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Stripping voltammetric and conductance measurements on corrosion and inhibition of copper in nitric acid
Materials Chemistry and Physics 71 (2001) 279–290
Stripping voltammetric and conductance measurements on corrosion and inhibition of copper in nitric acid M. Khodari a , M.M. Abou-krisha a,∗ , F.H. Assaf a , F.M. El-Cheikh b , A.A. Hussien a a
Chemistry Department, Faculty of Science, South Valley University, Qena, Egypt b Chemistry Department, Faculty of Science, Sohag, Egypt
Received 20 September 2000; received in revised form 5 January 2001; accepted 18 January 2001
Abstract The kinetics of copper dissolution in nitric acid especially, the phase of dissolution at the beginning of incubation period was investigated. Two techniques, the anodic stripping voltammetry and the conductance measurements, were used for the first time in this concern. Galvanostatic polarization measurements using very simple circuit used to study the corrosion process of copper in nitric acid. The effect of several factors was studied, namely nitric acid concentration, the copper surface area and stirring. The effect of inorganic substances and their role as inhibitors or promoters was also investigated. By applying the anodic stripping voltammetric technique it was found that the rate of copper dissolution during the incubation period is constant and time independent. Also, stirring prevents the corrosion of copper almost completely. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Copper; Dissolution rate; Inhibition; Anodic stripping voltammetry
1. Introduction The dissolution of copper in nitric acid is known to proceed in two stages [1]. During the first stage, the incubation period, the dissolution of copper extremely slow to be followed by the known analytical methods. The second stage is the active dissolution of copper. Numerous investigations have been carried out from different points of view on the dissolution of copper in acidic solutions [2–5]. Many techniques were suggested to investigate the dissolution and inhibition of Cu in different media [1,6,7]. In addition, the kinetics of copper dissolution in H2 SO4 solution containing different inorganic depolarizers was studied [6]. A first-order rate was proved for all depolarizers used. Smol’yaninov and Khitrov [8] stated that the corrosion rate of copper in HCl or H2 SO4 solution increased with the concentration of K2 Cr2 O7 . The change in energy of activation and the temperature coefficient of corrosion indicated that the rate of the process was determined both by diffusion and chemical effects. In the present work, the dissolution of copper in nitric acid was examined using stripping voltammetry [9,10], conductance and galvanostatic polarization techniques. Anodic stripping voltammetry used to study the dissolution in in∗ Corresponding author. E-mail address: mortaga [email protected] (M.M. Abou-krisha).
cubation period due to that analysis detect the very small ion concentrations. Conductance measurement technique is chosen to study the active dissolution, since the conductivity can be attributed to the decrease in the concentration of H+ which involved during the copper dissolution in HNO3 and can be easily detected by conductometer. However, galvanostatic polarization measurement using very simple circuit, used to study the corrosion process of copper in nitric acid and confirms the results obtained by the other two techniques. 2. Experimental The specimens of copper (99.9%) used in the present work were of thickness 0.80 mm, 1.0 cm in width and 5.0 cm in length. Copper specimens were washed in a tap water, in distilled water and dipped in pickling solution of the composition (1 H2 O:5 nitric acid) for about 15 s till effervescence covered the whole copper surface. Then washed thoroughly with tap water and finally with distilled water. This treatment was carried out immediately before starting the experiment. In stripping voltammetric measurements a computer-aided electrochemistry system potentiostat (EG and GPARC model 263), static mercury drop electrode (SMDE) and electroanalytical software models 270/250 version 4.0 (PARC) which controls the potentiostat via IEEE 488 GPIB were
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 2 9 1 - 7
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Fig. 1. Anodic stripping voltammograms for different concentrations of Cu2+ in the presence of 2.5 × 10−3 M HNO3 , at scan rate 50 mV s−1 , deposition potential −0.5 V and preconcentration time 30 s. (a) 0.00 M, (b) 1 × 10−5 M, (c) 2 × 10−5 M, (d) 3 × 10−5 M, (e) 4 × 10−5 M, (f) 5 × 10−5 M.
used. The SMDE model 303A contains three electrodes which were described elsewhere [10]. The anodic stripping voltammetric measurements were carried out by placing 50 cm3 of the used acid solution in a 50 cm3 beaker and thermostated at the working temperature. The cleaned copper specimen was then immersed vertically in the solution and the time started. Changes in the concentration of Cu2+ were followed by withdrawing 0.10 cm3 of the solution each time, completed to 100.0 cm3 by second distilled water and the current corresponding to the concentration of Cu2+ was recorded. For all stripping voltammetric measurements, the solution under investigation was degassed in a 10 cm3 cell for 8 min using purified nitrogen gas. Then the voltammogram was recorded after applying the required potential for a certain period of time. The concentration of Cu+2 was calculated from standard working curves. The standard working curves were made by adding known concentrations of Cu2+ to 10 cm3 of the diluted corrosive solution before immersing the copper specimen. The peak current corresponding to this concentration of Cu2+ was recorded after applying a definite potential for certain period of time. Representative anodic stripping voltammograms are shown in Fig. 1. In this work, a standard working curve (Fig. 2 as an example) was constructed for each concentration of nitric acid and the used inhibitors. The specimens of copper (99.9%) used in the conductance measurement technique were of dimensions 1.0 cm in width and 5.0 cm in length. In conductance technique a 50 cm3 of nitric acid solution was placed in a 50 cm3 beaker and thermostat at the working temperature. The cleaned Cu specimen was then immersed vertically in the solution and the time started. Changes in the conductance of nitric acid was followed by withdrawing 0.2 cm3 of the solution each time, which added to 10 cm3 distilled water in a test tube and the conductance of such solution was determined using digital conductometer (consort K120). The dilution of the original working nitric acid solution was carried out to put the
measured conductance values within the measuring range of the conductometer. In galvanostatic polarization technique, the working copper electrode was prepared from extra pure copper wire of diameter 0.1 cm and length 1 cm. The reference electrode was itself the counter electrode, which was made of copper sheet with surface area much greater (80 cm2 ) than that of the working electrode. The electric circuit for the galvanostatic polarization measurement is shown in Fig. 3. For each run, the working and the reference/counter electrodes were placed into 50.0 cm3 of the test solution. For cathodic galvanostatic polarization experiments, the negative pole of DC power supply (Delta Electronika CST 100) was connected to the working electrode while the positive pole was connected
Fig. 2. The anodic peak current of Cu2+ as a function of [Cu2+ ].
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Fig. 3. Circuitry for galvanostatic polarization measurements. R/C E: reference/counter electrode, W: working electrode.
to the reference/counter electrode. The same was carried out for anodic polarization measurements with change in the connection poles. The working and reference/counter electrodes were connected to the potentiometer (EIL-pH meter 7020). For each run, a certain current density was applied for 2 min, the potential difference between the working and the reference/counter electrodes was then measured. Then gradually increasing the current density each 2 min and the potential difference was then recorded. In this study, the counter and reference electrodes are one and the same electrodes. This was made possible because the surface area of the counter/reference electrode was several hundred times greater than the surface area of the working electrode. The counter/reference electrode is thus considered to be practically nonpolarizable. Moreover, the conductance of the nitric acid solutions is so high that the ohmic potential drop across the solution is negligible. In this work all chemicals used of AR grade and all measurements were carried out at 25 ± 0.5◦ C.
