Role of imposed potentials in threshold for caustic cracking susceptibility (KISCC): Investigations using circumferential notch tensile (CNT) testing

A fracture mechanics-based novel approach, i.e. circumferential notch tensile (CNT) testing has been employed for determination of threshold stress intensity factor for susceptibility of engineering materials to stress corrosion cracking (KISCC) using small specimens. Using CNT technique, KISCC of a carbon steel at an open circuit potential (Ecorr) in 500 g L−1 NaOH at 100 °C was determined to be 42.9 MPa m1/2. In order to establish the application of the CNT technique in understanding the mechanistic aspects of caustic cracking as well as for developing guidelines for mitigation, tests have also been performed under the imposed electrochemical potentials. An imposed potential in the active-passive potential regime (Ea-p) caused an extremely rapid failure (than observed at Ecorr) whereas, at an imposed potential in the passive region (Ep), the specimen did not fail even after relatively very long exposure time. The fractography of the CNT specimens tested at Ecorr and Ea-p presented evidence of SCC. The study has established the use of experimental CNT testing as a simple, relatively fast and cost-advantageous approach for generating the KISCC data, which are also consistent with the electrochemical mechanism for caustic cracking.