Effect of copper ions implantation on corrosion behavior of zirconium in 1 M H2SO4

International Journal of Refractory Metals & Hard Materials 25 (2007) 32–38 www.elsevier.com/locate/ijrmhm

Effect of copper ions implantation on corrosion behavior of zirconium in 1 M H2SO4 D.Q. Peng a

a,*

, X.D. Bai a, H. Sun a, B.S. Chen

b

Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China b Jianzhong Chemical Cooperation, Yibin 644000, China Received 24 August 2005; accepted 2 November 2005

Abstract In order to study the effect of copper ion implantation on the aqueous corrosion behavior, specimens of zirconium were implanted with copper 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. Transmission electron microscopy (TEM) was used to examine the micro-structure of the copper-implanted samples. The potentiodynamic polarization technique was employed to evaluate the aqueous corrosion resistance of the implanted zirconium samples in a 1 M H2SO4 solution. It was found that a significant improvement was achieved in the aqueous corrosion resistance of zirconium implanted with copper ions. While the corrosion resistance of implanted samples declined with increasing the fluence. Finally, the mechanism of the corrosion behavior of copperimplanted zirconium was discussed.  2005 Elsevier Ltd. All rights reserved. Keywords: Zirconium; Corrosion resistance; Copper ion implantation; X-ray photoemission spectroscopy (XPS); Auger electron spectroscopy (AES)

1. Introduction 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, 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 zirconium and its 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. [5]) that chromium implan*

Corresponding author. Tel.: +86 10 6277 2856; fax: +86 10 6277 2507. E-mail addresses: [email protected], [email protected] (D.Q. Peng). 0263-4368/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2005.11.011

tation could improve the corrosion resistance of iron. Additional studies of palladium implanted into titanium [6] and phosphorus implanted into iron [7] have confirmed that ion implantation with palladium or phosphorus may successfully improve the corrosion behavior. It was reported that copper implantation can change the optical properties of silica glasses and MgO crystal [8–13], and change the mechanical, structural properties and residual stress of polycrystalline alumina [14]. While relatively few papers have reported on the corrosion behavior of copper-implanted zirconium. In this paper, we report the results of a study of copper implantation on the aqueous corrosion behavior of zirconium. The valence states of copper, zirconium, and oxygen were analyzed by X-ray photoelectron spectroscopy (XPS), depths distributions of elements in the surface layer were determined by Auger electron spectrometry (AES), and the corrosion resistance of the implanted zirconium was investigated by the potentiodynamic method using a

D.Q. Peng et al. / International Journal of Refractory Metals & Hard Materials 25 (2007) 32–38

IM6e potentiostat. Transmission electron microscopy (TEM) was carried out to observe the micro-structure of the copper-implanted samples. The mechanism of corrosion behavior of copper-implanted zirconium in the aqueous solution is discussed. 2. Experimental procedure Zirconium samples were machined to a size of 10 mm · 10 mm from a 1 mm thick sheet of zirconium, fully annealed after cold rolling. The composition of zirconium is listed in Table 1. The zirconium 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 deionized 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 copper ions are expected to consist of 16% Cu+, 63% Cu2+, 20% Cu3+ and 1% Cu4+. Fluences of 1 · 1016, 5 · 1016, and 1 · 1017 ions/cm2 were used. The extraction voltage for the copper implantation was 40 kV. Therefore, the implantation energies were 40 KeV, 80 KeV, 120 KeV and 160 Kev for Cu+, Cu2+, Cu3+ and Cu4+, respectively. The samples were not cooled during implantation. Below the specimen, there is a thermocouple to measure the temperature of implanted samples. The implantation temperature therefore depended on the beam current density, which varied from 8.85 lA/cm2 to 26.6 lA/cm2. Table 2 summarized the implantation conditions used. As for preparation of TEM samples, at first, zirconium films with 50 nm thickness were evaporated on the surfaces of 15 mm · 10 mm · 1.5 mm crystalline sodium chloride. In order to get the uniform films, the depositing rate of zirconium films is controlled at 1.8 nm/min with electron beam evaporating zirconium target. The vacuum level of

