- Email: [email protected]
Effect of ultrasonic vibration on the dechromisation corrosion of a CuCr alloy in HCI solution
Available online at www.sciencedirect.com
RARE METALS Vol. 26, No. 4, Aug 2007, p . 398 E-mail: rm @ ustb.edu.cn
.:zO
-00
ScienceDirect
Effect of ultrasonic vibration on the dechromisation corrosion of a CuCr alloy in IZCl solution XU Tao", GAO Huawei 'I, LIU J i a n h d , and YU Cuiyan" 1) hpartment of Applied Technology,Daqing Petroleum Instilute, Daqing 163318. China
2) Logistics Groups Company, Yanshan University, Qinhuangdao O66004, China 3) Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdaoo66004,China (Received 2006-03-26)
Abstract: The effectof ultrasonic vibration on the dechromisation corrosion of a CuCr alloy in HCI solution was studied and the corrosion mechanisms were analyzed. It is found that ultrasonic vibration reduces the dechromisation incubation time, accelerates the dechromisation corrosion rate, decreases the temperature and concentration of HC1 solution, and when the dechromisation occurs it seriously weakens the microstructure of dechromisation layer. It is concluded that ultrasonic vibration can accelerate destruction of the passivation film on the Cr surface and increase the activities of Cl- and Cr. Key wonls: CuCr alloy; dechromisation corrosion; ultrasonic vibration; passivation film
[Zhisproject is financially supPofled by the scientific Research Fund of Heilongiiang Provincial Education Department, China (No. 11511020).]
1. Introduction With the application fields of Cu alloy extending, a higher requirement of intensity and corrosion resistance of Cu alloys is brought forward. A CuCr alloy has high intensity, excellent electric conductivity and heat conductivity, and high temperature corrosion resistance [ 1-91, so it is widely applied in the correlative fields. However, the corrosion study of CuCr alloys in the medium is scarcely reported. According to recent results, a CuCr alloy in HC1 solution under certain condition will cause serious dechromisation phenomenon [lo], but the understanding of the effect factors of dechromisation of a CuCr alloy is not comprehensive enough because of Cu alloy workpiece service in the eroding and vibrational medium. Therefore, this article studied the effects of ultrasonic vibration on the incubation time of dechromisation, the dechromisation rate, and the microstructure of dechromisation layer of a CuCr Correspondingauthor: XU Tao
E-mail:xt4535 @yahoo.corn.cn
alloy in HCl solution, and the effects of the lowest concentration and temperature of HCI solution during dechromisation. The results offer some reference data to the study of dealloying of a CuCr alloy, further understandings of dechromisation appearance and its effect factors of the CuCr alloy in HCl solution, and its application extending.
2.
Experimental
The experimental material is a CuCr alloy and its chemical components can be seen from Table 1. The samples' size is 5 mm x 5 mm x 10 mm. The every surface of samples was polished with 600# metallic sand papers, washed with water and degreased with alcohol. Then, the samples were immersed into HCl solutions with different concentrations and temperatures for corrosion. To test the effect of ultrasonic vibration on the dechromisation of the CuCr alloy, the experiment is carried out on a CQ50 ultrasonic
Xu T. et af., Effect of ultrasonic vibration on the dechromisation corrosion of a CuCr alloy in ... cleaning machine and its vibrational frequency is 0 kHz, 20 kHz, and 40 kHz, respectively. In this study the dechromisation of above-mentioned samples was observed, the incubation time of dechromisation (from samples being immersed into solution to air bubble coming into being) was tested for samples in different states in HCI solution with the lowest concentration and temperature at which dechromisation took place, and the dechromisation rate V ( V = h / t ) was calculated, where h represents the thickness of dechromisation layer (the hckness was measured vertically from the surface of the dechromisation sample to the position which has a obvious dividing line with the body matrix and its datum is the mean value of three measured results), r represents the dechromisation time (timed from air bubble separating out in the samples’ surface). Surface components and morphology after corrosion were observed and analyzed by means of ADVANTPXP-381 X-rays fluorescent spectrograph, Neophot2 1 metalloscope, and KYKY-2800 SEM (with EXD). Table 1. Composition of samplis surface in different wt.% conditions Sample’s condition
