Defects Responsible for Hydrogen Embrittlement in Austenitic Stainless Steel 304 by Positron Annihilation Lifetime Spectroscopy

Abstract

Hydrogen-related defects in the metastable austenitic stainless steel 304 were analyzed by positron annihilation lifetime spectroscopy to determine the factors responsible for hydrogen embrittlement. Hydrogen was introduced by the cathodic electrolysis method to the ~10-µm topmost layer which was then etched by electrochemical polishing to investigate the formation of hydrogen-induced defects in the bulk. Although hydrogen embrittlement did not occur simply upon removal of the hydrogen-charged layer, a decrease in ductility was confirmed by applying 10% tensile strain and subsequent polishing. In this latter sample a positron lifetime component of ~180 ps was detected, which is longer than that of dislocations, and disappeared upon annealing at 100°C. After further straining the sample until fracture, the formation of vacancy clusters was observed. These results suggest that hydrogen diffused to deeper regions upon application of tensile stress, where vacancy-hydrogen complexes were generated and developed into vacancy agglomerates. The detection of the precursors of the vacancy clusters, which are thought to lead to the brittle fracture, represents a fundamental step forward in the understanding of the hydrogen embrittlement process in austenitic stainless steels.