HIGH-RESOLUTION CHARACTERIZATIONS OF FILM FORMATION AND LOCALIZED CORROSION IN ALLOY 690 EXPOSED TO PWR PRIMARY WATER

摘要

High Cr alloy 690 has replaced lower Cr alloy 600 in many critical pressurized water reactor (PWR) components due to its superior resistance to intergranular stress corrosion cracking (IGSCC) in primary water environments. The increased Cr content (30 wt%) in alloy 690 is generally believed to promote formation of a protective chromia film on the surface that inhibits both corrosion and SCC. The current research investigates nanometer-scale corrosion/oxidation behavior at grain boundary and matrix regions. Unstressed specimens of cold-worked alloy 690 materials (shown to be susceptible to IGSCC in crack growth tests) have been exposed to simulated PWR primary water conditions at 360°C. Detailed surface and near-surface characterizations were performed on cross-section samples starting with low kV backscatter scanning electron microscopy (SEM) imaging to document the extent of localized corrosion. Selected surface and corrosion tip regions are then prepared by focused ion beam milling for transmission electron microscopy (TEM) and atom probe tomography (APT) analyses. A thin Cr-rich oxide film is detected across the entire surface, but a continuous chromia film only forms above high-energy grain boundaries. No IG corrosion is observed, however filamentary oxidation from the surface into the matrix is seen at distances ＞100 nm from the grain boundaries.