Cu
Original
50.48
CI-
Others
49.30 0.00
0.22
86.82
1.01
0.36
88.01 6.72
5.12
0.15
No dechromisation in HCI 11.81
Dechromisation in HCI
Cr
3. Results 3.1. Effects of ultrasonic vibration on the incubation time of dechromisation and the dechromisation rate of the CuCr alloy It can be seen from macroscopical observations that there are two corrosion phenomena of the CuCr alloy in ultrasonic vibration HCI solution. When the concentration and temperature of HCl solution are lower, the dechromisation of the CuCr alloy does not occur, and the samples’ surface looks light gray and silvery white particles appear. It can be seen from fluorescent spectrograph analysis that the Cu content in the samples’ surface decreases correspondingly as the content of Cr increases (Table 1). It can be seen from SEM observations that Cu phase
399
of the CuCr alloy is easier to corrode than Cr phase in the experimental conditions (Fig. 1). In Table 1, the corrosive condition of the non-dechromised CuCr alloy is 1.09 m0l.L-l HCl solution for 100 h, whereas the corrosive condition of dechromisation of the CuCr alloy is 3.27 mo1.L-l HCl solution for 2 h. The vibration frequency is 40 kHz, and temperature is 2OOC.
Fig. 1. SEM image of the non-dechromised sample.
When the concentration and temperature of HCl solution reach a certain value, dechromisation will occur after the dechromisation incubation time. Once the dechromisation occurs, it develops quickly. It can be seen from fluorescent spectrograph analysis that the Cr content in the samples’ surface decreases dramatically. Its dechromisation phenomena are that there are an amount of air bubbles appearing in the samples’ surface and the samples’ surface looks like bronze, whereas HCl solution is gradually bottle green. The incubation time of dechromisation of the CuCr alloy in different states was recorded, and the dechromisation region of the samples’ surface was abraded with a sand wheel after some time, and the interface between the dechromisation region and the body matrix was bare. The thickness of dechromisation layer was tested by metallographic observations and the dechromisation rate was calculated. The results are shown in Table 2. It can be seen that the higher the frequency of ultrasonic vibration is, the shorter the incubation time of dechromisation and the quicker the dechromisation rate in HC1 solution under the same condition. The facts show that ultrasonic vibration can accelerate the dechromisation of CuCr alloy.
RARE METALS, Vol. 26, No. 4, Aug 2007
400
Table 2. Dechromisation incubation time and dechromisation corrosion rate of a CuCr alloy in 2.74 mol-L-' HCI solution (20°C) under different states Item
Working frequency,f/ kHz 0
Dechromisationincuba113 tion time, t / s Dechromisationcorrosion 3.23 rate, V / (pmmin-')
20
40
75
52
3.58
3.74
3.2. Effects of ultrasonic vibration on the concentration and temperatureof solution during CuCr alloy dechromising It can be known from the experimental results that the dechromisation corrosion of the CuCr alloy will not wcur when the concentration (or temperature) of HCl solution is below a certain value. Fig. 2 shows the relationship between concentration and temperature of HC1 solution when dechromisation occurs in different states. It can be seen from Fig. 2 3.0 I
I
Temperature, T / "C
Fig. 2. Relationship between temperature and concentration of HCI solution.
