Enhancing corrosion resistance of AZ31 Mg alloy via redox-assisted MnOx incorporation into PEO coatings and hydrothermal LDH deposition

This work presents a novel redox-mediated approach for incorporating MnO x into plasma electrolytic oxidation (PEO) coatings to enhance the corrosion protection of AZ31 Mg alloy. MnO x particles were generated in situ by a KMnO 4 -glucose redox reaction, producing gluconate ions and MnO x species that augmented the electrical double layer, reduced porosity, and revealed a two-step manganese reduction pathway of redox and plasma reduction, which resulted in multi-oxidation-state Mn inclusions. Subsequent hydrothermal deposition of ZnFe LDH yielded unique tabular flake morphologies with narrowed interflake gaps on MnO x modified PEO surface, which enhanced sealing properties. XRD verified post-treated MnOx-incorporated surfaces exhibited a larger LDH crystallite size (24.31 nm) compared to bare PEO coatings (6.97 nm). Meanwhile, XPS revealed the presence of multi-oxidation-state MnOx, quantified as 41.3% MnO, 3.5% Mn 2 O 3 , and 55.2% MnO 2 . These findings were further supported by DFT calculations, which showed that MnO x incorporation increased the LDH binding energy from -0.844 eV Å -2 to -0.989 eV Å -2 , while DOS analysis indicated stronger chemisorption facilitated by Zn orbital contributions. Electrochemical evaluations established samples possessing combined MnO x modification and ZnFe Post-treatment as the most effective system, showing the lowest corrosion current of 0.6943 nA cm -2 , the highest total impedance of 3.08 × 10 9 Ω cm 2 , and a controlled corrosion mechanism, with substrate penetration only minimally detected until after 120 h of immersion in 3.5 wt% NaCl solution. These results underscore the role of redox-generated MnO x and ZnFe LDH in improving both adhesion and corrosion resistance. The findings not only provide mechanistic insight into MnO x -assisted LDH deposition but also highlight the potential of this redox strategy as an affordable and efficient route for developing advanced protective coatings on magnesium alloys.