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Impedance spectroscopic study of corrosion inhibition of copper by surfactants in the acidic solutions
Corrosion Science 45 (2003) 867–882 www.elsevier.com/locate/corsci
Impedance spectroscopic study of corrosion inhibition of copper by surfactants in the acidic solutions Houyi Ma a,b,*, Shenhao Chen b,c, Bingsheng Yin b, Shiyong Zhao b, Xiangqian Liu b a
c
Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, Shandong University, Jinan 250100, China b Department of Chemistry, Shandong University, Jinan, Shandong 250100, China State Key Laboratory for Corrosion and Protection of Metal, Shenyang 110015, China Received 1 September 2000; accepted 10 September 2002
Abstract The inhibitive action of the four surfactants, cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate, sodium oleate and polyoxyethylene sorbitan monooleate (TWEEN80), on the corrosion behavior of copper was investigated in aerated 0.5 mol dm3 H2 SO4 solutions, by means of electrochemical impedance spectroscopy. These surfactants acted as the mixed-type inhibitors and lowered the corrosion reactions by blocking the copper surface through electrostatic adsorption or chemisorption. The inhibitor effectiveness increased with the exposure time to aggressive solutions, reached a maximum and then decreased, which implies the orientation change of adsorbed surfactant molecules on the surface. CTAB inhibited most effectively the copper corrosion among the four surfactants. The copper surface was determined to be positively charged in sulfuric acid solutions at the corrosion potential, which is unfavourable for electrostatic adsorption of cationic surfactant, CTAB. The reason why CTAB gave the highest inhibition efficiency was attributed to the synergistic effect between bromide anions and positive quaternary ammonium ions. C16 H33 N(CH3 )þ 4 ions may electrostatically adsorbed on the copper surface covered with primarily adsorbed bromide ions. On the basis of the variation of impedance behaviors of copper in the surfactant-containing solutions with the immersion time, the adsorption model of the surfactants on the copper surface was proposed. Ó 2002 Elsevier Science Ltd. All rights reserved.
* Corresponding author. Address: Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, Shandong University, Jinan 250100, China. Fax: +86-531-8565167. E-mail address: [email protected] (H. Ma).
0010-938X/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 2 ) 0 0 1 7 5 - 0
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Keywords: Cetyltrimethylammonium bromide (CTAB); Inhibition efficiency (IE); Hemimicelle; Potential of zero charge (pzc); B. Electrochemical impedance spectroscopy (EIS)
1. Introduction Surfactants are molecules composed of polar hydrophilic group, the ‘‘head’’, attached to a nonpolar hydrophobic group, the ‘‘tail’’ [1]. This unique molecular architecture leads to a rich spectrum of complex self-assembling phenomena when surfactants are dissolved in polar or nonpolar solvents [2–4]. The adsorption behavior of surfactants at the solid/solution interface is somewhat similar to that at the gas/solution interface although the latter is more complicated than the former. It is reported that the interaction between hydrocarbon chains of surfactants will occur through van der Waals forces when the surface concentration of surfactant adsorbed on solid surface is high enough, forming an organized structure, the hemimicelle [5,6], which can be expected to decrease the corrosion reactions by blocking the surface of metals and alloys. Few reports about using surfactants to inhibit metal corrosion were found although surfactants have been widely used in chemical and light industry. Adsorption of inhibitors at the metal/solution interface is usually accepted as the formation of electrostatic or covalent bonding between the metal surface atoms and the adsorbates [5,7]. In aqueous solutions, the surfactant molecules may absorb on the solid surface either through electrostatic attraction or chemisorption depending on the charge of the solid surface and the free energy change of transferring hydrocarbon chains from water to the solid surface [5,6]. Luo and co-workersÕ results have shown that sodium dodecyl benzene sulfonate (SDBS) can adsorb on the surface of mild steel via electrostatic forces, whereas sodium oleate (SO) chemisorbs on the surface of mild steel in acidic solution [5]. The aim of this work is to investigate how the surfactants of different types, including the cationic surfactant––cetyltrimethylammonium bromide (CTAB), the anodic surfactants––sodium dodecyl sulfate (SDS) and SO, and the nonionic surfactant––polyoxyethylene sorbitan monooleate (TWEEN-80), inhibit the copper corrosion in aerated 0.5 mol dm3 H2 SO4 by using electrochemical impedance spectroscopy (EIS), together with other electrochemical techniques, and to propose the adsorption models of surfactants on the copper surface.
2. Experimental The copper electrode was made from 99.9% pure copper rods of 5.8 mm diameter. The rod specimen was embedded in epoxy resin mould and only its cross-section was allowed to contact the electrolyte solutions. Before each experiment, the electrode was briefly ground with #600 emery paper, followed by #2000 emery paper. After surface finishing, it was rinsed, washed with deionized water, alcohol and finally rinsed with triply distilled water.
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Solutions were prepared using triply distilled water and AR grade H2 SO4 , CTAB, SDS, SO and CP grade polyoxyethylene sorbitan monooleate (TWEEN-80) which was purified before the use. The cell for carrying out all EIS experiments was a traditional three-electrode cell. The main compartment held the working electrode and the reference electrode, a saturated calomel electrode (SCE), with a Luggin capillary leading to the surface of the working electrode. The other contained a platinum black counter electrode with 4 cm2 surface area. All potentials were measured with respect to the SCE. All electrochemical experiments were carried out in aerated solutions. The cell was submerged in an automatic temperature-controlled water bath to carry out polarization and EIS experiments at the constant temperature (22 °C). EIS measurements were performed at the corrosion potentials in the frequency range from 65 kHz to 10 mHz at five points per hertz decade with an AC voltage amplitude of 5 mV. All measurements were conducted using a Zahner IM6 electrochemical workstation.
3. Results and discussion 3.1. Copper corrosion in aerated sulfuric acid Fig. 1 shows a set of Nyquist impedance diagrams obtained from the copper electrode exposed for 5 min, 2, 5 and 12 h to the aerated 0.5 mol dm3 H2 SO4 solutions at the respective corrosion potential. The corrosion potentials kept an approximate constant during impedance measurements, and the values of corrosion potentials corresponding to the different immersion time given above were 88, 81, 77 and 86 mV (vs. SCE) in proper order. The impedance spectra measured in the case of the short immersion time (5 min and 2 h) exhibited a Warburg impedance, indicating the corrosion process involved the transport of reactants from the bulk
Fig. 1. Nyquist impedance spectra for copper in 0.5 mol dm3 H2 SO4 at the corrosion potentials after 5 min, 2, 5, 12 h of immersion. The corrosion potentials corresponding to different immersion time were 88, 81, 77 and 86 mV in proper order.
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solution to the copper/solution interface or transport of soluble corrosion products from the interface to the bulk solution in the early stage of corrosion. The Warburg impedance disappeared and the impedance spectrum only displayed a depressed capacitive loop 5 h later; moreover, the size of capacitive loop significantly increased with the further increase of the immersion time. The former shows that the mass transport has little influence on the corrosion reaction after 5 h of immersion, whereas latter indicates that the corrosion resistance is increasing. The pure sulfuric acid and surfactant-containing sulfuric acid solutions used in our experiments were all aerated. Theoretically, copper can hardly be corroded in the deoxygenated dilute sulfuric acid [8,9], as copper cannot displace hydrogen from acid solutions according to theories of chemical thermodynamics [9]. However, this situation will change in aerated sulfuric acid. Dissolved oxygen may be reduced on copper surface and this will enable some corrosion to take place [8,10–13]. It is a good approximation to ignore the hydrogen evolution reaction and only consider oxygen reduction in the aerated sulfuric acid solutions at potentials near the corrosion potential, according to Smyrl [9]. Cathodic reduction of oxygen can be expressed either by a direct 4e transfer as shown by Eq. (1) O2 þ 4Hþ þ 4e ¼ 2H2 O
ð1Þ
or by two consecutive 2e steps involving a reduction to hydrogen peroxide first O2 þ 2Hþ þ 2e ¼ H2 O2
ð2Þ
followed by a further reduction [14] H2 O2 þ 2Hþ þ 2e ¼ 2H2 O
ð3Þ
The transfer of oxygen from the bulk solution to the copper/solution interface will strongly affect rate of oxygen reduction reaction despite how oxygen reduction takes place, either in 4e transfer or two consecutive 2e transfer steps: Dissolution of copper in sulfuric acid is described by the following two continuous steps: K1
Cu e () CuðIÞads K1
K2
ðfast stepÞ
CuðIÞads e ! CuðIIÞ ðslow stepÞ
ð4Þ ð5Þ
where Cu(I)ads is an adsorbed species at the copper surface and does not diffuse into the bulk solution [9,15–19]. Consequently, the mass transport has little influence on dissolution of copper. As mentioned above, the appearance of the Warburg impedance at the corrosion potential in aerated sulfuric acid should be attributed to oxygen transport from the bulk solution to the copper surface. The changes of impedance spectra in size and shape with extension of the immersion time, including disappearance of the Warburg impedance and the increase of capacitive loop in size, show that a barrier gradually forms on the copper surface.
