Corrosion Science 50 (2008) 3615–3621
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Corrosion Science
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Effect of serine, threonine and glutamic acid on the corrosion of copper in aerated hydrochloric acid solution
Da-Quan Zhang a, Qi-Rui Cai a, Li-Xin Gao a, Kang Yong Lee b,* a Department of Environmental Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China b Stress Analysis and Failure Design Laboratory, School of Mechanical Engineering, Yonsei University, 134, Shinchon-dong, Seodaemun-gu, Seoul 120-749, Republic of Korea article info abstract
Article history: The corrosion inhibition of three amino acid compounds on copper was investigated by electrochemical Received 30 July 2008 method. They suppressed cathodic current densities and shifted the corrosion potential towards more Accepted 5 September 2008 negative values. The interaction between amino acid and copper surface was certified by reflected Available online 18 September 2008 FT-infrared spectroscopy. The quantum chemical parameters were obtained by PM3 semi-empirical calculation. Glutamic acid has the smaller net positive charge of nitrogen atom and the more net negative Keywords: charge of oxygen atoms. The improved inhibition of glutamic acid was due to the stabilization of its A. Copper adsorption on the copper surface by the oxygen atoms in its structure. B. Polarization Ó 2008 Elsevier Ltd. All rights reserved. B. IR spectroscopy C. Acid inhibition
1. Introduction Copper and its alloys are commonly employed as a material in heating and cooling systems due to their good thermal conductiv- The use of inhibitors is one of the most practical methods to ity and mechanical properties. Hydrochloric acid pickling is exten- protect metals from corrosion. Unfortunately, many common cor- sively used for the removal of rust and scale on heat transfer in rosion inhibitors are highly toxic and health-hazardable, such as several industrial processes [7,8]. Inhibition of amino acids for cop- chromates [1], nitrite [2], and aromatic heterocyclic compounds per corrosion in acidic media has also aroused interests. Among [3] etc. Their replacement by new environment-friendly inhibitors these amino acids, cysteine is the well-investigated. We studied is desirable. Amino acids are nontoxic, biodegradable, relatively the inhibitory action cysteine on copper corrosion in 0.5 M HCl, cheap and completely soluble in aqueous media. The number of and attributed inhibition to the adsorption on the copper surface publications about the inhibition effect of some amino acids on via the mercapto group in its molecular structure [9]. Ismail [10] corrosion of metals increases. El-Shafei studied the effect of six reported that the maximum inhibition efficiency of cysteine can a-amino acids for pitting corrosion of aluminum in chloride media achieve at about 84%, and the presence of Cu2+ ions increases the [4]. They reported that all the six compounds shift the pitting po- inhibition efficiency to 90% for copper in neutral and acidic chlo- tential (Ep) and the protection potential (Epp) towards more noble ride solution. Matos et al. [11] tested the effect of cysteine on the values, and arginine was the more effective inhibitor. Morad [5] anodic dissolution of copper in sulfuric acid media. They found that examined the effect of amino acids containing sulfur on the corro- cysteine is an inhibitor for copper dissolution in sulfuric acid media sion of mild steel in phosphoric acid solutions containing Cl ,F at low anodic polarization, and its inhibitory effect is based on the and Fe3+ ions. They found that the binary mixtures of Cl or F with formation of the cysteine–Cu (I) intermediate. Despite cysteine is a cysteine or methionine have the best inhibition efficiency exceed- very interesting corrosion inhibitor for copper, there remains rela- ing 90%. However, Ramakrishnaiah [6] measured the corrosion of tively few works directed towards the corrosion inhibition effect of steel in a number of amino acids in sodium chloride solutions at other amino acids for copper in acidic chloride media [12]. pH 8. He showed that while some amino acids were able to On the other hand, mechanistic information on corrosion and decrease corrosion, other such as aspartic acid actually seemed to inhibition processes is very important for proper selection of inhib- accelerate corrosion. In no case was the corrosion rate reduced itors. A systematic approach is necessary for characterization of the enough so that any of the amino acids would be capable of acting interaction between the organic inhibitor molecule and the metal as a practical corrosion inhibitor. or alloy [13]. Such an approach would include elucidation of the interaction by molecular orbital (MO) calculations of the relevant parameters. The semi-empirical calculation methods have been
