Effect of pH on Hydrogen Pick-Up and Corrosion in Zircaloy-4

Thermal desorption spectroscopy, secondary ion mass spectroscopy and scanning transmission electron microscopy have been used to investigate the effect of pH on corrosion and hydrogen pick-up behaviour in different samples of Zircaloy-4. Samples were autoclave-oxidised in pure water and at an elevated pH (with 50% deuterated water) when compared to commercial reactors. A characteristic desorption peak for hydrogen has been found at ~650 °C, which occurs when the difference in free energy between hydrogen in the metal and in the gas phase becomes positive. Electron energy loss spectroscopy provided us with a method to detect and measure the thickness of the following layers (from oxide to metal): ZrO2, a previously reported ZrO suboxide, an oxygen saturated zirconium region and the Zr metal. Overall, samples exposed to a high pH show a longer time to transition and contain far less hydrogen than those oxidised in pure water. A mechanistic explanation will be provided.