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The barrier is probably related to formation of the salt film of Cu(II) ions, as Cu2 O and CuO cannot exist in strong acidic solution. 3.2. Corrosion inhibition by surfactants 3.2.1. EIS Corrosion inhibition of copper by the four surfactants of different types was investigated. The four surfactants do not lead to the significant shift of copper corrosion potentials. Before each impedance measurement, the corrosion potential of copper was measured and all values of corrosion potentials in surfactant-containing solutions are marked in the plots shown by Fig. 2. Fig. 2 shows the complex plane impedance displays of copper at the corrosion potentials after various exposure times to 0.5 mol dm3 H2 SO4 solutions containing different surfactants. Obviously, the presence of the surfactants leads to changes of the impedance diagrams in both shape and size. In addition, the impedance display of copper in the presence of the surfactants depends on the immersion time. By comparing the Nyquist spectra obtained after 5 min of immersion in the presence or absence of the surfactants, it is found that the Warburg impedance disappeared in surfactant-containing solutions, even though concentration of the surfactants was relatively low (1:0 104 mol dm3 ), no matter which surfactant was used as inhibitor. Taking the impedance diagrams measured in the presence of 1:0 104 mol dm3 CTAB at different immersion time as a example (see Fig. 2(a)), it is found that, the
Fig. 2. Complex plane plots of the impedance data measured at respective corrosion potential after the copper electrode was exposed for different time to the 0.5 mol dm3 H2 SO4 solutions with surfactants of different types. The value of corrosion potential for each measurement was marked in the plots.
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size of the capacitive arc first increased with the immersion time up to the maximum in 2 h (see ‘‘ ’’ symbol ), and then decreased with further increase of immersion time until it reduced to almost the same size 24 h later (see ‘‘’’ symbol) as it was measured after 5 min of immersion. The same phenomena were also observed in the sulfuric acids containing three other surfactants. Especially in the solution with SDS, the size of capacitive arc more significantly reduced with immersion time 2 h later. When the immersion time was over 12 h, it became even smaller than that obtained after 5 min of immersion (see Fig. 2(c)). Usually, the charging–discharging process of the electric double layer is a very rapid process and the capacitive loop seen in the highest frequency is therefore associated with the double layer relaxation [20]. The impedance spectrum displaying a high frequency capacitive loop and the Warburg impedance in the low frequency (see Fig. 1(a)) can be analyzed with the equivalent circuit in Fig. 3(a) [21], in which Rt represents the charge-transfer resistance, W the Warburg impedance and Rs the solution resistance. One constant phase element (CPE) is substituted for the capacitive element to give a more accurate fit [22], as most capacitive loops are depressed semi-circles rather than regular semi-circles. The impedance spectra displaying one capacitive loop may be analyzed with the electrical circuit in Fig. 3(b) [21]. The physical meaning of Rs , Rt and CPEdl is the same with what they express in Fig. 3(a). The CPE is a special element whose immittance value is a function of the angular frequency, x, and whose phase is independent of the frequency. Its admittance and impedance are, respectively, expressed as YCPE ¼ Y0 ðjxÞ
a
ð6Þ
and ZCPE ¼ ð1=Y0 ÞðjxÞa
ð7Þ
where Y0 is the magnitude of the CPE, x the angular frequency and a the exponential term of the CPE [22–25]. Values of a are usually related to roughness of electrode surface. The smaller value of a, the higher the surface roughness of an electrode [22,24]. Considering that a CPE may be considered as a parallel combination of a pure capacitor and a resistor that is inversely proportional to the angular frequency, the value of capacitance, Ci , can therefore be calculated for a parallel circuit composed of a CPE (Qi ) and a resistor (Ri ), according to the following formula [22]: Y0i ¼ ðCi Ri Þai =Ri
ð8Þ
CPEdl
CPEdl Rs
Rs Rt
W (a)
Rt (b)
Fig. 3. (a) The equivalent circuit to fit the EIS for copper displaying a Warburg impedance; (b) the equivalent circuit to fit the EIS for copper displaying one capacitive loop.
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Table 1 Fitting the EIS for copper in 0.5 mol dm3 H2 SO4 after 2 h immersion by the equivalent circuit shown in Fig. 3(a) Immersion time (h)
Rt (X cm2 )
Rs (X cm2 )
Cdl (lF cm2 )
W (X cm2 )
2
1:14 103
0.71
3.93
5:59 105
Values of the elements of the equivalent circuit.
The impedance spectrum of copper in 0.5 mol dm3 H2 SO4 after 2 h of immersion was analysed by using the circuit in Fig. 3(a), and the double-layer capacitance (Cdl ) was calculated in terms of Eq. (8). Values of elements fitted and that of Cdl calculated are listed in Table 1. Similarly, all impedance spectra shown in Fig. 2 were fitted by the circuit shown in Fig. 3(b) and values of elements of the circuit corresponding to different corrosion systems, including values of Cdl , are listed in Table 2. The charge-transfer resistance, Rt , reflects the corrosion rate of a metal in corrosive solutions. The smaller Rt , the faster the corrosion reaction rate. Accordingly, Table 2 Fitting all impedance spectra in Fig. 2 by the equivalent circuit shown in Fig. 3(b) Cdl (lF cm2 )
IE (%)
0.5 mol dm3 H2 SO4 þ 1:0 104 mol dm3 CTAB 0.64 5 min 8:12 103 2h 1:36 104 0.62 5h 9:81 103 0.64 0.64 7h 1:02 104 24 h 8:34 103 0.62
12.3 12.4 14.1 14.6 15.9
96.1 97.6 96.8 96.9 96.2
0.5 mol dm3 H2 SO4 þ 1:0 104 mol dm3 SO 5 min 2:82 103 2h 6:71 103 5h 5:27 103 7h 5:32 103 12 h 5:03 103 24 h 4:83 103
0.65 0.66 0.63 0.73 0.74 0.74
10.5 8.25 7.72 7.54 7.54 7.31
88.7 95.3 93.9 94.0 93.7 93.4
0.5 mol dm3 H2 SO4 þ 1:0 104 mol dm3 SDS 5 min 3:78 103 0.51 0.61 2h 5:14 103 5h 4:27 103 0.64 7h 3:97 103 0.64 12 h 3:22 103 0.64 0.60 24 h 2:42 103
9.40 9.90 10.6 11.2 12.3 13.1
91.6 93.8 92.5 92.0 90.1 86.9
0.5 mol dm3 H2 SO4 þ 1:0 104 mol dm3 TWEEN-80 5 min 2:63 103 0.12 2h 4:66 103 0.13 5h 4:83 103 8:75 102 3 6:89 102 24 h 2:99 10
9.60 8.58 11.9 9.26
87.9 93.2 93.4 89.3
Immersion time
Rt (X cm2 )
Rs (X cm2 )
Values of the elements of the equivalent circuits in Fig. 3(b) and IEs of the four surfactants on the copper corrosion.
