Mechanism exploration of the influence of alternating loading frequency on hydrogen-induced cracking susceptibility of high strength pipeline steel

This study evaluated the HIC susceptibility of X70-grade acid-resistant (X70 MS) pipeline steel under various alternating loading frequency (f_a). The underlying mechanisms were elucidated by measuring the cathodic adsorbed hydrogen concentration (C_ad), subsurface hydrogen concentration (C₀) and apparent diffusivity (D_app), combined with X-ray diffraction (XRD) and bright-field transmission electron microscopy (BF-TEM) analyses. The results indicated that the HIC susceptibility of X70MS pipeline steel initially decreased and then increased with increasing f_a, exhibiting a critical frequency at f_a = 1.5 Hz. This behavior is attributed to the coupling effect between hydrogen and dislocations at different frequencies. When f_a < 1.5 Hz, the difference (∆v) between the dislocation velocity (v_D) and the critical velocity (v_cr) for hydrogen detachment from dislocations decreased as f_a increased, thereby weakening the hydrogen-pinning effect and shortening the cycle time to suppress hydrogen accumulation, resulting in reduced HIC susceptibility. In contrast, when f_a > 1.5 Hz, the dislocation velocity increased and deviated further from the critical velocity, leading to a moderate increase in ∆v. Under these conditions, hydrogen readily combined with dislocations to form Cottrell atmospheres, thereby increasing the HIC susceptibility.