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Studies of the corrosion inhibition of copper in Na2SO4 solution using polarization and electrochemical impedance spectroscopy
Materials Chemistry and Physics 121 (2010) 70–76
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Studies of the corrosion inhibition of copper in Na2 SO4 solution using polarization and electrochemical impedance spectroscopy E. Hamed ∗ Chemistry Department, Faculty of Science, Ain Shams University, Abbassia 11566, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received 20 May 2009 Accepted 24 December 2009 Keywords: Copper Potassium folate Corrosion inhibition EIS
a b s t r a c t Inhibition effect of different concentrations of potassium folate (green inhibitor) on copper corrosion in aerated Na2 SO4 solutions in the temperature range (15–55 ◦ C) was investigated by using potentiodynamic polarization and electrochemical impedance spectroscopy technique. The polarization curves showed that the addition of potassium folate inhibits copper corrosion as a result of physical adsorption of folate anions on the copper surface. The folate anion acts as an anodic-type inhibitor. The inhibition efficiency increases with increasing the folate anion concentration but decreases with increasing Na2 SO4 concentration and temperature. The adsorption of folate anions on copper surface follows Flory–Huggins isotherm and kinetic–thermodynamic model. The thermodynamic parameters for corrosion and adsorption processes in the absence and presence of the inhibitor were calculated and discussed. Effect of immersion time on the performance of the inhibitor was also tested in this investigation. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Copper has an excellent electrical and thermal conductivities, good corrosion resistance and mechanical workability. It is widely used in heating and cooling systems. Corrosion of copper can lead to many problems, the most being per formation that may result in coolant leakage. Scales and corrosion products have negative influence on heat-transfer, causing a decrease in heating efficiencies of the copper structures [1]. Thus, corrosion of copper and copper alloys and their inhibition in aqueous salt solutions have attracted the attention of a member of investigators [1–7]. The use of inorganic or organic inhibitors is one of the most practical methods for protection against corrosion of metals and their alloys. Most well known organic inhibitors such as those containing nitrogen, sulphur, oxygen atoms and aromatic rings [8–12]. To be effective an inhibitor must displace water molecules from the metal surface, interact with anodic and/or cathodic reaction sites to retard the partial oxidation and reduction corrosion reaction and prevent transportation of water and corrosive-active species to
∗ Tel.: +20 226822724; fax: +20 224831836. E-mail address: dr science [email protected]. 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.12.044
the surface [13,14]. In general, in aqueous solutions, the inhibitory action of organic inhibitors may be due their physical (electrostatic) adsorption or chemisorption onto the metal surface, depending on the charge of the metal surface, the electronic structure of organic inhibitor and the nature of the medium [15,16]. Several organic inhibitors for copper corrosion in aqueous sulphate solution at various pHs can be found in the literature [17–21]. However, the new environmental restrictions need to use organic inhibitors, acceptable from the environmental point of view (green inhibitors) [22]. The present work was aimed to find environment friendly non-toxic compound and inhibits copper corrosion in Na2 SO4 solution. Potassium folate is environmentally friendly compound and has applications in biological and pharmaceutical fields [23]. For this reason, we investigated the inhibition property of this compound using potentiodynamic polarization and electrochemical impedance spectroscopy measurements under different concentrations of both potassium folate and sodium sulphate, temperature and immersion time.
E. Hamed / Materials Chemistry and Physics 121 (2010) 70–76
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The structure of potassium folate is:
2. Experimental The working electrode used in the present work was made of spice pure cylindrical copper rod, welded with Cu wire for electrical connection and mounted into suitable glass tube using epoxy resin so that its cross-sectional area (0.5 cm2 ) was in contact with the test solution. The exposed area of the electrode was polished using emery papers of 1/0, 2/0, 3/0, washed with distilled water, degreased using pure alcohol, and bidistilled water and finally dried. A platinum wire was used as auxiliary electrode. A saturated calomel electrode (SEC) was used as reference electrode. The SCE was connected via a Luggin capillary, the tip of which was very close to the surface of the working electrode to minimizing the IR drop. All potentials are referred to SCE. The experiments were performed in Na2 SO4 solutions without and with different concentrations of potassium folate as inhibitor. All solutions were freshly prepared from analytical grade chemical reagents using doubly distilled water and were used without further purification. Each run was carried out at the required temperature (±1 ◦ C) using water thermostat. The potentiodynamic polarization curves were carried out at a scan rate of 2 mV s−1 . Impedance experiments were conducted using ac signal of amplitude mV peak to peak at open circuit potential (OCP) in the frequency range 0.5 Hz to 30 kHz. A potnetiostat/galvanostat EG & G model 273 lock-in amplifier (model 5210) and M352 corrosion software and M398 impedance software from EG&G Princeton Applied Research were used for polarization and impedance, respectively.
3. Results and discussion 3.1. Potentiodynamic polarization Potentiodynamic polarization curves of copper electrode in aerated sodium sulphate 1 M Na2 SO4 (blank) solution without and with various concentrations (0.002–0.01 M) of potassium folate at different temperatures (15–55 ◦ C) were recorded. These curves were swept from −500 up to 500 mV (SCE) with scan rate of 5 mV s−1 . Fig. 1 is an example. Inspections of the obtained data reveal that at a given temperature, the addition of potassium folate to the blank solution increases both cathodic and anodic overpotentials and shifts the cathodic and anodic current densities to lower values but has little effect on cathodic polarization. Such behavior indicates that the addition of potassium folate retards mainly the anodic dissolution of copper. In aerated solution, the cathodic reaction is either oxygen and/or water reduction reaction according [1].
values are listed in Table 1. It is clear from these data that, at a given temperature, the presence of potassium folate decreases the corrosion current density Icorr and shifts Ecorr slightly to more positive values. These changes increase with increasing the additive concentration. This means that potassium folate acts as anodic-type inhibitor for copper corrsosion in Na2 SO4 solution. The values of ba and bc remain approximately unchanged in presence of different concentrations of potassium folate indicating that this inhibitor has no significant influence on the mechanism of corrosion of copper in Na2 SO4 solution. In aqueous solution, potassium folate dissociates and yields folate anions. It is suggested that the negative folate anions are preferentially adsorbed on positively charged copper surface [24] via electrostatic interaction (physical adsorption). The presence of two carboxylic groups in folate anion is expected to impart high adsorption. Also such adsorption will be increased owing to the presence of several adsorption centers (as N and O atoms in folate anion). The adsorption of folate anions results in the formation of protective film of adsorbate on the copper surface. This protective film separates the metal surface from the corrosive medium and hence inhibits copper corrosion. However, it is observed that, at a given folate concentration, the values of Icorr in uninhibited and inhibited solutions increase with the increase in temperature. These results depict that the rate of corrosion in the absence and presence of folate species increase
O2 + 2H2 O + 4e− = 4OH− and/or
2H2 O + 2e− = H2 + 2OH−
However, the anodic dissolution of copper can be represented as follows [1]: Cu = Cu+ + e− Cu+ = Cu2+ + e− Electrochemical parameters such as corrosion current density (Icorr ), corrosion potential (Ecorr ) anodic (ba ) and cathodic (bc ) Tafel slopes in all cases were calculated from Tafel plots. The calculated
Fig. 1. Effect of temperature on potentiodynamic polarization curves of copper electrode in 1 M Na2 SO4 solution containing 0.01 M pot. folate.
