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Corrosion behavior of carbon-implanted zircaloy-4 in 1 M H2SO4
Nuclear Instruments and Methods in Physics Research B 234 (2005) 235–242 www.elsevier.com/locate/nimb
Corrosion behavior of carbon-implanted zircaloy-4 in 1 M H2SO4 D.Q. Peng a
a,*
, X.D. Bai a, F. Pan a, F. Sun a, B.S. Chen
b
Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China b Jianzhong Chemical Cooperation, Yibin 644000, PR China Received 6 August 2004; received in revised form 11 December 2004 Available online 17 March 2005
Abstract In order to study the effect of carbon ion implantation on the aqueous corrosion behavior, samples of zircaloy-4 were implanted with carbon ions with fluences ranging from 1 · 1016 to 1 · 1017 ions/cm2, using a metal vapor vacuum arc source (MEVVA) operated at an extraction voltage of 40 kV. The valence states and depth distributions of elements in the surface layer of the samples were analyzed by X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES), respectively. Glancing angle X-ray diffraction (GAXRD) was employed to examine the phase transformation due to the carbon ion implantation. The potentiodynamic polarization technique was employed to evaluate the aqueous corrosion resistance of implanted zircaloy-4 in a 1 M H2SO4 solution. It was found that a significant improvement was achieved in the aqueous corrosion behavior of zircaloy-4 implanted with carbon ions when the fluence is smaller than 5 · 1016 ions/cm2. The higher the fluence, the worse is the corrosion resistance. Finally, the mechanism of the corrosion behavior of carbon-implanted zircaloy-4 was discussed. 2005 Elsevier B.V. All rights reserved. PACS: 81.65.K; 79.60; 82.80.P; 81.65.M; 82.80.F Keywords: Zircaloy-4; Corrosion resistance; Carbon ion implantation; X-ray photoelectron spectroscopy (XPS); Auger electron spectroscopy (AES)
1. Introduction *
Corresponding author. Tel.: +86 10 6277 2856; fax: +86 10 6277 2507. E-mail addresses: [email protected], baixde@ mail.tsinighua.edu.cn (D.Q. Peng).
Zirconium alloys are widely used in the nuclear industry because of their low thermal neutron capture cross section, favorable mechanical properties, and good corrosion resistance. For example,
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.02.006
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D.Q. Peng et al. / Nucl. Instr. and Meth. in Phys. Res. B 234 (2005) 235–242
Zr-alloys can serve as fuel cladding, spreaders for fuel elements, and for core-structure materials. However, with the development of high burn-up fuel, improvements in the performance of Zr-alloys are increasingly required. It is well known that ion beam surface processing (IBP) techniques can significantly improve corrosion resistance [1–4]. Ion implantation in particular offers the possibility of introducing a controlled concentration of an element into a thin surface layer. It was first shown (by Ashworth et al.) that chromium implantation could improve the corrosion resistance of iron [5]. Many studies including those of palladiumimplanted into titanium [6] and phosphorusimplanted into iron [7], have confirmed that ion implantation with palladium or phosphorus may successfully improve the corrosion behavior. Although it was reported that carbon implantation improved the corrosion resistance and tribological properties of steels, titanium, aluminum [8–10]. Relatively few papers have reported on the corrosion behavior of carbon-implanted zircaloy-4. In this work we report the results of a study of carbon implantation on the aqueous corrosion behavior of zircaloy-4. The valence states of carbon, zirconium, and oxygen were analyzed by X-ray photoelectron spectroscopy (XPS); depth distributions of elements in the surface layer were obtained by Auger electron spectrometry (AES), and the corrosion resistance of the implanted zircaloy-4 was investigated by the potentiodynamic method using a IM6e potentiostat. 0.3 glancing angle X-ray diffraction (GAXRD) was employed to examine the phase transformation due to the carbon ion implantation. The mechanism of corrosion behavior of carbon-implanted zircaloy-4 in the aqueous is discussed.
2. Experimental procedure Zircaloy-4 samples were machined to a size of 10 mm · 10 mm from a 0.5 mm thick sheet of zircaloy-4, fully annealed after cold rolling. The composition of zircaloy-4 is Sn: 1.4 wt.%, Fe: 0.23 wt.%, Cr: 0.1 wt.%, Ni: 60 · 106 wt.%, balanced with zirconium. The zircaloy-4 samples were
mechanically polished using 200–800 grid emery paper. The samples were then degreased in acetone and ethanol, chemically polished in a solution of 10% HF, 30% HNO3 and 60% H2O by volume, and finally rinsed in de-ionized water. For implantation the samples were loaded onto an aluminum sample holder with a diameter of 12 cm. The vacuum level of the MEVVA (metal vapor vacuum arc) implanter target chamber was 1.3 · 103 Pa. Although the implantation system has no magnet analytic capability, the extracted carbon ions were measured to consist of 100% C+. Fluences of 1 · 1016, 5 · 1016, and 1 · 1017 ions/cm2 were used. The extraction voltage for the carbon implantation was 40 kV. Therefore, the implantation energies were 40 keV. The samples were not cooled during implantation. The implantation temperature therefore depended on the beam current density, which varied from 8.85 to 26.6 lA/cm2. Beneath the sample, there is a thermocouple to determine the temperature of the sample. Table 1 summarized the implantation conditions used. The valence states of carbon, zirconium, and oxygen on sample surfaces were analyzed by Xray photoelectron spectroscopy (XPS). The potentiodynamic polarization measurements were carried out to investigate the aqueous corrosion resistance of the carbon-implanted zircaloy-4. The potentiodynamic tests were performed in a 1 M H2SO4 solution using IM6e potentiostat (Zahner Electrik) at room temperature (25 C). The solution was stirred up with pure nitrogen for an hour before the measurement. The tested area was 1 cm2 and the scan rate was 2 mV/s. All electrochemical potential measurements were taken with respect to a saturated calomel electrode (SCE). Measurements were carried out using anode scan started in the cathodic region at approximately 0.4 V with respect to the SCE, and then scanned into the anodic region of approximately +2.0 V with respect to the SCE. Table 1 Implantation conditions Fluence (ions/m2)
1 · 1020
5 · 1020
1 · 1021
Beam current density (lA/cm2) Temperature (C)
8.85 100
17.7 220
26.6 250
D.Q. Peng et al. / Nucl. Instr. and Meth. in Phys. Res. B 234 (2005) 235–242
3. Results and discussion 2.0x105
Because of the systematic error in XPS measurements, the energy positions were adjusted by comparing the surface energy of absorbed C on the surface of the specimen with that of the standard binding energy (285.0 eV). The adjusted XPS spectra of C 1s, Zr 3d5/2 and O 1s are shown in Figs. 1–3 respectively. In Fig. 1 for carbon the highest peak is unimplanted sample, because there is more absorbed carbon on the surface of unim-
Intensity (c/s)
1.2x10
5
9.0x104
4 3 2
6.0x104
290
285
280
X Axis Title
Fig. 1. XPS spectra of C 1s peak of (1) as-received zircaloy-4 and implanted with carbon at fluence of: (2) 1 · 1016 ions/cm2; (3) 5 · 1016 ions/cm2; (4) 1 · 1017 ions/cm2.
