Determination of the fate of the current in the stress corrosion cracking of sensitized type 304SS in high temperature aqueous systems

Abstract

The experimental work reported in this paper was designed to resolve the question concerning the fate of the current that is generated during crack growth in engineering alloys in power plant environments. Two limiting possibilities exist: the cathodic processes that consume the positive ion current generated at the crack tip reside within the confines of the crack; or they reside on the external surfaces. Experiments were conducted which made use of sensitized type 304 stainless steel (SS) compact specimens C(T) that had been modified to provide external surfaces that are isolated from the specimen, so that the current flowing between the crack and the external cathode could be readily monitored. This was done by coating the C(T) specimens with baked-on polytetrafluoroethylene so that only the crack (after fatigue precracking) was exposed to the environment. Cathodes were then mounted on the sides of the C(T) specimen and the current was monitored as the stress intensity and water chemistry were varied. The experiments reported here clearly demonstrate that current flows from the crack to the external cathode during crack propagation in sensitized type 304SS in pure water at the elevated temperatures typical of light water reactor heat transport systems. Thus, in spite of the high resistivity of the environment, the cathodic reaction is not restricted to the flanks of the crack or to the crack mouth, but occurs predominantly on the external surfaces, as postulated in the coupled environment fracture model. New insight has been gained into the physical mechanism of crack growth in high temperature aqueous environments.