that ultrasonic vibration has some effects on the concentration and temperature of HC1 solution during the dechromisation of CuCr alloy. When the temperature is 20"C, the concentration is 2.74 mo1.L-I without ultrasonic vibration during dechromisation. However, at the same temperature, the concentrationsare 2.47 mo1.L-l and 2.19 rno1.L-l during dechromisationunder 20 kHz and 40 kHz ultrasonic vibration, respectively. When the concen-
tration is 1.64 mol.L-l, the temperature is 60°C during dechromisation without ultrasonic vibration. However, at the same concentration, the temperature is 40°C during dechromisation under 40 kHz ultrasonic vibration. The facts show that when the temperature (or concentration) is constant, the stronger the ultrasonic vibration, the lower the concentration (or temperature) is during dechromisation, that is to say, the easier the CuCr alloy dechromised. It can be a f f i i e d that ultrasonic vibration can increase the dechromisation tendency of CuCr alloy, which is obviously consistent with the results in section 3.1.
33. Effects of ultrasonic vibration on the dechromisation layer of the CuCr alloy It can be seen from macro-observations that the dechromisation samples' surface without ultrasonic vibration is smooth and compact whereas the dechromisation samples' surface under ultrasonic vibration is coarse, lax, and hollow. Fig. 3 shows two surface morphologies of the dechromised samples. By observing the microstructure of dechromisation layer of samples, it can be found that Cu phase exists in the dechromisation district, Cr phase fell out and some hollow remained. Compared with the microstructure of dechromisation layer in three different states, it can be found that the size of the hollow without ultrasonic vibration is smaller, far smaller than that of Cr phase in the original microstructure. Whereas the size of dechromisation layer under ultrasonic vibration is bigger, as big as that of Cr phase in the original microstructure, and the compactness of microstructure is less. These show the ultrasonic vibration aggravate the microstructure of dechromisation layer. The results are shown in Figs. 4 and 5. It can be indicated by EDX analysis of the CuCr alloy that the white bright region exists around dechromisation phase in two different states. The Cr content is 28.14 wt.%, far lower than that of Cr phase (99.78 wt.%). The fact indicates that the corrosion mechanism of dechromisation is coincident in the two different states, that is to say, the dechromisation corrosion first occurs around the interface of Cu phase and Cr phase, then gradually extends to the
Xu T.et aZ., Effect of ultrasonic vibration on the dechromisationcorrosion of a CuCr alloy in ... center of Cr phase until the Cr phase falls out com-
401
pletely.
Fig. 3. Morphologies of dechromised sample’s surface: (a) 0 kHz; (b) 40 kHz.
Fig. 4. Micmtructures of dechromisation layer: (a) 0 W ;(b) 40 kHz
Fig. 5. SEM images of dechromisation layer: (a) 0 IrHz,(b) 40 kHz.
4.
Discussion
It can be known from the potential-pH chart that the electrode potential of Cr metal i s lower than that of Cu metal in the acid region. In theory, Cr phase in the CuCr alloy is prior to corrosion in acid solutions. In fact, the dechromisation corrosion of the CuCr alloy does not immediately occur in hydrochloric acid. This may be caused by the passivation of Cr. After passivation, the electrode potential of Cr increases and the tendency of dechromistion corrosion
of the CuCr alloy decreases. Because C1- in the HCl solution has higher polarity and penetrability [ll], C1- can destroy the passivation layer of Cr surface. The increase of temperature and activated energy of atoms have benefits to the breakage of passivation layer. Therefore, when there i s a certain concentration C1- in the solutions in a certain temperature, the passivation layer is destroyed and Cr is activated. Because the activity of Cr is bigger than that of Cu after being activated, Cr first corrodes, that is to say, dechromisation corrosion occurs. It can be seen that
402
the stability of passivation layer on the Cr surface is a main reason that affects dechromisation corrosion. On one hand, ultrasonic vibration can bring liquid vibration to accelerate the movement of C1-. On the other hand, shock wave comes into being when air bubbles disappear and air bubbles vibrating strongly penetrate into the aperture and hollow of passivation layer under ultrasonic vibration. The phenomena can accelerate the breakage of passivation layer, abbreviate the incubation time of dechromisation, destroy the passivation layer even in low concentration and temperature of HCl solution, and increase the tendency of dechromisation of CuCr alloy. Subsequently, ultrasonic vibration penetrates into the bare cracks of the interface of Cu phase and Cr phase, which leads to vibration of Cr atoms, accelerates the moving rate of Cr atoms from in to out, and increases the dechromisation rate. The stronger the ultrasonic vibration is, the more evident the effect. For the microstructure of dechromisation layer, without ultrasonic vibration, the size of the aperture left when Cr phase is separated out to form the microstructure of dechromisation layer is smaller than that of Cr phase of the original microstructure. This indicates that Cu atoms take up the position of Cr atoms. Under ultrasonic vibration, the activity of Cr atoms increases, that may make the moving rate of Cr atoms from in to out far larger than that of Cu atoms. In addition, the deposit from Cu atoms to Cr atoms is destroyed because of the strong vibration, and the results in Fig. 5 happen. Its mechanism is to be discussed.