3.1. Treatment of the data Results of the variation of Cu2+ concentration accumulated in nitric acid solution during the incubation period with time are reported for different HNO3 acid concentrations in Table 1. In this table the correlation and regression [11] coefficients are also reported. It was found that Cu2+ concentration increased continuously with time. The correlation, between the concentration C and the time τ , can expressed statistically by the following formula: C = A + B τ
Power regression :
C = A0 + B 0 ln τ
C = A τ B
Exponential regression :
C = A∗ eB
(2) (3)
∗ τ
(4)
The interpretation of the constants are explained as follows: 1. The first relation shows the linear relationship, where A is denoted to the concentration of Cu2+ at τ = 0 whereas B is the concentration of Cu2+ per minute during the linear relation (i.e. the dissolution rate of Cu2+ expressed in mol dm−3 min−1 ). 2. In formula (2) A0 is the concentration of Cu2+ after 1 min. The meaning of the constant B0 was interpreted by using τ = e = 2.718281828 min, hence the formula (2) becomes Cat(τ =e) = A0(the concentration at τ =1) + B 0 ln τ B 0 = Cat(τ =e) − A0(the concentration at τ =1)
3. Results and discussion
Linear regression :
Logarithmic regression :
(1)
i.e. B0 is the concentration of Cu2+ after e minutes subtracted from its value at 1 min. 3. Also, the power regression (formula 3) becomes ln C = ln A + B ln τ at τ = 1, then C = A i.e. A is the concentration of CU2+ at τ = 1, and at τ = e the formula becomes ln Cat(τ =e) = ln A + B B = ln
Cat(τ =e) A
if Z = natural antilogarithm, then
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Table 1 The regression data for the dissolution of copper in 50.0 cm3 HNO3 of different concentrations at 25◦ C [HNO3 ] (M) 2.00
2.50
τ (min)
Cu2+
15.0 30.0 45.0 60.0 75.0 90.0 105.0 120.0
0.000310 0.00130 0.00350 0.00500 0.00713 0.00941 0.0108 0.0134
(M)
Linear regression equation y = A + Bx, i.e. C = A + B τ B 0.0001263 r 0.996806
3.00
τ (min)
Cu2+
15.0 30.0 45.0 60.0 75.0 90.0 105.0 120.0
0.000530 0.00480 0.00820 0.0159 0.0234 0.0321 0.0400 0.0474
τ (min) 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0
0.00337 0.00809 0.0135 0.0187 0.0244 0.0299 0.0337 0.0408 0.0460 0.0528
(M)
τ (min)
Cu2+ (M)
10.0 15.0 20.0 25.0 30.0 35.0 40.0
0.00509 0.0140 0.0229 0.0319 0.0408 0.0497 0.0586
0.0010879 0.9990655
0.0017847 0.9999995
Power regression equation y = AxB , i.e. C = Aτ B or ln C = ln A + B ln τ B 1.8008307 2.0931308 r 0.9929896 0.987934
1.8758386 0.9893259
1.7148472 0.989325
Exponential regression equation y = A eBx , i.e. C = A eBτ or ln C = ln A + Bτ B 0.0319750 0.0369336 r 0.9209804 0.91058728
0.0538497 0.9417963
0.0745531 0.9433452
Logarithmic regression equation y = A + B ln x, i.e. C = A + B ln τ B 0.0062110 0.0223828 r 0.938065 0.918677
0.0353002 0.9756823
0.0383855 0.980596
ν (mg cm−2 min−1 )
0.345
0.567
0.0401
0.0004629 0.992496
(M)
3.50 Cu2+
0.147
C , Cat(τ =e) = ZA(the concentration at t=1) , A b = Cτ =e − Aτ =1 = ZAτ =1 − Aτ =1
Z=
i.e. b is the concentration of Cu2+ after e minutes subtracted from its value at 1 min. 4. The meaning of the constants for the last formula are interpreted by the following: ln C = ln A∗ + B ∗ τ It is clear that at τ = 0, C = A∗ = the concentration of Cu2+ at τ = 0, and at τ = 1, the formula becomes ln Cat(τ =1) = ln A∗ + B ∗ B ∗ = ln
Cat(τ =1) , A∗
if M = natural antilogarithms, then
C , Cat(τ =1) = MA(the concentration at τ =0) , A∗ b∗ = Cτ =1 − Aτ =0 = MAτ =0 − Aτ =0 M=
i.e. b is the concentration of Cu2+ after 1 min subtracted from its value at time equal to 0. The correlation between C, the concentration of Cu2+ and τ , the time showed that a linear regression is the best to this
relation (r > 0.99). Thus Cu2+ concentration in solution is related to time best of all in the form: C = A + Bτ 3.2. The effect of nitric acid concentration Values of corrosion rates, ν of copper expressed in mg cm−2 min−1 were computed using the relation (5) from the values of the slope B (the corrosion rate of Cu2+ expressed in mol dm−3 min−1 ), the regression coefficient for the most fitting relation, namely the linear one, and reported in Table 1 ν
mol Cu2+ 1 dm3 mg Cu 3 = B × × 50 cm cm2 min 1000 cm3 dm3 min 63.5 g Cu 1 1000 mg × × ∗ 2× 2+ 1g A cm 1 mol Cu
(5)
where A∗ is the surface area of copper sheet. The linear character in the relation between C and time, τ points towards a constant corrosion rate during the whole incubation period which means a pseudo-zero-order process. This is quite logic as the depletion in nitric acid concentration is negligible. Nitric acid is being the main depolarizer in this stage of dissolution. The linearity, thus can hardly be attributed to a true zero-order kinetic.
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Fig. 4. The variation of conductance, K with time, τ for the dissolution of copper in 50 cm3 HNO3 of different concentrations at 25◦ C.
The variation of electric conductance with time as a result of copper dissolution (Fig. 4) in the presence of different HNO3 acid concentrations was measured. The collected data were treated using the different regression modes (Table 2), where conductance data were taken during the active period of copper dissolution only. The dissolution of copper in nitric acid during the incubation period was found to be so slow that almost no change in the conductance of solution was noticed. As catalytic intermediates accumulate near the copper surface, active dissolution starts accompanied with distinct decrease in conductance due to the exhaust of H3 O+ . The solution itself begins to acquire the characteristic blue coloration of hydrated Cu2+ . It is interesting to note that during the incubation period no blue coloration was noticed. The linear character of the relationship between conductance and time is quite evident as the correlation coefficient, r was found to be the highest for linear regression.
Table 2 Values of B, the regression coefficient and r, the correlation coefficient for the active dissolution of copper in 50.0 cm3 HNO3 of different concentrations at 25◦ C C (M)
Regression modes Power
Exponential
Logarithmic
Linear
2.00 B r
−0.275812 0.9824685
−0.000777 0.9886729
−3.353593 0.9842595
−0.009444 0.9898900
2.50 B r
−0.184169 0.9816836
−0.000990 0.9981029
−2.701341 0.98579100
−0.014476 0.9987239
3.00 B r
−0.2240000 0.9824210
−0.002046 0.9988846
−3.866970 0.9877855
−0.035164 0.9994389
3.50 B r
−0.300019 0.9670924
−0.003207 0.9979324
−5.224967 0.9807256
−0.055178 0.99965739
During the active period of the copper dissolution the rate with which conductance changes with time can be taken as the corrosion rate of copper. The intercept of straight lines (conductance vs. time) in Fig. 5 with values of conductance of the starting solution at τ = 0 (from Fig. 4) is taken as the incubation period, τ0 . It is clear that the incubation period decreases with the increase in nitric acid concentration. It is interesting to mention that the linear regression coefficient, B is taken for the corrosion rate with opposite sign, as B is always <1 (because conductance decreases all the time). Values of −B (resembling the corrosion rate (ν) of copper in the active period) were correlated with the nitric acid concentration (C). It was found that the rate of active dissolution of Cu is best of all expressed in the power function or the exponential function. For both of these expressions r was found to be 0.99. At the same time linear and logarithmic regressions gave rather lower values of r. Comparison of power functions for the dependence of corrosion rates of copper on nitric acid concentration for the incubation and active periods, reveals that the power is higher for the incubation period (4.775) than for active dissolution (3.290). This means that the dissolution of copper in the incubation period is more concentration dependent than the active dissolution. 3.3. The effect of inorganic substances In this part the effect of hydrazine N2 H4 and NO2 − was studied. The first is neither catalyst nor intermediate in the process of copper dissolution during the incubation period, while the second is the suggested intermediate catalyst according to El-Cheikh et al. [1]. 3.3.1. The effect of hydrazine The effect of different concentrations of hydrazine on the variation of Cu2+ concentration with time during the incubation period during interaction between copper metal and
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Fig. 5. The linear plot of conductance, K vs. time, τ for the active dissolution of copper in 50 cm3 HNO3 of different concentrations at 25◦ C.
3.0 M nitric acid was studied. The correlation of Cu2+ concentration with time for different hydrazine concentrations in 3.0 M HNO3 acid using the four regression modes gave very high values of r, the correlation coefficient for linear regression (Table 3). This once more confirms the linearity of the relationship between Cu2+ concentration and time during the incubation period under all conditions. Table 3 contains also values of the corrosion rate, ν in mg cm−2 min−1 for different concentrations of hydrazine in 3.0 M HNO3 acid. In this table the inhibition efficiencies {1% = [(νu − νi )/νu ] × 100, where ν u and ν i are the corrosion rates for copper in uninhibited and inhibited solutions, respectively} of different concentrations of hydrazine are also listed. The corrosion of copper in nitric acid was found to stop almost completely upon the addition of 0.01 M hydrazine (I,
the inhibition efficiency 99.8%). This is due to the formation of insoluble dark brown layer strongly adhered to the copper surface in solutions of nitric acid containing any traces of hydrazine. Apparently this layer blocks copper from further interaction with nitric acid. It is well known that hydrazine like NH3 can form coordination complexes with both Lewis and metal ions [12]. Just as with respect to the proton, electrostatic and in these cases, also steric considerations militate against bifunctional behavior. Some polymeric complexes having hydrazine bridges have been demonstrated in Fig. 6. The effect of temperature on the corrosion rate of copper was studied in a media containing 3.0 M HNO3 acid and 0.5 M hydrazine. From the variation of corrosion rate with temperature, the activation energy was calculated using Arrhenius equation as follows: K = A e−Ea /RT
Table 3 Values of B, r, ν (mg cm−2 min−1 ) and I (%) for the dissolution of copper in 50.0 cm3 of 3.0 M HNO3 in the absence and presence of different concentrations of hydrazine at 25◦ C C (M)
Regression mode (linear)
ν (mg cm−2 min−1 )
0.00 B r
0.0010879 0.9990655
0.345
0.010 B r
0.0000018 0.9976754
0.000572
99.83
0.100 B r
0.0000017 0.9993214
0.000540
99.84
0.500 B r
0.0000016 0.9980532
0.000508
99.85
1.00 B r
0.0000015 0.9967626
0.000476
99.86
I (%) –
where K is the specific reaction rate, A the frequency factor and Ea the activation energy. In our case either B, the linear regression coefficient for correlation of C and τ or ν, the corrosion rate is used in place of K in Arrhenius equation. The value of Ea obtained by correlating ln B with 1/T where the regression coefficient (slope) equal Ea /R then Ea = slope × R. The results showed no marked effect and the calculated activation energy is 24.8 kJ mol−1 . It is interesting to mention that the conductance of nitric acid solution containing different concentrations of hydrazine showed no significant changes even at quite long
Fig. 6. Sketch of the polymer complex of hydrazine-bridge.
