Intergranular stress corrosion cracking initiation and growth in mill-annealed Alloy 600 tubing in high-temperature caustic

Historically, pressurized water reactor (PWR) steam generator (SG) reliability has been dominated by degradation of alloy 600 (UNS N06600) tubing material. Stress corrosion cracking (SCC) crack initiation and crack growth rates (CGR) were measured in mill-annealed alloy 600 (UNS N06600) tubing as a function of the stress intensity factor (K) in 10% caustic at 315 C. Tests were conducted using internally pressurized smooth and precracked tubing. Samples were polarized to 150 mV (precracked tube test) or 225 mV (initiation test) with respect to a nickel electrode. Crack initiation and growth from the external tube surface were monitored using a multifrequency alternating current (AC) potential drop system. The AC potential drop system allowed detection of initiation from a smooth surface as well as the monitoring of crack extension in real time. In the case of precracked sample tests, the sample was precracked in fatigue from a sharp v-notch. CGR were obtained over the K range between 4 MPa√m and 18 MPa√m. Values for K were estimated based upon fractographic analysis of samples after testing and an estimate of the K-solution for a thin-walled tube. Average CGR ranged from 2 mm/y to 14 mm/y. CGR determined in this investigation represent the first SCC CGR data obtained in high-temperature caustic using actual steam generator tubing. Growth rates obtained fell within the overall range of the existing database for CGR (da/dt) in alloy 600. The data and analysis suggested a threshold value of K for K-driven crack growth of ∼ 4 MPa√m. However, since the scatter in existing data is very large and the conditions for most of the data are poorly known or not known at all, this was surprising. More importantly, from the standpoint of life prediction, it was observed that da/dt responded to and was a function of K for cracks as small as 0.2 mm in depth, and probably smaller