Fe 0.15 Cu 0.005 Ti 0.005 H 0.0025

Ni 0.007 Hf 0.01 U 0.00035 O 0.16

Cr 0.02 Mg 0.002 V 0.005 Zr Balanced

3. Results and discussion 3.1. The valence of the elements in the surface layer To compensate for a systematic error in XPS measurement, the energy positions were first adjusted by comparing the binding energy of the C 1s peak to that of the standard binding energy, 285 eV. The adjusted XPS spectra of C 1s, Zr 3d5/2, Cu 2p3/2, and O 1s are shown in Figs. 1–4 5

1.2x10

C1s 285 eV 1

4

9.0x10

Al 0.0075 Mn 0.005 W 0.01

B 0.00005 Mo 0.005 Cl 0.01

Cd 0.00005 Pb 0.013 C 0.027

Intensity (c/s)

Sn 0.005 Co 0.002 Si 0.012 N 0.0065

evaporator chamber (Balzers UTT400, Swiss manufactured) is 1.7 · 106 Pa. Then the crystalline sodium chloride with 50 nm zirconium film is implanted with copper accompanying with zirconium samples. After implantation, the crystalline sodium chloride with copper implanted zirconium film is dissolved in deionized water, the implanted zirconium films are gotten and the TEM samples are achieved. The valence states of copper, zirconium, and oxygen on sample surfaces were analyzed by X-ray photoelectron Spectroscopy (XPS). The micro-structure of implanted samples was investigated using transmission electron microscope (TEM). The potentiodynamic polarization measurements were carried out to investigate the aqueous corrosion resistance of the copper-implanted zirconium. 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 a 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.

2

Table 1 The composition of zirconium (wt.%)

(1)-as-received zirconium (2)-1E16Cu (3)-5E16Cu (4)-1E17Cu

4 3 4

6.0x10

291

288

285

282

Binding Energy (eV)

Table 2 Implantation conditions Fluence (ions/cm2) Beam current density (lA/cm2) Temperature (C)

33

1 · 1016 8.85 100

5 · 1016 17.7 220

1 · 1017 26.6 250

Fig. 1. The XPS spectra of C 1s peak in the implanted surface: (1) asreceived zirconium, (2) copper ions implanted with 1 · 1016 ions/cm2, (3) copper ions implanted with 5 · 1016 ions/cm2 and (4) copper ions implanted with 1 · 1017 ions/cm2.

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D.Q. Peng et al. / International Journal of Refractory Metals & Hard Materials 25 (2007) 32–38

5

(1)-as-received zirconium (2)-1E16Cu (3)-5E16Cu (4)-1E17Cu

3 2 4

5

1.0x10

1

5

2.0x10

Intensity (c/s)

Zr3d3/2 184.7 eV

1.5x10

Intensity (c/s)

O1s 530 eV

Zr3d 5/2 182.3 eV

5

2.0x10

O1s 531.4 eV 5

1.5x10

3 2 (1)-as-received zirconium (2)-1E16Cu 4 (3)-5E16Cu (4)-1E17Cu 1

5

1.0x10 4

5.0x10

2.4eV 4

5.0x10

0.0 188

186

184

182

180

537

178

534

531

528

525

Binding Energy (eV)

Binding Energy (eV)

Fig. 2. The XPS spectra of Zr 3d5/2 peak in the implanted surface: (1) asreceived zirconium, (2) copper ions implanted with 1 · 1016 ions/cm2, (3) copper ions implanted with 5 · 1016 ions/cm2 and (4) copper ions implanted with 1 · 1017 ions/cm2.

