Hydrogen embrittlement of Ni-Ti alloy in biological environment

It is known that Ni-Ti superelastic alloy is susceptible to environmental embrittlement in a corrosive atmosphere. Diffusion rate of hydrogen is thought to be one of the factors in deciding a service life. Accelerating test of hydrogen embrittlement was carried out, i.e., the alloys with a diameter of 0.56 mm were charged with hydrogen using an electrochemical system. The alloys with charging of 1 A/m(2) or 10 A/m(2) for 24 h or 120 h, respectively, were tested. Hardness numbers on cross-sectional area were measured. Distributions of hydrogen concentration were computed for infinite cylinder model by differential equation. If the hardness is linear to the concentration of hydride and/or hydrogen, diffusion constant of hydrogen through the Ni-Ti alloy could be estimated 9 x 10(-15) m(2)/s. Experimental results of the hardness and fractography support the estimated diffusion constant. Hydrogen embrittlement and fretting corrosion were discussed with the hydrogen intake.