* Corresponding author. Tel./fax: +82 2 2123 2813. used most successfully in providing theoretical consideration for E-mail address: [email protected] (K.Y. Lee). corrosion inhibitor actions [14–16]. Amino acids contain two
0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.09.007 3616 D.-Q. Zhang et al. / Corrosion Science 50 (2008) 3615–3621 functional groups of carboxyl and amino bonded to the same car- electrode reference electrode (SCE) was used for measurements. bon atom. This is beneficial to their molecular modification and Copper is a relatively noble metal. Nevertheless, it suffers severe structural assembling. The formation and organization of function- corrosion in aerated acidic media. During the measurements, the alized amino acid on the metal surfaces is of considerable impor- solution was not stirred or deaerated. The cell was open to air tance for the development of new assembled system for and the measurement was conducted at room temperature. All po- corrosion inhibition. Thus, amino acids can be used as model tential values were reported in mV (SCE). The WEs were mechan- compound for molecule design study of corrosion inhibitor [17]. ically polished with emery paper (#1, #4 and #6), rinsed with However, there is little information on the amino acid inhibition deionized water, degreased with ethanol, and dried at room tem- on copper surface by MO calculations [18]. perature. The degreased WE were inserted into the solution. After
In this paper, the efficiency of three kinds of amino acid the corrosion potential (Ecorr) was stable, it was then polarized compounds for the inhibition of copper corrosion in a 0.5 M HCl from 800 mV to +800 mV at 1 mV/s. The EIS experiments were solution was investigated. They are serine, threonine and glutamic performed at open circuit potential over a frequency range of acid, which structures are shown in Fig. 1. 0.05 Hz to 100 kHz. The sinusoidal potential perturbation was These amino acid compounds have three groups in their mole- 5 mV in amplitude. cules. Investigating the different action of these groups for copper corrosion can obtain a more quantitative knowledge about the sur- 2.3. Fourier transform infrared reflection test face films formed by adsorption of amino acids. For this purpose, the films were studied by using potentiodynamic polarization The copper specimens of 10 mm 10 mm 1 mm were curves and electrochemical impedance spectroscopy (EIS) meth- abraded with silicon carbide (SiC) paper and polished with dia- ods. The structure and bonding of amino acids on copper films mond paste to 1 lm. The samples were washed with ethanol of was investigated by reflected FT-infrared spectroscopy. Attempts analytical grade and then immediately immersed to 0.5 M HCl to elucidate the inhibition mechanism of these amino acids for solutions containing the 1 mM amino acids. After 48 h inhibitor the copper corrosion were done by PM3 (modified neglect of film-forming period, the copper specimens treated by amino acids diatomic overlap parametric method number) semi-empirical were transported for FT-infrared reflection spectra analysis. quantum chemical calculations. A background spectrum was recorded for a specimen without amino acids treatment. The infrared light from FT-IR spectrometer 2. Experimental bench was directed on the sample via plane and spherical windows. The angle of incidence was 80°. The reflect spectra were 2.1. Materials and apparatus obtained by using p-polarized light at a resolution of 4 cm 1. All subsequent spectrums were recorded in transmittance units
