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Electrically conducting paths in thick ( > 10 μm) zirconia films
*H JL3 --
jowual of nuclear mabrials
-F!iB ELSEVIER
Journal
of Nuclear
Materials
223 (199.5) 202-209
Letter to the Editors
Electrically conducting paths in thick ( > 10 km) zirconia films Brian Cox, Yin-Mei Wong, Thoai Hoang Centre
for Nuclear
Engineering,
University
Received
of Toronto,
15 September
184 College
1994; accepted
The manner in which porosity develops in post-transition (2 2 urn) oxide films on the Zircaloys, and the fate of the constituents (other than zirconium) of the second-phase particles (Fe, Cr, Ni) in such films has been the subject of much investigation [l-4]. It is known that the intermetallics oxidise later than the matrix [2,3], and it is argued [4] that iron atoms from these intermetallics act to stabilise the tetragonal zirconia phase in the oxide at and around the sites of these intermetallics, and thus affect the local breakdown of the oxide. Such regions in the oxide would, therefore, be expected to contain only a few pores when compared with oxide regions between intermetallics. Neither the precise location nor the valence of the iron atoms in the oxide is known with any certainty. When ac impedance techniques were used to study thick post-transition oxides in the past [5] one of the surprises was the number of examples of oxide films that short-circuited when the outer contact was made with a liquid metal alloy. In thin pretransition oxide films the intermetallic particles, with possibly only thin conducting oxides on their surfaces, have been argued to be the cause of these short-circuits 14-71. For oxides very much thicker than the intermetallic particle size, as is the case here, such an explanation is difficult to accept, since all the components of an intermetallic particle should be in their normal oxidation state near the outside of a thick porous oxide film. The ac impedance characteristics of short-circuits in thin anodic films [6], where intermetallics are thought to be the cause, and in N 1 km thermal oxides formed in steam on an alloy containing very few Zr/Fe intermetallics (Zr-2.5 wt% Nb), and rendered electrically conducting by heating in low po, environments [8], have both shown the typical features of Young impedances [9,10] when measured with metallic contacts. In the latter work, the development of clusters of small pores in the oxide was also observed as a result 0022-3115/95/$09.50 0 1995 Elsevier SSDI 0022-3115(94)00696-2
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of heating the specimens in vacua or in a low pressure hydrogen environment. In a previous study of thick post-transition oxides on Zircaloy-4 [ll] the conducting paths showed transmission line characteristics (slopes of - 0.5 for log 2 versus log f, and phase angles (4) of - 45”). However, the number of specimens in this study was limited, and results on a wider range of material fabrication routes and corrosion conditions were sought in this study. The specimens were selected from those oxidised in a previous study whose results are not generally available [12], and sufficient of the data obtained in this programme are presented here, with permission, to characterise the specimens. The specimens were short (1 cm) lengths of standard Zircaloyd fuel cladding tubes that were given either a stress-relief anneal, or a recrystallisation anneal after their final tube reduction. In addition samples from a batch given a late P-quench, and from a low-tin batch were included (Table 1). All the specimens studied here had received an initial chemical polishing in mixed nitric/hydrofluoric acids, and had been oxidised at 360°C (633 K) in stainless steel autoclaves in either pH 7 water or pH 12 LiOH. The oxidation curves are presented in Fig. 1, and the final oxide thicknesses measured by both ac impedance [6] and FTIR interferometry [13] as well as by weight gain are quoted in Table 2. Measurements with Hg and Pt contacts were made on small spots of N 6.6 mm2 area, while measurements in 1.0 molar ammonium nitrate electrolyte were made on the whole outer surface, with the inner surface and edges masked off. FTIR measurements were made on 250 X 150 urn spots. It can be seen that the interferometry results (assuming a refractive index, 17= 2.05, because the samples were too small in diameter to measure directly [13]) are in generally good agreement with the weight gains. In cases where this agreement is less good reserved
B. Con et al. /Journal Table 1 Characterisation Ingot Sn Fe Cr C 0
of Zircaloy-4
analyses
of Nuclear
Materials
223 (1995)
202-209
203
tubes
(wt%) 1.52 0.23 0.10 0.01 0.11
Tube analyses Designation H (wppm) N hpm) 0 (wppm) Mean particle Size (pm) No. density (10” mm’) Largest particle (km) Area fraction (%,) Fe : Cr ratio
L 12 20 1090
M 14 20 1070
0 10 30 1200
P 11 30 1200
R 7 18 1100
S 8 15 1550
T 16 14 1600
0.20
0.17
0.21
0.24
0.13
0.19
-
1.78 0.55 0.64 -
0.86 0.55 0.28 -
0.87 0.65 0.36 1.59
1.43 0.75 0.80 1.54
1.00 -
1.46
0.14 1.56
0.48 1.59
-
(T34,M14) this may indicate that oxide thicknesses on the outside and inside of the tubes differed significantly. All measurements (other than weight gains) apply to the outside surfaces of the tubes. ac impedance measurements using mercury contacts (employing a value of E = 13.5, for the effective dielectric constant, that has been found to correct approximately [5,6] for the effects of incomplete contact with typically rough ZrO, surfaces) were also close to but generally somewhat below the thicknesses estimated from the weight gains. It is concluded that the discrepancies here represent variations in the contact area of the mercury that were not accurately normalised for by using a single value for the effective dielectric constant.