3 4 2
Intensity (c/s)
1.0x105
(1) (2) (3) (4)
as-received Zr-4 1E 16C 5E 16C 1E 17C
1
5.0x104
186
1.0x105
5.0x104 540
535
530
525
Binding Energy (eV)
planted sample than that of implanted samples. On the other hand, the measured depth of XPS instrument is less than 10 nm. The adjusted binding energy of Zr 3d5/2 is 182.4 eV (Fig. 2), coincides with the standard values of ZrO2x [11], from which it is suggested that zircaloy-4 implanted carbon at the surface exists in the form of ZrO2x. Fig. 3 shows that the binding energy of O1s is 530 eV, which suggests the oxygen at the surface exists as ZrO2x as for implanted zircaloy-4. Oxygen comes from residual gas in vacuum chamber, because the vacuum level of the target chamber of the MEVVA implanter is not very high. 3.2. The depth profile of the elements in the implanted surface layer
Zr 3d5/2 182.4 eV
1.5x105
1.5x105
(1)-as-received Zr-4 (2)-1E 16C (3)-5E 16C (4)-1E 17C
O1s 530eV 1 2 3 4
Fig. 3. XPS spectra of O 1s peak of (1) as-received zircaloy-4 and implanted with carbon at fluence of: (2) 1 · 1016 ions/cm2; (3) 5 · 1016 ions/cm2; (4) 1 · 1017 ions/cm2.
C1s 285eV 1 (1)-as-recieved Zr-4 (2)-1E16C (3)-5E16C (4)-1E17C
Intensity (c/s)
3.1. The valence of the elements in the surface layer
237
183
180
Binding Energy (eV)
Fig. 2. XPS spectra of Zr 3d5/2 peak of (1) as-received zircaloy4 and implanted with carbon at fluence of: (2) 1 · 1016 ions/cm2; (3) 5 · 1016 ions/cm2; (4) 1 · 1017 ions/cm2.
AES measurements to determine the concentration of elements in the surface layer as a function of depth were carried out using a PHI-610/SAM spectrometer. For the AES measurements, the sputter rate was 30 nm/min. Curves (1)–(4) in Fig. 4 show AES spectra of zircaloy-4 implanted with carbon ions using fluences of 0, 1 · 1016, 5 · 1016, 1 · 1017 ions/cm2. The results show that the maximum thicknesses of the oxide film are 24, 150, 200, and 420 nm, respectively. The carbon range extends so much behind the peak-even in the low fluence case, which is probably due to recoil implantation during sputtering. A computer
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D.Q. Peng et al. / Nucl. Instr. and Meth. in Phys. Res. B 234 (2005) 235–242 100
90 Zr
90 Zr
80
ACP (at%)
ACP (at.%)
(1)
60 50 40 O
20
50 40
0.5
1.0
1.5
O
10
0
C
C
C
0
2.0
0
Sputter Time (min)
1
2
3
4
5
6
7
Sputter time (min)
100
100
Zr
Zr
90
90
80
80 70
70
(3)
60
ACP (at%)
ACP (at%)
Zr Zr
20 O
10 0.0
(2)
O
60
30
Zr
30
O
70
O
70
Zr
80
Zr
O
50 40 O
Zr
30
(4)
60
Zr
50
Zr
40
O
O
30 C
20
20
C
10
C
C
0 2
4
6
8
C
0
O 0
O
C
10
10
Sputter time (min)
0
5
10
15
20
Sputter time(min)
Fig. 4. AES depth profiles of zircaloy-4 implanted with carbon at (1) as-received zircaloy-4; (2) 1 · 1016 ions/cm2; (3) 5 · 1016 ions/cm2; (4) 1 · 1017 ions/cm2.
simulation program, TRIM-96, was used to calculate the expected distribution of carbon (Fig. 5). From Fig. 5, the peak position of carbon is
(Atoms/cm3)/(Atoms/cm2)
1.2x105 1.0x105 8.0x104 6.0x104 4.0x104 2.0x104 0.0 0
20
40
60
80
100
120
140
160
180
Depth (nm)
Fig. 5. Distribution range of carbon ions determined by TRIM-96.
