PNNL

Abstract

Abstract Intergranular (IG) attack and stress-corrosion cracks in alloy 600 tubing removed from the PWR steam generator #1 at Ringhals 2 have been characterized by analytical transmission electron microscopy (ATEM). Comparisons are made between environmentally induced cracks initiated on the primary-water ID surface versus those initiated on the secondary-water OD surface. General SCC crack morphologies were quite similar with branched IG cracking extending to approximately 50% through wall. Corrosion products in the open cracks were quite different with hydrated nickel phosphate seen filling the secondary-side crack, while the crack wall oxide in the primary-side crack was a Cr and Fe-rich spinel. Both samples revealed narrow (~10-nm wide), deeply penetrated, oxidized zones along most grain boundaries that intersect the open cracks. The local structures and chemistries in these corrosion-affected zones were examined by high-resolution TEM imaging, electron diffraction and fine-probe compositional analysis. These porous IG penetrations were nearly identical in appearance for both the primary- and secondary-side examples and contained Cr-rich oxides (Cr2O3 on the primary side and spinel plus Cr2O3 on the secondary side). Similarities between corrosion-induced structures for primary- and secondary-side cracking may indicate that the same degradation mechanism is operating in both cases. However, controlled experiments are needed where specific mechanisms can be properly distinguished.