Lithium Uptake and the Corrosion of Zirconium Alloys in Aqueous Lithium Hydroxide Solutions

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Abstract
Corrosion films on zirconium alloys were analyzed for lithium by Atomic Absorption Spectroscopy (AAS), Secondary Ion Mass Spectrometry (SIMS), and Infrared Reflection Absorption Spectroscopy (IRAS). The oxides grown in reactor in dilute lithium hydroxide solution, specimens cut from Zircaloy, and Zr-2.5Nb alloy pressure tubes removed from CANDU (CANada Deuterium Uranium, Registered Trademark) reactors showed low concentrations of lithium (4 to 50 ppm). The lithium was not leachable in a warm dilute acid. 6Li undergoes transmutation by the 6Li(n,t)4He reaction. However, SIMS profiles for 6Li and 7Li were identical through the bulk oxide and the isotopic ratio was close to the natural abundance value. The lithium in the oxide, existing as adsorbed lithium on the surface, has been in dynamic equilibrium with lithium in the coolant, and, in spite of many Effective Full Power Years (EFPY) of operation, lithium added to the CANDU coolant at ∼2.5 ppm is not concentrating in the oxides. On the other hand, corrosion films grown in the laboratory in concentrated lithium hydroxide solutions were very porous and contained hundreds of ppm of lithium in the oxide. A large fraction of the lithium in these oxides was leachable. In concentrated solutions, undissociated LiOH participates in the corrosion process, producing surface -OLi groups and Li2O. Dissolution of Li2O generates porosity, concentrates lithium in the solution inside the pores, and provides an easy access of the corrodant to the oxidealloy interface to maintain an accelerated corrosion.