Synergistic Protection Mechanism and Corrosion Resistance of Manganese Phosphate–Silane Composite Sealing Coating on AZ91D Magnesium Alloy

The poor corrosion resistance of magnesium alloys restricts their widespread application. In this study, a manganese phosphate conversion coating is first prepared on AZ91D magnesium alloy, followed by sealing with a KH550/KH570 composite silane coupling agent containing nano-SiO2. The morphology, structure, and corrosion resistance are characterized by SEM, EDS, FTIR, electrochemical measurements, and neutral salt spray tests. The single-phosphate coating exhibits microcracks and limited protection, whereas silane composite sealing significantly improves coating density. With 1.0 g/100 mL nano-SiO2, the coating shows a contact angle of 131°, a low-frequency impedance modulus of 105 Ω·cm2, a corrosion current density of 4.474 × 10−8 A/cm2, and only slight pulverization after 48 h salt spray exposure. The synergistic protection arises from the physical barrier of the silane cross-linked network, the labyrinth effect of nano-SiO2, and the spatial functional-group synergy between KH550 amino anchoring and KH570 alkyl hydrophobic shielding. This work provides an experimental basis for environmentally friendly phosphate–silane composite protection technology for magnesium alloys in automotive, aerospace, and 3C applications. From an industrial standpoint, the proposed waterborne and chromium-free phosphate–silane composite sealing route is compatible with conventional phosphating production lines and can be readily scaled up for magnesium alloy component protection.