Hydrogen-induced Vacancy Formation Process in Austenitic Stainless Steel 304

The interaction of hydrogen with lattice defects plays a crucial role in the hydrogen embrittlement mechanism, but the origin of hydrogen-related defects remains unclear. In this study we investigate the formation process of hydrogen-induced vacancies in austenitic stainless steel SUS 304 by positron annihilation lifetime spectroscopy. Positron lifetime measurements of hydrogen-charged samples subjected to tensile testing by different strains show that the formation of hydrogen-induced vacancies first appears when a strain of about 5% is applied. Electropolishing of the hydrogen-charged layer reveals the generation of vacancy-hydrogen complexes in the bulk underneath the hydrogen-charged layer, which develop into vacancy clusters by further application of stress. From the PALS results and complementary X-ray diffraction analysis, the dislocation density required for the formation of hydrogen-induced vacancies is quantitatively determined. Clarification of the conditions for the formation of hydrogen-induced vacancies provides important input and reference data for models of the hydrogen embrittlement in stainless steels.