5. Conclusions (1) Ultrasonic vibration can accelerate the breakage of passivation layer of Cr surface in the CuCr alloy and the activities of C1- and Cr atoms, and lead to abbreviating the incubation time of dechromisation of the CuCr alloy, increasing the dechromisation rate, and decreasing the concentration and temperature of HCl solution while the alloy dechromised. (2) Ultrasonic vibration makes the samples’ surface coarse and spongy, and makes the size of the aperture left when Cr phase moves in the micro-
RARE METALS, Vol. 26, No. 4, Aug 2007
structure of dechromisation layer as big as that of Cr phase in the original microstructure, far bigger than that of the aperture left when Cr phase moved without ultrasonic vibration. (3) Controlling the concentration and temperature of HCl solution can avoid dechromisation corrosion of the CuCr alloy under ultrasonic vibration.
References Jin Y., Adachi K., and Takeuchi T., Correlation between the electrical conductivity and aging treatment for a Cu-15 wt% Cr alloy composite formed in-situ, Mater. Lett., 1997,32 (5-6):307. Cooper K.P, Ayers J.D, and Malzahnkampe J.C., Microstructural evolution and thermal stability in rapidly solidified high-chromium-containing coppers, Mater. Sci. Eng., 1991,A142 221. Rider W.F., Influence of composition and Cr particle size of CuCr contacts on chopping current, con-tact resistance and breakdown voltage in vacuum interrupter, IEEE Trans. CPMT, 1989,12 (2): 273. Niu Y., G.F., and Douglas D.L., The air oxidation of two-phase Cu-Cr alloys at 700-90O0C, Oxid. Met., 1997,48: 357. Fu G.Y., Niu Y., and Wu W.T., Oxidation of two-phase Cu-Cr alloys with different microstructures, Trans. Nonferrous Met. SOC. China (in Chinese), 2001, ll (3): 332. Zhang C.Y., Wang J., and Zhang H., Deoxidization of CuCrv alloys p r e p d by vacuum induction melting, Trans. Nonferrous Met. SOC. China (in Chinese), 2001, ll (3): 337. Wang L.B., Zhang C.G., and Ding B.G., Influence of W of C adding on the microstructure of CuCr25 alloy, Rare Met. Mater. Eng. (in Chinese), 2003,32 (1): 4 1. Wang J., Zhang C.G., and Ding B.G., Microstructures and properties of CuCrzs alloys melted by vacuum induction, Rare Met. Mater. Eng. (in Chinese). 2001,30 (4): 290. Cao Z.Q.. and Niu Y., Effect of grain size on the oxidation behavior of Cu-20Ni-20Cr alloy, Rare Me?. Mater. Eng. (in Chinese), 2003,32 (12): 1016. [lo] Xu T., Chen L., and Chen Y., Dechromisation of Cu-Cr alloy in acid solutions containing C1-, Trans. Nonferrous Met. China (in Chinese), 2004, 14 (3): 520. [ 111 Li C.J., Study on kinetics of low carbon steel hydrochloric acid corrosion, J. Daqing Petr. Inst. (in Chinese), 1999,23 (3): 28.
