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 AZ91D 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 (i(corr)) of (2.884 x 10(-6) +/- 0.345 x 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 degrees, and its i(corr) decreased to (3.524 x 10(-7) +/- 0.214 x 10(-7)) A.cm(-2) compared with a single pH 11.2 film. The SLLZ coating exhibited high durability and corrosion protection, even after 15 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 long-term anti-corrosion performance.

Automated summary

The in-situ synthesis of Mg-Al layered double hydroxide (LDH) conversion coatings and the addition of various modifiers resulted in the successful construction of a hydrophobic film with excellent corrosion resistance.