Diffusion hydrogen absorption

The role of the stress in embrittlement and stress-corrosion processes has been examined in some detail by employing the slow strain-rate technique . Specimens of alloy 7179-T651 tested in air or in vacuum after pre-exposure to water at 70° C or in water at various potentials at ambient temperature exhibited a reversible embrittlement in excess of that arising from testing in moist air . The embrittlement was attributed to hydrogen absorption, and recovery was thought to be due to loss of hydrogen (particularly under vacuum) or to diffusion to traps. Potentiostatic tests revealed... 

Said this, we can let the reader to recall Fig. 1.15, where we depicted amorphous-like phase regions at grain boundaries as the pathways open for preferential diffusion of hydrogen atoms. Apparently, an alloy can benefit from some fraction of amorphous phase to improve kinetics of hydrogen absorption, but complete amorphization of crystalline lattice lowers capacity for storing hydrogen [156]. Mechanochemical activation is therefore a complex process where kinetic and thermodynamic effects must be firstly well understood, and then optimized. 

Hydrides. According to X-ray and neutron diffraction and metallographic studies of the Nb-H system,524 the H may be considered a lattice gas with phase transitions. In the a-, a -, j6-, and -phases of the system, H occupies tetrahedral interlattice positions. Whereas direct reaction between niobium metal and hydrogen occurs only after repeated activation of the metal by hydrogen absorption at ca. 7 atm and 350 °C, NbH2 is formed at temperatures as low as 22 °C in mixtures of LaNi5H6 7 and Nb.525 The extraordinary catalytic effect of the lanthanum-nickel complex is attributed to the presence of surface-absorbed atomic hydrogen species which are able to diffuse into the niobium lattice. There has been a review of the T a-H system.526... 

And again the residual stresses change the symmetry of the task the diffusion process in the hollow cylinder obeys the law of flat symmetry. The hydrogen absorption rate increases from the side of the outer boundary. Acceleration of the process kinetics is caused by reducing the compression stresses in the radial direction, which are changed with the tensile stresses. The solution of tasks (5) and (6) is well known. Thus, it is not difficult to obtain the solution of task (4). This solution describes the hydrogen absorption kinetics from the two surfaces of the hollow cylinder at given character of the residual stresses distribution... 

These results were obtained, using hydrogen absorption into pure aluminum between 300 and 1050 °C and were calculated by temporal process degassing. Eichenauer et al. also reported that the square root Sievert s law is satisfied within the limit of error in the solid and in the liquid state. Utilizing the fact that diffusion in the metal is responsible for the hydrogen degassing rate of the solid aluminum they deduced the diffusion coefficient to be D = 0.11 exp (—9780/ RT) cm s . The heat of solution was calculated from these measurements by Birnbaum et al. [7] and Ichimura et al. [8] and found to be in the range of + 0.6 to + 0.7 eV. 

Experimental results for hydrogen diffusion coefficients and their relevance for the kinetics of hydrogen absorption are presented in Section 26.3. Section 26.4... 

---