Probing hydrogen content in steel using the thermoelectric effect

The issue of hydrogenation of steel is a serious technological problem. Due to its small atomic radius, hydrogen is difficult to analyze quantitatively and qualitatively. Non-destructive estimation of hydrogen content would significantly improve the quality control process. In this work, we attempt to develop a new methodology for the determination of hydrogen content in 4340 steel using the thermoelectric effect. For this purpose, we investigated the mechanisms of hydrogenation, structural and microstructural changes caused by hydrogenation, and performed theoretical calculations of the influence of hydrogen on the electronic structure of steel. Measurements of the Seebeck coefficient were performed using the specially developed mu m-resolution scanning thermoelectric microscope (SThM). The KKR-CPA calculations show that hydrogen located at T-voids strongly modifies electronic properties and influences the Seebeck coefficient of the material. Ultraviolet photoelectron spectroscopy (UPS) proves that hydrogenated samples exhibit the valence band onset shift towards higher binding energies concerning the non-hydrogenated materials. Theoretical and experimental studies of the Seebeck coefficient dependence on the total hydrogen content confirm a measurable decrease in the Seebeck coefficient (18.5 mu V/K for 0.136 ppm and 13.5 mu V/K for 1.03 ppm). Plotted calibration curves for 4340 steel were used for preliminary kinetic studies of the spontaneous dehydrogenation in this material. This work demonstrates that the thermoelectric effect can be used as the basis of a new fast and non-destructive analytical method for the determination of the hydrogen content in 4340 steel.