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the inhibition efficiency (IE) of the surfactants on copper corrosion is calculated from the following formula [26]: IE ¼ ðRt R0t Þ=Rt 100
ð9Þ
where Rt is the charge-transfer resistance in the inhibited solutions and R0t the one in the uninhibited solutions, and IE values calculated are also listed in Table 2. Obviously, CTAB is the most effective inhibitor, SO the second, TWEEN-80 the third and SDS the worst, among the four surfactants. The impedance spectra in the surfactant-containing solutions at the corrosion potential do not display new low frequency capacitive or inductive loops, implying that the four surfactants are all the mixed-type inhibitors to copper corrosion in sulfuric acid. Their inhibition action is realized by the geometric coverage of surfactants on the surface of copper. In this way, IE can also be considered as the surface coverage by the surfactant molecules, h. Fig. 4 describes dependence of h on the immersion time for each surfactant. 3.2.2. Polarization curves Fig. 5 represents the polarization curves for the copper electrode in 0.5 mol dm3 H2 SO4 solutions with and without surfactants. The addition of two surfactants (CTAB and TWEEN-80) in 0.5 mol dm3 H2 SO4 solution shifts respectively the corrosion potential of copper cathodically and anodically to a small extent, but markedly lowers both anodic and cathodic current densities. The corrosion potential
Fig. 4. The dependence of the surface coverage by the different surfactants (h) on the immersion time.
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Fig. 5. Polarization curves for the copper electrode in 0.5 mol dm3 H2 SO4 solutions with and without surfactants at 22 °C.
and corrosion current density for the three corrosion systems were determined by the Tafel extrapolation method. The values of corrosion current density obtained are, respectively, 1:71 103 mA cm2 in the surfactant-free H2 SO4 solution, 1:69 104 mA cm2 in CTAB-containing solution and 2:65 104 mA cm2 in the presence of TWEEN-80. Based on the following formula [26]: IEð%Þ ¼
icorr i0corr 100 icorr
ð10Þ
where icorr and i0corr are, respectively, the corrosion current density in the absence or presence of surfactants, the IE of CTAB and TWEEN-80 to copper corrosion after 2 h of immersion was calculated and values of IE are 90.1% and 84.5%, respectively, in approximate agreement with values of IE obtained by using Eq. (9). 3.3. The pzc of copper in the sulfuric acid Adsorption of surfactants on a corroding metal depends mainly on the charge of the metal surface, the charge or the dipole moment of surfactants, and the adsorption of other ionic species if it is electrostatic in nature [5]. The potential of zero charge (pzc) plays a very important role in the electrostatic adsorption process. The pzc data of solid metallic electrode have been limited so far. The pzc for copper in 0.01 mol dm3 Na2 SO4 is 214 mV vs. SCE according to Bockris and ReddyÕs
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report [27], whereas the pzc for copper in H2 SO4 solutions has not been found in references. The first capacitive loop in high frequency range usually originates from the relaxation of the double-layer capacitance under the conditions (i) there are no rapid Faradaic pseudo-capacitive processes which can be coupled with the double layer relaxation [20] and (ii) there are no the specific adsorption of ions. The two conditions can be met in sulfuric acid solution. Thus, EIS offers a good method to determine the pzc of copper in sulfuric acid. We have measured a series of impedance spectra for the copper electrode in 0.5 mol dm3 H2 SO4 solution in a wide potential range between 512 and 20 mV (vs. SCE), analyzed these impedance spectra by means of IM6 impedance analysis software and plotted the double-layer capacitance against potential (see Fig. 6) in the light of Luo and co-workersÕ method [5]. The pzc of copper in 0.5 mol dm3 H2 SO4 is estimated to be 140 mV from the dependence of the double-layer capacitance on potential shown in Fig. 6, which is more negative than the corrosion potential (96 mV). This means that the copper surface is positively charged at the corrosion potential. If only considering the electrostatic attraction, electrostatic adsorption of anionic surfactant, SDS and SO, is favoured, while it is unlikely for positive quaternary ammonium ions to directly adsorb on the surface. The reason that CTAB gives the highest IE is related to the synergistic effect resulting from the specific adsorption of bromide ions at the copper/solution interface [28].
Fig. 6. Double-layer capacitance of copper as a function of potential in 0.5 mol dm3 H2 SO4 solution.
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3.4. The adsorption model of surfactants on the copper surface 3.4.1. SDS The inhibitive action of SDS in sulfuric acid solution results from physical (electrostatic) adsorption of the negatively charged C12 H25 SO 4 to the positively charged copper surface, forming a barrier on the copper surface. On the basis of Luo and co-workersÕ model, the IE should increases continuously with the immersion time since more C12 H25 SO2 4 ions will electrostatically adsorb on the copper surface with the increase of immersion time. However, the experimental results were not so, and the inhibition action of SDS decreased with further increasing the immersion time 2 h later. It is reported that diameter of the head of a C12 H25 SO 4 ion is 0.5 nm, and length of the hydrocarbon chain is 2.1 nm [29]. The orientation of adsorbed C12 H25 SO 4 ions on the surface greatly affects the inhibitory efficiency of SDS. We believe that the chemisorption of n-dodecyl and the electrostatic attraction of the C12 H25 SO 4 ions on copper surface take place simultaneously in the early stage, as shown in Fig. 7(a). This type of form of adsorption is favourable for free energy change of transferring hydrocarbon chains from water to the surface, by means of which the adsorbed C12 H25 SO 4 ions can cover much area, thereby inhibiting more effectively the copper corrosion. However, when more C12 H25 SO 4 ions electrostatically adsorb on the surface, the adsorption density of surfactant becomes so high that interaction between tails of C12 H25 SO 4 will occur through van der Waals force. The hydrocarbon chains of many adsorbed ions are believed to leave the surface and aggregate to form hemimicelle (see Fig. 7(b)). This will cause the decrease of the effective area covered by C12 H25 SO 4 ions to some extent. In this way, the IE of SDS first increases
(a)
-
-
-
-
+ + + + + + + + + + ++ + (b)
- - -
-
-
- -
-
the copper/solution interface SDS Fig. 7. Illustration of adsorption of and C12 H25 SO 4 ions at copper/solution interface. (a) Adsorption as single ions in the case of short immersion time; (b) hemimicelle formation at some sites of the surface in the case of long immersion time.
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with the immersion time, reaching a maximum at certain time, and then decreases with further increasing the immersion time. 3.4.2. CTAB The halides have been proved to be the most effective derivatives as they increase the inhibiting tendency of positive quaternary ammonium ion by the well known synergistic effect [28,30,31]. Addition of CTAB will introduce bromide ions to the sulfuric acid solutions. Bromide ions will first adsorbed at copper/solution interface through electrostatic attraction force because of the excess of positive charge at the copper/solution interface at the corrosion potential, leading to that the change on the solution side of the interface changes from positive to negative. Thus, the quaternary ammonium cations, C16 H33 N(CH3 )þ 3 ions, can be electrostatically adsorbed on the copper surface covered with primarily adsorbed bromide ions. The adsorption of CTAB at the copper/solution interface cannot be simply consider as an electrostatic adsorption attributed to the synergistic effect of bromide ions, the chemisorption of n-cetyl on copper surface must be considered at the same time. Adsorption model of CTAB, as shown by Fig. 8(a), is similar to that of SDS. The only difference between them is that C16 H33 N(CH3 )þ 3 ions electrostatically adsorb on the copper surface covered with primarily adsorbed bromide ions. The orientation change of the adsorbed C16 H33 N(CH3 )þ 4 ions on the copper surface with the immersion time is also similar to what C12 H25 SO 4 ions do (see Fig. 8(b)). þ Compared with C12 H25 SO ion, the head of C H N(CH 16 33 3 )4 ion is larger and the 4 hydrocarbon is longer. Consequently, the IE of CTAB is much higher than that of SDS. (a)
+ + + + - + + + + + + + + + + + + + (b)
+ + + + ++ + + - - -- + + + + + + + + + + + + + ++ - Fig. 8. Illustration of adsorption of and C16 H33 (CH3 )þ 3 ions at copper/solution interface. (a) Adsorption as single ions in the case of short immersion time; (b) hemimicelle formation at some sites of the surface in the case of long immersion time.