RARE METALS Vol. 26, No. 4, Aug 2007, p . 398 E-mail: rm @ ustb.edu.cn
.:zO
-00
ScienceDirect
Effect of ultrasonic vibration on the dechromisation corrosion of a CuCr alloy in IZCl solution XU Tao", GAO Huawei 'I, LIU J i a n h d , and YU Cuiyan" 1) hpartment of Applied Technology,Daqing Petroleum Instilute, Daqing 163318. China
2) Logistics Groups Company, Yanshan University, Qinhuangdao O66004, China 3) Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdaoo66004,China (Received 2006-03-26)
Abstract: The effectof ultrasonic vibration on the dechromisation corrosion of a CuCr alloy in HCI solution was studied and the corrosion mechanisms were analyzed. It is found that ultrasonic vibration reduces the dechromisation incubation time, accelerates the dechromisation corrosion rate, decreases the temperature and concentration of HC1 solution, and when the dechromisation occurs it seriously weakens the microstructure of dechromisation layer. It is concluded that ultrasonic vibration can accelerate destruction of the passivation film on the Cr surface and increase the activities of Cl- and Cr. Key wonls: CuCr alloy; dechromisation corrosion; ultrasonic vibration; passivation film
[Zhisproject is financially supPofled by the scientific Research Fund of Heilongiiang Provincial Education Department, China (No. 11511020).]
1. Introduction With the application fields of Cu alloy extending, a higher requirement of intensity and corrosion resistance of Cu alloys is brought forward. A CuCr alloy has high intensity, excellent electric conductivity and heat conductivity, and high temperature corrosion resistance [ 1-91, so it is widely applied in the correlative fields. However, the corrosion study of CuCr alloys in the medium is scarcely reported. According to recent results, a CuCr alloy in HC1 solution under certain condition will cause serious dechromisation phenomenon [lo], but the understanding of the effect factors of dechromisation of a CuCr alloy is not comprehensive enough because of Cu alloy workpiece service in the eroding and vibrational medium. Therefore, this article studied the effects of ultrasonic vibration on the incubation time of dechromisation, the dechromisation rate, and the microstructure of dechromisation layer of a CuCr Correspondingauthor: XU Tao
E-mail:xt4535 @yahoo.corn.cn
alloy in HCl solution, and the effects of the lowest concentration and temperature of HCI solution during dechromisation. The results offer some reference data to the study of dealloying of a CuCr alloy, further understandings of dechromisation appearance and its effect factors of the CuCr alloy in HCl solution, and its application extending.
2.
Experimental
The experimental material is a CuCr alloy and its chemical components can be seen from Table 1. The samples' size is 5 mm x 5 mm x 10 mm. The every surface of samples was polished with 600# metallic sand papers, washed with water and degreased with alcohol. Then, the samples were immersed into HCl solutions with different concentrations and temperatures for corrosion. To test the effect of ultrasonic vibration on the dechromisation of the CuCr alloy, the experiment is carried out on a CQ50 ultrasonic
Xu T. et af., Effect of ultrasonic vibration on the dechromisation corrosion of a CuCr alloy in ... cleaning machine and its vibrational frequency is 0 kHz, 20 kHz, and 40 kHz, respectively. In this study the dechromisation of above-mentioned samples was observed, the incubation time of dechromisation (from samples being immersed into solution to air bubble coming into being) was tested for samples in different states in HCI solution with the lowest concentration and temperature at which dechromisation took place, and the dechromisation rate V ( V = h / t ) was calculated, where h represents the thickness of dechromisation layer (the hckness was measured vertically from the surface of the dechromisation sample to the position which has a obvious dividing line with the body matrix and its datum is the mean value of three measured results), r represents the dechromisation time (timed from air bubble separating out in the samples’ surface). Surface components and morphology after corrosion were observed and analyzed by means of ADVANTPXP-381 X-rays fluorescent spectrograph, Neophot2 1 metalloscope, and KYKY-2800 SEM (with EXD). Table 1. Composition of samplis surface in different wt.% conditions Sample’s condition
Cu
Original
50.48
CI-
