Corrosion-Induced Degradation of Low-Cycle Fatigue Behavior in Q690qNH Weathering Steel: Coupling of Hierarchical Rust Layer Evolution and Surface Damage

Marine atmospheric corrosion can compromise the fatigue reliability of high-strength weathering steel structures, yet the coupled effects of protective rust-layer evolution and corrosion-pit damage on low-cycle fatigue behavior remain insufficiently understood. This study systematically investigates the degradation mechanism of low-cycle fatigue (LCF) performance in high-strength weathering steel Q690qNH under a simulated marine atmospheric environment, with particular emphasis on the coupling among hierarchical rust-layer evolution, corrosion-pit damage, and fatigue failure. The results show that the rust layer gradually evolves from an unstable γ-FeOOH dominated structure into a dense and stable α-FeOOH/Fe3O4 structure during corrosion. Accompanied by Cr enrichment in the inner rust layer, a protective inner rust-layer structure is developed, leading to a significant reduction in the corrosion rate. Meanwhile, corrosion pits formed during the early stage of exposure induce localized stress concentration and plastic strain accumulation under cyclic loading, thereby accelerating cyclic softening and significantly reducing fatigue life. At a strain amplitude of 0.8%, the fatigue life decreases by 31.9%, 55.2%, 60.5%, and 66.7% after 7, 30, 45, and 60 days of corrosion exposure, respectively. Given the limited replication, the reported fatigue-life reductions represent observed trends and require confirmation through additional testing. In uncorroded specimens, fatigue cracks mainly initiate at ferrite/M-A constituent interfaces, whereas corrosion pits become the dominant crack initiation sites after corrosion exposure; nevertheless, subsequent crack propagation remains governed by grain boundaries and microstructural characteristics. These results demonstrate that the LCF performance of Q690qNH weathering steel is jointly governed by the competing effects of corrosion-pit damage accumulation and the formation of a protective rust layer.