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Comparative study on the electrochemical performance of the Cu–30Ni and Cu–20Zn–10Ni alloys
Journal of Alloys and Compounds 491 (2010) 92–97
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Comparative study on the electrochemical performance of the Cu–30Ni and Cu–20Zn–10Ni alloys Xiao-Zhong Zhou, Chu-Ping Deng, Yu-Chang Su ∗ School of Materials Science and Engineering, Central South University, Changsha 410083, Hunan, PR China
a r t i c l e
i n f o
Article history: Received 12 August 2009 Received in revised form 23 October 2009 Accepted 2 November 2009 Available online 6 November 2009 Keywords: Copper alloy Corrosion Electrochemical impedance spectroscopy
a b s t r a c t The electrochemical performance of the Cu–20Zn–10Ni alloy was compared with that of the Cu–30Ni alloy in a 3.5 wt.% sodium chloride solution. Electrochemical impedance spectroscopy (EIS) measurements show that, interestingly, both the increasing rates of the thickness and resistance of the passive film formed on the Cu–20Zn–10Ni alloy are larger than those of the passive film formed on the Cu–30Ni alloy during the immersion of the two alloys in the 3.5 wt.% NaCl solution. The passive film formed on the Cu–20Zn–10Ni alloy surface is found to have a higher resistance and thicker depth than that formed on the Cu–30Ni alloy surface after the immersion of the two alloys in the 3.5 wt.% NaCl solution for 120 h. To some extent, the Cu–20Zn–10Ni alloy could substitute for the Cu–30Ni alloy in a corrosive environment containing chloride ions. According to X-ray diffraction and scanning electron microscopy studies, the corrosion product films of the Cu–20Zn–10Ni alloy were found to consist of two layers, with the inner layer and the outer layer being composed of cuprous oxide and (Cu, Zn)2 Cl(OH)3 , respectively, while the corrosion product films of the Cu–30Ni alloy were found to consist mainly of (Cu, Ni)2 Cl(OH)3 . © 2009 Elsevier B.V. All rights reserved.
1. Introduction Copper-based alloys can be found in a wide variety of applications because of their good physical properties, excellent corrosion resistance, and reasonable price. For example, copper–zinc alloys are used as materials for condenser and heat-exchange tubes in power-generation industry. Copper–nickel alloys are used as materials for condenser tubes in marine applications, where anti-erosion and anti-corrosion are very important. The corrosion behavior depends heavily on the nature of a passivation film formed on the alloy surface. However, these alloys will suffer from accelerated corrosion when exposed to seawater for the presence of chloride ions. The structure of the passivation film formed on the alloy surface under a simulated seawater environment was found to consist of two layers, which are an outer CuO layer with chemisorbed water molecules and traces of chloride ions, and an inner Cu2 O layer containing Ni2+ and Ni3+ [1]. Nickel silver is a general term for the alloys that contain copper, nickel, and zinc. Other common names are Chinese silver, German silver, nickel bronze, and alpaca silver. These alloys are hard, corrosion resistant, and ductile, and actually contain no elemental
∗ Corresponding author. Tel.: +86 731 88830735; fax: +86 731 88830785. E-mail address: [email protected] (Y.-C. Su). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.002
silver. Nickel silver first became popular as a base metal for silver plated cutlery and other silverware, especially electro-plated nickel silver (EPNS), jewellery, and coins. Its industrial and technical uses include marine fittings and plumbing fixtures because of its corrosion resistance, and heating coils because of its high electrical resistance. The commercial Cu–18Ni–20Zn alloy, known as nickel silver, is used for making various tools, optical parts, and jewellery. There are numerous publications on the electrochemical behavior and corrosion resistance of copper–nickel alloys [2–4] and copper–zinc alloys [5–7]. However, in contrast to the previous binary alloys, electrochemical studies on the Cu–20Zn–10Ni alloy are scarce. Several papers on nickel silver have been published [8,9]. However, these studies are focused on the characterization of the morphological [10] and thermodynamical [11,12] properties of the electrodeposited alloys. This present work is a comparative study on the electrochemical performance of the copper–nickel alloys after substituting zinc for nickel in a sodium chloride solution, in the hope to clarify the mechanism of corrosion processes taking place at the electrode/solution interface and to probe into the possibility that the Cu–20Zn–10Ni alloy could substitute for the Cu–30Ni alloy in a corrosive environment containing chloride ions. Various characterization methods including electrochemical impedance spectroscopy (EIS), X-ray diffraction (XRD), and scanning electron microscopy (SEM), are used.
X.-Z. Zhou et al. / Journal of Alloys and Compounds 491 (2010) 92–97 Table 1 Composition of the copper alloys used in this work. Designation
Cu–20Zn–10Ni Cu–30Ni
3. Results and discussion 3.1. Impedance measurement
Composition/(wt.%) Cu
Zn
Bal. Bal.
19.4 –
93
Ni
Fe
Mn
9.60 29.21
0.80 0.91
0.49 0.54
2. Experimental 2.1. Materials and solutions Two copper-based alloys of Cu–30Ni and Cu–20Zn–10Ni were used for the current study. The Cu–30Ni alloy is a commercial alloy purchased from Changsha Copper-Aluminum Material, Ltd., China. The Cu–20Zn–10Ni alloy was prepared in an intermediate frequency furnace using 99.99% nickel, 99.99% copper, 99.99% zinc, 99.99% iron, and 99.99% manganese as starting materials. These starting materials were melted, and then chilled into a 25 mm × 120 mm × 200 mm water-cooled iron crucible to obtain a cast ingot. The cast ingot was homogenized at 850 ◦ C for 3 h, followed by being hot-rolled down to 6 mm in thickness. Subsequently, the hot-rolled sheet was annealed at 600 ◦ C for 1 h, followed by being cold-rolled to 70% reduction. Finally, the cold-rolled sheet was annealed at 680 ◦ C for 1 h. Lamellar coupons with the size of 20 mm × 12 mm × 1.8 mm used for the electrochemical measurements were cut from the two above alloy sheets, respectively. Lamellar coupons with the size of 10 mm × 10 mm × 1.8 mm were also cut from the two above alloy sheets, respectively, and then were immersed in a 3.5 wt.% sodium chloride (NaCl) solution for 20 days, ready for XRD and SEM analyses. The nominal chemical composition of the two above alloy sheets is presented in Table 1. One end of the lamellar coupons used for the electrochemical measurements was fixed with a copper wire lead by mechanical jamming, and then the lamellar coupons were mounted into glass tubes by two-component epoxy resin, leaving a contact surface area of 1 cm2 to the solution. The lamellar coupons available for the immersion and the electrochemical measurements were wet-polished mechanically using a series of emery papers with grit up to 800, and then rubbed against smooth cloth using alumina (0.5 m) until a bright mirror surface was obtained. The polished lamellar coupons were rinsed with triply distilled water for three times, and dried in air. After treatment, the lamellar coupons were found to have a uniform surface without grease and oxide impurities, and they were then highly susceptible to corrosion. The as-prepared lamellar coupons were immediately transferred to a vacuum desiccator before use.
