Fate of the Coupling Current in Stress Corrosion Cracking - Mechanistic and Corrosion Control Implications

Abstract Strong coupling between the cavity (crack, pit, or crevice) internal and external surfaces, as required by the differential aeration hypothesis for localized corrosion, 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, IGSCC in the same sensitized alloy in thiosulfate solutions at ambient temperature, and caustic cracking in AISI 4340 high strength steel at 70°C. Coupling is manifest as a current flowing from the crack to the external surfaces, where it is consumed by the reduction of a cathodic depolarizer, such as oxygen, water, or hydrogen ion. Examination of this current, which is easily measured using a sensitive zero resistance ammeter, 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 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. This information has provided a clearer view of the fracture mechanisms, which in all three cases (IGSCC in sensitized stainless steel in BWR environments and in thiosulfate solution and caustic cracking in AISI 4340) appear to involve brittle micro fracture events of a few micrometers to a few tens of micrometers in size. These data are more consistent with hydrogen- induced fracture than they are with a slip/dissolution mechanism, even when the external environment is oxidizing in nature.