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Intergranular Stress Corrosion Cracking in Nuclear Power Plants

Intergranuler and Transgranular Stress Corrosion Cracking

Description and Experience
Intergranular stress corrosion cracking (IGSCC) is stress corrosion cracking where the grain boundaries of a susceptible material are cracked due to stress and an agressive environment [1-3]. Transgranular stress corrosion cracking (TGSCC) is the same as IGSCC, but the grain boundaries are not preferentially attacked [1, 2]. Instead, cracking is induced by preferential attack along certain crystallographic planes within grains in metals. TGSCC is brittle in nature, like IGSCC, and usually occurs in the presence of halogens [4]. Such cracking can cause leakage, shutdowns, augmented in service inspection, analysis and repairs, and eventual replacement, depending on the component. A particular case where IGSCC has occurred is in austenitic stainless steels in oxidizing environments, such as the boiling water reactor (BWR) coolant [1]. IGSCC in Type 304 stainless steel piping and in Type 304 and Alloy 600 safe-ends has been a concern [1, 3]. Welding [1] , and shrink-fit [4] residual stresses can be major contributors to the driving force in stress corrosion cracking.

Bolting materials, such as SAE 4340 modifications and Ni-Cr-Fe X-750 are also susceptible to IGSCC and can result in relatively rapid failure at times [5]. Pressurized water reactor (PWR) boron injection tank nozzles, manway liners, boric acid piping and high pressure feedwater heaters [6], as well as PWR steam generator Alloy 600 tubes [7], have suffered IGSCC. There have also been cases of ammonia SCC of copper alloys, and IGSCC of stainless steels in condenser tubes [8].

Material Selection, Treatment and Testinq
Very significant strides can be made to prevent IGSCC and TGSCC through judicious material selection. Considerable testing (3,9] has been performed to justify the use of certain materials in potential IGSCC situations. For example, the low carbon austenitic stainless steels such as 316 Nuclear Grade, 347 Nuclear Grade, and CF3 and CF3M castings have been described [9,10] as being resistant to IGSCC. 304NG, 304L and 316L are also considered [10] adequate. Inconel 82 is the only nickel base weld metal considered to be resistant [10]. Austenitic stainless steels should be solution heat treated and water quenched after welding and hot or cold forming, although this procedure may be mollified when procedures are qualified for the above IGSCC-resistant materials. Fabrication procedure qualifications and material acceptance testing may be in accordance with constant extension rate testing [3] and ASTM A262 practices A and E [11, 12].
Increased resistance to IGSCC may be gained for ferritic bolting and turbine materials by utilizing reduced yield strength (less than approximately 120 ksi minimum specified) where practical[(5] and by utilizing materials produced by high purity melting procedures (13]. Control of bolting lubricants and degreasing to prevent contamination with sulfur and other harmful environments is essential [5].

Operation and Desiqn Limitations
Minimizing applied loads and residual stresses reduces one of the major driving forces for IGSCC [1]. Also, for temperatures below 200°F, IGSCC is not a great concern for austenitic stainless steels in oxidizing water environments [1, 10]. Towards this goal, weld joints should be designed to avoid integral back-up rings or back-up bars which are left in place and can cause a crevice. As mentioned above, cold working, grinding, and bending should be avoided, unless followed by a qualified heat treatment procedure. During plant startup or extended downtime due to repair, replacement or long refueling outages, plant systems cool down and the water becomes quite oxidizing unless the primary and secondary systems are made inert [2]. Also, sulfide ions can cause hydrogen embrittlement in the higher strength steels (eg.bolting), and sulfur-related ions can lead to IGSCC in sensitized austenitic stainless steels even at low temperatures in oxidizing environments [2].

Mitigatinq Actions and Remedies
Numerous mitigating actions and remedies have been proposed and studied with regard to IGSCC and TGSCC. This is especially evident with regard to BWR austenitic stainless steel piping, and many of these remedies may also be applied to other components.