M. Khodari et al. / Materials Chemistry and Physics 71 (2001) 279–290 Table 4 Values of B, r, ν (mg cm−2 min−1 ) and I (%) for the dissolution of copper in 50.0 cm3 of 3.0 M HNO3 in the absence and presence of different concentrations of NO2 − at 25◦ C C (M)
Regression mode (linear)
ν (mg cm−2 min−1 )
0.00 B r
0.000462 0.992497
0.147
0.000477 0.995661
0.152
−3.4
1 × 10−3 B r
0.000503 0.995802
0.160
−8.8
1 × 10−2 B r
0.000601 0.999470
0.191
−29.9
1 × 10−4 B r
I (%)
285
Table 5 The correlation between 1/T and ln B for the calculation of the activation energy for the dissolution of copper in 50.0 cm3 of 2.5 M HNO3 in the absence and presence of 1 × 10−4 M NO2 − T (◦ C)
1/T
ln B 2.5 M HNO3
2.5 M HNO3 + 1 × 10−4 M NO2 −
−7.68 −7.30 −6.48 −5.34
−7.65 −6.94 −6.03 −5.34
−0.988488 −8858 73.6
−0.996434 −8738 72.6
–
periods of time after the immersion of copper metal in solution. 3.3.2. The effect of NO2 − Concentrations ranged from 1 × 10−4 to 1 × 10−2 M of NO2 − was studied. Higher concentrations do not allow reasonable rates of corrosion as NO2 − are intermediate catalyst in the corrosion process of copper in nitric acid. Results of correlation of Cu2+ concentration with time during the incubation period of copper dissolution in 2.5 M HNO3 acid in the presence of different concentrations of NO2 − are listed in Table 4. From these data one can observe the promoting effect of NO2 − on the dissolution of copper during the incubation period. 0.01 M of NO2 − was found sufficient to increase corrosion rate by 33%. This behavior confirms beyond any doubt the catalytic role of NO2 − as intermediates in the corrosion process of copper in HNO3 acid. The increase in temperature caused a parallel increase in the corrosion rate of copper in 2.50 M HNO3 acid in presence of 1×10−4 M NO2 − . It was found that the activation energy of copper dissolution in HNO3 acid during the incubation period does not change upon the addition of NO2 − (Table 5). This can simply attributed to the fact that the nature of the corrosion process does not suffer any principle changes by addition of NO2 − as these ions are formed during the course of copper corrosion in nitric acid. It is quite clear from conductance measurements that the addition of very small amounts of (1 × 10−4 –1 × 10−2 M) NO2 − caused a clear rise in the corrosion rate of copper in 2.5 M HNO3 acid. The variation of conductance with time for different concentrations of NO2 − was correlated. The linear character is proved here for the active dissolution of copper in the presence of NO2 − . The rate of conductance decreasing is greater with the higher the NO2 − concentration. This may be due to the increase in the concentration of the vital ions NO2 − which are one of the most important intermediates and very strong depolarizer.
25 30 40 50
3.36 × 10−3 3.30 × 10−3 3.19 × 10−3 3.10 × 10−3
r B Ea (kJ mol−1 )
Nitrite ions also affect the incubation period as can be seen from Table 6. It seems that NO2 − added from outside the solution makes long story of the accumulation of the catalytic intermediates shorter. The rise of temperature caused an increase in the active corrosion rate of copper in the presence of NO2 − . In this concern we managed to calculate the activation energy of the process. The activation energy was found to be a little pit lower than that found for free 2.5 M HNO3 acid (53.7 compared to 56.0 kJ mol−1 ). It is interesting to note that the increase of temperature besides increasing the corrosion rate it brought about a significant decrease in the incubation period (Table 7). The activation energy of the incubation period proceeding was computed by correlating ln 1/τ0 vs. 1/T (Arrhenius equation). The computed value of Ea was found to be 67.9 kJ mol−1 . This is very close to the value reported for the same process in 2.50 M HNO3 acid only (67.9 compared to 69.2 kJ mol−1 ). The fact that the addition of NO2 − does not cause appreciable variation in activation energy of active copper dissolution or proceeding of the incubation period means that no considerable changes in Table 6 The effect of NO2 − concentration on the incubation period for the dissolution of copper in 50.0 cm3 of 2.5 M HNO3 at 25◦ C [NO2 − ] (M) τ0 (min)
0.00 90.0
1×10−4 70.6
1×10−3 64.0
1×10−2 52.9
Table 7 Values of τ0 , the incubation period and Ea , the activation energy (from correlating 1/T with ln 1/τ0 ) for the dissolution of copper in 50.0 cm3 of 2.5 M HNO3 containing 1 × 10−4 M NO2 − at different temperatures T (◦ C)
τ0 (min)
1/T
ln 1/τ0
25 30 40 50
70.6 28.2 13.5 7.6
3.36 × 10−3 3.30 × 10−3 3.19 × 10−3 3.10 × 10−3
−4.257 −3.334 −2.663 −2.028
r = −0.981 Ea (kJ mol−1 ) = 67.9
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Fig. 7. The linear plot of C, the Cu+2 concentration in 50.0 ml of 2.50 M HNO3 vs. time, τ for the dissolution of copper of different surface areas (10.0,15.0,20.0 and 25.0 cm2 ) at 25◦ C.
the nature of the corrosion process. In this case, the NO2 − themselves are being the main catalytic intermediate and the depolarizer of the process [1]. 3.4. The effect of copper surface area The effect of the copper surface area on the corrosion process in the free acid was studied. Different surface areas were used viz. 10.0, 15.0, 20.0 and 25.0 cm2 . Fig. 7 demonstrates once more the linear relationship between Cu2+ concentration and time. Here again the linear regression gives the best correlation coefficient r (Table 8). The corrosion rate, which is taken as either the linear regression coefficient B (mol dm−3 min−1 ), or ν (mg cm−2 min−1 ) was found to be linearly dependent on the surface area. This is a well-accepted behavior for heterogeneous processes. By correlating surface area with B or ν a linear regression was found to fit with r = 0.9922. The fact that r for surface Table 8 The linear regression data for the dissolution of copper of different surface areas (10.0, 15.0, 20.0 and 25.0 cm2 ) in 50.0 cm3 of 3.0 M HNO3 at 25◦ C (mg cm−2
min−1 )
area with corrosion rate is not very close to unity may be attributed to the increasing effect of accumulated intermediates (NO2 ) as the surface area increases in a limited volume of HNO3 solution. The values of ν illustrate this possibility. As the surface area increases from 10.0 to 25.0 cm2 , the corrosion rate was found to increase slightly from 0.147 to 0.168 mg cm−2 min−1 showing that the increase in the concentration of accumulated intermediates affects the rate with which copper dissolves. The conductance measurements showed that the increase in surface area of copper metal leads to a parallel increase in the active corrosion rate in HNO3 acid. This is quite clear from the data listed in Table 9. From this table, the linear correlation coefficient is proving once more the linear behavior of conductance with time. By correlating the corrosion rate with the examined surface area, an excellent correlation coefficient for linear Table 9 Values of B, the regression coefficient and r, the correlation coefficient for the active dissolution of copper of different surface areas (10.0, 15.0, 20.0 and 25.0 cm2 ) in 50.0 cm3 of 2.5 M HNO3 at 25◦ C Surface area (cm2 )
Regression modes
Regression mode (linear)
ν
B r
0.0004629 0.9924978
0.147
10.0 B r
−0.184169 −0.000990 0.9816836 0.998102
15.0 cm2 B r
0.0006982 0.9911868
0.148
15.0 B r
−0.232087 −0.001635 −3.244583 −0.022727 0.9803397 0.9990577 0.9866151 0.9997038
20.0 cm2 B r
0.0009413 0.9935613
0.149
20.0 B r
−0.244271 −0.002263 −3.412854 −0.031444 0.9704112 0.9956325 0.9788717 0.9984780
25.0 cm2 B r
0.0013246 0.9933435
0.168
25.0 B r
−0.324921 −0.003011 −4.312328 −0.039666 0.9695784 0.9950360 0.9806236 0.9986258
Surface area 10.0 cm2
Power
Exponential
Logarithmic
Linear
−2.701341 −0.014476 0.9857910 0.998723
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Fig. 8. The linear plot of conductance, K with time, τ for the active dissolution of copper of different surface areas (10.0,15.0,20.0 and 25.0 cm2 ) in 50.0 ml of 2.5 M HNO3 at 25◦ C.
regression was obtained. Perfect linear dependence of −B, the linear regression coefficient (taken as a corrosion rate), of copper surface means that the active period of dissolution proceeds after the accumulation of sufficient amount of the catalytic intermediates (NO2 − ) and the factor of the surface area is purely cumulative, compared to the effect of copper surface area on the corrosion rate during the incubation period, where the role of surface area is also to enhance the formation of the catalytic intermediates. A careful examination of the obtained results shows that the incubation
period depends on the copper surface area. Fig. 8 shows the greater the surface area the shorter the incubation period (the incubation period at 10.0 cm2 (τ0 )4 > at 25.0 cm2 (τ0 )1 ). 3.5. The effect of stirring The dissolution of copper metal in HNO3 acid is known to be very sensitive to stirring. This is simply due to the continuous removal of the traces of catalytic intermediates (NO2 − ) from the vicinity of the copper surface by stirring.