78 nm. When the fluence is 1 · 1016 ions/cm2, the experimental result coincides with that of calculated value basically. When the fluence is greater than 5 · 1016 ions/cm2, the peak positions of experiments are bigger than that of calculated value, 78 nm. These phenomena can be attributed to the thermal diffusion of carbon. Because the implantation was not employed at liquid nitrogen temperature, but at room temperature, the temperature of implanted samples was mainly depended on the beam current densities. From the Table 1, it can be known that the temperature is about 100 C as for 1 · 1016 ions/cm2 fluence, carbon diffuse a little. When the fluence is greater than 1 · 1016 ions/cm2, the beam current densities are much bigger that of 1 · 1016 ions/cm2 fluence, the temperature of them are over 200 C, therefore the carbon diffuse severely. According to Lawrence H. Van Vlack [12], D = D0eE/kT. D is diffusivity, D0 is constant, irrespective to the temperature; E
D.Q. Peng et al. / Nucl. Instr. and Meth. in Phys. Res. B 234 (2005) 235–242 (111)
(002)
(101)
(111)
(1)-as-received Zr-4 (2)-1E16C (3)-5E16C (112) (4)-1E17C
-α-Zr -H-ZrO 0.35 -M-ZrO 2
4
(002) (111)
Intensity (c/s)
is activation energy of atoms diffusion. k is BoltzmanÕs constant, equal to 13.8 · 1024 K. T is temperature (K). From the equality, the higher the implantation temperature, the larger is the diffusivity of carbon. From AES measurements, the carbon implantation enhanced the oxidization of zircaloy-4 samples. The larger the fluence, the higher is the implantation temperature, the longer is the duration time, and subsequently the thicker is the zirconia oxide film of implanted samples.
239
3
(002) (111)
(112)
2
(002) (101)
(102)
1
3.3. The phase structure in the implanted surface layer Glancing angle X-ray diffraction (GAXRD) was employed to investigate the phase structure characters in the implanted surface layer due to the carbon addition. From the AES of samples, the depth extent for the three fluences carbon-implanted zircaloy-4 was about a few hundreds of nanometers. If conventional XRD is used, then the information is dominated by the matrix, while the information contributed by the thin film will be very little. Glancing angle X-ray diffraction (GAXRD) with 0.3 glancing angle was employed in our experiment to study the phase characters of the carbon implantation zircaloy-4. An incident angle of 0.3 corresponded to a theoretical penetration depth of about 55 nm in the scale. So the 0.3-angle was determined for the incident angle. Fig. 6 shows the GAXRD spectra at 0.3 incident angle of the as-received and as-implanted samples. Table 2 shows experimental and tabulated values of the d-spacings of carbon-implanted zircaloy-4. From the observation of Fig. 6 and Table 2, it is clear that the as-received zircaloy-4 is composed of hexagonal alpha zirconium. Following implantation at a fluence of 1 · 1016 ions/ cm2 hexagonal zircaonia (H-ZrO0.35) is firstly formed. The amount of H-ZrO0.35 decreases at the intermediate fluence without simultaneous appearance of M-ZrO2, there is a possibility that the system was misaligned when spectrum (3) in Fig. 6 was recorded. While when the fluence reaches to 1 · 1017 ions/cm2, there produces new monoclinic zirconia (M-ZrO2). From the results of GAXRD, zirconium carbide was not observed, which is because the car-
30
35
40
45
50
diffraction angle (2-theta)
Fig. 6. GAXRD spectra at 0.3 incident angle of (1) as-received zircaloy-4 and implanted with C at (2) 1 · 1016 ions/cm2; (3) 5 · 1016 ions/cm2; and (4) 1 · 1017 ions/cm2. a-Zr represents hexagonal alpha metallic zirconium; H represents hexagonal zirconia (H-ZrO0.35); M represents monoclinic zirconia (M-ZrO2).
Table 2 Experimental and tabulated values of the d-spacings of carbonimplanted zircaloy-4 ˚ ) experimental values dhkl (A ˚ ) tabulated values (hkl) dhkl (A a-Zr 2.56 2.45 1.89
2.57 2.46 1.89
002 101 102
H-ZrO0.35 2.58 2.47 1.90
2.60 2.48 1.91
002 111 112
M-ZrO2 2.94 2.61
2.94 2.62
101 111
bon fluence is not large enough and the implantation temperature is not very high, while zircaloy-4 is oxidized and the zirconia films become thicker and thicker with raising the implantation fluences. 3.4. The electrochemical behavior of the carbonimplanted zircaloy-4 The potentiodynamic polarization curves of the as-received zircaloy-4 and the zircaloy-4 implanted with carbon-ions to a fluence ranging from
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Current Density (A/cm2)
Table 3 The relationship between the fluence and the passive current density of carbon ion implanted zircaloy-4
(1)-as-received Zr-4 (2)-1E16C (3)-5E16C (4)-1E17C
1E-4 1E-5
4 1
1E-6 1E-7
3 2
Fluence (ions/cm2)
Passive current density (A/cm2)
As-received zircaloy-4 1 · 1016 5 · 1016 1 · 1017
7.73 · 107 1.64 · 107 4.11 · 107 1.86 · 106
1E-8 1E-9 1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Potential (V vs SCE)
Fig. 7. Potentiodynamic polarization curves in 1 M H2SO4 solution of the as-received zircaloy-4 and zircaloy-4 implanted with carbon ions at a fluence of: (1) as-received zircaloy-4; (2) 1 · 1016 ions/cm2; (3) 5 · 1016 ions/cm2; 1 · 1017 ions/cm2.