Stress remedies include methods such as heat sink welding (HSW), and mechanical stress improvement [10, 14, 15]. These methods all basically depend on placing the inside diameter of piping in a compressive residual stress state to resist IGSCC Environmental remedies include oxygen control during startup, deaeration, and hydrogen water chemistry methods [10, 14, 15].

Mitigating actions for condenser tubes made of copper base alloys include keeping the tubing clean, optimizing fluid velocity, flushing during outages, automatic cleaning, avoiding polluted water, preventing marine growth intrusions, avoiding scale formation, considering ferrous sulfate, and using resistant materials [8]. With high performance stainless steels and titanium tubes, the primary measure is to prevent overheating [8].

When SCC does occur, there are also other measures which may be taken, including weld overlay repairs [10]. Weld overlays involve the deposition of a corrosion resistant material around the piping outside surface, over the defect location, to provide structural reinforcement [10]. Care must be taken in applying these methods, such that subsequent component life is maximized.

References
1. Klepfer, H. H., “Investigation of Cause of Cracking in Austenitic Stainless Steel Piping” Volume 1, GE, NEDO-21000-1, 75NED35, Class 1, July 1975.
2. “LWR Structural Materials Degradation Mechanisms – Preliminary Assessment of BWR Internals Life Limiting Concerns” Structural Integrity Associates, Draft Report EPRI RP2643-5, Feb. 1986.
3. Hale, D. A., et al., “The Growth and Stability of Stress Corrosion Cracks in Large-Diameter BWR Piping” Volume 2: Appendices, EPRI NP-2472, Vol. 2, July 1982.
4. Lyle, Jr., F. F., “Steam Turbine Disc Cracking Experience” Volumes 1-7, EPRI NP-2429-LD, June 1982.
5. Runtga, R., “Stress Corrosion Cracking of Alternate Bolting Alloys” EPRI RP-2058-12, Draft Final Report, Sept. 1984.
6. Beavers, J. A., et al., “Corrosion-Related Failures in Feedwater Heaters” EPRI CS-3184, July 1983.
7. Greene, S. J. and Paine, J. P. N., Nuclear Technology, Vol. 55, 1981, p. 10.
8. Syrett, B. C., “Prevention of Condenser Failures – The State of the Art” EPRI RD-2282-SR, Mar. 1982.
9. Alexander, J., et al., “Alternative Alloys for BWR Pipe Applications” EPRI NP-2671-LD, Oct. 1982.
10. Hazelton, W. S., “Technical Report on Material Selection andProcessing Guidelines for BWR Coolant Pressure Boundary Piping” Draft Report, U. S. NRC NUREG-0313, Rev. 2, June 1986.
11. Copeland, J. F., and Sayre, E. D., “The Application of Low Carbon Type 316 Stainless Steel for BWR Recirculation Piping Systems” MPC-15 Symposium at the Winter Annual Meeting of ASME, Nov. 16 – 21, 1980, pp. 95 – 105.
12. Clarke, W. L., et al., “Comparative Methods for Measuring Degree of Sensitization in Stainless Steel” ASTM STP 656, 1978, pp. 99 – 132.
13. Viswanathan, R. and Hudak, Jr., S., “The Effect of Impurities and Strength Level on Hydrogen Induced Cracking in a Low Alloy Turbine Steel” Metallurgical Transactions A, Vol. 8A, Oct. 1977, pp. 1633 – 1637.
14. Danko, J. C. and Smith, R. E., “Proceedings: Seminar on Countermeasures for Pipe Cracking in BWRs” Vol. 1 – 4, EPRI WS-79-174, May 1980.
15- Hughes, N. R. and Giannuzzi, A. J., “Evaluation of Near-Term BWR Piping Remedies” Vol. 1 and 2, EPRI NP-1222, Nov. 1979.

 

O’Donnell Consulting Engineers can assist you identify and mitigate Stress Corrosion Cracking, keeping your plant operating efficiently and cost-effectively. See Stress Corrosion Cracking Mitigation.

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