Table 10 The regression data for the dissolution of copper in 50.0 cm3 of 2.5 M HNO3 with and without stirring at 25◦ C τ (min)
Concentration of Cu2+ (M) Without stirring
Stirring at position 1
Stirring at position 3
Stirring at position 5
15.0 30.0 45.0 60.0 75.0 90.0 105.0 120.0
0.000530 0.00480 0.00820 0.0159 0.0234 0.0321 0.0400 0.0474
0.0000980 0.000181 0.000263 0.000364 0.000461 0.000532 0.000641 0.000736
0.0000810 0.000141 0.000216 0.000299 0.000379 0.000449 0.000539 0.000598
0.0000720 0.000130 0.000190 0.000275 0.000355 0.000435 0.000515 0.000585
Linear regression B r
0.0004629 0.992496
0.00000609 0.9993466
0.00000507 0.9992404
0.00000502 0.9986464
Power regression B r
2.093108 0.987934
0.9761647 0.9991233
0.9890163 0.9982582
1.03329889 0.997501
Exponential regression B r
0.0369336 0.91058728
0.0180842 0.9668638
0.0183580 0.9679052
0.0192762 0.9720269
Logarithmic regression B r
0.0223828 0.918677
0.000302 0.952274
0.0002525 0.9529076
0.0002483 0.94476923
ν (mg cm−2 min−1 )
0.147
0.00193
0.00161
0.00159
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Fig. 9. (a) The anodic galvanostatic polarization curves for copper electrode (0.314 cm2 ) in 50 cm3 HNO3 of different concentrations at 25◦ C. (b) The cathodic galvanostatic polarization curves for copper electrode (0.314 cm2 ) in 50 cm3 HNO3 of different concentrations at 25◦ C.
Therefore the formations of these catalytic intermediates being extremely slow. Results of the effect of stirring on the corrosion of copper (Table 10) during the incubation period was found that a sudden drop in the corrosion rate was exhibited as we started stirring at position 1 of the magnetic stirrer, i.e. the slowest speed of stirring (rpm = 50). The corrosion rate of copper in the incubation period dropped almost 70 times from 0.157 to 0.00193 mg cm−2 min−1 upon the introduction of slight stirring. Slight stirring is thus sufficient to prevent the accumulation of the catalytic intermediates at the copper surface.
1. Values of cathodic polarization are generally much higher than anodic ones under the same conditions and the same concentration. 2. The higher the concentration of HNO3 acid the lower is the polarization value for both anodic and cathodic behavior. 3. Cathodic polarization curves for the higher range of concentrations does not exhibit limiting currents, while the lower concentrations well developed limiting currents. This may be due to a second reaction taking place. 4. Cathodic polarization were found to obey Tafel’s equation at low current values
3.6. Galvanostatic polarization behavior Anodic and cathodic galvanostatic polarization curves for 2.0–3.5 M HNO3 acid are displayed in Fig. 9a and b. The same curves for concentrations of HNO3 acid in the range 0.2–0.8 M are shown in Fig. 10a and b. From these figures it is obvious that
η = a + b log i where η, the polarization, a and b are Tafel’s constants. RT RT a = 2.303 log i0 , b = 2.303 αzF αzF
M. Khodari et al. / Materials Chemistry and Physics 71 (2001) 279–290
289
Fig. 10. (a) The anodic galvanostatic polarization curves for copper electrode (0.314 cm2 ) in 50 cm3 HNO3 of different concentrations at 25◦ C. (b) The cathodic galvanostatic polarization curves for copper electrode (0.314 cm2 ) in 50 cm3 HNO3 of different concentrations at 25◦ C.
where z is the number of moles of electrons, F the Faraday number and α the transference coefficient or in other words, the degree of irreversibility. The correlation of the cathodic current i, with the concentration of nitric acid, C at constant values of polarization in the range of limiting currents and close to them gave the highest correlation coefficient for the power regression: i = ACB . Values of B were found to be very close to unity for polarization values (Table 11) in the limiting current range (600–1000 mV). At 400 mV cathodic polarization, B value was found less than that 0.880. This can be explained as due to full diffusion control in the limiting current range, while prior to that the control is still mixed kinetic/diffusion one. The current dependence on nitric acid concentration with a power close to unity means a first order overall cathodic reduction process. The cathodic polarization value η were correlated to ln i (applying Tafel’s equation) for the cathodic process. Values of r, the correlation coefficient (Table 12) were found very high and indicate the applicability of Tafel’s equation and
in turn the electrochemical control at low current and polarization values. Values of Tafel’s constants a and b were recorded in Table 12. It can be seen from this table that the values of b varied with the concentration of nitric acid where higher figures correspond to lower concentrations. This fact may be related to the possible changes in the degree of irreversibility α with concentration of nitric acid. Table 11 The correlation between C, the concentration of HNO3 and i, the cathodic (anodic) current density at constant polarization potential (−400, −600, −800 and −1000 mV) C (M)
0.200 0.400 0.600 0.800 r B
i −400
−600
−800
−1000
12.7 21.4 31.0 44.0
20.5 41.1 60.6 80.8
23.5 44.7 70.7 87.3
23.8 48.3 75.0 93.8
0.995 0.880
1.000 0.988
1.000 0.965
1.000 1.000
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M. Khodari et al. / Materials Chemistry and Physics 71 (2001) 279–290
Table 12 Values of a and b, Tafel’s constants and r, the correlation coefficient for copper cathode (0.314 cm2 ) in 50.0 cm3 HNO3 of different concentrations at 25◦ C C (M)
a (mV)
b (mV)
r
0.200 0.400 0.600 0.800 2.00 2.50 3.00 3.50
−140 −20 −64 −107 −157 −130 −79 −20
−260 −288 −197 −166 −120 −106 −97 −121
0.950 0.997 0.990 0.998 0.988 0.982 0.992 0.993
2.
3.
4. The anodic galvanostatic polarization curves do not obey Tafel’s equation. For comparison we correlated i and C at anodic polarization of 400 mV and found that the power regression to fit best of all with r very high and B = 0.642. The power of the concentration term for the anodic current is distinctly less than the cathodic (0.642 compared to 0.965–1.00). This means that nitric acid is not the main anodic depolarizer as it is a cathodic one. The effect of hydrazine addition on the cathodic and anodic polarization of copper in nitric acid was studied. A sudden shift in cathodic and anodic polarization was noticed upon the introduction of any concentration of hydrazine. This may be due to corrosion retarding effect of hydrazine on both anodic and cathodic reactions (mixed inhibitor). The further increase in hydrazine concentration does not give rise to distinct increase in cathodic or anodic polarization. This is explained on the same basis as mentioned before. On the other hand, the addition of NO2 − ions did not affect the anodic or cathodic polarization. This may be attributed that NO2 − ions are themselves a cathodic depolarizer for copper corrosion which is formed and accumulated during the dissolution of copper in HNO3 .
4. Conclusions 1. The present work shows that both of the two used methods (stripping and conductance measurements) are satisfactory to study the two steps (incubation and active). The stripping analysis was found more acceptable one for studying the incubation period due to that analysis
5.
6.
7.
detect the very small concentrations of dissolved Cu2+ . Whereas in the active step, the dissolved Cu2+ need more and more dilution to detect its concentration. The more dilution leads in that case to some errors. On the other hand, the change in conductance measurements during the incubation period is very slow and unclear, whereas in the active step is clear. The very small rate of copper dissolution during the incubation period and the rate of active dissolution of copper are constant and time independent under all conditions. The conductance of solution varies linearly with time, which means a constant rate in time or a pseudozero-order kinetic. Rate of copper corrosion depends on surface area in a more or less perfect linear manner. Stirring prevents the corrosion of copper completely. The rate of copper corrosion is suppressed thousand times upon moderate stirring. Hydrazine added in nitric acid solution, even in minor quantities forms a dark brown, rigid, hard and adherent layer at the copper surface which blocks the corrosion of copper almost completely. The addition of the NO2 − promotes the copper corrosion in the incubation period and active dissolution, but does not affect the activation energy of the process, as these ions are themselves the intermediate catalyst formed during the incubation period.
