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Review on effect of cyclic loading on environmental assisted cracking of alloy 600 in typical nuclear coolant waters
Corrosion Science 43 (2001) 2265±2279
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Review on eect of cyclic loading on environmental assisted cracking of alloy 600 in typical nuclear coolant waters J. Congleton, E.A. Charles *, G. Sui Corrosion Research Centre, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK Received 1 September 2000; accepted 11 January 2001
Abstract Corrosion fatigue data for alloy 600 in simulated primary side, pressurised water reactor (PWR) coolant and in oxygenated pure water (OPW) have been reviewed. Higher crack growth rates occur in these environments than in air and the former show a dependence upon frequency of loading, m, and R ratio. The da=dN versus DK plots have a tendency to show plateau regions where da=dN remains constant with increasing DK. The enhancements of crack growth over those in air due to cyclic loading are larger in OPW than in simulated PWR primary side water. If corrosion fatigue of alloy 600 was perceived of importance for reactor components, the existing data base will need to be extended and it would be necessary to generate more air fatigue data than is currently available. Conventional S±N fatigue and corrosion fatigue data from smooth surface specimens may also be required, collected taking due regard of the possible eects of R and m mentioned above. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Alloy; Nickel; Corrosion fatigue; Stress corrosion
1. Introduction Environmental assisted cracking (EAC) of alloy 600 in primary and secondary side waters of pressurised water reactors (PWR) and in oxygenated pure water (OPW) could in¯uence the life-span of nuclear reactor components. This has been
*
Corresponding author. Tel.: +44-0191-222-6000; fax: +44-0191-222-7153. E-mail address: [email protected] (E.A. Charles).
0010-938X/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 1 ) 0 0 0 2 3 - 3
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known for more than 20 years and an extensive set of data pertaining to such cracking has been generated. However, few data exist concerning crack growth rates likely to be observed if cyclic loading is imposed in addition to the environments. The present review summarises published data available in the open literature on the combined eects of cyclic loading and environments on cracking of alloy 600 in various nuclear coolant systems and addresses the need for obtaining further cyclic loading data for EAC of alloy 600. There are two useful benchmarks for assessing the cyclic loading data. First, a reference data line for air fatigue is needed to assess the eect of environments on the rate of cracking. Second, a comparable reference curve for stress corrosion cracking (SCC), i.e. EAC under static load conditions, is required. With regard to SCC, there is a large amount of scatter in the available crack growth rate data, so a theoretically based model for the SCC crack growth in alloy 600 in the relevant environments would be helpful. Such a model has been proposed by Scott an Le Calvar [1] who considered three possible alternative mechanisms for cracking, namely internal oxidation leading to bubble formation on grain boundaries, internal oxidation to give a brittle intergranular region and embrittlement by oxygen adsorption on grain boundaries. Although gas bubble formation leading to a weakening of the grain boundaries is unlikely to be the relevant mechanism for intergranular cracking of alloy 600, their model based on this premise can be used to generate an equation that ®ts the available SCC data satisfactorily. Scott and Le Calvar assumed that crack growth was a stress driven mechanism that depended upon the size of the Ôprocess zoneÕ at the crack tip. A time for unstable growth of bubbles, that is inversely proportional to the square of the applied stress but proportional to the square root of a diusion depth, was combined with an estimate of 2 the plastic zone size at a crack tip, taken as proportional K=rp . The critical diffusion depth was chosen as a process zone size at the crack tip, set as a fraction of the plane strain plastic zone size for which the outer limit contour is a stress rp . A similar equation could be generated for any diusion related mechanism that showed a p 2 x=r dependence. Bubble formation could be replaced by oxide crystallites formed on the grain boundaries [2] or by grain boundary decohesion occurring intermittently with a stress dependent increment of growth occurring at a frequency controlled by diusion of oxygen into the active grain boundary. The Scott and Le Calvar equation is dx dt
81kTD0 512c3 a2
s dNS rp KI 6pz
where k is the Boltzmann constant, T the absolute temperature, D0 the grain boundary diusion coecient for oxygen, NS the surface solubility of oxygen, c the surface energy of nickel, a the interatomic spacing, z the number of sites explored per gas atom jump, rp a parameter that is related to the yield strength of the material and KI is the applied crack tip mode I stress intensity factor.
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2. Data survey Among more than 200 papers on cracking of alloy 600 reviewed, 12 papers [3±14] contained corrosion fatigue data. These 12 papers provided 380 records, each containing a crack growth rate and an associated DK, and in many cases the R ratio (minimum stress/maximum stress), loading frequency m, test temperature, environmental conditions and details relating to the source of the information. 2.1. Air fatigue data for alloy 600 The data base for fatigue crack growth of alloy 600 in air from pre-cracked specimens, consisting of 152 records taken from Refs. [3±11], were obtained in two cyclic frequency ranges, speci®cally from 0.0167 to 1 Hz and from 10 to 16.67 Hz, at R values from 0.05 to 0.85 and at temperatures from 25°C to 320°C. The da=dN versus DK data obtained at temperatures from 288°C to 320°C showed no noticeable temperature eects and are presented collectively in Fig. 1 as the open square symbols, while the room temperature data are shown as the full symbols. There is scatter in the data. No signi®cant reduction of the scatter results if da=dN , is plotted against an eective DK by correcting for R eects using either the Elberpor Shrive relationships [15,16]. There appears to be a threshold DK of 4.5 MPa m for fatigue in air at room temperature but insucient data was available to decide upon the existence or otherwise of a threshold DK for the tests at the high temperatures. As corrosion fatigue tends to occur only under relatively low frequency loading conditions, we have chosen a reference air curve by least squares ®tting the low
Fig. 1. da=dN versus DK with the average low frequency trend line shown, alloy 600 under cyclic loading in air.
