Identification of Trapping Sites in High-Strength Steels by Secondary Ion Mass Spectrometry for Thermally Desorbed Hydrogen

Abstract

The trapping sites of hydrogen detected by Thermal Desorption Spectrometry (TDS) have been identified by Secondary Ion Mass Spectrometry (SIMS). High-strength steel specimens were given applied loads in D2O and 20% NH4SCN solution at 323 K. In SIMS analysis, deuterium ions can be detected with greater sensitivity than hydrogen ions and measurement can be started in a matter of minutes. TDS analysis shows that hydrogen thermal desorption rate has two peaks, corresponding to trap activation energies of 20.7∼22.5 kJ/mol and 82.2∼87.4 kJ/mol for the lower and higher temperature peaks, respectively. These values are close to 26.8 kJ/mol reported in the literature as the trap activation energy for desorption from dislocations and ≥72.3 kJ/mol from the interfaces of inclusions and precipitates. By applying SIMS image-analysis to specimens cooled after the respective peak temperatures, we could identify the trapping sites corresponding to the lower temperature peak as sites within the matrix such as defects including dislocations, and those corresponding to the higher temperature peaks as inclusion interfaces, precipitate interfaces, and segregation bands of phosphorus. These SIMS results confirm the location of the trapping sites to be the same as those estimated from trap activation energy by TDS.