Enhanced corrosion protection of magnesium alloy via in situ Mg-Al LDH coating modified by core-shell structured Zn-Al LDH@ZIF-8

The Mg-Al layered double hydroxide(LDH)conversion coatings were first synthesized in situ to modify the AZ91 D alloy through urea hydrolysis to adjust the pH values(9.4,10.4,11.2 and 11.4).The pH 11.2 Mg-Al LDH possessed the best compactness and good crystallinity compared to other in situ LDH coatings and obtained the lowest corrosion current density(icorr) of(2.884 × 10-6± 0.345 × 10-6) A·cm-2,which was attributed to the anion-exchange reaction of LDH and the physical barrier against corrosion owing to the twisted penetration pathway of the interlaced LDH sheets.Core-shell structured Zn-Al LDH@ZIF-8 powder modified with stearic acid(SA) was further wrapped with polyvinylidene fluoride(PVDF) to prepare a hydrophobic double-layered coating on the underlying pH 11.2 Mg-Al LDH(SLLZ).The water contact angle(CA) of the SLLZ coating reached 105.6°,and its icorrdecreased to(3.524 × 10-7± 0.214 × 10-7) A·cm-2compared with a single pH 11.2 film.The SLLZ coating exhibited high durability and corrosion protection,even after15 days of immersion in NaCl solution.The PVDF,SA and ZIF-8 nano-shells contributed to good hydrophobicity,effectively forming a physical barrier.The Zn-Al LDH core and underlying in situ Mg-Al LDH were beneficial for synergistically promoting anion-exchange reaction between the intercalated anions of LDH and chlorides in corrosive media.This work provides a promising approach that combines core-shell LDH@ZIF-8 with LDH film on a Mg alloy surface to construct a hydrophobic film with excellent longterm anti-corrosion performance.