894
17.6954 13.2938 17.9325 22.1880 22.8561
1.8776 4.2694 3.9665 3.4484 2.6373
Fig. 8. Concentration of copper present in the solution after leaching of copper for various inhibitors with biocide mixture.
The present investigation, SATP with mixture exhibited a very low tendency (1.227 ppm) to leach metal ions compared to the blank (11.776 ppm). This can be attributed mainly to the stable complex film that inhibits the metal dissolution by forming a barrier layer between the metal and the environment, thus preventing the bare metal contacting solution. The amount of copper leached out from metal with all inhibitors shows the following order, PBATP > BATP > CATP > SATP. The results are good agreements with electrochemical measurements. 3.4. Surface evaluation study Copper specimens were immersed in various test solutions for a period of 30 days. After 30 days, the specimens were taken out and dried. The nature of the film formed on the surface on the metal specimens was analysed by the various surface analytical techniques. Compositional analysis of the corrosion products films were performed, where possible, using FT-IR, X-ray diffraction and Energy dispersive X-ray analysis. 3.4.1. Analysis of FT-IR spectra FT-IR of SATP is presented in Fig. 9a. The stretching mode of P@O bond gives rise to a strong band in the region 1320–1140 cm1 as expected [20]. The two prominent bands at 1080 and 930 cm1 are attributed to the H2 PO3 group of the
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Fig. 9. Infrared transmission spectra of (a) SATP and (b) CTAB. Infrared reflection absorption spectrum of the film formed on the copper substrate after immersion in the solution containing (c) SATP and (d) mixture of SATP + Mo + CTAB.
molecule. The band at 1080 cm1 is assigned to the P–O stretch of the ionic species and the other at 930 cm1 is assigned to the P–OH stretch [21,22]. The reflectance absorption spectra of the film formed on copper immersed in the environment consisting of 4 ppm of SATP and 4 ppm of SATP + 5 ppm of Molybdate + 15 ppm of CTAB are given in Fig. 9c and d. The band at 930 cm1 almost disappeared in Fig. 9c and d compared to Fig. 9a. This can be attributed to a P–O–M bond in which a free Pþ –O interacts with metallic species. The fact that the band at 930 cm1 due to P–OH stretching is weak further indicates the possibility of a P–O–M (P–O–Cu) bond [23].
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The FT-IR spectrum of pure CTAB was given in Fig. 9b. The small absorption band due to 2854 cm1 represents CH2 stretching frequency. The aliphatic C–H stretches cause absorption at 2923 and 2854 cm1 . The band at 1126 cm1 corresponds to C–N stretching. In addition to the metal–inhibitor complex, the CTAB– Cu2þ complex may also deposit as a layer on the metal and as a part of the protective film. Nitrogen containing organic heterocyclic compounds are considered to be excellent complex or chelate forming substances with metals of the transition series [24]. It is known that aromatic triazoles are effective inhibitors for copper and its alloys. The protective action of the triazole is based on the formation of a semipermeable, insoluble, polymeric copper–triazole complex film on the copper surface. The polymeric complex was formed by covalent and coordinate covalent bonds. The formation of a complex is the result of the reaction between the triazole ring and the metal surface [25]. In this case it may be viewed as a result of Lewis acid–base electron exchange, resulting in the formation of 5- and 6-membered metal inhibitor ring complexes, as shown in the following figure.
3.4.2. X-ray diffraction (XRD) The X-ray diffraction (XRD) method can be used not only for identification of crystalline phases in corrosion products but also for their quantitative phase analysis based on measuring the intensity of a single diffraction line or even all the lines in the pattern [26]. Identification was carried out using a search-and-match fit of the XRD data to the patterns in the JCPDF database. The X-ray diffraction patterns of the films formed on the copper surface immersed in neutral aqueous solution in absence and presence of inhibitor (SATP) and biocide are shown in Fig. 10 respectively. In the absence of inhibitor, the XRD pattern of the copper shows peaks at 36.6, 43, 50.2, 61.6, 74 and 89. The XRD pattern of inhibited surface (Fig. 10b) has no characteristic peaks. This indicates the amorphous nature of the surface film. 3.4.3. Energy dispersive X-ray analysis (EDXA) Energy dispersive X-ray analysis (EDXA) technique was employed in order to get additional information on the inhibition mechanisms. The results obtained from these techniques showed that the corrosion inhibition process was related to the development of an inhibitor film over the metal surface.
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Fig. 10. XRD pattern obtained on the surface film formed on copper at the end of 30 days in different environment. Curve: (a) blank and (b) SATP with molybdate and CTAB.
