Determination of the equivalent hydrogen fugacity during electrochemical charging of 3.5NiCrMoV steel

A new thermal desorption spectroscopy (TDS) apparatus was used to identify the hydrogen trapping peaks, and to measure the hydrogen concentrations in 3.5NiCrMoV steel after hydrogen charging electrochemically, and in gaseous hydrogen. The hydrogen concentration increased with (i) increasingly negative charging potential and (ii) increasing hydrogen gas pressure. The equivalent hydrogen fugacity versus charging overpotential was derived. There was a difference in the diffusible hydrogen traps activated during electrochemical and gas phase charging, attributed to the difference in the hydrogen fugacity. A two-site model for Sieverts' Law explained the positive Y-intercept, as the density of already filled hydrogen traps.