Predictions of ingress of hydrogen isotopes for zirconium alloys should not be based on solubility limits

The equilibrium hydrogen solvus in the Zr-H system is still in doubt after 60 years of experiments and debate. Currently, there are two ‘equilibrium’ solvi: one for heating and one for cooling. Two solvi are not allowed at equilibrium by Gibbs’ Phase Rule: there can be only one solvus. The ‘true’ solvus is often associated with the temperature associated with the maximum slope of the differential scanning calorimetry endothermic heat flow curve, called TSSD. This association stems from isothermal experiments where hydrogen moves from a source into the metal. Previous work has concluded that hydrogen stops moving when the TSSD temperature reaches the isothermal temperature. This stoppage is interpreted to occur when equilibrium is reached and, thus, the TSSD temperature is the equilibrium solvus. In this paper, long-time isothermal experiments are presented that show hydrogen movement continues well after the TSSD temperature reaches the isothermal temperature. Implications for predicting hydrogen isotope ingress for zirconium alloys are discussed.