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frequency ( 6 1 Hz) data only, but ignoring points near the threshold DK. This generated the equation: da 3:0 10 9 DK 3:14 dN i.e. a linear relationship between log da=dN and log DK. If in contrast to the above, corrosion fatigue curves show plateau regions where da=dN remains essentially constant as DK increases, it is possible that a stress corrosion component of growth is dominating da=dN at these DK values. To consider this possibility it is useful to convert the air fatigue data to da=dt versus Kmax , as shown in Fig. 2, because stress corrosion crack growth rates which are dominated by Kmax . The spread in crack growth rates displayed in Fig. 2 is mainly due to the eect of cyclic frequency, m, because da=dt was calculated using da da m dt dN For instance, the data for R 0:05 at 25°C shows that a tenfold increase in frequency generates a tenfold increase in crack growth rate. Corrosion fatigue is occurring if cyclic loading tests in a water environment generate higher crack growth rates than tests with equivalent loading conditions performed in air. The expected dierences between corrosion fatigue data and air fatigue data are that for corrosion fatigue, m, the R ratio and the wave form should all aect da=dN whereas there is likely to be only small eects from these variables in air fatigue, as indicated in Fig. 1. Also, plateau regions often exist at intermediate DK values in corrosion fatigue but not in air fatigue da=dN versus DK plots.
Fig. 2. da=dt versus Kmax with R values and frequencies shown, alloy 600 under cyclic loading in air.
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2.2. Data for cracking of alloy 600 obtained in water environments EAC data for alloy 600 under cyclic loading exists for tests in OPW containing up to 8 ppm dissolved oxygen, mainly at 288°C, and in simulated PWR primary water at 320°C or 325°C [7±14]. The data are plotted as da=dN versus DK and da=dt versus DK in Figs. 3 and 4 respectively. In Fig. 3, the average low frequency air line and the scatter band for air fatigue data are also shown. Stress corrosion data from Refs. [17±24] relating to tests at various temperatures and with a range of material conditions, span about three orders of magnitude at most K values. Because SCC of alloy 600 is a thermally activated process, much of the data can be normalised to 300°C by assuming an apparent activation energy, from the wide range of reported data [25±33], of 180 kJ/mol. An Arrhenius relationship using this value yields the plot shown in Fig. 5, where the 149°C and 209°C data have been omitted because they do not extrapolate sensibly using the above activation energy. The data that were obtained in hydrogen/steam environments at temperatures in the range 360±420°C extrapolate to very low crack growth rates at 300°C unless a lower activation energy of 90 kJ/mol is used for those data. The Scott and Le Calvar curve for 300°C has been included in Fig. 5. It follows the general trend of the data set and is close to an upper bound line for all data. Alternatively, the SCC data in PWR and OPW environments can be least squares ®tted to the equation da 3:0 10 dt
11
K 1:93
Fig. 3. da=dN versus DK, alloy 600 under cyclic loading in PWR and OPW, from Refs. [7±15]. Also shown is the average low frequency air line, and the scatter band for air fatigue data as dashed lines.
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Fig. 4. da=dt versus DK with frequencies shown, alloy 600 under cyclic loading in PWR and OPW, from Refs. [7±15].
Fig. 5. da=dt versus K at 300°C, SCC of alloy 600 in PWR, OPW and hydrogen/steam, data adjusted by assuming an apparent activation energy of 180 kJ/mol for the water environments except for low temperature data and of 90 kJ/mol for the hydrogen/steam environment.
albeit with considerable scatter in the data presumably in part due to dierences in cold worked state of the starting materials and variations in microstructural condition. However, the data are useful in helping to establishment of the role, if any, of SCC in corrosion fatigue crack growth.
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Fig. 6. Comparison of cyclic loading EAC data and SCC data, alloy 600 in PWR and OPW.
The cyclic loading crack growth rate data converted to da=dt using the relationship da=dt mda=dN are plotted together with the SCC data in Fig. 6. Despite the scatter in the data, it is clear that the corrosion fatigue data lie mainly above the SCC data, especially at the lower K values, because of a large contribution from mechanically induced growth in the cyclic loading tests. The published air fatigue data are from tests performed at frequencies either 6 1 or P 10 Hz and the separate sets can be ®tted to the equations da 4 1:2 10 9 Kmax dt
for m P 10 Hz;
and da 4:3 10 dt
12
4 Kmax
for m 6 1 Hz:
respectively, as shown in Fig. 2. These two equations can be used to test a simple superposition law for corrosion fatigue of the form da da da dt cyclic loading EAC dt SCC dt fatigue in air and this has been done for the high and low frequency cases in Fig. 7. This graph illustrates that, in most cases, the SCC crack growth rates were too low to cause any noticeable enhancement of the mechanically induced crack growth rates from the cyclic loading. Signi®cant increases in crack growth rate due to the environment p only occurred for the low frequency tests in the Kmax range from about 10 to 30 MPa m.
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Fig. 7. da=dt versus K, calculated cyclic loading EAC data, alloy 600.
