Preparation And Properties Of Plasma Electrolytic Oxidation Composite Coatings On The Surface Of Mg-Li Alloys

Owing to the unique characteristics of high strength-to-weight ratio, high dimensionalstability, high specific stiffness, good machining property, excellent magnetic screen andshock resistance ability, as the lightest structural materials, magnesium–lithium alloy （Mg–Lialloys） have application prospects in the fields such as aeronautics, astronautics, weapons,electronics and automobiles. However, due to the high chemical reactivity of magnesium andlithium, it is susceptible to localized pitting corrosion, which lead to corrosion occurs withdestructive consequences for structural integrity of Mg–Li alloys, especially in aggressiveenvironments. Hence, it is specially promising to develop a simple and highly effectiveapproach to delay the onset of corrosion on Mg–Li alloys.We adjust different current density to obtain a PEO coating with excellent anti-corrosionability by changing the properties of inner barrier layer and outer porous layer, such aschemical and phase composition, and hardness of PEO coatings. We perform a detailedanalysis on surface and cross-sectional morphologies, chemical and phase compositions,anti-corrosion properties and formation mechanism of composite coatings on Mg–Li alloyprepared by combined plasma electrolytic oxidation and chemical conversion techniques,plasma electrolytic oxidation and self-assembled monolayers by SEM, EDX, XPS, TF XRD,ATR-FTIR, potentiodynamic polarization and EIS measurements.In this contribution, the effect of current density on plasma electrolytic oxidation process,morphologies, compositions and anti-corrosion properties of coatings are elucidated. X-rayphotoelectron spectroscopy and X-ray diffraction analysis show that coatings prepared atdifferent current densities are composed of MgO,-Mg₂SiO₄and-Mg₂SiO₄phase. Oxidecoating obtained at5A/dm2with a relatively higher degree of compactness and the highestMgO/Mg₂SiO₄ratio offers a superior corrosion resistance in potentiodynamic polarization andEIS tests.Two kinds of novel plasma electrolytic oxidation/chemical conversion composite coatingshave been successfully fabricated onto the surface of Mg–Li alloy. Yellow plasma electrolyticoxidation/cerium and lanthanum conversion composite coating shows a large amount offlake-like particles stack to each other and distribute randomly on the surface of PEO coating.A lot of cubic-like particles instead of flake-like particles distribute randomly on the surface of white plasma electrolytic oxidation/stannate conversion composite coating. Barrierproperties of composite coatings are evaluated by potentiodynamic polarization andelectrochemical impedance spectroscopy in3.5wt.%NaCl solution. Results indicate that theanti-corrosion properties of two composite coatings improve significantly in contrast toMg Li alloy substrate. Furthermore, plasma electrolytic oxidation/stannate conversioncomposite coating exhibits superior stability during long-term immersion test.Based on the vitally important water repellency induced by superhydrophobic surfaces, wesuccessfully extended our studies by incorporating two separate but equally attractivetechniques （PEO and Self-Assemble Monolayers） together, combining the remarkableadhesive and anti-corrosion abilities of PEO coating with promising organophosphonateself-assemble monolayers, thus giving rise to the excellent water repellent surface propertieswith exceptional corrosion-resistant coating. The procedure was conducted under mildconditions, and no intricate apparatus were needed. Typical porous oxide coating is coveredby nano-and microstructured sheets, which indicates that phosphonic acid molecules canchemisorb and form stable self-assembled monolayers onto the surface of PEO coating.Electrochemical studies indicates that once the porous PEO coating surface is grafted byorganophosphonate molecule, the solid/liquid interface has a blocking function, effectivelypreventing the charge-transfer reactions take place.The results show that although a slightdecrease in contact angles value after4months storage in air, the superior superhydrophobicproperties of composite coatings are still maintained, denoting that superhydrophobiccomposite coatings have exceptional long-term stability.