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Cathodic sites on Zircaloy surfaces
Journal of Nuclear Materials 189 (1992) 362-369 North-Holland
Letter to the Editors
Cathodic sites on Zircaloy surfaces Brian Cox, Fernande
Gauducheau
* and Yin-Mei Wong
Centre for Nuclear Engineering, University of Toronto, 184 College Street, Toronto, Ontario, Canada M5S IA4 Received 9 March 1992; accepted 20 May 1992
Nodular corrosion in Boiling Water Reactors (BWRS) is a localised form of corrosion giving rise in its initial stages, to the formation of lenticular nodules of thick oxide of variable frequency from batch to batch of either the Zircaloy-2 fuel cladding [1], or the Zircaloy4 fuel channels [2]. Severe examples of this phenomenon are associated with poor water chemistry, especially the presence of high levels of copper and some other impurities [3,4], and can lead to through wall oxidation of the fuel cladding by Crud Induced Localised Corrosion (CILC) failures [5]. Batches of good and poor cladding are usually discriminated by one of a number of variants on an - 500°C (773 K), - 10 MPa steam test, which generates oxide nodules with a superficial similarity to those observed in BWRs [6,7]. In BWRs it is evident that scratches on the fuel cladding surface represent preferred sites for nodule
* Exchange student from Bcole Nationale Superieure de Caen, France.
nucleation (fig. 11, whereas typical nodule distributions obtained in high temperature, high pressure steam show less preference for nucleation on scratches (fig. 2). A high temperature steam test is unable to model the chemical factors leading to CILC failures, and also appears to be insensitive to electrochemical effects, since nodules nucleating on scratches are relatively infrequent in steam tests compared with in-reactor observations. Attempts to identify sites for nodule nucleation about which there was general agreement have, so far, been unsuccessful [&lo]. The potential of a freshly-pickled zirconium sample immersed in an electrolyte at room temperature increases rapidly from < -1.0 V (SCE) to 0 to + 100 mV (SCE) in oxygen saturated solutions, or -400 to -500 mV (SCE) in hydrogen saturated solutions, as a thin protective oxide forms [11,12]. In - 300°C water, since most areas of the surface will be at more noble potentials because of the effect of a thick unbroken oxide film, scratches may represent the most cathodic sites on the cladding surface during the period until a
Fig. 1. Nodular corrosion on BWR fuel cladding, showing continuous nodular oxide along scratches (X 2). 0022-3115/92/$05.00
0 1992 - Elsevier Science Publishers B.V. All rights reserved
B. Cox et al. / Cathodic sites on Zircaloy swfaces
thick protective oxide film reforms on the scratches. Other evidence suggests that nodules nucleate early in the life of the fuel cladding, and that no further ones nucleate unless the cladding is scratched at the next shutdown [9]. It may be reasonable to infer from this that other sites where discrete nodules nucleate are also cathodic to the remainder of the surface. However, no persuasive evidence for the nature of these sites has been published. Of the species present in BWR water either Cu2+ or NiZf could potentially be reduced at cathodic sites on Zircaloy surfaces since their redox potentials in 300°C water lie between those expected for oxide covered and oxide free Zircaloy surfaces in this medium [13,14]. Oxidation of the Zircaloys in high temperature water in the laboratory is surprisingly free of evidence for electrochemical factors affecting the oxidation rate. Corrosion rates are relatively independent of pH from 1-14 (measured at RT) unless specific chemical species such as LiOH are present [15-171, and no clear effects of polarisation have been reported [18]. Nevertheless, in-reactor there are a number of features that suggest a major influence of electrochemical or galvanic effects on the frequency and severity of nodule formation [2,19-241. These observations suggest that some of the factors that may inhibit the appearance of electrochemical effects in the laboratory (e.g. the low conductivity of the water and the ZrO, films) may be significantly relaxed in the high radiation fields in reactors. The frequency with which, in BWRs, the nodules initiate at scratches suggests that sites- that are relatively cathodic to the rest of the surface are important for nodule nucleation. This paper presents a preliminary attempt to decorate cathodic sites on Zircaloy surfaces with copper.
Fig. 2. Typical appearance of nodular corrosion in a 500”C/1500 psi test showing less dependence of the nodule distribution on scratches than in fig. 1. Light bands are light reflections.