Attempts to identify the surface films using X-ray diffraction were not successful, possibly because of the poor crystallinity of the material, but more probably due to the fact that the films were very thin. The cross section analyses of the corrosion layers were performed by EDXA. Mapping of C, O, P and Cu was carried out to investigate the distribution of these elements in the surface layers. EDXA analysis
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167
Fig. 11. EDXA spectrum of copper in natural lake water: (a) blank and (b) SATP with biocide mixture.
(Fig. 11b) confirmed that a phosphorus-containing layer was formed on copper when it was exposed to treated natural lake water. This observation clearly proves
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that the inhibition was due to the formation of an insoluble stable film through the process of complexation of the organic molecules on the metal surface.
4. Conclusions 1. System IV has higher efficiencies than system III and elucidates the minimal interference between biocide and inhibitors system. 2. The inhibition efficiencies calculated from ac measurement show the same trend as those observed from dc polarisation results. 3. The dissolution of copper in presence of SATP with biocide mixture is low compared to the blank. 4. FT-IR and EDXA shows that the inhibition is due to the formation of an insoluble stable film through the process of complexation of the organic molecules on the metal surface.
Acknowledgement The authors greatly acknowledge Tamil Nadu State Council for Science and Technology (TNSCST) for providing financial assistance.
References [1] W. Qafsaoui, G. Mankwski, P. Leterrible, F. Dabosi, in: Proc. of the Int. Sym. on Control of Copper and Copper Alloys Oxidation, Rouen, France, 1992. [2] P.E. Francis, W.K. Cheung, R.C. Pemberton, in: Proc. of the 11th Int. Corr. Cong., Florence, Italy, vol. 5, Associazione Italiana di Metallurgia, 1990, p. 363. [3] E. Stupnisek-Lisac, A. Gazioda, M. Madzarac, Electrochim. Acta 47 (2002) 4189. [4] Y.I. Kuznetzov, Organic Inhibitors of Corrosion of Metals, Plenum Press, NY, 1996. [5] F. Ammelot, C. Faiud, E.M.M. Sutter, Electrochim. Acta 42 (1997) 3565. [6] C.W. Yan, H.C. Lin, C.N. Cao, Electrochim. Acta 45 (2000) 2815. [7] M. Fonsati, F. Zucchi, G. Trabanelli, Electrochim. Acta 44 (1998) 311. [8] A. Frignani, L. Tommesani, G. Brunoro, M. Fogagnolo, Corros. Sci 41 (1999) 1205. [9] N. Huynth, T. Notoya, S.E. Bottle, D.P. Schweinsberg, Corros. Sci 42 (2000) 259. [10] M.K. Patterson, G.R. Husted, A. Rutkowski, et al., Ultrapure Water 8 (1991) 18. [11] W.G. Charaklis, in: W.G. Charaklis, K.C. Marshall (Eds.), Biofllms, Willey, New York, 1990, p. 671. [12] S. Ramesh, S. Rajeswari, S. Maruthamuthu, in: Proc. of National Convention on Corros. (CORCON-2002), East Asia Pacific Reg. Conf., Goa, India, 2002. [13] S. Ramesh, S. Rajeswari, Can. Metall. Quart. 42 (2003) 377. [14] S. Ramesh, S. Rajeswari, Electrochim. Acta 49 (2004) 811. [15] E. Khamis, M. Atea, Corrosion 50 (2) (1994) 106. [16] A.S. Fouda, M.N. Moussa, F.I. Taha, Al El-Neamaa, Corros. Sci. 26 (9) (1986) 719. [17] D.P. Schwesberg, V. Ashworth, Corros. Sci. 28 (1988) 639. [18] H. Ma, S. Chen, S. Zhao, X. Liu, D. Li, J. Electrochem. Soc. 148 (11) (2001) B482. [19] Basic of A.C. Impedance measurements, Application note-AC-1, Egand G. Princeton Applied Research, Electrochemical Instrument Division, Princeton, NJ, 1982.
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169
[20] M.A. Quraishi, M. Wajid Khan, M. Ajmal, S. Muralidharan, S. Venkatakrishna lyer, Anti-Corros. Methods Mater. 43 (1996) 5. [21] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman spectroscopy, third ed., Academic Press, New York, 1990. [22] Y. Gonzlez, M.C. Lafont, N. Pebere, G. Chatainiel, J. Roy, T. Bouissou, Corros. Sci 37 (1998) 1823. [23] R.O. Carter III, C.A. Giercgak, R.A. Dickie, Appl. Spectrosc. 40 (1986) 649. [24] N.K. Patel, J. Electrochem. Soc., India 21 (1972) 136. [25] N. Eldakar, K. Nobe, Corrosion 36 (1981) 271. [26] R. Jenkins, R.L. Snyder, Introduction to X-ray Powder Diffractometry, John Wiley & Sons, New York, 1996.
