Atomic Force Infrared Microscopy (AFM-IR), also known as Photothermal Induced Resonance (PTIR), is a scanning probe spectroscopic technique that enables chemical characterization with submicron spatial resolution. This capability is especially valuable in corrosion science, where the chemical composition and phase distribution of metal alloy microstructures play a critical role in corrosion initiation, progression, and the formation of reaction products—key factors that determine long-term material performance. In this study, PTIR and infrared microspectroscopy were employed to investigate the influence of microstructure on corrosion product formation on ZnAlMg-coated steel exposed to SO₂-containing humid air. The analysis revealed that MgSO₄·7H₂O and MgSO₃·nH₂O predominantly form in eutectic regions. High-resolution PTIR measurements further showed preferential corrosion of microscopic MgZn₂ phases within the lamellar binary eutectic. This localized corrosion is attributed to micro-galvanic coupling, where MgZn₂ acts as the anodic site and Zn-rich phases serve as cathodic regions. These findings underscore the significant role of microstructural phase distribution in corrosion behavior and demonstrate the utility of PTIR for spatially resolved chemical analysis for complex metallic alloys.