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3.4.3. SO Most SO transforms into oleic acid (CH3 (CH2 )7 CH@CH(CH)7 COOH) in sulfuric acid solution. In fact, the oleic acid is in the form of colloid precipitate due to its quite low solubility. It is reported that colloid precipitates of oleic acid are charged positively [5] when pH value of solution is below 3. Since the copper surface is charged positively, it is impossible for the colloid particles to adsorb on the surface directly through electrostatic attraction. Oleate ions (C17 H33 COO ) with negative charge in the solution will first adsorb on the copper surface through the electrostatic attraction force. The chemisorption of oleic acids on the surface should also take place at the same time according to Luo et al. [5]. Then, the colloid particles may adsorb on the copper surface covered with oleic acids via van der Waals force. The dependence of IE of OS on the immersion time suggests that there exists the orientation change of the adsorbed oleate ions or oleic acid with the immersion time, like what the adsorbed C12 H25 SO 4 ions do in Fig. 7(b). That SO exhibited the higher IE than SDS did is probably related to the longer hydrocarbon chain and presence of ‘‘–CH@CH–’’ group in the SO molecule. ‘‘–CH@CH–’’ group provides p-bond orbital. The p-bond orbital adsorption will certainly occur during adsorption of oleate ions or oleic acid at copper/solution interface, which is very advantageous to enhancement of IE of SO.
3.4.4. TWEEN-80 TWEEN-80 is a nonionic surfactant with the longer hydrocarbon chains and ring groups, whose molecular structure is shown in Fig. 9. Unlike SDS and CTAB, the TWEEN-80 molecules do not adsorb on the copper through the electrostatic attraction forces. Therefore, the inhibitive action to copper is attributed to its chemisorption at copper/solution interface. Considering copper surface is hydrophilic, TWEEN-80 may chemisorb at copper/solution interface via hydrogen bond between the –OH groups in TWEEN-80 molecules and the water molecules adsorbed on the surface. The adsorption strength of TWEEN-80 on copper surface should be lower than that of SDS since adsorption of the former can only realized via hydrogen bond. Therefore, its inhibition is as low as that of SDS despite the fact although its molecular area is larger than that of SDS.
O H2C HO(CH2CH2O)w HC
(OCH2CH2)xOH
O
CH-CH-CH2(OCH2CH2)yOC-C17H33 CH-(OCH2CH2)zOH ω +x+y+z=21-26
Fig. 9. Structure scheme of the TWEEN-80 molecule.
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3.5. Effect of temperature on IE of CTAB Fig. 10(a)–(c) represent respectively the influence of immersion time on the impedance spectra of copper electrode in 0.5 mol dm3 H2 SO4 þ 1:0 104 mol dm3 CTAB solution at 30, 40 and 50 °C. In all cases the Nyquist diagram shows a depressed semi-circles. By comparing the impedance spectra in Fig. 2(a) with those in Fig. 10(a)–(c), it is seen that the diameter of semi-circle reduces with increasing the
Fig. 10. Complex plane plots of the impedance data obtained from the copper electrode exposed for different time to 0.5 mol dm3 H2 SO4 þ 1:0 104 mol dm3 CTAB solutions at 30, 40 and 50 °C. All impedance measurements were carried out at corrosion potentials.
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temperature under otherwise identical conditions. The variation of impedance spectra in size with increasing immersion time at 30 and 40 °C is very similar to that occurring at 22 °C shown by Fig. 2(a). At the higher temperature (40 °C), the diameter of semi-circles begins to decline with the immersion time after 30 min of immersion. The experimental phenomena shown in Fig. 10 are associated with change of orientation of adsorbed C16 H33 N(CH3 )þ 3 ions on copper surface. The higher the temperature, the shorter the time required for the transition of n-cetyl group from lying on the surface to being normal to the surface.
4. Conclusion The four surfactants, CTAB, SDS, OE and TWEEN-80, regardless of the surfactant type, inhibit the copper corrosion to some extent in acidic solutions. CTAB exhibit the highest IE among these surfactants. All impedance spectra do not display new low frequency capacitive or inductive loop at the corrosion potentials, indicating that the surfactants act as the mixed-type inhibitor against copper corrosion in sulfuric acid. The copper surface is positive charged in sulfuric solutions at corrosion potential, which is favourable for adsorption of anionic surfactants and unfavourable for adsorption of cationic surfactants. The reason why CTAB can inhibit most effectively the copper corrosion is ascribed to the synergistic effect between bromide anions and C16 H33 N(CH3 )þ 4 positive ions. The immersion time of the electrode have a great influence on the IE of the surfactants. The surfactant ions may adsorb on the copper surface through both chemisorption and electrostatic attraction when the immersion time is short, by means of which surfactant molecules can cover more sites on the copper surface, giving rise to the higher IE. On the contrary, more surfactant molecules adsorbed on the copper surface will leave the surface and aggregate to form hemimicelle, leading to reduction of the effective coverage by the surfactant molecules, when the immersion time is long. Thus, the IE decreases with the immersion time despite the fact more the surfactant molecules adsorb at copper/ solution interface.
Acknowledgements This project was supported by the Visiting Scholar Foundation of Key Laboratory in Shandong University, the Chinese National Natural Science Fund (20173033) and Special Funds for the Major State Basic Research Projects (G 19990650).
