The mechanism of stress corrosion cracking in sensitized austenitic stainless steels in nuclear power reactor heat transport circuits

Extensive work over almost the past hundred years has suggested that the stress corrosion cracking of metals and alloys in aqueous environments is primarily an electrochemical phenomenon falling within the realm of the differential aeration hypothesis (DAH). An important feature of the DAH is that the local anode and the local cathode are spatially separated, with the former existing within the crack enclave (on the crack flanks and at the crack tip) and the latter existing on the bold, external surfaces. Because of the need to compensate the positive charge being deposited into the crack cavity from metal dissolution, anions (e.g, Cl^-) are transported into the crack, a process that is manifest as a positive current flowing from the crack to the external surfaces, where it is consumed by hydrogen ion, water, and/or oxygen reduction. Thus, strong electrochemical coupling exists between the crack internal and external surfaces and this coupling has been observed in stress corrosion cracking in a variety of systems, including IGSCC in sensitized Type 304 SS in simulated BWR coolant environments at 288°C. Examination of this “coupling current” shows that it contains “structured” noise superimposed upon a mean. In the case of the sensitized stainless steel in the high temperature aqueous environment, the mean current is found to be linearly related to the crack propagation rate. Furthermore, the noise in the current is found to yield a wealth of information on the fracture events that occur at the crack tip, including their frequency, temporal relationship with other events, and size. This information has provided a clearer view of the fracture mechanisms.