Passivation Current Density (A/cm2)
2.0x10-6
Zero Current Density (Ei=0,V vs SCE)
1E-10
0.8
0.6
0.4
0.2
0.0
-6
1.5x10
2.0x1016
4.0x1016
6.0x1016
8.0x1016
1.0x1017
2
Fluence (ions/cm )
Fig. 9. Dependence of the zero current potential (Ei=0) on the implanted fluences.
-6
1.0x10
5.0x10-7
Table 4 The relationship between zero current potential and the fluences 0.0 0.0
16
2.0x10
16
4.0x10
16
6.0x10
16
8.0x10
Fluence (ions/cm2)
Zero current potential (Ei=0, V versus SCE)
As-received Zr-4 1 · 1016 5 · 1016 1 · 1017
0.2453 0.9534 0.8454 0.8074
17
1.0x10
Fluence (ions/cm2)
Fig. 8. Dependence of the passivation current density, ip, on the implanted fluences.
1 · 1016 to 1 · 1017 ions/cm2 are compared in Fig. 7. Fig. 8 plots the passive current density ip as a function of fluence (values are summarized in Table 3). From Figs. 7 and 8, it was found that a significant improvement of corrosion resistance was achieved as for the implanted zircaloy-4 when the fluence is smaller than 5 · 1016 ions/cm2. The higher the fluence, the bigger is the passive current density; subsequently the worse is the corrosion resistance of implanted samples. When the fluence reached 1 · 1017 ions/cm2, the corrosion resistance of implanted sample is worse than that of as-re-
ceived zircaloy-4. Fig. 9 plots the zero current potential as a function fluence (values are summarized in Table 4), which shows that all the zero current potentials of the implanted samples are more positive than that of the as-received zircaloy-4, while the larger the fluence, the lower is the zero current potential, and subsequently the worse is the corrosion resistance of the implanted sample. The results reflected by passive current densities are consistent with those reflected by zero current potentials as for implanted samples.
D.Q. Peng et al. / Nucl. Instr. and Meth. in Phys. Res. B 234 (2005) 235–242
3.5. The mechanism of the aqueous corrosion behavior by carbon ion implantation It is well known that the formation of the passive film on surface of the zircaloy-4 is an oxidation process. According to Pourbaix [13], there is an oxidation reaction that can take place at the zircaloy-4 anode Zr ðmÞ ¼ Zr4þ ðaqÞ þ 4e
ð1Þ
The cathodic reaction may be 2Hþ þ 2e ¼ H2 ðgÞ
ð2Þ
As analyzed above, as for the unimplanted sample, it is alpha zirconium, there is little oxidation protection in the surface of the sample, and anodic reaction of zirconium is easy to occur, subsequently the corrosion resistance of zircaloy-4 is bad. From the AES results, the implantation of carbon ions promoted the oxidization of zirconium, the zirconia film acted as a barrier to reduce the anodic reaction. When the fluence is 1 · 1016 ions/cm2, there produced hexagonal zirconia (H-ZrO0.35), and anodic reaction of zirconium is most difficult to occur, subsequently the corrosion resistance of sample implanted using a fluence of 1 · 1016 ions/cm2 is the best. When the fluence is 5 · 1016 ions/cm2, the intensity of hexagonal zirconia (H-ZrO0.35) declined, and the anodic reaction of zirconium becomes easy, subsequently the corrosion resistance of implanted sample becomes worse. When the fluence reaches to 1 · 1017 ions/ cm2, there produces new monoclinic zirconia (MZrO2), the anodic reaction of zirconium become easier and easier, subsequently, the corrosion resistance of it becomes the worst. The corrosion resistance of sample implanted with fluence of 1 · 1017 ions/cm2 is even worse than that of the as-received zircaloy-4 sample.
4. Conclusions The carbon was implanted into the zircaloy-4 sample, from XPS and AES, it was found that the carbon ion implantation promoted the oxidation in the surface layer. From GAXRD, the unimplanted sample is hexagonal alpha zirconium;
241
there firstly produces hexagonal zirconia (HZrO0.35) for fluence of 1 · 1016 ions/cm2, the intensity of H-ZrO0.35 declined with increasing the fluence, when the fluence reaches to 1 · 1017 ions/ cm2, there produces new monoclinic zirconia (MZrO2). A significant improvement of corrosion resistance was achieved when the implantation fluence of carbon is smaller than 5 · 1016 ions/cm2. The corrosion resistance of sample implanted with 1 · 1016 ions/cm2 is the best in all the samples, which is attribute to the oxidation protection of hexagonal zirconia (H-ZrO0.35). When the fluence reaches to 1 · 1017 ions/cm2, the corrosion behavior of implanted sample is even worse than that of as-received zircaloy-4, which is attributed to the appearance of monoclinic zirconia (M-ZrO2). The large the fluence, the worse is the corrosion resistance.
Acknowledgements Special thanks should be given to the Ministry of Science and Technology of China for research funding (MSTC no. G 2000067207-1). The authors would like to thank Professor Andrew Godfrey for his great help and the Analysis Center of Tsinghua University for the partial financial support.
References [1] S.J. Lee, H.S. Kwon, W. Kim, B.H. Choi, Mater. Sci. Eng. A 263 (1991) 23. [2] J. Xu, X. Bai, Y. Fan, J. Mater. Sci. 35 (2000) 6225. [3] Y. Etoh, S. Shimada, K. Kikuchi, J. Nucl. Sci. Technol. 29 (12) (1992) 1173. [4] J. Xu, X. Bai, A. Jin, Y. Fan, J. Mater. Sci. Lett. 19 (2000) 1633. [5] V. Ashworth, D. Baxter, W.A. Grant, R.P.M. Procter, T.C. Wellington, Corros. Sci. 16 (1976) 775. [6] G.K. Huble, E. Mccafferty, Corros. Sci. 20 (1980) 103. [7] X.D. Bai, D.H. Zhu, B.X. Liu, Nucl. Instr. and Meth. B 103 (1995) 440. [8] Y. Li, T. Zhang, X. Wang, Surf. Coat. Technol. 128–129 (2000) 205. [9] J.I. Onate, F. Alonso, A. Garcia, Thin Solid films 317 (1998) 471. [10] N.J. Mikklelsen, C.A. Straede, Surf. Coat. Technol. 51 (1992) 152.
