<p>This paper investigated the corrosion resistance and mechanical properties of laser-welded T-joints under two types of corrosion environments, i.e., the neutral salt spray corrosion (NSSC) and simulated seawater immersion corrosion (SSIC). The heterogeneous microstructure, corrosion morphology, production, electrochemical curve, mechanical response and fracture characteristics were extensively investigated by a series of tests to clarify the evolution of corrosion mechanism with environment and its effect on mechanical properties. Results show that the corrosion products were FeOOH + Fe₃O₄ and CaCO₃ + Fe₃O₄ under NSSC and SSIC, respectively. The corrosion rate initially increased and subsequently decreased with the accumulation of corrosion time regardless of environments, but it was always lower under SSIC than that under NSSC. The corrosion resistance of welding material (WM) was inferior to that of base metal (BM) due to geometric discontinuity and lath martensite. Therefore, different with tensile fracture happened at BM for uncorroded T-joint, it trended to transfer to WM with the going of corrosion, and transgranular corrosion characters was found at WM. But it will return to BM with intergranular corrosion characters for T-joint under NSSC when corrosion time enough due to that the accumulation of corrosion products at WM blocked the continued penetration of ions, and the corrosion spread toward BM. A quantitative corrosion parameters analysis was conducted to reveal the relationship between the fracture location and the accumulated corrosion damage, and develop the time- and location-dependent corrosion and fracture mechanism.</p>