EFFECTS OF LIOH ON THE MICROSTRUCTURE OF THE OXIDE FORMED DURING THE AQUEOUS CORROSION OF A ZR-2.5 WT-PERCENT NB ALLOY

Zr-2.5 wt% Nb pressure tubing specimens were exposed to pressurized li thiated water environments containing 4.8 and 1.2 g LiOH per litre dei onized water, a pure deionized water at 573 K for times up to 2790 h. The weight gains were measured and the microstructure of the oxide for med at the oxide-metal interface was characterized using SEM and TEM t echniques. The corrosion behaviour in a pressurized lithiated water so lution containing 4.8 g LiOH/L H2O is characterized by two stages, nam ely an initial stage exhibiting parabolic weight gain kinetics and a s econd stage with linear weight gain kinetics. However, there is no tra nsition from parabolic to linear kinetics for corrosion in a pressuriz ed lithiated water solution containing 1.2 g LiOH/L H2O or during corr osion in deionized water. There thus appears to be a critical LiOH con centration above which accelerated corrosion occurs. The SEM and TEM o bservations show that the microstructure of oxide formed in pressurize d lithiated water containing 4.8 g LiOH/L H2O is significantly differe nt from that of the oxide formed in pressurized lithiated water contai ning 1.2 g LiOH/L H2O or in deionized water. Different corrosion mecha nisms at the alpha-Zr grain boundaries are shown to be operative for p ressurized lithiated water with differing LiOH concentrations.