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In this study, an Al₂O₃3D/5083 Al composite was fabricated by infiltrating a molten 5083 Al alloy into a three-dimensional alumina reticulated porosity ceramics skeleton preform (Al₂O₃3D) using a pressureless infiltration method. The corrosion resistance of 5083 Al alloy and Al₂O₃3D/5083 Al in NaCl solution were compared via electrochemical impedance spectroscopy (EIS), dynamic polarization potential (PDP), and neutral salt spray (NSS) tests. The microstructure of the two materials were investigated by 3D X-ray microscope and scanning electron microscopy aiming at understanding the corrosion mechanisms. Results show that an Al₂O₃3D/5083 Al composite consists of interpenetrating structure of 3D-continuous matrices of continuous networks 5083 Al alloy and Al₂O₃3D phase. A large area of strong interfaces of 5083 Al and Al₂O₃3D exist in the Al₂O₃3D/5083 Al composite. The corrosion development process can be divided into the initial period, the development period, and the stability period. Al₂O₃3D used as reinforcement in Al₂O₃3D/5083 Al composite improves the corrosion resistance of Al₂O₃3D/5083 Al composite via electrochemistry tests. Thus, the corrosion resistance of Al₂O₃3D/5083 Al is higher than that of 5083 Al alloy. The NSS test results indicate that the corrosion resistance of Al₂O₃3D/5083 Al was lower than that of 5083 Al alloy during the initial period, higher than that of 5083 Al alloy during the development period, and there was no obvious difference in corrosion resistance during the stability period. It is considered that the elements in 5083 Al alloy infiltrated into the Al₂O₃3D/5083 Al composite are segregated, and the uniform distribution of the segregated elements leads to galvanic corrosion during the corrosion initial period. The perfect combination of interfaces of Al₂O₃3D and the 5083 Al alloy matrix promotes excellent corrosion resistance during the stability period.