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

TitleIntergranular stress corrosion cracking initiation and growth in mill-annealed alloy 600 tubing in high-temperature caustic
Publication TypeJournal Article
Year of Publication1998
AuthorsBrisson, BW, Ballinger, R, McIlree, AR
JournalCorrosion
Volume54
Issue7
Pagination504 - 514
Date Published1998/07//
Abstract

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 degrees 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 root m and 18 MPA root 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. (0.079 in./y to 0.55 in. /y) Estimates of the uncertainty in CGR and K were made. 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 approximate to 4 MPa root m. However, since the scatter In existing data is very large and the conditions (K, stress, and pH) for most of the data are poorly known or not known at all, this was not 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 (0.008 in.) in depth, and probably smaller. Initiation from a smooth surface occurred at multiple sites, followed by linking to form a main crack front.