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[11] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg (Eds.), A Reference Book of Standard Data for Use in X-ray Photoelectron Spectroscopy, PerkinElmer corporation, Physical Electronics Division, 6509 Flying Cloud Drive, Eden Prairie, Minnesota 55344, 1992, p. 100.
[12] Lawrence H. Van Vlack. Elements of Materials Science and Engineering, fourth ed., The University of Michigan Ann Arbor, Michigan, Addison-Wesley Publishing Company, 1980, p. 97. [13] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, N.A.C.E., Houstan TX, 1974, p. 223.
Corrosion behavior of carbon-implanted zircaloy-4 in 1 M H2SO4 D.Q. Peng a
a,*
, X.D. Bai a, F. Pan a, F. Sun a, B.S. Chen
b
Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China b Jianzhong Chemical Cooperation, Yibin 644000, PR China Received 6 August 2004; received in revised form 11 December 2004 Available online 17 March 2005
Abstract In order to study the effect of carbon ion implantation on the aqueous corrosion behavior, samples of zircaloy-4 were implanted with carbon ions with fluences ranging from 1 · 1016 to 1 · 1017 ions/cm2, using a metal vapor vacuum arc source (MEVVA) operated at an extraction voltage of 40 kV. The valence states and depth distributions of elements in the surface layer of the samples were analyzed by X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES), respectively. Glancing angle X-ray diffraction (GAXRD) was employed to examine the phase transformation due to the carbon ion implantation. The potentiodynamic polarization technique was employed to evaluate the aqueous corrosion resistance of implanted zircaloy-4 in a 1 M H2SO4 solution. It was found that a significant improvement was achieved in the aqueous corrosion behavior of zircaloy-4 implanted with carbon ions when the fluence is smaller than 5 · 1016 ions/cm2. The higher the fluence, the worse is the corrosion resistance. Finally, the mechanism of the corrosion behavior of carbon-implanted zircaloy-4 was discussed. 2005 Elsevier B.V. All rights reserved. PACS: 81.65.K; 79.60; 82.80.P; 81.65.M; 82.80.F Keywords: Zircaloy-4; Corrosion resistance; Carbon ion implantation; X-ray photoelectron spectroscopy (XPS); Auger electron spectroscopy (AES)
1. Introduction *
Corresponding author. Tel.: +86 10 6277 2856; fax: +86 10 6277 2507. E-mail addresses: [email protected], baixde@ mail.tsinighua.edu.cn (D.Q. Peng).
Zirconium alloys are widely used in the nuclear industry because of their low thermal neutron capture cross section, favorable mechanical properties, and good corrosion resistance. For example,
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.02.006
236
D.Q. Peng et al. / Nucl. Instr. and Meth. in Phys. Res. B 234 (2005) 235–242
Zr-alloys can serve as fuel cladding, spreaders for fuel elements, and for core-structure materials. However, with the development of high burn-up fuel, improvements in the performance of Zr-alloys are increasingly required. It is well known that ion beam surface processing (IBP) techniques can significantly improve corrosion resistance [1–4]. Ion implantation in particular offers the possibility of introducing a controlled concentration of an element into a thin surface layer. It was first shown (by Ashworth et al.) that chromium implantation could improve the corrosion resistance of iron [5]. Many studies including those of palladiumimplanted into titanium [6] and phosphorusimplanted into iron [7], have confirmed that ion implantation with palladium or phosphorus may successfully improve the corrosion behavior. Although it was reported that carbon implantation improved the corrosion resistance and tribological properties of steels, titanium, aluminum [8–10]. Relatively few papers have reported on the corrosion behavior of carbon-implanted zircaloy-4. In this work we report the results of a study of carbon implantation on the aqueous corrosion behavior of zircaloy-4. The valence states of carbon, zirconium, and oxygen were analyzed by X-ray photoelectron spectroscopy (XPS); depth distributions of elements in the surface layer were obtained by Auger electron spectrometry (AES), and the corrosion resistance of the implanted zircaloy-4 was investigated by the potentiodynamic method using a IM6e potentiostat. 0.3 glancing angle X-ray diffraction (GAXRD) was employed to examine the phase transformation due to the carbon ion implantation. The mechanism of corrosion behavior of carbon-implanted zircaloy-4 in the aqueous is discussed.