Others
49.30 0.00
0.22
86.82
1.01
0.36
88.01 6.72
5.12
0.15
No dechromisation in HCI 11.81
Dechromisation in HCI
Cr
3. Results 3.1. Effects of ultrasonic vibration on the incubation time of dechromisation and the dechromisation rate of the CuCr alloy It can be seen from macroscopical observations that there are two corrosion phenomena of the CuCr alloy in ultrasonic vibration HCI solution. When the concentration and temperature of HCl solution are lower, the dechromisation of the CuCr alloy does not occur, and the samples’ surface looks light gray and silvery white particles appear. It can be seen from fluorescent spectrograph analysis that the Cu content in the samples’ surface decreases correspondingly as the content of Cr increases (Table 1). It can be seen from SEM observations that Cu phase
399
of the CuCr alloy is easier to corrode than Cr phase in the experimental conditions (Fig. 1). In Table 1, the corrosive condition of the non-dechromised CuCr alloy is 1.09 m0l.L-l HCl solution for 100 h, whereas the corrosive condition of dechromisation of the CuCr alloy is 3.27 mo1.L-l HCl solution for 2 h. The vibration frequency is 40 kHz, and temperature is 2OOC.
Fig. 1. SEM image of the non-dechromised sample.
When the concentration and temperature of HCl solution reach a certain value, dechromisation will occur after the dechromisation incubation time. Once the dechromisation occurs, it develops quickly. It can be seen from fluorescent spectrograph analysis that the Cr content in the samples’ surface decreases dramatically. Its dechromisation phenomena are that there are an amount of air bubbles appearing in the samples’ surface and the samples’ surface looks like bronze, whereas HCl solution is gradually bottle green. The incubation time of dechromisation of the CuCr alloy in different states was recorded, and the dechromisation region of the samples’ surface was abraded with a sand wheel after some time, and the interface between the dechromisation region and the body matrix was bare. The thickness of dechromisation layer was tested by metallographic observations and the dechromisation rate was calculated. The results are shown in Table 2. It can be seen that the higher the frequency of ultrasonic vibration is, the shorter the incubation time of dechromisation and the quicker the dechromisation rate in HC1 solution under the same condition. The facts show that ultrasonic vibration can accelerate the dechromisation of CuCr alloy.
RARE METALS, Vol. 26, No. 4, Aug 2007
400
Table 2. Dechromisation incubation time and dechromisation corrosion rate of a CuCr alloy in 2.74 mol-L-' HCI solution (20°C) under different states Item
Working frequency,f/ kHz 0
Dechromisationincuba113 tion time, t / s Dechromisationcorrosion 3.23 rate, V / (pmmin-')
20
40
75
52
3.58
3.74
3.2. Effects of ultrasonic vibration on the concentration and temperatureof solution during CuCr alloy dechromising It can be known from the experimental results that the dechromisation corrosion of the CuCr alloy will not wcur when the concentration (or temperature) of HCl solution is below a certain value. Fig. 2 shows the relationship between concentration and temperature of HC1 solution when dechromisation occurs in different states. It can be seen from Fig. 2 3.0 I
I
Temperature, T / "C
Fig. 2. Relationship between temperature and concentration of HCI solution.
that ultrasonic vibration has some effects on the concentration and temperature of HC1 solution during the dechromisation of CuCr alloy. When the temperature is 20"C, the concentration is 2.74 mo1.L-I without ultrasonic vibration during dechromisation. However, at the same temperature, the concentrationsare 2.47 mo1.L-l and 2.19 rno1.L-l during dechromisationunder 20 kHz and 40 kHz ultrasonic vibration, respectively. When the concen-
tration is 1.64 mol.L-l, the temperature is 60°C during dechromisation without ultrasonic vibration. However, at the same concentration, the temperature is 40°C during dechromisation under 40 kHz ultrasonic vibration. The facts show that when the temperature (or concentration) is constant, the stronger the ultrasonic vibration, the lower the concentration (or temperature) is during dechromisation, that is to say, the easier the CuCr alloy dechromised. It can be a f f i i e d that ultrasonic vibration can increase the dechromisation tendency of CuCr alloy, which is obviously consistent with the results in section 3.1.