2.2. Electrochemical instrumentation and measurement A conventional three-electrode cell with a volume of 300 ml was used to measure impedance spectra. All the potential values were defined with respect to the saturated calomel electrode (SCE) in this paper. The lamellar coupons with the size of 20 mm × 12 mm × 1.8 mm cut from the Cu–30Ni alloy and the Cu–20Zn–10Ni alloy sheets were used as the working electrodes, and a platinum sheet was used as the corresponding counter electrodes. A 3.5 wt.% sodium chloride (NaCl) solution was used as the electrolyte. The electrolyte solution was not deaerated during the measurements. All the experiments were conducted under ambient temperature of about 25 ◦ C. EIS measurements were performed using a potentiostat/galvanostat of Princeton Applied Research (PAR) EG&G Model 263A and a PerkinElmer Instruments Model 5210 dual phase lock-in amplifier controlled by PowerSuite software. To compare the different corrosion behavior of the two above alloys in the 3.5 wt.% NaCl solution as a function of time, freely corroded lamellar coupons were immersed in the 3.5 wt.% NaCl solution for 1, 24 or 120 h. To investigate corrosion parameters, the samples were allowed to stabilize at the open-circuit potential (OCP) for 1 h. The EIS measurements were recorded at the OCP with a 5 mV AC perturbation between 100 kHz and 50 mHz. The ZSimpWin software from PAR was used for modeling the EIS data.
2.3. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies Surface analyses with XRD and SEM were carried out to understand the nature and the composition of the corrosion product layers formed both on the Cu–30Ni alloy and the Cu–20Zn–10Ni alloy. The lamellar coupons were exposed to the 3.5 wt.% NaCl solution for 20 days. At the end of the experiment, the lamellar coupons were quickly removed, rinsed with distilled water, dried in the air, and finally transferred to a vacuum desiccator before the surface analyses. XRD patterns were recorded with the D/Max-2500 X-ray diffractometer using Cu K␣ radiation ( = 0.15406 nm) at a scan rate of 8◦ /min. SEM observations were performed using the Sirion 200 microscopy equipped with an energy dispersion analysis of X-ray (EDAX).
EIS is a powerful and non-destructive electrochemical technique for identifying electrochemical reactions and corrosion products at the electrode/electrolyte interface. EIS spectra are generally displayed either in the form of a Nyquist (i.e., complex plane) plot or a Bode plot [13]. The EIS enables us to fit the experimental results to a pure electronic model to interpret the electrochemical system under investigation. The correlation of the experimental impedance plot with an equivalent circuit allows verification of the mechanistic model for the system. Such a correlation renders calculation of numerical values corresponding to the physicochemical properties of the electrochemical system [14–16]. Herein, the EIS spectra are recorded at the OCP after immersion for 1, 24 or 120 h, in the hope to identify the corrosion products formed both on the two above alloy surfaces in the 3.5 wt.% NaCl solution. Fig. 1 shows the EIS spectra in the 3.5 wt.% NaCl solution as a function of time for the two types of alloys. As shown in Fig. 1a, high frequency inductive behavior was caused by instrumental artifacts, notably capacitance associated with the current measuring resistor [17]. It is remarkable to see from the figure that the diameters of semicircles for both the Cu–30Ni alloy and the Cu–20Ni–10Ni alloy increase with the exposure time from 1 to 120 h, which suggests an increase in the charge transfer resistance or the oxide film resistance with time. After substituting zinc for nickel, the Nyquist plot for the Cu–20Zn–10Ni alloy is very similar to that for the Cu–30Ni alloy during the exposure, but the diameters of semicircles for the Cu–20Zn–10Ni alloy are larger than those for the Cu–30Ni alloy after immersion for 1, 24 and 120 h, respectively. This indicates an increase in the charge transfer or the oxide film resistance for the alloys after substituting zinc for nickel in the 3.5 wt.% NaCl solution. Both the total impedance Bode plot and the phase angle Bode plot are also shown in Fig. 1. From the phase angle Bode plots (Fig. 1c), it can be seen that these plots show two overlapped phase maxima at intermediate and low frequencies. The time constant at high frequencies is believed to originate from the electrical double layer (i.e., EDL), while the one at low frequencies is believed to originate from the formation of a protective corrosion product film [2,18–22]. The changes in phase maxima with time indicate the differences in the relaxation time constants at different exposure times. From the corresponding phase angle Bode plots, it can be seen that the angle values at the low frequency zone increase with the immersion time, which indicates a decrease in the corrosion rate of both the alloys with time. The total impedance plots (Fig. 1b) show that the impedance for the Cu–20Zn–10Ni alloy is higher than that for the Cu–30Ni alloy, which indicates that the protective film on the Cu–20Zn–10Ni alloy has more protective ability than that of the Cu–30Ni alloy. From the total impedance plots, the impedance of both the two alloys increases rapidly with time, which indicates that the protective films formed on the two alloy surfaces are protective. According to the AC-circuit theory, an impedance plot for a given electrochemical system can be correlated to one or more equivalent circuits. The impedance data were analyzed using software provided with the impedance system where the dispersion formula was used. Fig. 2 shows the equivalent circuit used to analyze the impedance data of the alloy electrodes in the 3.5 wt.% NaCl solution [23], which can be satisfactorily used for fitting the impedance spectra. The quality of the fits is judged by the Chi-square (2 ) value. For a simple equivalent circuit consisting of a parallel combination of a capacitor, Cdl , and a resistor, Rct , representing the charge transfer resistance, in series with a resistor, Rs , representing the solution resistance, the electrode impedance, Z, in this case can be
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Fig. 1. Impedance spectra of the Cu–30Ni alloy and the Cu–20Zn–10Ni alloy recorded at the open-circuit potential in the 3.5 wt.% NaCl solution after immersion of 1 h (squares), 24 h (circles) and 120 h (upper triangles): (a) Nyquist plots, (b) total impedance Bode plots, and (c) phase angle Bode plots.
expressed as follows: Z = Rs +
Rct 1 + (2Rct fCdl )
n,
(1)
where n denotes an empirical parameter (0 ≤ n ≤ 1) and f is the frequency in Hz. The above relation is known as the dispersion formula and it takes into account the deviation from the ideal RCbehavior in terms of a distribution of time constants due to surface inhomogeneity, roughness, adsorption of inhibitors, formation of porous layers, and variation in properties or compositions of surface layers [24–27]. Here n can be used as a measure of the surface inhomogeneity.
Equivalent circuit in Fig. 2 models two relaxation time constants Rs Li (Q1 [Rct (Q2 Rf )]). The symbol Q signifies the possibility of a nonideal capacitance (CPE, constant phase element) with varying n. The CPE is a special element, whose value is a function of the angular frequency, ω, whereas whose phase is independent of the frequency. The admittance and impedance components are as follows: n
YCPE = Y0 (jω) ,
Q = ZCPE =
1 Y0 (jω)
(2)
n.