Tim
Fig. 1. Oxidation
1.21 1.21 0.10 0.10 0.17
curves
Impedance measurements using evaporated platinum contacts should avoid the contact area problems experienced with mercury and oxide thicknesses were, therefore, calculated using E = 22 (the best value for the dielectric constant of dense ZrO, [14,15]. Oxide thickness values for the Pt (and Hg) contacts were calculated by extrapolating the log Z versus log f (Bode) plots from 2 X lo5 Hz to log f= 0 at the observed slope (Fig. 2) at high frequency. Despite the expected absence of contact area effects with Pt contacts the results are generally lower than the oxide thicknesses determined by weight gain or interferometry. It is thought that these low values may result from the high electrical conductivity of the inner layers of
Tnu3
(ally.)
for Zircaloy-4
specimens
at 360°C
(633 K) in (a) pH 7 water,
(b) pH
(day@
12 LiOH
solution.
204
B. Cox et al. /Journal
Table 2 Oxide thickness measurements Specimen Oxidation designation conditions
L16 L36 P16 R16 T34
pH pH pH pH pH
12 LiOH, 12 LiOH, 12 LiOH, 12 LiOH, 12 LiOH,
Ml4 014 P14 R13 s13
pH pH pH pH pH
7 H,O, 7 H,O, 7 H,O, 7 H,O, 7 H,O,
360°C 360°C 360°C 360°C 360°C
360°C 360°C 360°C 360°C 360°C
of Nuclear
Metallurgical condition
Materials
223 (1995)
202-209
Oxide thickness (urn) from Aw/15
81.2 20.3 37.3 37.3 21.3 17.1 18.7 15.8 34.5 12.3
SRA SRA
RXA B-quench, SRA Low-Sn, SRA RXA SRA RXA P-quench, SRA Low-Sn, SRA
the oxide film, that form at low local oxygen partial pressures. These layers may be too conducting to contribute to the total impedance thickness at the upper frequency employed here 161. If this is the correct explanation for the low results using Pt contacts, then a similar effect with Hg contacts is included in the use of E = 13.5 for these measurements. The oxide thicknesses measured in ammonium nitrate electrolyte (Table 2) were the initial and final values obtained at lo3 Hz during soaking in the elec-
FTIR Impedance (71 = 2.05) Hg Pt
NH,NO,
(E = 22)
(E = 13.5)
(E = 22)
Initial
Final
Bode plot
80.3 21.0 37.8 37.8 30.8
80.1 21.1 29.8 26.1 22.2
48.8 5.3 17.0 14.5 3.1
35 5.0 13.0 13.3 8.7
9.3 1.7 3.2 6.3 1.2
16.2 14.9 3.8 35.6 21.0
22.1 19.0 15.6 38.1 12.2
19.7 19.7 9.0 34.1 11.7
15.0 15.2 14.6 33.6 10.9
13.0 1.3 4.3 31.7 2.2
10.2 0.5 2.4 13.2 1.2
12.8 8.8 18.7 11.9 2.8
trolyte until the impedance came to equilibrium (Fig. 3), together with the thickness obtained at equilibrium electrolyte penetration from the Bode plots (Fig. 4) of log Z versus log f by extrapolating the data from the highest frequency region (2 X lo5 Hz) back to the log f = 0 at a slope of - 1. These calculations apparently give results close to the total thickness obtained with other techniques when the Bode plots clearly show a second peak at high frequency, and much higher than the final thickness measured by a medium frequency
c 12
3
4
5 12 log Rmwulsy
3
4
5
6
cw
Fig. 2. Bode plots comparing results obtained with evaporated platinum contacts (solid lines) and mercury contacts (dashed lines). Two spots per sample were measured with Pt. The spots measured for Pt and Hg were not necessarily identical, but were as close as could be achieved. (a-d) Specimens oxidised in pH 12 LiOH, (e,f) oxidised in pH 7 water. Specimens (e) and (f) were selected to show the minimum and maximum differences respectively between results obtained with platinum and mercury contacts.
B. Cox et al. /Journal
of Nuclear
Fig. 3. Changesin impedancewith time of immersion in 1 molar NH,NO, for specimensoxidisedin pH 7 water (a) and pH 12 LiOH (b). technique (e.g. lo3 Hz) during soaking, which has been equated previously with the residual impervious barrier layer oxide thickness [16,17]. It is not clear at present
Materials
223 (1995)
205
202-209
whether such an interpretation is valid if the values of oxide thickness determined from measurements with Pt contacts are low because of the high conductivity of the inner layers of these thick oxides. The initial oxide thickness, measured immediately after immersion should be close to the total thickness derived from the weight gain if highly conducting inner layers are not a factor, as should the extrapolated values from the Bode plot at high frequencies (> lo5 Hz). In these experiments the initial oxide thicknesses measured immediately after immersion in ammonium nitrate were close to but somewhat below those obtained using Pt contacts for oxides formed in pH 12 LiOH, and (with the exception of specimens R13, M14) were well below the Pt contact values and showed little further decrease with time for specimens oxidised in pH 7 water. In instances where a double peak was not clearly present in the Bode plots (e.g. P16, S13), the final extrapolated value from 2 x lo5 Hz in ammonium nitrate was much lower than the total oxide thickness. It is possible that a peak at even higher frequencies may have been missed, but previous searches for such peaks have proved fruitless [6]. There are at least two possible explanations for these low initial readings on immersion in ammonium nitrate. The first would be that the initial reading can never represent an apparently thicker oxide than that measured with Pt contacts, because the lower than expected values with Pt may result from the inner part of the oxide being too conducting to contribute to the total impedance thickness. Such an explanation would satisfy the generally close agreement between Pt and initial NH,NO, thickness for oxides formed in pH 12
90 75 60
4.5 30 15
0 90 75 60 15 30 15 2
3
4
5 12 log -=w=ncy
3
4
5
6'
(HZ)
Fig. 4. Bode plots for Z and 4 measuredafter saturatingin NH,NO, electrolytefor specimensoxidisedin pH 7 water (a) and pH 12 LiOH (b).
