Microstructure And Corrosion Degradation Behavior Of Magnesium Alloy With Microarc Oxidation And Hydrothermal Process Treated

Biodegradable magnesium alloy has attractive application in the short term implants（such as the bone plate or bolt）,angiocarpy,and cavity intervention stents fields.But the corrosion degradation rate of magnesium alloy implants is too fast,which results in the loss of mechanical integrity before the complete tissue healing.This thesis designs and fabricates the microarc oxidation ceramic coatings of Si-Ca and Si-K systems on the surface of magnesium alloy in order to improve the corrosion resistance.To further enhance the surface densification and biological activity and of microarc oxidation porous coating,bioactive outer layer that involves apatite-type compounds has been synthesized using the hydrothermal method on the coating surface of Si-Ca and Si-K systems to enhance the corrosion resistance by sealing hole and inducing the growth of bone.The growth mechanism and microstructure of different microarc oxidation coatings and microarc oxidation-hydrothermal duplex coatings have been investigated through electrochemical polarization and long term immersion test in simulated body fluid（SBF）to evaluate the corrosion degradation,activating characteristic of different coating systems and reveal the relative mechanism.Finally,the corrosion degradation behaviors of several typical magnesium alloy implanted devices applied by the certain coating have been explored.The results indicate that electrolyte composition and electrical parameters affect the microstructure and corrosion resistance of microarc oxidation coatings on magnesium alloy surface.The Si-Ca coating prepared in electrolyte Na₂Si O₃-Ca（H₂PO₄）₂ is mainly composed of MgO,Mg₂Si O₄ and Ca Si O₃.The coating thickness increases from 7mm to 24mm with the increase of concentration of Ca（H₂PO₄）₂ from5g/L to 15g/L in electrolyte under the same voltage（450V）,meanwhile the content of elements Ca and P increases in the coatings.In the same electrolyte,the thickness and Ca,P content of coatings increase with the increase of voltage from 450 V to 500 V.The Si-K coating prepared in electrolyte Na₂Si O₃-KOH is mainly composed of Mg₂Si O₄ and MgO,and coating thickness increases from 2mm to 15mm with the increase of voltage,meanwhile,the micropore diameter of the coating surface increases.Microarc oxidation coatings have significantly enhanced the corrosion resistance of magnesium alloy substrate.The Si-Ca and Si-K bioactive ceramic coating systems reduce corrosion current by 3⁴ order of magnitudes,and the polarization resistance is 2³ order of magnitudes higher than the uncoated substrate.After long term soaking in SBF for 28 days,the corrosion resistance of different coating systems increases with the increasing coating thickness.The thicker Si-Ca15 coating with more content of Ca and P is almost not corroded,while the uncoated substrate is corroded and presents porous morphology.The corrosion process of microarc oxidation coated magnesium alloy in SBF can be divided into the following steps:a)coating dissolution and deposition in the early erosion,microcracks generate in the porous loose layer of surface;b)the corrosive ions in SBF contact the compact layer through the pores and cracks,then the compact layer is broken and corrosion channel through the coating is formed,and then the corrosion reaction occurs in the underlined substrate;c)corrosion products pile up at the interfaces;d)corrosion products are mainly composed of Mg（OH）₂,Ca CO₃,and（Ca,Mg）₃PO₄,the stress field caused by bulk accumulation of corrosion products caused the coating to peel off,in which the bare corrosion pits form.The micro pores on the microarc oxidation coated magnesium alloy surface are the key factor that results in corrosion damagement.In order to seal the micro pores on the coating surface and further improve the corrosion resistance,the outer layers with Mg（OH）₂,Ca HPO₄,and Ca₃（PO₄）₂ are hydrothermal synthesized on the Si-Ca and Si-K microarc oxidation bottom layer surface with the thickness of 10mm,respectively.The growth characteristic of outer layers on the microarc oxidation coating surface is affected by the hydrothermal activation treatment conditions（p H,hydrothermal temperature and hydrothermal time）.Ca HPO₄ is easier to be hydrothermal synthesized in acidic hydrothermal solution,thus the amount of Ca₃（PO₄）₂ and HA is less.While in the neutral or alkaline hydrothermal solution,the Mg（OH）₂ is preferential to produce,which is also beneficial to promote the formation of Ca₃（PO₄）₂ and HA products.The phase compositions of coatings have obvious regular changes during soaking corrosion in SBF.Ca HPO₄ and（Ca,Mg）₃（PO₄）₂ in hydrothermal coatings prone to produce the dissolution and ion exchange,which leads to the formation of Mg₃（PO₄）₂and transform to Ca₃（PO₄）₂.As Ca₃（PO₄）₂ can not exist in SBF steadily,and transforms into the more stable phase,Ca₁₀（PO₄）₆（OH）₂.The corrosion rate is relative high before Ca₁₀（PO₄）₆（OH）₂ is induced to precipitate onto the surface.If the hydrothermal coatings dissolve off and microarc oxidation bottom layer is exposed totally,Ca₁₀（PO₄）₆（OH）₂ can not be induced sequentially,which results in the acceleration of corrosion rate.If Ca₁₀（PO₄）₆（OH）₂ is successfully induced to precipitate onto the surface,corrosion rate of coatings will keep low during the long-time immersion corrosion process.It has demonstrated that outer layer treated by hydrothermal activation on the bottom microarc oxidation layer can observably reduce the corrosion degradation and enhance the bioactivity.The corrosion current of the sample with Ca（NO₃）₂-K₂HPO₄hydrothermal system on Si-Ca microarc oxidation layer reduces to 9.695×10^-6A/cm²,and corrosion potential increases to-0.92 V.The coatings keep perfect during the soaking corrosion process,and the weight of sample increases until the fourth week.The weight loss reached 24.97 mg/cm² after 8 weeks,which is 6 times lower than the magnesium alloy.The corrosion current of the sample with Ca（NO₃）₂-KH₂PO₄hydrothermal system on Si-K microarc oxidation layer reduces to 7.651×10^-6A/cm²,and corrosion potential increases to-0.82 V,its corrosion resistance has been greatly improved.The coatings keep perfect during the soaking corrosion process.The weight loss rate is one order of magnitude lower than magnesium alloy after 8 weeks,and Ca₁₀（PO₄）₆（OH）₂ can be rapidly induced to form only in one week.The exploratory investigation on the corrosion degradation of typical medical devices with coated magnesium alloys,such as bone plate,bone fixing bolt and suture silk,indicates that microarc oxidation coating can significantly decrease the degradation rate of magnesium alloy devices in SBF,thus maintain the good mechanical properties of magnesium alloy.After soaking corrosion for 4 weeks,the tensile strength of magnesium alloy sheet with Si-Ca15 and Si-Ca10 coatings still maintains 139.4 MPa and 134.4 MPa,while the tensile strength of magnesium alloy without coatings reduces to 32.7 MPa due to the severe corrosion.The coatings can restrain the corrosion degradation of magnesium wire with the diameter of 600μm and 300μm.The corrosion generates on the local defects in magnesium wire or coating defects regions after soaking corrosion for 36 h,which results in corrosion and breaking of magnesium wire.Large areas of corrosion pits on the magnesium alloy screw generate on the plate and thread after soaking corrosion for 4 weeks,while only local pitting generates on the thread of the coated screw.