(3)
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Table 2 Fitting parameters for the Cu–30Ni alloy and the Cu–20Zn–10Ni alloy in the 3.5 wt.% NaCl solution at different time intervals at 25 ◦ C. Parameters
Rs ( cm2 ) Li (H cm2 ) Q1 (−1 cm−2 sn ) n1 Rct ( cm2 ) Q2 (−1 cm−2 sn ) n2 Rf (k cm2 )
Cu–30Ni
Cu–20Zn–10Ni
1h
24 h
120 h
1h
24 h
120 h
0.71 1.55 2.23 × 10−5 1 32.17 1.24 × 10−4 0.60 5.11
0.44 1.77 3.13 × 10−5 1 95.76 1.06 × 10−4 0.65 6.80
1.23 2.21 9.45 × 10−6 0.96 29.68 8.00 × 10−5 0.70 12.81
1.1 1.55 5.71 × 10−5 0.83 216.90 5.55 × 10−5 0.43 8.58
1.2 2.02 6.52 × 10−5 0.82 133.50 2.70 × 10−5 0.70 10.08
1.1 2.20 3.90 × 10−5 0.82 35.52 1.09 × 10−5 0.81 25.86
Here, j is the imaginary number with j2 = −1, ω is the angular frequency [28]. The factor n, defined as the CPE power, is an adjustable parameter that lies between 0 and 1. For n = 1, the CPE describes an ideal capacitor with Y0 equal to the capacitance C; for n = 0, the CPE is an ideal resistor; for n = 0.5, the CPE represents a Warburg impedance with diffusion character; for 0.5 < n < 1, the CPE describes a frequency dispersion of time constants due to local inhomogeneity on the surface. Generally, it is believed that the CPE is related to some type of heterogeneity of the electrode surface as well as the fractal nature (i.e., roughness or porosity) of the surface [29]. In the first process (Q1 Rct ) at higher frequencies in the 3.5 wt.% NaCl solution, parameter Q1 represents the capacitive behavior at the electrolyte/metal interface (i.e. the electrical double layer, EDL) and Rct represents the corresponding charge transfer resistance. In the second detected process, Q2 represents the capacitive behavior of the passive film formed, coupled with resistance due to ionic paths through oxide film Rf . To fit the impedance spectra of the two alloys in the 3.5 wt.% NaCl solution, the equivalent circuit in Fig. 2 is used to fit the impedance data for the Cu–20Zn–10Ni alloy and the Cu–30Ni alloy. The 2 are all below 10−3 . The calculated equivalent circuit parameters for the two alloys in the 3.5 wt.% NaCl solution are presented in Table 2. As shown in Table 2, the resistance, Rf , of the passivation film increases with time, indicating formation of a protective film. Radford et al. [30] reported that a layer of protective film will be formed on the surface of the Cu–30Ni alloy within a very short time. It can be seen that the Q1 exponent n1 of the Cu–30Ni alloy are equal or close to 1, which indicates presence of an approximate ideal capacitor for the Cu–30Ni alloy. The values of n2 for the Cu–20Zn–10Ni alloy increase with time, indicating a possible increase in surface homogeneities. In order to study the changes in corrosion rate and in capacitance of the film formed, the capacitance was calculated from Q using the equation C = [R1−n Q]1/n [31]. The values of the EDL capacity (C1 ) and of the film capacity (C2 ) calculated by us are presented in Table 3. The capacitance of the electrical double layer, C1 , is proportional to the corrosion rate. As shown in Table 3, it is clear that the corrosion rate of the Cu–30Ni alloy increases with the exposure time from 1 to 24 h, and then decreases with
Fig. 2. The equivalent circuit model used in the fitting of the impedance data of the alloys, Rs = solution resistance, Rct = charge transfer resistance of the electrical double layer (EDL), Q1 = constant phase element (CPE) of the electrical double layer (EDL), Rf = resistance of the passive film formed on the metal surface and Q2 = constant phase element (CPE) of the passive film layer formed on the metal surface.
Table 3 Capacitance C1 and C2 calculated from the EIS data. The values of C1 and C2 were calculated from the EIS spectra of the Cu–30Ni alloy and the Cu–20Zn–10Ni alloy recorded in the 3.5 wt.% NaCl solution at different time intervals at 25 ◦ C. Parameters
C1 (F cm2 ) C2 (F cm2 )
Cu–30Ni
Cu–20Zn–10Ni
1h
24 h
120 h
1h
24 h
120 h
22.3 91.2
31.3 89.0
6.7 80.9
23.2 20.6
23.0 15.6
9.2 8.1
the exposure time from 24 to 120 h; whereas the corrosion rate of the Cu–20Zn–10Ni alloy decreases with the immersion time from 1 to 120 h. However, it should be noted that the values of Rct cannot provide reliable information due to the data deviation of the fitting process. Although the actual value of the dielectric constant with the passivation film is difficult to estimate, a change of C2 can be used as an indicator of a change in the passivation film thickness. Assuming that the dielectric constant does not change with the different parameters under investigation, the reciprocal capacitance of the passivation film, 1/C2 , will be directly proportional to the thickness of the passivation film. The thickness, 1/C2 , and the resistance, Rf , of the passivation film increase with the increase of the time of immersion, thus indicating a continuous growth of the passive film with time. However, it is interesting to note that the increasing rate of the thickness of the passivation film formed on the Cu–20Zn–10Ni alloy surface is larger than that of the thickness and resistance of the passive film formed on the Cu–30Ni alloy surface, as indicated in Table 3. The impedance (Z) for the Cu–20Zn–10Ni alloy increases from 8.58 k cm2 after immersion for 1 h to about 26.0 k cm2 after immersion for 120 h, while the impedance (Z) for the Cu–30Ni alloy increases from 5.11 k cm2 after immersion for 1 h to about 13.0 k cm2 after immersion for
Fig. 3. XRD patterns obtained from the surfaces after 456 h of immersion in the 3.5 wt.% NaCl solution (a) Cu–20Zn–10Ni alloy, (b) Cu–30Ni alloy.
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Fig. 4. SEM micrograph of corrosion products formed on the Cu–20Zn–10Ni alloy (a and b) and the Cu–30Ni alloy (c and d) after immersion in the 3.5 wt.% NaCl solution for a period of 456 h, and EDAX spectra of the outer layer (e) and the inner layer (f) of the corrosion products formed on the Cu–20Zn–10Ni alloy.