The upper bound corrosion fatigue curve from Fig. 6 is plotted in Fig. 7 along with the average air fatigue curves for low and high frequency loading and the data points in Fig. 7 are the sums of the calculated average da=dt values and the measured SCC da=dt values for the same Kmax . It is appreciated that cracks under cyclic loading will grow by striation formation whereas SCC will mainly exhibit intergranular crack growth but it was considered instructive to compare the relative crack growth rates despite the dierence in mechanisms. The in¯uence of R ratio on the corrosion fatigue data is demonstrated in Figs. 8 and 9 where da=dN is plotted versus DK, for simulated PWR and OPW water data respectively. In both data sets, the expected R dependence of corrosion fatigue data is apparent, i.e. that enhancement increases with R. However, some of the data lie below the average low frequency air line but do lie within the wide scatter band for the air fatigue data set plotted in Fig. 1.
3. Discussion The aims of the present review were to establish whether or not a signi®cant corrosion fatigue enhancement existed for alloy 600 specimens fatigued in simulated nuclear cooling waters. The extent of corrosion fatigue enhancement can be illustrated by drawing upper bound lines for all data in both PWR and OPW environments, as shown in Fig. 10, compared to the average air fatigue crack growth rate line and an upper bound line for all data from Fig. 1. Signi®cant environmental enhancement of crack growth exist compared to the average air line and a smaller but signi®cant corrosion fatigue enhancement can still be seen if the upper bound
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Fig. 8. da=dN versus DK, alloy 600 under cyclic loading in PWR primary side water.
Fig. 9. da=dN versus DK, alloy 600 under cyclic loading in OPW.
lines for the corrosion fatigue data are compared with the upper bound line for all of the air fatigue data. Plotting data as da=dN versus DK gives no information about the eect of frequency of loading on the amount of corrosion assisted enhancement of crack growth rates. However, plotting da=dtenvironment versus da=dtair , as suggested by Shoji and Takahashi [34], includes the frequency eect and such plots are shown in Fig. 11. This diagram indicates that the enhancements are larger and extend over a wider range of frequencies in OPW than in PWR water conditions.
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Fig. 10. da=dN versus DK upper bound trend lines for corrosion fatigue of alloy 600, including constant maximum K data.
Fig. 11. da=dt in environment versus da=dt in air for cyclic loading tests in PWR primary side water and OPW.
The da=dN versus DK data is both R and cyclic frequency dependent, as shown in Figs. 3, 8 and 9. It is dicult to separate the eects of R, cyclic frequency and temperature in the limited data that are available, but the trends are similar to those shown for corrosion fatigue data for reactor pressure vessel (RPV) steels [34]. However, for RPV steels the air fatigue crack growth rates have been quite fully investigated whereas for alloy 600 the data are quite sparse and show a lot of scatter.
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Fig. 12. da=dN versus DK, low frequency R < 0:3, alloy 600 under cyclic loading in OPW.
Thus, the time domain representations shown in Fig. 11 are limited by the uncertainty of the true air fatigue rate for a given DK. Errors in that calculation can have a large eect on the apparent enhancement due to the environment at a given crack tip strain rate. For instance, if the low R value R 6 0:3 cyclic loading data in OPW water is plotted and compared with the average low frequency air line that was used to generate the time domain plots, Fig. 12, it can be seen that many data lie below the air crack growth rate line. This suggests that the air crack growth rates have been overestimated at low DK values by using the average low frequency air curve equation because the cyclic loading data obtained in the water environments should not be less than those in air under equivalent loading conditions. Thus, if cyclic loading of alloy 600 in PWR and OPW environments is perceived as of some importance, it seems necessary to establish a well validated air fatigue reference curve in addition to conducting further cyclic loading experiments in the water environments. With respect to the generation of further cyclic loading data, it appears that the roles of frequency and R value need to be explored, as was found necessary for RPV steels. Turning now to the R dependence shown by the data in Figs. 8 and 9, it is possible that this is due to closure eects on DK. At high R the crack enclave is essentially open to the environment throughout the loading cycle and dissolution at the crack tip can occur throughout the stressing cycle whereas at low R crack closure and eective pumping of the solution from the crack enclave might occur. An eective DK can be calculated using an Elber [15] type approach and such data are plotted in Fig. 13 for the OPW and in Fig. 14 for the PWR data respectively using an eective DK obtained from the equation DKeffective DK 0:35 0:65R
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Fig. 13. da=dN versus eective DK, alloy 600 under cyclic loading in OPW.
Fig. 14. da=dN versus eective DK, alloy 600 under cyclic loading in PWR primary side water.
