Corrosion Behaviour of SS304L at Unapproachable Regions of Fillet Weld Joints in Fast Reactor Reprocessing Application

Abstract

The Purex process is opted for reprocessing of spent fuel from fast breeder reactors. The process employs nitric acid with concentrations up to 11M. Nitric Acid Grade (NAG) SS304L is opted as material of construction for equipments, process tanks and piping of such process plants so as to minimise the corrosion losses due to nitric acid. Practice C test as per ASTM A262-15(2021) (Standard Practices for Detecting Susceptibility to Intergrannular Attack in Austenitic Stainless Steels), with maximum average corrosion rate of 15 mils/year (mpy) for five cycles of 48 hrs, and 18 mpy in any cycle, is one of the criteria for qualification of material among others. A similar acceptance criterion is adopted for qualification of undiluted weld metal deposit of filler material and welding procedure specification (WPS) used therein. However, conventional tests may not always accurately represent the corrosion behaviour of surfaces at all locations. One of such are the surfaces which abut each other, and hence are not approachable for visual examination and thereafter cleaning, such as unwelded open abutted surfaces of fillet weld joint (root), partial penetration (pp) weld joints facing process fluids in abutted portions not in a welded portion of joint. Multiple factors like remains of heat tints, adjacent inter-spaces and type of surface finish may collectively contribute to corrosion rates exceeding acceptable limits. This study delves into the often-neglected corrosion behaviour of these challenging regions. The samples with abutting surfaces are prepared by using EDM cutting and and Gas tungsten arc welding (GTAW) and subjected to corrosion test. The samples made by using base material, filler and WPS that already qualified for corrosion test for groove weld joints and hence fillet weld joints as per ASME BPVC sec IX (2021). Higher corrosion rates are being observed in these weld joints than standard test specimens during study.