Abstract: With the extension of urban rail transit systems, more and more buried gas pipelines will suffer from stray current interference due to contact with adjacent metro systems. Stray current interference will generate a hydrogen evolution reaction through cathodic current flow, which leads to severe hydrogen embrittlement hazards on pipeline steel. In this work, we used scanning electron microscopy to obtain the morphology of fracture surfaces, which is ascribed to hydrogen embrittlement for Q235 pipeline steel under stray current interference, considering varying defect area and environmental pH value. The fracture surfaces were then quantitatively characterized by multifractal methods, including multifractal spectrum and generalized fractal dimension. A novel method was proposed to accurately describe the surface morphology of hydrogen-induced fracture by combining spectral width Δα and generalized dimension threshold width ΔD_q. The results indicate that HE fracture surfaces of Q235 pipeline steels under stray current interference show obvious self-similar and multifractal characteristics. Moreover, multifractal spectrum f(α)-α and generalized fractal dimension D(q)-q exhibit high-accuracy performance in depicting the roughness and homogeneity of complex fracture surfaces, considering varying defect area and environmental pH value. Spectral width Δα and generalized dimension threshold width ΔD_q show a good negative correlation with hydrogen embrittlement susceptibility. The underlying mechanism was analyzed through the energy-absorbing effect during the transition process from ductile to brittle fracture. The findings of this research offer a new perspective that describes the fracture surface through probability distribution. The proposed approach provides a potential application for hydrogen embrittlement susceptibility evaluation through an on-site screening technique.

Key words: Stray current, Fracture surface, Hydrogen embrittlement susceptibility, Pipeline steel, Multifractal and generalized fractals