33. Effects of ultrasonic vibration on the dechromisation layer of the CuCr alloy It can be seen from macro-observations that the dechromisation samples' surface without ultrasonic vibration is smooth and compact whereas the dechromisation samples' surface under ultrasonic vibration is coarse, lax, and hollow. Fig. 3 shows two surface morphologies of the dechromised samples. By observing the microstructure of dechromisation layer of samples, it can be found that Cu phase exists in the dechromisation district, Cr phase fell out and some hollow remained. Compared with the microstructure of dechromisation layer in three different states, it can be found that the size of the hollow without ultrasonic vibration is smaller, far smaller than that of Cr phase in the original microstructure. Whereas the size of dechromisation layer under ultrasonic vibration is bigger, as big as that of Cr phase in the original microstructure, and the compactness of microstructure is less. These show the ultrasonic vibration aggravate the microstructure of dechromisation layer. The results are shown in Figs. 4 and 5. It can be indicated by EDX analysis of the CuCr alloy that the white bright region exists around dechromisation phase in two different states. The Cr content is 28.14 wt.%, far lower than that of Cr phase (99.78 wt.%). The fact indicates that the corrosion mechanism of dechromisation is coincident in the two different states, that is to say, the dechromisation corrosion first occurs around the interface of Cu phase and Cr phase, then gradually extends to the
Xu T.et aZ., Effect of ultrasonic vibration on the dechromisationcorrosion of a CuCr alloy in ... center of Cr phase until the Cr phase falls out com-
401
pletely.
Fig. 3. Morphologies of dechromised sample’s surface: (a) 0 kHz; (b) 40 kHz.
Fig. 4. Micmtructures of dechromisation layer: (a) 0 W ;(b) 40 kHz
Fig. 5. SEM images of dechromisation layer: (a) 0 IrHz,(b) 40 kHz.
4.
Discussion
It can be known from the potential-pH chart that the electrode potential of Cr metal i s lower than that of Cu metal in the acid region. In theory, Cr phase in the CuCr alloy is prior to corrosion in acid solutions. In fact, the dechromisation corrosion of the CuCr alloy does not immediately occur in hydrochloric acid. This may be caused by the passivation of Cr. After passivation, the electrode potential of Cr increases and the tendency of dechromistion corrosion
of the CuCr alloy decreases. Because C1- in the HCl solution has higher polarity and penetrability [ll], C1- can destroy the passivation layer of Cr surface. The increase of temperature and activated energy of atoms have benefits to the breakage of passivation layer. Therefore, when there i s a certain concentration C1- in the solutions in a certain temperature, the passivation layer is destroyed and Cr is activated. Because the activity of Cr is bigger than that of Cu after being activated, Cr first corrodes, that is to say, dechromisation corrosion occurs. It can be seen that
402
the stability of passivation layer on the Cr surface is a main reason that affects dechromisation corrosion. On one hand, ultrasonic vibration can bring liquid vibration to accelerate the movement of C1-. On the other hand, shock wave comes into being when air bubbles disappear and air bubbles vibrating strongly penetrate into the aperture and hollow of passivation layer under ultrasonic vibration. The phenomena can accelerate the breakage of passivation layer, abbreviate the incubation time of dechromisation, destroy the passivation layer even in low concentration and temperature of HCl solution, and increase the tendency of dechromisation of CuCr alloy. Subsequently, ultrasonic vibration penetrates into the bare cracks of the interface of Cu phase and Cr phase, which leads to vibration of Cr atoms, accelerates the moving rate of Cr atoms from in to out, and increases the dechromisation rate. The stronger the ultrasonic vibration is, the more evident the effect. For the microstructure of dechromisation layer, without ultrasonic vibration, the size of the aperture left when Cr phase is separated out to form the microstructure of dechromisation layer is smaller than that of Cr phase of the original microstructure. This indicates that Cu atoms take up the position of Cr atoms. Under ultrasonic vibration, the activity of Cr atoms increases, that may make the moving rate of Cr atoms from in to out far larger than that of Cu atoms. In addition, the deposit from Cu atoms to Cr atoms is destroyed because of the strong vibration, and the results in Fig. 5 happen. Its mechanism is to be discussed.