363
Table 1 Analyses of Zircaloy-2 and Zircaloy-4 Batches Material:
Zircaloy-2 (Bh) Zircaloy4 (Q)
Form:
Cold-rolled and Fuel-clad annealed sheet tubing
Analysis Sn Fe Cr Ni 0 C H N Al Si
1.45% 0.140% 0.090% 0.050% 1000 ppm 80 ppm 10 Ppm 20 Ppm < 40 ppm 70 Ppm
Weight gains (mg/dm’) 4oo”C/3 day, 10 MPa steam 17, 17 SOO’C/l day, 10 MPa steam 218, 750
1.54% 0.235% 0.103% < 50 ppm 1100 ppm 123 ppm 10 ppm 20 Ppm _ < 80 ppm 17,18 143,149
Specimens were selected from one batch of Zircaloy-2 sheet (designated Bh), and one batch of Zircaloy4 fuel cladding with a very fine precipitate size distribution (designated Q). Analysis of these materials, together with their weight gains in 400 and 500°C high-pressure steam tests are given in table 1. Specimens were spot-welded to lengths of Zircaloy-2 wire and were chemically polished in nitric/ hydrofluoric acid solution prior to testing so that the grain structure and second-phase particles were readily visible. Specimens were examined initially by cyclic voltammetry at 25°C (298 K) in an 0.1 molar K,S0,/0.005 molar CuSO, solution to establish the potential at which Cu2+ reduction on the surfaces first occurred. Results showed that Cu was being deposited over a broad range of potential between -0.8 and - 1.0 V (versus saturated Hg/HgSO,). The specimens were then cleaned in hot 50% nitric acid solution, to remove any residual copper and cathodically polarised at 25°C (298 K) in 0.1 molar CuSO, solution at 1 mA/cm2 in order to ensure that copper deposition would be extensive. The first polarisation was for 15 min, this was subsequently reduced in steps to 1 min, in order to reduce the size and number of the copper particles deposited on the surface. The specimens were initially exposed in the chemically polished condition, and the distribution of copper particles on the surface was examined in a scanning electron microscope (Hitachi Model S570). These deposits were again removed by dissolution in hot 50% HNO, and the speci-
364
B. Cox et al. / Cathodic sites on Zircaloy surfaces
mens were anodised at 25°C (298 K) to - 30 V in the same CuSO, solution that was used for cathodic polarisation. Microhardness indents were applied to areas of the surface close to active copper deposition sites before dissolution of the copper deposits so that the sites of the copper deposits could be identified subsequently. Successive deposition and removal, using progressively shorter cathodic polarisation times were continued until only a few very small copper deposits were obtained. After the first cathodic polarisation of the chemically polished surfaces extensive deposition of copper was found to be nonuniformly distributed over the specimen surfaces. Deposition occurred preferentially along prominent scratches on the surfaces (fig. 3) and resulted in almost continuous copper deposits at these sites. Some isolated copper deposits were observed, but these were too large compared with the metal grain size and after dissolution it was not possible to establish unequivocally the sites upon which these had nucleated. Anodic oxidation to give a uniform oxide film resulted in smaller and fewer copper deposits forming. Deposits on scratches were now few, although the same prominent scratches still nucleated a few discrete copper deposits, as did a few other sites. Reduction in the size of the copper deposits improved the prospects for identifying their initiation sites (figs. 4 and 5). Inspection of the nucleation site in fig. 5 shows that it was close to where the prominent scratch disappeared at a grain boundary and that the oxide film was heavily cracked in this area, even though it had been formed after the scratch (fig. 6). It was impossible to say which of the many fine cracks might have remained unpassivated and served to nucleate the copper particle. Other regions of the same scratch, even when less heavily indented showed similar arrays of cracks, but less prominently visible. It may be that the areas of less severe cracking repassivated more completely than the few residual sites on scratches that continued to nucleate copper deposits. When the specimen was reanodised at a slightly higher voltage and held at the formation voltage for a longer time (to try to repair any residual weak spots in the oxide) this active site was eliminated. Fia. 3. Coooer deoosits on Zircaloy surfaces (a, b) Zircaloy-4, Q; (c) Zircaloy-2,-Bh. (a) As-picklkd surface cathodically PO: larised at 1 mA/cm* for 1.5 min. fb, c) Surfaces after two successive anodisations at _ 30 V in O.lM C&O,, the second one with a 20-min hold at the formation voltage, followed by 1-min cathodic polarisation at 1 mA/cm’.