206
B. Cox et al. /Journal
of Nuclear
Materials
LiOH, but would not explain the very low initial thickness results for oxides grown in pH 7 water. A second explanation of these low initial values, obtained following immersion in NH,NO, electrolyte could be that the oxides formed in pH 7 water contained more large cracks than for those formed in pH 12 LiOH. Such cracks could allow a very rapid penetration of the electrolyte during the first few seconds of immersion, before an initial reading was obtained. It was decided that the presence of such cracks could be readily detected by electron microscope replicas taken from the specimen surfaces. Inspection of replicas in the transmission electron microscope (Fig. 5) showed the almost complete absence of large cracks in the outer surfaces of any of the oxides. Arrays of small pores were seen that were so near to the detection limit for pores of small diameter by the formvar-carbon replication technique that it was concluded that only a small fraction of the total numbers of pores present were being revealed. This situation arises because, for pores of successivelysmaller diameter, the small spikes of plastic extracted from the pores (and on which the characteristic appearance of
223 (1995)
202-209
the pores in the microscope depends) break off successively nearer to the outer surface of the oxide until a pore becomes indistinguishable from the fine surface roughness resulting from the surface oxide crystallites. This same problem has beset other recent work on oxides formed in higher concentrations of LiOH[18-201 and seriously limits the ability to characterise the surface porosity in oxides where pore sizes are very small ( < 10 nm diameter). With these difficulties in mind, therefore, we can conclude that there were no large cracks present that could explain the ac impedance results obtained after immersion in NH,NO, electrolyte. Many pores were observed in these oxides, whether they were formed in pH 7 water or pH 12 LiOH, and these often appeared to be in localised clusters. However, this could have been a result of local variations in the resolution of very small pores, and it was concluded that estimates of pore frequencies and diameters made from these replicas would not be reliable. The characteristic appearances of the clusters of pores that were observed are shown in Fig. 5, where the difficulties in distinguishing pores from surface roughness are evident.
a.
b.
d.
e.
Fig. 5. Replica electron micrographsof specimensurfacesshowingclustersof pores in the oxide films formed in pH 7 water at 360°C;(a) M14, (b) 014 and pH 12 LiOH, (c) L36, (d-f) T34.
B. Cox et al. /Journal
of Nuclear
Only one specimen (T34) showed any unusual distribution of pores. In addition to the usual clusters of small pores (Fig. 5d) it showed concentrated arrays of larger pores that appeared to lie along prior metal grain boundaries, although these “grains” were much larger than any individual grains observed metallographically in this batch, where the largest recrystallised grains in the partially recrystallised structure were only lo-12 pm diameter. The cause of these unique arrays of pores remains unknown. Using the limited range of frequencies available routinely for ac impedance measurements, it appeared that, unlike the situation in a previous study [ll], there was no evidence here for conducting paths in the oxides formed in pH 7 water, based on the results with either Hg or Pt contacts (Fig. 2), and that the impedance spectra obtained after saturation with 1.0 molar ammonium nitrate showed typical evidence for two layered oxide films [21-261 in the form of double peaks in the phase angle plots. However, there is no real agreement on how to interpret these two layers in physical terms. The simplest explanation is to see them
Materials
223 (1995)
202-209
207
as a layer of porous oxide on top of an impervious barrier film. However, if this interpretation is correct then the highest frequency data should represent the total oxide thickness. These results show that this is not always the case, and that in thick thermally formed oxide films the layers of oxide close to the oxide-metal interface may always be too conducting to contribute to the total impedance of the oxide. This would render the interpretation of one of the layers as the “barrierlayer” improbable since it would be impossible to determine how much, if any, of the conducting layer was free of small pores. An alternative interpretation of the two layers may be that they represent contributions from the impedance of the pores and the intervening oxide in parallel with each other. This situation might arise if the pores were too small for the normal electrolyte conductivity to operate within them. Further work will be needed to resolve this, but at least one of the published models for the impedance of thermally formed ZrO, films [23] considers the two capacitative components in the oxide to be in parallel. The impedance spectra obtained with Pt contacts
1 L36
9 0
. -‘-.-‘-k
Y,
7-
‘.
‘-._,
‘..
‘k
65 16
A
,,,,\
6-
Fig. 6. Bode plots of results for selected oxides formed in pH 12 LiOH obtained with platinum contacts when the frequency range was extended down to 10m3 Hz using a Solartron impedance spectrum analyser. (a) Specimen L36, (b) specimen R16, (c) specimen T34.
