Isomorphous substitution behavior of RE³⁺ in M/MgAlRE LDHs and its protective and self-healing effects for micro-arc oxidized Mg-Gd-Y-Zn alloy

The present study investigated the isomorphous substitution behaviors of MgAl layered double hydroxides (LDHs) with three different trivalent rare-earth (RE) ions (RE³⁺ = La³⁺, Y³⁺, Ce³⁺). Firstly, a micro-arc oxidation (MAO) layer was developed on a Mg-Gd-Y-Zn alloy, followed by the in-situ growth of MgAl LDHs sealing layer. By correlating coating morphology, structure, composition, and corrosion resistance across varying rare-earth substitution times, optimal durations were determined to synthesize MAO/MgAlRE LDHs coatings with superior comprehensive properties. The results showed that the optimal duration required for the in-situ growth and RE³⁺ substitution was 12 h of hydrothermal treatment in the respective solutions. The isomorphous substitution mechanism of RE³⁺ involves “LDHs dissolution–Adjusted morphology regrowth–RE³⁺ substitution recrystallization”. After isomorphous substitution, the laminate thickness of LDHs increased from 4.77 Å to 5.05 Å, 5.24 Å and 4.89 Å, respectively, and the corresponding isomorphous substitution rates were 5.87%, 9.85%, and 2.52%. In addition, all MAO/MgAlRE LDHs coatings demonstrated self-healing capabilities, with La³⁺ providing the most significant enhancement.