that ensures that DKeffective is similar in magnitude to DK for high R values but signi®cantly smaller than DK for low R values. Using an eective DK groups the OPW data quite closely together but has less eect on the PWR data. However, there was less PWR than OPW data in the data base. The fact that the cyclic loading data obtained in the environments tend to show plateau regions in which da=dN is relatively constant with increasing DK also implies that the speci®c environment in which the tests are performed will in¯uence da=dN . This is apparent because the enhancements in growth appear larger for OPW than
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PWR conditions despite the fact that the tests in PWR waters were carried out at slightly higher temperatures than those in OPW. This again parallels the situation for RPV steels where the electrode potential achieved by the steel, as in¯uenced by the oxygen content of the water, aects the crack growth rate. The mechanism for EAC for alloy 600 is not the same as that for RPV steels but the existing data suggests that water chemistry, electrode potential, loading waveform, hold times etc. may be factors that will need to be considered in the generation of further cyclic loading data for alloy 600. Fig. 11 gives some guidance to the eects of cyclic loading on crack initiation but the onset of enhancement shown there is related to the, at present, uncertain air fatigue crack growth rate equation used to calculate the equivalent air growth rate for a set of corrosion fatigue loading conditions. Also, these data refer to growth rates from specimens having pre-existing fatigue cracks and do not address the question of whether or not cracks will initiate more readily in these environments at smooth surfaces under fatigue loading than under static loading. Thus, it would be useful to establish an air fatigue design line for smooth specimens and a suitably factored corrosion fatigue line in the form of conventional S±N (stress versus number of cycles to fail) plots in any future work. 4. Conclusions 1. The available cyclic loading data in the open literature suggests that alloy 600 shows enhanced crack growth rates relative to those in air when tested in PWR primary coolant water and in OPW. 2. The enhancements are larger in OPW than in PWR primary side water. 3. The data show the characteristic tendencies for corrosion fatigue, i.e. a dependence upon frequency of loading and R ratio exists and the da=dN versus DK plots have a tendency to show plateau regions where da=dN remains relatively constant with increasing DK. 4. If suitable design curves need to be developed, which depends upon the loading to which components made from alloy 600 are exposed, the current data base on corrosion fatigue of alloy 600 will need to be extended, taking due account of the eects of frequency of loading, R ratio, cyclic wave form, environment and temperature. 5. It would seem advantageous to also generate a data base for air and corrosion fatigue S±N data from smooth surface specimens, taking due regard of the various parameters mentioned above. Acknowledgements The authors would like to acknowledge Dr. G.P. Airey for his helpful comments. This research was funded by British Energy on behalf of the Industry Management Committee (IMC).
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References [1] P. Scott, M. Le Calvar, Some possible mechanisms of intergranular stress corrosion cracking of alloy 600 in PWR primary water, in: R.E. Gold, E.P. Simonen (Eds.), Proceedings of the Sixth International Symposium on Environmental Degradation of Materials in Nuclear Power Systems±± Water Reactors, San Diego, CA, 1±5 August, 1993, pp. 657±667. [2] C.H. Shen, P.G. Shewmon, Met. Trans. A 21A (5) (1990) 1261. [3] M.O. Speidel, Fatigue crack growth at high temperature, in: P.R. Sahm, M.O. Speidel (Eds.), High Temperature Materials in Gas Turbines, Elsevier, Amsterdam, 1974, pp. 207±251. [4] L.A. James, Int. J. Pressure Vessels Piping 5 (4) (1977) 241. [5] P. Shahinian, Met. Technol. 5 (11) (1978) 372. [6] G.S. Was, H.H. Tischner, R.M. Latanision, R.M. Pelloux, Met. Trans. A 12 (8) (1981) 1409. [7] C. Amzallag, G. Baudry, J.L. Bernard, Eects of PWR environment on the fatigue crack growth of dierent stainless steels and inconel type alloys, Proceedings of The International Atomic Energy Agency Specialists' Meeting on Subcritical Crack Growth, NUREG/CP-0044, vol. 1, 1983, pp. 263± 294. [8] G. Nakao, H. Yamasaki, S. Kimura, Fatigue crack propagation behaviour of alloy 600 and 690 in high temperature pressurised water, International Cyclic Crack Growth Committee (ICCGR)Japanese Committee on Fatigue (JCF) Joint Meeting, JAERI sponsored, Mito, Japan, May 1985. [9] H. Nagano, K. Tokimasa, K. Tanaka, H. Tsuge, Trans. Iron Steel Inst. Jpn. 28 (4) (1988) 338. [10] R.G. Ballinger, R.M. Latanision, W.C. Moshier, R.M. Pelloux, The Eect of Heat Treatment and Environment on Corrosion Fatigue±±Volume 2: Alloy 600, EPRI TR-102436, 1993. [11] Per Lidar, Aspects of crack growth in structural materials in light water reactors, Ph.D. Thesis, Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, Sweden, 1997. [12] D.A. Hale, C.W. Jewett, J.N. Kass, J. Engng. Mater. Technol. 