208
B. Cox et al. /Journal
of Nuclear
for oxides grown in pH 12 LiOH all showed declining phase angles and slopes for the log Z versus log f plots at the lowest frequency initially used (12 Hz). In order to determine whether or not this represented Young impedance type behaviour, as had been found with thinner oxide films [6,8,23], the measurements were extended to 10e3 Hz (Fig. 6) when it became evident that these oxides were showing the presence of conducting paths with the characteristics of transmission lines (i.e., phase angle - 45”, slope of log Z versus log f -0.5). There was no electrolyte present during these measurements, and similar characteristics were not seen at low frequencies when oxides were saturated with 1.0 molar ammonium nitrate, so it is concluded that these conducting “transmission lines” in the oxide may arise from the pore structure that is penetrated by the electrolyte. If the conducting “transmission lines” resulted from a thin conducting layer on the pore walls, then the presence of electrolyte in them might interfere with this condition. The characteristics of the conducting paths were similar, but their occurrence only in oxides formed in LiOH in this study was dissimilar to the previous study [ll], where the oxidation was in pH 7 water or steam, and this type of behaviour was observed for all specimens. At present, neither a physical explanation of these features nor a clear understanding of the factors leading to their observation can be reached. In summary, therefore, examinations of thick thermal oxides formed in either pH 7 water or pH 12 LiOH at 360°C (633 K) on a series of batches of Zircaloy-4 fuel cladding with different fabrication routes have shown that: Differences in oxide characteristics resulting from the conditions of oxide growth were larger than those resulting from differences in fabrication route. Oxides formed in pH 7 water showed good capacitative behaviour when measured with metallic outer contacts (unlike the specimens in a previous study [ll]), whereas oxides formed in pH 12 LiOH showed conduction paths with transmission line (Warburg impedance) characteristics. A comparison of several measures of the oxide thickness showed good agreement between weight gain and FTIR interferometry for the total oxide thickness. ac impedance measurements with Pt contacts often gave lower values than these, suggesting that the inner layers of these oxides might be too conducting to contribute to the total impedance. ac impedance measurements following immersion in 1.0 molar NH,NO, sometimes failed to measure the total oxide thickness, probably for the same reason, since no large cracks could be found that could provide an alternative explanation. Pores in the oxides, revealed by electron microscope replicas, were very small and near the limit of reso-
Materials
223 (1995)
202-209
lution. They appeared to be clustered, but this might be an artefact resulting from an inability to reveal the smallest pores.
Acknowledgements The authors are indebted to the Natural Sciences and Engineering Research Council of Canada and the CANDU Owner’s Group for the support that allowed this study to be carried out; to the Nuclear Fuel Industry Research Group for permission to include some earlier results obtained under their auspices, and to Prof. S. Thorpe for the use of the Solartron Impedance Spectrometer.
References
ill B. Cox, Advances in Corrosion Science and Technology, vol. V, eds. M.G. Fontana and R.W. Staehle (Plenum, New York, 1976) p. 173. Dl D. Pecheur, F. Lefebvre, A.T. Motta and C. Lemaignan, J. Nucl. Mater. 189 (1992) 318. [31 D. Pecheur, F. Lefebvre, A.T. Motta, C. Lemaignan and D. Charquet, Oxidation of intermetallic precipitates in Zircaloy-4: impact of irradiation, Proc. 10th Int. Symp. on Zirconium in the Nuclear Industry, Baltimore, June 1993, ASTM-STP, to be published. [41 J. Godlewski, How the tetragonal zirconia phase is stabilised in oxide scale formed on zirconium alloy corroded at 400°C in steam, Proc. 10th Int. Symp. on Zirconium in the Nuclear Industry, Baltimore, June 1993, ASTM-STP, to be published. 151B. Cox, The Use of Electrical Methods for Investigating the Growth and Breakdown of Oxide Films on Zirconium Alloys, Canadian Report, AECL-2668 (1967). [61 B. Cox, F. Gascoin and Y.-M. Wong, J. Nucl. Mater., to be published. [71 P.J. Shirvington, J. Nucl. Mater., 37 (1970) 177. @I B. Cox and Y.-M. Wong, Degradation of Oxide Films on a Zr-2.5 wt% Nb Alloy, University of Toronto, Report CNEUT-93-03 (1993). [91 L. Young, Anodic Oxide Films (Academic Press, New York, 1961) pp. 253-267. [lOI H. Gohr, Ber. Bunsenges, Phys. Chem. 85 (1981) 274. [ill B. Cox and Y. Yamaguchi, J. Nucl. Mater. 210 (1994) 303.
1121 B. Cox, N. Ramasubramanian, V.C. Ling, Zircaloy Corrosion Properties under LWR Coolant Conditions, part 1, Electric Power Research Institute, Report EPRINP6979-D (1990). [131 B. Cox, Y.-M. Wong, J. Nucl. Mater. 199 (1993) 258. [141 P.J. Harrop and J.N. Wanklyn, Brit. J. Appl. Phys. 18 (1967) 739. [151 B. Cox, Brit. J. Appl. Phys. (J. Phys. D), ser. 2, vol. 1 (1968) 671. Ml J.N. Wanklyn, CF. Britton, D.R. Silvester and N.J.M. Wilkins, J. Electrochem. Sot. 110 (1963) 856.
B. Cox et al. /Journal
of Nuclear
[17] H.J. Beie, F. Garzarolli, H. Ruhmann, H.J. Sell, A. Mitwalsky and S. Hofler, Examination of the corrosion mechanism of zirconium alloys, Proc. 10th Int. Symp. on Zirconium in the Nuclear Industry, Baltimore, June 1993, ASTM-STP, to be published. [18] B. Cox and Y.-M. Wong, ASTM-STP 1132 (1991) 643. [19] B. Cox and C. Wu, J. Nucl. Mater. 199 (1993) 272. [20] B. Cox and C. Wu, J. Nucl. Mater., to be published. [21] 0. Gebhardt, Electrochim. Acta 38 (1993) 633.
