Investigations On The Sealing Of MAO Coating And Corrosion Resistance In SBF For AZ31Magnesium Alloy

With good biocompatibility and biodegradability, magnesium alloys are widelydeveloped. But its wider application is limited by the poor corrosion resistance in simulatedbody fluid （SBF）. A porous oxide coating could be prepared on the surface of magnesiumalloy by a micro-arc oxidation technology （MAO）, which will protect the substrate from beingcorroded. But some sealing treatments after MAO treatments must also be necessary tofurther improve its corrosion resistance.Micro-arc oxidation process was explored, the process parameters were fixed on and theMAO was carried out on the AZ31magnesium alloy. Some sealing technologies, such aselectrochemical deposition, alkali heat treatment, silane treatment and high-pressure steamtreatment, were applied on the porous MgO coating formed by MAO and the processparameters were ascertained via tests. The surface morphologies, phase structures andcompositions of the coatings are characterized by SEM, EDS and XRD. The thickness of thecoatings was measured with thickness tester and the bonding strength between the substrateand the coating was evaluated by the tension test. The corrosion resistance of the alloy withdifferent surface modifications was evaluated by measuring the hydrogen evolution rate inSBF.It shows that a uniform MgO coating, with the thickness of3-4μm and the diameter lessthan1μm, can be obtained on the surface of AZ31alloy by micro-arc oxidation in40g/LNaOH solution at DC current3A/cm²for30s. It is measured that the bonding strengthbetween the AZ31alloy substrate and the MAO coating is larger than25MPa. A layer ofCaHPO4·2H2O could be prepared on the surface of MAO coating by the electrochemicaldeposition under DC voltage4V for5min, in the500mL electrolyte composed of7.2gCa（NO₃）2，2.5g NH₄H₂PO₄and2ml H₂O₂with pH5. The deposition with the thickness of10μm exhibits flake-like. The thickness of the flake is less than10μm. They arrangeperpendicular and are well bonded to the porous MgO coating. KH550（NH₂CH₂CH₂CH₂Si（OC₂H₅）₃） is modified with water and ethanol. A uniform layer oforganosilicon compound with thickness of about14μm could be prepared on the surface ofMAO coating by silane treatment via dip coating process followed by heat treatment at200℃for3hrs. The SEM morphology shows that the organosilicon compound is well bonded to theporous MgO coating.The AZ31alloy MAO coated is treated under high-pressure steam1M NaOH solutionat200℃for5hrs. It shows an effective sealing of the porous MgO. The AZ31alloy MAOcoated experiences an alkali heat treatment, i. e., immersed in the saturated solution ofNaHCO₃and MgCO₃for24hrs and then is heated at500℃for5hrs. The porous MgO coatingis sealed and the composite coating becomes denser. The high-pressure steam treatment andthe alkali heat treatment can attributed a sealing of the porous MgO with an in situ chemical reaction inside the substrate and the MgO coating. The thickness of the formed layer varieslittle and the composite coating is not as uniformly and even as that formed by theelectrochemical deposition and the silane treatment.It shows that the corrosion resistance of AZ31magnesium in SBF could be greatlyimproved by the MAO. The corrosion resistance of the MAO coated alloy sealed by theelectrochemical deposition or the silane treatment is higher than that sealed by thehigh-pressure steam treatment or the alkali heat treatment.