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E. Hamed / Materials Chemistry and Physics 121 (2010) 70–76
Table 1 The electrochemical parameters and the corresponding inhibition efficiency for corrosion of Cu electrode in 1 M Na2 SO4 solution in the absence and presence of different concentrations of the inhibitor and at different temperatures. Temperature ◦ C ◦
15
25◦
35◦
45◦
55◦
C mol L−1
Ecorr V
Icorr A cm−2
ba V dec−1
bc V dec−1
P%
000 0.002 0.004 0.006 0.01
−0.324 −0.290 −0.288 −0.282 −0.278
4.370 2.620 2.106 1.708 1.141
0.277 0.270 0.269 0.267 0.264
0.334 0.299 0.284 0.276 0.269
40.0 51.8 60.9 73.9
000 0.002 0.004 0.006 0.01
−0.308 −0.271 −0.264 −0.253 −0.226
4.964 3.514 2.909 2.412 2.224
0.394 0.303 0.286 0.277 0.270
0.408 0.342 0.310 0.294 0.282
29.2 41.4 51.4 55.2
000 0.002 0.004 0.006 0.01
−0.291 −0.259 −0.247 −0.238 −0.192
5.876 4.419 3.780 3.390 2.726
0.493 0.400 0.384 0.354 0.331
0.540 0.488 0.401 0.334 0.317
24.8 35.6 42.3 53.6
000 0.002 0.004 0.006 0.010
−0.287 −0.217 −0.204 −0.199 −0.178
7.928 6.517 6.020 4.773 4.427
0.521 0.480 0.428 0.399 0.374
0.583 0.520 0.448 0.433 0.395
17.8 20.3 39.8 44.2
000 0.002 0.004 0.006 0.010
−0.259 −0.202 −0.194 −0.178 −0.166
8.836 7.903 7.242 6.020 5.440
0.582 0.532 0.498 0.445 0.403
0.622 0.577 0.533 0.498 0.456
10.6 18.0 31.9 38.4
with increasing temperature. The values of Icorr for copper are higher in uninhibited solution than in inhibited solution revealing that folate anions inhibit the copper corrosion in the temperature range used. The effect of the concentration of Na2 SO4 on the potentiodynamic polarization in the absence and presence of fixed concentration of potassium folate (0.006 M) was investigated at 30 ◦ C. Values Icorr and Ecorr , ba and bc obtained from Tafel analysis method are given in Table 1. It is observed that in the absence and the presence of fixed concentration of the inhibitor, the value of Icorr increases and Ecorr shift towards more negative potentials with increasing Na2 SO4 concentration. The increase in the value of Icorr indicates that copper corrosion increases with increasing SO4 2− concentration. Since Icorr is directly proportional to the corrosion rate, the inhibition efficiency (P%) of folate anions, in all cases, was calculated using the equation:
P% = 1 −
Icorr o Icorr
× 100
(1)
o where Icorr and Icorr are the corrosion current densities in the absence and presence of potassium folate, respectively. The calculated values of P% are included in Table 1. Fig. 2 shows the relation between P% vs the logarithmic concentration of the inhibitor at different temperatures. The data reveal that the inhibition efficiency of folate ions enhances with increasing its concentration in the solution. The increase in inhibition efficiency observed at higher inhibitor concentration indicating that more inhibitors are adsorbed onto the metal surface, thus providing wider surface coverage. On the other hand, it is clear that the inhibition efficiency decreases with the increase in temperature. This trend is attributed to desorption of some inhibitor ions from the surface reflecting the physical nature of adsorption (physisorption). Fig. 3 illustrates the variation of P% vs the concentration of Na2 SO4 which containing fixed concentration of the inhibitor at 30 ◦ C. The data demonstrate that the inhibition efficiency of folate anions decreases with increasing the concentration of SO4 2− anions. This behavior may
be attributed to the greater aggressiveness of Na2 SO4 solution at higher concentrations. It seems that the adsorption of folate anions takes place on the metal surface in competition with the adsorption of SO4 2− anions. 3.2. Activation energies The corrosion reaction could be related to an Arrhenius-type of process according to the equation: Icorr = A expEa /2.303RT
(2)
where A is the Arrhenius pre-exponential factor, Ea is the apparent activation energy of corrosion process, T is the absolute temperature and R is the universal gas constant. The value of Ea was calculated from the slope of log Icorr vs 1/T for the uninhibited and inhibited solutions as shown in Fig. 4. The calculated values of Ea are given in Table 2. The value of Ea in the blank solution was 5.94 kJ mol−1 . The addition of potassium folate to the sulphate solution enhances Ea
Fig. 2. Variation of the inhibition efficiency (P%) with logarithmic concentrations of the inhibitor (log Cinhib ) for copper electrode in 1 M Na2 SO4 solution at different temperatures.
E. Hamed / Materials Chemistry and Physics 121 (2010) 70–76
73
Fig. 5. Transition state plots of log[Icorr /T] vs 1/T for copper in 1 M Na2 SO4 without and with different concentrations of pot. folate.
Fig. 3. Variation of the inhibition efficiency (P%) with the concentrations of Na2 SO4 containing constant concentration 0.006 M of pot. folate at 30 ◦ C.
given in Table 2. The positive sign of Ha reflects the endothermic nature of the dissolution process. The large and negative sign of Sa in the absence and presence of the inhibitor implying that the activated complex in the rate-determining step represents association rather than dissociation, meaning that a decrease in disorder takes place, going from reaction to he activated complex [26]. 3.3. Adsorption isotherms The inhibition action of many organic compounds is assumed to be assigned to their adsorption on metal surface. Therefore, investigation of adsorption isotherms can provide important information on the adsorption of folate anions on copper surface in Na2 SO4 solution. In an attempt to find the most suitable adsorption, the surface coverage ( = P/100) was subjected to different adsorption isotherms. It is found that the corrosion data fit well to Flory–Huggins and kinetic–thermodynamic model isotherms at all the temperatures used. The Flory–Huggins isotherm [27] is given by
Fig. 4. Arrhenius plot of log corrosion (Icorr ) vs 1/T for copper in 1 M Na2 SO4 without and with different concentrations of pot. folate.