Materials
223 (1995)
202-209
209
[22] 0. Gebhardt, J. Nucl. Mater. 203 (1993) 17. [23] H. Gohr, J. Schaller and C.-R. Schiller, Electrochim. Acta 38 (1993) 1961. [24] L. Durand-Keklikian, G. Cragnolino and D.D. McDonald, Corr. Sci. 32 (1991) 347. [25] Idem, Corr. Sci. 32 (1991) 361. [26] J.A. Bardwell and M.A. McKubre, Electrochim. Acta 36 (1991) 647.
jowual of nuclear mabrials
-F!iB ELSEVIER
Journal
of Nuclear
Materials
223 (199.5) 202-209
Letter to the Editors
Electrically conducting paths in thick ( > 10 km) zirconia films Brian Cox, Yin-Mei Wong, Thoai Hoang Centre
for Nuclear
Engineering,
University
Received
of Toronto,
15 September
184 College
1994; accepted
The manner in which porosity develops in post-transition (2 2 urn) oxide films on the Zircaloys, and the fate of the constituents (other than zirconium) of the second-phase particles (Fe, Cr, Ni) in such films has been the subject of much investigation [l-4]. It is known that the intermetallics oxidise later than the matrix [2,3], and it is argued [4] that iron atoms from these intermetallics act to stabilise the tetragonal zirconia phase in the oxide at and around the sites of these intermetallics, and thus affect the local breakdown of the oxide. Such regions in the oxide would, therefore, be expected to contain only a few pores when compared with oxide regions between intermetallics. Neither the precise location nor the valence of the iron atoms in the oxide is known with any certainty. When ac impedance techniques were used to study thick post-transition oxides in the past [5] one of the surprises was the number of examples of oxide films that short-circuited when the outer contact was made with a liquid metal alloy. In thin pretransition oxide films the intermetallic particles, with possibly only thin conducting oxides on their surfaces, have been argued to be the cause of these short-circuits 14-71. For oxides very much thicker than the intermetallic particle size, as is the case here, such an explanation is difficult to accept, since all the components of an intermetallic particle should be in their normal oxidation state near the outside of a thick porous oxide film. The ac impedance characteristics of short-circuits in thin anodic films [6], where intermetallics are thought to be the cause, and in N 1 km thermal oxides formed in steam on an alloy containing very few Zr/Fe intermetallics (Zr-2.5 wt% Nb), and rendered electrically conducting by heating in low po, environments [8], have both shown the typical features of Young impedances [9,10] when measured with metallic contacts. In the latter work, the development of clusters of small pores in the oxide was also observed as a result 0022-3115/95/$09.50 0 1995 Elsevier SSDI 0022-3115(94)00696-2
Science
B.V.
All rights
Street,
Toronto,
5 December
Ontario,
Canada
M5S IA4
1994
of heating the specimens in vacua or in a low pressure hydrogen environment. In a previous study of thick post-transition oxides on Zircaloy-4 [ll] the conducting paths showed transmission line characteristics (slopes of - 0.5 for log 2 versus log f, and phase angles (4) of - 45”). However, the number of specimens in this study was limited, and results on a wider range of material fabrication routes and corrosion conditions were sought in this study. The specimens were selected from those oxidised in a previous study whose results are not generally available [12], and sufficient of the data obtained in this programme are presented here, with permission, to characterise the specimens. The specimens were short (1 cm) lengths of standard Zircaloyd fuel cladding tubes that were given either a stress-relief anneal, or a recrystallisation anneal after their final tube reduction. In addition samples from a batch given a late P-quench, and from a low-tin batch were included (Table 1). All the specimens studied here had received an initial chemical polishing in mixed nitric/hydrofluoric acids, and had been oxidised at 360°C (633 K) in stainless steel autoclaves in either pH 7 water or pH 12 LiOH. The oxidation curves are presented in Fig. 1, and the final oxide thicknesses measured by both ac impedance [6] and FTIR interferometry [13] as well as by weight gain are quoted in Table 2. Measurements with Hg and Pt contacts were made on small spots of N 6.6 mm2 area, while measurements in 1.0 molar ammonium nitrate electrolyte were made on the whole outer surface, with the inner surface and edges masked off. FTIR measurements were made on 250 X 150 urn spots. It can be seen that the interferometry results (assuming a refractive index, 17= 2.05, because the samples were too small in diameter to measure directly [13]) are in generally good agreement with the weight gains. In cases where this agreement is less good reserved
B. Con et al. /Journal Table 1 Characterisation Ingot Sn Fe Cr C 0
of Zircaloy-4
analyses
of Nuclear
Materials
223 (1995)
202-209
203
tubes
(wt%) 1.52 0.23 0.10 0.01 0.11
Tube analyses Designation H (wppm) N hpm) 0 (wppm) Mean particle Size (pm) No. density (10” mm’) Largest particle (km) Area fraction (%,) Fe : Cr ratio
L 12 20 1090
M 14 20 1070
0 10 30 1200
P 11 30 1200
R 7 18 1100
S 8 15 1550
T 16 14 1600
0.20
0.17
0.21
0.24
0.13
0.19
-
1.78 0.55 0.64 -
0.86 0.55 0.28 -
0.87 0.65 0.36 1.59
1.43 0.75 0.80 1.54
1.00 -
1.46
0.14 1.56
0.48 1.59
-
(T34,M14) this may indicate that oxide thicknesses on the outside and inside of the tubes differed significantly. All measurements (other than weight gains) apply to the outside surfaces of the tubes. ac impedance measurements using mercury contacts (employing a value of E = 13.5, for the effective dielectric constant, that has been found to correct approximately [5,6] for the effects of incomplete contact with typically rough ZrO, surfaces) were also close to but generally somewhat below the thicknesses estimated from the weight gains. It is concluded that the discrepancies here represent variations in the contact area of the mercury that were not accurately normalised for by using a single value for the effective dielectric constant.