B. Cox et al. / Cathodic sites on Zircaloy surfaces
365
Fig. 4. Copper deposit approximately 8 urn diameter (a) and the same site (b) after dissolving it. Note initiation site may have centre one of three pits in centre of grain (arrow) Zircaloy-2 (Bh).
Fig. 5. Isolated copper deposit (a) on a residual scratch after anodisation to 30 V in O.lM CuS04 and (b) the initiation site (arrow) after copper dissolution.
366
B. Cox et al. / Cathodic sites on Zircaloy surfaces
Fig. 6. High magnification view of initiation site seen in fig. 5. Note that it is not possible to establish which of the many cracks was active.
Fig. 7. Copper deposit (a) that appears to have nucteated at the almost triangular pit seen in the centre of(b).
B. Cox et al. / Cathodic sites on Zircaloy surfaces
The microhardness indents that had been put in after the first anodisation, also proved to be preferred sites for copper deposition. Cracks in the anodic oxide, similar to those seen at scratches, were found to be present at these indents, and required similar long anodisation hold times to repassivate and deactivate them against copper deposition. Once the scratches and microhardness indents had been adequately repassivated, and the polarisation times were further reduced, only a few initiation sites copper deposits were seen. None of these were at either scratches or indents. The frequency of these appeared to be higher on Bh than on Q specimens (fig. 3), and in the same order as the batches resistance to a 500°C steam test. However, the variability from area to area of either material in copper particle frequency rendered any firm conclusion difficult, since a complete particle count was not performed on either material. Once small deposits were obtained they were checked by EDAX to confirm that they were still deposited copper, and after dissolution, examination of the deposition sites showed some of them to be characteristically elongated, angular, deep pits (fig. 7). SEM examination showed these and other pits (e.g. fig. 4b), that initiated deposits were typically empty of second phase particles, and the only feature that distinguished
361
them from shallower pits still containing Zr(Fe/Cr/Ni) intermetallic particles was the common presence of cracks in the oxide around the perimeters of the deeper more angular pits (fig. 8). However, many more pits in this class had peripheral cracks than the number that initiated copper deposits. Thus, as for active cracks that remained at scratches or micruhardness indents, the criteria for the presence or absence of unpassivated cracks that could nucleate copper deposits at these pits could not be established. The observation that prominent scratches were the most preferred sites for copper deposition was hardly surprising. Zircaloy surfaces, even in the as chemically polished condition, always bear a thin, air-formed oxide film N 2-3 nm thick [25,26], and severe scratches are the most probable sites for flaws to persist in this film and in subsequently formed oxides. Such flaws are also expected to be cathodic sites. That the flaws along scratches in the as-pickled surfaces were sufficiently frequent for an almost continuous band of copper to be deposited may be analogous to the continuous bands of nodular oxide formed along such scratches in-reactor (fig. 1). The finding that microhardness indents in a thicker anodic oxide film also represent preferred sites
for copper deposition, and that cracks in the oxide were evident at these indents, further supports the
Fig. 8. Deep angular pits resulting from the etching-out of second-phase particles showing that most have peripheral cracks in the oxide formed on them even though only a few are active sites for copper deposition. Pit at bottom right in (b) is arrowed in fig. 4b.