and the extent of the enhancement is proportional to the inhibitor concentration implying that the energy barrier for corrosion process enhances with inhibitor concentration. This observation further supports the proposal physical adsorption mechanism. The adsorbed inhibitor species block the surface sites with lower Ea values and pushed the corrosion reaction to surface sites with higher Ea values [25]. The activation thermodynamic parameters of copper dissolution namely, the enthalpy of activation (Ha ) and entropy of activation (Sa ) can be obtained from the transition state equation: Icorr =
RT exp(Sa /R) exp(−Ha /RT ) Nh
log
= log xKads + x log(1 − ) C
(4)
and the kinetic–thermodynamic model isotherm [28] is given by log
= log K + y log C 1−
(5)
where Kads is the adsorption constant, x is the number of surface water molecules replaced by one inhibitor species, C is the inhibitor concentration in the bulk of the solution, (1/y) is the number of active sites occupied by one inhibitor species and [(K )1/y = Kads ]. Curves fitting folate anions to the Flory–Huggins isotherm equation (4) is shown in Fig. 6 where straight lines were obtained by plotting log(/c) vs log(1 − ). Fig. 7 shows the curves fitting on
(3)
where N and h are Avogadro’s number and Plank’s constant, respectively. A straight line was obtained when log(Icorr /T) was plotted vs 1/T as shown in Fig. 5. From the slope and intercept of the straight line plot, the values of Ha and Sa , respectively were obtained and Table 2 The values of activation parameter Ea , Ha and Sa for Cu electrode in 1 M Na2 SO4 solution in the absence and presence of different concentrations of the inhibitor at 30 ◦ C. C (M) 000 2 × 10−3 4 × 10−3 6 × 10−3 1 × 10−2
Ea (kJ mol−1 )
Ha (kJ mol−1 )
Sa (J mol−1 K−1 )
5.94 8.098 10.642 12.19 13.92
11.13 18.58 22.58 24.72 26.42
−136.4 −114.58 −102.91 −97.70 −93.29
Fig. 6. Flory–Huggins isotherm model of folate anions on copper surface at different temperatures.
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E. Hamed / Materials Chemistry and Physics 121 (2010) 70–76
Fig. 7. Thermodynamic–kinetic model of folate anions on copper surface at different temperatures.
folate anions to the kinetic equation (5) and the linear relationship between log(/1 − ) vs log C. The values of Kads , x and (1/y) were calculated from these two isotherms and are given in Table 3. A good agreement between the two isotherms concerning the values of Kads was obtained. It is clear that the values of Kads are relatively small and decrease with increasing temperature indicating that the interactions between the adsorbed inhibitor and the metal surface are weakend and consequently, the adsorbed inhibitors become easily desorbed. These data explain the decrease in the inhibition efficiency with increasing temperature. The number of x (obtained from Flory–Huggins isotherm) and 1/y (obtained from kinetic isotherm) are approximately equal one under all conditions investigated. This suggests that each folate anion adsorbs occupied one active sit and replace an adsorbed water molecule. The free energy of adsorption Gads on copper surface can be calculated from the equation [29] Kads =
1 exp(Gads /RT ) 55.5
(6)
where 55.5 is the molar concentration of water in the solution. The calculated values of Gads are given in Table 3. The negative sign of Gads implies spontaneous adsorption. The values of Gads are around 20 kJ mol−1 indicating electrostatic interaction between the negatively charged folate ions and the positively charged copper surface (physisorption). 3.4. Impedance measurements Electrochemical impedance spectroscopy (EIS) measurements were conducted to give more insight on the inhibition behaviour of potassium folate on copper corrosion in Na2 SO4 solution. Nyquist plots recorded for copper in 1 M Na2 SO4 at OCP without and with various concentrations of the inhibitor and temperatures are shown in Figs. 8 and 9, respectively. In all cases, only on depressed capacitive semicircle has been observed, indicating that corrosion of copper in uninhibited and inhibited solutions is under chargetransfer control. These semicircles are assigned to the time constant of charge-transfer and to double layer capacitance. The semicircles at high frequencies are generally associated with the relaxation of the capacitors of electrical double layers with their diameters represents the charge-transfer resistances [30,31]. From these Nyquist plots, the values of the charge-transfer resistance (Rct ) were Table 3 Adsorption constant (Kads ), change of free energy (Gads ), number of active sites (x) Flory–Huggins kinetic–thermodynamic model. Temperature (K) x 288 298 308 318 328
Kads
Fig. 8. EIS diagrams of copper at Ecorr in 1 M Na2 SO4 in the absence and presence of different concentration of pot. folate at 30 ◦ C.
Gads (kJ mol−1 ) 1/y
1.16 248.9 −22.83 0.95 161.7 −21.79 1.17 121.5 −21.11 1.23 82.6 −20.19 1.48 62.3 −19.51
0.98 1.05 0.99 1.00 1.09
Kads
Gads (kJ mol−1 )
276.9 160.9 118.5 81.03 57.00
−23.08 −22.54 −22.51 −22.23 −20.31
Fig. 9. EIS diagrams of copper at Ecorr in 1 M Na2 SO4 + 0.006 M pot. folate at different temperatures.
obtained from the difference in real component (Z ) of impedance at lower frequencies. Also the double layer capacitances (Cdl ) were calculated by the Eq. (7) [31]. Cdl = (2fmax Rct )
−1
(7)
where fmax is the frequency value at which the imaginary component (Z ) of impedance is maximum. The calculated values of Rct and Cdl are listed in Tables 4 and 5. The equivalent circuit model employed for analyzing Nyquist responses is shown in Fig. 10. In this circuit a constant phase element (CPE) was used in place of a capacitor to compensate for inhomogeneity in the system [32]. The circuit model consisted of the solutions resistance Rs , the charge-transfer resistance of the interfacial corrosion Rct and the constant phase element CPE. Table 4 Effect of inhibitor concentration on impedance parameters and the corresponding inhibition efficiency for copper in 1 M Na2 SO4 at 30 ◦ C. Conc. folate
Cdl × 10−4 F
Rct × 103 ohm cm
P%
0 0.002 0.004 0.006 0.008 0.01
3.974 2.773 2.374 2.216 1.921 1.71
1.012 1.589 1.674 1.803 2.071 2.311
36.73 40.3 44.24 51.65 56.97
E. Hamed / Materials Chemistry and Physics 121 (2010) 70–76
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Table 5 Effect of temperature on impedance parameters and their corresponding inhibition efficiency for copper in 1 M Na2 SO4 containing 0.006 M pot. folate at different temperatures. Temp. C0
1 M Na2 SO4 0
1 M Na2 SO4 + 0.006 M pot. folate −2
Cdl (blank) (F cm 15 25 35 45 55
2.239 2.407 4.005 4.761 5.15
)
−2
0
Rct (blank) (ohm cm 2.332 2.091 1.471 0.979 0.686
P% = 1 −
Rct 0 Rct
× 100
Cdl (F cm 0.816 1.164 2.291 2.833 3.190
The value of the frequency power n of CPE used obtained by computer is close to 0.9 and consequently the CPE can be assumed to correspond to capacities behaviors. In Figs. 8 and 9 symbols represent the actual data while the solid lines represent the best fit using the equivalent circuit shown in Fig. 10. The data, reveal that the addition of folate inhibitor to the sulphate solution at any given temperature enhances the value of Rct but reduces the value of Cdl . These changes in the impedance parameters increase with increasing the inhibitor concentration. The increase in the value of Rct is ascribed to the adsorption of folate anions and displace the water molecules and other anions originally adsorbed on the copper surface. Such process may suggest the formation of protective film of inhibitor on the metal surface. This protective film impedes the charge-transfer across the metal/solution interface. On the other hand, the decrease in the value of Cdl could be related to a decrease in the local dielectric constant and/or an increase in the thickness of the electrical double layer, suggests that inhibitor acts by adsorption on the metal/solution interface [33]. On the other hand, the diameter of the Nyquist loop decreases with increasing temperature as a result of increasing the rate of corrosion and desorption of some folate anions from the copper surface and consequently decreases the surface coverage. These changes in the impedance parameters associated the addition of the inhibitors confirm the suggestion that folate anions act as adsorption inhibitor. Since the value of (1/Rct ) is directly proportional to the corrosion rate, the inhibition efficiency (P%) of folate anions for copper in 1 M Na2 So4 solution were calculated from Rct values obtained from impedance data at different inhibition concentrations and temperature using the following equation [34].