Tim
Fig. 1. Oxidation
1.21 1.21 0.10 0.10 0.17
curves
Impedance measurements using evaporated platinum contacts should avoid the contact area problems experienced with mercury and oxide thicknesses were, therefore, calculated using E = 22 (the best value for the dielectric constant of dense ZrO, [14,15]. Oxide thickness values for the Pt (and Hg) contacts were calculated by extrapolating the log Z versus log f (Bode) plots from 2 X lo5 Hz to log f= 0 at the observed slope (Fig. 2) at high frequency. Despite the expected absence of contact area effects with Pt contacts the results are generally lower than the oxide thicknesses determined by weight gain or interferometry. It is thought that these low values may result from the high electrical conductivity of the inner layers of
Tnu3
(ally.)
for Zircaloy-4
specimens
at 360°C
(633 K) in (a) pH 7 water,
(b) pH
(day@
12 LiOH
solution.
204
B. Cox et al. /Journal
Table 2 Oxide thickness measurements Specimen Oxidation designation conditions
L16 L36 P16 R16 T34
pH pH pH pH pH
12 LiOH, 12 LiOH, 12 LiOH, 12 LiOH, 12 LiOH,
Ml4 014 P14 R13 s13
pH pH pH pH pH
7 H,O, 7 H,O, 7 H,O, 7 H,O, 7 H,O,
360°C 360°C 360°C 360°C 360°C
360°C 360°C 360°C 360°C 360°C
of Nuclear
Metallurgical condition
Materials
223 (1995)
202-209
Oxide thickness (urn) from Aw/15
81.2 20.3 37.3 37.3 21.3 17.1 18.7 15.8 34.5 12.3
SRA SRA
RXA B-quench, SRA Low-Sn, SRA RXA SRA RXA P-quench, SRA Low-Sn, SRA
the oxide film, that form at low local oxygen partial pressures. These layers may be too conducting to contribute to the total impedance thickness at the upper frequency employed here 161. If this is the correct explanation for the low results using Pt contacts, then a similar effect with Hg contacts is included in the use of E = 13.5 for these measurements. The oxide thicknesses measured in ammonium nitrate electrolyte (Table 2) were the initial and final values obtained at lo3 Hz during soaking in the elec-
FTIR Impedance (71 = 2.05) Hg Pt
NH,NO,
(E = 22)
(E = 13.5)
(E = 22)
Initial
Final
Bode plot
80.3 21.0 37.8 37.8 30.8
80.1 21.1 29.8 26.1 22.2
48.8 5.3 17.0 14.5 3.1
35 5.0 13.0 13.3 8.7
9.3 1.7 3.2 6.3 1.2
16.2 14.9 3.8 35.6 21.0
22.1 19.0 15.6 38.1 12.2
19.7 19.7 9.0 34.1 11.7
15.0 15.2 14.6 33.6 10.9
13.0 1.3 4.3 31.7 2.2
10.2 0.5 2.4 13.2 1.2
12.8 8.8 18.7 11.9 2.8
trolyte until the impedance came to equilibrium (Fig. 3), together with the thickness obtained at equilibrium electrolyte penetration from the Bode plots (Fig. 4) of log Z versus log f by extrapolating the data from the highest frequency region (2 X lo5 Hz) back to the log f = 0 at a slope of - 1. These calculations apparently give results close to the total thickness obtained with other techniques when the Bode plots clearly show a second peak at high frequency, and much higher than the final thickness measured by a medium frequency
c 12
3
4
5 12 log Rmwulsy
3
4
5
6
cw
Fig. 2. Bode plots comparing results obtained with evaporated platinum contacts (solid lines) and mercury contacts (dashed lines). Two spots per sample were measured with Pt. The spots measured for Pt and Hg were not necessarily identical, but were as close as could be achieved. (a-d) Specimens oxidised in pH 12 LiOH, (e,f) oxidised in pH 7 water. Specimens (e) and (f) were selected to show the minimum and maximum differences respectively between results obtained with platinum and mercury contacts.
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Fig. 3. Changesin impedancewith time of immersion in 1 molar NH,NO, for specimensoxidisedin pH 7 water (a) and pH 12 LiOH (b). technique (e.g. lo3 Hz) during soaking, which has been equated previously with the residual impervious barrier layer oxide thickness [16,17]. It is not clear at present
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whether such an interpretation is valid if the values of oxide thickness determined from measurements with Pt contacts are low because of the high conductivity of the inner layers of these thick oxides. The initial oxide thickness, measured immediately after immersion should be close to the total thickness derived from the weight gain if highly conducting inner layers are not a factor, as should the extrapolated values from the Bode plot at high frequencies (> lo5 Hz). In these experiments the initial oxide thicknesses measured immediately after immersion in ammonium nitrate were close to but somewhat below those obtained using Pt contacts for oxides formed in pH 12 LiOH, and (with the exception of specimens R13, M14) were well below the Pt contact values and showed little further decrease with time for specimens oxidised in pH 7 water. In instances where a double peak was not clearly present in the Bode plots (e.g. P16, S13), the final extrapolated value from 2 x lo5 Hz in ammonium nitrate was much lower than the total oxide thickness. It is possible that a peak at even higher frequencies may have been missed, but previous searches for such peaks have proved fruitless [6]. There are at least two possible explanations for these low initial readings on immersion in ammonium nitrate. The first would be that the initial reading can never represent an apparently thicker oxide than that measured with Pt contacts, because the lower than expected values with Pt may result from the inner part of the oxide being too conducting to contribute to the total impedance thickness. Such an explanation would satisfy the generally close agreement between Pt and initial NH,NO, thickness for oxides formed in pH 12
90 75 60
4.5 30 15
0 90 75 60 15 30 15 2
3
4
5 12 log -=w=ncy
3
4
5
6'
(HZ)
Fig. 4. Bode plots for Z and 4 measuredafter saturatingin NH,NO, electrolytefor specimensoxidisedin pH 7 water (a) and pH 12 LiOH (b).