Evaluation of inhibitors and biocide on the corrosion control of copper in neutral aqueous environment S. Ramesh *, S. Rajeswari Department of Analytical Chemistry, University of Madras, Guindy Campus, Chennai 600 025, India Received 2 October 2003; accepted 10 May 2004 Available online 6 July 2004
Abstract Polarisation and impedance measurements were performed on copper in neutral aqueous solution with and without inhibitors and biocide. The inhibitors used were 3-benzylidene amino 1,2,4-triazole phosphonate (BATP), 3-cinnamalidene amino 1,2,4-triazole phosphonate (CATP), 3-salicylalidene amino 1,2,4-triazole phosphonate (SATP) and 3-paranitro benzylidene amino 1,2,4-triazole phosphonate (PBATP). Effect of inhibitors and biocide against the corrosion of copper in neutral aqueous solution has been studied. Results elucidate the minimal interference between biocide and inhibitors system. Surface evaluation techniques like FT-IR, XRD and EDXA were used to determine the nature of the protective film formed on the metal surface. 2004 Elsevier Ltd. All rights reserved. Keywords: Corrosion inhibition; Copper; Triazole phosphonate; Biocide
1. Introduction Copper is commonly used in heating and cooling systems due to its excellent thermal conductivity and mechanical workability. However, it is susceptible to different forms of corrosion such as pitting, induced by different corrosive species such as chloride, sulphate and nitrate ions [1,2]. Biological fouling decreases thermal efficiency and can cause localised corrosion attack; hence, resistance to biofouling is sometimes important.
*
Corresponding author. Tel.: +91-44-22351137x221; fax: +91-44-22352494. E-mail address: [email protected] (S. Ramesh).
0010-938X/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.05.013
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The corrosion inhibition of copper and its alloys is an important topic; thus, in the presence of an organic inhibitor, the properties of an oxide protective layer can be modified. Corrosion inhibitors can effectively prevent further metal dissolution. Organic compounds cannot be designed especially as anodic or cathodic inhibitors. They are mixed inhibitors that inhibit both anodic and the cathodic reactions. The effectiveness of organic inhibitors is related to the extent to which they adsorb and cover the metal oxide surface. The adsorption depends on the structure of the compounds, the surface charge on the metal, and on the type of the electrolyte [3]. Heterocyclic organic compounds containing nitrogen, sulphur or oxygen atoms are often used to protect metals form corrosion. Thus, azoles have been intensively investigated as effective corrosion inhibitors [4–9]. In aquatic systems, aerobic and anaerobic microorganisms may degrade metallic substrates. The role of microbes in corrosion processes is mainly due to metabolisms associated with microbial growth and reproduction. In the presence of microbes, biofilm formation begins immediately after exopolymeric substance excretion and cell adhesion. The microbial colonisation markedly modifies the corrosion behaviour of the metallic substrate. Biofilms influence the surface, liquid phase and the interfacial process, which results in contamination of water systems [10] and reduction of heat transfer [11]. Most industries add biocides and inhibitors at the same point in cooling water systems. Generally, for the control of fouling and corrosion, continuous addition of inhibitors as well as the addition of biocides every week or once in a fortnight is explored. It is often not known whether interference between biocides and inhibitors will lead to an adverse effect. Hence, it is quite essential to study the possible interference between an inhibitor and a biocide for a cooling water system. In the present study synthesised triazole phosphonates were used as inhibitors along with molybdate and cetyl trimethyl ammonium bromide (CTAB) is used as biocide.
2. Experimental 2.1. Synthesis of triazole derivatives All the tested azoles were synthesised [12–14] by condensing 3-amino 1,2,4triazole, hypophosphorus acid with various aldehydes (benzaldehyde, cinnamaldehyde, salicylaldehyde and p-nitro benzaldehyde). The products were characterised by their FT-IR and NMR spectra. The structures of the compounds are given in Fig. 1. A schematic representation of the synthesis is shown below.
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Fig. 1. Structure and name of the triazole derivatives.