References [1] F.M. El-Cheikh, S.A. Khalil, M.A. El-Manguch, A.O. Hadi, Ann. Chim. 73 (1983) 75. [2] D. Tromans, J.C. Silva, Corrosion 53 (1997) 171. [3] R.N. Singh, N. Verma, W.R. Singh, Corrosion 45 (1989) 222. [4] M. Georgiadon, R. Alkire, J. Electrochem. Soc. 140 (1993) 1340. [5] S.L.F.A. Da Costa, S.M.L. Agstinho, Corrosion 45 (1989) 472. [6] S.A. Khalil, M.A. El-Manguch, F.M. El-Cheikh, The Libyan J. Sci. 12 (1983) 1. [7] I.S. Smol’yaninov, V.A. Khitrov, Zh. Prikl. Khim. 37 (1964) 696. [8] F.M. El-Cheikh, S.A. Khalil, H.A. Omar, The Libyan J. Sci. 14 (1985) 15. [9] J. Wang, Stripping Analysis Principles, Instrumentation and Applications, VCH, Weinheim, 1985. [10] M. Khodari, Electroanalysis 15 (1998) 1061. [11] F.M. El-Cheikh, F.H. Assaf, M. Khodari, M.A. Gandour, M.M. Abou-krisha, Bull. Soc. Jpn. 67 (1994) 2286–2297. [12] Ctton, Wilkinson, Advanced Inorganic Chemistry, 3rd Edition, p. 350.
Stripping voltammetric and conductance measurements on corrosion and inhibition of copper in nitric acid M. Khodari a , M.M. Abou-krisha a,∗ , F.H. Assaf a , F.M. El-Cheikh b , A.A. Hussien a a
Chemistry Department, Faculty of Science, South Valley University, Qena, Egypt b Chemistry Department, Faculty of Science, Sohag, Egypt
Received 20 September 2000; received in revised form 5 January 2001; accepted 18 January 2001
Abstract The kinetics of copper dissolution in nitric acid especially, the phase of dissolution at the beginning of incubation period was investigated. Two techniques, the anodic stripping voltammetry and the conductance measurements, were used for the first time in this concern. Galvanostatic polarization measurements using very simple circuit used to study the corrosion process of copper in nitric acid. The effect of several factors was studied, namely nitric acid concentration, the copper surface area and stirring. The effect of inorganic substances and their role as inhibitors or promoters was also investigated. By applying the anodic stripping voltammetric technique it was found that the rate of copper dissolution during the incubation period is constant and time independent. Also, stirring prevents the corrosion of copper almost completely. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Copper; Dissolution rate; Inhibition; Anodic stripping voltammetry
1. Introduction The dissolution of copper in nitric acid is known to proceed in two stages [1]. During the first stage, the incubation period, the dissolution of copper extremely slow to be followed by the known analytical methods. The second stage is the active dissolution of copper. Numerous investigations have been carried out from different points of view on the dissolution of copper in acidic solutions [2–5]. Many techniques were suggested to investigate the dissolution and inhibition of Cu in different media [1,6,7]. In addition, the kinetics of copper dissolution in H2 SO4 solution containing different inorganic depolarizers was studied [6]. A first-order rate was proved for all depolarizers used. Smol’yaninov and Khitrov [8] stated that the corrosion rate of copper in HCl or H2 SO4 solution increased with the concentration of K2 Cr2 O7 . The change in energy of activation and the temperature coefficient of corrosion indicated that the rate of the process was determined both by diffusion and chemical effects. In the present work, the dissolution of copper in nitric acid was examined using stripping voltammetry [9,10], conductance and galvanostatic polarization techniques. Anodic stripping voltammetry used to study the dissolution in in∗ Corresponding author. E-mail address: mortaga [email protected] (M.M. Abou-krisha).
cubation period due to that analysis detect the very small ion concentrations. Conductance measurement technique is chosen to study the active dissolution, since the conductivity can be attributed to the decrease in the concentration of H+ which involved during the copper dissolution in HNO3 and can be easily detected by conductometer. However, galvanostatic polarization measurement using very simple circuit, used to study the corrosion process of copper in nitric acid and confirms the results obtained by the other two techniques. 2. Experimental The specimens of copper (99.9%) used in the present work were of thickness 0.80 mm, 1.0 cm in width and 5.0 cm in length. Copper specimens were washed in a tap water, in distilled water and dipped in pickling solution of the composition (1 H2 O:5 nitric acid) for about 15 s till effervescence covered the whole copper surface. Then washed thoroughly with tap water and finally with distilled water. This treatment was carried out immediately before starting the experiment. In stripping voltammetric measurements a computer-aided electrochemistry system potentiostat (EG and GPARC model 263), static mercury drop electrode (SMDE) and electroanalytical software models 270/250 version 4.0 (PARC) which controls the potentiostat via IEEE 488 GPIB were
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 2 9 1 - 7
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Fig. 1. Anodic stripping voltammograms for different concentrations of Cu2+ in the presence of 2.5 × 10−3 M HNO3 , at scan rate 50 mV s−1 , deposition potential −0.5 V and preconcentration time 30 s. (a) 0.00 M, (b) 1 × 10−5 M, (c) 2 × 10−5 M, (d) 3 × 10−5 M, (e) 4 × 10−5 M, (f) 5 × 10−5 M.
used. The SMDE model 303A contains three electrodes which were described elsewhere [10]. The anodic stripping voltammetric measurements were carried out by placing 50 cm3 of the used acid solution in a 50 cm3 beaker and thermostated at the working temperature. The cleaned copper specimen was then immersed vertically in the solution and the time started. Changes in the concentration of Cu2+ were followed by withdrawing 0.10 cm3 of the solution each time, completed to 100.0 cm3 by second distilled water and the current corresponding to the concentration of Cu2+ was recorded. For all stripping voltammetric measurements, the solution under investigation was degassed in a 10 cm3 cell for 8 min using purified nitrogen gas. Then the voltammogram was recorded after applying the required potential for a certain period of time. The concentration of Cu+2 was calculated from standard working curves. The standard working curves were made by adding known concentrations of Cu2+ to 10 cm3 of the diluted corrosive solution before immersing the copper specimen. The peak current corresponding to this concentration of Cu2+ was recorded after applying a definite potential for certain period of time. Representative anodic stripping voltammograms are shown in Fig. 1. In this work, a standard working curve (Fig. 2 as an example) was constructed for each concentration of nitric acid and the used inhibitors. The specimens of copper (99.9%) used in the conductance measurement technique were of dimensions 1.0 cm in width and 5.0 cm in length. In conductance technique a 50 cm3 of nitric acid solution was placed in a 50 cm3 beaker and thermostat at the working temperature. The cleaned Cu specimen was then immersed vertically in the solution and the time started. Changes in the conductance of nitric acid was followed by withdrawing 0.2 cm3 of the solution each time, which added to 10 cm3 distilled water in a test tube and the conductance of such solution was determined using digital conductometer (consort K120). The dilution of the original working nitric acid solution was carried out to put the
measured conductance values within the measuring range of the conductometer. In galvanostatic polarization technique, the working copper electrode was prepared from extra pure copper wire of diameter 0.1 cm and length 1 cm. The reference electrode was itself the counter electrode, which was made of copper sheet with surface area much greater (80 cm2 ) than that of the working electrode. The electric circuit for the galvanostatic polarization measurement is shown in Fig. 3. For each run, the working and the reference/counter electrodes were placed into 50.0 cm3 of the test solution. For cathodic galvanostatic polarization experiments, the negative pole of DC power supply (Delta Electronika CST 100) was connected to the working electrode while the positive pole was connected
Fig. 2. The anodic peak current of Cu2+ as a function of [Cu2+ ].
M. Khodari et al. / Materials Chemistry and Physics 71 (2001) 279–290
281
Fig. 3. Circuitry for galvanostatic polarization measurements. R/C E: reference/counter electrode, W: working electrode.
to the reference/counter electrode. The same was carried out for anodic polarization measurements with change in the connection poles. The working and reference/counter electrodes were connected to the potentiometer (EIL-pH meter 7020). For each run, a certain current density was applied for 2 min, the potential difference between the working and the reference/counter electrodes was then measured. Then gradually increasing the current density each 2 min and the potential difference was then recorded. In this study, the counter and reference electrodes are one and the same electrodes. This was made possible because the surface area of the counter/reference electrode was several hundred times greater than the surface area of the working electrode. The counter/reference electrode is thus considered to be practically nonpolarizable. Moreover, the conductance of the nitric acid solutions is so high that the ohmic potential drop across the solution is negligible. In this work all chemicals used of AR grade and all measurements were carried out at 25 ± 0.5◦ C.
