1991 Volume 40 Issue 12 Pages 827-833
The embrittlement during the electrolytic hydrogen charging at 25-55°C and its recovery during hydrogen outgassing at 65-110°C after the charging were examined on Monel metal in some details with regard to tensile properties, fracture surface, and X-ray diffraction. A thin layer of hydride formed on the surface during the charging. It was unstable and disappeared outgassing, leaving local lattice strain in the underlying metal. Brittle fracture always occurred at grain boundaries in tensile test. Tensile strength and strain at fracture decreased with the increases in charging time and temperature. There was a relation, h∝ t1/2, between the depth of brittle layer (h) and charging time (t). The activation energy for the diffusion of hydrogen atoms obtained from the temperature dependence of h vs. t1/2 was estimated to be 10.7kcal/mol. On the other hand, the activation energy for recovery process, calculated from Arrhenius plot of the reciprocat time of 50% recovery of the strain at fracture, was evaluated to be 10.5kcal/mol. A linear relation was held between the mean depth of brittle layer and tensile strength and between the depth and strain at fracture, regrdless of charging and outgassing. These results show that the processes of both embrittlement and its recovery are rate-determined by the diffusion of hydrogen atoms in the metal.
