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

Recent results are presented demonstrating the application of cross-section al analytical transmission electron microscopy (ATEM) to corrosion and crac king in high-temperature water environments. Microstructural, chemical and crystallographic characterization of buried interfaces at near-atomic resol utions is shown to reveal evidence for unexpected local environments, corro sion reactions and material transformations. Information obtained by a wide variety of high-resolution imaging and analysis methods indicates the proc esses occurring during crack advance and provides insights into the mechani sms 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, i s believed to indicate a major role of active corrosion in the stress corro sion 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 id entify the local electrochemistry promoting degradation in complex service environments. Solution impurities such as lead are found in high concentrat ions at nanometer-wide reaction zones in samples from secondary water envir onments, indicating water access at leading edges of the attack and the inf luence of these impurities on the corrosion processes. Results for specific samples are used to demonstrate the ability of cross-sectional ATEM to rev eal new details of buried corrosion structures that cannot be detected by o ther methods. Copyright (C) 2001 John Wiley & Sons, Ltd.