Hydrogen Embrittlement as a Surface Phenomenon in Deformed Metals

Hydrogen charging is widely used in industry for testing metals intended for natural gas pipelines and hydrogen energetics. Here we study the distribution of mechanical damage in a specimen resulting from hydrogen charging in a neutral solution based on hydrogen-induced cracking tests according to NACE standard TM0284. The investigation is carried out by acoustoelasticity (acoustic damage detection). The basic relationships are derived for the time delay in acoustic sounding and for damage tensor components. The dependences of the principal damage tensor components on the hydrogen charging time are plotted. The volume distribution of hydrogen concentrations after hydrogen charging is measured. It is found that the skin effect due to hydrogen charging is closely related to a similar effect of damage concentration in a 100-µm-thick surface layer. This means that hydrogen embrittlement induced by artificial hydrogen charging in aqueous electrolytes can be considered as a surface phenomenon. This conclusion allows us to verify the adequacy of hydrogen-induced cracking tests for metals, mechanical models of hydrogen embrittlement, and methods for evaluating and predicting the effect of the hydrogen-containing corrosive environment on the mechanical properties of metals and metal structures. The test results will surely differ considerably from the field evidence due to different hydrogen charging times. It needs to be ascertained if this difference is significant for the fracture mechanism of the specimens. The strong surface effect observed in the experiments on artificial hydrogen charging of metals requires further investigation and comparison.