)
−2
(8)
0 and R are the charge-transfer values without and with where Rct ct inhibitor, respectively. The calculated values of P% are given in Tables 4 and 5. It I found that P% increases with the inhibitor concentration but decreases with increasing temperature. It is clear that inhibition efficiencies obtained from ac impedance agree well with those obtained from potentiodynamic polarization.
Fig. 10. Electrical equivalent circuit representing copper/Na2 SO4 without and with inhibitor.
)
P% −2
Rct (ohm cm 6.401 4.325 2.572 1.645 1.107
) 63.57 51.65 42.8 40.5 38.05
Fig. 11. Variation of Rct with the immersion time for a copper electrode in 1 M Na2 SO4 solution containing 0.006 M pot. folate at 30 ◦ C.
The effect of immersion time on the performance of potassium folate as an inhibitor for corrosion of copper in Na2 SO4 solution was tested via EIS measurements. The experiments were carried out in 1 M Na2 SO4 containing 0.006 M potassium folate at OCP and 30 ◦ C. Nyquist plots were recorded after certain time intervals up to 1320 min (obtained Nyquist plots are not shown here). Only one semicircle was observed in all cases. The semicircle diameter depends on the immersion time, initially, the diameter increases considerably with immersion time and then tends to achieve a steady state value. The impedance parameters Rct and Cdl were calculated from the Nyquist plots. Figs. 11 and 12 show the variation of both charge-transfer resistance and double layer capacitance values against the immersion time. It is clear from Figs. 11 and 12 with respect that Rct increased and Cdl decreased rapidly during the initial 180 min and then they tend to attain steady state values afterward. The increase in Rct and the decrease in Cdl indicates the continuous adsorption of the inhibitor and enhancement of the surface coverage and hence the inhibition efficiency of folate ions increases. It seems that after 180 min, the protection film becomes perfectly covered the electrode surface and hence the inhibition efficiency remains high and nearly constant up to 1320 min.
Fig. 12. Variation of Cdl with the immersion time for a copper electrode in 1 M Na2 SO4 solution containing 0.006 M pot. folate at 30 ◦ C.
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E. Hamed / Materials Chemistry and Physics 121 (2010) 70–76
4. Conclusions • Potassium folate (non-toxic) has been tested as an inhibitor for copper corrosion in Na2 SO4 solution using potentiodynamic polarization and EIS measurements. • Potentiodynamic polarization curves indicated that the presence of potassium folate inhibits the corrosion of copper in this medium. Inhibition of copper corrosion is attributed to the adsorption of folate anions on the copper surface and formation of barrier film thus suppress the copper corrosion. This inhibitor acts as an anodic-type inhibitor. • The activation energy Ea , enthalpy of activation and entropy of activation Sa for copper dissolution were calculated in the absence and presence of folate anions. The presence of the inhibitor enhances the thermodynamic activation parameters. • Adsorption of the inhibitor anion on copper surface obeys Flory–Huggins isotherm and kinetic–thermodynamic model. The thermodynamic parameters for involves electrostatic interaction between the charged inhibition anion and the charged metal surface. • The EIS measurements have also confirmed the inhibition action of folate anions. • The inhibition efficiency of folate anion increases with increasing folate concentration and time of immersion but decreases with increasing the Na2 SO4 concentration and temperature. • All results obtained from potentiodynamic polarization and from EIS measurements are in good agreement. References [1] F.K. Crundwell, Electrochim. Acta 37 (1992) 2101. [2] A. Dafli, B. Hammouti, R. Mokhlisse, S. Kertit, Corros. Sci. 45 (2003) 1619. [3] M. Abdallah, A.Y. Eletre, A.I. Mead, J. Electrochem. Soc. India 45 (1996) 71.