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[2] C.A. Bunton, in: K.L. Mittal, D.O. Shah (Eds.), Surfactants in Solution, Vol. 11, Plenum Press, New York and London, 1990, p. 329. [3] S.I. Ahmad, S. Friberg, J. Amer. Chem. Soc. 94 (1972) 5169. [4] P. Ekwall, L. Mandell, K. Fontell, J. Colloid. Interface Sci. 29 (1969) 639. [5] H. Luo, Y.C. Guan, K.N. Han, Corrosion 54 (1998) 619. [6] C.A. Miller, S. Qutubuddin, in: H.-F. Eicke, C.D. Parfitt (Eds.), Interfacial Phenomena in Apolar Media, Surfactant Science Series, Vol. 21, Markel Dekker Inc., New York and Basel, 1987, p. 166. [7] G. Trabanelli, in: F. Mansfeld (Ed.), Corrosion Mechanism, Marcel Dekker Inc, New York, 1987, p. 119. [8] G. Quartarone, G. Moretti, T. Bellomi, G. Capobianco, A. Zingales, Corrosion 54 (1998) 606. [9] W.H. Smyrl, in: J.OÕM. Bockris, B.E. Conway, E. Yeager, R.E. White (Eds.), Comprehensive Treatise of Electrochemistry, Vol. 4, Plenum Press, New York, 1981, p. 116. [10] W.D. Bjorndahl, K. Nobe, Corrosion 40 (1984) 82. [11] H.P. Dhar, R.E. White, G. Burnell, L.R. Cornwell, R.B. Griffin, R. Darby, Corrosion 41 (1985) 317. [12] R. Schumacher, A. Muller, W. Stockel, J. Electroanal. Chem. 219 (1987) 311. [13] A.H. Moreira, A.V. Benedetti, P.L. Calot, P.T.A. Sumodja, Electrochim. Acta 38 (1993) 981. [14] P. Jinturkar, Y.C. Guan, K.N. Han, Corrosion 54 (1984) 106. [15] E. Mattsson, J.OÕM. Bockris, Trans. Faraday Soc. 55 (1959) 1586. [16] G.G.O. Cordeiro, O.E. Barcia, O.R. Mattos, Electrochim. Acta 38 (1993) 319. [17] D.K.Y. Wang, B.A.W. Coller, D.R. Macfarlane, Electrochim. Acta 38 (1993) 2121. [18] R. Caban, T.W. Chapman, J. Electrochem. Soc. 124 (1977) 1371. [19] T. Hurlen, G. Ottesen, A. Staurset, Electrochim. Acta 23 (1978) 23. [20] O.E. Barcia, O.R. Mattos, Electrochim. Acta 35 (1990) 1601. [21] S. Li, S. Chen, S. Lei, H. Ma, R. Yu, D. Liu, Corros. Sci. 41 (1999) 1273. [22] X. Wu, H. Ma, S. Chen, Z. Xu, A. Sui, J. Electrochem. Soc. 146 (1999) 1847. [23] P. Zoltowski, J. Electroanal. Chem. 433 (1998) 149. [24] A.V. Benedetti, P.T.A. Sumodjo, K. Nobe, P.L. Cabot, M.G. Proud, Electrochim. Acta 40 (1995) 2657. [25] M. Leibig, T.C. Halsey, Electrochim. Acta 38 (1993) 1985. [26] S. Li, Y. Wang, S. Chen, R. Yu, S. Lei, H. Ma, D. Liu, Corros. Sci. 41 (1999) 1769. [27] J.OÕM. Bockris, A.K.N. Reddy, in: J.OÕM. Bockris, B.E. Conway, E. Yeager, R.E. White (Eds.), Modern Electrochemistry, Vol. 2, Plenum Press, New York, 1970, p. 708. [28] D.P. Schweisberg, V. Ashworth, Corros. Sci. 28 (1988) 539. [29] B. Zhu, Z. Zhao, Basic Interface Chemistry, Chemical Industry Press, Beijing, 1996, p. 72, (in Chinese). [30] A. Frignani, F. Zucchi, C. Monticelli, Br. Corros. J. 18 (1983) 19. [31] I.L. Rozenfeld, Corrosion Inhibitors, Mcgraw-Hill, New York, 1981, p. 109.
Impedance spectroscopic study of corrosion inhibition of copper by surfactants in the acidic solutions Houyi Ma a,b,*, Shenhao Chen b,c, Bingsheng Yin b, Shiyong Zhao b, Xiangqian Liu b a
c
Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, Shandong University, Jinan 250100, China b Department of Chemistry, Shandong University, Jinan, Shandong 250100, China State Key Laboratory for Corrosion and Protection of Metal, Shenyang 110015, China Received 1 September 2000; accepted 10 September 2002
Abstract The inhibitive action of the four surfactants, cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate, sodium oleate and polyoxyethylene sorbitan monooleate (TWEEN80), on the corrosion behavior of copper was investigated in aerated 0.5 mol dm3 H2 SO4 solutions, by means of electrochemical impedance spectroscopy. These surfactants acted as the mixed-type inhibitors and lowered the corrosion reactions by blocking the copper surface through electrostatic adsorption or chemisorption. The inhibitor effectiveness increased with the exposure time to aggressive solutions, reached a maximum and then decreased, which implies the orientation change of adsorbed surfactant molecules on the surface. CTAB inhibited most effectively the copper corrosion among the four surfactants. The copper surface was determined to be positively charged in sulfuric acid solutions at the corrosion potential, which is unfavourable for electrostatic adsorption of cationic surfactant, CTAB. The reason why CTAB gave the highest inhibition efficiency was attributed to the synergistic effect between bromide anions and positive quaternary ammonium ions. C16 H33 N(CH3 )þ 4 ions may electrostatically adsorbed on the copper surface covered with primarily adsorbed bromide ions. On the basis of the variation of impedance behaviors of copper in the surfactant-containing solutions with the immersion time, the adsorption model of the surfactants on the copper surface was proposed. Ó 2002 Elsevier Science Ltd. All rights reserved.
* Corresponding author. Address: Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, Shandong University, Jinan 250100, China. Fax: +86-531-8565167. E-mail address: [email protected] (H. Ma).
0010-938X/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 2 ) 0 0 1 7 5 - 0
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Keywords: Cetyltrimethylammonium bromide (CTAB); Inhibition efficiency (IE); Hemimicelle; Potential of zero charge (pzc); B. Electrochemical impedance spectroscopy (EIS)
1. Introduction Surfactants are molecules composed of polar hydrophilic group, the ‘‘head’’, attached to a nonpolar hydrophobic group, the ‘‘tail’’ [1]. This unique molecular architecture leads to a rich spectrum of complex self-assembling phenomena when surfactants are dissolved in polar or nonpolar solvents [2–4]. The adsorption behavior of surfactants at the solid/solution interface is somewhat similar to that at the gas/solution interface although the latter is more complicated than the former. It is reported that the interaction between hydrocarbon chains of surfactants will occur through van der Waals forces when the surface concentration of surfactant adsorbed on solid surface is high enough, forming an organized structure, the hemimicelle [5,6], which can be expected to decrease the corrosion reactions by blocking the surface of metals and alloys. Few reports about using surfactants to inhibit metal corrosion were found although surfactants have been widely used in chemical and light industry. Adsorption of inhibitors at the metal/solution interface is usually accepted as the formation of electrostatic or covalent bonding between the metal surface atoms and the adsorbates [5,7]. In aqueous solutions, the surfactant molecules may absorb on the solid surface either through electrostatic attraction or chemisorption depending on the charge of the solid surface and the free energy change of transferring hydrocarbon chains from water to the solid surface [5,6]. Luo and co-workersÕ results have shown that sodium dodecyl benzene sulfonate (SDBS) can adsorb on the surface of mild steel via electrostatic forces, whereas sodium oleate (SO) chemisorbs on the surface of mild steel in acidic solution [5]. The aim of this work is to investigate how the surfactants of different types, including the cationic surfactant––cetyltrimethylammonium bromide (CTAB), the anodic surfactants––sodium dodecyl sulfate (SDS) and SO, and the nonionic surfactant––polyoxyethylene sorbitan monooleate (TWEEN-80), inhibit the copper corrosion in aerated 0.5 mol dm3 H2 SO4 by using electrochemical impedance spectroscopy (EIS), together with other electrochemical techniques, and to propose the adsorption models of surfactants on the copper surface.
2. Experimental The copper electrode was made from 99.9% pure copper rods of 5.8 mm diameter. The rod specimen was embedded in epoxy resin mould and only its cross-section was allowed to contact the electrolyte solutions. Before each experiment, the electrode was briefly ground with #600 emery paper, followed by #2000 emery paper. After surface finishing, it was rinsed, washed with deionized water, alcohol and finally rinsed with triply distilled water.
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Solutions were prepared using triply distilled water and AR grade H2 SO4 , CTAB, SDS, SO and CP grade polyoxyethylene sorbitan monooleate (TWEEN-80) which was purified before the use. The cell for carrying out all EIS experiments was a traditional three-electrode cell. The main compartment held the working electrode and the reference electrode, a saturated calomel electrode (SCE), with a Luggin capillary leading to the surface of the working electrode. The other contained a platinum black counter electrode with 4 cm2 surface area. All potentials were measured with respect to the SCE. All electrochemical experiments were carried out in aerated solutions. The cell was submerged in an automatic temperature-controlled water bath to carry out polarization and EIS experiments at the constant temperature (22 °C). EIS measurements were performed at the corrosion potentials in the frequency range from 65 kHz to 10 mHz at five points per hertz decade with an AC voltage amplitude of 5 mV. All measurements were conducted using a Zahner IM6 electrochemical workstation.