3.1. Treatment of the data Results of the variation of Cu2+ concentration accumulated in nitric acid solution during the incubation period with time are reported for different HNO3 acid concentrations in Table 1. In this table the correlation and regression [11] coefficients are also reported. It was found that Cu2+ concentration increased continuously with time. The correlation, between the concentration C and the time τ , can expressed statistically by the following formula: C = A + B τ
Power regression :
C = A0 + B 0 ln τ
C = A τ B
Exponential regression :
C = A∗ eB
(2) (3)
∗ τ
(4)
The interpretation of the constants are explained as follows: 1. The first relation shows the linear relationship, where A is denoted to the concentration of Cu2+ at τ = 0 whereas B is the concentration of Cu2+ per minute during the linear relation (i.e. the dissolution rate of Cu2+ expressed in mol dm−3 min−1 ). 2. In formula (2) A0 is the concentration of Cu2+ after 1 min. The meaning of the constant B0 was interpreted by using τ = e = 2.718281828 min, hence the formula (2) becomes Cat(τ =e) = A0(the concentration at τ =1) + B 0 ln τ B 0 = Cat(τ =e) − A0(the concentration at τ =1)
3. Results and discussion
Linear regression :
Logarithmic regression :
(1)
i.e. B0 is the concentration of Cu2+ after e minutes subtracted from its value at 1 min. 3. Also, the power regression (formula 3) becomes ln C = ln A + B ln τ at τ = 1, then C = A i.e. A is the concentration of CU2+ at τ = 1, and at τ = e the formula becomes ln Cat(τ =e) = ln A + B B = ln
Cat(τ =e) A
if Z = natural antilogarithm, then
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Table 1 The regression data for the dissolution of copper in 50.0 cm3 HNO3 of different concentrations at 25◦ C [HNO3 ] (M) 2.00
2.50
τ (min)
Cu2+
15.0 30.0 45.0 60.0 75.0 90.0 105.0 120.0
0.000310 0.00130 0.00350 0.00500 0.00713 0.00941 0.0108 0.0134
(M)
Linear regression equation y = A + Bx, i.e. C = A + B τ B 0.0001263 r 0.996806
3.00
τ (min)
Cu2+
15.0 30.0 45.0 60.0 75.0 90.0 105.0 120.0
0.000530 0.00480 0.00820 0.0159 0.0234 0.0321 0.0400 0.0474
τ (min) 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0
0.00337 0.00809 0.0135 0.0187 0.0244 0.0299 0.0337 0.0408 0.0460 0.0528
(M)
τ (min)
Cu2+ (M)
10.0 15.0 20.0 25.0 30.0 35.0 40.0
0.00509 0.0140 0.0229 0.0319 0.0408 0.0497 0.0586
0.0010879 0.9990655
0.0017847 0.9999995
Power regression equation y = AxB , i.e. C = Aτ B or ln C = ln A + B ln τ B 1.8008307 2.0931308 r 0.9929896 0.987934
1.8758386 0.9893259
1.7148472 0.989325
Exponential regression equation y = A eBx , i.e. C = A eBτ or ln C = ln A + Bτ B 0.0319750 0.0369336 r 0.9209804 0.91058728
0.0538497 0.9417963
0.0745531 0.9433452
Logarithmic regression equation y = A + B ln x, i.e. C = A + B ln τ B 0.0062110 0.0223828 r 0.938065 0.918677
0.0353002 0.9756823
0.0383855 0.980596
ν (mg cm−2 min−1 )
0.345
0.567
0.0401
0.0004629 0.992496
(M)
3.50 Cu2+
0.147
C , Cat(τ =e) = ZA(the concentration at t=1) , A b = Cτ =e − Aτ =1 = ZAτ =1 − Aτ =1
Z=
i.e. b is the concentration of Cu2+ after e minutes subtracted from its value at 1 min. 4. The meaning of the constants for the last formula are interpreted by the following: ln C = ln A∗ + B ∗ τ It is clear that at τ = 0, C = A∗ = the concentration of Cu2+ at τ = 0, and at τ = 1, the formula becomes ln Cat(τ =1) = ln A∗ + B ∗ B ∗ = ln
Cat(τ =1) , A∗
if M = natural antilogarithms, then
C , Cat(τ =1) = MA(the concentration at τ =0) , A∗ b∗ = Cτ =1 − Aτ =0 = MAτ =0 − Aτ =0 M=
i.e. b is the concentration of Cu2+ after 1 min subtracted from its value at time equal to 0. The correlation between C, the concentration of Cu2+ and τ , the time showed that a linear regression is the best to this
relation (r > 0.99). Thus Cu2+ concentration in solution is related to time best of all in the form: C = A + Bτ 3.2. The effect of nitric acid concentration Values of corrosion rates, ν of copper expressed in mg cm−2 min−1 were computed using the relation (5) from the values of the slope B (the corrosion rate of Cu2+ expressed in mol dm−3 min−1 ), the regression coefficient for the most fitting relation, namely the linear one, and reported in Table 1 ν
mol Cu2+ 1 dm3 mg Cu 3 = B × × 50 cm cm2 min 1000 cm3 dm3 min 63.5 g Cu 1 1000 mg × × ∗ 2× 2+ 1g A cm 1 mol Cu
(5)
where A∗ is the surface area of copper sheet. The linear character in the relation between C and time, τ points towards a constant corrosion rate during the whole incubation period which means a pseudo-zero-order process. This is quite logic as the depletion in nitric acid concentration is negligible. Nitric acid is being the main depolarizer in this stage of dissolution. The linearity, thus can hardly be attributed to a true zero-order kinetic.
M. Khodari et al. / Materials Chemistry and Physics 71 (2001) 279–290
283
Fig. 4. The variation of conductance, K with time, τ for the dissolution of copper in 50 cm3 HNO3 of different concentrations at 25◦ C.
The variation of electric conductance with time as a result of copper dissolution (Fig. 4) in the presence of different HNO3 acid concentrations was measured. The collected data were treated using the different regression modes (Table 2), where conductance data were taken during the active period of copper dissolution only. The dissolution of copper in nitric acid during the incubation period was found to be so slow that almost no change in the conductance of solution was noticed. As catalytic intermediates accumulate near the copper surface, active dissolution starts accompanied with distinct decrease in conductance due to the exhaust of H3 O+ . The solution itself begins to acquire the characteristic blue coloration of hydrated Cu2+ . It is interesting to note that during the incubation period no blue coloration was noticed. The linear character of the relationship between conductance and time is quite evident as the correlation coefficient, r was found to be the highest for linear regression.
Table 2 Values of B, the regression coefficient and r, the correlation coefficient for the active dissolution of copper in 50.0 cm3 HNO3 of different concentrations at 25◦ C C (M)
Regression modes Power
Exponential
Logarithmic
Linear
2.00 B r
−0.275812 0.9824685
−0.000777 0.9886729
−3.353593 0.9842595
−0.009444 0.9898900
2.50 B r
−0.184169 0.9816836
−0.000990 0.9981029
−2.701341 0.98579100
−0.014476 0.9987239
3.00 B r
−0.2240000 0.9824210
−0.002046 0.9988846
−3.866970 0.9877855
−0.035164 0.9994389
3.50 B r
−0.300019 0.9670924
−0.003207 0.9979324
−5.224967 0.9807256
−0.055178 0.99965739
During the active period of the copper dissolution the rate with which conductance changes with time can be taken as the corrosion rate of copper. The intercept of straight lines (conductance vs. time) in Fig. 5 with values of conductance of the starting solution at τ = 0 (from Fig. 4) is taken as the incubation period, τ0 . It is clear that the incubation period decreases with the increase in nitric acid concentration. It is interesting to mention that the linear regression coefficient, B is taken for the corrosion rate with opposite sign, as B is always <1 (because conductance decreases all the time). Values of −B (resembling the corrosion rate (ν) of copper in the active period) were correlated with the nitric acid concentration (C). It was found that the rate of active dissolution of Cu is best of all expressed in the power function or the exponential function. For both of these expressions r was found to be 0.99. At the same time linear and logarithmic regressions gave rather lower values of r. Comparison of power functions for the dependence of corrosion rates of copper on nitric acid concentration for the incubation and active periods, reveals that the power is higher for the incubation period (4.775) than for active dissolution (3.290). This means that the dissolution of copper in the incubation period is more concentration dependent than the active dissolution. 3.3. The effect of inorganic substances In this part the effect of hydrazine N2 H4 and NO2 − was studied. The first is neither catalyst nor intermediate in the process of copper dissolution during the incubation period, while the second is the suggested intermediate catalyst according to El-Cheikh et al. [1]. 3.3.1. The effect of hydrazine The effect of different concentrations of hydrazine on the variation of Cu2+ concentration with time during the incubation period during interaction between copper metal and
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Fig. 5. The linear plot of conductance, K vs. time, τ for the active dissolution of copper in 50 cm3 HNO3 of different concentrations at 25◦ C.
3.0 M nitric acid was studied. The correlation of Cu2+ concentration with time for different hydrazine concentrations in 3.0 M HNO3 acid using the four regression modes gave very high values of r, the correlation coefficient for linear regression (Table 3). This once more confirms the linearity of the relationship between Cu2+ concentration and time during the incubation period under all conditions. Table 3 contains also values of the corrosion rate, ν in mg cm−2 min−1 for different concentrations of hydrazine in 3.0 M HNO3 acid. In this table the inhibition efficiencies {1% = [(νu − νi )/νu ] × 100, where ν u and ν i are the corrosion rates for copper in uninhibited and inhibited solutions, respectively} of different concentrations of hydrazine are also listed. The corrosion of copper in nitric acid was found to stop almost completely upon the addition of 0.01 M hydrazine (I,
the inhibition efficiency 99.8%). This is due to the formation of insoluble dark brown layer strongly adhered to the copper surface in solutions of nitric acid containing any traces of hydrazine. Apparently this layer blocks copper from further interaction with nitric acid. It is well known that hydrazine like NH3 can form coordination complexes with both Lewis and metal ions [12]. Just as with respect to the proton, electrostatic and in these cases, also steric considerations militate against bifunctional behavior. Some polymeric complexes having hydrazine bridges have been demonstrated in Fig. 6. The effect of temperature on the corrosion rate of copper was studied in a media containing 3.0 M HNO3 acid and 0.5 M hydrazine. From the variation of corrosion rate with temperature, the activation energy was calculated using Arrhenius equation as follows: K = A e−Ea /RT
Table 3 Values of B, r, ν (mg cm−2 min−1 ) and I (%) for the dissolution of copper in 50.0 cm3 of 3.0 M HNO3 in the absence and presence of different concentrations of hydrazine at 25◦ C C (M)
Regression mode (linear)
ν (mg cm−2 min−1 )
0.00 B r
0.0010879 0.9990655
0.345
0.010 B r
0.0000018 0.9976754
0.000572
99.83
0.100 B r
0.0000017 0.9993214
0.000540
99.84
0.500 B r
0.0000016 0.9980532
0.000508
99.85
1.00 B r
0.0000015 0.9967626
0.000476
99.86
I (%) –
where K is the specific reaction rate, A the frequency factor and Ea the activation energy. In our case either B, the linear regression coefficient for correlation of C and τ or ν, the corrosion rate is used in place of K in Arrhenius equation. The value of Ea obtained by correlating ln B with 1/T where the regression coefficient (slope) equal Ea /R then Ea = slope × R. The results showed no marked effect and the calculated activation energy is 24.8 kJ mol−1 . It is interesting to mention that the conductance of nitric acid solution containing different concentrations of hydrazine showed no significant changes even at quite long
Fig. 6. Sketch of the polymer complex of hydrazine-bridge.
