Effects of Prior Cold Work on Hydrogen Trapping and Diffusion in API X-70 Line Pipe Steel During Electrochemical Charging

The effect of prior cold work (10, 20, and 40% reduction in thickness) on hydrogen diffusion and trapping was investigated using both Devanathan permeation and thermal desorption methods. The first rise transient during diffusion-controlled permeation marks the slowest Deff (1.8×10−7 cm2/s in as-received API X-70 steel), which is indicative of the greatest degree of trapping by both irreversible and reversible traps. Faster Deff during all subsequent decay and rise transients (3.0 to 4.2×10−6 cm2/s in as-received API X-70 steel) indicates partial trap filling/release from reversible traps and permanent filling of irreversible traps after the first rise transient. Cold work substantially increased trapping as evident from both slower permeation and reduced Deff (2.0 to 4.2×10−7 cm2/s in cold-worked API X-70 steels) as well as by thermal desorption. Thermal desorption spectroscopy indicates one relatively reversible and one room temperature irreversible trap state in the cold-worked steels with desorption activation energies of 13.9±0.8 and 19.9±0.8 kJ/mol, respectively. The reversible trap state was the dominant absorber of H in the cold-worked materials.