368
B. Cox et al. / Cathodic sites on Zircaloy surfaces
argument that such flaws can be very persistent. In the same way small cracks at geometrically angular pits wouid also be expected to be persistent cathodic sites. Although this work was done at room temperature, it should be noted that similar cracks around surface pits are observed during corrosion in high temperature water (fig. 9) and are also persistent [27], though the extent to which they repassivate has not been tested. The angular pits may be characteristic of the shape and size of zirconium silicide/ phosphide precipitates in the Zircaloys [28,29]. Other deep circular pits may possibly have contained zirconium carbide particles, since such precipitates are reported to be close to spherical when seen in the Zircaloys 128,291.The chemical analyses of these batches of Zircaloy show that the quantities of Si and C may be high enough for a few of these particles to be present in these materials [28,30]. Precipitates of the normal Zr,(Fe/Ni) and Zr(Fe/Cr), phases were clearly visible, typically appeared to lie in relatively shallow circular pits, and yet were not sites for nucleating copper deposition. Only empty angular pits that had peripheral cracks nucleated copper deposits (figs. 4, 7 and 81, and only a few of these were active. Reduction of cupric ions on Zircaloy surfaces at room temperature was found to occur most readily at prominent scratches or other sites where unpassivated
cracks were present in the oxide film. If these sites were sufficiently well oxidised to prevent copper deposition at them, then the next most cathodic sites at which copper deposition occurs appear to be deep, angular pits, where second phase particles have been etched out of the surface by the initial pickling treatment. These pits are sufficiently angular for a persistent crack to form even with thin anodic oxide films present on the surface. No copper deposits were observed at sites where second phase particles were still visible in the surfaces. Thus, all the cathodic sites identified in this study have been related to flaws or cracks in the oxide where unprotective oxide may have persisted. Although this work was done at low temperature, and cannot therefore be extrapolated to high temperature water conditions, the same type of angular pits with peripheral cracks have been seen after prolonged oxidation in high temperature water [27], although no evidence about how well these cracks were passivated is available.
Acknowledgements Two authors (B.C. and Y.-M.W.) are grateful to the Natural Sciences and Engineering Research Council of Canada and the Canadian Nuclear Industry for finan-
Fig. 9. Persistent cracks formed at deep pits during prolonged oxidation (339 days) of Zircaloy-2 in 360°C water. Oxide thickness - 10 pm.
6. Cox et al. / Cathodicsiteson Zircaloysurfaces
cial support under which this work was performed; the other author (F.G.) is grateful to IAESTE for financial support of her summer exchange programme. References [l] R.C. Nelson, The corrosion of Zircaloy-2 fuel element cladding in a boiling water environment, Proc. AEC Symp. on Zirconium Alloy Development, Castlewood, CA, November 1962, GEAP-4089, vol. II, paper 17. [2] Y. Kuroda, Corrosion of fuel channels at extended fuel burnups, poster presentation at 9th Int. Symp. on Zirconium in the Nuclear Industry, Kobe, Japan, November 1990, ASTM-SIP 1132, p. 785. [3] A. Garlick, R. Sumerling and G.L. Shires, J. Brit. Nucl. Energy Sot. 16 (1977) 77. [4] BWR Normal Water Chemistry Guidelines: 1986 Revision, Electric Power Research Institute, Report EPRI NP-4946~SR (1988). [S] M.O. Marlowe, J.S. Armijo, B. Cheng and R.B. Adamson, Proc. ANS Topical Meeting on LWR Fuel Performance, Orlando, FL, April 1985, pp. 3-73. [6] Papers on nodular corrosion in Proc. 7th Int. Symp. on Zirconium in the Nuclear Industry, Strasbourg, June 1985, ASTM-STP 939. [7] Papers on nodular corrosion in Proc. 8th Int. Symp. on Zirconium in the Nuclear Industry, San Diego, CA, June 1988, ASTM-SIP 1023. [8] B. Cox, Proc. Int. Conf. on Reactor Materials Science, Alushta, Crimea, May 1978, vol. V (Atominfo~, Moscow, 1978) p. 90. [9] B. Cheng and R.B. Adamson, Proc. 7th Int. Symp. on Zirconium in the Nuclear Industry, Strasbourg, June 1985, ASTM-STP 939, p. 387. [lo] D. Charquet, R. Tricot and J.