The purpose of this work was to reveal the effect of silanization pre - treatment on the adhesive force and the early protective properties of the whole coating system before coating water - based polyurethane (PU) varnish on surface of magnesium alloy (MA). A silane conversion (SC) coating was prepared on the AZ91D MA surface during the process of silanization pre - treatment. Then, the magnesium alloy surfaces without and with silanization pre - treatment were coated with waterborne polyurethane varnish. After the varnish was fully cured into a coating, the "polyurethane/magnesium alloy (PU/MA)" system and the "polyurethane/silane film/magnesium alloy (PU/SC/MA)" system were subjected to pull - out adhesion test and electrochemical impedance spectroscopy test to evaluate the effect of silanization pre - treatment on the adhesion and early protective properties of waterborne polyurethane coatings. Besides, the relevant mechanisms were also analyzed and discussed.Results showed that due to the silanization pre - treatment, the average adhesive force (Fa) of water - based PU coating on the MA surface increased from 12.74 MPa to 18.52 MPa. In 3.5%(mass fraction) NaCl solution, both the coating resistance (Rc) and the impedance value at low frequency (|Z|0.01) of PU/MA system and PU/SC/MA/system decreased gradually with the extension of immersion time, but Rc and |Z|0.01 of the PU/SC/MA system were greater than those of the PU/MA system. The above results illustrated that the silanization pre - treatment and the presence of silane conversion film had significant effects on improving the adhesion of waterborne polyurethane coatings on the surface of magnesium alloys and the protective properties of magnesium alloy substrate. Further analysis and discussion revealed that the main mechanism lied in the occurrence of “nucleophilic addition reaction” and “intramolecular rearrangement reaction” in the isocyanate group (-NCO) in the waterborne polyurethane coating and amino (-NH2) in the silane conversion film， -NH-CO-NH- bond was formed between the waterborne polyurethane coating and the silane conversion film. Coupled with the -Si-O-Mg- bond between the silane conversion film and the magnesium alloy substrate, the interaction between the waterborne polyurethane coating and the magnesium alloy substrate was changed from physical bonding without silanization pretreatment to chemical binding with silanization pre - treatment.