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LiOH, but would not explain the very low initial thickness results for oxides grown in pH 7 water. A second explanation of these low initial values, obtained following immersion in NH,NO, electrolyte could be that the oxides formed in pH 7 water contained more large cracks than for those formed in pH 12 LiOH. Such cracks could allow a very rapid penetration of the electrolyte during the first few seconds of immersion, before an initial reading was obtained. It was decided that the presence of such cracks could be readily detected by electron microscope replicas taken from the specimen surfaces. Inspection of replicas in the transmission electron microscope (Fig. 5) showed the almost complete absence of large cracks in the outer surfaces of any of the oxides. Arrays of small pores were seen that were so near to the detection limit for pores of small diameter by the formvar-carbon replication technique that it was concluded that only a small fraction of the total numbers of pores present were being revealed. This situation arises because, for pores of successivelysmaller diameter, the small spikes of plastic extracted from the pores (and on which the characteristic appearance of
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the pores in the microscope depends) break off successively nearer to the outer surface of the oxide until a pore becomes indistinguishable from the fine surface roughness resulting from the surface oxide crystallites. This same problem has beset other recent work on oxides formed in higher concentrations of LiOH[18-201 and seriously limits the ability to characterise the surface porosity in oxides where pore sizes are very small ( < 10 nm diameter). With these difficulties in mind, therefore, we can conclude that there were no large cracks present that could explain the ac impedance results obtained after immersion in NH,NO, electrolyte. Many pores were observed in these oxides, whether they were formed in pH 7 water or pH 12 LiOH, and these often appeared to be in localised clusters. However, this could have been a result of local variations in the resolution of very small pores, and it was concluded that estimates of pore frequencies and diameters made from these replicas would not be reliable. The characteristic appearances of the clusters of pores that were observed are shown in Fig. 5, where the difficulties in distinguishing pores from surface roughness are evident.
a.
b.
d.
e.
Fig. 5. Replica electron micrographsof specimensurfacesshowingclustersof pores in the oxide films formed in pH 7 water at 360°C;(a) M14, (b) 014 and pH 12 LiOH, (c) L36, (d-f) T34.
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Only one specimen (T34) showed any unusual distribution of pores. In addition to the usual clusters of small pores (Fig. 5d) it showed concentrated arrays of larger pores that appeared to lie along prior metal grain boundaries, although these “grains” were much larger than any individual grains observed metallographically in this batch, where the largest recrystallised grains in the partially recrystallised structure were only lo-12 pm diameter. The cause of these unique arrays of pores remains unknown. Using the limited range of frequencies available routinely for ac impedance measurements, it appeared that, unlike the situation in a previous study [ll], there was no evidence here for conducting paths in the oxides formed in pH 7 water, based on the results with either Hg or Pt contacts (Fig. 2), and that the impedance spectra obtained after saturation with 1.0 molar ammonium nitrate showed typical evidence for two layered oxide films [21-261 in the form of double peaks in the phase angle plots. However, there is no real agreement on how to interpret these two layers in physical terms. The simplest explanation is to see them
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as a layer of porous oxide on top of an impervious barrier film. However, if this interpretation is correct then the highest frequency data should represent the total oxide thickness. These results show that this is not always the case, and that in thick thermally formed oxide films the layers of oxide close to the oxide-metal interface may always be too conducting to contribute to the total impedance of the oxide. This would render the interpretation of one of the layers as the “barrierlayer” improbable since it would be impossible to determine how much, if any, of the conducting layer was free of small pores. An alternative interpretation of the two layers may be that they represent contributions from the impedance of the pores and the intervening oxide in parallel with each other. This situation might arise if the pores were too small for the normal electrolyte conductivity to operate within them. Further work will be needed to resolve this, but at least one of the published models for the impedance of thermally formed ZrO, films [23] considers the two capacitative components in the oxide to be in parallel. The impedance spectra obtained with Pt contacts
1 L36
9 0
. -‘-.-‘-k
Y,
7-
‘.
‘-._,
‘..
‘k
65 16
A
,,,,\
6-
Fig. 6. Bode plots of results for selected oxides formed in pH 12 LiOH obtained with platinum contacts when the frequency range was extended down to 10m3 Hz using a Solartron impedance spectrum analyser. (a) Specimen L36, (b) specimen R16, (c) specimen T34.
