Kyeong Ho Kim

The effect of hydride on the corrosion of Zircaloy-4 in aqueous LiOH solution

Abstract Commercial Zircaloy-4 sheets were charged with 230–250 wppm hydrogen by the gas-charging method and were homogenized at 400°C for 72 h in vacuum. The hydrogen charged specimens were corroded in pure water and aqueous LiOH solutions using static autoclaves at 350°C, and then characterized by measuring their weight gains with the corrosion time and observing their microstructures using an optical microscope (O/M) and a scanning electron microscope (SEM). The elemental depth profiles for hydrogen and lithium were measured using a secondary ion mass spectrometry (SIMS) to confirm their distribution at the oxide/metal interface. As the concentration of Li ions in the aqueous LiOH solution increased to more than 30 ppm, the normal Zircaloy-4 specimens corroded abruptly and heavily, forming more Zr-hydrides by the larger amount of absorbed hydrogen at higher Li ion concentrations. The specimens that had been charged with amounts of hydrogen greater than its solubility limit corroded early with a more rapid accerleration than normal specimens, regardless of the corrosion solutions. At longer corrosion times under 220 wppm Li ion concentration conditions, however, normal specimens showed a rather accelerated corrosion rate compared to the hydrogen-charged specimens. These slower corrosion rates of the hydrogen-charged specimens at longer corrosion times under 220 wppm Li ion concentration conditions may be due to the pre-existent Zr-hydride, which causes the hydrogen pick-up into the specimen to be depressed after the formation of the oxide with an appropriate thickness.

Cation incorporation into zirconium oxide in LiOH, NaOH, and KOH solutions

To investigate the cation incorporation into zirconium oxide, SIMS analysis was performed on the specimens prepared to have an equal oxide thickness in LiOH, NaOH, and KOH solutions. Even though they have an equal oxide thickness in LiOH, NaOH, and KOH solutions, the penetration depth of cation into the oxide decreased with an increase in the ionic radius of cation. The cation is considered to control the corrosion in alkali hydroxide solutions and its effect is dependent on the concentration of alkali and the oxide thickness. The slight enhancement of the corrosion rate at a low concentration is thought to be caused by cation incorporation into oxide, while the significant acceleration at a high concentration is due to the transformation of oxide microstructures that would be also induced by cation incorporation into oxide.