Cr-assisted rust densification governs hydrogen entry–retention trade-off in pipeline steels

Wet-dry atmospheric cycling drives hydrogen ingress into repurposed constructional steels, yet corrosion layers often act as static boundaries. This study investigated the hydrogen co-evolution of API X65 and P110 steels exposed to a representative wet-dry environment. Continuous, Cr-enriched inner layer formed on P110 after 7 cyclic exposure, about 3 times thicker than on X65 even after 21 cycles. After 21 cycles, total hydrogen content in P110 reaches 9.2 ± 0.4 ppm versus 3.6 ± 0.3 ppm in X65. The hydrogen content in the corroded P110 research plateaus after around 29 cycles while that in X65 continues increasing. Thermal desorption bands (160-360 °C) and Kelvin probe shifts indicate redistribution toward stronger traps at the interface of oxide/P110 substrate, whereas a larger near-rust-surface diffusible fraction is present in X65. These findings help a barrier-trap trade-off mechanism in which Cr-assisted densification reduces hydrogen ingress while enhancing deep retention under cyclic atmospheric corrosion.