Environmentally Assisted Cracking of Ni-Base Alloys

The Ni-base alloys used as LWR construction material have experienced stress corrosion cracking (SCC). In the fall of 1991, for example, a leak was discovered in the penetration to the pressure vessel head at the Bugey 3 plant in France. Metallurgical evaluations indicated that the leak was caused by primary-water SCC. The main crack had initiated in Alloy 600 base metal and propagated into the Alloy 182 weld metal. Subsequent inspections of penetrations in the control rod drive mechanisms of domestic and foreign PWRs identified a small number (<5% of the penetrations inspected) with axial cracks. None of the cracks was through-wall, and until recently, no more leaks occurred in pressure-vessel head penetrations. From the summer of 2000, SCC has occurred in penetrations of control rod drive mechanisms for several PWRs in the U.S., as well as in outlet nozzle-to-pipe dissimilar weld metals. More recently, cracks have been found in bottom-mounted instrumentation nozzles.

Long-term operating experience indicates that although wrought Ni-base Alloy 600 is susceptible to SCC, until recently, the weld metal (Alloys 82 and 182) used with Alloy 600 had not shown environmentally assisted cracking. However, laboratory tests indicate that in PWR coolant environments, the SCC susceptibility of Alloy 182 may be greater than that of Alloy 600, and the SCC susceptibility of Alloy 82 may be comparable to that of Alloy 600. This apparent inconsistency between field and laboratory experience is an issue that needs further investigation. An Argonne program is underway to evaluate the resistance of Ni-base alloys and their welds to environmentally assisted cracking in simulated LWR coolant environments. Information has been obtained on the effect of temperature, load ratio, and stress intensity on environmentally assisted cracking of these alloys in simulated LWR environments.

To obtain a qualitative understanding of the degree of enhancement and the range of conditions over which significant environmental enhancement is observed, it is helpful to plot the observed crack growth rates (CGRs) against those expected in air under the same mechanical loading conditions, i.e., the same stress intensity range, cyclic stress ratio, and rise time. The next figure shows corrosion fatigue results for various heats of Alloy 600 and 82 weld metal tested in a simulated PWR environment. There is significant environmental enhancement of the CGR for Ni-base alloys at both low- and high-DO levels. The next figure (a) shows that, for Alloy 600 at CGRs greater than ≈10-9 m/s, the mechanical driving forces dominate; at lower CGRs, the environmental contributions dominate. These same observations do not appear to be valid for Alloy 82 in a PWR environment (next figure, (b)).

(a)(See full-size image)

(b)(See full-size image) Corrosion fatigue data for Alloy 600 (a) and Alloy 82 (b) in low-dissolved-oxygen (<10 ppb) water.

Corrosion and Mechanics of Materials: Light Water Reactors
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