Fig. 4. The XPS spectra of O 1s peak in the implanted surface: (1) asreceived zirconium, (2) copper ions implanted with 1 · 1016 ions/cm2, (3) copper ions implanted with 5 · 1016 ions/cm2 and (4) copper ions implanted with 1 · 1017 ions/cm2.

3.2. The depth profile of the elements in the implanted surface layer Cu 2p 3/2 933.7 eV

5

1.8x10

Cu 2p1/2 953.7 eV

(1)-1E16Cu (2)-5E16Cu (3)-1E17Cu

Intensity (c/s)

5

1.5x10

5

1.2x10

3 2 4

1

9.0x10

20 eV 965

960

955

950

945

940

935

930

925

Binding Energy(eV)

Fig. 3. The XPS spectra of Cu 2p3/2 peak in the implanted surface: (1) copper ions implanted with 5 · 1016 ions/cm2 and (2) copper ions implanted with 1 · 1017 ions/cm2.

respectively. The adjusted binding energy of Zr 3d5/2 is 182.4 eV (Fig. 2), coincides with the standard values of ZrO2 [15], from which it is suggested that zirconium implanted copper at the surface exists in the form of ZrO2. Fig. 3 shows that the binding energy of Cu 2p3/2 is 933.7 eV, which suggests the copper at the surface exists as CuO as for implanted zirconium. From Fig. 4, there are two peaks: 530 eV and 531.4 eV as for as-received zirconium, while there is one peak: 530 eV as for copper implanted zirconium samples. Oxygen comes from residual gas in vacuum chamber, because the vacuum level of the target chamber of the MEVVA implanter is not very high. From the AES, we can determine that the fluence of copper is higher, and the penetration depth of oxygen is greater.

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 approx. 30 nm/min. Curves (1)–(4) in Fig. 5 show AES spectra of zirconium implanted with copper ions using fluences of 0, 1 · 1016, 5 · 1016, 1 · 1017 ions/cm2. The results show that the depths of oxygen in the oxide films are 18, 90, 195, and 222 nm, respectively. When the fluence is 1 · 1016 ions/cm2, the concentration of copper is too low to detect, but noises. The peak concentrations of copper are approx. 5% and 10%, corresponding to fluences of 5 · 1016 and 1 · 1017 ions/cm2, respectively. The depth corresponding to the peak concentration of copper with 5 · 1016, 1 · 1017 ions/cm2 fluences are approx. 78, 90 nm, respectively. A computer simulation program, TRIM-96, was used to calculate the expected distribution of copper (Fig. 6). From Fig. 6, the peak position of copper is 40 nm. The peak positions of experiments are bigger than that of calculated value, 40 nm. These phenomena can be attributed to the thermal diffusion of copper. 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 Table 1, it can be known that the temperature is about 100 C as for 1 · 1016 ions/cm2 fluence, copper 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 copper diffuse severely. From AES measurements, the copper implantation enhanced the oxidization of zirconium samples, the larger the fluence, the thicker is the zirconia oxide film of implanted samples.

D.Q. Peng et al. / International Journal of Refractory Metals & Hard Materials 25 (2007) 32–38

35

100 100

Zr

90

Zr

90

80

Zr

80

70

(1)

O

70

O

ACP (at%)

ACP (at.%)

60 60 50 40 30

Zr

O

40

Zr Zr

30 20

20

O

O

10

10

O

Cu

Cu

0

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0

2.0

1

2

4

5

100

100

Zr

90

6

Zr

90 80

80

70

70

O

60

(3)

O

ACP (at%)

60 50

Zr

40 30

3

Sputter Time (min)

Sputter Time (min)

ACP (at%)

(2)

50

Zr

O

(4)

O

50 40

Zr

Zr

30 20

20

O

O

10

10

Cu

Cu 0

0 0

2

4

6

8

10

12

14

16

0

2

4

6

8

10

12

14

Sputter Time (min)

Sputter Time (min)

1.8x10

5

1.6x10

5

1.4x10 1.2x10 1.0x10

5

5

5

3

2

(Atoms/cm )/(Atoms/cm )

Fig. 5. AES spectra of zirconium implanted with copper at (1) as-received zirconium; (2) 1 · 1016 ions/cm2; (3) 5 · 1016 ions/cm2 and (4) 1 · 1017 ions/cm2.