120 h, due to the formation of the passivation film. This indicates that the corrosion resistance of the alloy within the 3.5 wt.% NaCl solution increases after substituting zinc for nickel. To some extent, the Cu–20Zn–10Ni alloy could substitute for Cu–30Ni alloy in a corrosive environment containing chloride ions. Both the resistance and the thickness of the passivation film increase with the immersion time due to the growth of the passivation film for the two alloys. 3.2. Surface characterization studies In order to understand the nature of the corrosion product film, XRD patterns were obtained from the Cu–30Ni alloy and the Cu–20Zn–10Ni alloy surfaces after the immersion for 456 h
in the 3.5 wt.% NaCl solution, as shown in Fig. 3. The results showed that the corrosion products of the Cu–30Ni alloy and the Cu–20Zn–10Ni alloys are mainly composed of (Cu, Ni)2 Cl(OH)3 and (Cu, Zn)2 Cl(OH)3 , respectively. After the immersion in the 3.5 wt.% NaCl solution for 20 days, the surface corrosion layers was then examined using the SEM/EDAX analyses, as shown in Fig. 4. After immersion of 20 days, the surface of the Cu–30Ni alloy is covered completely and uniformly with a layer of corrosion products, and the surface of the Cu–20Zn–10Ni alloy is covered with the corrosion products. According to the XRD patterns (Fig. 3) and the EDAX results (Fig. 4e and f) the corrosion product film of the Cu–30Ni alloy consists mainly of (Cu, Ni)2 Cl(OH)3 (JCPDS 50-1560), while the corrosion product film of the Cu–20Zn–10Ni alloy consists mainly of (Cu, Zn)2 Cl(OH)3 (JCPDS
X.-Z. Zhou et al. / Journal of Alloys and Compounds 491 (2010) 92–97
50-1558). A fine needle-like structure is observed under the film of (Cu, Zn)2 Cl(OH)3 (Fig. 4b). EDAX analysis showed that these fine needles are constituted by Cu2 O [32]. The good corrosion resistance of copper alloy in chloride solution was found to be attributed to the formation of a duplex oxide film comprising an outer layer of cupric hydroxyl chloride, Cu2 (OH)3 Cl, overlaying a compact inner layer of cuprous oxide, Cu2 O [18,20]. Based on its low electronic conductivity, the inner Cu2 O layer is claimed to be mainly responsible for the good corrosion resistance for copper alloys. Therefore, in the 3.5 wt.% NaCl solution, the corrosion resistance of the Cu–20Zn–10Ni alloy, whose corrosion product films were found to consist of two layers, with the inner layer and the outer layer being composed of Cu2 O (Cu, Zn)2 Cl(OH)3 , respectively, is better than that of the Cu–30Ni alloy, whose corrosion product films were found to mainly consist of one layer being composed of (Cu, Ni)2 Cl(OH)3 , and it is in accord well with the results of the EIS. Due to its defective structure (p-type semiconductor), the inner Cu2 O layer can accept large amounts of foreign ions, i.e. nickel or chloride ions [33,34]. The incorporation of Ni in the lattice defects of Cu2 O decreases its ionic and electronic conductivity [2,22]. North et al. [2] found that the film on copper exposed to the 3.4% NaCl solution consisted of Cu2 O with traces of Cu2 (OH)3 Cl on top of Cu2 O at longer immersion times. According to Kato et al. [18], the less protective Atacamite (Cu2 (OH)3 Cl) was dominant in the corrosion product film after a long exposure time. After the incorporation of nickel or zinc in the lattice defects of Cu2 (OH)3 Cl, the relatively more protective (Cu, Ni)2 Cl(OH)3 or the relatively less (Cu, Zn)2 Cl(OH)3 would be dominant in the corrosion product film after a long exposure time. 4. Conclusions The electrochemical performance of the Cu–20Zn–10Ni alloy was compared with that of the Cu–30Ni alloy in the 3.5 wt.% NaCl solution by the EIS, XRD and SEM/EDAX in the present work. As a consequence, after substituting zinc for nickel, both the increasing rate of the thickness and the resistance of the passive film formed on the Cu–20Zn–10Ni alloy are found to be larger than those of the passive film formed on the Cu–30Ni alloy during the immersion of the two alloys in the 3.5 wt.% NaCl solution. The passive film formed on the Cu–20Zn–10Ni alloy surface is found to have a higher resistance and thicker depth than that formed on the Cu–30Ni alloy surface, and the impedance increases from 13.0 k cm2 for the Cu–30Ni alloy to 26.0 k cm2 for the Cu–20Zn–10Ni alloy after the immersion in the 3.5 wt.% NaCl solution for 120 h. The corrosion resistance of the Cu–20Zn–10Ni alloy, whose corrosion product films consist of two layers, with the inner layer and the outer layer being composed of Cu2 O and (Cu, Zn)2 Cl(OH)3 , respectively, is better than
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that of the Cu–30Ni alloy, whose corrosion product films mainly consist of only one layer being composed of (Cu, Ni)2 Cl(OH)3 , and the inner corrosion product layer of Cu2 O is mainly responsible for the good corrosion resistance. To some extent, the Cu–20Zn–10Ni alloy could substitute for Cu–30Ni alloy in a corrosive environment containing chloride ions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
P.K. Chauhan, H.S. Gadiyar, Corros. Sci. 25 (1985) 55–68. R.F. North, M.J. Pryor, Corros. Sci. 10 (1970) 297–311. P. Druska, H.-H. Strehblow, Surf. Inter. Anal. 23 (1995) 440–450. W.A. Badawy, K.M. Ismail, A.M. Fathi, Electrochim. Acta 50 (2005) 3603–3608. J. Morales, P. Esparza, S. Gonzalez, J.E. Garcia, R.C. Salvarezza, A.J. Arvia, Corros. Sci. 37 (1995) 231–239. J. Morales, G.T. Fernandez, P. Esparze, S. Gonzalez, R.C. Salvarezza, A.J. Arvia, Corros. Sci. 40 (1998) 177–190. R.M. El-Sherif, K.M. Ismail, W.A. Badawy, Electrochim. Acta 49 (2004) 5139–5150. I. Miloˇsev, T. Kosec, Electrochim. Acta 52 (2007) 6799–6810. T. Kosec, I. Miloˇsev, Mater. Chem. Phys. 104 (2007) 44–49. F. El-Chiekh, M.T. El-Haty, H. Minoura, A.A. Montaser, Electrochim. Acta 50 (2005) 2857–2864. M. Jiang, C.P. Wang, X.J. Liu, I. Ohnuma, R. Kainuma, G.P. Vassilev, K. Ishida, J. Phys. Chem. Solids 66 (2005) 246–250. J. Miettinen, Calphad 27 (2003) 147–152. W.S. Tait, An Introduction to Electrochemical Corrosion Testing for Practicing Engineers and Scientists, University of Wisconsin-Madison, Racine, USA, 1994. J.R. Macdonald, Impedance Spectroscopy, 3rd ed., John Wiley & Sons, New York, 1987. K.M. Ismail, A.A. El-Moneim, W.A. Badawy, J. Electrochem. Soc. 148 (2001) 81–87. K.M. Ismail, R.M. El-Sherif, W.A. Badawy, Corrosion 61 (2005) 411–419. “Corrections for Potentiostat Response.”, Technical Note 201, Princeton Applied Research. C. Kato, B.G. Ateya, J.E. Castle, H.W. Pickering, J. Electrochem. Soc. 127 (1980) 1897–1903. Y.Z. Wang, A.M. Beccaria, G. Poggi, Corros. Sci. 36 (1994) 1277–1288. P. Druska, H.H. Strehblow, S. Golledge, Corros. Sci. 38 (1996) 835–851. J.M. Popplewell, R.J. Hart, J.A. Ford, Corros. Sci. 13 (1973) 295–309. R.G. Blundy, M.J. Pryor, Corros. Sci. 12 (1972) 65–75. X.H. Zhang, S.O. Pehkonen, N. Kocherginsky, G.A. Ellis, Corros. Sci. 44 (2002) 2507–2528. A. Popova, E. Sokolova, S. Raicheva, M. Christov, Corros. Sci. 45 (2003) 33–58. A.E. Bohe, J.R. Vilche, K. Juettner, W.J. Lorenz, W. Paatsch, Electrochim. Acta 34 (1989) 1443–1448. K. Hladky, L.M. Calow, J.L. Dawson, Br. Corros. J. 15 (1980) 20–25. J. Hitzig, J. Titz, K. Juettner, W.J. Lorenz, E. Schmidt, Electrochim. Acta 29 (1984) 287–296. Y.S. Tan, M.P. Srinivasan, S.O. Pehkonen, S.Y.M. Chooi, Corros. Sci. 48 (2006) 840–862. J.R. Macdonald, Impedance Spectroscopy—Emphasizing Solid Materials and Systems, Wiley, New York, 1987 (Chapter 1–4). G.J.W. Radford, F.C. Walsh, J.R. Smith, C.D.S. Tuck, S.A. Campbell, Developments in Marine Corrosion, Royal Society of Chemistry, Cambridge, 1998, pp. 41–63. D. Kek-Merl, J. Lappalainen, H.L. Tuller, J. Electrochem. Soc. 153 (2006) J15–J20. A. Srivastava, R. Balasubramaniam, Mater. Charact. 55 (2005) 127–135. H. Shih, H.W. Pickering, J. Electrochem. Soc. 134 (1987) 1949–1957. C. Kato, B.G. Ateya, J.E. Castle, H.W. Pickering, J. Electrochem. Soc. 127 (1980) 1890–1896.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Comparative study on the electrochemical performance of the Cu–30Ni and Cu–20Zn–10Ni alloys Xiao-Zhong Zhou, Chu-Ping Deng, Yu-Chang Su ∗ School of Materials Science and Engineering, Central South University, Changsha 410083, Hunan, PR China
a r t i c l e
i n f o
Article history: Received 12 August 2009 Received in revised form 23 October 2009 Accepted 2 November 2009 Available online 6 November 2009 Keywords: Copper alloy Corrosion Electrochemical impedance spectroscopy
a b s t r a c t The electrochemical performance of the Cu–20Zn–10Ni alloy was compared with that of the Cu–30Ni alloy in a 3.5 wt.% sodium chloride solution. Electrochemical impedance spectroscopy (EIS) measurements show that, interestingly, both the increasing rates of the thickness and resistance of the passive film formed on the Cu–20Zn–10Ni alloy are larger than those of the passive film formed on the Cu–30Ni alloy during the immersion of the two alloys in the 3.5 wt.% NaCl solution. The passive film formed on the Cu–20Zn–10Ni alloy surface is found to have a higher resistance and thicker depth than that formed on the Cu–30Ni alloy surface after the immersion of the two alloys in the 3.5 wt.% NaCl solution for 120 h. To some extent, the Cu–20Zn–10Ni alloy could substitute for the Cu–30Ni alloy in a corrosive environment containing chloride ions. According to X-ray diffraction and scanning electron microscopy studies, the corrosion product films of the Cu–20Zn–10Ni alloy were found to consist of two layers, with the inner layer and the outer layer being composed of cuprous oxide and (Cu, Zn)2 Cl(OH)3 , respectively, while the corrosion product films of the Cu–30Ni alloy were found to consist mainly of (Cu, Ni)2 Cl(OH)3 . © 2009 Elsevier B.V. All rights reserved.