101 (7) (1979) 191. [13] P.L. Andresen, Corrosion 47 (12) (1991) 917. [14] L.A. James, W.J. Mills, Fatigue-crack propagation behaviour of wrought alloy 600 and welddeposited EN82H in an elevated temperature aqueous environment, PVP-Volume 303, Service Experience, Structural Integrity, Severe Accidents, and Erosion in Nuclear and Fossil Plants, ASME 1995, pp. 21±36. [15] W. Elber, Damage tolerance in aircraft structures, ASTM STP 486, Philadelphia, 1977, pp. 230±241. [16] J. Shrive, The stress ratio eect on fatigue crack growth in 2024-T3 Alclad and the relation to crack closure, Delft University of Technology, Department of Aerospace Engineering Memorandum M336, August 1979. [17] D.A. Hale, J. Engng. Mater. Technol. 108 (1) (1986) 44. [18] R.B. Rebak, A.R. McIlree, Z. Szklarska-Smialowska, Eects of pH and stress intensity on crack growth rate in alloy 600 in lithiated borated water at high temperatures, in: D. Cubicciotti, E.P. Simonen, R. Gold (Eds.), Proceedings of the Fifth International Symposium on Environmental Degradation of Materials in Nuclear Power Systems±±Water Reactors, Monterey, CA, 25±29 August 1991, pp. 511±517. [19] T.B. Cassagne, A. Gelpi, Crack growth rate measurements on alloy 600 steam generator tubes in steam and primary water, in: D. Cubicciotti, E.P. Simonen, R. Gold (Eds.), Proceedings of the Fifth International Symposium on Environmental Degradation of Materials in Nuclear Power Systems±± Water Reactors, Monterey, CA, 25±29 August 1991, pp. 518±524. [20] R.G. Lott, R.J. Jacko, R.E. Gold, Primary water stress corrosion crack growth rates in alloy 600 steam generator tubing, in: D. Cubicciotti, E.P. Simonen, R. Gold (Eds.), Proceedings of the Fifth International Symposium on Environmental Degradation of Materials in Nuclear Power Systems±± Water Reactors, Monterey, CA, 25±29 August 1991, pp. 525±532. [21] Y. Shen, P.G. Shewmon, Corrosion 47 (9) (1991) 712. [22] T. Cassagne, A. Gelpi, Crack growth rate measurements on alloy 600 steam generator tubing in primary and hydrogenated water, in: R.E. Gold, E.P. Simonen (Eds.), Proceedings of the Sixth
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[23] [24] [25] [26] [27]
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International Symposium on Environmental Degradation of Materials in Nuclear Power Systems±± Water Reactors, San Diego, CA, 1±5 August 1993, pp. 679±686. W.H. Bamford, J.P. Foster, G.V. Rao, Crack growth and microstructural characterisation of alloy 600 head penetration materials, WCAP 13929, Westinghouse Energy Systems, October 1993. R.B. Rebak, Z. Szklarska-Smialowska, Corrosion 50 (5) (1994) 378. D. van Rooyen, T. S. Bulischeck, Stress corrosion cracking of inconel 600 tubing in deaerated high temperature water, Third South African Corrosion Conference, March 1980, pp. 18.1±18.31. T.S. Bulischeck, D. van Rooyen, Corrosion 37 (10) (1981) 597. R. Bandy, D. van Rooyen, Initiation and propagation of stress corrosion cracking of alloy 600 in high temperature water, Proceedings of International Symposium on Environmental Degradation of Materials in Nuclear Power Systems±±Water Reactors, Myrtle Beach, SC, 22±25 August, 1983, pp. 763±774. R. Bandy, D. van Rooyen, Corrosion 40 (8) (1984) 425. U.C. Kim, D. van Rooyen, The stress corrosion cracking of inconel 600 steam generator tubing in primary water and related conditions, private communication, October 1985. J. Congleton, R.N. Parkins, B. Hemsworth, Nucl. Engng. Design 103 (3) (1987) 301. T. Yonezawa, N. Sasaguri, K. Onimura, Eects of metallurgical factors on stress corrosion cracking of nickel-base alloys in high temperature water, Proceedings of 1988 JAIF International Conference on Water Chemistry in Nuclear Power Plants, vol. 2, 1988, pp. 490±495. J.M. Boursier, O de Bouvier, J.M. Gras, D. Noel, R. Rios, F. Vaillant, SCC of alloy 600 in high temperature water: a study of mechanisms, in: T. Magnin, J.M. Gras (Eds.), Corrosion, Deformation and Interaction Conference, CDI'92, Fontainbleau, France, October 1992. R.B. Rebak, Z. Xia, Z. Szklarska-Smialowska, Corrosion 51 (9) (1995) 689. T. Shoji, H. Takahashi, M. Suzuki, T. Kondo, J. Engng. Mater. Technol. Trans. ASME 103 (4) (1981) 298.
www.elsevier.com/locate/corsci
Review on eect of cyclic loading on environmental assisted cracking of alloy 600 in typical nuclear coolant waters J. Congleton, E.A. Charles *, G. Sui Corrosion Research Centre, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK Received 1 September 2000; accepted 11 January 2001
Abstract Corrosion fatigue data for alloy 600 in simulated primary side, pressurised water reactor (PWR) coolant and in oxygenated pure water (OPW) have been reviewed. Higher crack growth rates occur in these environments than in air and the former show a dependence upon frequency of loading, m, and R ratio. The da=dN versus DK plots have a tendency to show plateau regions where da=dN remains constant with increasing DK. The enhancements of crack growth over those in air due to cyclic loading are larger in OPW than in simulated PWR primary side water. If corrosion fatigue of alloy 600 was perceived of importance for reactor components, the existing data base will need to be extended and it would be necessary to generate more air fatigue data than is currently available. Conventional S±N fatigue and corrosion fatigue data from smooth surface specimens may also be required, collected taking due regard of the possible eects of R and m mentioned above. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Alloy; Nickel; Corrosion fatigue; Stress corrosion
1. Introduction Environmental assisted cracking (EAC) of alloy 600 in primary and secondary side waters of pressurised water reactors (PWR) and in oxygenated pure water (OPW) could in¯uence the life-span of nuclear reactor components. This has been
*
Corresponding author. Tel.: +44-0191-222-6000; fax: +44-0191-222-7153. E-mail address: [email protected] (E.A. Charles).