8.0x10

4

6.0x10

4

4.0x10

4

2.0x10

4

0.0 0

20

40

60

80

100

120

140

Depth (nm)

Fig. 6. Distribution range of copper ions determined by TRIM-96.

3.3. The micro-structure of copper implanted zirconium in the surface layer The micro-structures of copper implanted samples were examined by TEM. Although the grain size of the sputtered film may be different with that of the 1.0 mm sheet,

the transmission structure of the 1.0 mm sheet can be simulated by TEM studies of the sprayed film when the implanted fluence is high enough [7]. The thickness of all the sprayed zirconium films is 50 nm. Fig. 7(1a and 1b) shows the amorphous structure obtained for the unimplanted zirconium. Fig. 7(2a–4a) are bright field images of implanted samples, Fig. 7(2b–4b) are selected area diffraction (SAD) pattern. Fig. 7(2a and 2b) shows the partially amorphous and crystal structures when the fluence is 1 · 1016 ions/cm2. Most of the polycrystalline particles are spherical with 17 nm diameter. The implantation temperature in this case is low, only 100 C, therefore, the polycrystalline particles grow a little. When the fluence is 5 · 1016 ions/cm2, the implanted surface layer changed into polycrystalline structure. Most of the polycrystalline particles are spherical and square with diameters ranging from 20 to 35 nm. This is probably attributed to the high temperature during the implantation. The implantation temperature in this fluence is high, up to 220 C, therefore, the polycrystalline particles grow much. When the fluence reached 1 · 1017 ions/cm2, the implanted surface layer is still polycrystalline structure with 30 nm average diameter in the implanted surface layer. This is also attributed to the high temperature during the implantation. In

36

D.Q. Peng et al. / International Journal of Refractory Metals & Hard Materials 25 (2007) 32–38

Fig. 7. The TEM bright field images and SAD patterns of zirconium implanted copper ions at fluence of from 0 to 1 · 1017 ions/cm2: (a) bright field image, (b) TEM selected area electron diffraction patterns (SAD); (1) as-received zirconium, (2) 1 · 1016 ions/cm2, (3) 5 · 1016 ions/cm2 and (4) 1 · 1017 ions/cm2.

this fluence, the implantation temperature reached to 250 C, which leaded to the growth of polycrystalline particles. 3.4. The electrochemical behavior of the copper-implanted zirconium Compared with each other, the potentiodynamic polarization curves of the as-received zirconium and the zirconium implanted with copper-ions to a fluence ranging from 1 · 1016 to 1 · 1017 ions/cm2 are summarized in Fig. 8. Fig. 9 plots the passive current density ip as a function of fluence (values are summarized in Table 3). From

Figs. 8 and 9, it was found that a significant improvement of corrosion resistance was achieved as for the implanted zirconium. While the larger the fluence, the worse is the corrosion resistance. Fig. 10 plots the zero current potential as a function fluence (values are summarized in Table 4), which shows that all the zero current potential of the implanted samples is more positive than that of the asreceived zirconium, and subsequently the better is the corrosion resistance of the implanted samples. The larger the fluence, the more negative is the zero current potential of the implanted sample. The results reflect by passive current densities are consistent with those reflect by zero current potentials as for samples.