1. Introduction Copper-based alloys can be found in a wide variety of applications because of their good physical properties, excellent corrosion resistance, and reasonable price. For example, copper–zinc alloys are used as materials for condenser and heat-exchange tubes in power-generation industry. Copper–nickel alloys are used as materials for condenser tubes in marine applications, where anti-erosion and anti-corrosion are very important. The corrosion behavior depends heavily on the nature of a passivation film formed on the alloy surface. However, these alloys will suffer from accelerated corrosion when exposed to seawater for the presence of chloride ions. The structure of the passivation film formed on the alloy surface under a simulated seawater environment was found to consist of two layers, which are an outer CuO layer with chemisorbed water molecules and traces of chloride ions, and an inner Cu2 O layer containing Ni2+ and Ni3+ [1]. Nickel silver is a general term for the alloys that contain copper, nickel, and zinc. Other common names are Chinese silver, German silver, nickel bronze, and alpaca silver. These alloys are hard, corrosion resistant, and ductile, and actually contain no elemental
∗ Corresponding author. Tel.: +86 731 88830735; fax: +86 731 88830785. E-mail address: [email protected] (Y.-C. Su). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.11.002
silver. Nickel silver first became popular as a base metal for silver plated cutlery and other silverware, especially electro-plated nickel silver (EPNS), jewellery, and coins. Its industrial and technical uses include marine fittings and plumbing fixtures because of its corrosion resistance, and heating coils because of its high electrical resistance. The commercial Cu–18Ni–20Zn alloy, known as nickel silver, is used for making various tools, optical parts, and jewellery. There are numerous publications on the electrochemical behavior and corrosion resistance of copper–nickel alloys [2–4] and copper–zinc alloys [5–7]. However, in contrast to the previous binary alloys, electrochemical studies on the Cu–20Zn–10Ni alloy are scarce. Several papers on nickel silver have been published [8,9]. However, these studies are focused on the characterization of the morphological [10] and thermodynamical [11,12] properties of the electrodeposited alloys. This present work is a comparative study on the electrochemical performance of the copper–nickel alloys after substituting zinc for nickel in a sodium chloride solution, in the hope to clarify the mechanism of corrosion processes taking place at the electrode/solution interface and to probe into the possibility that the Cu–20Zn–10Ni alloy could substitute for the Cu–30Ni alloy in a corrosive environment containing chloride ions. Various characterization methods including electrochemical impedance spectroscopy (EIS), X-ray diffraction (XRD), and scanning electron microscopy (SEM), are used.
X.-Z. Zhou et al. / Journal of Alloys and Compounds 491 (2010) 92–97 Table 1 Composition of the copper alloys used in this work. Designation
Cu–20Zn–10Ni Cu–30Ni
3. Results and discussion 3.1. Impedance measurement
Composition/(wt.%) Cu
Zn
Bal. Bal.
19.4 –
93
Ni
Fe
Mn
9.60 29.21
0.80 0.91
0.49 0.54
2. Experimental 2.1. Materials and solutions Two copper-based alloys of Cu–30Ni and Cu–20Zn–10Ni were used for the current study. The Cu–30Ni alloy is a commercial alloy purchased from Changsha Copper-Aluminum Material, Ltd., China. The Cu–20Zn–10Ni alloy was prepared in an intermediate frequency furnace using 99.99% nickel, 99.99% copper, 99.99% zinc, 99.99% iron, and 99.99% manganese as starting materials. These starting materials were melted, and then chilled into a 25 mm × 120 mm × 200 mm water-cooled iron crucible to obtain a cast ingot. The cast ingot was homogenized at 850 ◦ C for 3 h, followed by being hot-rolled down to 6 mm in thickness. Subsequently, the hot-rolled sheet was annealed at 600 ◦ C for 1 h, followed by being cold-rolled to 70% reduction. Finally, the cold-rolled sheet was annealed at 680 ◦ C for 1 h. Lamellar coupons with the size of 20 mm × 12 mm × 1.8 mm used for the electrochemical measurements were cut from the two above alloy sheets, respectively. Lamellar coupons with the size of 10 mm × 10 mm × 1.8 mm were also cut from the two above alloy sheets, respectively, and then were immersed in a 3.5 wt.% sodium chloride (NaCl) solution for 20 days, ready for XRD and SEM analyses. The nominal chemical composition of the two above alloy sheets is presented in Table 1. One end of the lamellar coupons used for the electrochemical measurements was fixed with a copper wire lead by mechanical jamming, and then the lamellar coupons were mounted into glass tubes by two-component epoxy resin, leaving a contact surface area of 1 cm2 to the solution. The lamellar coupons available for the immersion and the electrochemical measurements were wet-polished mechanically using a series of emery papers with grit up to 800, and then rubbed against smooth cloth using alumina (0.5 m) until a bright mirror surface was obtained. The polished lamellar coupons were rinsed with triply distilled water for three times, and dried in air. After treatment, the lamellar coupons were found to have a uniform surface without grease and oxide impurities, and they were then highly susceptible to corrosion. The as-prepared lamellar coupons were immediately transferred to a vacuum desiccator before use.
