Type of presentation: Poster

MS-14-P-3122 New Insights into SCC Initiation in Alloy 600 using Advanced Analytical Electron Microscopy

Bertali G.1, Burke M. G.1, Scenini F.1, Zhong X. L.1
1Materials Performance Centre, University of Manchester, Machester UK 1
m.g.burke@manchester.ac.uk

Stress corrosion cracking (SCC) is an important failure phenomenon in a variety of alloys used in power generation applications. In light water reactor (LWR) plants, materials such as austenitic stainless steels and Ni-Cr-Fe alloys (Alloy 600) can be susceptible to Primary Water SCC (PWSCC). Although there is an extensive amount of data on PWSCC crack growth rates and fracture, initiation of SCC continues to be a topic of research, particularly in terms of localized intergranular oxidation.  In this study, Alloy 600 (Ni-16Cr-9Fe-0.045C) was exposed for 66 and 120 h to a hydrogenated steam environment at atmospheric pressure and 480°C (O2 partial pressure = 9.88x10-26 atm which is lower than the dissociation pressure of Ni/NiO at 480°C). This reducing environment was used to avoid protective Ni-rich surface oxide formation and to accelerate the intergranular oxidation that is generally occurs at lower temperatures in a LWR environment [1]. This oxidation system successfully simulated PWR oxide morphologies [2-4]. Field emission gun (FEG) scanning electron microscopy (SEM), focused ion beam (FIB) and analytical electron microscopy techniques were used to characterize the type and extent of preferential oxidation associated with SCC initiation.

The exposed solution-annealed and water-quenched samples were evaluated in an FEI Quanta 650 SEM equipped with Oxford Instruments SDD and EBSD systems. A variety of grain boundary (GB) oxide morphologies were observed, reflecting the type of grain boundary, Fig. 1.   Surface oxide morphology from FIB cross-sections was correlated with intergranular oxidation susceptibility.  More detailed analyses were performed using the FEI Titan G2 80-200 aberration-corrected S/TEM with Super EDX.  STEM-EDX microanalysis confirmed the presence of interconnected subsurface Cr-rich oxides and intergranular Cr-rich oxide (Fig.2), with a Ti/Al-rich oxide delineating the original grain boundary location. Analyses showed strong correlation between the surface GB oxide morphology and its susceptibility to intergranular oxides. Microstructural evidence of grain boundary migration and the associated depletions and enrichments that develop also appear to be notable factors in the preferential GB oxidation/SCC initiation.

References

1. Scott PM, Le Calvar M., in Proc. 6th Intl Symp. on Environ. Deg. of Matls in Nuclear Power Systems- Water Reactors; 1993, 657.

2. Scenini F, et al., Proc.12th Intl Symp. on Environ. Deg. of Matls in Nuclear Power Systems- Water Reactors 2005, 891.

3. Bertali G, et al., Proc.16th Intl Symp. on Environ. Deg. of Matls in Nuclear Power Systems- Water Reactors 2013, in press.

4. Lindsay J, et al., Proc.16th Intl Symp. on Environ. Deg. of Matls in Nuclear Power Systems- Water Reactors, 2013, in press.

Fig. 1: Secondary electron images showing (a) the surface morphology of the GB oxide, Ni surface nodules, and nodule-free zone (NFZ); and (b) a FIB cross-section through the oxidized GB.

Fig. 2: HAADF STEM image and corresponding Titan EDX spectrum images for Cr, O and Ni showing the oxidation and a Ni-rich nodule at the surface.