The corrosion performance for different areas on the cross-section of high strength low alloy (HSLA) thick steel plate was comparatively assessed in simulated marine environments via neutral salt spray tests, electrochemical impedance spectroscopy, and microstructural analyses (SEM, XRD, EBSD). It is especially concerned the corrosion behavior of the top surface layer (lath martensite, LM) and the center region (granular bainite (GB)/lath bainite (LB) mixed microstructure) of the thick plate. Results demonstrated that the LM-dominated surface exhibited superior long-term corrosion resistance, with a higher free-corrosion potential and lower corrosion current density in contrast to that of the center ones. The LM structure may facilitate the formation of a dense α-FeOOH rust layer, therewith effectively blocking Cl^- and O₂ ingress, while the heterogeneous GB-LB center initially delayed corrosion via low-energy coincidence site lattice (CSL) grain boundaries but suffered accelerated degradation due to microcracks and residual metastable γ-FeOOH caused by internal stress. Weight loss analysis revealed the surface's corrosion rate of the top surface decreased from 0.672 mm/a (6 h) to 0.214 mm/a (168 h), outperforming the center. XRD and SEM confirmed higher α-FeOOH/γ-FeOOH + Fe₃O₄ ratios (α/γ*) in the surface rust layer, indicating the enhanced stability. EBSD highlighted fewer high-angle grain boundaries and higher localized strain in the LM structure, reducing the Cl^- inward diffusion, whereas due to the heterogeneous phase interfaces and grain boundary defects in the GB-LB center's mixed microstructure, although the corrosion was initially delayed by the relatively large number of low-energy CSL grain boundaries, but the internal stress in the rust layer later could induce microcracks, leading to a decline in protective performance. Besides, electrochemical tests further validated the top surface's superior barrier properties, with higher charge transfer resistance. This work elucidates the critical role of microstructure homogeneity and grain boundary characteristics for R&D of corrosion resistant marine-grade HSLA steels.