Feasibility of on-line monitoring of stress corrosion cracking in rotating components. Final report

A feasibility study of real-time monitoring of stress corrosion cracking (SCC) of turbine disk materials was conducted. The main objective of the program was to evaluate the viability of the monitoring approach for steam turbine applications. The first phase of the monitoring investigation was conducted using compact tensile specimens in laboratory test rigs. Tests were undertaken in two types of environment, namely steam condensate and sodium hydroxide (NAOH) solutions. A prototype data transmission unit was used to transmit corrosion signals across an air gap to demonstrate that, if required, the corrosion data could be gathered from the shaft of a rotating turbine. The laboratory program showed that electrochemical signals characteristic of SCC can be detected in the test environments. The corrosion. results were confirmed by post-exposure metallographic examination. The trial of the instrument that transmitted and received data across an air gap was also successful. The approach was found to be more sensitive than either electromechanical or potential drop methods for the detection of SCC. In the second phase of the investigation, on-line electrochemical corrosion monitoring was conducted on a 500-MW steam turbine of a coal-fired generating station. The plant trial indicated that corrosion signals generated from a sensor couldmore » be recorded in a plant without interference from external electric fields. Characteristic SCC signals were detected over a three-month period from the stressed specimen. Variability in the magnitude and the frequency of the SCC events was observed, which showed a relationship with operational factors.« less