-F. Wadier, Proc. 8th Int. Symp. on Zirconium in the Nuclear Industry, San Diego, June 1988, ASTM-STP 1023, p. 374. [ill R.E. Meyer, J. Electrochem. Sot. 106 (19591930. [12] F. Hine, M. Yasuda and H. Sato, Corrosion 33 (1977) 181. [13] D.D. MacDonald, G.R. Shierman and P. Butler, The Thermodynamics of Metal-Water Systems at Elevated Temperatures, part 1: The Water and Copper-Water Systems, Atomic Energy of Canada Ltd., report AECL4136 (1972). [14] D.D. MacDonald, The Thermodynamics of Metal-Water Systems at Elevated Temperatures, part 4: The Nickel-
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Water System, Atomic Energy of Canada Ltd., Report AECL-4139 (1972). H-51 G.H. Jenks, in: Fluid Fuel Reactors, eds. J.A. Lane, H.G. McPherson and F. Maslon (Addison Wesley, Reading, MA, 1958) pp. 232-245. WI B. Cox, K. Alcock and F.W. Derrick, J. Electrochem. Sot. 108 (1961) 129. D71 H. Coriou, L. Grail, F. Meunier, M. Pelraz and H. Willermoz, J. Nucl. Mater. 7 (1962) 328. [=I B. Cox and P.J. Shi~ington, J. Nucf. Mater. 35 (1970) 211. [I91 R. Sumerling, A. Garlick, A. Stuttard, J.M. Hartog, F.W. Dowse and P. Sims, Proc. 4th Int. Conf. on Zirconium in the Nuclear Industry, Stratford-upon-Avon, UK, June 1978, ASTM-SIP 681, p. 107. ml D.J. Ferret& W.S.A. Black and T.E. Blackman, Proc. 2nd Int. Topical Meeting on Nuclear Power Plant Thermal Hydraulics and Operations, Tokyo, April 1986, pp. 5-87. ml P.A. Platonov, A.P. Eperin, A.N. Ivanov, LA. Frolov, B.S. Rodchenkov and LA. Voronin, Some results of investigating the deformation behaviour of Zr-2.5 wt% niobium pressure tubes in the MR reactor and the operating RBMK units, presented at 8th Int. Symp. on Zirconium in the Nuclear Industry, San Diego, June 1988. WI D. Franklin and C.-Y. Li, Proc. 7th Int. Symp. on Zirconium in the Nuclear Industry, Strasbourg, June 1985, p. 206. 1231 A.B. Johnson, Jr., Zirconium Alloy Oxidation and Hydriding Under Irradiation: review of Pacific Northwest Laboratories’ Test program Results, Electric Power Research Institute, Report EPRI NP-5132 (1987). I241 A.B. Johnson, Jr., in: Reviews in Coatings and Corrosion, ed. J. Yahalom, vol. I, no. 4 (1975) p, 300. Lw R. Kaufmann, H. Klewe-Nebenius, H. Moers, G. Pfennig, H. Jenett and H.J. Acke, Surf. Interf. Anal. 11 (1988) 502. [261 L. Kumar, D.D. Sarma and S. ~ummacher, Appl. Surf. Sci. 32 (1988) 309. D71 B. Cox, J. Nucl. Mater. 29 (1969) 50. 1281 D. Charquet and E. Alheritiere, Proc. KTG Workshop on Second Phase Particles and Matrix Properties of the Zircaloys, Erlangen, Germany, July 1985, p. 9. t291 C.T. Wang and P.E. Danielson, Mater. Charact. 24 (1990) 87. i301 D. Charquet, R. Hahn, E. Ortlieb, J.-P. Gros and J.-F. Wadier, Proc. 8th Int. Symp. on Zirconium in the Nuclear Industry, San Diego, CA, June 1988, ASTM-SIP 1023, p. 405.
Letter to the Editors
Cathodic sites on Zircaloy surfaces Brian Cox, Fernande
Gauducheau
* and Yin-Mei Wong
Centre for Nuclear Engineering, University of Toronto, 184 College Street, Toronto, Ontario, Canada M5S IA4 Received 9 March 1992; accepted 20 May 1992
Nodular corrosion in Boiling Water Reactors (BWRS) is a localised form of corrosion giving rise in its initial stages, to the formation of lenticular nodules of thick oxide of variable frequency from batch to batch of either the Zircaloy-2 fuel cladding [1], or the Zircaloy4 fuel channels [2]. Severe examples of this phenomenon are associated with poor water chemistry, especially the presence of high levels of copper and some other impurities [3,4], and can lead to through wall oxidation of the fuel cladding by Crud Induced Localised Corrosion (CILC) failures [5]. Batches of good and poor cladding are usually discriminated by one of a number of variants on an - 500°C (773 K), - 10 MPa steam test, which generates oxide nodules with a superficial similarity to those observed in BWRs [6,7]. In BWRs it is evident that scratches on the fuel cladding surface represent preferred sites for nodule
* Exchange student from Bcole Nationale Superieure de Caen, France.