0010-938X/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 1 ) 0 0 0 2 3 - 3
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known for more than 20 years and an extensive set of data pertaining to such cracking has been generated. However, few data exist concerning crack growth rates likely to be observed if cyclic loading is imposed in addition to the environments. The present review summarises published data available in the open literature on the combined eects of cyclic loading and environments on cracking of alloy 600 in various nuclear coolant systems and addresses the need for obtaining further cyclic loading data for EAC of alloy 600. There are two useful benchmarks for assessing the cyclic loading data. First, a reference data line for air fatigue is needed to assess the eect of environments on the rate of cracking. Second, a comparable reference curve for stress corrosion cracking (SCC), i.e. EAC under static load conditions, is required. With regard to SCC, there is a large amount of scatter in the available crack growth rate data, so a theoretically based model for the SCC crack growth in alloy 600 in the relevant environments would be helpful. Such a model has been proposed by Scott an Le Calvar [1] who considered three possible alternative mechanisms for cracking, namely internal oxidation leading to bubble formation on grain boundaries, internal oxidation to give a brittle intergranular region and embrittlement by oxygen adsorption on grain boundaries. Although gas bubble formation leading to a weakening of the grain boundaries is unlikely to be the relevant mechanism for intergranular cracking of alloy 600, their model based on this premise can be used to generate an equation that ®ts the available SCC data satisfactorily. Scott and Le Calvar assumed that crack growth was a stress driven mechanism that depended upon the size of the Ôprocess zoneÕ at the crack tip. A time for unstable growth of bubbles, that is inversely proportional to the square of the applied stress but proportional to the square root of a diusion depth, was combined with an estimate of 2 the plastic zone size at a crack tip, taken as proportional K=rp . The critical diffusion depth was chosen as a process zone size at the crack tip, set as a fraction of the plane strain plastic zone size for which the outer limit contour is a stress rp . A similar equation could be generated for any diusion related mechanism that showed a p 2 x=r dependence. Bubble formation could be replaced by oxide crystallites formed on the grain boundaries [2] or by grain boundary decohesion occurring intermittently with a stress dependent increment of growth occurring at a frequency controlled by diusion of oxygen into the active grain boundary. The Scott and Le Calvar equation is dx dt
81kTD0 512c3 a2
s dNS rp KI 6pz
where k is the Boltzmann constant, T the absolute temperature, D0 the grain boundary diusion coecient for oxygen, NS the surface solubility of oxygen, c the surface energy of nickel, a the interatomic spacing, z the number of sites explored per gas atom jump, rp a parameter that is related to the yield strength of the material and KI is the applied crack tip mode I stress intensity factor.
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2. Data survey Among more than 200 papers on cracking of alloy 600 reviewed, 12 papers [3±14] contained corrosion fatigue data. These 12 papers provided 380 records, each containing a crack growth rate and an associated DK, and in many cases the R ratio (minimum stress/maximum stress), loading frequency m, test temperature, environmental conditions and details relating to the source of the information. 2.1. Air fatigue data for alloy 600 The data base for fatigue crack growth of alloy 600 in air from pre-cracked specimens, consisting of 152 records taken from Refs. [3±11], were obtained in two cyclic frequency ranges, speci®cally from 0.0167 to 1 Hz and from 10 to 16.67 Hz, at R values from 0.05 to 0.85 and at temperatures from 25°C to 320°C. The da=dN versus DK data obtained at temperatures from 288°C to 320°C showed no noticeable temperature eects and are presented collectively in Fig. 1 as the open square symbols, while the room temperature data are shown as the full symbols. There is scatter in the data. No signi®cant reduction of the scatter results if da=dN , is plotted against an eective DK by correcting for R eects using either the Elberpor Shrive relationships [15,16]. There appears to be a threshold DK of 4.5 MPa m for fatigue in air at room temperature but insucient data was available to decide upon the existence or otherwise of a threshold DK for the tests at the high temperatures. As corrosion fatigue tends to occur only under relatively low frequency loading conditions, we have chosen a reference air curve by least squares ®tting the low
Fig. 1. da=dN versus DK with the average low frequency trend line shown, alloy 600 under cyclic loading in air.
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frequency ( 6 1 Hz) data only, but ignoring points near the threshold DK. This generated the equation: da 3:0 10 9 DK 3:14 dN i.e. a linear relationship between log da=dN and log DK. If in contrast to the above, corrosion fatigue curves show plateau regions where da=dN remains essentially constant as DK increases, it is possible that a stress corrosion component of growth is dominating da=dN at these DK values. To consider this possibility it is useful to convert the air fatigue data to da=dt versus Kmax , as shown in Fig. 2, because stress corrosion crack growth rates which are dominated by Kmax . The spread in crack growth rates displayed in Fig. 2 is mainly due to the eect of cyclic frequency, m, because da=dt was calculated using da da m dt dN For instance, the data for R 0:05 at 25°C shows that a tenfold increase in frequency generates a tenfold increase in crack growth rate. Corrosion fatigue is occurring if cyclic loading tests in a water environment generate higher crack growth rates than tests with equivalent loading conditions performed in air. The expected dierences between corrosion fatigue data and air fatigue data are that for corrosion fatigue, m, the R ratio and the wave form should all aect da=dN whereas there is likely to be only small eects from these variables in air fatigue, as indicated in Fig. 1. Also, plateau regions often exist at intermediate DK values in corrosion fatigue but not in air fatigue da=dN versus DK plots.
Fig. 2. da=dt versus Kmax with R values and frequencies shown, alloy 600 under cyclic loading in air.