2.2. Sample preparation Copper specimens of size 1.0 cm · 1.0 cm · 0.3 cm having the following compositions were used: P––0.021%, Fe––0.07%, Ni––0.004%, Zn––1.59%, Cr––0.006%, As––0.003%, S––0.06% and Cu––balance. Before each experiment the specimens were mechanically abraded with silicon carbide papers (from grades 120 to 1000), washed with distilled water, degreased with acetone and dried at room temperature, then placed in a test solution. Lake water was used as the test solution and a typical analysis of this electrolyte is given in Table 1. A copper specimen was immersed in a test solution for a period of three days. Each test was done in triplicate. After immersion period the efficiencies were assessed for the following systems.
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System
Material used
Medium
Parameters of interest
System I System II System III
Inhibitors with Mo CTAB Inhibitors with molybdate and CTAB (biocide)
Lake water Lake water Lake water
System IV
CTAB (biocide) with Lake water inhibitors and molybdate
Inhibition efficiency Biocide efficiency Interference in the inhibitors and biocide (inhibitors and biocide are added at same time) Biocide are added first, after 24 h inhibitors were added
Table 1 Analysis of lake water (electrolyte) Parameter
Value
pH Conductivity (lS/cm) Alkalinity Total hardness (ppm) Ca (ppm) Mg (ppm) Chloride (ppm) Sulphate (ppm)
7.99 1002 201 236 59 21 170 30
Total dissolved solids (ppm)
738
2.3. Potentiodynamic polarisation measurement Potentiodynamic polarisation studies were carried out using a Vibrant Potentiostat/Galvanostat model No. VSM/CS/30 at a scan rate of 1 mV/s under static condition. A platinum electrode and a saturated calomel electrode (SCE) were used as auxiliary and reference electrodes respectively. The working electrode (WE) was a copper specimen of area 1 cm2 . All the experiments were carried out at 30 ± 2 C with natural lake water as an electrolyte. Polarisation studies were carried out in lake water containing various concentrations of additives (systems I–IV).
2.4. Electrochemical impedance measurement Ac impedance investigations were carried out using EG&G PARC Potentiostat Model 6310 analyser with M 398 software at 30 ± 2 C. Electrochemical impedance spectroscopy was employed in the region of 10 kHz to 100 mHz with the perturbation amplitude of 10 mV.
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2.5. Accelerated-leaching study (ICP-AES) In this study the concentration of metal ions present in the test solution was determined after ageing the WE for 1 h at the impressed steady state potential. The analysis was carried out using inductively coupled argon plasma-atomic emission spectroscopy (ICP-AES)––Thermo Jarrel Ash-Atom Scan, USA. 2.6. Surface evaluation study Copper specimens were immersed in various test solutions for a period of 30 days. After this period, the specimens were removed and dried. The nature of the film formed on the copper surface was ascertained by the following surface analytical techniques. 2.6.1. FT-IR Fourier Transform Infrared (FT-IR) spectra were recorded with a Perkin-Elmer 1600 FT-IR spectrophotometer. 2.6.2. X-ray diffraction (XRD) XRD patterns of the film formed on the metal surface were recorded using a computer controlled X-ray powder diffractometer, JEOL JDX 8030, with CuKa at a rating of 40 kV, 20 mA. (Ni-filtered) radiation (k ¼ 1:5418 A) 2.6.3. Energy dispersive X-ray analysis (EDXA) The surface film formed on the metal specimen was examined by energy dispersive X-ray analysis (EDXA). This was carried out with a Philips 501 SEM in conjugation with an energy dispersive spectrometer. The spectra were recorded on samples immersed for a period of 30 days in natural lake water with and without the inhibitors and biocide.
3. Results and discussion 3.1. Potentiodynamic polarisation measurements Cathodic and anodic polarisation of copper in lake water in the presence and absence of various concentrations of triazole derivatives after three days were carried out. Each inhibitor was studied at different concentration levels (2, 3, 4, 5, 6, 8 and 10 ppm). The inhibition efficiency was found to increase appreciably with increase in concentration of inhibitor after which it decreased. The optimum concentrations of inhibitors were evaluated based on inhibition efficiency. The potentiodynamic polarisation parameters of copper immersed in lake water for all the systems are given in Tables 2–4 and corresponding polarisation curves are also shown in Figs. 2–6. The corrosion current density was decreased considerably in the presence of inhibitor. In the presence of inhibitors, the corrosion potential
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Table 2 Potentiodynamic polarisation parameters of copper in lake water with and without the optimum concentration inhibitors (system I) Inhibitors
Ecorr (mV)
icorr (lA/cm2 )
Corrosion rate (mpy)
I.E. (%)
Blank BATP CATP SATP PBATP BATP + Mo CATP + Mo SATP + Mo PBATP + Mo
)156 )230 )220 )218 )185 )254 )200 )180 )253
1.6 0.46 0.43 0.40 0.60 0.44 0.41 0.38 0.55
0.735 0.211 0.197 0.183 0.275 0.202 0.188 0.174 0.252
– 71.25 73.12 75.00 62.50 72.50 74.37 76.25 65.62
I.E.––inhibition efficiency.
