SEM-FIB and TEM Characterization of the Microstructure and Oxides in Base Metal at Weld Roots of EDF Safety Injection Pipeline

Oxides and microstructure from welds of safety injection pipeline have been studied by Transmission Electron Microscopy in order to provide a better understanding of the SCC mechanism. In particular, the potential contribution of an effect of the dissolved oxygen, the presence or absence of pollutants, the sensitization and the deformation of the material have been investigated. For oxides on the internal pipe surfaces of base metal at the weld roots, results show a duplex structure typical of a PWR environment. It consists of mostly Fe-rich spinels in the external oxide layer and Cr-rich spinels in the internal oxide layer. Both oxide types also contain Ni. In addition, a Ni enrichment in the base metal at the interface with oxides is also noted, typical of a PWR environment. Oxides at the crack tips are mainly (Cr, Fe, Ni)-rich spinels with local amorphous signatures. No evidence of a contribution of dissolved oxygen was highlighted. No pollutants were detected as well as no peculiar sensitization signature (no Cr depletion along grain boundaries, no Cr-rich carbide). Deformation features are characteristic of a strain-hardened stainless steel. Further investigations are still ongoing on different welds to better understand SCC mechanism on auxiliary lines’ welds.