2.2. Electrochemical instrumentation and measurement A conventional three-electrode cell with a volume of 300 ml was used to measure impedance spectra. All the potential values were defined with respect to the saturated calomel electrode (SCE) in this paper. The lamellar coupons with the size of 20 mm × 12 mm × 1.8 mm cut from the Cu–30Ni alloy and the Cu–20Zn–10Ni alloy sheets were used as the working electrodes, and a platinum sheet was used as the corresponding counter electrodes. A 3.5 wt.% sodium chloride (NaCl) solution was used as the electrolyte. The electrolyte solution was not deaerated during the measurements. All the experiments were conducted under ambient temperature of about 25 ◦ C. EIS measurements were performed using a potentiostat/galvanostat of Princeton Applied Research (PAR) EG&G Model 263A and a PerkinElmer Instruments Model 5210 dual phase lock-in amplifier controlled by PowerSuite software. To compare the different corrosion behavior of the two above alloys in the 3.5 wt.% NaCl solution as a function of time, freely corroded lamellar coupons were immersed in the 3.5 wt.% NaCl solution for 1, 24 or 120 h. To investigate corrosion parameters, the samples were allowed to stabilize at the open-circuit potential (OCP) for 1 h. The EIS measurements were recorded at the OCP with a 5 mV AC perturbation between 100 kHz and 50 mHz. The ZSimpWin software from PAR was used for modeling the EIS data.
2.3. X-ray diffraction (XRD) and scanning electron microscopy (SEM) studies Surface analyses with XRD and SEM were carried out to understand the nature and the composition of the corrosion product layers formed both on the Cu–30Ni alloy and the Cu–20Zn–10Ni alloy. The lamellar coupons were exposed to the 3.5 wt.% NaCl solution for 20 days. At the end of the experiment, the lamellar coupons were quickly removed, rinsed with distilled water, dried in the air, and finally transferred to a vacuum desiccator before the surface analyses. XRD patterns were recorded with the D/Max-2500 X-ray diffractometer using Cu K␣ radiation ( = 0.15406 nm) at a scan rate of 8◦ /min. SEM observations were performed using the Sirion 200 microscopy equipped with an energy dispersion analysis of X-ray (EDAX).
EIS is a powerful and non-destructive electrochemical technique for identifying electrochemical reactions and corrosion products at the electrode/electrolyte interface. EIS spectra are generally displayed either in the form of a Nyquist (i.e., complex plane) plot or a Bode plot [13]. The EIS enables us to fit the experimental results to a pure electronic model to interpret the electrochemical system under investigation. The correlation of the experimental impedance plot with an equivalent circuit allows verification of the mechanistic model for the system. Such a correlation renders calculation of numerical values corresponding to the physicochemical properties of the electrochemical system [14–16]. Herein, the EIS spectra are recorded at the OCP after immersion for 1, 24 or 120 h, in the hope to identify the corrosion products formed both on the two above alloy surfaces in the 3.5 wt.% NaCl solution. Fig. 1 shows the EIS spectra in the 3.5 wt.% NaCl solution as a function of time for the two types of alloys. As shown in Fig. 1a, high frequency inductive behavior was caused by instrumental artifacts, notably capacitance associated with the current measuring resistor [17]. It is remarkable to see from the figure that the diameters of semicircles for both the Cu–30Ni alloy and the Cu–20Ni–10Ni alloy increase with the exposure time from 1 to 120 h, which suggests an increase in the charge transfer resistance or the oxide film resistance with time. After substituting zinc for nickel, the Nyquist plot for the Cu–20Zn–10Ni alloy is very similar to that for the Cu–30Ni alloy during the exposure, but the diameters of semicircles for the Cu–20Zn–10Ni alloy are larger than those for the Cu–30Ni alloy after immersion for 1, 24 and 120 h, respectively. This indicates an increase in the charge transfer or the oxide film resistance for the alloys after substituting zinc for nickel in the 3.5 wt.% NaCl solution. Both the total impedance Bode plot and the phase angle Bode plot are also shown in Fig. 1. From the phase angle Bode plots (Fig. 1c), it can be seen that these plots show two overlapped phase maxima at intermediate and low frequencies. The time constant at high frequencies is believed to originate from the electrical double layer (i.e., EDL), while the one at low frequencies is believed to originate from the formation of a protective corrosion product film [2,18–22]. The changes in phase maxima with time indicate the differences in the relaxation time constants at different exposure times. From the corresponding phase angle Bode plots, it can be seen that the angle values at the low frequency zone increase with the immersion time, which indicates a decrease in the corrosion rate of both the alloys with time. The total impedance plots (Fig. 1b) show that the impedance for the Cu–20Zn–10Ni alloy is higher than that for the Cu–30Ni alloy, which indicates that the protective film on the Cu–20Zn–10Ni alloy has more protective ability than that of the Cu–30Ni alloy. From the total impedance plots, the impedance of both the two alloys increases rapidly with time, which indicates that the protective films formed on the two alloy surfaces are protective. According to the AC-circuit theory, an impedance plot for a given electrochemical system can be correlated to one or more equivalent circuits. The impedance data were analyzed using software provided with the impedance system where the dispersion formula was used. Fig. 2 shows the equivalent circuit used to analyze the impedance data of the alloy electrodes in the 3.5 wt.% NaCl solution [23], which can be satisfactorily used for fitting the impedance spectra. The quality of the fits is judged by the Chi-square (2 ) value. For a simple equivalent circuit consisting of a parallel combination of a capacitor, Cdl , and a resistor, Rct , representing the charge transfer resistance, in series with a resistor, Rs , representing the solution resistance, the electrode impedance, Z, in this case can be
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Fig. 1. Impedance spectra of the Cu–30Ni alloy and the Cu–20Zn–10Ni alloy recorded at the open-circuit potential in the 3.5 wt.% NaCl solution after immersion of 1 h (squares), 24 h (circles) and 120 h (upper triangles): (a) Nyquist plots, (b) total impedance Bode plots, and (c) phase angle Bode plots.
expressed as follows: Z = Rs +
Rct 1 + (2Rct fCdl )
n,
(1)
where n denotes an empirical parameter (0 ≤ n ≤ 1) and f is the frequency in Hz. The above relation is known as the dispersion formula and it takes into account the deviation from the ideal RCbehavior in terms of a distribution of time constants due to surface inhomogeneity, roughness, adsorption of inhibitors, formation of porous layers, and variation in properties or compositions of surface layers [24–27]. Here n can be used as a measure of the surface inhomogeneity.
Equivalent circuit in Fig. 2 models two relaxation time constants Rs Li (Q1 [Rct (Q2 Rf )]). The symbol Q signifies the possibility of a nonideal capacitance (CPE, constant phase element) with varying n. The CPE is a special element, whose value is a function of the angular frequency, ω, whereas whose phase is independent of the frequency. The admittance and impedance components are as follows: n
YCPE = Y0 (jω) ,
Q = ZCPE =
1 Y0 (jω)
(2)
n.
