KEYNOTE: The Electrochemistry of Stress Corrosion Cracking

Extensive work over the past hundred years has shown that the stress corrosion cracking of metals and alloys in aqueous environments falls 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 enclave 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. The mean current is found to be linearly related to the crack propagation rate and, indeed, the measurement of the coupling current may provide a sensitive method of measuring crack growth 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. For example, IGSCC in sensitized stainless steel in BWR environments appears to involve brittle micro fracture events of a few micrometers to a few tens of micrometers in size.