[4] J. Mabrour, M. Akssira, M. Azzi, M. Zertoubi, N. Saib, A. Messaoudi, A. Albizane, S. Tahiri, Corros. Sci. 46 (2004) 1833. [5] D. Kuron, H.J. Rother, H. Graefen, Werkst. Korros. 32 (1981) 409. [6] E. Geler, D.S. Azambuja, Corros. Sci. 42 (2000) 631. [7] L. Nunez, E. Reguera, F. Corvo, E. Gonzalez, C. Vazquez, Corros. Sci. 47 (2005) 461. [8] W. Li, Q. He, C. Pei, B. Hov, Electrochim. Acta 52 (2007) 6386. [9] M. Stoll, R.D. Webster, J. Appl. Electrochem. 34 (2004) 225. [10] C. Wang, S. Chem, S. Zhao, J. Electrochem. Soc. 151 (2004) B11. [11] G. Kardas, Mater. Sci. 41 (2005) 337. [12] K.F. Khaled, N. Hecberman, Electrochim. Acta 49 (2004) 485. [13] A.Y. El-Etre, M. Abdallah, Corros. Sci. 42 (2000) 731. [14] M. Abdallah, Corros. Sci. 44 (2002) 717. [15] F. Bentiss, M. Lagrenee, M. Trainsel, Corrosion 56 (2000) 733. [16] J. Cruz, R. Martinez, J. Genesca, E. Gracia-Ochoa, J. Electroanal. Chem. 566 (2004) 111. [17] Nebil Souissi, Ezzeddine Triki, Corros. Sci. 50 (2008) 231. [18] N. Huynh, S.E. Bottle, T. Notoya, D.P. Schweinsberg, Corros. Sci. 42 (2000) 259. [19] M.M. EL-Naggar, J. Mater. Sci. 35 (2000) 6189. [20] W. Qafsaoui, Ch. Blanc, J. Roques, N. Pébère, A. Srhiri, C. Mijoule, G. Mankowski, J. Appl. Electrochem. 31 (2001) 223. [21] R. Youda, H. Nishihara, K. Aramaki, Electrochim. Acta 35 (1990) 1011. [22] M. Lebrini, M. Traisnel, M. Lagrenee, B. Mernari, F. Bentiss, Corros. Sci. 50 (2008) 473. [23] A.L. Lehninger, Principle of Biochemistry, Worth, New York, 1982. [24] J.O.M. Bockris, A.K.N. Reddy, B.E. Conway, E. Yeager, R.E. White (Eds.), Modern Electrochemistry, vol. 2, 1970, 708. [25] S.S. Abd El Rehim, M.A. Ibrahim, K.F. Khaled, J. Appl. Electrochem. 29 (1999) 593. [26] J. Marsh, Advanced Organic Chemistry, 3rd ed., Wiley, Eastern New Delhi, 1988. [27] H. Dhar, B. Conway, K. Joshi, Electrochim. Acta 18 (1973) 789. [28] A.A. El-Awady, B.A. Abd El-Nabey, S.G. Aziz, J. Electrochem. Soc. 139 (1992) 2149. [29] D. Do, Adsorption Analysis: Equilibria and Kinetics, Imperial College Press, 1998, p. 10. [30] E.M. Sherif, S.-M. Park, Electrochim. Acta 51 (2006) 4665. [31] H. Ma, S. Chem, L. Niu, S. Zhao, S. Li, Di Li, J. Appl. Electrochem. 32 (2002) 65. [32] E.M. Sherif, Su-Moon Park, Electrochim. Acta 51 (2006) 4665. [33] F. Mansfield, Corrosion Mechanisms, Marcel Dekker, New York, 1987, p. 119. [34] X. Wu, H. Ma, S. Chem, Z. Xu, A. Sui, J. Electrochem. Soc. 146 (1999) 1847.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Studies of the corrosion inhibition of copper in Na2 SO4 solution using polarization and electrochemical impedance spectroscopy E. Hamed ∗ Chemistry Department, Faculty of Science, Ain Shams University, Abbassia 11566, Cairo, Egypt
a r t i c l e
i n f o
Article history: Received 20 May 2009 Accepted 24 December 2009 Keywords: Copper Potassium folate Corrosion inhibition EIS
a b s t r a c t Inhibition effect of different concentrations of potassium folate (green inhibitor) on copper corrosion in aerated Na2 SO4 solutions in the temperature range (15–55 ◦ C) was investigated by using potentiodynamic polarization and electrochemical impedance spectroscopy technique. The polarization curves showed that the addition of potassium folate inhibits copper corrosion as a result of physical adsorption of folate anions on the copper surface. The folate anion acts as an anodic-type inhibitor. The inhibition efficiency increases with increasing the folate anion concentration but decreases with increasing Na2 SO4 concentration and temperature. The adsorption of folate anions on copper surface follows Flory–Huggins isotherm and kinetic–thermodynamic model. The thermodynamic parameters for corrosion and adsorption processes in the absence and presence of the inhibitor were calculated and discussed. Effect of immersion time on the performance of the inhibitor was also tested in this investigation. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Copper has an excellent electrical and thermal conductivities, good corrosion resistance and mechanical workability. It is widely used in heating and cooling systems. Corrosion of copper can lead to many problems, the most being per formation that may result in coolant leakage. Scales and corrosion products have negative influence on heat-transfer, causing a decrease in heating efficiencies of the copper structures [1]. Thus, corrosion of copper and copper alloys and their inhibition in aqueous salt solutions have attracted the attention of a member of investigators [1–7]. The use of inorganic or organic inhibitors is one of the most practical methods for protection against corrosion of metals and their alloys. Most well known organic inhibitors such as those containing nitrogen, sulphur, oxygen atoms and aromatic rings [8–12]. To be effective an inhibitor must displace water molecules from the metal surface, interact with anodic and/or cathodic reaction sites to retard the partial oxidation and reduction corrosion reaction and prevent transportation of water and corrosive-active species to
∗ Tel.: +20 226822724; fax: +20 224831836. E-mail address: dr science [email protected]. 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.12.044
the surface [13,14]. In general, in aqueous solutions, the inhibitory action of organic inhibitors may be due their physical (electrostatic) adsorption or chemisorption onto the metal surface, depending on the charge of the metal surface, the electronic structure of organic inhibitor and the nature of the medium [15,16]. Several organic inhibitors for copper corrosion in aqueous sulphate solution at various pHs can be found in the literature [17–21]. However, the new environmental restrictions need to use organic inhibitors, acceptable from the environmental point of view (green inhibitors) [22]. The present work was aimed to find environment friendly non-toxic compound and inhibits copper corrosion in Na2 SO4 solution. Potassium folate is environmentally friendly compound and has applications in biological and pharmaceutical fields [23]. For this reason, we investigated the inhibition property of this compound using potentiodynamic polarization and electrochemical impedance spectroscopy measurements under different concentrations of both potassium folate and sodium sulphate, temperature and immersion time.
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The structure of potassium folate is:
2. Experimental The working electrode used in the present work was made of spice pure cylindrical copper rod, welded with Cu wire for electrical connection and mounted into suitable glass tube using epoxy resin so that its cross-sectional area (0.5 cm2 ) was in contact with the test solution. The exposed area of the electrode was polished using emery papers of 1/0, 2/0, 3/0, washed with distilled water, degreased using pure alcohol, and bidistilled water and finally dried. A platinum wire was used as auxiliary electrode. A saturated calomel electrode (SEC) was used as reference electrode. The SCE was connected via a Luggin capillary, the tip of which was very close to the surface of the working electrode to minimizing the IR drop. All potentials are referred to SCE. The experiments were performed in Na2 SO4 solutions without and with different concentrations of potassium folate as inhibitor. All solutions were freshly prepared from analytical grade chemical reagents using doubly distilled water and were used without further purification. Each run was carried out at the required temperature (±1 ◦ C) using water thermostat. The potentiodynamic polarization curves were carried out at a scan rate of 2 mV s−1 . Impedance experiments were conducted using ac signal of amplitude mV peak to peak at open circuit potential (OCP) in the frequency range 0.5 Hz to 30 kHz. A potnetiostat/galvanostat EG & G model 273 lock-in amplifier (model 5210) and M352 corrosion software and M398 impedance software from EG&G Princeton Applied Research were used for polarization and impedance, respectively.