(3)
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Table 2 Fitting parameters for the Cu–30Ni alloy and the Cu–20Zn–10Ni alloy in the 3.5 wt.% NaCl solution at different time intervals at 25 ◦ C. Parameters
Rs ( cm2 ) Li (H cm2 ) Q1 (−1 cm−2 sn ) n1 Rct ( cm2 ) Q2 (−1 cm−2 sn ) n2 Rf (k cm2 )
Cu–30Ni
Cu–20Zn–10Ni
1h
24 h
120 h
1h
24 h
120 h
0.71 1.55 2.23 × 10−5 1 32.17 1.24 × 10−4 0.60 5.11
0.44 1.77 3.13 × 10−5 1 95.76 1.06 × 10−4 0.65 6.80
1.23 2.21 9.45 × 10−6 0.96 29.68 8.00 × 10−5 0.70 12.81
1.1 1.55 5.71 × 10−5 0.83 216.90 5.55 × 10−5 0.43 8.58
1.2 2.02 6.52 × 10−5 0.82 133.50 2.70 × 10−5 0.70 10.08
1.1 2.20 3.90 × 10−5 0.82 35.52 1.09 × 10−5 0.81 25.86
Here, j is the imaginary number with j2 = −1, ω is the angular frequency [28]. The factor n, defined as the CPE power, is an adjustable parameter that lies between 0 and 1. For n = 1, the CPE describes an ideal capacitor with Y0 equal to the capacitance C; for n = 0, the CPE is an ideal resistor; for n = 0.5, the CPE represents a Warburg impedance with diffusion character; for 0.5 < n < 1, the CPE describes a frequency dispersion of time constants due to local inhomogeneity on the surface. Generally, it is believed that the CPE is related to some type of heterogeneity of the electrode surface as well as the fractal nature (i.e., roughness or porosity) of the surface [29]. In the first process (Q1 Rct ) at higher frequencies in the 3.5 wt.% NaCl solution, parameter Q1 represents the capacitive behavior at the electrolyte/metal interface (i.e. the electrical double layer, EDL) and Rct represents the corresponding charge transfer resistance. In the second detected process, Q2 represents the capacitive behavior of the passive film formed, coupled with resistance due to ionic paths through oxide film Rf . To fit the impedance spectra of the two alloys in the 3.5 wt.% NaCl solution, the equivalent circuit in Fig. 2 is used to fit the impedance data for the Cu–20Zn–10Ni alloy and the Cu–30Ni alloy. The 2 are all below 10−3 . The calculated equivalent circuit parameters for the two alloys in the 3.5 wt.% NaCl solution are presented in Table 2. As shown in Table 2, the resistance, Rf , of the passivation film increases with time, indicating formation of a protective film. Radford et al. [30] reported that a layer of protective film will be formed on the surface of the Cu–30Ni alloy within a very short time. It can be seen that the Q1 exponent n1 of the Cu–30Ni alloy are equal or close to 1, which indicates presence of an approximate ideal capacitor for the Cu–30Ni alloy. The values of n2 for the Cu–20Zn–10Ni alloy increase with time, indicating a possible increase in surface homogeneities. In order to study the changes in corrosion rate and in capacitance of the film formed, the capacitance was calculated from Q using the equation C = [R1−n Q]1/n [31]. The values of the EDL capacity (C1 ) and of the film capacity (C2 ) calculated by us are presented in Table 3. The capacitance of the electrical double layer, C1 , is proportional to the corrosion rate. As shown in Table 3, it is clear that the corrosion rate of the Cu–30Ni alloy increases with the exposure time from 1 to 24 h, and then decreases with
Fig. 2. The equivalent circuit model used in the fitting of the impedance data of the alloys, Rs = solution resistance, Rct = charge transfer resistance of the electrical double layer (EDL), Q1 = constant phase element (CPE) of the electrical double layer (EDL), Rf = resistance of the passive film formed on the metal surface and Q2 = constant phase element (CPE) of the passive film layer formed on the metal surface.
Table 3 Capacitance C1 and C2 calculated from the EIS data. The values of C1 and C2 were calculated from the EIS spectra of the Cu–30Ni alloy and the Cu–20Zn–10Ni alloy recorded in the 3.5 wt.% NaCl solution at different time intervals at 25 ◦ C. Parameters
C1 (F cm2 ) C2 (F cm2 )
Cu–30Ni
Cu–20Zn–10Ni
1h
24 h
120 h
1h
24 h
120 h
22.3 91.2
31.3 89.0
6.7 80.9
23.2 20.6
23.0 15.6
9.2 8.1
the exposure time from 24 to 120 h; whereas the corrosion rate of the Cu–20Zn–10Ni alloy decreases with the immersion time from 1 to 120 h. However, it should be noted that the values of Rct cannot provide reliable information due to the data deviation of the fitting process. Although the actual value of the dielectric constant with the passivation film is difficult to estimate, a change of C2 can be used as an indicator of a change in the passivation film thickness. Assuming that the dielectric constant does not change with the different parameters under investigation, the reciprocal capacitance of the passivation film, 1/C2 , will be directly proportional to the thickness of the passivation film. The thickness, 1/C2 , and the resistance, Rf , of the passivation film increase with the increase of the time of immersion, thus indicating a continuous growth of the passive film with time. However, it is interesting to note that the increasing rate of the thickness of the passivation film formed on the Cu–20Zn–10Ni alloy surface is larger than that of the thickness and resistance of the passive film formed on the Cu–30Ni alloy surface, as indicated in Table 3. The impedance (Z) for the Cu–20Zn–10Ni alloy increases from 8.58 k cm2 after immersion for 1 h to about 26.0 k cm2 after immersion for 120 h, while the impedance (Z) for the Cu–30Ni alloy increases from 5.11 k cm2 after immersion for 1 h to about 13.0 k cm2 after immersion for
Fig. 3. XRD patterns obtained from the surfaces after 456 h of immersion in the 3.5 wt.% NaCl solution (a) Cu–20Zn–10Ni alloy, (b) Cu–30Ni alloy.
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Fig. 4. SEM micrograph of corrosion products formed on the Cu–20Zn–10Ni alloy (a and b) and the Cu–30Ni alloy (c and d) after immersion in the 3.5 wt.% NaCl solution for a period of 456 h, and EDAX spectra of the outer layer (e) and the inner layer (f) of the corrosion products formed on the Cu–20Zn–10Ni alloy.