2. Experimental procedure Zircaloy-4 samples were machined to a size of 10 mm · 10 mm from a 0.5 mm thick sheet of zircaloy-4, fully annealed after cold rolling. The composition of zircaloy-4 is Sn: 1.4 wt.%, Fe: 0.23 wt.%, Cr: 0.1 wt.%, Ni: 60 · 106 wt.%, balanced with zirconium. The zircaloy-4 samples were
mechanically polished using 200–800 grid emery paper. The samples were then degreased in acetone and ethanol, chemically polished in a solution of 10% HF, 30% HNO3 and 60% H2O by volume, and finally rinsed in de-ionized water. For implantation the samples were loaded onto an aluminum sample holder with a diameter of 12 cm. The vacuum level of the MEVVA (metal vapor vacuum arc) implanter target chamber was 1.3 · 103 Pa. Although the implantation system has no magnet analytic capability, the extracted carbon ions were measured to consist of 100% C+. Fluences of 1 · 1016, 5 · 1016, and 1 · 1017 ions/cm2 were used. The extraction voltage for the carbon implantation was 40 kV. Therefore, the implantation energies were 40 keV. The samples were not cooled during implantation. The implantation temperature therefore depended on the beam current density, which varied from 8.85 to 26.6 lA/cm2. Beneath the sample, there is a thermocouple to determine the temperature of the sample. Table 1 summarized the implantation conditions used. The valence states of carbon, zirconium, and oxygen on sample surfaces were analyzed by Xray photoelectron spectroscopy (XPS). The potentiodynamic polarization measurements were carried out to investigate the aqueous corrosion resistance of the carbon-implanted zircaloy-4. The potentiodynamic tests were performed in a 1 M H2SO4 solution using IM6e potentiostat (Zahner Electrik) at room temperature (25 C). The solution was stirred up with pure nitrogen for an hour before the measurement. The tested area was 1 cm2 and the scan rate was 2 mV/s. All electrochemical potential measurements were taken with respect to a saturated calomel electrode (SCE). Measurements were carried out using anode scan started in the cathodic region at approximately 0.4 V with respect to the SCE, and then scanned into the anodic region of approximately +2.0 V with respect to the SCE. Table 1 Implantation conditions Fluence (ions/m2)
1 · 1020
5 · 1020
1 · 1021
Beam current density (lA/cm2) Temperature (C)
8.85 100
17.7 220
26.6 250
D.Q. Peng et al. / Nucl. Instr. and Meth. in Phys. Res. B 234 (2005) 235–242
3. Results and discussion 2.0x105
Because of the systematic error in XPS measurements, the energy positions were adjusted by comparing the surface energy of absorbed C on the surface of the specimen with that of the standard binding energy (285.0 eV). The adjusted XPS spectra of C 1s, Zr 3d5/2 and O 1s are shown in Figs. 1–3 respectively. In Fig. 1 for carbon the highest peak is unimplanted sample, because there is more absorbed carbon on the surface of unim-
Intensity (c/s)
1.2x10
5
9.0x104
4 3 2
6.0x104
290
285
280
X Axis Title
Fig. 1. XPS spectra of C 1s peak of (1) as-received zircaloy-4 and implanted with carbon at fluence of: (2) 1 · 1016 ions/cm2; (3) 5 · 1016 ions/cm2; (4) 1 · 1017 ions/cm2.
3 4 2
Intensity (c/s)
1.0x105
(1) (2) (3) (4)
as-received Zr-4 1E 16C 5E 16C 1E 17C
1
5.0x104
186
1.0x105
5.0x104 540
535
530
525
Binding Energy (eV)
planted sample than that of implanted samples. On the other hand, the measured depth of XPS instrument is less than 10 nm. The adjusted binding energy of Zr 3d5/2 is 182.4 eV (Fig. 2), coincides with the standard values of ZrO2x [11], from which it is suggested that zircaloy-4 implanted carbon at the surface exists in the form of ZrO2x. Fig. 3 shows that the binding energy of O1s is 530 eV, which suggests the oxygen at the surface exists as ZrO2x as for implanted zircaloy-4. Oxygen comes from residual gas in vacuum chamber, because the vacuum level of the target chamber of the MEVVA implanter is not very high. 3.2. The depth profile of the elements in the implanted surface layer
Zr 3d5/2 182.4 eV
1.5x105
1.5x105
(1)-as-received Zr-4 (2)-1E 16C (3)-5E 16C (4)-1E 17C
O1s 530eV 1 2 3 4
Fig. 3. XPS spectra of O 1s peak of (1) as-received zircaloy-4 and implanted with carbon at fluence of: (2) 1 · 1016 ions/cm2; (3) 5 · 1016 ions/cm2; (4) 1 · 1017 ions/cm2.
C1s 285eV 1 (1)-as-recieved Zr-4 (2)-1E16C (3)-5E16C (4)-1E17C
Intensity (c/s)
3.1. The valence of the elements in the surface layer
237
183
180
Binding Energy (eV)
Fig. 2. XPS spectra of Zr 3d5/2 peak of (1) as-received zircaloy4 and implanted with carbon at fluence of: (2) 1 · 1016 ions/cm2; (3) 5 · 1016 ions/cm2; (4) 1 · 1017 ions/cm2.
AES measurements to determine the concentration of elements in the surface layer as a function of depth were carried out using a PHI-610/SAM spectrometer. For the AES measurements, the sputter rate was 30 nm/min. Curves (1)–(4) in Fig. 4 show AES spectra of zircaloy-4 implanted with carbon ions using fluences of 0, 1 · 1016, 5 · 1016, 1 · 1017 ions/cm2. The results show that the maximum thicknesses of the oxide film are 24, 150, 200, and 420 nm, respectively. The carbon range extends so much behind the peak-even in the low fluence case, which is probably due to recoil implantation during sputtering. A computer
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D.Q. Peng et al. / Nucl. Instr. and Meth. in Phys. Res. B 234 (2005) 235–242 100
90 Zr
90 Zr
80
ACP (at%)
ACP (at.%)
(1)
60 50 40 O
20
50 40
0.5
1.0
1.5
O
10
0
C
C
C
0
2.0
0
Sputter Time (min)
1
2
3
4
5
6
7
Sputter time (min)
100
100
Zr
Zr
90
90
80
80 70
70
(3)
60
ACP (at%)
ACP (at%)
Zr Zr
20 O
10 0.0
(2)
O
60
30
Zr
30
O
70
O
70
Zr
80
Zr
O
50 40 O
Zr
30
(4)
60
Zr
50
Zr
40
O
O
30 C
20
20
C
10
C
C
0 2
4
6
8
C
0
O 0
O
C
10
10
Sputter time (min)
0
5
10
15
20
Sputter time(min)
Fig. 4. AES depth profiles of zircaloy-4 implanted with carbon at (1) as-received zircaloy-4; (2) 1 · 1016 ions/cm2; (3) 5 · 1016 ions/cm2; (4) 1 · 1017 ions/cm2.
simulation program, TRIM-96, was used to calculate the expected distribution of carbon (Fig. 5). From Fig. 5, the peak position of carbon is
(Atoms/cm3)/(Atoms/cm2)
1.2x105 1.0x105 8.0x104 6.0x104 4.0x104 2.0x104 0.0 0
20
40
60
80
100
120
140
160
180
Depth (nm)
Fig. 5. Distribution range of carbon ions determined by TRIM-96.
