Detection of stress-corrosion crack initiation in the BWR environment by electrochemical noise

The objective of this research was to investigate the use of electrochemical noise (EN) for detecting stress corrosion crack (SCC) initiation in boiling water reactor (BWR) environments. Initial experiments examined the response of sensitized Type 304 stainless steel (SS) in slow strain-rate tensile (SSRT) tests in 288 degrees C/10.4 MPa water, a laboratory simulation of the BWR environment. This combination of specimen condition and geometry assured abundant nucleation of intergranular cracks, with controllable propagation and arrest via changes in either loading or environment. The SSRT tests provided an ideal platform for optimizing the electrochemical cell configuration, while establishing the nature of electrochemical potential and current noise (EPN and ECN) responses to those events. The standard deviation of electrochemical potential, a measure of EPN amplitude, proved to be the best indicator of SCC initiation, but the degree of correlation depended upon both the periodicity of the calculation and the electrode configuration. Subsequent experiments examined EN signal response as a function of crack depth by adapting the SSRT protocol to blunt-notch compact-tension (CT) specimens in a system that employed reversing de potential drop to monitor crack advance continuously. Consistent with poor electrochemical coupling to the tip of high-aspect-ratio cracks, a reference-electrode wire in the notch near the blunt tip sensed no useful EPN beyond a crack depth of about 0.1 mm.