nucleation (fig. 11, whereas typical nodule distributions obtained in high temperature, high pressure steam show less preference for nucleation on scratches (fig. 2). A high temperature steam test is unable to model the chemical factors leading to CILC failures, and also appears to be insensitive to electrochemical effects, since nodules nucleating on scratches are relatively infrequent in steam tests compared with in-reactor observations. Attempts to identify sites for nodule nucleation about which there was general agreement have, so far, been unsuccessful [&lo]. The potential of a freshly-pickled zirconium sample immersed in an electrolyte at room temperature increases rapidly from < -1.0 V (SCE) to 0 to + 100 mV (SCE) in oxygen saturated solutions, or -400 to -500 mV (SCE) in hydrogen saturated solutions, as a thin protective oxide forms [11,12]. In - 300°C water, since most areas of the surface will be at more noble potentials because of the effect of a thick unbroken oxide film, scratches may represent the most cathodic sites on the cladding surface during the period until a
Fig. 1. Nodular corrosion on BWR fuel cladding, showing continuous nodular oxide along scratches (X 2). 0022-3115/92/$05.00
0 1992 - Elsevier Science Publishers B.V. All rights reserved
B. Cox et al. / Cathodic sites on Zircaloy swfaces
thick protective oxide film reforms on the scratches. Other evidence suggests that nodules nucleate early in the life of the fuel cladding, and that no further ones nucleate unless the cladding is scratched at the next shutdown [9]. It may be reasonable to infer from this that other sites where discrete nodules nucleate are also cathodic to the remainder of the surface. However, no persuasive evidence for the nature of these sites has been published. Of the species present in BWR water either Cu2+ or NiZf could potentially be reduced at cathodic sites on Zircaloy surfaces since their redox potentials in 300°C water lie between those expected for oxide covered and oxide free Zircaloy surfaces in this medium [13,14]. Oxidation of the Zircaloys in high temperature water in the laboratory is surprisingly free of evidence for electrochemical factors affecting the oxidation rate. Corrosion rates are relatively independent of pH from 1-14 (measured at RT) unless specific chemical species such as LiOH are present [15-171, and no clear effects of polarisation have been reported [18]. Nevertheless, in-reactor there are a number of features that suggest a major influence of electrochemical or galvanic effects on the frequency and severity of nodule formation [2,19-241. These observations suggest that some of the factors that may inhibit the appearance of electrochemical effects in the laboratory (e.g. the low conductivity of the water and the ZrO, films) may be significantly relaxed in the high radiation fields in reactors. The frequency with which, in BWRs, the nodules initiate at scratches suggests that sites- that are relatively cathodic to the rest of the surface are important for nodule nucleation. This paper presents a preliminary attempt to decorate cathodic sites on Zircaloy surfaces with copper.
Fig. 2. Typical appearance of nodular corrosion in a 500”C/1500 psi test showing less dependence of the nodule distribution on scratches than in fig. 1. Light bands are light reflections.
363
Table 1 Analyses of Zircaloy-2 and Zircaloy-4 Batches Material:
Zircaloy-2 (Bh) Zircaloy4 (Q)
Form:
Cold-rolled and Fuel-clad annealed sheet tubing
Analysis Sn Fe Cr Ni 0 C H N Al Si
1.45% 0.140% 0.090% 0.050% 1000 ppm 80 ppm 10 Ppm 20 Ppm < 40 ppm 70 Ppm
Weight gains (mg/dm’) 4oo”C/3 day, 10 MPa steam 17, 17 SOO’C/l day, 10 MPa steam 218, 750
1.54% 0.235% 0.103% < 50 ppm 1100 ppm 123 ppm 10 ppm 20 Ppm _ < 80 ppm 17,18 143,149