3. Results and discussion 3.1. Potentiodynamic polarization Potentiodynamic polarization curves of copper electrode in aerated sodium sulphate 1 M Na2 SO4 (blank) solution without and with various concentrations (0.002–0.01 M) of potassium folate at different temperatures (15–55 ◦ C) were recorded. These curves were swept from −500 up to 500 mV (SCE) with scan rate of 5 mV s−1 . Fig. 1 is an example. Inspections of the obtained data reveal that at a given temperature, the addition of potassium folate to the blank solution increases both cathodic and anodic overpotentials and shifts the cathodic and anodic current densities to lower values but has little effect on cathodic polarization. Such behavior indicates that the addition of potassium folate retards mainly the anodic dissolution of copper. In aerated solution, the cathodic reaction is either oxygen and/or water reduction reaction according [1].
values are listed in Table 1. It is clear from these data that, at a given temperature, the presence of potassium folate decreases the corrosion current density Icorr and shifts Ecorr slightly to more positive values. These changes increase with increasing the additive concentration. This means that potassium folate acts as anodic-type inhibitor for copper corrsosion in Na2 SO4 solution. The values of ba and bc remain approximately unchanged in presence of different concentrations of potassium folate indicating that this inhibitor has no significant influence on the mechanism of corrosion of copper in Na2 SO4 solution. In aqueous solution, potassium folate dissociates and yields folate anions. It is suggested that the negative folate anions are preferentially adsorbed on positively charged copper surface [24] via electrostatic interaction (physical adsorption). The presence of two carboxylic groups in folate anion is expected to impart high adsorption. Also such adsorption will be increased owing to the presence of several adsorption centers (as N and O atoms in folate anion). The adsorption of folate anions results in the formation of protective film of adsorbate on the copper surface. This protective film separates the metal surface from the corrosive medium and hence inhibits copper corrosion. However, it is observed that, at a given folate concentration, the values of Icorr in uninhibited and inhibited solutions increase with the increase in temperature. These results depict that the rate of corrosion in the absence and presence of folate species increase
O2 + 2H2 O + 4e− = 4OH− and/or
2H2 O + 2e− = H2 + 2OH−
However, the anodic dissolution of copper can be represented as follows [1]: Cu = Cu+ + e− Cu+ = Cu2+ + e− Electrochemical parameters such as corrosion current density (Icorr ), corrosion potential (Ecorr ) anodic (ba ) and cathodic (bc ) Tafel slopes in all cases were calculated from Tafel plots. The calculated
Fig. 1. Effect of temperature on potentiodynamic polarization curves of copper electrode in 1 M Na2 SO4 solution containing 0.01 M pot. folate.
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Table 1 The electrochemical parameters and the corresponding inhibition efficiency for corrosion of Cu electrode in 1 M Na2 SO4 solution in the absence and presence of different concentrations of the inhibitor and at different temperatures. Temperature ◦ C ◦
15
25◦
35◦
45◦
55◦
C mol L−1
Ecorr V
Icorr A cm−2
ba V dec−1
bc V dec−1
P%
000 0.002 0.004 0.006 0.01
−0.324 −0.290 −0.288 −0.282 −0.278
4.370 2.620 2.106 1.708 1.141
0.277 0.270 0.269 0.267 0.264
0.334 0.299 0.284 0.276 0.269
40.0 51.8 60.9 73.9
000 0.002 0.004 0.006 0.01
−0.308 −0.271 −0.264 −0.253 −0.226
4.964 3.514 2.909 2.412 2.224
0.394 0.303 0.286 0.277 0.270
0.408 0.342 0.310 0.294 0.282
29.2 41.4 51.4 55.2
000 0.002 0.004 0.006 0.01
−0.291 −0.259 −0.247 −0.238 −0.192
5.876 4.419 3.780 3.390 2.726
0.493 0.400 0.384 0.354 0.331
0.540 0.488 0.401 0.334 0.317
24.8 35.6 42.3 53.6
000 0.002 0.004 0.006 0.010
−0.287 −0.217 −0.204 −0.199 −0.178
7.928 6.517 6.020 4.773 4.427
0.521 0.480 0.428 0.399 0.374
0.583 0.520 0.448 0.433 0.395
17.8 20.3 39.8 44.2
000 0.002 0.004 0.006 0.010
−0.259 −0.202 −0.194 −0.178 −0.166
8.836 7.903 7.242 6.020 5.440
0.582 0.532 0.498 0.445 0.403
0.622 0.577 0.533 0.498 0.456
10.6 18.0 31.9 38.4
with increasing temperature. The values of Icorr for copper are higher in uninhibited solution than in inhibited solution revealing that folate anions inhibit the copper corrosion in the temperature range used. The effect of the concentration of Na2 SO4 on the potentiodynamic polarization in the absence and presence of fixed concentration of potassium folate (0.006 M) was investigated at 30 ◦ C. Values Icorr and Ecorr , ba and bc obtained from Tafel analysis method are given in Table 1. It is observed that in the absence and the presence of fixed concentration of the inhibitor, the value of Icorr increases and Ecorr shift towards more negative potentials with increasing Na2 SO4 concentration. The increase in the value of Icorr indicates that copper corrosion increases with increasing SO4 2− concentration. Since Icorr is directly proportional to the corrosion rate, the inhibition efficiency (P%) of folate anions, in all cases, was calculated using the equation:
P% = 1 −
Icorr o Icorr
× 100
(1)
o where Icorr and Icorr are the corrosion current densities in the absence and presence of potassium folate, respectively. The calculated values of P% are included in Table 1. Fig. 2 shows the relation between P% vs the logarithmic concentration of the inhibitor at different temperatures. The data reveal that the inhibition efficiency of folate ions enhances with increasing its concentration in the solution. The increase in inhibition efficiency observed at higher inhibitor concentration indicating that more inhibitors are adsorbed onto the metal surface, thus providing wider surface coverage. On the other hand, it is clear that the inhibition efficiency decreases with the increase in temperature. This trend is attributed to desorption of some inhibitor ions from the surface reflecting the physical nature of adsorption (physisorption). Fig. 3 illustrates the variation of P% vs the concentration of Na2 SO4 which containing fixed concentration of the inhibitor at 30 ◦ C. The data demonstrate that the inhibition efficiency of folate anions decreases with increasing the concentration of SO4 2− anions. This behavior may
be attributed to the greater aggressiveness of Na2 SO4 solution at higher concentrations. It seems that the adsorption of folate anions takes place on the metal surface in competition with the adsorption of SO4 2− anions. 3.2. Activation energies The corrosion reaction could be related to an Arrhenius-type of process according to the equation: Icorr = A expEa /2.303RT
(2)
where A is the Arrhenius pre-exponential factor, Ea is the apparent activation energy of corrosion process, T is the absolute temperature and R is the universal gas constant. The value of Ea was calculated from the slope of log Icorr vs 1/T for the uninhibited and inhibited solutions as shown in Fig. 4. The calculated values of Ea are given in Table 2. The value of Ea in the blank solution was 5.94 kJ mol−1 . The addition of potassium folate to the sulphate solution enhances Ea
Fig. 2. Variation of the inhibition efficiency (P%) with logarithmic concentrations of the inhibitor (log Cinhib ) for copper electrode in 1 M Na2 SO4 solution at different temperatures.