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for oxides grown in pH 12 LiOH all showed declining phase angles and slopes for the log Z versus log f plots at the lowest frequency initially used (12 Hz). In order to determine whether or not this represented Young impedance type behaviour, as had been found with thinner oxide films [6,8,23], the measurements were extended to 10e3 Hz (Fig. 6) when it became evident that these oxides were showing the presence of conducting paths with the characteristics of transmission lines (i.e., phase angle - 45”, slope of log Z versus log f -0.5). There was no electrolyte present during these measurements, and similar characteristics were not seen at low frequencies when oxides were saturated with 1.0 molar ammonium nitrate, so it is concluded that these conducting “transmission lines” in the oxide may arise from the pore structure that is penetrated by the electrolyte. If the conducting “transmission lines” resulted from a thin conducting layer on the pore walls, then the presence of electrolyte in them might interfere with this condition. The characteristics of the conducting paths were similar, but their occurrence only in oxides formed in LiOH in this study was dissimilar to the previous study [ll], where the oxidation was in pH 7 water or steam, and this type of behaviour was observed for all specimens. At present, neither a physical explanation of these features nor a clear understanding of the factors leading to their observation can be reached. In summary, therefore, examinations of thick thermal oxides formed in either pH 7 water or pH 12 LiOH at 360°C (633 K) on a series of batches of Zircaloy-4 fuel cladding with different fabrication routes have shown that: Differences in oxide characteristics resulting from the conditions of oxide growth were larger than those resulting from differences in fabrication route. Oxides formed in pH 7 water showed good capacitative behaviour when measured with metallic outer contacts (unlike the specimens in a previous study [ll]), whereas oxides formed in pH 12 LiOH showed conduction paths with transmission line (Warburg impedance) characteristics. A comparison of several measures of the oxide thickness showed good agreement between weight gain and FTIR interferometry for the total oxide thickness. ac impedance measurements with Pt contacts often gave lower values than these, suggesting that the inner layers of these oxides might be too conducting to contribute to the total impedance. ac impedance measurements following immersion in 1.0 molar NH,NO, sometimes failed to measure the total oxide thickness, probably for the same reason, since no large cracks could be found that could provide an alternative explanation. Pores in the oxides, revealed by electron microscope replicas, were very small and near the limit of reso-
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lution. They appeared to be clustered, but this might be an artefact resulting from an inability to reveal the smallest pores.
Acknowledgements The authors are indebted to the Natural Sciences and Engineering Research Council of Canada and the CANDU Owner’s Group for the support that allowed this study to be carried out; to the Nuclear Fuel Industry Research Group for permission to include some earlier results obtained under their auspices, and to Prof. S. Thorpe for the use of the Solartron Impedance Spectrometer.
References
ill B. Cox, Advances in Corrosion Science and Technology, vol. V, eds. M.G. Fontana and R.W. Staehle (Plenum, New York, 1976) p. 173. Dl D. Pecheur, F. Lefebvre, A.T. Motta and C. Lemaignan, J. Nucl. Mater. 189 (1992) 318. [31 D. Pecheur, F. Lefebvre, A.T. Motta, C. Lemaignan and D. Charquet, Oxidation of intermetallic precipitates in Zircaloy-4: impact of irradiation, Proc. 10th Int. Symp. on Zirconium in the Nuclear Industry, Baltimore, June 1993, ASTM-STP, to be published. [41 J. Godlewski, How the tetragonal zirconia phase is stabilised in oxide scale formed on zirconium alloy corroded at 400°C in steam, Proc. 10th Int. Symp. on Zirconium in the Nuclear Industry, Baltimore, June 1993, ASTM-STP, to be published. 151B. Cox, The Use of Electrical Methods for Investigating the Growth and Breakdown of Oxide Films on Zirconium Alloys, Canadian Report, AECL-2668 (1967). [61 B. Cox, F. Gascoin and Y.-M. Wong, J. Nucl. Mater., to be published. [71 P.J. Shirvington, J. Nucl. Mater., 37 (1970) 177. @I B. Cox and Y.-M. Wong, Degradation of Oxide Films on a Zr-2.5 wt% Nb Alloy, University of Toronto, Report CNEUT-93-03 (1993). [91 L. Young, Anodic Oxide Films (Academic Press, New York, 1961) pp. 253-267. [lOI H. Gohr, Ber. Bunsenges, Phys. Chem. 85 (1981) 274. [ill B. Cox and Y. Yamaguchi, J. Nucl. Mater. 210 (1994) 303.
1121 B. Cox, N. Ramasubramanian, V.C. Ling, Zircaloy Corrosion Properties under LWR Coolant Conditions, part 1, Electric Power Research Institute, Report EPRINP6979-D (1990). [131 B. Cox, Y.-M. Wong, J. Nucl. Mater. 199 (1993) 258. [141 P.J. Harrop and J.N. Wanklyn, Brit. J. Appl. Phys. 18 (1967) 739. [151 B. Cox, Brit. J. Appl. Phys. (J. Phys. D), ser. 2, vol. 1 (1968) 671. Ml J.N. Wanklyn, CF. Britton, D.R. Silvester and N.J.M. Wilkins, J. Electrochem. Sot. 110 (1963) 856.
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[17] H.J. Beie, F. Garzarolli, H. Ruhmann, H.J. Sell, A. Mitwalsky and S. Hofler, Examination of the corrosion mechanism of zirconium alloys, Proc. 10th Int. Symp. on Zirconium in the Nuclear Industry, Baltimore, June 1993, ASTM-STP, to be published. [18] B. Cox and Y.-M. Wong, ASTM-STP 1132 (1991) 643. [19] B. Cox and C. Wu, J. Nucl. Mater. 199 (1993) 272. [20] B. Cox and C. Wu, J. Nucl. Mater., to be published. [21] 0. Gebhardt, Electrochim. Acta 38 (1993) 633.
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[22] 0. Gebhardt, J. Nucl. Mater. 203 (1993) 17. [23] H. Gohr, J. Schaller and C.-R. Schiller, Electrochim. Acta 38 (1993) 1961. [24] L. Durand-Keklikian, G. Cragnolino and D.D. McDonald, Corr. Sci. 32 (1991) 347. [25] Idem, Corr. Sci. 32 (1991) 361. [26] J.A. Bardwell and M.A. McKubre, Electrochim. Acta 36 (1991) 647.