The role of pearlite lamellae spacing on hydrogen permeation

Hydrogen embrittlement (HE) presents a significant challenge in materials science, notably reducing the ductility and load-bearing capacity of susceptible metals and leading to premature failures. The phenomenon is particularly problematic in ferritic and ferritic-pearlitic steels used in pipeline applications, due to their body-centered cubic (b.c.c) crystal structure which facilitates rapid hydrogen diffusion and accumulation. This study quantitatively examines how pearlite lamellae spacing affects hydrogen permeation in fully eutectoid AISI 1080 steel, isolating the effect from other microstructural factors. Three heat treatments produced coarse, medium, and fine pearlite with interlamellar spacings of 452 ± 11 nm, 338 ± 17 nm, and 223 ± 10 nm, respectively. Electrochemical permeation tests showed that as spacing decreased, steady-state permeation current rose from 0.0025 A/m 2 to 0.0040 A/m 2 , while the effective diffusion coefficient dropped from 1.7 × 10 −11 m 2 /s to 8.1 × 10 −12 m 2 /s. Near-surface hydrogen concentration increased by about 60%, and trap density ranged from 1.2 × 10 26 m −3 to 1.4 × 10 27 m −3 , shifting from irreversible to reversible traps. Statistical analysis confirmed significant differences among all spacing conditions. Overall, a 48% reduction in interlamellar spacing slowed hydrogen diffusion but increased flux and retention, revealing a critical spacing threshold that controls hydrogen trapping and providing a framework for designing steels with improved resistance to HE. • Isolated effect of pearlite lamellae spacing on hydrogen permeation. • Fine pearlite slows diffusion but increases hydrogen retention (C 0R ) and flux. • Coarse pearlite enables faster diffusion with lower hydrogen retention (C 0R ). • Critical spacing shifts balance between reversible and irreversible traps.