The materials used for test was pure copper (99.99%) supplied (It/I0), where It is the reflected intensity of the sample surface trea- in the form of a drawn copper rod. The working electrode (WE) ted by amino acids and I0 is the reflectivity of the background spec- for electrochemistry measurements was sealed with epoxy resin trum. The recorded spectra were compared with those of the pure so that only the circular cross section (1 cm2) of the rod was substances selected from Sadtler Spectral Handbooks. exposed. Amino acids were analytical grade reagent (AR) and used as received. It has been reported that inhibition efficiencies in- 2.4. Calculation method crease as the amino acids concentration increases and can have a remarkable effect at 1 mM in HCl solution [5,10]. The concentra- Most semi-empirical MO programs are based on the linear com- tion of the amino acids in the solution was set at 1 m mol L 1 for bination of atomic orbital by self-consistent field (LCAO-SCF) type the electrochemical measurement. The aggressive environment of procedure. The PM3 quantum chemical method used in the cur- used was 0.5 M HCl solution prepared from AR chemicals and rent work is based on one of the levels of integral approximation deionized water. known as the neglect of diatomic differential overlap (NDDO). All Electrochemical polarization curves and electrochemical the calculations were carried out with the help of complete geom- impedance spectroscopy (EIS) measurements were performed in etry optimization. Initial estimates of the geometry data were ob- a three-electrode cell using a PARC M283 potentiostat (EG&G), tained from the Pcmodel 6.0 packet program, and subsequent PM 3 PARC Model 1025 frequency response analyzer, and M352 and calculations were performed with MOPAC 7.0 packet program on a M398 software packages, respectively. Infrared spectra were PC implemented with a 1.5 GHz Intel Pentium Processor. recorded on Nicolet Nexus 470 IR spectrometer in the range from 1 1 500 cm to 4000 cm . 3. Results and discussion
2.2. Electrochemical measurements 3.1. Polarization curve measurements
A three-electrode cell, employing a copper rod-working elec- Fig. 2 shows typical polarization curves for copper in aerated trode (WE), platinum foil counter electrode, and saturated calomel 0.5 mol L 1 HCl solution with and without amino acids after 1 h immersion. Anodic dissolution of copper in chloride media has been studied extensively [19–23]. The accepted anodic reaction at a 0.5 M HCl solution is the dissolution of copper through oxidation of Cu (0) to Cu+ Cu ! Cuþ þ e ð1Þ Then Cu+ reacts with chloride ion from the solution to form CuCl Cuþ þ Cl ! CuCl ð2Þ Insoluble CuCl precipitates on the copper surface. The CuCl species Fig. 1. Molecular structures of amino acids (serine, threonine and glutamic acid). has poor adhesion, is unable to produce enough protection for the D.-Q. Zhang et al. / Corrosion Science 50 (2008) 3615–3621 3617
Fig. 2. Polarization behavior of a copper electrode in 0.5 M HCl without (a) and with 1 mM of serine (b), threonine (c), or glutamic acid (d) in 0.5 M HCl solution.
copper surface, and transforms to the sparingly soluble cuprous Table 1 Corrosion parameters and inhibition efficiency (IE) obtained by ParCalc of M352 chloride complex, CuCl2 [24] software for copper in 0.5 M HCl CuCl þ Cl ! CuCl2 ð3Þ 2 1 Electrodes Ecorr/mV(SCE) Icorr/lAcm ba/mV dec IE It has also been reported that the CuCl2 adsorbed on the surface dis- Blank 221.7 17.08 66.1 – solves by further oxidation [22] 1 mM serine 254.0 7.743 76.9 54.7 1 mM threonine 249.8 2.830 70.5 83.4 2þ CuCl2;ads ! Cu þ 2Cl þ e ð4Þ 1 mM glutamic acid 280.7 1.641 68.8 90.4 The anodic curve for the copper electrode in 0.5 M HCl blank solution exhibits three distinct regions, which are the active dissolution (apparent Tafel region), the transition region, and the was determined by extrapolation of the anodic Tafel lines. The cor- limiting current region. The limiting current plateau indicates a rosion inhibition efficiency (IE) was calculated from the corre- role of a diffusion-limiting rate, probably both the transport of sponding electrochemical polarization curves measurements chloride (Cl ) to the surface and the diffusion of (CuCl2 ) in the according to solution. An anodic current peak appeared at a potential of about icorr icorrðinhÞ 25 mV (SCE) is related to the CuCl film formation. It is reported IE ¼ 100 ð7Þ that the anodic dissolution of copper in the acidic chloride solution icorr is controlled by both electrodissolution of copper and diffusion of where icorr is the corrosion current density for the metal in the blank CuCl2 to the solution bulk [22,24]. solution and icorr(inh) is its value in the presence of the inhibitor. The cathodic corrosion reaction in an aerated acidic chloride The values of IE for the three amino acids are also included in solution is Table 1.
