SCC initiation in cold worked Alloy 690TT under simulated PWR primary water. 3rd report. Crack initiation of cavity induced specimens prior to SCC initiation test

Cavities formed on grain boundaries of Alloy 690TT may lead to SCC initiation when exposed to PWR primary water. To investigate the possibility whether SCC initiate due to cavity formation, a constant load was applied on 20% cold worked Alloy 690TT blunt notched CT specimens in 450℃ air for short time to enhance the precursor of the initiation and then tested in simulated PWR primary water. 220h of cavity formation and subsequent 2,269h testing in 360℃ simulated PWR primary water revealed tiny fissures on the notch tip indicating that the cavity formation before the test enhanced precursor of SCC initiation since it was earlier than the time (40,635h) of the test without the cavity formation. By observing the time dependence of the gap of these fissures, it was found that the gap monotonically increased until 18,037h indicating that cavity formation or corrosion process evolved under these fissures during the SCC initiation test. 200h of cavity formation and subsequent 15,661h testing in 320℃ simulated PWR primary water revealed grain boundary openings at two points on the notch tip which has not evolved into fissures. It was much longer than the testing at 360℃ indicating Arrhenius temperature dependence on the SCC initiation process. (author)