Corrosion Cracking: When Does it Really Start?
Research at PNNL aims to identify the onset of stress corrosion cracking in pressurized water nuclear reactors
Pressurized water nuclear reactors in the United States generate about 13 percent of U.S. electricity. Though efficient, these reactors face a unique challenge with stress corrosion cracking (SCC). This type of corrosion is one of the primary life-limiting degradation mechanisms of nickel-base alloy pressure boundary components, such as instrumentation and control rod nozzles, the welds that attach these nozzles to the reactor vessel, and welds that connect feedwater piping to the reactor vessel. As interest grows in a more sustainable and efficient fleet of nuclear reactors across the world, there is increasing interest in characterizing SCC initiation response.
Ongoing research at PNNL uses PNNL-designed and built equipment to study SCC initiation times for relevant alloys, and also perform microscopic studies to characterize and understand what leads up to crack initiation. This builds upon the growing body of SCC crack growth rate measurements conducted over the last several decades. The crack growth rate data were used to determine inspection intervals that assume cracks exist from the moment the reactor first starts up, but PNNL is looking to better understand SCC initiation so that a more quantitative basis for inspection intervals can be established.
At the heart of PNNL’s research are seven SCC initiation test systems that feature active loading and in-situ monitoring to detect initiation. Detection is accomplished by monitoring the resistance across the gauge section of the test specimens. As cracks nucleate, grow and coalesce, the resistance of the specimen increases. This contrasts the often-used "cook-and-look" method of monitoring for initiation, where a statically loaded specimen is periodically removed to observe whether cracking has occurred.
PNNL’s in-situ test method provides a more precise measurement of SCC initiation time. It also allows for interruptions to observe surface or cross section microstructures at known fractions of the initiation time. This provides for a better understanding and linkage of the response between microscopic and macroscopic initiation.
Testing revealed an initiation response that varies strongly with material starting state. Relevant alloys tested with a low-strength starting condition are very resistant to SCC initiation; however, small amounts of cold-work consistent with bending—needed to align components or that which occurs due to post-fabrication grinding of welds and adjacent structures—dramatically reduces SCC initiation time.
Detailed characterizations of cracking both on the surface and in cross-section are another key element of this research. By using smaller size specimens that fit into a scanning electron microscope (SEM), researchers can fully map the specimen surface and track the formation and growth of cracks, allowing such information to be synchronized with the initiation time that corresponds to when cracks begin to rapidly grow. Images from the SEM confirm that large cracks form by a combination of growth and coalescence, and that extensive cracking does occur before fast crack growth begins.
The combination of high-resolution SCC initiation testing and detailed examinations confirms the hypothesized initiation response of the first generation nickel-base alloys. New information is also being obtained on second-generation replacement alloys that are more resistant to SCC initiation. In addition to playing a role in regulatory decisions related to inspections, outcomes from these studies may influence the choice of material, starting condition, and assembly techniques for new reactors.
Published: October 12, 2015