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2.2. Data for cracking of alloy 600 obtained in water environments EAC data for alloy 600 under cyclic loading exists for tests in OPW containing up to 8 ppm dissolved oxygen, mainly at 288°C, and in simulated PWR primary water at 320°C or 325°C [7±14]. The data are plotted as da=dN versus DK and da=dt versus DK in Figs. 3 and 4 respectively. In Fig. 3, the average low frequency air line and the scatter band for air fatigue data are also shown. Stress corrosion data from Refs. [17±24] relating to tests at various temperatures and with a range of material conditions, span about three orders of magnitude at most K values. Because SCC of alloy 600 is a thermally activated process, much of the data can be normalised to 300°C by assuming an apparent activation energy, from the wide range of reported data [25±33], of 180 kJ/mol. An Arrhenius relationship using this value yields the plot shown in Fig. 5, where the 149°C and 209°C data have been omitted because they do not extrapolate sensibly using the above activation energy. The data that were obtained in hydrogen/steam environments at temperatures in the range 360±420°C extrapolate to very low crack growth rates at 300°C unless a lower activation energy of 90 kJ/mol is used for those data. The Scott and Le Calvar curve for 300°C has been included in Fig. 5. It follows the general trend of the data set and is close to an upper bound line for all data. Alternatively, the SCC data in PWR and OPW environments can be least squares ®tted to the equation da 3:0 10 dt
11
K 1:93
Fig. 3. da=dN versus DK, alloy 600 under cyclic loading in PWR and OPW, from Refs. [7±15]. Also shown is the average low frequency air line, and the scatter band for air fatigue data as dashed lines.
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Fig. 4. da=dt versus DK with frequencies shown, alloy 600 under cyclic loading in PWR and OPW, from Refs. [7±15].
Fig. 5. da=dt versus K at 300°C, SCC of alloy 600 in PWR, OPW and hydrogen/steam, data adjusted by assuming an apparent activation energy of 180 kJ/mol for the water environments except for low temperature data and of 90 kJ/mol for the hydrogen/steam environment.
albeit with considerable scatter in the data presumably in part due to dierences in cold worked state of the starting materials and variations in microstructural condition. However, the data are useful in helping to establishment of the role, if any, of SCC in corrosion fatigue crack growth.
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Fig. 6. Comparison of cyclic loading EAC data and SCC data, alloy 600 in PWR and OPW.
The cyclic loading crack growth rate data converted to da=dt using the relationship da=dt mda=dN are plotted together with the SCC data in Fig. 6. Despite the scatter in the data, it is clear that the corrosion fatigue data lie mainly above the SCC data, especially at the lower K values, because of a large contribution from mechanically induced growth in the cyclic loading tests. The published air fatigue data are from tests performed at frequencies either 6 1 or P 10 Hz and the separate sets can be ®tted to the equations da 4 1:2 10 9 Kmax dt
for m P 10 Hz;
and da 4:3 10 dt
12
4 Kmax
for m 6 1 Hz:
respectively, as shown in Fig. 2. These two equations can be used to test a simple superposition law for corrosion fatigue of the form da da da dt cyclic loading EAC dt SCC dt fatigue in air and this has been done for the high and low frequency cases in Fig. 7. This graph illustrates that, in most cases, the SCC crack growth rates were too low to cause any noticeable enhancement of the mechanically induced crack growth rates from the cyclic loading. Signi®cant increases in crack growth rate due to the environment p only occurred for the low frequency tests in the Kmax range from about 10 to 30 MPa m.
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Fig. 7. da=dt versus K, calculated cyclic loading EAC data, alloy 600.
The upper bound corrosion fatigue curve from Fig. 6 is plotted in Fig. 7 along with the average air fatigue curves for low and high frequency loading and the data points in Fig. 7 are the sums of the calculated average da=dt values and the measured SCC da=dt values for the same Kmax . It is appreciated that cracks under cyclic loading will grow by striation formation whereas SCC will mainly exhibit intergranular crack growth but it was considered instructive to compare the relative crack growth rates despite the dierence in mechanisms. The in¯uence of R ratio on the corrosion fatigue data is demonstrated in Figs. 8 and 9 where da=dN is plotted versus DK, for simulated PWR and OPW water data respectively. In both data sets, the expected R dependence of corrosion fatigue data is apparent, i.e. that enhancement increases with R. However, some of the data lie below the average low frequency air line but do lie within the wide scatter band for the air fatigue data set plotted in Fig. 1.
3. Discussion The aims of the present review were to establish whether or not a signi®cant corrosion fatigue enhancement existed for alloy 600 specimens fatigued in simulated nuclear cooling waters. The extent of corrosion fatigue enhancement can be illustrated by drawing upper bound lines for all data in both PWR and OPW environments, as shown in Fig. 10, compared to the average air fatigue crack growth rate line and an upper bound line for all data from Fig. 1. Signi®cant environmental enhancement of crack growth exist compared to the average air line and a smaller but signi®cant corrosion fatigue enhancement can still be seen if the upper bound
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Fig. 8. da=dN versus DK, alloy 600 under cyclic loading in PWR primary side water.
Fig. 9. da=dN versus DK, alloy 600 under cyclic loading in OPW.
lines for the corrosion fatigue data are compared with the upper bound line for all of the air fatigue data. Plotting data as da=dN versus DK gives no information about the eect of frequency of loading on the amount of corrosion assisted enhancement of crack growth rates. However, plotting da=dtenvironment versus da=dtair , as suggested by Shoji and Takahashi [34], includes the frequency eect and such plots are shown in Fig. 11. This diagram indicates that the enhancements are larger and extend over a wider range of frequencies in OPW than in PWR water conditions.
