protective oxides are produced in the alloy 600 cracks and attacked boundaries. Cracks are generally quite tight (tip openings down to a few nm) in both alloys, but the 316SS samples reveal a high dislocation density at all crack tips suggesting that plasticity is an essential part of the SCC process. This work has demonstrated the ability of ATEM to reveal new and important details of buried corrosion and SCC structures that cannot be detected by other methods. However, the current observations must be considered work in progress and additional work is needed to properly establish mechanisms controlling IGSCC. In particular, ATEM characterization of degradation structures must be performed on samples where mechanisms are better distinguished. Tests in high-temperature aqueous and gaseous environments should be performed under well-controlled solution chemistries and electrochemical conditions to establish a library of the corrosion signatures. CONCLUSIONS Cross-sectional ATEM has been used to effectively characterize corrosion and cracking in hightemperature water or steam environments. A wide variety of high-resolution imaging and analysis methods are employed to elucidate processes occurring during crack advance and provides insights into the mechanisms controlling environmental degradation. Fundamental differences are detected between corrosion structures in Fe-base and Ni-base stainless alloys. Deeply attacked grain boundaries off the main cracks are found in alloy 600 samples indicating a major role of IG corrosion in the SCC process. Corroded boundaries were filled with nanocrystalline oxides to the leading edges of attack. Precipitates in alloy 600 are also attacked and can be used to identify the local electrochemistry promoting degradation in complex service environments. Solution impurities such as Pb are found in high concentrations at nm-width reaction zones in samples from secondary-water environments documenting water access at leading edges of the attack and indicating how impurities influence the corrosion processes. Results presented demonstrate the ability of cross-sectional ATEM to reveal new details of buried corrosion structures that cannot be detected by other methods. ACKNOWLEDGEMENTS The technical assistance of C. E. Chamberlin and V. Y. Gertsman are recognized along with helpful discussions with P. M. Scott. Primary support comes from the Materials Sciences Branch, Office of Basic Energy Sciences, U.S. Department of Energy (DOE) under contract DE-AC06-76RLO 1830 with Battelle Memorial Institute. Additional support is acknowledged from EPRI, the B&W Owners Group Chemistry Committee and the DOE Office of Nuclear Energy, Science and Technology. REFERENCES 1. Lewis, N., Perry, D.J. and Bunch, M.L. (1995) In: Proc. Microscopy and Microanalysis ed., Bailey, G.W. et al., Jones and Begnell Publishing, New York, 550. 2. Lewis, N., Attanasio, S., Morton, D.S. and Young, G.A. (2001) In: Proc. Staehle Symposium on Chemistry and Electrochemistry of Corrosion and Stress Corrosion, ed. Jones, R.H. TMS, p. 421. 3. Thomas, L.E., Charlot, L.A. and Bruemmer, S.M. (1996) In: Proc. New Techniques for Characterizing Corrosion and Stress Corrosion, Eds. Jones, R.H. and Baer, D.R. TMS, p. 175. 4. Thomas, L.E. and Bruemmer, S.M. (2000) Corrosion J., 56, 572. 5. Thomas, L.E. and Bruemmer, S.M. (2000) In: Proc. 9th Int. Conf. Environmental Degradation of Materials in Nuclear Power Systems-Water Reactors, ed. Bruemmer, S., Ford, F.P., TMS, p. 41. 6. Bruemmer, S.M. and Thomas, L.E. (2001) In: Proc. Staehle Symposium on Chemistry and Electrochemistry of Corrosion and Stress Corrosion, ed. Jones, R.H. TMS, p. 123. 7. Bruemmer, S.M. and Thomas, L.E. (2001) J. Surface and Interface Analysis, in press. 8. Robertson, J. (1991) Corrosion Sci., 32-4, 443. 9. Stellwag, B. (1998) Corrosion Sci., 40-2/3 337. 10. Gertsman, V.Y. and Bruemmer, S.M. (2001) Acta Metallurgica, 49, 1589. 11. Karlsson, L. (1988) Acta Metalurgica, 36, 1.
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