þ The values of IE in Table 1 show that threonine and glutamic 4H þ O2 þ 4e ! 2H2O ð5Þ acid have good protection for copper in 0.5 M HCl solution. The The cathodic curve close to the corrosion potential for blank can constancy of this anodic slope indicates that the mechanism of be ascribed to the reduction of dissolved oxygen present in the test the anodic reaction is not changed by the addition of amino acids. solution. The total corrosion reaction of copper in acidic chloride The cathodic reaction appears a pseudo-Tafel region. The cathodic solutions is as follows portion of the polarization curve is a composite and represents
þ 2þ copper ion re-deposition and oxygen reduction. The cathodic cur- 2Cu þ 4H þ 4Cl þ O2 ! 2Cu þ 4Cl þ 2H2O ð6Þ rent densities were greatly decreased near Ecorr region, and the It is clear from the potentiodynamic polarization experiments inhibition action of amino acids under examination decreases in that the presence of amino acids decreases the corrosion rate, i.e. the order: glutamic acid > threonine > serine. It reports that the the value of icorr decreases. Furthermore, Ecorr shifts more negative amino acids act as inhibitors through adsorption on the metal sur- to 254.0 mV(SCE), 249.8 mV(SCE), and 280.7 mV(SCE) for ser- face [25,26]. These amino acids can be protonated at the amine ine, threonine and glutamic acid, respectively. Particularly, the group in acidic solution, and can be adsorbed to cathodic sites of cathodic reaction is inhibited to a larger extent than the anodic copper. Thus, they hinder the cathodic reaction and acts as catho- reaction. Since the transfer of oxygen from the bulk solution to dic inhibitors. the copper/solution interface will strongly affect the rate of oxygen reduction, it can be inferred that the adsorbed layer behaves as a 3.2. Electrochemical impedance spectroscopy (EIS) cathodic inhibitor to Cu corrosion by retarding the transfer of O2 to the cathodic sites of the Cu surface. The corresponding corrosion To get further information concerning the inhibition process potentials (Ecorr), corrosion currents (icorr), anodic Tafel slopes (ba) and to confirm the potentiodynamic polarization experiments, is calculated by ParCalc method of Model352 corrosion analysis impedance measurements on the copper electrode were per- software and listed in Table 1. The corrosion current density (icorr) formed open to air at the open circuit potential. Fig. 3 shows the 3618 D.-Q. Zhang et al. / Corrosion Science 50 (2008) 3615–3621
Fig. 3. Nyquist plots of a copper electrode in 0.5 M HCl without (a) and with 1 mM of serine (b), threonine (c), or glutamic acid (d) in 0.5 M HCl solution.
limited low frequency range [29]. The parameters obtained by fit- ting the equivalent circuit are listed in Table 2. For the Cu/HCl system, the cathodic reactions and anodic reac- tion occurring on the copper surface at the open circuit potential
were the reduction of O2 and dissolution of copper, respectively [30]. The diffusion processes involved in this potential region was described as being limited by copper ions through the surface
Fig. 4. Equivalent circuit model used to fit the impedance displaying Warburg film [31] or O2 diffusion in solution [32]. Fig. 3 clearly shows that impedance. the shape of the impedance plots for inhibited electrodes are not substantially different from those of uninhibited electrodes. These results confirm the suggestion that the inhibitor does not alter the Nyquist plots for copper with and without amino acids in 0.5 M corrosion mechanism of copper in HCl solutions [33]. It inhibits HCl solution. A high frequency (HF) depressed semicircle was corrosion primarily through its adsorption on the metal surface. observed by a line portion in the low frequency (LF) region. The The corrosion resistance of each of the samples was determined Nyquist plots show a depressed semicircular shape with their by the total polarization resistance R . R is given by [34] centers below the real axis. This behavior is typical for solid metal p p electrodes that show frequency dispersion of the impedance data. Rp ¼ lim ReE¼Ecorr ð8Þ x !0 The equivalent circuit model with two time constants is employed for this system in Fig. 4. where Re represents the real part of the complex Faradaic imped-
