Incorporating reinforcements can improve the mechanical properties of Al matrix composites but make them susceptible to corrosion attacks by facilitating the formation of a galvanic cell network along the interface. The optimization of the interfacial structure improves the corrosion resistance of the composites. Herein, a solid-state thermite reaction integrated with a heat treatment strategy was developed to create plain interfaces, encompassing the grain boundary (GB) and reinforcement–matrix interface, in an Al–CuO composite system. The intergranular corrosion behavior and susceptibility to stress corrosion cracking of the composite were comprehensively examined by immersion/electrochemical corrosion tests and microstructural characterizations. The intragranular distribution of nanosized secondary phases, i.e., Al₂O₃ and precipitates, was investigated to address their contribution to the continuous galvanic cell detachment along the GBs and interruption of the corrosion path, thus boosting the corrosion duration. In addition, the tightly bonded Al₂O₃–Al interface with low strain energy distinctly lowers local susceptibility to corrosion. This study illuminates the correlation between interface characteristics and corrosion mechanisms in the composites, which can shed light on the development of high-corrosion-resistant composites.