Hydrogen Embrittlement of Austenitic Stainless Steels-Influence of hydrogen charging, microstructure and low-temperature carburizing

Doctoral thesis, 2026

Austenitic stainless steels (ASS) are widely applied in hydrogen-related infrastructures because of their excellent corrosion resistance, low hydrogen diffusivity and good ductility. However, ASS still faces the risk of failure when exposed to hydrogen environments for extended periods. More specifically, hydrogen reduces ductility and fracture toughness, leading to sudden fracture of steel, a condition known as hydrogen embrittlement (HE), thus posing a critical challenge to the hydrogen economy. Microstructure and surface treatment are key factors affecting HE because they influence hydrogen diffusion and uptake, thus affecting deformation and mechanical properties. This thesis study systematically investigates the effects hydrogen charging, microstructure and low-temperature carburizing (LTC) on the HE of ASS. HE behavior was evaluated by introducing hydrogen into steel using cathodic charging combined with slow strain rate tensile tests. Scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) techniques were used for analyzing the microstructure and fracture morphology, along with hydrogen content, to reveal the failure mechanism. The main results of this study indicate that:

1) Hydrogen uptake, microstructure, and surface morphology are influenced by current density and electrolyte. During the hydrogen charging process in H₂SO₄ electrolyte, high current density can lead to surface cracking, martensitic phase transformation, and increased hydrogen content due to high hydrogen fugacity and surface stress. Increasing surface roughness can reduce hydrogen uptake and suppress surface cracking. Hydrogen charging in NaCl and NaOH electrolytes reveal intact surfaces and low hydrogen content.

2) LTC treatment introduces an approximately 22 μm thick expanded austenite layer on the surface of ASS, exhibiting lattice expansion, high hardness and high carbon concentration. The expanded austenite effectively suppresses hydrogen and strain-induced martensitic phase transformation. The effect of LTC on hydrogen-induced cracking and HE depends on the surface carbon concentration. Low carbon concentration in expanded austenite reduced surface cracking, HE and hydrogen uptake, while high carbon concentration leads to severe surface cracking, decomposition of expanded austenite, and increased hydrogen uptake and HE.

3) Hydrogen, similar to interstitial carbon, strengthens ASS thin film but introduces brittleness. Hydrogen embrittlement and carbon embrittlement share similarities, both being caused by stress cracking induced by high concentration gradients. The coexistence of both leads to softening, attributed to a decrease in lattice parameters and precipitation. Hydrogen triggered precipitation, deformation twins, accelerated local deformation with enhanced orientation-related slip.