A reason why fitness-for-service assessments of hydrogen isotopes in pressure tubes are not conservative

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 MS. This association stems from isothermal experiments where hydrogen flows from a source into the metal. Previous work has indicated that hydrogen stops flowing when the TSSD MS temperature reaches the isothermal temperature. This stoppage is claimed to occur when equilibrium is reached and, thus, the TSSD MS temperature is the equilibrium solvus. In this presentation, long-time isothermal experiments are presented that show hydrogen ingress continues well after the TSSD MS temperature reaches the isothermal temperature. Implications for fitness-for-service assessments are discussed.