Enhancing stress corrosion resistance in austenitic stainless steel through low-temperature oxy-nitriding surface modification: Investigation on H2S-induced phenomenon

The study focused on investigating the phenomenon of stress corrosion cracking induced by H2S, while also examining the influence of low-temperature oxy-nitriding (LTON) surface modification on the diffusion behavior of hydrogen atoms through electrochemical hydrogen charging experiments. Following the hydrogen charging process, the concentration of diffusible hydrogen in the base material (BM) was measured at 5.41 ppm (ppm), which reduced to 1.10 ppm after implementing LTON treatment. Clearly, the surface-modified layer effectively hindered the diffusion of hydrogen atoms within the steel substrate. In the case of BM specimens exposed to H2S stress corrosion, the duration of exposure significantly impacted the formation of corrosion products. Additionally, the externally applied tensile stress played a pivotal role in the cracking behavior, as increasing stress levels led to the emergence of deformation bands outside the C-ring and ultimately initiated cracks at the base of the pits. Notably, H2S stress corrosion pits accumulated oxides and sulfides. Excessive hydrogen supersaturation potentially induced localized plastic deformation. Furthermore, due to the presence of sulfide ions, hydrogen atoms could not recombine to form hydrogen molecules, resulting in an accumulation of hydrogen atoms on the steel surface. The diffusion of hydrogen atoms within the steel material strongly influenced the initiation and propagation of stress corrosion cracking. Importantly, the surface modification layer established by LTON treatment exhibited remarkable capability in enhancing the steel material's resistance against H2S-induced stress corrosion. This was achieved by effectively preventing acidic medium attack and impeding the penetration of hydrogen atoms.