Understanding hydrogen diffusion and trapping behaviour in low alloy steels for gaseous hydrogen storage and transport

Abstract

The master’s thesis was commissioned by LUT University and Swerim AB, partnered with SSAB Europe. The main goal was to quantify and map hydrogen desorption using advanced techniques and relating them to the trapping sites in low-carbon steels. The microstructures of the low-carbon steels were characterized by light optical microscope (LOM) and scanning electron microscope (SEM). The specimens were hydrogen-charged by two methods: in electrolyte solution and in a pressurized hydrogen gaseous environment. Thermal desorption spectroscopy (TDS) quantified the hydrogen content on the charged specimens, further analysis was the deconvolution of the curves in hydrogen peaks, identifying the different trapping sites of each steel. Scanning kelvin probe force microscopy (SKPFM) located where hydrogen desorption had higher intensity. Desorption energy calculated from TDS results were correlated with SKPFM analysis to associate each peak energy to a trapping site.

Hydrogen was trapped most effectively in the ferritic-pearlitic (FP) low-carbon steel. Hydrogen was seen to be in dislocations and vacancies of cementite in the pearlite and only released when the temperature reached 400 °C. The ferritic-bainitic (FB) and the bainitic (B) low-carbon steels showed only weak traps, releasing hydrogen below 200 °C, coming mainly from interstitial lattices and dislocations of the ferrite. The activation energy (Eₐ) of lattices and dislocations in ferrite was 30.9 kJ.mol⁻¹ and 41.2 kJ.mol⁻¹, whereas dislocations and vacancies in the cementite were 53.2 kJ.mol⁻¹ and 66.4 kJ.mol⁻¹. SKFPM revealed that pearlite trapped hydrogen longer than ferrite. These results can be used to forecast the amount of hydrogen content and its peaks as well as designing alloys which minimise hydrogen embrittlement by creating appropriate deep trap sites.