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Fig. 10. da=dN versus DK upper bound trend lines for corrosion fatigue of alloy 600, including constant maximum K data.
Fig. 11. da=dt in environment versus da=dt in air for cyclic loading tests in PWR primary side water and OPW.
The da=dN versus DK data is both R and cyclic frequency dependent, as shown in Figs. 3, 8 and 9. It is dicult to separate the eects of R, cyclic frequency and temperature in the limited data that are available, but the trends are similar to those shown for corrosion fatigue data for reactor pressure vessel (RPV) steels [34]. However, for RPV steels the air fatigue crack growth rates have been quite fully investigated whereas for alloy 600 the data are quite sparse and show a lot of scatter.
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Fig. 12. da=dN versus DK, low frequency R < 0:3, alloy 600 under cyclic loading in OPW.
Thus, the time domain representations shown in Fig. 11 are limited by the uncertainty of the true air fatigue rate for a given DK. Errors in that calculation can have a large eect on the apparent enhancement due to the environment at a given crack tip strain rate. For instance, if the low R value R 6 0:3 cyclic loading data in OPW water is plotted and compared with the average low frequency air line that was used to generate the time domain plots, Fig. 12, it can be seen that many data lie below the air crack growth rate line. This suggests that the air crack growth rates have been overestimated at low DK values by using the average low frequency air curve equation because the cyclic loading data obtained in the water environments should not be less than those in air under equivalent loading conditions. Thus, if cyclic loading of alloy 600 in PWR and OPW environments is perceived as of some importance, it seems necessary to establish a well validated air fatigue reference curve in addition to conducting further cyclic loading experiments in the water environments. With respect to the generation of further cyclic loading data, it appears that the roles of frequency and R value need to be explored, as was found necessary for RPV steels. Turning now to the R dependence shown by the data in Figs. 8 and 9, it is possible that this is due to closure eects on DK. At high R the crack enclave is essentially open to the environment throughout the loading cycle and dissolution at the crack tip can occur throughout the stressing cycle whereas at low R crack closure and eective pumping of the solution from the crack enclave might occur. An eective DK can be calculated using an Elber [15] type approach and such data are plotted in Fig. 13 for the OPW and in Fig. 14 for the PWR data respectively using an eective DK obtained from the equation DKeffective DK 0:35 0:65R
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Fig. 13. da=dN versus eective DK, alloy 600 under cyclic loading in OPW.
Fig. 14. da=dN versus eective DK, alloy 600 under cyclic loading in PWR primary side water.
that ensures that DKeffective is similar in magnitude to DK for high R values but signi®cantly smaller than DK for low R values. Using an eective DK groups the OPW data quite closely together but has less eect on the PWR data. However, there was less PWR than OPW data in the data base. The fact that the cyclic loading data obtained in the environments tend to show plateau regions in which da=dN is relatively constant with increasing DK also implies that the speci®c environment in which the tests are performed will in¯uence da=dN . This is apparent because the enhancements in growth appear larger for OPW than
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PWR conditions despite the fact that the tests in PWR waters were carried out at slightly higher temperatures than those in OPW. This again parallels the situation for RPV steels where the electrode potential achieved by the steel, as in¯uenced by the oxygen content of the water, aects the crack growth rate. The mechanism for EAC for alloy 600 is not the same as that for RPV steels but the existing data suggests that water chemistry, electrode potential, loading waveform, hold times etc. may be factors that will need to be considered in the generation of further cyclic loading data for alloy 600. Fig. 11 gives some guidance to the eects of cyclic loading on crack initiation but the onset of enhancement shown there is related to the, at present, uncertain air fatigue crack growth rate equation used to calculate the equivalent air growth rate for a set of corrosion fatigue loading conditions. Also, these data refer to growth rates from specimens having pre-existing fatigue cracks and do not address the question of whether or not cracks will initiate more readily in these environments at smooth surfaces under fatigue loading than under static loading. Thus, it would be useful to establish an air fatigue design line for smooth specimens and a suitably factored corrosion fatigue line in the form of conventional S±N (stress versus number of cycles to fail) plots in any future work. 4. Conclusions 1. The available cyclic loading data in the open literature suggests that alloy 600 shows enhanced crack growth rates relative to those in air when tested in PWR primary coolant water and in OPW. 2. The enhancements are larger in OPW than in PWR primary side water. 3. The data show the characteristic tendencies for corrosion fatigue, i.e. a dependence upon frequency of loading and R ratio exists and the da=dN versus DK plots have a tendency to show plateau regions where da=dN remains relatively constant with increasing DK. 4. If suitable design curves need to be developed, which depends upon the loading to which components made from alloy 600 are exposed, the current data base on corrosion fatigue of alloy 600 will need to be extended, taking due account of the eects of frequency of loading, R ratio, cyclic wave form, environment and temperature. 5. It would seem advantageous to also generate a data base for air and corrosion fatigue S±N data from smooth surface specimens, taking due regard of the various parameters mentioned above. Acknowledgements The authors would like to acknowledge Dr. G.P. Airey for his helpful comments. This research was funded by British Energy on behalf of the Industry Management Committee (IMC).
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