5. Conclusions (1) Ultrasonic vibration can accelerate the breakage of passivation layer of Cr surface in the CuCr alloy and the activities of C1- and Cr atoms, and lead to abbreviating the incubation time of dechromisation of the CuCr alloy, increasing the dechromisation rate, and decreasing the concentration and temperature of HCl solution while the alloy dechromised. (2) Ultrasonic vibration makes the samples’ surface coarse and spongy, and makes the size of the aperture left when Cr phase moves in the micro-
RARE METALS, Vol. 26, No. 4, Aug 2007
structure of dechromisation layer as big as that of Cr phase in the original microstructure, far bigger than that of the aperture left when Cr phase moved without ultrasonic vibration. (3) Controlling the concentration and temperature of HCl solution can avoid dechromisation corrosion of the CuCr alloy under ultrasonic vibration.
References Jin Y., Adachi K., and Takeuchi T., Correlation between the electrical conductivity and aging treatment for a Cu-15 wt% Cr alloy composite formed in-situ, Mater. Lett., 1997,32 (5-6):307. Cooper K.P, Ayers J.D, and Malzahnkampe J.C., Microstructural evolution and thermal stability in rapidly solidified high-chromium-containing coppers, Mater. Sci. Eng., 1991,A142 221. Rider W.F., Influence of composition and Cr particle size of CuCr contacts on chopping current, con-tact resistance and breakdown voltage in vacuum interrupter, IEEE Trans. CPMT, 1989,12 (2): 273. Niu Y., G.F., and Douglas D.L., The air oxidation of two-phase Cu-Cr alloys at 700-90O0C, Oxid. Met., 1997,48: 357. Fu G.Y., Niu Y., and Wu W.T., Oxidation of two-phase Cu-Cr alloys with different microstructures, Trans. Nonferrous Met. SOC. China (in Chinese), 2001, ll (3): 332. Zhang C.Y., Wang J., and Zhang H., Deoxidization of CuCrv alloys p r e p d by vacuum induction melting, Trans. Nonferrous Met. SOC. China (in Chinese), 2001, ll (3): 337. Wang L.B., Zhang C.G., and Ding B.G., Influence of W of C adding on the microstructure of CuCr25 alloy, Rare Met. Mater. Eng. (in Chinese), 2003,32 (1): 4 1. Wang J., Zhang C.G., and Ding B.G., Microstructures and properties of CuCrzs alloys melted by vacuum induction, Rare Met. Mater. Eng. (in Chinese). 2001,30 (4): 290. Cao Z.Q.. and Niu Y., Effect of grain size on the oxidation behavior of Cu-20Ni-20Cr alloy, Rare Me?. Mater. Eng. (in Chinese), 2003,32 (12): 1016. [lo] Xu T., Chen L., and Chen Y., Dechromisation of Cu-Cr alloy in acid solutions containing C1-, Trans. Nonferrous Met. China (in Chinese), 2004, 14 (3): 520. [ 111 Li C.J., Study on kinetics of low carbon steel hydrochloric acid corrosion, J. Daqing Petr. Inst. (in Chinese), 1999,23 (3): 28.