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Fig. 5. Transition state plots of log[Icorr /T] vs 1/T for copper in 1 M Na2 SO4 without and with different concentrations of pot. folate.
Fig. 3. Variation of the inhibition efficiency (P%) with the concentrations of Na2 SO4 containing constant concentration 0.006 M of pot. folate at 30 ◦ C.
given in Table 2. The positive sign of Ha reflects the endothermic nature of the dissolution process. The large and negative sign of Sa in the absence and presence of the inhibitor implying that the activated complex in the rate-determining step represents association rather than dissociation, meaning that a decrease in disorder takes place, going from reaction to he activated complex [26]. 3.3. Adsorption isotherms The inhibition action of many organic compounds is assumed to be assigned to their adsorption on metal surface. Therefore, investigation of adsorption isotherms can provide important information on the adsorption of folate anions on copper surface in Na2 SO4 solution. In an attempt to find the most suitable adsorption, the surface coverage ( = P/100) was subjected to different adsorption isotherms. It is found that the corrosion data fit well to Flory–Huggins and kinetic–thermodynamic model isotherms at all the temperatures used. The Flory–Huggins isotherm [27] is given by
Fig. 4. Arrhenius plot of log corrosion (Icorr ) vs 1/T for copper in 1 M Na2 SO4 without and with different concentrations of pot. folate.
and the extent of the enhancement is proportional to the inhibitor concentration implying that the energy barrier for corrosion process enhances with inhibitor concentration. This observation further supports the proposal physical adsorption mechanism. The adsorbed inhibitor species block the surface sites with lower Ea values and pushed the corrosion reaction to surface sites with higher Ea values [25]. The activation thermodynamic parameters of copper dissolution namely, the enthalpy of activation (Ha ) and entropy of activation (Sa ) can be obtained from the transition state equation: Icorr =
RT exp(Sa /R) exp(−Ha /RT ) Nh
log
= log xKads + x log(1 − ) C
(4)
and the kinetic–thermodynamic model isotherm [28] is given by log
= log K + y log C 1−
(5)
where Kads is the adsorption constant, x is the number of surface water molecules replaced by one inhibitor species, C is the inhibitor concentration in the bulk of the solution, (1/y) is the number of active sites occupied by one inhibitor species and [(K )1/y = Kads ]. Curves fitting folate anions to the Flory–Huggins isotherm equation (4) is shown in Fig. 6 where straight lines were obtained by plotting log(/c) vs log(1 − ). Fig. 7 shows the curves fitting on
(3)
where N and h are Avogadro’s number and Plank’s constant, respectively. A straight line was obtained when log(Icorr /T) was plotted vs 1/T as shown in Fig. 5. From the slope and intercept of the straight line plot, the values of Ha and Sa , respectively were obtained and Table 2 The values of activation parameter Ea , Ha and Sa for Cu electrode in 1 M Na2 SO4 solution in the absence and presence of different concentrations of the inhibitor at 30 ◦ C. C (M) 000 2 × 10−3 4 × 10−3 6 × 10−3 1 × 10−2
Ea (kJ mol−1 )
Ha (kJ mol−1 )
Sa (J mol−1 K−1 )
5.94 8.098 10.642 12.19 13.92
11.13 18.58 22.58 24.72 26.42
−136.4 −114.58 −102.91 −97.70 −93.29
Fig. 6. Flory–Huggins isotherm model of folate anions on copper surface at different temperatures.
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E. Hamed / Materials Chemistry and Physics 121 (2010) 70–76
Fig. 7. Thermodynamic–kinetic model of folate anions on copper surface at different temperatures.
folate anions to the kinetic equation (5) and the linear relationship between log(/1 − ) vs log C. The values of Kads , x and (1/y) were calculated from these two isotherms and are given in Table 3. A good agreement between the two isotherms concerning the values of Kads was obtained. It is clear that the values of Kads are relatively small and decrease with increasing temperature indicating that the interactions between the adsorbed inhibitor and the metal surface are weakend and consequently, the adsorbed inhibitors become easily desorbed. These data explain the decrease in the inhibition efficiency with increasing temperature. The number of x (obtained from Flory–Huggins isotherm) and 1/y (obtained from kinetic isotherm) are approximately equal one under all conditions investigated. This suggests that each folate anion adsorbs occupied one active sit and replace an adsorbed water molecule. The free energy of adsorption Gads on copper surface can be calculated from the equation [29] Kads =
1 exp(Gads /RT ) 55.5
(6)
where 55.5 is the molar concentration of water in the solution. The calculated values of Gads are given in Table 3. The negative sign of Gads implies spontaneous adsorption. The values of Gads are around 20 kJ mol−1 indicating electrostatic interaction between the negatively charged folate ions and the positively charged copper surface (physisorption). 3.4. Impedance measurements Electrochemical impedance spectroscopy (EIS) measurements were conducted to give more insight on the inhibition behaviour of potassium folate on copper corrosion in Na2 SO4 solution. Nyquist plots recorded for copper in 1 M Na2 SO4 at OCP without and with various concentrations of the inhibitor and temperatures are shown in Figs. 8 and 9, respectively. In all cases, only on depressed capacitive semicircle has been observed, indicating that corrosion of copper in uninhibited and inhibited solutions is under chargetransfer control. These semicircles are assigned to the time constant of charge-transfer and to double layer capacitance. The semicircles at high frequencies are generally associated with the relaxation of the capacitors of electrical double layers with their diameters represents the charge-transfer resistances [30,31]. From these Nyquist plots, the values of the charge-transfer resistance (Rct ) were Table 3 Adsorption constant (Kads ), change of free energy (Gads ), number of active sites (x) Flory–Huggins kinetic–thermodynamic model. Temperature (K) x 288 298 308 318 328
Kads
Fig. 8. EIS diagrams of copper at Ecorr in 1 M Na2 SO4 in the absence and presence of different concentration of pot. folate at 30 ◦ C.