Specimens were selected from one batch of Zircaloy-2 sheet (designated Bh), and one batch of Zircaloy4 fuel cladding with a very fine precipitate size distribution (designated Q). Analysis of these materials, together with their weight gains in 400 and 500°C high-pressure steam tests are given in table 1. Specimens were spot-welded to lengths of Zircaloy-2 wire and were chemically polished in nitric/ hydrofluoric acid solution prior to testing so that the grain structure and second-phase particles were readily visible. Specimens were examined initially by cyclic voltammetry at 25°C (298 K) in an 0.1 molar K,S0,/0.005 molar CuSO, solution to establish the potential at which Cu2+ reduction on the surfaces first occurred. Results showed that Cu was being deposited over a broad range of potential between -0.8 and - 1.0 V (versus saturated Hg/HgSO,). The specimens were then cleaned in hot 50% nitric acid solution, to remove any residual copper and cathodically polarised at 25°C (298 K) in 0.1 molar CuSO, solution at 1 mA/cm2 in order to ensure that copper deposition would be extensive. The first polarisation was for 15 min, this was subsequently reduced in steps to 1 min, in order to reduce the size and number of the copper particles deposited on the surface. The specimens were initially exposed in the chemically polished condition, and the distribution of copper particles on the surface was examined in a scanning electron microscope (Hitachi Model S570). These deposits were again removed by dissolution in hot 50% HNO, and the speci-
364
B. Cox et al. / Cathodic sites on Zircaloy surfaces
mens were anodised at 25°C (298 K) to - 30 V in the same CuSO, solution that was used for cathodic polarisation. Microhardness indents were applied to areas of the surface close to active copper deposition sites before dissolution of the copper deposits so that the sites of the copper deposits could be identified subsequently. Successive deposition and removal, using progressively shorter cathodic polarisation times were continued until only a few very small copper deposits were obtained. After the first cathodic polarisation of the chemically polished surfaces extensive deposition of copper was found to be nonuniformly distributed over the specimen surfaces. Deposition occurred preferentially along prominent scratches on the surfaces (fig. 3) and resulted in almost continuous copper deposits at these sites. Some isolated copper deposits were observed, but these were too large compared with the metal grain size and after dissolution it was not possible to establish unequivocally the sites upon which these had nucleated. Anodic oxidation to give a uniform oxide film resulted in smaller and fewer copper deposits forming. Deposits on scratches were now few, although the same prominent scratches still nucleated a few discrete copper deposits, as did a few other sites. Reduction in the size of the copper deposits improved the prospects for identifying their initiation sites (figs. 4 and 5). Inspection of the nucleation site in fig. 5 shows that it was close to where the prominent scratch disappeared at a grain boundary and that the oxide film was heavily cracked in this area, even though it had been formed after the scratch (fig. 6). It was impossible to say which of the many fine cracks might have remained unpassivated and served to nucleate the copper particle. Other regions of the same scratch, even when less heavily indented showed similar arrays of cracks, but less prominently visible. It may be that the areas of less severe cracking repassivated more completely than the few residual sites on scratches that continued to nucleate copper deposits. When the specimen was reanodised at a slightly higher voltage and held at the formation voltage for a longer time (to try to repair any residual weak spots in the oxide) this active site was eliminated. Fia. 3. Coooer deoosits on Zircaloy surfaces (a, b) Zircaloy-4, Q; (c) Zircaloy-2,-Bh. (a) As-picklkd surface cathodically PO: larised at 1 mA/cm* for 1.5 min. fb, c) Surfaces after two successive anodisations at _ 30 V in O.lM C&O,, the second one with a 20-min hold at the formation voltage, followed by 1-min cathodic polarisation at 1 mA/cm’.
B. Cox et al. / Cathodic sites on Zircaloy surfaces
365
Fig. 4. Copper deposit approximately 8 urn diameter (a) and the same site (b) after dissolving it. Note initiation site may have centre one of three pits in centre of grain (arrow) Zircaloy-2 (Bh).
Fig. 5. Isolated copper deposit (a) on a residual scratch after anodisation to 30 V in O.lM CuS04 and (b) the initiation site (arrow) after copper dissolution.
366
B. Cox et al. / Cathodic sites on Zircaloy surfaces
Fig. 6. High magnification view of initiation site seen in fig. 5. Note that it is not possible to establish which of the many cracks was active.
Fig. 7. Copper deposit (a) that appears to have nucteated at the almost triangular pit seen in the centre of(b).
