Microstructural evaluation and corrosion behaviour of Inconel 82 and Inconel 625 overlays on 304L stainless steel weld used in nuclear fuel reprocessing plant

Weld overlays are considered the most cost-effective surface modification technique to mitigate corrosion in the nuclear industry. Considering this, an attempt to overlay the 304L weld metal at some critical weld locations of the nuclear fuel reprocessing plant with Inconel alloy was initiated. The work focuses on the microstructural evaluation and corrosion behaviour of Inconel 82 and Inconel 625 weld overlays on 304L stainless steel (SS) weld metal in a nitric acid and chloride medium. Scanning electron micrographs of the 304L SS weld metal revealed a duplex microstructure containing δ-ferrite as a minor phase in the austenite matrix. The microstructure of the Inconel 82 and Inconel 625 weld overlays was found to be fully austenitic, with Nb-rich laves phases at the inter-dendritic region. The hardness of the Inconel weld overlays was found to be higher than that of the 304L SS weld metal. X-ray diffraction measurements showed austenite and δ-ferrite in the 304L SS weld metal and a single austenite phase in the Inconel weld overlays. Potentiodynamic anodic polarization experiments carried out in 4M and 8M nitric acid indicated superior passive film stability in the Inconel weld overlays when compared to the 304L SS weld metal. Comparatively, Inconel 625 weld overlay was found to be much superior to Inconel 82 weld overlay in nitric acid medium. The 304L SS weld metal exhibited inferior pitting resistance in chloride medium by virtue of the high δ-ferrite present in the weld fusion zone. The pitting resistance of the Inconel 82 weld overlay marginally improved, while a substantial enhancement occurred in the Inconel 625 weld overlay.