Diffusion and trapping of hydrogen in pipeline steels

The presence of hydrogen into steels is well known to be detrimental, in particular for ferritic steels: typical phenomena are stress corrosion cracking due to “hydrogen embrittlement” mechanism, worsening of mechanical properties (i.e. significant decrease of ductility and /or fracture strength), delayed fracture or irreversible damage (like blistering). The thesis deals with hydrogen diffusion of two pipeline steels, a carbon-manganese (API 5L X65) and a low alloyed (ASTM A182 F22), in three different metallurgical microstructures (quenched, quenched and tempered and annealed) and mechanical states (plastically deformed and under the fatigue loading), compared with those obtained on C-Mn steel not produced for sour service (API 5L grade B). Tests were carried out by electrochemical permeation techniques proposed by Devanathan and Stachurski with three different diffusion coefficient measurement methods: 1) standard ISO charge method; 2) partial charge/discharge method (after long polarisation); 3) discharge method. The charging method enabled to measure an apparent diffusion coefficient, influenced by both lattice diffusion and trapping. The diffusion coefficient obtained by partial charge/discharge and the first stage of complete discharge method was close to the one of pure iron, then it can be reasonably assumed to be the lattice diffusion coefficient of hydrogen, dependent only on the hydrogen migration processes in the crystal lattice regular sites. When the microstructure was modified by means of thermal treatment, the lowest diffusion coefficient was found for quenched, intermediate for quenched and tempered and the greatest one for annealed steel. The highest lattice hydrogen concentration belongs to the martensitic steel. Since the transport phenomena are hindered in martensitic steel, the hydrogen concentration can be locally increased thus rendering the steel more susceptible to cracking. However, the cracking susceptibility and related consequences cannot be easily interpreted by diffusivity of hydrogen alone. D lattice was reduced in plastically deformed steel, and this effect was more evident with increasing level of plastic strain. The binding energy of reversible traps obtained by fitting procedure based upon the McNabb and Foster and Oriani`s models was very similar (-34.4 ± 2.0 kJ/mole) and can be associated with dislocation or grain boundaries

Diffusione e intrappolamento dell’idrogeno in acciai per condotte