Stress corrosion cracking of sensitized Type 304 stainless steel in thiosulfate solution: I. Fate of the coupling current

This work explores the fate of the coupling current generated during stress corrosion cracking of sensitized Type 304 stainless steel (304SS) in thiosulfate solution at ambient temperature. The coupling current, which is generated inside the crack, may be consumed (as required by charge conservation) by cathodic reactions at two different locations: inside the crack (e.g., on the crack flanks) and on the external surfaces. Experiments were conducted using sensitized Type 304SS compact tension specimens C(T) modified to provide external surfaces isolated from the specimen itself, so that the coupling current flowing between the crack and the external cathode could be directly monitored using a zero-resistance ammeter. The results demonstrate that significant coupling current flows from the crack to the external surface, consistent with the differential aeration hypothesis for localized corrosion. Coupling current data from specimens heat treated at 650 degreesC for 4, 14, and 24 h show that 4 h of sensitizing heat treatment apparently does not induce significant sensitization, while 14 h of heat treatment is sufficient to produce a fully sensitized microstructure, equivalent to that obtained after 24 h of heating. Finally, the crack was determined to grow via discrete microfracture events at the crack front with a dimension of the order of the grain size. The observations are more consistent with crack growth being due to a hydrogen induced fracture mechanism rather than slip/dissolution. (C) 2003 Elsevier Science Ltd. All rights reserved.