SIMS STUDIES OF HYDROGEN DIFFUSION THROUGH OXIDES ON ZR-NB ALLOY

Hydrogen migration has been followed through the thermally-grown oxide on a Zr-2.5% Nb alloy. A secondary ion mass spectrometer, calibrated for deuterium, was used to measure the concentration as a function of depth into the film. Thin film oxide specimens, grown in steam to approximately 1-mu-m thickness, were heated to 350-degrees-C and exposed to deuterium gas at pressures ranging from 6 x 10(-3) to 6 Pa and times from 30 min to 870 min. Some irreversible uptake was detected for all exposures using SIMS. At low exposures, the shape of the deuterium concentration profile is Fickian and diffusion coefficients have been calculated. In this low-exposure regime, the effect of temperatures between 280-degrees-C and 350-degrees-C on the diffusion equation has been measured. At longer exposures, the rate of deuterium ingress was sharply curtailed and a more complex diffusion profile was observed. Using Raman spectroscopy during a depth profile, the ZrO2 component of the oxide is found to change from a monoclinic-tetragonal mixture at the oxide surface to a primarily tetragonal oxide near the oxide/metal interface. Such changes in phase probably contribute to the complex deuterium diffusion kinetics observed during the longer exposures of the oxide film to deuterium gas.