M. Khodari et al. / Materials Chemistry and Physics 71 (2001) 279–290 Table 4 Values of B, r, ν (mg cm−2 min−1 ) and I (%) for the dissolution of copper in 50.0 cm3 of 3.0 M HNO3 in the absence and presence of different concentrations of NO2 − at 25◦ C C (M)
Regression mode (linear)
ν (mg cm−2 min−1 )
0.00 B r
0.000462 0.992497
0.147
0.000477 0.995661
0.152
−3.4
1 × 10−3 B r
0.000503 0.995802
0.160
−8.8
1 × 10−2 B r
0.000601 0.999470
0.191
−29.9
1 × 10−4 B r
I (%)
285
Table 5 The correlation between 1/T and ln B for the calculation of the activation energy for the dissolution of copper in 50.0 cm3 of 2.5 M HNO3 in the absence and presence of 1 × 10−4 M NO2 − T (◦ C)
1/T
ln B 2.5 M HNO3
2.5 M HNO3 + 1 × 10−4 M NO2 −
−7.68 −7.30 −6.48 −5.34
−7.65 −6.94 −6.03 −5.34
−0.988488 −8858 73.6
−0.996434 −8738 72.6
–
periods of time after the immersion of copper metal in solution. 3.3.2. The effect of NO2 − Concentrations ranged from 1 × 10−4 to 1 × 10−2 M of NO2 − was studied. Higher concentrations do not allow reasonable rates of corrosion as NO2 − are intermediate catalyst in the corrosion process of copper in nitric acid. Results of correlation of Cu2+ concentration with time during the incubation period of copper dissolution in 2.5 M HNO3 acid in the presence of different concentrations of NO2 − are listed in Table 4. From these data one can observe the promoting effect of NO2 − on the dissolution of copper during the incubation period. 0.01 M of NO2 − was found sufficient to increase corrosion rate by 33%. This behavior confirms beyond any doubt the catalytic role of NO2 − as intermediates in the corrosion process of copper in HNO3 acid. The increase in temperature caused a parallel increase in the corrosion rate of copper in 2.50 M HNO3 acid in presence of 1×10−4 M NO2 − . It was found that the activation energy of copper dissolution in HNO3 acid during the incubation period does not change upon the addition of NO2 − (Table 5). This can simply attributed to the fact that the nature of the corrosion process does not suffer any principle changes by addition of NO2 − as these ions are formed during the course of copper corrosion in nitric acid. It is quite clear from conductance measurements that the addition of very small amounts of (1 × 10−4 –1 × 10−2 M) NO2 − caused a clear rise in the corrosion rate of copper in 2.5 M HNO3 acid. The variation of conductance with time for different concentrations of NO2 − was correlated. The linear character is proved here for the active dissolution of copper in the presence of NO2 − . The rate of conductance decreasing is greater with the higher the NO2 − concentration. This may be due to the increase in the concentration of the vital ions NO2 − which are one of the most important intermediates and very strong depolarizer.
25 30 40 50
3.36 × 10−3 3.30 × 10−3 3.19 × 10−3 3.10 × 10−3
r B Ea (kJ mol−1 )
Nitrite ions also affect the incubation period as can be seen from Table 6. It seems that NO2 − added from outside the solution makes long story of the accumulation of the catalytic intermediates shorter. The rise of temperature caused an increase in the active corrosion rate of copper in the presence of NO2 − . In this concern we managed to calculate the activation energy of the process. The activation energy was found to be a little pit lower than that found for free 2.5 M HNO3 acid (53.7 compared to 56.0 kJ mol−1 ). It is interesting to note that the increase of temperature besides increasing the corrosion rate it brought about a significant decrease in the incubation period (Table 7). The activation energy of the incubation period proceeding was computed by correlating ln 1/τ0 vs. 1/T (Arrhenius equation). The computed value of Ea was found to be 67.9 kJ mol−1 . This is very close to the value reported for the same process in 2.50 M HNO3 acid only (67.9 compared to 69.2 kJ mol−1 ). The fact that the addition of NO2 − does not cause appreciable variation in activation energy of active copper dissolution or proceeding of the incubation period means that no considerable changes in Table 6 The effect of NO2 − concentration on the incubation period for the dissolution of copper in 50.0 cm3 of 2.5 M HNO3 at 25◦ C [NO2 − ] (M) τ0 (min)
0.00 90.0
1×10−4 70.6
1×10−3 64.0
1×10−2 52.9
Table 7 Values of τ0 , the incubation period and Ea , the activation energy (from correlating 1/T with ln 1/τ0 ) for the dissolution of copper in 50.0 cm3 of 2.5 M HNO3 containing 1 × 10−4 M NO2 − at different temperatures T (◦ C)
τ0 (min)
1/T
ln 1/τ0
25 30 40 50
70.6 28.2 13.5 7.6
3.36 × 10−3 3.30 × 10−3 3.19 × 10−3 3.10 × 10−3
−4.257 −3.334 −2.663 −2.028
r = −0.981 Ea (kJ mol−1 ) = 67.9
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Fig. 7. The linear plot of C, the Cu+2 concentration in 50.0 ml of 2.50 M HNO3 vs. time, τ for the dissolution of copper of different surface areas (10.0,15.0,20.0 and 25.0 cm2 ) at 25◦ C.
the nature of the corrosion process. In this case, the NO2 − themselves are being the main catalytic intermediate and the depolarizer of the process [1]. 3.4. The effect of copper surface area The effect of the copper surface area on the corrosion process in the free acid was studied. Different surface areas were used viz. 10.0, 15.0, 20.0 and 25.0 cm2 . Fig. 7 demonstrates once more the linear relationship between Cu2+ concentration and time. Here again the linear regression gives the best correlation coefficient r (Table 8). The corrosion rate, which is taken as either the linear regression coefficient B (mol dm−3 min−1 ), or ν (mg cm−2 min−1 ) was found to be linearly dependent on the surface area. This is a well-accepted behavior for heterogeneous processes. By correlating surface area with B or ν a linear regression was found to fit with r = 0.9922. The fact that r for surface Table 8 The linear regression data for the dissolution of copper of different surface areas (10.0, 15.0, 20.0 and 25.0 cm2 ) in 50.0 cm3 of 3.0 M HNO3 at 25◦ C (mg cm−2
min−1 )
area with corrosion rate is not very close to unity may be attributed to the increasing effect of accumulated intermediates (NO2 ) as the surface area increases in a limited volume of HNO3 solution. The values of ν illustrate this possibility. As the surface area increases from 10.0 to 25.0 cm2 , the corrosion rate was found to increase slightly from 0.147 to 0.168 mg cm−2 min−1 showing that the increase in the concentration of accumulated intermediates affects the rate with which copper dissolves. The conductance measurements showed that the increase in surface area of copper metal leads to a parallel increase in the active corrosion rate in HNO3 acid. This is quite clear from the data listed in Table 9. From this table, the linear correlation coefficient is proving once more the linear behavior of conductance with time. By correlating the corrosion rate with the examined surface area, an excellent correlation coefficient for linear Table 9 Values of B, the regression coefficient and r, the correlation coefficient for the active dissolution of copper of different surface areas (10.0, 15.0, 20.0 and 25.0 cm2 ) in 50.0 cm3 of 2.5 M HNO3 at 25◦ C Surface area (cm2 )
Regression modes
Regression mode (linear)
ν
B r
0.0004629 0.9924978
0.147
10.0 B r
−0.184169 −0.000990 0.9816836 0.998102
15.0 cm2 B r
0.0006982 0.9911868
0.148
15.0 B r
−0.232087 −0.001635 −3.244583 −0.022727 0.9803397 0.9990577 0.9866151 0.9997038
20.0 cm2 B r
0.0009413 0.9935613
0.149
20.0 B r
−0.244271 −0.002263 −3.412854 −0.031444 0.9704112 0.9956325 0.9788717 0.9984780
25.0 cm2 B r
0.0013246 0.9933435
0.168
25.0 B r
−0.324921 −0.003011 −4.312328 −0.039666 0.9695784 0.9950360 0.9806236 0.9986258
Surface area 10.0 cm2
Power
Exponential
Logarithmic
Linear
−2.701341 −0.014476 0.9857910 0.998723
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287
Fig. 8. The linear plot of conductance, K with time, τ for the active dissolution of copper of different surface areas (10.0,15.0,20.0 and 25.0 cm2 ) in 50.0 ml of 2.5 M HNO3 at 25◦ C.
regression was obtained. Perfect linear dependence of −B, the linear regression coefficient (taken as a corrosion rate), of copper surface means that the active period of dissolution proceeds after the accumulation of sufficient amount of the catalytic intermediates (NO2 − ) and the factor of the surface area is purely cumulative, compared to the effect of copper surface area on the corrosion rate during the incubation period, where the role of surface area is also to enhance the formation of the catalytic intermediates. A careful examination of the obtained results shows that the incubation
period depends on the copper surface area. Fig. 8 shows the greater the surface area the shorter the incubation period (the incubation period at 10.0 cm2 (τ0 )4 > at 25.0 cm2 (τ0 )1 ). 3.5. The effect of stirring The dissolution of copper metal in HNO3 acid is known to be very sensitive to stirring. This is simply due to the continuous removal of the traces of catalytic intermediates (NO2 − ) from the vicinity of the copper surface by stirring.