It contains of solution resistance Rs in series connection with ance, and x corresponds to the angular velocity of the AC signal two time constants [27,28]. The first time constant, R1C1, in the (x =2pf, where f is frequency, Hz). The total polarization resistance high frequency region is proposed to be a result of the fast charge Rp values can be simply defined as the sum of R1 and R2 [22]. In or- transfer process of copper dissolution, R1 being the charge transfer der to confirm the potentiodynamic polarization results, the corro- resistance and C1 the double layer capacitance. The second time sion inhibition efficiency (IE) was also calculated from the constant in the low impedance region results from mass transport corresponding electrochemical impedance data according to through the oxide film, C2 is the capacitance of the surface film, and Rp;inh Rp R2 is the surface layer resistance. A Warburg impedance, Zw,is IE ¼ 100 ð9Þ Rp;inh introduced for a diffusion process. Since electrochemical systems show various types of inhomogeneity [21], the double layer capac- Where Rp,inh and Rp is the total polarization resistance in 0.5 M HCl itance can be better substituted by Q the constant phase element with and without amino acids present, respectively. The IE values (CPE). The CPE element was introduced formally only for fitting thus calculated are also listed in Table 2. impedance data. The corrosion behavior of copper in this solution The larger polarization resistance (Rp) in presence of amino was influenced, to some extent, by mass transport since the LF lin- acids indicates their inhibition effects. The value of Rp in the pres- ear portion is believed to be Warburg impedance. Nyquist plots do ence of glutamic acid was greater than that with serine or threo- not show a straight-line portion at low frequencies because of the nine alone. The inhibition efficiencies of threonine and glutamic
Table 2
Electrochemical impedance parameters and IE values of a copper electrode in 0.5 M HCl after 1 h immersion
2 2 2 0.5 2 Inhibitor Rs/Xcm Q1 R1//Xcm Q2 R2/kX cm W/S s cm IE
n 2 n -2 Y0/S s cm n1 Y0/S s cm n2 Blank 1.48 197.1E–6 0.79 18.7 1041E–6 0.63 0.356 0.0174 – 1 mM serine 1.37 136.1E–6 0.81 45.2 610.8E–6 0.58 2.11 0.0047 83.1 1 mM threonine 1.48 31.4E–6 0.86 146 229.3E–6 0.64 2.89 0.0032 87.7 1 mM glutamic acid 1.58 45.9E–6 0.85 163 164.4E–6 0.63 6.39 0.0039 94.5 D.-Q. Zhang et al. / Corrosion Science 50 (2008) 3615–3621 3619 acid calculated from impedance data are very close to those ob- Table 4 1 tained from potentiodynamic polarization measurements. Hence, Frequencies (cm ) of FT-IR spectra selected from Sadtler Spectral Handbooks þ þ the trend obtained by using these two different methods for effi- Compounds mNH4 mas,COO dNH4 ms,COO mC–N mC–O ciencies of serine, threonine and glutamic acid were the same. Serine 3021 1659 1507 1437 1312 1284 Threonine 3136 1631 1462 1418 1347 1250 3.3. FT-IR reflection measurement Glutamic acid 3059 1643 1516 1418 1352 1231
The FT-IR technique provides information concerning both the strength of the bonds between atoms and is a useful tool to study ence of amino acid in the surface film. The band at 650 cm 1 in the interaction between the organic compounds and the metal sur- Fig. 5 is ascribable to the Cu (I) oxide (Cu2O) formation in the cop- face [35,36]. Fig. 5 shows the FT-IR reflection spectra recorded after per surface. A weak band near 3300 cm 1 is assigned to the asym-
48 h immersion of copper with different amino acids in 0.5 M HCl metric NH2 stretching mode. In comparison with the data in Table þ solutions. Table 3 collects the fundamental frequencies of the 4, a shift towards higher frequency values of NH3 stretching bands corresponding reflection spectra recorded for copper specimen. is shown, attributable to the lack of hydrogen bonds in the The standard IR spectra of the pure amino acids selected from Sad- adsorbed product. The two bands of carboxyl groups are shown tler Spectral Handbooks were also listed in Table 4 for comparison. in Table 3. This can be explained the carboxyl groups are in forms The amino acids are characterized by the presence of bands of of both ACOOH and COO in 0.5 M HCl solution. The bands of CAN þ A the NH3 and COO group in their molecule [37]. As shown in Table bond and C O are shifted towards lower frequency values, due to 4 characteristic bands of amino acids are also found in the FT-IR the strong interaction of the molecule at the copper surface reflection spectrum of copper specimen. This confirms the pres- through both the protonated nitrogen in amino group and the oxygen in carboxyl or hydroxyl group. These observations are taken as indirect evidence for the formation of strong surface chemical bonds, i.e., CuAN and CuAO in the interaction of amino acids with the copper surface.
