Hydrogen trapping phenomena in metals with B.C.C. and F.C.C. crystals structures by the desorption thermal analysis technique

Abstract

The types and amounts of lattice defects acting as trapping sites of hydrogen in metals with b.c.c. and f.c.c. structures and those induced by hydrogen during cathodic charging have been determined by the thermal analysis technique. Trapping parameters such as the trap activation energy, trap binding energy and trap density for each trap have been examined by using the mathematical models derived from the existing trap theory. From the results of thermal analysis of b.c.c.-structured metals, it is observed that the interfaces of non-metallic inclusions such as iron oxides, Al 2O 3, MnS and TiC are strong trapping sites for hydrogen with a high trap activation energy while the lattice imperfections, grain boundaries, dislocations and microvoids are shallow traps. The energy levels of hydrogen around each trapping site are as follows: for a strong trapping site, the saddle point energy is higher than the activation energy of lattice diffusion, which means that trapping and detrapping of hydrogen are difficult but, in a shallow trapping site, the saddle point energy is lower so that trapping and detrapping are easy. In pure nickel with an f.c.c. crystal structure, dislocation and grain boundaries have low trap activation energies compared with the activation energy for lattice diffusion. Equations for lattice solubility and diffusivity under atmospheric hydrogen pressure in pure nickel have been obtained by the thermal analysis technique and are as follows: C(( H atom) atom -1)= 1.57 × 10 -3 exp − 11.80 kJ RT D( cm 2 s -1)=7.5 × 10 -3 exp − 39.23 kJ RT It is found that internal microcracks or microvoids are generated and are the major trapping sites of hydrogen when pure iron is cathodically charged.