Microarc Oxidation Coating On Mg-7Li Alloy And Its Corrosive And Tribological Properties

Mg-Li alloys, with good comprehensive properties, are suitable light weightfabrication materials. However, the poor corrosion resistance and wear performancebecome the main factor that has restricted their further applications. Surfacemodification for Mg-Li alloys is the best way to improve their performances.Using microarc oxidation treatment to improve the performance of Mg-Li alloy hasa great significance to enhance the national competitiveness in the field of aerospace,military power,electronic and automotive. In this study, the surface treatment wascarried out on Mg-7Li alloy by optimized microarc oxidation （MAO） process.Different additives were selected to enhance the growth rate and improve themicrostructure of the coatings. Finally, the ceramic coating prepared with phyticacid additive exhibits excellent corrosion and wear resistance. To discuss theformation mechanism of MAO coating, the morphology and phase compositionwere investigated by means of TEM, AFM, SEM, EDS, XRD and XPS. Themorphology of discharged microarc was analyzed by digital imaging technology.The corrosion resistance, impedance change and the corrosion mechanism of thebare alloy and the MAO coating were discussed by potentiodynamic polarizationcurves and Electrochemical Impedance Spectroscopy （EIS）. The friction and wearproperties of the coatings were measured using a ball-on-disk wear tester.In order to determine technological parameters of the MAO coating on Mg-7Lialloy, the influences of the Na2SiO3·9H2O concentration in the electrolyte andelectrical parameters on the structure of the coating were investigated. Resultsshowed that when the Na2SiO3·9H2O concentration varied from10g/L to30g/L, thethickness of the MAO coatings on Mg-7Li alloy increased from8to23μm. With arise in voltage and duty cycle as well as time, the growth rate and the thickness ofthe coatings increased, while the number of the micropores decreased and thecoating surface became coarse. On the contrary, the rise of frequency diminished thecoating thickness and made the surface smooth. The coatings prepared inNa2SiO3·9H2O electrolyte were mainly composed of nanoscale MgO, Li₂O₂andMg₂SiO₄. The optimized parameters are constant voltage of400-450V, oxidationtime of30min, frequency of200-400Hz, duty cycle of10-15%and Na2SiO3·9H2Oconcentration of15-20g/L.To adjust the structure and increase the thickness of the coating, the additivessuch as sodium citrate, sodium borate, sodium tungstate, sodium fluoride and phyticacid (C₆H₁₈O₂₄P₆) were added to the electrolyte, and effects of these additives on thecoating structure were discussed. Results showed that the use of additives improved the thickness of the coatings and reduced the quantity of surface micropores. Amongthem, the C₆H₁₈O₂₄P₆additive contributed maximum thickness of27μm to thecoatings. Chelation of phytic acid adsorbed Mg²⁺and Li+to form chain structure,which had improved growth rate of the coating.Morphology of microarc discharge affects growth process of the coatings.Results of ordinary digital imaging analysis showed that with increase inconcentration of sodium silicate, supply voltage, charging time and addition ofadditives, the discharged arc reduced, united and the diameter of micoarc increased.Analysis of high-speed video showed that discharge breakdown continued to the endof the pulse during a pulse-period in Na2SiO3solution, while discharge breakdownhappened at the same position repeatedly with intermittent time up to160ms indifferent pulse-period. The increase of voltage accelerated the migration ofdischarge location. Addition of sodium borate and1.5mL/L phytic acid resulted infast migration of discharge location, and sodium fluoride as well as phytic acidmade discharge occur at the same position repeatedly and extended intermittent time.Therefore, addition of phytic acid caused the repeated intermittent time of dischargebreakdown up to220ms.According to analysis of the microstructure, composition and element bindingenergy of the coatings in combination with morphology of discharged microarc, it isfound that the growth process of the coating involved the formation of doubleelectric layer, discharge of plasma, occurrence of compound reaction at hightemperature and pressure, continuous discharge of internal hidden arc and the coldquenching of the solution to the melts. Adding the additives to Na2SiO3·9H2Osolution could promote the growth rate of the coating during the breakdown process.The chelation of phytic acid made it absorb more Mg and Li ions to form chainstructure, improving the growth rate of the coating and causing the increase ofMg₂SiO₄content in the coating.Polarization curve and impedance spectroscopy are used to evaluate thecorrosion resistant of Mg-Li alloy. The researches on the polarization curve andimpedance of the Mg-7Li alloy and the coatings in3.5wt.%NaCl solution indicatedthat the corrosion potential（Ecorr） and corrosion current density（Icorr） of Mg-7Li alloywere-1.5857V and2.235×10^-4A/cm², respectively. The maximum impedance ofMg-7Li alloy was250ohm·cm². The Ecorrof the coating with both differentNa2SiO3·9H2O concentration and additive was higher than that of the substrate, andthe Icorrof the coating was lower than that of the substrate. Among them, the Ecorrofthe coating with3ml/L C₆H₁₈O₂₄P₆was-1.4761V which had shifted109.6mVtowards positive direction relative to the substrate, and its Icorrwas7.204×10-7A·cm-2which had reduced by3orders of magnitude compared to that of substrate. The impedance of the MAO coating prepared in Na2SiO3·9H2Oelectrolyte was10times higher than that of substrate, and the impedance of thecoating prepared in Na2SiO3·9H2O-C₆H₁₈O₂₄P₆electrolyte was10050ohm·cm²inNaCl solution after0.5h. The EIS test of the substrate and the coating immersed for2-120h indicated that the impedance of the substrate gradually decreased withincreasing time, and reached70ohm·cm²for120h when the substrate becamepowders due to corrosion damage. However, the impedance of MAO coating was15times that of substrate in the initial stage of immersion, and still maintained300ohm·cm²after120h immersion. Based on above data, the corrosion mechanism ofMAO coatings involved that the corrosion solution promoted the coating dissolutionby penetration into any porosity. The corrosion process included the penetration ofthe solution into the micropores, priority absorption of the Cl-ions on coating,formation of soluble chloride salt, coating peeling and overall corrosion.Under the dry friction test condition, the friction coefficient of Mg-7Li alloyranged from0.08to0.20. Its friction and wear process included the repeated actionsof plough wear, oxidation wear, adhesive wear as well as abrasive wear. After MAOtreatment, the friction coefficient of all the coating was lower than that of thesubstrate, improving the wear performance significantly. With the increase of theNa2SiO3·9H2O concentration in the electrolyte, the friction coefficient increased andwide-range fluctuations appeared. The friction coefficient of the additive-dopedcoatings less than0.15was lower than that of the coatings with20g/LNa2SiO3·9H2O.The friction and wear mechanism of the MAO coating on Mg-7Lialloy was resulted from the repeated actions of impact vibration on porous layersurface, wear or fracture of coating’s micro-protrusion as well as abrasive wearamong coating, peeling and SiO2ball.