3. Results and discussion 3.1. Copper corrosion in aerated sulfuric acid Fig. 1 shows a set of Nyquist impedance diagrams obtained from the copper electrode exposed for 5 min, 2, 5 and 12 h to the aerated 0.5 mol dm3 H2 SO4 solutions at the respective corrosion potential. The corrosion potentials kept an approximate constant during impedance measurements, and the values of corrosion potentials corresponding to the different immersion time given above were 88, 81, 77 and 86 mV (vs. SCE) in proper order. The impedance spectra measured in the case of the short immersion time (5 min and 2 h) exhibited a Warburg impedance, indicating the corrosion process involved the transport of reactants from the bulk
Fig. 1. Nyquist impedance spectra for copper in 0.5 mol dm3 H2 SO4 at the corrosion potentials after 5 min, 2, 5, 12 h of immersion. The corrosion potentials corresponding to different immersion time were 88, 81, 77 and 86 mV in proper order.
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solution to the copper/solution interface or transport of soluble corrosion products from the interface to the bulk solution in the early stage of corrosion. The Warburg impedance disappeared and the impedance spectrum only displayed a depressed capacitive loop 5 h later; moreover, the size of capacitive loop significantly increased with the further increase of the immersion time. The former shows that the mass transport has little influence on the corrosion reaction after 5 h of immersion, whereas latter indicates that the corrosion resistance is increasing. The pure sulfuric acid and surfactant-containing sulfuric acid solutions used in our experiments were all aerated. Theoretically, copper can hardly be corroded in the deoxygenated dilute sulfuric acid [8,9], as copper cannot displace hydrogen from acid solutions according to theories of chemical thermodynamics [9]. However, this situation will change in aerated sulfuric acid. Dissolved oxygen may be reduced on copper surface and this will enable some corrosion to take place [8,10–13]. It is a good approximation to ignore the hydrogen evolution reaction and only consider oxygen reduction in the aerated sulfuric acid solutions at potentials near the corrosion potential, according to Smyrl [9]. Cathodic reduction of oxygen can be expressed either by a direct 4e transfer as shown by Eq. (1) O2 þ 4Hþ þ 4e ¼ 2H2 O
ð1Þ
or by two consecutive 2e steps involving a reduction to hydrogen peroxide first O2 þ 2Hþ þ 2e ¼ H2 O2
ð2Þ
followed by a further reduction [14] H2 O2 þ 2Hþ þ 2e ¼ 2H2 O
ð3Þ
The transfer of oxygen from the bulk solution to the copper/solution interface will strongly affect rate of oxygen reduction reaction despite how oxygen reduction takes place, either in 4e transfer or two consecutive 2e transfer steps: Dissolution of copper in sulfuric acid is described by the following two continuous steps: K1
Cu e () CuðIÞads K1
K2
ðfast stepÞ
CuðIÞads e ! CuðIIÞ ðslow stepÞ
ð4Þ ð5Þ
where Cu(I)ads is an adsorbed species at the copper surface and does not diffuse into the bulk solution [9,15–19]. Consequently, the mass transport has little influence on dissolution of copper. As mentioned above, the appearance of the Warburg impedance at the corrosion potential in aerated sulfuric acid should be attributed to oxygen transport from the bulk solution to the copper surface. The changes of impedance spectra in size and shape with extension of the immersion time, including disappearance of the Warburg impedance and the increase of capacitive loop in size, show that a barrier gradually forms on the copper surface.
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The barrier is probably related to formation of the salt film of Cu(II) ions, as Cu2 O and CuO cannot exist in strong acidic solution. 3.2. Corrosion inhibition by surfactants 3.2.1. EIS Corrosion inhibition of copper by the four surfactants of different types was investigated. The four surfactants do not lead to the significant shift of copper corrosion potentials. Before each impedance measurement, the corrosion potential of copper was measured and all values of corrosion potentials in surfactant-containing solutions are marked in the plots shown by Fig. 2. Fig. 2 shows the complex plane impedance displays of copper at the corrosion potentials after various exposure times to 0.5 mol dm3 H2 SO4 solutions containing different surfactants. Obviously, the presence of the surfactants leads to changes of the impedance diagrams in both shape and size. In addition, the impedance display of copper in the presence of the surfactants depends on the immersion time. By comparing the Nyquist spectra obtained after 5 min of immersion in the presence or absence of the surfactants, it is found that the Warburg impedance disappeared in surfactant-containing solutions, even though concentration of the surfactants was relatively low (1:0 104 mol dm3 ), no matter which surfactant was used as inhibitor. Taking the impedance diagrams measured in the presence of 1:0 104 mol dm3 CTAB at different immersion time as a example (see Fig. 2(a)), it is found that, the
Fig. 2. Complex plane plots of the impedance data measured at respective corrosion potential after the copper electrode was exposed for different time to the 0.5 mol dm3 H2 SO4 solutions with surfactants of different types. The value of corrosion potential for each measurement was marked in the plots.
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size of the capacitive arc first increased with the immersion time up to the maximum in 2 h (see ‘‘ ’’ symbol ), and then decreased with further increase of immersion time until it reduced to almost the same size 24 h later (see ‘‘’’ symbol) as it was measured after 5 min of immersion. The same phenomena were also observed in the sulfuric acids containing three other surfactants. Especially in the solution with SDS, the size of capacitive arc more significantly reduced with immersion time 2 h later. When the immersion time was over 12 h, it became even smaller than that obtained after 5 min of immersion (see Fig. 2(c)). Usually, the charging–discharging process of the electric double layer is a very rapid process and the capacitive loop seen in the highest frequency is therefore associated with the double layer relaxation [20]. The impedance spectrum displaying a high frequency capacitive loop and the Warburg impedance in the low frequency (see Fig. 1(a)) can be analyzed with the equivalent circuit in Fig. 3(a) [21], in which Rt represents the charge-transfer resistance, W the Warburg impedance and Rs the solution resistance. One constant phase element (CPE) is substituted for the capacitive element to give a more accurate fit [22], as most capacitive loops are depressed semi-circles rather than regular semi-circles. The impedance spectra displaying one capacitive loop may be analyzed with the electrical circuit in Fig. 3(b) [21]. The physical meaning of Rs , Rt and CPEdl is the same with what they express in Fig. 3(a). The CPE is a special element whose immittance value is a function of the angular frequency, x, and whose phase is independent of the frequency. Its admittance and impedance are, respectively, expressed as YCPE ¼ Y0 ðjxÞ
a
ð6Þ
and ZCPE ¼ ð1=Y0 ÞðjxÞa
ð7Þ
where Y0 is the magnitude of the CPE, x the angular frequency and a the exponential term of the CPE [22–25]. Values of a are usually related to roughness of electrode surface. The smaller value of a, the higher the surface roughness of an electrode [22,24]. Considering that a CPE may be considered as a parallel combination of a pure capacitor and a resistor that is inversely proportional to the angular frequency, the value of capacitance, Ci , can therefore be calculated for a parallel circuit composed of a CPE (Qi ) and a resistor (Ri ), according to the following formula [22]: Y0i ¼ ðCi Ri Þai =Ri
ð8Þ
CPEdl
CPEdl Rs
Rs Rt
W (a)
Rt (b)
Fig. 3. (a) The equivalent circuit to fit the EIS for copper displaying a Warburg impedance; (b) the equivalent circuit to fit the EIS for copper displaying one capacitive loop.
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Table 1 Fitting the EIS for copper in 0.5 mol dm3 H2 SO4 after 2 h immersion by the equivalent circuit shown in Fig. 3(a) Immersion time (h)
Rt (X cm2 )
Rs (X cm2 )
Cdl (lF cm2 )
W (X cm2 )
2
1:14 103
0.71
3.93
5:59 105
Values of the elements of the equivalent circuit.