Gads (kJ mol−1 ) 1/y
1.16 248.9 −22.83 0.95 161.7 −21.79 1.17 121.5 −21.11 1.23 82.6 −20.19 1.48 62.3 −19.51
0.98 1.05 0.99 1.00 1.09
Kads
Gads (kJ mol−1 )
276.9 160.9 118.5 81.03 57.00
−23.08 −22.54 −22.51 −22.23 −20.31
Fig. 9. EIS diagrams of copper at Ecorr in 1 M Na2 SO4 + 0.006 M pot. folate at different temperatures.
obtained from the difference in real component (Z ) of impedance at lower frequencies. Also the double layer capacitances (Cdl ) were calculated by the Eq. (7) [31]. Cdl = (2fmax Rct )
−1
(7)
where fmax is the frequency value at which the imaginary component (Z ) of impedance is maximum. The calculated values of Rct and Cdl are listed in Tables 4 and 5. The equivalent circuit model employed for analyzing Nyquist responses is shown in Fig. 10. In this circuit a constant phase element (CPE) was used in place of a capacitor to compensate for inhomogeneity in the system [32]. The circuit model consisted of the solutions resistance Rs , the charge-transfer resistance of the interfacial corrosion Rct and the constant phase element CPE. Table 4 Effect of inhibitor concentration on impedance parameters and the corresponding inhibition efficiency for copper in 1 M Na2 SO4 at 30 ◦ C. Conc. folate
Cdl × 10−4 F
Rct × 103 ohm cm
P%
0 0.002 0.004 0.006 0.008 0.01
3.974 2.773 2.374 2.216 1.921 1.71
1.012 1.589 1.674 1.803 2.071 2.311
36.73 40.3 44.24 51.65 56.97
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Table 5 Effect of temperature on impedance parameters and their corresponding inhibition efficiency for copper in 1 M Na2 SO4 containing 0.006 M pot. folate at different temperatures. Temp. C0
1 M Na2 SO4 0
1 M Na2 SO4 + 0.006 M pot. folate −2
Cdl (blank) (F cm 15 25 35 45 55
2.239 2.407 4.005 4.761 5.15
)
−2
0
Rct (blank) (ohm cm 2.332 2.091 1.471 0.979 0.686
P% = 1 −
Rct 0 Rct
× 100
Cdl (F cm 0.816 1.164 2.291 2.833 3.190
The value of the frequency power n of CPE used obtained by computer is close to 0.9 and consequently the CPE can be assumed to correspond to capacities behaviors. In Figs. 8 and 9 symbols represent the actual data while the solid lines represent the best fit using the equivalent circuit shown in Fig. 10. The data, reveal that the addition of folate inhibitor to the sulphate solution at any given temperature enhances the value of Rct but reduces the value of Cdl . These changes in the impedance parameters increase with increasing the inhibitor concentration. The increase in the value of Rct is ascribed to the adsorption of folate anions and displace the water molecules and other anions originally adsorbed on the copper surface. Such process may suggest the formation of protective film of inhibitor on the metal surface. This protective film impedes the charge-transfer across the metal/solution interface. On the other hand, the decrease in the value of Cdl could be related to a decrease in the local dielectric constant and/or an increase in the thickness of the electrical double layer, suggests that inhibitor acts by adsorption on the metal/solution interface [33]. On the other hand, the diameter of the Nyquist loop decreases with increasing temperature as a result of increasing the rate of corrosion and desorption of some folate anions from the copper surface and consequently decreases the surface coverage. These changes in the impedance parameters associated the addition of the inhibitors confirm the suggestion that folate anions act as adsorption inhibitor. Since the value of (1/Rct ) is directly proportional to the corrosion rate, the inhibition efficiency (P%) of folate anions for copper in 1 M Na2 So4 solution were calculated from Rct values obtained from impedance data at different inhibition concentrations and temperature using the following equation [34].
)
−2
(8)
0 and R are the charge-transfer values without and with where Rct ct inhibitor, respectively. The calculated values of P% are given in Tables 4 and 5. It I found that P% increases with the inhibitor concentration but decreases with increasing temperature. It is clear that inhibition efficiencies obtained from ac impedance agree well with those obtained from potentiodynamic polarization.
Fig. 10. Electrical equivalent circuit representing copper/Na2 SO4 without and with inhibitor.
)
P% −2
Rct (ohm cm 6.401 4.325 2.572 1.645 1.107
) 63.57 51.65 42.8 40.5 38.05
Fig. 11. Variation of Rct with the immersion time for a copper electrode in 1 M Na2 SO4 solution containing 0.006 M pot. folate at 30 ◦ C.
The effect of immersion time on the performance of potassium folate as an inhibitor for corrosion of copper in Na2 SO4 solution was tested via EIS measurements. The experiments were carried out in 1 M Na2 SO4 containing 0.006 M potassium folate at OCP and 30 ◦ C. Nyquist plots were recorded after certain time intervals up to 1320 min (obtained Nyquist plots are not shown here). Only one semicircle was observed in all cases. The semicircle diameter depends on the immersion time, initially, the diameter increases considerably with immersion time and then tends to achieve a steady state value. The impedance parameters Rct and Cdl were calculated from the Nyquist plots. Figs. 11 and 12 show the variation of both charge-transfer resistance and double layer capacitance values against the immersion time. It is clear from Figs. 11 and 12 with respect that Rct increased and Cdl decreased rapidly during the initial 180 min and then they tend to attain steady state values afterward. The increase in Rct and the decrease in Cdl indicates the continuous adsorption of the inhibitor and enhancement of the surface coverage and hence the inhibition efficiency of folate ions increases. It seems that after 180 min, the protection film becomes perfectly covered the electrode surface and hence the inhibition efficiency remains high and nearly constant up to 1320 min.
Fig. 12. Variation of Cdl with the immersion time for a copper electrode in 1 M Na2 SO4 solution containing 0.006 M pot. folate at 30 ◦ C.
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E. Hamed / Materials Chemistry and Physics 121 (2010) 70–76
4. Conclusions • Potassium folate (non-toxic) has been tested as an inhibitor for copper corrosion in Na2 SO4 solution using potentiodynamic polarization and EIS measurements. • Potentiodynamic polarization curves indicated that the presence of potassium folate inhibits the corrosion of copper in this medium. Inhibition of copper corrosion is attributed to the adsorption of folate anions on the copper surface and formation of barrier film thus suppress the copper corrosion. This inhibitor acts as an anodic-type inhibitor. • The activation energy Ea , enthalpy of activation and entropy of activation Sa for copper dissolution were calculated in the absence and presence of folate anions. The presence of the inhibitor enhances the thermodynamic activation parameters. • Adsorption of the inhibitor anion on copper surface obeys Flory–Huggins isotherm and kinetic–thermodynamic model. The thermodynamic parameters for involves electrostatic interaction between the charged inhibition anion and the charged metal surface. • The EIS measurements have also confirmed the inhibition action of folate anions. • The inhibition efficiency of folate anion increases with increasing folate concentration and time of immersion but decreases with increasing the Na2 SO4 concentration and temperature. • All results obtained from potentiodynamic polarization and from EIS measurements are in good agreement. References [1] F.K. Crundwell, Electrochim. Acta 37 (1992) 2101. [2] A. Dafli, B. Hammouti, R. Mokhlisse, S. Kertit, Corros. Sci. 45 (2003) 1619. [3] M. Abdallah, A.Y. Eletre, A.I. Mead, J. Electrochem. Soc. India 45 (1996) 71.
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