Table 3 Potentiodynamic polarisation parameters of copper in lake water with and without the biocide (system II) Biocide
Conc. (ppm)
Ecorr (mV)
icorr (lA/cm2 )
Corrosion rate (mpy)
I.E. (%)
Blank
–
)160 )138 )130 )127 )161
1.6 0.6 0.5 0.36 0.75
0.73 0.27 0.22 0.16 0.34
– 62.50 68.75 77.50 53.12
CTAB
5 10 15 20
Table 4 Potentiodynamic polarisation parameters of copper in lake water mixing of biocide + inhibitor (systems III and IV) Inhibitors
Ecorr (mV)
icorr (lA/cm2 )
Corrosion rate (mpy)
I.E. (%)
Blank BATP CATP SATP PBATP
)156 )188 )173 )190 )180
1.6 0.43 0.39 0.35 0.46
0.73 0.197 0.179 0.160 0.211
– 73.12 75.62 78.12 71.25
Inhibitors addition after killing bacterial by addition of biocide (system IV) BATP )256 0.30 0.137 CATP )218 0.28 0.128 SATP )258 0.26 0.119 PBATP )254 0.38 0.174
81.25 82.50 83.75 76.25
slightly shifts towards the active direction in comparison to the result obtained in the absence of the inhibitor. Both the anodic and cathodic current densities were decreased indicating that all the tested inhibitors suppressed both the anodic and cathodic reactions, although mainly the cathodic one. Optimum concentrations were determined for all investigated compounds by Tafel extrapolation method. For SATP and CATP, the corrosion inhibition efficiency of 75% and 73.12% respectively was obtained at 4 ppm concentration. The
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Fig. 2. Potentiodynamic polarisation curves of copper in lake water with and without optimum concentration of molybdate and biocide.
inhibiting effect decreases at lower and higher concentrations and the compounds act as activators at the highest investigated concentration. The higher inhibition efficiency shown by SATP compared to PBATP may be attributed to the increased electron density leading to electron transfer mechanism from functional group to metal surface, producing greater coordinate bonding with a greater adsorption and inhibition efficiency. Similar explanation has been sought to explain the discrepancy in the order of inhibition efficiency by triazoline derivative [15]. SATP is the most efficient inhibitor due to the presence of electron releasing character of OH group compared to the other substituents. PBATP has the lowest protection efficiency due to the coplanarity of p-NO2 group with the phenyl ring, which imparts maximum electron withdrawal [16]. Most investigators have evaluated the inhibitive action of molybdate. Molybdate have been shown to be corrosion inhibitors in neutral or basic solution similar to cooling tower water. The inhibition efficiency with the addition of molybdate at various concentration levels to the lake water was determined. An optimum concentration of 5 ppm gave a maximum inhibition efficiency of 50.40%.
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Fig. 3. Potentiodynamic polarisation curves of copper in lake water with and without optimum concentration of BATP and biocide.
Addition of molybdate controls both cathodic as well as anodic reaction. But along with inhibitor they also predominantly control the cathodic reaction site. Influence of molybdate with inhibitors on corrosion is presented in Table 2. In presence of molybdate, the efficacy of BATP and CATP are increased to 72.50% and 74.37% respectively. Similarly SATP showed a higher efficiency of 76.25% in presence of molybdate than SATP alone (75%). The synergism is an enhanced inhibition, which may be related either to interaction between the inhibitor compounds or to interaction between the inhibitor compound and one of the ions present in the aqueous media. In system II, the inhibitive effect of CTAB on copper corrosion in neutral aqueous solution was investigated. The reason for selecting CTAB as biocide is that CTAB is not only a typical cationic surfactant, which is commercially available, but is also a quaternary ammonium salt with a long hydrocarbon chain, whose homologues have been used extensively as biocides and inhibitors [17,18]. In particular it is reported that the halides are the most effective derivatives since they increased the inhibiting tendency of the positive quaternary ammonium ions through a synergistic effect [17]. By comparing polarisation curves in the absence and presence of CTAB, it was observed that an increase in CTAB concentration shifted the open circuit potential in the positive direction with a lowering of both the anodic and cathodic current
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Fig. 4. Potentiodynamic polarisation curves of copper in lake water with optimum concentration of CATP and biocide.
