<p>Interfacial adhesion of epoxy resin-aluminum foil composites is critical for electronic packaging and aerospace applications. Trivalent chromium (Cr(III)) conversion coatings have emerged as critical alternatives to toxic and carcinogenic hexavalent chromium treatments, where phosphoric acid (H₃PO₄) serves as the primary acidic component to promote coating formation. However, single H₃PO₄ systems exhibit limited Cr(III) deposition efficiency, resulting in inadequate adhesion performance. This study developed binary acid systems combining H₃PO₄ with HCl, HNO₃, or HF for Cr(III) conversion treatment. Multi-scale characterization techniques including SEM/EDS, XPS, contact angle measurements, T-peel tests, and molecular dynamics simulations elucidated synergistic mechanisms governing Cr deposition, coating composition, and interfacial adhesion. Among the three combinations, the HF-H₃PO₄ (P + F) system achieved optimal results by facilitating HF attack on oxide layers, exposing Al–Fe intermetallics and attaining the highest Cr₂O₃content. The P + F system increased the surface free energy by 79.2%, reduced the contact angle to 22.1°, and achieved an adhesion strength of 55.3 N/15 mm, surpassing the single-acid H₃PO₄ system by 122.1% and the untreated foil by 250.0%. MD simulations provided theoretical insights into how Cr₂O₃ enrichment enhances adhesion at the atomic and molecular level. This work reveals that the enhanced interfacial adhesion is a synergistic result of physical interlocking caused by acid etching, improved wettability, and altered chemical compositions. Furthermore, by eliminating the interference of physical roughness, MD simulations help to theoretically isolate and evaluate the critical chemical contribution of Cr₂O₃ enrichment in interfacial adhesion through more pronounced Lewis acid–base interactions, providing guidance for developing high-performance Cr(VI)-free conversion processes.</p>