D.Q. Peng et al. / International Journal of Refractory Metals & Hard Materials 25 (2007) 32–38 (1)-as-received zirconium (2)-1E16Cu (3)-5E16Cu (4)-1E17Cu

2

Current Density (A/cm )

1E-4

1.4

Zero Current Potential (Ei=0,V vs SCE)

1E-3

1E-5

1

1E-6

4

3 2

1E-7 1E-8 1E-9 1E-10

-0.5

0.0

0.5

1.0

37

1.5

2.0

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0

16

2.0 x10

4.0x10

16

6.0x10

16

16

8.0 x10

17

1.0 x10

2

Potential (V vs SCE)

Fluence (ions/cm )

Fig. 8. The potentiodynamic polarization curves of the as-received zirconium and zirconium implanted with copper ions at a fluence range from 1 · 1016 to 1 · 1017 ions/cm2: (1) as-received zirconium, (2) 1 · 1016 ions/cm2, (3) 5 · 1016 ions/cm2 and (4) 1 · 1017 ions/cm2.

Fig. 10. The dependence of the zero current potential (Ei=0) on the implanted fluences.

Table 4 The relationship between zero current potential and the fluences -6

2

Passivation Current Density (A/cm )

1.6x10

1.2x10

8.0x10

-6

Fluence (ions/cm2)

Zero current potential (Ei=0, V vs SCE)

As-received Zr 1 · 1016 5 · 1016 1 · 1017

0.09425 1.309 0.8054 0.2884

to Pourbaix [16], there is an oxidation reaction that can take place at the zirconium anode:

-7

ZrðmÞ ¼ Zr4þ ðaqÞ þ 4e 4.0x10

ð1Þ

The cathodic reactions may be:

-7

2Hþ þ 2e ¼ H2 ðgÞ 0.0

16

2.0x10

4.0x10

16

6.0x10

16

16

8.0x10

17

1.0x10

Fluence (ions/cm 2)

Fig. 9. The dependence of the passivation current density, ip, on the implanted fluences.

Table 3 The relationship between the displacement per atom (dpa) and the passive current density of copper ion implanted zirconium Fluence (ions/cm2)

dpa

Passive current density (A/cm2)

As-received zirconium 1 · 1016 5 · 1016 1 · 1017

0 39.9 199.7 399.4

1.33 · 106 2.95 · 107 5.43 · 107 7.34 · 107

3.5. The mechanism of the aqueous corrosion behavior by copper ion implantation It is well known that the formation of the passive film on surface of the zirconium is an oxidation process. According

ð2Þ

As for the unimplanted sample, it is hexagonal 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 zirconium is bad. The zirconia oxide films increased from 18 nm to 222 nm with increasing fluences from 0 to 1 · 1017 ions/ cm2, respectively. The zirconia film acted as a barrier to reduce the anodic reaction. Subsequently, the corrosion resistance of implanted samples is better than that of the unimplanted sample. From XPS, the implanted copper existed in the form of CuO, the oxide dispersoid addition also acted as a barrier to reduce the migration and dissolution of zirconium. So, the passive current density decreases. On the other hand, when the target metal was implanted with ions, the surface layer was damaged. Different irritation ions lead to different damage efficiency. The depth dependence of damage level in the units of displacement per atom (dpa) was calculated by TRIM-96 computer program. The damage events were calculated and shown in Fig. 11. In the outermost layer, the damage efficiency of

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D.Q. Peng et al. / International Journal of Refractory Metals & Hard Materials 25 (2007) 32–38 2.5

COLLISION EVENTS Vacancies Produced (Kinchen-Pease)

Number/Ion/Angstrom

2.0

1.5

structure, most of the polycrystalline particles in the implanted samples are spherical and square with diameters ranging from 20 to 35 nm. A significant improvement of corrosion resistance was achieved as for implanted samples compared with as-received zirconium; this is attributed to the oxidation protection. While the corrosion resistance of implanted samples declined with increasing the fluence, the reason is the irradiation damage.