The impedance spectrum of copper in 0.5 mol dm3 H2 SO4 after 2 h of immersion was analysed by using the circuit in Fig. 3(a), and the double-layer capacitance (Cdl ) was calculated in terms of Eq. (8). Values of elements fitted and that of Cdl calculated are listed in Table 1. Similarly, all impedance spectra shown in Fig. 2 were fitted by the circuit shown in Fig. 3(b) and values of elements of the circuit corresponding to different corrosion systems, including values of Cdl , are listed in Table 2. The charge-transfer resistance, Rt , reflects the corrosion rate of a metal in corrosive solutions. The smaller Rt , the faster the corrosion reaction rate. Accordingly, Table 2 Fitting all impedance spectra in Fig. 2 by the equivalent circuit shown in Fig. 3(b) Cdl (lF cm2 )
IE (%)
0.5 mol dm3 H2 SO4 þ 1:0 104 mol dm3 CTAB 0.64 5 min 8:12 103 2h 1:36 104 0.62 5h 9:81 103 0.64 0.64 7h 1:02 104 24 h 8:34 103 0.62
12.3 12.4 14.1 14.6 15.9
96.1 97.6 96.8 96.9 96.2
0.5 mol dm3 H2 SO4 þ 1:0 104 mol dm3 SO 5 min 2:82 103 2h 6:71 103 5h 5:27 103 7h 5:32 103 12 h 5:03 103 24 h 4:83 103
0.65 0.66 0.63 0.73 0.74 0.74
10.5 8.25 7.72 7.54 7.54 7.31
88.7 95.3 93.9 94.0 93.7 93.4
0.5 mol dm3 H2 SO4 þ 1:0 104 mol dm3 SDS 5 min 3:78 103 0.51 0.61 2h 5:14 103 5h 4:27 103 0.64 7h 3:97 103 0.64 12 h 3:22 103 0.64 0.60 24 h 2:42 103
9.40 9.90 10.6 11.2 12.3 13.1
91.6 93.8 92.5 92.0 90.1 86.9
0.5 mol dm3 H2 SO4 þ 1:0 104 mol dm3 TWEEN-80 5 min 2:63 103 0.12 2h 4:66 103 0.13 5h 4:83 103 8:75 102 3 6:89 102 24 h 2:99 10
9.60 8.58 11.9 9.26
87.9 93.2 93.4 89.3
Immersion time
Rt (X cm2 )
Rs (X cm2 )
Values of the elements of the equivalent circuits in Fig. 3(b) and IEs of the four surfactants on the copper corrosion.
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the inhibition efficiency (IE) of the surfactants on copper corrosion is calculated from the following formula [26]: IE ¼ ðRt R0t Þ=Rt 100
ð9Þ
where Rt is the charge-transfer resistance in the inhibited solutions and R0t the one in the uninhibited solutions, and IE values calculated are also listed in Table 2. Obviously, CTAB is the most effective inhibitor, SO the second, TWEEN-80 the third and SDS the worst, among the four surfactants. The impedance spectra in the surfactant-containing solutions at the corrosion potential do not display new low frequency capacitive or inductive loops, implying that the four surfactants are all the mixed-type inhibitors to copper corrosion in sulfuric acid. Their inhibition action is realized by the geometric coverage of surfactants on the surface of copper. In this way, IE can also be considered as the surface coverage by the surfactant molecules, h. Fig. 4 describes dependence of h on the immersion time for each surfactant. 3.2.2. Polarization curves Fig. 5 represents the polarization curves for the copper electrode in 0.5 mol dm3 H2 SO4 solutions with and without surfactants. The addition of two surfactants (CTAB and TWEEN-80) in 0.5 mol dm3 H2 SO4 solution shifts respectively the corrosion potential of copper cathodically and anodically to a small extent, but markedly lowers both anodic and cathodic current densities. The corrosion potential
Fig. 4. The dependence of the surface coverage by the different surfactants (h) on the immersion time.
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Fig. 5. Polarization curves for the copper electrode in 0.5 mol dm3 H2 SO4 solutions with and without surfactants at 22 °C.
and corrosion current density for the three corrosion systems were determined by the Tafel extrapolation method. The values of corrosion current density obtained are, respectively, 1:71 103 mA cm2 in the surfactant-free H2 SO4 solution, 1:69 104 mA cm2 in CTAB-containing solution and 2:65 104 mA cm2 in the presence of TWEEN-80. Based on the following formula [26]: IEð%Þ ¼
icorr i0corr 100 icorr
ð10Þ
where icorr and i0corr are, respectively, the corrosion current density in the absence or presence of surfactants, the IE of CTAB and TWEEN-80 to copper corrosion after 2 h of immersion was calculated and values of IE are 90.1% and 84.5%, respectively, in approximate agreement with values of IE obtained by using Eq. (9). 3.3. The pzc of copper in the sulfuric acid Adsorption of surfactants on a corroding metal depends mainly on the charge of the metal surface, the charge or the dipole moment of surfactants, and the adsorption of other ionic species if it is electrostatic in nature [5]. The potential of zero charge (pzc) plays a very important role in the electrostatic adsorption process. The pzc data of solid metallic electrode have been limited so far. The pzc for copper in 0.01 mol dm3 Na2 SO4 is 214 mV vs. SCE according to Bockris and ReddyÕs
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report [27], whereas the pzc for copper in H2 SO4 solutions has not been found in references. The first capacitive loop in high frequency range usually originates from the relaxation of the double-layer capacitance under the conditions (i) there are no rapid Faradaic pseudo-capacitive processes which can be coupled with the double layer relaxation [20] and (ii) there are no the specific adsorption of ions. The two conditions can be met in sulfuric acid solution. Thus, EIS offers a good method to determine the pzc of copper in sulfuric acid. We have measured a series of impedance spectra for the copper electrode in 0.5 mol dm3 H2 SO4 solution in a wide potential range between 512 and 20 mV (vs. SCE), analyzed these impedance spectra by means of IM6 impedance analysis software and plotted the double-layer capacitance against potential (see Fig. 6) in the light of Luo and co-workersÕ method [5]. The pzc of copper in 0.5 mol dm3 H2 SO4 is estimated to be 140 mV from the dependence of the double-layer capacitance on potential shown in Fig. 6, which is more negative than the corrosion potential (96 mV). This means that the copper surface is positively charged at the corrosion potential. If only considering the electrostatic attraction, electrostatic adsorption of anionic surfactant, SDS and SO, is favoured, while it is unlikely for positive quaternary ammonium ions to directly adsorb on the surface. The reason that CTAB gives the highest IE is related to the synergistic effect resulting from the specific adsorption of bromide ions at the copper/solution interface [28].
Fig. 6. Double-layer capacitance of copper as a function of potential in 0.5 mol dm3 H2 SO4 solution.
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3.4. The adsorption model of surfactants on the copper surface 3.4.1. SDS The inhibitive action of SDS in sulfuric acid solution results from physical (electrostatic) adsorption of the negatively charged C12 H25 SO 4 to the positively charged copper surface, forming a barrier on the copper surface. On the basis of Luo and co-workersÕ model, the IE should increases continuously with the immersion time since more C12 H25 SO2 4 ions will electrostatically adsorb on the copper surface with the increase of immersion time. However, the experimental results were not so, and the inhibition action of SDS decreased with further increasing the immersion time 2 h later. It is reported that diameter of the head of a C12 H25 SO 4 ion is 0.5 nm, and length of the hydrocarbon chain is 2.1 nm [29]. The orientation of adsorbed C12 H25 SO 4 ions on the surface greatly affects the inhibitory efficiency of SDS. We believe that the chemisorption of n-dodecyl and the electrostatic attraction of the C12 H25 SO 4 ions on copper surface take place simultaneously in the early stage, as shown in Fig. 7(a). This type of form of adsorption is favourable for free energy change of transferring hydrocarbon chains from water to the surface, by means of which the adsorbed C12 H25 SO 4 ions can cover much area, thereby inhibiting more effectively the copper corrosion. However, when more C12 H25 SO 4 ions electrostatically adsorb on the surface, the adsorption density of surfactant becomes so high that interaction between tails of C12 H25 SO 4 will occur through van der Waals force. The hydrocarbon chains of many adsorbed ions are believed to leave the surface and aggregate to form hemimicelle (see Fig. 7(b)). This will cause the decrease of the effective area covered by C12 H25 SO 4 ions to some extent. In this way, the IE of SDS first increases
(a)
-
-
-
-
+ + + + + + + + + + ++ + (b)
- - -
-
-
- -
-
the copper/solution interface SDS Fig. 7. Illustration of adsorption of and C12 H25 SO 4 ions at copper/solution interface. (a) Adsorption as single ions in the case of short immersion time; (b) hemimicelle formation at some sites of the surface in the case of long immersion time.