densities. The corrosion potential and corrosion current density for copper in CTAB were obtained and the results are listed in Table 3. The results revealed that the I.E. increases with the CTAB concentration up to a maximum (77.5%) in a concentration of 15 ppm and then it decreases. In system III (Table 4) the inhibition efficiency was about 78.12% and 75.62% respectively for SATP and CATP. In system IV, the corrosion inhibition percentage was higher compared to system III are shown in Table 4. The inhibition efficiencies of SATP and PBATP (system IV) were about 83.75% and 76.25% respectively. From the observation of results, SATP found to act as better inhibitor. Besides in system IV, the inhibitor recorded higher efficiencies than system III and elucidates the low interference between biocide and inhibitors system. In system III, the biocide (CTAB) adsorption may disturb the absorption of triazole phosphonate, which may interfere with the inhibitor action. But in system IV the interference between biocide and inhibitor was least. The I.E. of all inhibitors with biocide decreased in the following order SATP > CATP > BATP > PBTAB. The polarisation curve for the blended mixture (system IV) indicates a shift in the corrosion potential towards the negative values of potential compared with the control sample. The cathodic region shows a decrease in the current density as a
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Fig. 5. Potentiodynamic polarisation curves of copper in lake water with and without optimum concentration of SATP and biocide.
result of cathodic inhibition. However, according to corrosion potential the inhibitor trended towards cathodic inhibition. 3.2. Electrochemical impedance spectroscopy (EIS) The admittance plots of copper immersed in natural aqueous environment (lake water) in the absence and presence of inhibitors (4 ppm of BATP, CATP, SATP and 5 ppm of PBATP, 5 ppm of molybdate and CTAB-15 ppm) are shown in Fig. 7. As the admittance plots were not semicircles, these plots were not used for calculating the impedance parameters. The electrochemical parameters were obtained by using the semicircle fitting method [19]. This can be achieved by selecting the best fitting for the semicircle on the complex plane (Nyquist) plot. The data was plotted and analysed using the software Z view version 1.5b, 1996, Scribner Associates Inc. As seen from the impedance results the increase in resistance in the presence of SATP (compared to inhibitor-free solution) is related to the corrosion protection effect of the molecule. The value of Cdl decreases in the presence of SATP with biocide mixture, indicating that the surface oxide layer thickness decreases and changes the influence of the oxide layer on the kinetics of the electrode process.
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161
Fig. 6. Potentiodynamic polarisation curves of copper in lake water with optimum concentration of PBATP and biocide.
The EIS parameters were calculated and presented in Table 5. Copper with CTAB + SATP + Mo mixture exhibited higher resistance and impedance but lower capacitance than for the blank. The inhibition efficiencies calculated from ac measurement show the same trend as those observed from dc polarisation results. Maximum efficiency was noticed for the compounds with the methoxy phenyl substituents, since the basicity of the coordinating atoms were increased by electron donating groups. The electron withdrawing groups such as p-nitro group retards this electron transfer process, which results in decreased inhibition efficiency. The results are followed the same order, were observed in dc polarisation measurement. 3.3. Accelerated-leaching studies (ICP-AES) To determine the quantity of leached copper, accelerated-leaching studies were carried out on the WE. In the leaching studies, the WE was immersed in the test solution for 1 h at the impressed potential of )200 mV. At the end of each experiment, the test solution was analysed for metal ion (Cu), by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The results of the accelerated-leaching study are presented in Fig. 8, which show the concentration of copper leached from the metal. Among the options studied, the
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Fig. 7. (a) Nyquist and (b) Bode plot of copper in lake water with optimum concentration of various inhibitors with biocide mixture (system IV).
most effective treatment for reducing copper dissolution was the application of 4 ppm of SATP + 5 ppm of molybdate with 15 ppm of CTAB to lake water. The less effective treatment in this study was the application of only PBATP with biocide to water, although copper dissolution was somewhat reduced.
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Table 5 Electrochemical impedance parameters of copper immersed in lake water with optimum concentration of inhibitors and biocide (system IV) Compounds
Rct 105 (X)
Cdl 106 (F)
Total impedance · 104
Blank SATP CATP BATP PBATP
0.2055 0.4371 0.4192 0.3666 0.2894
17.6954 13.2938 17.9325 22.1880 22.8561
1.8776 4.2694 3.9665 3.4484 2.6373
Fig. 8. Concentration of copper present in the solution after leaching of copper for various inhibitors with biocide mixture.