1.0

Acknowledgements 0.5

0.0 0

20

40

60

80

100

120

140

Depth (nm)

Fig. 11. The depth dependence of damage level of copper ions implantation.

tin ion implantation is 1.7093 number/ion/angstrom. The damage level (represented by dpa) will be changed by the implanted fluence, shown in Table 3. Up to now, many implantation corrosion mechanisms were proposed, such as the mechanism of enhancement of conductivity of oxide film by the change of energy band under implantation [17], electric conduction of the precipitates in the oxide films [18], the enhancement of oxide film cracks under implantation [19]. The basic effect of implantation is to create a lot of disorder of lattice atoms in oxide film, which changes the shape of the energy band and add some local conductive states inside the forbidden band along the collision tracts in the oxide films. The greater the fluence increases, the more serve the disorder forms, the more conductivity of the oxide film are and finally the easier the electrochemical corrosion. Therefore the implantation enhanced the corrosion. The larger the fluence, the worse is the corrosion resistance of implanted samples. 4. Conclusions The copper was implanted into the zirconium sample, from XPS, the implanted copper existed in the form of CuO; zirconium existed in the form of ZrO2. From AES, it was found that the copper ion implantation promoted the oxidation in the surface layer. The larger the fluence, the thicker is the oxide film. From the TEM, it is fact that micro-structure change from amorphous to polycrystalline

This work was supported by the National Nature Science Foundation of China (Grants No. 50501011) and China Postdoctoral Science Foundation (37th batch, No. 2005037079). Authors would like to thank Analysis Center of Tsinghua University for partial financial support. References [1] Lee SJ, Kwon HS, Kim W, Choi BH. Mater Sci Eng 1991;A263:23–31. [2] Xu Jian, Bai Xinde, Fan Yudian. J Mater Sci 2000;35:6225–9. [3] Etoh Y, Shimada S, Kikuchi K. J Nucl Sci Technol 1992;29(12):1173. [4] Xu Jian, Bai Xinde, Jin An, Fan Yudian. J Mater Sci Lett 2000;19:1633–5. [5] Ashworth V, Baxter D, Grant WA, Procter RPM, Wellington TC. Corros Sci 1976;16:775. [6] Huble GK, Mccafferty E. Corros Sci 1980;20:103. [7] Bai XD, Zhu DH, Liu BX. Nucl Instr Meth Phys Res B 1995;103:440. [8] Stepanov AL, Hole DE, Townsend PD. Nucl Instr Meth Phys Res B 2002;191:468–72. [9] Savoini B, Caceres D, Gonzalez R, Chen Y. Nucl Instr Meth Phys Res B 2004;218:148–52. [10] Kishimoto N, Gritsyna VT, Takeda Y, Lee CG, Saito T. Nucl Instr Meth Phys Res B 1998;141:299–303. [11] Nakao Setsuo, Saitoh Kazuo, Ikeyama Masami, Niwa Hiroaki. Nucl Instr Meth Phys Res B 1998;141:246–51. [12] Ikeyama M, Nakao S, Tazawa M. Surf Coat Technol 2002;158– 159:720–4. [13] Fukumi Kohei, Chayahara Akiyoshi, Ohora Kenji, Kitamura Naoyuki. Nucl Instr Meth Phys Res B 1999;149:77–80. [14] Halitim F, Ikhlef N, Boudoukha L, Fantozzi G. Thin Solid Films 1997;300:197–201. [15] Wagner CD, Riggs WM, Davis LE, Moulder JF, Muilenberg GE (editors), A reference book of standard data for use in X-ray photoelectron spectroscopy, published by Perkin–Elmer corporation, Physical Electronics Division, 6509 Flying Cloud Drive, Eden Prairie, Minnesota 55344, p. 100. [16] Pourbaix M. Atlas Electrochem Equil Aqueous Solutions 1974:223. N.A.C.E., Houstan, TX. [17] Stimming U. Electrochem Acta 1986;31(4):415. [18] Shirvington PJ. J Nucl Mater 1970;37:177. [19] Plot RA. J Nucl Mater 1980;91:332.