Characterization of Hydrogen Thermal Desorption Behavior and Enhancement of Hydrogen Embrittlement in Ni-Ti Superelastic Alloy Induced by Cathodic Hydrogen Charging in the Presence of Chloride Ions

Characterization of hydrogen thermal desorption behavior and enhancement of hydrogen embrittlement in Ni-Ti superelastic alloy have been investigated in terms of the type of cathodic charging in easy-to-handle 0.15 M NaCl, KCl, Na2SO4, and NaHCO3 solutions. Immediately after hydrogen charging at approximately 200 mass ppm, the amounts of hydrogen desorbed in the low-temperature region (150-400 degrees C) for specimens charged in NaCl and KCl solutions are larger than those charged in Na2SO4 and NaHCO3 solutions. Even upon aging at room temperature after hydrogen charging, the shoulder in the low-temperature side (400-500 degrees C) of the desorption peak remains for specimens charged in NaCl and KCl solutions. In cyclic tensile deformation in the stress plateau region caused by stress-induced martensite and reverse transformations, fracture occurs after 50-60 cycles irrespective of the type of solution immediately after hydrogen charging. Upon aging, the cycle numbers to fracture of specimens charged in NaCl and KCl solutions (800-900 cycles) are smaller than those charged in Na2SO4 and NaHCO3 solutions (approximately 1000 cycles). The present results suggest that hydrogen states, which strongly interact with martensite transformation, are charged to a greater degree in the presence of chloride ions, thereby enhancing hydrogen embrittlement.