B. Cox et al. / Cathodic sites on Zircaloy surfaces
The microhardness indents that had been put in after the first anodisation, also proved to be preferred sites for copper deposition. Cracks in the anodic oxide, similar to those seen at scratches, were found to be present at these indents, and required similar long anodisation hold times to repassivate and deactivate them against copper deposition. Once the scratches and microhardness indents had been adequately repassivated, and the polarisation times were further reduced, only a few initiation sites copper deposits were seen. None of these were at either scratches or indents. The frequency of these appeared to be higher on Bh than on Q specimens (fig. 3), and in the same order as the batches resistance to a 500°C steam test. However, the variability from area to area of either material in copper particle frequency rendered any firm conclusion difficult, since a complete particle count was not performed on either material. Once small deposits were obtained they were checked by EDAX to confirm that they were still deposited copper, and after dissolution, examination of the deposition sites showed some of them to be characteristically elongated, angular, deep pits (fig. 7). SEM examination showed these and other pits (e.g. fig. 4b), that initiated deposits were typically empty of second phase particles, and the only feature that distinguished
361
them from shallower pits still containing Zr(Fe/Cr/Ni) intermetallic particles was the common presence of cracks in the oxide around the perimeters of the deeper more angular pits (fig. 8). However, many more pits in this class had peripheral cracks than the number that initiated copper deposits. Thus, as for active cracks that remained at scratches or micruhardness indents, the criteria for the presence or absence of unpassivated cracks that could nucleate copper deposits at these pits could not be established. The observation that prominent scratches were the most preferred sites for copper deposition was hardly surprising. Zircaloy surfaces, even in the as chemically polished condition, always bear a thin, air-formed oxide film N 2-3 nm thick [25,26], and severe scratches are the most probable sites for flaws to persist in this film and in subsequently formed oxides. Such flaws are also expected to be cathodic sites. That the flaws along scratches in the as-pickled surfaces were sufficiently frequent for an almost continuous band of copper to be deposited may be analogous to the continuous bands of nodular oxide formed along such scratches in-reactor (fig. 1). The finding that microhardness indents in a thicker anodic oxide film also represent preferred sites
for copper deposition, and that cracks in the oxide were evident at these indents, further supports the
Fig. 8. Deep angular pits resulting from the etching-out of second-phase particles showing that most have peripheral cracks in the oxide formed on them even though only a few are active sites for copper deposition. Pit at bottom right in (b) is arrowed in fig. 4b.
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B. Cox et al. / Cathodic sites on Zircaloy surfaces
argument that such flaws can be very persistent. In the same way small cracks at geometrically angular pits wouid also be expected to be persistent cathodic sites. Although this work was done at room temperature, it should be noted that similar cracks around surface pits are observed during corrosion in high temperature water (fig. 9) and are also persistent [27], though the extent to which they repassivate has not been tested. The angular pits may be characteristic of the shape and size of zirconium silicide/ phosphide precipitates in the Zircaloys [28,29]. Other deep circular pits may possibly have contained zirconium carbide particles, since such precipitates are reported to be close to spherical when seen in the Zircaloys 128,291.The chemical analyses of these batches of Zircaloy show that the quantities of Si and C may be high enough for a few of these particles to be present in these materials [28,30]. Precipitates of the normal Zr,(Fe/Ni) and Zr(Fe/Cr), phases were clearly visible, typically appeared to lie in relatively shallow circular pits, and yet were not sites for nucleating copper deposition. Only empty angular pits that had peripheral cracks nucleated copper deposits (figs. 4, 7 and 81, and only a few of these were active. Reduction of cupric ions on Zircaloy surfaces at room temperature was found to occur most readily at prominent scratches or other sites where unpassivated
cracks were present in the oxide film. If these sites were sufficiently well oxidised to prevent copper deposition at them, then the next most cathodic sites at which copper deposition occurs appear to be deep, angular pits, where second phase particles have been etched out of the surface by the initial pickling treatment. These pits are sufficiently angular for a persistent crack to form even with thin anodic oxide films present on the surface. No copper deposits were observed at sites where second phase particles were still visible in the surfaces. Thus, all the cathodic sites identified in this study have been related to flaws or cracks in the oxide where unprotective oxide may have persisted. Although this work was done at low temperature, and cannot therefore be extrapolated to high temperature water conditions, the same type of angular pits with peripheral cracks have been seen after prolonged oxidation in high temperature water [27], although no evidence about how well these cracks were passivated is available.
Acknowledgements Two authors (B.C. and Y.-M.W.) are grateful to the Natural Sciences and Engineering Research Council of Canada and the Canadian Nuclear Industry for finan-
Fig. 9. Persistent cracks formed at deep pits during prolonged oxidation (339 days) of Zircaloy-2 in 360°C water. Oxide thickness - 10 pm.
6. Cox et al. / Cathodicsiteson Zircaloysurfaces
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