Thermal Desorption Spectroscopy Evaluation of the Hydrogen-Trapping Capacity of NbC and NbN Precipitates

In the current study, ferritic steels containing NbC or NbN precipitates were investigated. The materials were subjected to various heat treatments, giving rise to different precipitate size distributions as determined by transmission electron microscopy. Both NbC and NbN precipitates act as hydrogen traps. The steels were hydrogen charged both electrochemically and/or from the gaseous hydrogen source, followed by multiple thermal desorption spectroscopy (TDS) measurements. Electrochemical charging gave rise to a low-temperature peak [323 K to 523 K (50 °C to 250 °C)], originating from the hydrogen trapped near grain boundaries, with activation energy ranging between 24 and 33 kJ/mol, and at small NbC (39 to 48 kJ/mol) or NbN precipitates (23 to 24 kJ/mol). Gaseous charging caused a high-temperature TDS peak [723 K to 923 K (450 °C to 650 °C)], which was attributed to the presence of incoherent precipitates. The activation energy for NbC precipitates, charged in a hydrogen atmosphere, ranged between 63 and 68 kJ/mol and between 100 and 143 kJ/mol for NbN precipitates.