High-resolution analytical electron microscopy characterization of corrosion and cracking at buried interfaces

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 characterization of buried interfaces at near-atomic resolutions is shown to reveal evidence for unexpected local environments, corrosion reactions and material transformations. Information obtained by a wide variety of high-resolution imaging and analysis methods indicates the processes occurring 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-based alloy 600 are presented to illustrate the potential for 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 the narrow intergranular penetrations and crack tips with widths of 10 nm or less indicate influences of the grain boundary characteristics and 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 lead are found in high concentrations at nanometer-wide reaction zones in samples from 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. Copyright (C) 2001 John Wiley & Sons, Ltd.