120 h, due to the formation of the passivation film. This indicates that the corrosion resistance of the alloy within the 3.5 wt.% NaCl solution increases after substituting zinc for nickel. To some extent, the Cu–20Zn–10Ni alloy could substitute for Cu–30Ni alloy in a corrosive environment containing chloride ions. Both the resistance and the thickness of the passivation film increase with the immersion time due to the growth of the passivation film for the two alloys. 3.2. Surface characterization studies In order to understand the nature of the corrosion product film, XRD patterns were obtained from the Cu–30Ni alloy and the Cu–20Zn–10Ni alloy surfaces after the immersion for 456 h
in the 3.5 wt.% NaCl solution, as shown in Fig. 3. The results showed that the corrosion products of the Cu–30Ni alloy and the Cu–20Zn–10Ni alloys are mainly composed of (Cu, Ni)2 Cl(OH)3 and (Cu, Zn)2 Cl(OH)3 , respectively. After the immersion in the 3.5 wt.% NaCl solution for 20 days, the surface corrosion layers was then examined using the SEM/EDAX analyses, as shown in Fig. 4. After immersion of 20 days, the surface of the Cu–30Ni alloy is covered completely and uniformly with a layer of corrosion products, and the surface of the Cu–20Zn–10Ni alloy is covered with the corrosion products. According to the XRD patterns (Fig. 3) and the EDAX results (Fig. 4e and f) the corrosion product film of the Cu–30Ni alloy consists mainly of (Cu, Ni)2 Cl(OH)3 (JCPDS 50-1560), while the corrosion product film of the Cu–20Zn–10Ni alloy consists mainly of (Cu, Zn)2 Cl(OH)3 (JCPDS
X.-Z. Zhou et al. / Journal of Alloys and Compounds 491 (2010) 92–97
50-1558). A fine needle-like structure is observed under the film of (Cu, Zn)2 Cl(OH)3 (Fig. 4b). EDAX analysis showed that these fine needles are constituted by Cu2 O [32]. The good corrosion resistance of copper alloy in chloride solution was found to be attributed to the formation of a duplex oxide film comprising an outer layer of cupric hydroxyl chloride, Cu2 (OH)3 Cl, overlaying a compact inner layer of cuprous oxide, Cu2 O [18,20]. Based on its low electronic conductivity, the inner Cu2 O layer is claimed to be mainly responsible for the good corrosion resistance for copper alloys. Therefore, in the 3.5 wt.% NaCl solution, the corrosion resistance of the Cu–20Zn–10Ni alloy, whose corrosion product films were found to consist of two layers, with the inner layer and the outer layer being composed of Cu2 O (Cu, Zn)2 Cl(OH)3 , respectively, is better than that of the Cu–30Ni alloy, whose corrosion product films were found to mainly consist of one layer being composed of (Cu, Ni)2 Cl(OH)3 , and it is in accord well with the results of the EIS. Due to its defective structure (p-type semiconductor), the inner Cu2 O layer can accept large amounts of foreign ions, i.e. nickel or chloride ions [33,34]. The incorporation of Ni in the lattice defects of Cu2 O decreases its ionic and electronic conductivity [2,22]. North et al. [2] found that the film on copper exposed to the 3.4% NaCl solution consisted of Cu2 O with traces of Cu2 (OH)3 Cl on top of Cu2 O at longer immersion times. According to Kato et al. [18], the less protective Atacamite (Cu2 (OH)3 Cl) was dominant in the corrosion product film after a long exposure time. After the incorporation of nickel or zinc in the lattice defects of Cu2 (OH)3 Cl, the relatively more protective (Cu, Ni)2 Cl(OH)3 or the relatively less (Cu, Zn)2 Cl(OH)3 would be dominant in the corrosion product film after a long exposure time. 4. Conclusions The electrochemical performance of the Cu–20Zn–10Ni alloy was compared with that of the Cu–30Ni alloy in the 3.5 wt.% NaCl solution by the EIS, XRD and SEM/EDAX in the present work. As a consequence, after substituting zinc for nickel, both the increasing rate of the thickness and the resistance of the passive film formed on the Cu–20Zn–10Ni alloy are found to be larger than those of the passive film formed on the Cu–30Ni alloy during the immersion of the two alloys in the 3.5 wt.% NaCl solution. The passive film formed on the Cu–20Zn–10Ni alloy surface is found to have a higher resistance and thicker depth than that formed on the Cu–30Ni alloy surface, and the impedance increases from 13.0 k cm2 for the Cu–30Ni alloy to 26.0 k cm2 for the Cu–20Zn–10Ni alloy after the immersion in the 3.5 wt.% NaCl solution for 120 h. The corrosion resistance of the Cu–20Zn–10Ni alloy, whose corrosion product films consist of two layers, with the inner layer and the outer layer being composed of Cu2 O and (Cu, Zn)2 Cl(OH)3 , respectively, is better than
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that of the Cu–30Ni alloy, whose corrosion product films mainly consist of only one layer being composed of (Cu, Ni)2 Cl(OH)3 , and the inner corrosion product layer of Cu2 O is mainly responsible for the good corrosion resistance. To some extent, the Cu–20Zn–10Ni alloy could substitute for Cu–30Ni alloy in a corrosive environment containing chloride ions. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34]
P.K. Chauhan, H.S. Gadiyar, Corros. Sci. 25 (1985) 55–68. R.F. North, M.J. Pryor, Corros. Sci. 10 (1970) 297–311. P. Druska, H.-H. Strehblow, Surf. Inter. Anal. 23 (1995) 440–450. W.A. Badawy, K.M. Ismail, A.M. Fathi, Electrochim. Acta 50 (2005) 3603–3608. J. Morales, P. Esparza, S. Gonzalez, J.E. Garcia, R.C. Salvarezza, A.J. Arvia, Corros. Sci. 37 (1995) 231–239. J. Morales, G.T. Fernandez, P. Esparze, S. Gonzalez, R.C. Salvarezza, A.J. Arvia, Corros. Sci. 40 (1998) 177–190. R.M. El-Sherif, K.M. Ismail, W.A. Badawy, Electrochim. Acta 49 (2004) 5139–5150. I. Miloˇsev, T. Kosec, Electrochim. Acta 52 (2007) 6799–6810. T. Kosec, I. Miloˇsev, Mater. Chem. Phys. 104 (2007) 44–49. F. El-Chiekh, M.T. El-Haty, H. Minoura, A.A. Montaser, Electrochim. Acta 50 (2005) 2857–2864. M. Jiang, C.P. Wang, X.J. Liu, I. Ohnuma, R. Kainuma, G.P. Vassilev, K. Ishida, J. Phys. Chem. Solids 66 (2005) 246–250. J. Miettinen, Calphad 27 (2003) 147–152. W.S. Tait, An Introduction to Electrochemical Corrosion Testing for Practicing Engineers and Scientists, University of Wisconsin-Madison, Racine, USA, 1994. J.R. Macdonald, Impedance Spectroscopy, 3rd ed., John Wiley & Sons, New York, 1987. K.M. Ismail, A.A. El-Moneim, W.A. Badawy, J. Electrochem. Soc. 148 (2001) 81–87. K.M. Ismail, R.M. El-Sherif, W.A. Badawy, Corrosion 61 (2005) 411–419. “Corrections for Potentiostat Response.”, Technical Note 201, Princeton Applied Research. C. Kato, B.G. Ateya, J.E. Castle, H.W. Pickering, J. Electrochem. Soc. 127 (1980) 1897–1903. Y.Z. Wang, A.M. Beccaria, G. Poggi, Corros. Sci. 36 (1994) 1277–1288. P. Druska, H.H. Strehblow, S. Golledge, Corros. Sci. 38 (1996) 835–851. J.M. Popplewell, R.J. Hart, J.A. Ford, Corros. Sci. 13 (1973) 295–309. R.G. Blundy, M.J. Pryor, Corros. Sci. 12 (1972) 65–75. X.H. Zhang, S.O. Pehkonen, N. Kocherginsky, G.A. Ellis, Corros. Sci. 44 (2002) 2507–2528. A. Popova, E. Sokolova, S. Raicheva, M. Christov, Corros. Sci. 45 (2003) 33–58. A.E. Bohe, J.R. Vilche, K. Juettner, W.J. Lorenz, W. Paatsch, Electrochim. Acta 34 (1989) 1443–1448. K. Hladky, L.M. Calow, J.L. Dawson, Br. Corros. J. 15 (1980) 20–25. J. Hitzig, J. Titz, K. Juettner, W.J. Lorenz, E. Schmidt, Electrochim. Acta 29 (1984) 287–296. Y.S. Tan, M.P. Srinivasan, S.O. Pehkonen, S.Y.M. Chooi, Corros. Sci. 48 (2006) 840–862. J.R. Macdonald, Impedance Spectroscopy—Emphasizing Solid Materials and Systems, Wiley, New York, 1987 (Chapter 1–4). G.J.W. Radford, F.C. Walsh, J.R. Smith, C.D.S. Tuck, S.A. Campbell, Developments in Marine Corrosion, Royal Society of Chemistry, Cambridge, 1998, pp. 41–63. D. Kek-Merl, J. Lappalainen, H.L. Tuller, J. Electrochem. Soc. 153 (2006) J15–J20. A. Srivastava, R. Balasubramaniam, Mater. Charact. 55 (2005) 127–135. H. Shih, H.W. Pickering, J. Electrochem. Soc. 134 (1987) 1949–1957. C. Kato, B.G. Ateya, J.E. Castle, H.W. Pickering, J. Electrochem. Soc. 127 (1980) 1890–1896.