The present investigation, SATP with mixture exhibited a very low tendency (1.227 ppm) to leach metal ions compared to the blank (11.776 ppm). This can be attributed mainly to the stable complex film that inhibits the metal dissolution by forming a barrier layer between the metal and the environment, thus preventing the bare metal contacting solution. The amount of copper leached out from metal with all inhibitors shows the following order, PBATP > BATP > CATP > SATP. The results are good agreements with electrochemical measurements. 3.4. Surface evaluation study Copper specimens were immersed in various test solutions for a period of 30 days. After 30 days, the specimens were taken out and dried. The nature of the film formed on the surface on the metal specimens was analysed by the various surface analytical techniques. Compositional analysis of the corrosion products films were performed, where possible, using FT-IR, X-ray diffraction and Energy dispersive X-ray analysis. 3.4.1. Analysis of FT-IR spectra FT-IR of SATP is presented in Fig. 9a. The stretching mode of P@O bond gives rise to a strong band in the region 1320–1140 cm1 as expected [20]. The two prominent bands at 1080 and 930 cm1 are attributed to the H2 PO3 group of the
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Fig. 9. Infrared transmission spectra of (a) SATP and (b) CTAB. Infrared reflection absorption spectrum of the film formed on the copper substrate after immersion in the solution containing (c) SATP and (d) mixture of SATP + Mo + CTAB.
molecule. The band at 1080 cm1 is assigned to the P–O stretch of the ionic species and the other at 930 cm1 is assigned to the P–OH stretch [21,22]. The reflectance absorption spectra of the film formed on copper immersed in the environment consisting of 4 ppm of SATP and 4 ppm of SATP + 5 ppm of Molybdate + 15 ppm of CTAB are given in Fig. 9c and d. The band at 930 cm1 almost disappeared in Fig. 9c and d compared to Fig. 9a. This can be attributed to a P–O–M bond in which a free Pþ –O interacts with metallic species. The fact that the band at 930 cm1 due to P–OH stretching is weak further indicates the possibility of a P–O–M (P–O–Cu) bond [23].
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The FT-IR spectrum of pure CTAB was given in Fig. 9b. The small absorption band due to 2854 cm1 represents CH2 stretching frequency. The aliphatic C–H stretches cause absorption at 2923 and 2854 cm1 . The band at 1126 cm1 corresponds to C–N stretching. In addition to the metal–inhibitor complex, the CTAB– Cu2þ complex may also deposit as a layer on the metal and as a part of the protective film. Nitrogen containing organic heterocyclic compounds are considered to be excellent complex or chelate forming substances with metals of the transition series [24]. It is known that aromatic triazoles are effective inhibitors for copper and its alloys. The protective action of the triazole is based on the formation of a semipermeable, insoluble, polymeric copper–triazole complex film on the copper surface. The polymeric complex was formed by covalent and coordinate covalent bonds. The formation of a complex is the result of the reaction between the triazole ring and the metal surface [25]. In this case it may be viewed as a result of Lewis acid–base electron exchange, resulting in the formation of 5- and 6-membered metal inhibitor ring complexes, as shown in the following figure.
3.4.2. X-ray diffraction (XRD) The X-ray diffraction (XRD) method can be used not only for identification of crystalline phases in corrosion products but also for their quantitative phase analysis based on measuring the intensity of a single diffraction line or even all the lines in the pattern [26]. Identification was carried out using a search-and-match fit of the XRD data to the patterns in the JCPDF database. The X-ray diffraction patterns of the films formed on the copper surface immersed in neutral aqueous solution in absence and presence of inhibitor (SATP) and biocide are shown in Fig. 10 respectively. In the absence of inhibitor, the XRD pattern of the copper shows peaks at 36.6, 43, 50.2, 61.6, 74 and 89. The XRD pattern of inhibited surface (Fig. 10b) has no characteristic peaks. This indicates the amorphous nature of the surface film. 3.4.3. Energy dispersive X-ray analysis (EDXA) Energy dispersive X-ray analysis (EDXA) technique was employed in order to get additional information on the inhibition mechanisms. The results obtained from these techniques showed that the corrosion inhibition process was related to the development of an inhibitor film over the metal surface.
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Fig. 10. XRD pattern obtained on the surface film formed on copper at the end of 30 days in different environment. Curve: (a) blank and (b) SATP with molybdate and CTAB.
Attempts to identify the surface films using X-ray diffraction were not successful, possibly because of the poor crystallinity of the material, but more probably due to the fact that the films were very thin. The cross section analyses of the corrosion layers were performed by EDXA. Mapping of C, O, P and Cu was carried out to investigate the distribution of these elements in the surface layers. EDXA analysis
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167
Fig. 11. EDXA spectrum of copper in natural lake water: (a) blank and (b) SATP with biocide mixture.