78 nm. When the fluence is 1 · 1016 ions/cm2, the experimental result coincides with that of calculated value basically. When the fluence is greater than 5 · 1016 ions/cm2, the peak positions of experiments are bigger than that of calculated value, 78 nm. These phenomena can be attributed to the thermal diffusion of carbon. Because the implantation was not employed at liquid nitrogen temperature, but at room temperature, the temperature of implanted samples was mainly depended on the beam current densities. From the Table 1, it can be known that the temperature is about 100 C as for 1 · 1016 ions/cm2 fluence, carbon diffuse a little. When the fluence is greater than 1 · 1016 ions/cm2, the beam current densities are much bigger that of 1 · 1016 ions/cm2 fluence, the temperature of them are over 200 C, therefore the carbon diffuse severely. According to Lawrence H. Van Vlack [12], D = D0eE/kT. D is diffusivity, D0 is constant, irrespective to the temperature; E
D.Q. Peng et al. / Nucl. Instr. and Meth. in Phys. Res. B 234 (2005) 235–242 (111)
(002)
(101)
(111)
(1)-as-received Zr-4 (2)-1E16C (3)-5E16C (112) (4)-1E17C
-α-Zr -H-ZrO 0.35 -M-ZrO 2
4
(002) (111)
Intensity (c/s)
is activation energy of atoms diffusion. k is BoltzmanÕs constant, equal to 13.8 · 1024 K. T is temperature (K). From the equality, the higher the implantation temperature, the larger is the diffusivity of carbon. From AES measurements, the carbon implantation enhanced the oxidization of zircaloy-4 samples. The larger the fluence, the higher is the implantation temperature, the longer is the duration time, and subsequently the thicker is the zirconia oxide film of implanted samples.
239
3
(002) (111)
(112)
2
(002) (101)
(102)
1
3.3. The phase structure in the implanted surface layer Glancing angle X-ray diffraction (GAXRD) was employed to investigate the phase structure characters in the implanted surface layer due to the carbon addition. From the AES of samples, the depth extent for the three fluences carbon-implanted zircaloy-4 was about a few hundreds of nanometers. If conventional XRD is used, then the information is dominated by the matrix, while the information contributed by the thin film will be very little. Glancing angle X-ray diffraction (GAXRD) with 0.3 glancing angle was employed in our experiment to study the phase characters of the carbon implantation zircaloy-4. An incident angle of 0.3 corresponded to a theoretical penetration depth of about 55 nm in the scale. So the 0.3-angle was determined for the incident angle. Fig. 6 shows the GAXRD spectra at 0.3 incident angle of the as-received and as-implanted samples. Table 2 shows experimental and tabulated values of the d-spacings of carbon-implanted zircaloy-4. From the observation of Fig. 6 and Table 2, it is clear that the as-received zircaloy-4 is composed of hexagonal alpha zirconium. Following implantation at a fluence of 1 · 1016 ions/ cm2 hexagonal zircaonia (H-ZrO0.35) is firstly formed. The amount of H-ZrO0.35 decreases at the intermediate fluence without simultaneous appearance of M-ZrO2, there is a possibility that the system was misaligned when spectrum (3) in Fig. 6 was recorded. While when the fluence reaches to 1 · 1017 ions/cm2, there produces new monoclinic zirconia (M-ZrO2). From the results of GAXRD, zirconium carbide was not observed, which is because the car-
30
35
40
45
50
diffraction angle (2-theta)
Fig. 6. GAXRD spectra at 0.3 incident angle of (1) as-received zircaloy-4 and implanted with C at (2) 1 · 1016 ions/cm2; (3) 5 · 1016 ions/cm2; and (4) 1 · 1017 ions/cm2. a-Zr represents hexagonal alpha metallic zirconium; H represents hexagonal zirconia (H-ZrO0.35); M represents monoclinic zirconia (M-ZrO2).
Table 2 Experimental and tabulated values of the d-spacings of carbonimplanted zircaloy-4 ˚ ) experimental values dhkl (A ˚ ) tabulated values (hkl) dhkl (A a-Zr 2.56 2.45 1.89
2.57 2.46 1.89
002 101 102
H-ZrO0.35 2.58 2.47 1.90
2.60 2.48 1.91
002 111 112
M-ZrO2 2.94 2.61
2.94 2.62
101 111
bon fluence is not large enough and the implantation temperature is not very high, while zircaloy-4 is oxidized and the zirconia films become thicker and thicker with raising the implantation fluences. 3.4. The electrochemical behavior of the carbonimplanted zircaloy-4 The potentiodynamic polarization curves of the as-received zircaloy-4 and the zircaloy-4 implanted with carbon-ions to a fluence ranging from
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Current Density (A/cm2)
Table 3 The relationship between the fluence and the passive current density of carbon ion implanted zircaloy-4
(1)-as-received Zr-4 (2)-1E16C (3)-5E16C (4)-1E17C
1E-4 1E-5
4 1
1E-6 1E-7
3 2
Fluence (ions/cm2)
Passive current density (A/cm2)
As-received zircaloy-4 1 · 1016 5 · 1016 1 · 1017
7.73 · 107 1.64 · 107 4.11 · 107 1.86 · 106
1E-8 1E-9 1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Potential (V vs SCE)
Fig. 7. Potentiodynamic polarization curves in 1 M H2SO4 solution of the as-received zircaloy-4 and zircaloy-4 implanted with carbon ions at a fluence of: (1) as-received zircaloy-4; (2) 1 · 1016 ions/cm2; (3) 5 · 1016 ions/cm2; 1 · 1017 ions/cm2.
