Corrosion behavior of ZrO2-incorporated PEO coatings on magnesium–lithium alloy: Parameter optimization and stress-induced phase transformation

Magnesium-lithium alloys (Mg–Li alloys) are prone to severe corrosion due to their high chemical reactivity, especially in chloride-rich environments, which significantly limits their engineering applications despite their ultra-low density and excellent mechanical properties. Plasma electrolytic oxidation (PEO) technology offers promising ceramic coating solutions for corrosion protection of magnesium-lithium alloys, yet systematic process optimization and understanding of protection mechanisms remain limited. This study employs the Taguchi method to optimize PEO parameters for magnesium alloy LA103Z while elucidating a novel stress-induced crack closure mechanism based on ZrO 2 phase transformation. The experimental design comprised three factors at three levels: treatment duration (5–15 min), termination voltage (300–400 V), and (NH 4 ) 2 ZrF 6 concentration (2–6 g/L), evaluated using an L9(3 3 ) orthogonal array. Potentiodynamic polarization tests assessed corrosion resistance, while comprehensive microstructural characterization revealed the protective mechanisms. Results demonstrate that optimal coating parameters (15 min, 400 V, 6 g/L (NH 4 ) 2 ZrF 6 ) yield corrosion resistance 614 times superior to the substrate. Analysis of variance indicates termination voltage as the dominant factor (59.15 % contribution), followed by (NH 4 ) 2 ZrF 6 concentration (35.68 %) and treatment duration (5.17 %). Transmission electron microscopy analysis reveals a gradient ZrO 2 distribution creating a multi-layered protection system. Notably, stress-induced transformation from metastable tetragonal ZrO 2 to stable monoclinic ZrO 2 generates 3–5 % volumetric expansion, promoting microcrack closure and stress-induced crack closure behavior. This transformation toughening mechanism, combined with the chemical inertness of ZrO 2 and physical barrier effects, establishes a synergistic protection system accounting for the exceptional corrosion resistance. The findings provide both practical optimization guidelines and fundamental insights into ZrO 2 -enhanced stress-induced crack closure coatings for lightweight alloy protection.