INTERGRANULAR STRESS-CORROSION CRACKING INITIATION AND GROWTH IN MILL-ANNEALED ALLOY-600 TUBING IN HIGH-TEMPERATURE CAUSTIC

Stress corrosion cracking (SCC) crack initiation and crack growth rate s (CGR) were measured in mill-annealed alloy 600 (UNS N06600) tubing a s a function of the stress intensity factor (K) in 10% caustic at 315 degrees C. Tests were conducted using internally pressurized smooth an d 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 monit ored using a multifrequency alternating current (AC) potential drop sy stem. The AC potential drop system allowed detection of initiation fro m a smooth surface as well as the monitoring of crack extension in rea l time. In the case of precracked sample tests, the sample was precrac ked in fatigue from a sharp v-notch. CGR were obtained over the K rang e 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 esti mate 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 unce rtainty in CGR and K were made. CGR determined in this investigation r epresent the first SCC CGR data obtained in high-temperature caustic u sing actual steam generator tubing. Growth rates obtained fell within the overall range of the existing database for CGR (da/dt) in alloy 60 0. 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 scatt er In existing data is very large and the conditions (K, stress, and p H) for most of the data are poorly known or not known at all, this was not surprising. More importantly, from the standpoint of life predict ion, it was observed that da/dt responded to and was a function of K f or cracks as small as 0.2 mm (0.008 in.) in depth, and probably smalle r. Initiation from a smooth surface occurred at multiple sites, follow ed by linking to form a main crack front.