Corrosion analysis of welded Nb-1% Zr-0.1% C alloy in lead-bismuth eutectic solution

Abstract

This study aims at checking the suitability of Nb-1% Zr-0.1% C alloy as coolant channel for carrying molten lead-bismuth eutectic (LBE). Molten LBE is used to carry heat from the reactors to heat exchangers in the compact high temperature reactors. Corrosion properties of the base metal (BM), heat-affected zone (HAZ) and fusion zone (FZ) of the weld of niobium alloy were compared at the temperature of 1050 °C in LBE solution in open-air condition. The presence of lead and bismuth at grain-boundary and triple junctions of the grain boundaries is evitable in both the BM and FZ, and the material loss can be attributed to oxidation corrosion. Nb2O5 and NbO2 being brittle and permeable oxides of niobium did not inherit the characteristics of hindering further oxidation-corrosion at elevated temperature. The corrosion rates of FZ, HAZ and BM were observed to be equal to 24.16 mm/year, 20.83 mm/y and 27.80 mm/y, respectively, and this difference occurred mainly due to the formation of nitrides of zirconium (ZrN) and carbides of zirconium (ZrC) and niobium (NbC). ZrO2 could be confirmed by the micro-hardness profile, which appeared to be flat throughout the FZ and BM. The formation of these carbide and nitride phases was confirmed through XPS and XRD analyses. The differences of surface roughness values expressed in root mean square (Sq.) values, determined before and after the corrosion, were found to be equal to 0.267 and 0.230 for the base metal and weld zone, respectively. It signifies that FZ was more corrosion-resistant than BM. It is not recommended to use this alloy in open-air condition for the coolant channels, due to the high corrosion rate in the BM, FZ and HAZ, which leads to material loss beyond the acceptable limit.