ICF100890OR HIGH-RESOLUTION ANALYTICAL ELECTRON MICROSCOPY CHARACTERIZATION OF STRESS CORROSION CRACK TIPS S. M. Bruemmer and L. E. Thomas Pacific Northwest National Laboratory P.O. Box 999, Richland, WA 99352 ABSTRACT Recent results are presented demonstrating the application of cross-sectional analytical transmission electron microscopy (ATEM) to corrosion and cracking in high-temperature water environments. Microstructural, chemical and crystallographic characterizations of buried interfaces at near-atomic resolutions are shown to reveal evidence for unexpected local environments, corrosion reactions and material transformations. Information obtained by high-resolution imaging and analysis indicates the corrosion and deformation processes during crack advance and provides insights into the mechanisms controlling environmental degradation. Examples of intergranular attack and cracking in type 316 austenitic stainless steel and Ni-base alloy 600 are presented to illustrate the value of this approach. The presence of deeply attacked grain boundaries off the main cracks, revealed by TEM, is believed to indicate a major role of active corrosion in the stress-corrosion cracking (SCC) process. Corroded boundaries were filled with oxides to the leading edges of attack. Analyses of the oxide films and impurities in intergranular penetrations and crack tips with widths of 10 nm or less indicate influences of the grain boundary characteristics and the water chemistry. Boundary and precipitate corrosion structures 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 steam-generator secondary-water environments indicating water access at leading edges of the attack and the influence of these impurities on the corrosion processes. Results for specific samples are used to demonstrate the ability of cross-sectional ATEM to reveal new details of buried corrosion structures that cannot be detected by other methods. KEYWORDS Crack tips, stress corrosion cracking, corrosion, intergranular fracture, grain boundary, segregation, corrosion products, passive films, solution impurities, deformation. INTRODUCTION The fundamental basis for mechanistic understanding and modeling of stress corrosion cracking (SCC) remains in question for many systems. Specific mechanisms controlling SCC can vary with changes in alloy characteristics, applied/residual stress or environmental conditions. The local crack electrochemistry, crack-tip mechanics and material metallurgy are the main factors controlling crack growth. These localized properties are difficult or impossible to measure in active cracks. Nevertheless, it is essential to quantitatively interrogate these crack-tip conditions if mechanistic understanding is to be obtained.
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