(Fig. 11b) confirmed that a phosphorus-containing layer was formed on copper when it was exposed to treated natural lake water. This observation clearly proves
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that the inhibition was due to the formation of an insoluble stable film through the process of complexation of the organic molecules on the metal surface.
4. Conclusions 1. System IV has higher efficiencies than system III and elucidates the minimal interference between biocide and inhibitors system. 2. The inhibition efficiencies calculated from ac measurement show the same trend as those observed from dc polarisation results. 3. The dissolution of copper in presence of SATP with biocide mixture is low compared to the blank. 4. FT-IR and EDXA shows that the inhibition is due to the formation of an insoluble stable film through the process of complexation of the organic molecules on the metal surface.
Acknowledgement The authors greatly acknowledge Tamil Nadu State Council for Science and Technology (TNSCST) for providing financial assistance.
References [1] W. Qafsaoui, G. Mankwski, P. Leterrible, F. Dabosi, in: Proc. of the Int. Sym. on Control of Copper and Copper Alloys Oxidation, Rouen, France, 1992. [2] P.E. Francis, W.K. Cheung, R.C. Pemberton, in: Proc. of the 11th Int. Corr. Cong., Florence, Italy, vol. 5, Associazione Italiana di Metallurgia, 1990, p. 363. [3] E. Stupnisek-Lisac, A. Gazioda, M. Madzarac, Electrochim. Acta 47 (2002) 4189. [4] Y.I. Kuznetzov, Organic Inhibitors of Corrosion of Metals, Plenum Press, NY, 1996. [5] F. Ammelot, C. Faiud, E.M.M. Sutter, Electrochim. Acta 42 (1997) 3565. [6] C.W. Yan, H.C. Lin, C.N. Cao, Electrochim. Acta 45 (2000) 2815. [7] M. Fonsati, F. Zucchi, G. Trabanelli, Electrochim. Acta 44 (1998) 311. [8] A. Frignani, L. Tommesani, G. Brunoro, M. Fogagnolo, Corros. Sci 41 (1999) 1205. [9] N. Huynth, T. Notoya, S.E. Bottle, D.P. Schweinsberg, Corros. Sci 42 (2000) 259. [10] M.K. Patterson, G.R. Husted, A. Rutkowski, et al., Ultrapure Water 8 (1991) 18. [11] W.G. Charaklis, in: W.G. Charaklis, K.C. Marshall (Eds.), Biofllms, Willey, New York, 1990, p. 671. [12] S. Ramesh, S. Rajeswari, S. Maruthamuthu, in: Proc. of National Convention on Corros. (CORCON-2002), East Asia Pacific Reg. Conf., Goa, India, 2002. [13] S. Ramesh, S. Rajeswari, Can. Metall. Quart. 42 (2003) 377. [14] S. Ramesh, S. Rajeswari, Electrochim. Acta 49 (2004) 811. [15] E. Khamis, M. Atea, Corrosion 50 (2) (1994) 106. [16] A.S. Fouda, M.N. Moussa, F.I. Taha, Al El-Neamaa, Corros. Sci. 26 (9) (1986) 719. [17] D.P. Schwesberg, V. Ashworth, Corros. Sci. 28 (1988) 639. [18] H. Ma, S. Chen, S. Zhao, X. Liu, D. Li, J. Electrochem. Soc. 148 (11) (2001) B482. [19] Basic of A.C. Impedance measurements, Application note-AC-1, Egand G. Princeton Applied Research, Electrochemical Instrument Division, Princeton, NJ, 1982.
S. Ramesh, S. Rajeswari / Corrosion Science 47 (2005) 151–169
169
[20] M.A. Quraishi, M. Wajid Khan, M. Ajmal, S. Muralidharan, S. Venkatakrishna lyer, Anti-Corros. Methods Mater. 43 (1996) 5. [21] N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman spectroscopy, third ed., Academic Press, New York, 1990. [22] Y. Gonzlez, M.C. Lafont, N. Pebere, G. Chatainiel, J. Roy, T. Bouissou, Corros. Sci 37 (1998) 1823. [23] R.O. Carter III, C.A. Giercgak, R.A. Dickie, Appl. Spectrosc. 40 (1986) 649. [24] N.K. Patel, J. Electrochem. Soc., India 21 (1972) 136. [25] N. Eldakar, K. Nobe, Corrosion 36 (1981) 271. [26] R. Jenkins, R.L. Snyder, Introduction to X-ray Powder Diffractometry, John Wiley & Sons, New York, 1996.