Laser cladding layers and diamond-like carbon (DLC) films are widely employed to improve the corrosion resistance of marine steels. However, defects such as pores and cracks in these coatings can act as initiation sites for localized corrosion during prolonged exposure to marine environments, thereby accelerating substrate failure. In this study, 304 stainless steel (304 SS) was first treated by laser cladding and subsequently coated with various nitrogen-doped DLC films deposited by plasma-enhanced chemical vapor deposition to form composite coatings. The localized failure mechanisms of these composite coatings under long-term exposure to sodium chloride solutions were investigated through electrochemical measurements, microstructural characterization, thermodynamic analysis, and numerical simulation. The results demonstrate that the DLC layer effectively seals surface defects in the laser-cladded layer and enhances barrier performance owing to its high chemical inertness. Nitrogen incorporated significantly alters the corrosion behavior by consuming locally generated H+ ions to form NH4+, thereby mitigating acidification within defect microzones. This shift in local pH promotes the formation of compact spinel-type oxides (e.g., Fe3O4, FeCr2O4, and NiFe2O4) instead of porous hematite, effectively suppressing autocatalytic localized corrosion. Increasing nitrogen content enhances the density of active sites in the DLC matrix, accelerates spinel nucleation, and facilitates earlier passivation. In situ electrochemical noise analysis reveals a transition from stable pitting in undoped coatings to uniform corrosion and eventual passivation in N-doped coatings. A reaction–diffusion model based on the Nernst–Planck framework further clarifies the relationship between microzone pH evolution and oxide stability. These findings elucidate the synergistic mechanism between laser-induced microstructural heterogeneity and nitrogen-regulated local chemistry, providing a mechanistic basis for durability prediction and optimized design of laser cladding/DLC composite coatings for marine applications.