Lithium Uptake and the Accelerated Corrosion of Zirconium Alloys

Abstract

The corrosion of zirconium alloys in aqueous lithiated solutions is sensitive to the concentration of the alkali and the temperature. In concentrated solutions, ⪢10-1 M in lithium hydroxide (LiOH) (700-ppm lithium) and at temperatures >573 K, accelerated corrosion occurs at quite an early stage. Our investigations indicate that the accelerated corrosion is caused by the generation of porosity, rather than the dissolution of lithium, in the growing oxide. Specimens of standard Zircaloy-4 fuel cladding and Zr-2.5 wt% Nb pressure tube materials were corroded in lithium hydroxide solutions, 10 -3to 1 M in concentration, at 589 K. Impedance measurements, polarizations in molten lithium nitrate-lithium hydroxide (LiNO3-LiOH) and scanning electron microscopy of the alloy-oxide interface indicated a high level of porosity, right from the initial stages, for oxide films grown in the concentrated solutions. The oxides, when analyzed by atomic absorption spectroscopy, revealed the presence of a few 100 ppm of lithium, too small to account for the accelerated corrosion by a mechanism of solid solution of lithium in zirconia. X-ray powder patterns of the oxides showed peaks for only monoclinic zirconia, but occasionally peaks for LiOH ∙ H2O and LiOH were also observed. The counts for lithium, detected by secondary ion mass spectrometry, decreased when specimens cut from the same corroded samples were leached in nitric acid. It is concluded from these observations that a major part of lithium is physically held in the porous oxide. Lithium hydroxide is not completely dissociated in aqueous solutions; with increasing concentration and temperature, an increasingly larger proportion of the alkali remains undissociated. It is suggested that the accelerated corrosion in concentrated solutions is caused by the participation of the undissociated alkali in the reactions occurring on the surfaces of the zirconia crystallites. The undissociated LiOH and hydroxyl ions react at an anion vacancy to produce Zr-OLi. These surface -OLi groups could retard the normal recrystallization and growth of oxide crystallites and thus maintain a high intercrystalline grain boundary area for oxygen diffusion to occur. They could also generate porosity in the growing oxide by reacting to form lithium oxide (Li2O), a soluble product.