Abstract: To reveal the pitting initiation mechanism of Al-Li alloys under simulated seawater immersion and clarify the electrochemical heterogeneity and interfacial coupling effects inside multiphase particles, a systematic investigation was carried out on the pitting behavior and corresponding influence mechanism of Al-Li alloys immersed in simulated seawater. This work aimed to accurately identify the phase compositions, potential differences, surface activity and atomic-scale electronic structural characteristics of intermetallic compounds in Al-Li alloys, establish the correlation laws between multiphase microstructures and localized corrosion susceptibility, and provide a theoretical basis and data support for the composition design, heat treatment process optimization and corrosion protection technologies of Al-Li alloys. Electron backscatter diffraction phase mapping was adopted to precisely characterize the phase compositions of intermetallic compounds in Al-Li alloys. A simulated coupling system composed of intermetallic compound particles and a pure Al matrix was fabricated to achieve precise regulation of interfacial structures and eliminate interference induced by other precipitated phases and defects. Scanning electron microscopy combined with energy dispersive spectroscopy was utilized to observe pitting morphologies and localized corrosion characteristics. Scanning Kelvin probe, scanning vibrating electrode technique and electrochemical impedance spectroscopy(EIS) were employed to test potential distribution, local current density and interfacial charge-transfer performances. First-principles calculations based on density functional theory were implemented to analyze surface energy, adsorption energies of oxygen and chloride ions, electron localization function and charge density difference, and reveal the differentiated corrosion mechanism of each phase at the atomic and electronic scales. The results show that intermetallic compounds in Al-Li alloys consist of three phases, including Al₄Cu₉, Cu₃Fe₁₇ and Fe₁₉Mn. The electrode potential of Al₄Cu₉ is approximately 300 mV higher than that of the aluminum matrix. This phase acts as a cathode and accelerates preferential dissolution of the surrounding matrix to form annular trenches. The potentials of the Cu₃Fe₁₇ and Fe₁₉Mn are roughly 300 mV lower than that of the aluminum matrix, and these two anodic phases dissolve preferentially to generate pitting pits. EIS test results indicate that the charge transfer resistance of the Al₄Cu₉ is markedly higher than those of the other two phases, which endow this phase with better corrosion resistance. First-principles calculations confirm that the Al₄Cu₉ (330) crystal plane possesses the lowest surface energy at 1.293×10²¹ eV/m² (12.93 eV/Ų). The phase exhibits the strongest structural stability. O₂ presents low adsorption energies on the Cu₃Fe₁₇ (110) crystal plane and the Fe₁₉Mn (310) crystal plane, which promotes the proceeding of oxygen reduction reactions. Analyses of electron localization function and charge density difference demonstrate that the Al₄Cu₉ phase has stronger interatomic bonding and interfacial electron enrichment. The other two phases display weak interatomic bonding and obvious electron loss, corresponding to higher corrosion activity. This work illustrates the multi-galvanic coupling corrosion mechanism within intermetallic compounds and verifies that electrochemical heterogeneity of internal phases serves as the core factor triggering pitting corrosion of Al-Li alloys. The research findings establish the intrinsic correlations among phase structure, surface activity, electronic characteristics and corrosion behavior. The results can provide theoretical support for regulating precipitated phases and enhancing the pitting resistance of Al-Li alloys and offer references for durability design and service life prediction of Al-Li alloy structural components applied in marine environments.