Passivation Current Density (A/cm2)
2.0x10-6
Zero Current Density (Ei=0,V vs SCE)
1E-10
0.8
0.6
0.4
0.2
0.0
-6
1.5x10
2.0x1016
4.0x1016
6.0x1016
8.0x1016
1.0x1017
2
Fluence (ions/cm )
Fig. 9. Dependence of the zero current potential (Ei=0) on the implanted fluences.
-6
1.0x10
5.0x10-7
Table 4 The relationship between zero current potential and the fluences 0.0 0.0
16
2.0x10
16
4.0x10
16
6.0x10
16
8.0x10
Fluence (ions/cm2)
Zero current potential (Ei=0, V versus SCE)
As-received Zr-4 1 · 1016 5 · 1016 1 · 1017
0.2453 0.9534 0.8454 0.8074
17
1.0x10
Fluence (ions/cm2)
Fig. 8. Dependence of the passivation current density, ip, on the implanted fluences.
1 · 1016 to 1 · 1017 ions/cm2 are compared in Fig. 7. Fig. 8 plots the passive current density ip as a function of fluence (values are summarized in Table 3). From Figs. 7 and 8, it was found that a significant improvement of corrosion resistance was achieved as for the implanted zircaloy-4 when the fluence is smaller than 5 · 1016 ions/cm2. The higher the fluence, the bigger is the passive current density; subsequently the worse is the corrosion resistance of implanted samples. When the fluence reached 1 · 1017 ions/cm2, the corrosion resistance of implanted sample is worse than that of as-re-
ceived zircaloy-4. Fig. 9 plots the zero current potential as a function fluence (values are summarized in Table 4), which shows that all the zero current potentials of the implanted samples are more positive than that of the as-received zircaloy-4, while the larger the fluence, the lower is the zero current potential, and subsequently the worse is the corrosion resistance of the implanted sample. The results reflected by passive current densities are consistent with those reflected by zero current potentials as for implanted samples.
D.Q. Peng et al. / Nucl. Instr. and Meth. in Phys. Res. B 234 (2005) 235–242
3.5. The mechanism of the aqueous corrosion behavior by carbon ion implantation It is well known that the formation of the passive film on surface of the zircaloy-4 is an oxidation process. According to Pourbaix [13], there is an oxidation reaction that can take place at the zircaloy-4 anode Zr ðmÞ ¼ Zr4þ ðaqÞ þ 4e
ð1Þ
The cathodic reaction may be 2Hþ þ 2e ¼ H2 ðgÞ
ð2Þ
As analyzed above, as for the unimplanted sample, it is alpha zirconium, there is little oxidation protection in the surface of the sample, and anodic reaction of zirconium is easy to occur, subsequently the corrosion resistance of zircaloy-4 is bad. From the AES results, the implantation of carbon ions promoted the oxidization of zirconium, the zirconia film acted as a barrier to reduce the anodic reaction. When the fluence is 1 · 1016 ions/cm2, there produced hexagonal zirconia (H-ZrO0.35), and anodic reaction of zirconium is most difficult to occur, subsequently the corrosion resistance of sample implanted using a fluence of 1 · 1016 ions/cm2 is the best. When the fluence is 5 · 1016 ions/cm2, the intensity of hexagonal zirconia (H-ZrO0.35) declined, and the anodic reaction of zirconium becomes easy, subsequently the corrosion resistance of implanted sample becomes worse. When the fluence reaches to 1 · 1017 ions/ cm2, there produces new monoclinic zirconia (MZrO2), the anodic reaction of zirconium become easier and easier, subsequently, the corrosion resistance of it becomes the worst. The corrosion resistance of sample implanted with fluence of 1 · 1017 ions/cm2 is even worse than that of the as-received zircaloy-4 sample.
4. Conclusions The carbon was implanted into the zircaloy-4 sample, from XPS and AES, it was found that the carbon ion implantation promoted the oxidation in the surface layer. From GAXRD, the unimplanted sample is hexagonal alpha zirconium;
241
there firstly produces hexagonal zirconia (HZrO0.35) for fluence of 1 · 1016 ions/cm2, the intensity of H-ZrO0.35 declined with increasing the fluence, when the fluence reaches to 1 · 1017 ions/ cm2, there produces new monoclinic zirconia (MZrO2). A significant improvement of corrosion resistance was achieved when the implantation fluence of carbon is smaller than 5 · 1016 ions/cm2. The corrosion resistance of sample implanted with 1 · 1016 ions/cm2 is the best in all the samples, which is attribute to the oxidation protection of hexagonal zirconia (H-ZrO0.35). When the fluence reaches to 1 · 1017 ions/cm2, the corrosion behavior of implanted sample is even worse than that of as-received zircaloy-4, which is attributed to the appearance of monoclinic zirconia (M-ZrO2). The large the fluence, the worse is the corrosion resistance.
Acknowledgements Special thanks should be given to the Ministry of Science and Technology of China for research funding (MSTC no. G 2000067207-1). The authors would like to thank Professor Andrew Godfrey for his great help and the Analysis Center of Tsinghua University for the partial financial support.
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