The widespread application of aluminum alloys in automotive lightweighting is frequently compromised by corrosion initiated at coating defects, yet the specific failure mechanisms governed by defect morphology remain difficult to isolate via traditional experimentation. This study employs a two-dimensional multiphysics coupling model, constructed on the COMSOL platform, to quantitatively investigate the corrosion evolution of epoxy-coated aluminum alloys containing two representative defect types: interfacial delamination and through-thickness bubbles. By integrating electrochemical kinetic parameters, the simulation elucidates how defect geometry modulates potential fields, current density distributions, and ionic transport within the “coating–electrolyte–substrate” system. Results indicate that delamination thickness positively correlates with corrosion rates; thicker cavities (50μm) significantly reduce ohmic resistance, thereby accelerating micro-galvanic coupling between the α-Al matrix and β-phase and expanding the corrosion domain. Furthermore, a distinct size effect was observed for through-thickness bubbles: large-scale defects (50μm) facilitate unimpeded mass transport, leading to severe lateral propagation and extensive substrate loss due to stable charge loops. Conversely, small-scale defects (20μm) induce a pronounced occluded cell effect, generating high local current densities that drive rapid vertical penetration, identifying them as the critical factor for deep-base pitting. These findings provide a theoretical basis for optimizing coating integrity and predicting localized corrosion behaviors in aluminum alloy structures.