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with the immersion time, reaching a maximum at certain time, and then decreases with further increasing the immersion time. 3.4.2. CTAB The halides have been proved to be the most effective derivatives as they increase the inhibiting tendency of positive quaternary ammonium ion by the well known synergistic effect [28,30,31]. Addition of CTAB will introduce bromide ions to the sulfuric acid solutions. Bromide ions will first adsorbed at copper/solution interface through electrostatic attraction force because of the excess of positive charge at the copper/solution interface at the corrosion potential, leading to that the change on the solution side of the interface changes from positive to negative. Thus, the quaternary ammonium cations, C16 H33 N(CH3 )þ 3 ions, can be electrostatically adsorbed on the copper surface covered with primarily adsorbed bromide ions. The adsorption of CTAB at the copper/solution interface cannot be simply consider as an electrostatic adsorption attributed to the synergistic effect of bromide ions, the chemisorption of n-cetyl on copper surface must be considered at the same time. Adsorption model of CTAB, as shown by Fig. 8(a), is similar to that of SDS. The only difference between them is that C16 H33 N(CH3 )þ 3 ions electrostatically adsorb on the copper surface covered with primarily adsorbed bromide ions. The orientation change of the adsorbed C16 H33 N(CH3 )þ 4 ions on the copper surface with the immersion time is also similar to what C12 H25 SO 4 ions do (see Fig. 8(b)). þ Compared with C12 H25 SO ion, the head of C H N(CH 16 33 3 )4 ion is larger and the 4 hydrocarbon is longer. Consequently, the IE of CTAB is much higher than that of SDS. (a)
+ + + + - + + + + + + + + + + + + + (b)
+ + + + ++ + + - - -- + + + + + + + + + + + + + ++ - Fig. 8. Illustration of adsorption of and C16 H33 (CH3 )þ 3 ions at copper/solution interface. (a) Adsorption as single ions in the case of short immersion time; (b) hemimicelle formation at some sites of the surface in the case of long immersion time.
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3.4.3. SO Most SO transforms into oleic acid (CH3 (CH2 )7 CH@CH(CH)7 COOH) in sulfuric acid solution. In fact, the oleic acid is in the form of colloid precipitate due to its quite low solubility. It is reported that colloid precipitates of oleic acid are charged positively [5] when pH value of solution is below 3. Since the copper surface is charged positively, it is impossible for the colloid particles to adsorb on the surface directly through electrostatic attraction. Oleate ions (C17 H33 COO ) with negative charge in the solution will first adsorb on the copper surface through the electrostatic attraction force. The chemisorption of oleic acids on the surface should also take place at the same time according to Luo et al. [5]. Then, the colloid particles may adsorb on the copper surface covered with oleic acids via van der Waals force. The dependence of IE of OS on the immersion time suggests that there exists the orientation change of the adsorbed oleate ions or oleic acid with the immersion time, like what the adsorbed C12 H25 SO 4 ions do in Fig. 7(b). That SO exhibited the higher IE than SDS did is probably related to the longer hydrocarbon chain and presence of ‘‘–CH@CH–’’ group in the SO molecule. ‘‘–CH@CH–’’ group provides p-bond orbital. The p-bond orbital adsorption will certainly occur during adsorption of oleate ions or oleic acid at copper/solution interface, which is very advantageous to enhancement of IE of SO.
3.4.4. TWEEN-80 TWEEN-80 is a nonionic surfactant with the longer hydrocarbon chains and ring groups, whose molecular structure is shown in Fig. 9. Unlike SDS and CTAB, the TWEEN-80 molecules do not adsorb on the copper through the electrostatic attraction forces. Therefore, the inhibitive action to copper is attributed to its chemisorption at copper/solution interface. Considering copper surface is hydrophilic, TWEEN-80 may chemisorb at copper/solution interface via hydrogen bond between the –OH groups in TWEEN-80 molecules and the water molecules adsorbed on the surface. The adsorption strength of TWEEN-80 on copper surface should be lower than that of SDS since adsorption of the former can only realized via hydrogen bond. Therefore, its inhibition is as low as that of SDS despite the fact although its molecular area is larger than that of SDS.
O H2C HO(CH2CH2O)w HC
(OCH2CH2)xOH
O
CH-CH-CH2(OCH2CH2)yOC-C17H33 CH-(OCH2CH2)zOH ω +x+y+z=21-26
Fig. 9. Structure scheme of the TWEEN-80 molecule.
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3.5. Effect of temperature on IE of CTAB Fig. 10(a)–(c) represent respectively the influence of immersion time on the impedance spectra of copper electrode in 0.5 mol dm3 H2 SO4 þ 1:0 104 mol dm3 CTAB solution at 30, 40 and 50 °C. In all cases the Nyquist diagram shows a depressed semi-circles. By comparing the impedance spectra in Fig. 2(a) with those in Fig. 10(a)–(c), it is seen that the diameter of semi-circle reduces with increasing the
Fig. 10. Complex plane plots of the impedance data obtained from the copper electrode exposed for different time to 0.5 mol dm3 H2 SO4 þ 1:0 104 mol dm3 CTAB solutions at 30, 40 and 50 °C. All impedance measurements were carried out at corrosion potentials.
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temperature under otherwise identical conditions. The variation of impedance spectra in size with increasing immersion time at 30 and 40 °C is very similar to that occurring at 22 °C shown by Fig. 2(a). At the higher temperature (40 °C), the diameter of semi-circles begins to decline with the immersion time after 30 min of immersion. The experimental phenomena shown in Fig. 10 are associated with change of orientation of adsorbed C16 H33 N(CH3 )þ 3 ions on copper surface. The higher the temperature, the shorter the time required for the transition of n-cetyl group from lying on the surface to being normal to the surface.
4. Conclusion The four surfactants, CTAB, SDS, OE and TWEEN-80, regardless of the surfactant type, inhibit the copper corrosion to some extent in acidic solutions. CTAB exhibit the highest IE among these surfactants. All impedance spectra do not display new low frequency capacitive or inductive loop at the corrosion potentials, indicating that the surfactants act as the mixed-type inhibitor against copper corrosion in sulfuric acid. The copper surface is positive charged in sulfuric solutions at corrosion potential, which is favourable for adsorption of anionic surfactants and unfavourable for adsorption of cationic surfactants. The reason why CTAB can inhibit most effectively the copper corrosion is ascribed to the synergistic effect between bromide anions and C16 H33 N(CH3 )þ 4 positive ions. The immersion time of the electrode have a great influence on the IE of the surfactants. The surfactant ions may adsorb on the copper surface through both chemisorption and electrostatic attraction when the immersion time is short, by means of which surfactant molecules can cover more sites on the copper surface, giving rise to the higher IE. On the contrary, more surfactant molecules adsorbed on the copper surface will leave the surface and aggregate to form hemimicelle, leading to reduction of the effective coverage by the surfactant molecules, when the immersion time is long. Thus, the IE decreases with the immersion time despite the fact more the surfactant molecules adsorb at copper/ solution interface.
Acknowledgements This project was supported by the Visiting Scholar Foundation of Key Laboratory in Shandong University, the Chinese National Natural Science Fund (20173033) and Special Funds for the Major State Basic Research Projects (G 19990650).
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