3.4. PM3 quantum chemical calculations
In aqueous solutions, ionization of amino acids depends on pH. At the isoelectric pH, the molecules have no net charge, and the zwitter ion structure is dominant. Below or above this isoelectric pH, the molecules are cationic or anionic, respectively [38]. These amino acids present in its protonated form in 0.5 M HCl solution. Therefore, the protonated form of these amino acids was used for quantum chemical calculations. Calculations were carried out at the Restricted Hartree-Fock level (RHF) using PM 3 semi-empirical SCF-MO methods in the MOPAC 7.0 program. RHF means the re- stricted Hartree–Fock Hamiltonian is to be used [15]. The stable optimum conformation geometry obtained by PM3 calculations is shown in Fig. 6.
Fig. 5. FT-IR reflection spectrum of copper specimen after 48 h immersion in 0.5 M HCl solution with different amino acids: serine (a), threonine (b), and glutamic acid (c).
Table 3 Frequencies (cm 1) of FT-IR reflection spectra recorded on copper after 48 h immersion in 0.5 M HCl solution of different amino acids
þ þ Copper surface treated by mNH4 mas,COO dNH4 ms,COO mC–N mC–O Serine 3346 1732, 1677 1497 1392 1298 1180 Threonine 3338 1732, 1677 1452 1389 1298 1176 Glutamic acid 3340 1732, 1676 1495 1387 1299 1184 Fig. 6. The optimum conformations of protonated form of serine, threonine and glutamic acid. 3620 D.-Q. Zhang et al. / Corrosion Science 50 (2008) 3615–3621
Table 5 Quantum chemical parameters of protonated form of serine, threonine and glutamic acid calculated by PM3 semi-empirical method
Inhibitor EHOMO/eV ELUMO/eV Net atomic charges Dipole moment /lD
gO gN Serine 15.348 10.987 O1 O5 O7 – N6 7.744 0.034 0.262 0.370 1.021 Threonine 15.680 10.357 O5 O6 O8 – N7 5.545 0.303 0.201 0.353 1.001 Glutamic acid 15.419 11.137 O1 O7 O8 O10 N9 8.358 0.275 0.244 0.301 0.362 0.950
It is possible to observe that in all case the carbon skeleton in 4. Conclusion amino acids molecule was almost planar. Liedberg et al. [37] investigated the molecular orientation and coordination of the Serine, threonine and glutamic acid show corrosion inhibition amino acid–metal complexes formation by Infrared Reflection property against copper corrosion in 0.5 M HCl solution. They ren- A spectra. The orientation of the C C axes were found to be essen- der Ecorr more negative and strongly decrease the cathodic current tially parallel to the copper surface. Values of energy of the high- density for copper corrosion. The corrosion inhibition can be attrib- est occupied molecular orbital (EHOMO), energy of the lowest uted to the adsorption of molecules through both the nitrogen and unoccupied molecular orbital (ELUMO), dipole moments of mole- the oxygen atoms, which forms a blocking barrier to copper corro- cules, and the electron density for heteroatoms of the molecular sion. The improved inhibition efficiency of glutamic acid is affected are collected in Table 5. by the negative charge centers of oxygen atom in its molecule. It has been reported that amino acid presents in its proton- ated form in acidic solution and can be attracted to the cathodic Acknowledgments sites on the metal surface. A physical adsorption of protonated amino group on the active corrosion sited play an important role Supports from Shanghai leading academic discipline project in its inhibition [10] and its inhibition efficiency improves with (P1304) and the National Science Foundation of China the increase of positive charge on nitrogen atom [39]. This con- (20776083) are gratefully acknowledged. One of the authors clusion, however, does not fit well with data presented in Table (ZHANG) wishes to thank the Korea Federation of Science and 5. 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