THE EFFECT OF HYDRIDE ON THE CORROSION OF ZIRCALOY-4 IN AQUEOUS LIOH SOLUTION

Commercial Zircaloy-4 sheets were charged with 230-250 wppm hydrogen b y the gas-charging method and were homogenized at 400 degrees C for 72 h in vacuum. The hydrogen charged specimens were corroded in pure wat er and aqueous LiOH solutions using static autoclaves at 350 degrees C , and then characterized by measuring their weight gains with the corr osion time and observing their microstructures using an optical micros cope (O/M) and a scanning electron microscope (SEM). The elemental dep th profiles for hydrogen and lithium were measured using a secondary i on 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 specimen s corroded abruptly and heavily, forming more Zr-hydrides by the large r amount of absorbed hydrogen at higher Li ion concentrations. The spe cimens 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 cor rosion times under 220 wppm Li ion concentration conditions. however, normal specimens showed a rather accelerated corrosion rate compared t o 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-hydrid e, which causes the hydrogen pick-up into the specimen to be depressed after the formation of the oxide with an appropriate thickness. (C) 1 998 Elsevier Science B.V. All rights reserved.