Table 10 The regression data for the dissolution of copper in 50.0 cm3 of 2.5 M HNO3 with and without stirring at 25◦ C τ (min)
Concentration of Cu2+ (M) Without stirring
Stirring at position 1
Stirring at position 3
Stirring at position 5
15.0 30.0 45.0 60.0 75.0 90.0 105.0 120.0
0.000530 0.00480 0.00820 0.0159 0.0234 0.0321 0.0400 0.0474
0.0000980 0.000181 0.000263 0.000364 0.000461 0.000532 0.000641 0.000736
0.0000810 0.000141 0.000216 0.000299 0.000379 0.000449 0.000539 0.000598
0.0000720 0.000130 0.000190 0.000275 0.000355 0.000435 0.000515 0.000585
Linear regression B r
0.0004629 0.992496
0.00000609 0.9993466
0.00000507 0.9992404
0.00000502 0.9986464
Power regression B r
2.093108 0.987934
0.9761647 0.9991233
0.9890163 0.9982582
1.03329889 0.997501
Exponential regression B r
0.0369336 0.91058728
0.0180842 0.9668638
0.0183580 0.9679052
0.0192762 0.9720269
Logarithmic regression B r
0.0223828 0.918677
0.000302 0.952274
0.0002525 0.9529076
0.0002483 0.94476923
ν (mg cm−2 min−1 )
0.147
0.00193
0.00161
0.00159
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Fig. 9. (a) The anodic galvanostatic polarization curves for copper electrode (0.314 cm2 ) in 50 cm3 HNO3 of different concentrations at 25◦ C. (b) The cathodic galvanostatic polarization curves for copper electrode (0.314 cm2 ) in 50 cm3 HNO3 of different concentrations at 25◦ C.
Therefore the formations of these catalytic intermediates being extremely slow. Results of the effect of stirring on the corrosion of copper (Table 10) during the incubation period was found that a sudden drop in the corrosion rate was exhibited as we started stirring at position 1 of the magnetic stirrer, i.e. the slowest speed of stirring (rpm = 50). The corrosion rate of copper in the incubation period dropped almost 70 times from 0.157 to 0.00193 mg cm−2 min−1 upon the introduction of slight stirring. Slight stirring is thus sufficient to prevent the accumulation of the catalytic intermediates at the copper surface.
1. Values of cathodic polarization are generally much higher than anodic ones under the same conditions and the same concentration. 2. The higher the concentration of HNO3 acid the lower is the polarization value for both anodic and cathodic behavior. 3. Cathodic polarization curves for the higher range of concentrations does not exhibit limiting currents, while the lower concentrations well developed limiting currents. This may be due to a second reaction taking place. 4. Cathodic polarization were found to obey Tafel’s equation at low current values
3.6. Galvanostatic polarization behavior Anodic and cathodic galvanostatic polarization curves for 2.0–3.5 M HNO3 acid are displayed in Fig. 9a and b. The same curves for concentrations of HNO3 acid in the range 0.2–0.8 M are shown in Fig. 10a and b. From these figures it is obvious that
η = a + b log i where η, the polarization, a and b are Tafel’s constants. RT RT a = 2.303 log i0 , b = 2.303 αzF αzF
M. Khodari et al. / Materials Chemistry and Physics 71 (2001) 279–290
289
Fig. 10. (a) The anodic galvanostatic polarization curves for copper electrode (0.314 cm2 ) in 50 cm3 HNO3 of different concentrations at 25◦ C. (b) The cathodic galvanostatic polarization curves for copper electrode (0.314 cm2 ) in 50 cm3 HNO3 of different concentrations at 25◦ C.
where z is the number of moles of electrons, F the Faraday number and α the transference coefficient or in other words, the degree of irreversibility. The correlation of the cathodic current i, with the concentration of nitric acid, C at constant values of polarization in the range of limiting currents and close to them gave the highest correlation coefficient for the power regression: i = ACB . Values of B were found to be very close to unity for polarization values (Table 11) in the limiting current range (600–1000 mV). At 400 mV cathodic polarization, B value was found less than that 0.880. This can be explained as due to full diffusion control in the limiting current range, while prior to that the control is still mixed kinetic/diffusion one. The current dependence on nitric acid concentration with a power close to unity means a first order overall cathodic reduction process. The cathodic polarization value η were correlated to ln i (applying Tafel’s equation) for the cathodic process. Values of r, the correlation coefficient (Table 12) were found very high and indicate the applicability of Tafel’s equation and
in turn the electrochemical control at low current and polarization values. Values of Tafel’s constants a and b were recorded in Table 12. It can be seen from this table that the values of b varied with the concentration of nitric acid where higher figures correspond to lower concentrations. This fact may be related to the possible changes in the degree of irreversibility α with concentration of nitric acid. Table 11 The correlation between C, the concentration of HNO3 and i, the cathodic (anodic) current density at constant polarization potential (−400, −600, −800 and −1000 mV) C (M)
0.200 0.400 0.600 0.800 r B
i −400
−600
−800
−1000
12.7 21.4 31.0 44.0
20.5 41.1 60.6 80.8
23.5 44.7 70.7 87.3
23.8 48.3 75.0 93.8
0.995 0.880
1.000 0.988
1.000 0.965
1.000 1.000
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Table 12 Values of a and b, Tafel’s constants and r, the correlation coefficient for copper cathode (0.314 cm2 ) in 50.0 cm3 HNO3 of different concentrations at 25◦ C C (M)
a (mV)
b (mV)
r
0.200 0.400 0.600 0.800 2.00 2.50 3.00 3.50
−140 −20 −64 −107 −157 −130 −79 −20
−260 −288 −197 −166 −120 −106 −97 −121
0.950 0.997 0.990 0.998 0.988 0.982 0.992 0.993
2.
3.
4. The anodic galvanostatic polarization curves do not obey Tafel’s equation. For comparison we correlated i and C at anodic polarization of 400 mV and found that the power regression to fit best of all with r very high and B = 0.642. The power of the concentration term for the anodic current is distinctly less than the cathodic (0.642 compared to 0.965–1.00). This means that nitric acid is not the main anodic depolarizer as it is a cathodic one. The effect of hydrazine addition on the cathodic and anodic polarization of copper in nitric acid was studied. A sudden shift in cathodic and anodic polarization was noticed upon the introduction of any concentration of hydrazine. This may be due to corrosion retarding effect of hydrazine on both anodic and cathodic reactions (mixed inhibitor). The further increase in hydrazine concentration does not give rise to distinct increase in cathodic or anodic polarization. This is explained on the same basis as mentioned before. On the other hand, the addition of NO2 − ions did not affect the anodic or cathodic polarization. This may be attributed that NO2 − ions are themselves a cathodic depolarizer for copper corrosion which is formed and accumulated during the dissolution of copper in HNO3 .
4. Conclusions 1. The present work shows that both of the two used methods (stripping and conductance measurements) are satisfactory to study the two steps (incubation and active). The stripping analysis was found more acceptable one for studying the incubation period due to that analysis
5.
6.
7.
detect the very small concentrations of dissolved Cu2+ . Whereas in the active step, the dissolved Cu2+ need more and more dilution to detect its concentration. The more dilution leads in that case to some errors. On the other hand, the change in conductance measurements during the incubation period is very slow and unclear, whereas in the active step is clear. The very small rate of copper dissolution during the incubation period and the rate of active dissolution of copper are constant and time independent under all conditions. The conductance of solution varies linearly with time, which means a constant rate in time or a pseudozero-order kinetic. Rate of copper corrosion depends on surface area in a more or less perfect linear manner. Stirring prevents the corrosion of copper completely. The rate of copper corrosion is suppressed thousand times upon moderate stirring. Hydrazine added in nitric acid solution, even in minor quantities forms a dark brown, rigid, hard and adherent layer at the copper surface which blocks the corrosion of copper almost completely. The addition of the NO2 − promotes the copper corrosion in the incubation period and active dissolution, but does not affect the activation energy of the process, as these ions are themselves the intermediate catalyst formed during the incubation period.
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